1. Art and Diseño

Linux Fun
Paul Cobbaut
Linux Fun
Paul Cobbaut
lt-2.0
Publication date Fri 06 Feb 2015 01:02:26 AM CET
Abstract
This book is meant to be used in an instructor-led training. For self-study, the intent is to read
this book next to a working Linux computer so you can immediately do every subject, practicing
each command.
This book is aimed at novice Linux system administrators (and might be interesting and useful
for home users that want to know a bit more about their Linux system). However, this book
is not meant as an introduction to Linux desktop applications like text editors, browsers, mail
clients, multimedia or office applications.
More information and free .pdf available at http://linux-training.be .
Feel free to contact the author:
• Paul Cobbaut: [email protected], http://www.linkedin.com/in/cobbaut
Contributors to the Linux Training project are:
• Serge van Ginderachter: [email protected], build scripts; infrastructure setup; minor stuff
• Hendrik De Vloed: [email protected], buildheader.pl script
We'd also like to thank our reviewers:
• Wouter Verhelst: [email protected], http://grep.be
• Geert
Goossens:
geertgoossens
[email protected],
http://www.linkedin.com/in/
• Elie De Brauwer: [email protected], http://www.de-brauwer.be
• Christophe Vandeplas: [email protected], http://christophe.vandeplas.com
• Bert Desmet: [email protected], http://bdesmet.be
• Rich Yonts: [email protected],
Copyright 2007-2015 Paul Cobbaut
Permission is granted to copy, distribute and/or modify this document under the terms of the
GNU Free Documentation License, Version 1.3 or any later version published by the Free
Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts. A copy of the license is included in the section entitled 'GNU Free Documentation
License'.
Table of Contents
I. introduction to Linux ............................................................................................................. 1
1. Linux history ............................................................................................................. 3
1.1. Linux history .................................................................................................... 4
2. distributions ............................................................................................................... 5
2.1. Red Hat ........................................................................................................... 6
2.2. Ubuntu ............................................................................................................. 6
2.3. Debian ............................................................................................................. 6
2.4. Other ............................................................................................................... 6
2.5. Which to choose ? ............................................................................................. 7
3. licensing ..................................................................................................................... 8
3.1. about software licenses ....................................................................................... 9
3.2. public domain software and freeware ..................................................................... 9
3.3. Free Software or Open Source Software ................................................................. 9
3.4. GNU General Public License ............................................................................. 10
3.5. using GPLv3 software ....................................................................................... 10
3.6. BSD license .................................................................................................... 11
3.7. other licenses ................................................................................................... 11
3.8. combination of software licenses ......................................................................... 11
II. installing Linux ................................................................................................................. 12
4. installing Debian 8 .................................................................................................... 14
4.1. Debian ........................................................................................................... 15
4.2. Downloading ................................................................................................... 15
4.3. virtualbox networking ....................................................................................... 31
4.4. setting the hostname ......................................................................................... 33
4.5. adding a static ip address ................................................................................... 33
4.6. Debian package management .............................................................................. 34
5. installing CentOS 7 ................................................................................................... 35
5.1. download a CentOS 7 image .............................................................................. 36
5.2. Virtualbox ....................................................................................................... 38
5.3. CentOS 7 installing .......................................................................................... 43
5.4. CentOS 7 first logon ......................................................................................... 51
5.5. Virtualbox network interface .............................................................................. 52
5.6. configuring the network .................................................................................... 53
5.7. adding one static ip address ............................................................................... 53
5.8. package management ........................................................................................ 54
5.9. logon from Linux and MacOSX .......................................................................... 55
5.10. logon from MS Windows ................................................................................. 55
6. getting Linux at home ............................................................................................... 57
6.1. download a Linux CD image .............................................................................. 58
6.2. download Virtualbox ........................................................................................ 58
6.3. create a virtual machine .................................................................................... 59
6.4. attach the CD image ......................................................................................... 64
6.5. install Linux .................................................................................................... 67
III. first steps on the command line ........................................................................................... 68
7. man pages ................................................................................................................ 70
7.1. man $command ............................................................................................... 71
7.2. man $configfile ................................................................................................ 71
7.3. man $daemon .................................................................................................. 71
7.4. man -k (apropos) .............................................................................................. 71
7.5. whatis ............................................................................................................ 71
7.6. whereis ........................................................................................................... 71
7.7. man sections ................................................................................................... 72
7.8. man $section $file ............................................................................................ 72
7.9. man man ........................................................................................................ 72
7.10. mandb .......................................................................................................... 72
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8. working with directories ............................................................................................ 73
8.1. pwd ............................................................................................................... 74
8.2. cd .................................................................................................................. 74
8.3. absolute and relative paths ................................................................................. 75
8.4. path completion ............................................................................................... 76
8.5. ls ................................................................................................................... 76
8.6. mkdir ............................................................................................................. 78
8.7. rmdir .............................................................................................................. 78
8.8. practice: working with directories ........................................................................ 80
8.9. solution: working with directories ....................................................................... 81
9. working with files ..................................................................................................... 83
9.1. all files are case sensitive .................................................................................. 84
9.2. everything is a file ........................................................................................... 84
9.3. file ................................................................................................................ 84
9.4. touch .............................................................................................................. 85
9.5. rm ................................................................................................................. 86
9.6. cp .................................................................................................................. 87
9.7. mv ................................................................................................................. 88
9.8. rename ........................................................................................................... 89
9.9. practice: working with files ................................................................................ 90
9.10. solution: working with files .............................................................................. 91
10. working with file contents ........................................................................................ 93
10.1. head ............................................................................................................. 94
10.2. tail ............................................................................................................... 94
10.3. cat ............................................................................................................... 95
10.4. tac ............................................................................................................... 96
10.5. more and less ................................................................................................ 97
10.6. strings .......................................................................................................... 97
10.7. practice: file contents ...................................................................................... 98
10.8. solution: file contents ...................................................................................... 99
11. the Linux file tree .................................................................................................. 100
11.1. filesystem hierarchy standard .......................................................................... 101
11.2. man hier ...................................................................................................... 101
11.3. the root directory / ........................................................................................ 101
11.4. binary directories .......................................................................................... 102
11.5. configuration directories ................................................................................. 104
11.6. data directories ............................................................................................. 106
11.7. in memory directories .................................................................................... 108
11.8. /usr Unix System Resources ............................................................................ 113
11.9. /var variable data .......................................................................................... 115
11.10. practice: file system tree ............................................................................... 117
11.11. solution: file system tree ............................................................................... 119
IV. shell expansion ............................................................................................................... 121
12. commands and arguments ...................................................................................... 124
12.1. arguments .................................................................................................... 125
12.2. white space removal ...................................................................................... 125
12.3. single quotes ................................................................................................ 126
12.4. double quotes ............................................................................................... 126
12.5. echo and quotes ............................................................................................ 126
12.6. commands ................................................................................................... 127
12.7. aliases ......................................................................................................... 128
12.8. displaying shell expansion .............................................................................. 129
12.9. practice: commands and arguments .................................................................. 130
12.10. solution: commands and arguments ................................................................ 132
13. control operators ................................................................................................... 134
13.1. ; semicolon .................................................................................................. 135
13.2. & ampersand ................................................................................................ 135
13.3. $? dollar question mark .................................................................................. 135
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13.4. && double ampersand ...................................................................................
13.5. || double vertical bar ......................................................................................
13.6. combining && and || .....................................................................................
13.7. # pound sign ................................................................................................
13.8. \ escaping special characters ...........................................................................
13.9. practice: control operators ..............................................................................
13.10. solution: control operators .............................................................................
14. shell variables ........................................................................................................
14.1. $ dollar sign ................................................................................................
14.2. case sensitive ...............................................................................................
14.3. creating variables ..........................................................................................
14.4. quotes .........................................................................................................
14.5. set ..............................................................................................................
14.6. unset ...........................................................................................................
14.7. $PS1 ...........................................................................................................
14.8. $PATH .......................................................................................................
14.9. env .............................................................................................................
14.10. export ........................................................................................................
14.11. delineate variables .......................................................................................
14.12. unbound variables .......................................................................................
14.13. practice: shell variables ................................................................................
14.14. solution: shell variables ................................................................................
15. shell embedding and options ...................................................................................
15.1. shell embedding ............................................................................................
15.2. shell options ................................................................................................
15.3. practice: shell embedding ...............................................................................
15.4. solution: shell embedding ...............................................................................
16. shell history ...........................................................................................................
16.1. repeating the last command ............................................................................
16.2. repeating other commands ..............................................................................
16.3. history ........................................................................................................
16.4. !n ...............................................................................................................
16.5. Ctrl-r ..........................................................................................................
16.6. $HISTSIZE ..................................................................................................
16.7. $HISTFILE ..................................................................................................
16.8. $HISTFILESIZE ...........................................................................................
16.9. prevent recording a command .........................................................................
16.10. (optional)regular expressions .........................................................................
16.11. (optional) Korn shell history ..........................................................................
16.12. practice: shell history ...................................................................................
16.13. solution: shell history ...................................................................................
17. file globbing ..........................................................................................................
17.1. * asterisk .....................................................................................................
17.2. ? question mark ............................................................................................
17.3. [] square brackets ..........................................................................................
17.4. a-z and 0-9 ranges ........................................................................................
17.5. $LANG and square brackets ...........................................................................
17.6. preventing file globbing .................................................................................
17.7. practice: shell globbing ..................................................................................
17.8. solution: shell globbing ..................................................................................
V. pipes and commands .........................................................................................................
18. I/O redirection .......................................................................................................
18.1. stdin, stdout, and stderr ..................................................................................
18.2. output redirection ..........................................................................................
18.3. error redirection ............................................................................................
18.4. output redirection and pipes ............................................................................
18.5. joining stdout and stderr .................................................................................
18.6. input redirection ...........................................................................................
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18.7. confusing redirection ..................................................................................... 177
18.8. quick file clear ............................................................................................. 177
18.9. practice: input/output redirection ...................................................................... 178
18.10. solution: input/output redirection .................................................................... 179
19. filters .................................................................................................................... 180
19.1. cat .............................................................................................................. 181
19.2. tee .............................................................................................................. 181
19.3. grep ............................................................................................................ 181
19.4. cut .............................................................................................................. 183
19.5. tr ................................................................................................................ 183
19.6. wc .............................................................................................................. 184
19.7. sort ............................................................................................................. 185
19.8. uniq ............................................................................................................ 186
19.9. comm ......................................................................................................... 187
19.10. od ............................................................................................................. 188
19.11. sed ............................................................................................................ 189
19.12. pipe examples ............................................................................................. 190
19.13. practice: filters ............................................................................................ 191
19.14. solution: filters ............................................................................................ 192
20. basic Unix tools ..................................................................................................... 194
20.1. find ............................................................................................................ 195
20.2. locate .......................................................................................................... 196
20.3. date ............................................................................................................ 196
20.4. cal .............................................................................................................. 197
20.5. sleep ........................................................................................................... 197
20.6. time ............................................................................................................ 198
20.7. gzip - gunzip ................................................................................................ 199
20.8. zcat - zmore ................................................................................................. 199
20.9. bzip2 - bunzip2 ............................................................................................ 200
20.10. bzcat - bzmore ............................................................................................ 200
20.11. practice: basic Unix tools .............................................................................. 201
20.12. solution: basic Unix tools ............................................................................. 202
21. regular expressions ................................................................................................ 204
21.1. regex versions .............................................................................................. 205
21.2. grep ............................................................................................................ 206
21.3. rename ........................................................................................................ 211
21.4. sed ............................................................................................................. 214
21.5. bash history ................................................................................................. 218
VI. vi ................................................................................................................................. 219
22. Introduction to vi .................................................................................................. 221
22.1. command mode and insert mode ...................................................................... 222
22.2. start typing (a A i I o O) ................................................................................ 222
22.3. replace and delete a character (r x X) ............................................................... 223
22.4. undo and repeat (u .) ..................................................................................... 223
22.5. cut, copy and paste a line (dd yy p P) ............................................................... 223
22.6. cut, copy and paste lines (3dd 2yy) .................................................................. 224
22.7. start and end of a line (0 or ^ and $) ................................................................ 224
22.8. join two lines (J) and more ............................................................................. 224
22.9. words (w b) ................................................................................................. 225
22.10. save (or not) and exit (:w :q :q! ) .................................................................... 225
22.11. Searching (/ ?) ............................................................................................ 225
22.12. replace all ( :1,$ s/foo/bar/g ) ......................................................................... 226
22.13. reading files (:r :r !cmd) ................................................................................ 226
22.14. text buffers ................................................................................................ 226
22.15. multiple files .............................................................................................. 226
22.16. abbreviations .............................................................................................. 227
22.17. key mappings ............................................................................................. 228
22.18. setting options ............................................................................................ 228
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22.19. practice: vi(m) ............................................................................................
22.20. solution: vi(m) ............................................................................................
VII. scripting .......................................................................................................................
23. scripting introduction .............................................................................................
23.1. prerequisites .................................................................................................
23.2. hello world ..................................................................................................
23.3. she-bang ......................................................................................................
23.4. comment .....................................................................................................
23.5. variables ......................................................................................................
23.6. sourcing a script ...........................................................................................
23.7. troubleshooting a script ..................................................................................
23.8. prevent setuid root spoofing ............................................................................
23.9. practice: introduction to scripting .....................................................................
23.10. solution: introduction to scripting ...................................................................
24. scripting loops .......................................................................................................
24.1. test [ ] .........................................................................................................
24.2. if then else ..................................................................................................
24.3. if then elif ...................................................................................................
24.4. for loop .......................................................................................................
24.5. while loop ...................................................................................................
24.6. until loop .....................................................................................................
24.7. practice: scripting tests and loops .....................................................................
24.8. solution: scripting tests and loops .....................................................................
25. scripting parameters ..............................................................................................
25.1. script parameters ...........................................................................................
25.2. shift through parameters .................................................................................
25.3. runtime input ...............................................................................................
25.4. sourcing a config file .....................................................................................
25.5. get script options with getopts .........................................................................
25.6. get shell options with shopt ............................................................................
25.7. practice: parameters and options ......................................................................
25.8. solution: parameters and options ......................................................................
26. more scripting .......................................................................................................
26.1. eval ............................................................................................................
26.2. (( )) ............................................................................................................
26.3. let ..............................................................................................................
26.4. case ............................................................................................................
26.5. shell functions ..............................................................................................
26.6. practice : more scripting .................................................................................
26.7. solution : more scripting .................................................................................
VIII. local user management ...................................................................................................
27. introduction to users ..............................................................................................
27.1. whoami .......................................................................................................
27.2. who ............................................................................................................
27.3. who am i .....................................................................................................
27.4. w ...............................................................................................................
27.5. id ...............................................................................................................
27.6. su to another user .........................................................................................
27.7. su to root ....................................................................................................
27.8. su as root ....................................................................................................
27.9. su - $username .............................................................................................
27.10. su - ...........................................................................................................
27.11. run a program as another user .......................................................................
27.12. visudo .......................................................................................................
27.13. sudo su - ...................................................................................................
27.14. sudo logging ..............................................................................................
27.15. practice: introduction to users ........................................................................
27.16. solution: introduction to users ........................................................................
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28. user management ................................................................................................... 273
28.1. user management .......................................................................................... 274
28.2. /etc/passwd ................................................................................................... 274
28.3. root ............................................................................................................ 274
28.4. useradd ....................................................................................................... 275
28.5. /etc/default/useradd ........................................................................................ 275
28.6. userdel ........................................................................................................ 275
28.7. usermod ...................................................................................................... 275
28.8. creating home directories ................................................................................ 276
28.9. /etc/skel/ ...................................................................................................... 276
28.10. deleting home directories .............................................................................. 276
28.11. login shell .................................................................................................. 277
28.12. chsh .......................................................................................................... 277
28.13. practice: user management ............................................................................ 278
28.14. solution: user management ............................................................................ 279
29. user passwords ...................................................................................................... 281
29.1. passwd ........................................................................................................ 282
29.2. shadow file .................................................................................................. 282
29.3. encryption with passwd .................................................................................. 283
29.4. encryption with openssl .................................................................................. 283
29.5. encryption with crypt ..................................................................................... 284
29.6. /etc/login.defs ............................................................................................... 285
29.7. chage .......................................................................................................... 285
29.8. disabling a password ..................................................................................... 286
29.9. editing local files .......................................................................................... 286
29.10. practice: user passwords ............................................................................... 287
29.11. solution: user passwords ............................................................................... 288
30. user profiles .......................................................................................................... 290
30.1. system profile .............................................................................................. 291
30.2. ~/.bash_profile .............................................................................................. 291
30.3. ~/.bash_login ................................................................................................ 292
30.4. ~/.profile ..................................................................................................... 292
30.5. ~/.bashrc ..................................................................................................... 292
30.6. ~/.bash_logout .............................................................................................. 293
30.7. Debian overview ........................................................................................... 294
30.8. RHEL5 overview .......................................................................................... 294
30.9. practice: user profiles .................................................................................... 295
30.10. solution: user profiles ................................................................................... 296
31. groups .................................................................................................................. 297
31.1. groupadd ..................................................................................................... 298
31.2. group file .................................................................................................... 298
31.3. groups ......................................................................................................... 298
31.4. usermod ...................................................................................................... 299
31.5. groupmod .................................................................................................... 299
31.6. groupdel ...................................................................................................... 299
31.7. gpasswd ...................................................................................................... 300
31.8. newgrp ........................................................................................................ 301
31.9. vigr ............................................................................................................ 301
31.10. practice: groups .......................................................................................... 302
31.11. solution: groups .......................................................................................... 303
IX. file security .................................................................................................................... 304
32. standard file permissions ........................................................................................ 306
32.1. file ownership .............................................................................................. 307
32.2. list of special files ......................................................................................... 309
32.3. permissions .................................................................................................. 310
32.4. practice: standard file permissions .................................................................... 315
32.5. solution: standard file permissions .................................................................... 316
33. advanced file permissions ....................................................................................... 318
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33.1. sticky bit on directory ....................................................................................
33.2. setgid bit on directory ....................................................................................
33.3. setgid and setuid on regular files .....................................................................
33.4. setuid on sudo ..............................................................................................
33.5. practice: sticky, setuid and setgid bits ...............................................................
33.6. solution: sticky, setuid and setgid bits ...............................................................
34. access control lists ..................................................................................................
34.1. acl in /etc/fstab .............................................................................................
34.2. getfacl .........................................................................................................
34.3. setfacl .........................................................................................................
34.4. remove an acl entry .......................................................................................
34.5. remove the complete acl ................................................................................
34.6. the acl mask ................................................................................................
34.7. eiciel ..........................................................................................................
35. file links ................................................................................................................
35.1. inodes .........................................................................................................
35.2. about directories ...........................................................................................
35.3. hard links ....................................................................................................
35.4. symbolic links ..............................................................................................
35.5. removing links .............................................................................................
35.6. practice : links ..............................................................................................
35.7. solution : links ..............................................................................................
X. process management .........................................................................................................
36. introduction to processes ........................................................................................
36.1. terminology .................................................................................................
36.2. basic process management ..............................................................................
36.3. signalling processes .......................................................................................
36.4. practice : basic process management .................................................................
36.5. solution : basic process management .................................................................
37. process priorities ...................................................................................................
37.1. priority and nice values ..................................................................................
37.2. practice : process priorities .............................................................................
37.3. solution : process priorities .............................................................................
38. background jobs ....................................................................................................
38.1. background processes ....................................................................................
38.2. practice : background processes .......................................................................
38.3. solution : background processes .......................................................................
XI. disk management ............................................................................................................
39. disk devices ...........................................................................................................
39.1. terminology .................................................................................................
39.2. device naming ..............................................................................................
39.3. discovering disk devices .................................................................................
39.4. erasing a hard disk ........................................................................................
39.5. advanced hard disk settings ............................................................................
39.6. practice: hard disk devices ..............................................................................
39.7. solution: hard disk devices ..............................................................................
40. disk partitions .......................................................................................................
40.1. about partitions .............................................................................................
40.2. discovering partitions .....................................................................................
40.3. partitioning new disks ....................................................................................
40.4. about the partition table .................................................................................
40.5. practice: partitions .........................................................................................
40.6. solution: partitions ........................................................................................
41. file systems ............................................................................................................
41.1. about file systems .........................................................................................
41.2. common file systems .....................................................................................
41.3. putting a file system on a partition ...................................................................
41.4. tuning a file system .......................................................................................
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41.5. checking a file system ...................................................................................
41.6. practice: file systems .....................................................................................
41.7. solution: file systems .....................................................................................
mounting ..............................................................................................................
42.1. mounting local file systems ............................................................................
42.2. displaying mounted file systems ......................................................................
42.3. from start to finish ........................................................................................
42.4. permanent mounts .........................................................................................
42.5. securing mounts ............................................................................................
42.6. mounting remote file systems ..........................................................................
42.7. practice: mounting file systems ........................................................................
42.8. solution: mounting file systems .......................................................................
troubleshooting tools ..............................................................................................
43.1. lsof .............................................................................................................
43.2. fuser ...........................................................................................................
43.3. chroot .........................................................................................................
43.4. iostat ..........................................................................................................
43.5. iotop ...........................................................................................................
43.6. vmstat .........................................................................................................
43.7. practice: troubleshooting tools .........................................................................
43.8. solution: troubleshooting tools .........................................................................
introduction to uuid's .............................................................................................
44.1. about unique objects ......................................................................................
44.2. tune2fs ........................................................................................................
44.3. uuid ............................................................................................................
44.4. uuid in /etc/fstab ...........................................................................................
44.5. uuid as a boot device .....................................................................................
44.6. practice: uuid and filesystems ..........................................................................
44.7. solution: uuid and filesystems .........................................................................
introduction to raid ................................................................................................
45.1. hardware or software .....................................................................................
45.2. raid levels ....................................................................................................
45.3. building a software raid5 array ........................................................................
45.4. practice: raid ................................................................................................
45.5. solution: raid ................................................................................................
logical volume management ....................................................................................
46.1. introduction to lvm ........................................................................................
46.2. lvm terminology ...........................................................................................
46.3. example: using lvm .......................................................................................
46.4. example: extend a logical volume ....................................................................
46.5. example: resize a physical Volume ...................................................................
46.6. example: mirror a logical volume .....................................................................
46.7. example: snapshot a logical volume .................................................................
46.8. verifying existing physical volumes ..................................................................
46.9. verifying existing volume groups .....................................................................
46.10. verifying existing logical volumes ..................................................................
46.11. manage physical volumes .............................................................................
46.12. manage volume groups .................................................................................
46.13. manage logical volumes ...............................................................................
46.14. practice : lvm .............................................................................................
46.15. solution : lvm .............................................................................................
iSCSI devices .........................................................................................................
47.1. iSCSI terminology ........................................................................................
47.2. iSCSI Target in RHEL/CentOS .......................................................................
47.3. iSCSI Initiator in RHEL/CentOS .....................................................................
47.4. iSCSI target on Debian ..................................................................................
47.5. iSCSI target setup with dd files .......................................................................
47.6. ISCSI initiator on ubuntu ...............................................................................
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47.7. using iSCSI devices ......................................................................................
47.8. practice: iSCSI devices ..................................................................................
47.9. solution: iSCSI devices ..................................................................................
48. introduction to multipathing ...................................................................................
48.1. install multipath ............................................................................................
48.2. configure multipath .......................................................................................
48.3. network .......................................................................................................
48.4. start multipathd and iscsi ................................................................................
48.5. multipath list ................................................................................................
48.6. using the device ............................................................................................
48.7. practice: multipathing ....................................................................................
48.8. solution: multipathing ....................................................................................
XII. boot management ...........................................................................................................
49. bootloader .............................................................................................................
49.1. boot terminology ..........................................................................................
49.2. grub ............................................................................................................
49.3. grub2 ..........................................................................................................
49.4. lilo .............................................................................................................
49.5. practice: bootloader .......................................................................................
49.6. solution: bootloader .......................................................................................
50. init and runlevels ...................................................................................................
50.1. system init(ialization) ....................................................................................
50.2. daemon or demon ? .......................................................................................
50.3. starting and stopping daemons .........................................................................
50.4. chkconfig ....................................................................................................
50.5. update-rc.d ...................................................................................................
50.6. bum ............................................................................................................
50.7. runlevels .....................................................................................................
50.8. systemd .......................................................................................................
50.9. practice: init .................................................................................................
50.10. solution : init ..............................................................................................
XIII. system management ......................................................................................................
51. scheduling .............................................................................................................
51.1. one time jobs with at .....................................................................................
51.2. cron ............................................................................................................
51.3. practice : scheduling ......................................................................................
51.4. solution : scheduling ......................................................................................
52. logging ..................................................................................................................
52.1. login logging ................................................................................................
52.2. syslogd ........................................................................................................
52.3. logger .........................................................................................................
52.4. watching logs ...............................................................................................
52.5. rotating logs .................................................................................................
52.6. practice : logging ..........................................................................................
52.7. solution : logging ..........................................................................................
53. memory management .............................................................................................
53.1. displaying memory and cache .........................................................................
53.2. managing swap space ....................................................................................
53.3. monitoring memory with vmstat ......................................................................
53.4. practice : memory .........................................................................................
53.5. solution : memory .........................................................................................
54. resource monitoring ...............................................................................................
54.1. four basic resources .......................................................................................
54.2. top .............................................................................................................
54.3. free ............................................................................................................
54.4. watch ..........................................................................................................
54.5. vmstat .........................................................................................................
54.6. iostat ..........................................................................................................
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54.7. mpstat .........................................................................................................
54.8. sadc and sar .................................................................................................
54.9. ntop ............................................................................................................
54.10. iftop ..........................................................................................................
54.11. iptraf .........................................................................................................
54.12. nmon ........................................................................................................
54.13. htop ..........................................................................................................
55. package management .............................................................................................
55.1. package terminology ......................................................................................
55.2. deb package management ...............................................................................
55.3. apt-get ........................................................................................................
55.4. aptitude .......................................................................................................
55.5. apt ..............................................................................................................
55.6. rpm ............................................................................................................
55.7. yum ............................................................................................................
55.8. alien ...........................................................................................................
55.9. downloading software outside the repository ......................................................
55.10. compiling software ......................................................................................
55.11. practice: package management .......................................................................
55.12. solution: package management .......................................................................
XIV. network management .....................................................................................................
56. general networking ................................................................................................
56.1. network layers ..............................................................................................
56.2. unicast, multicast, broadcast, anycast ................................................................
56.3. lan-wan-man ................................................................................................
56.4. internet - intranet - extranet ............................................................................
56.5. tcp/ip ..........................................................................................................
57. interface configuration ...........................................................................................
57.1. to gui or not to gui .......................................................................................
57.2. Debian/Ubuntu nic configuration .....................................................................
57.3. Red Hat/Fedora nic configuration ....................................................................
57.4. ifconfig .......................................................................................................
57.5. hostname .....................................................................................................
57.6. arp .............................................................................................................
57.7. route ...........................................................................................................
57.8. ping ............................................................................................................
57.9. optional: ethtool ............................................................................................
57.10. practice: interface configuration .....................................................................
57.11. solution: interface configuration .....................................................................
58. network sniffing .....................................................................................................
58.1. wireshark .....................................................................................................
58.2. tcpdump ......................................................................................................
58.3. practice: network sniffing ...............................................................................
58.4. solution: network sniffing ...............................................................................
59. binding and bonding ..............................................................................................
59.1. binding on Redhat/Fedora ...............................................................................
59.2. binding on Debian/Ubuntu ..............................................................................
59.3. bonding on Redhat/Fedora ..............................................................................
59.4. bonding on Debian/Ubuntu .............................................................................
59.5. practice: binding and bonding .........................................................................
59.6. solution: binding and bonding .........................................................................
60. ssh client and server ..............................................................................................
60.1. about ssh .....................................................................................................
60.2. log on to a remote server ...............................................................................
60.3. executing a command in remote ......................................................................
60.4. scp .............................................................................................................
60.5. setting up passwordless ssh .............................................................................
60.6. X forwarding via ssh .....................................................................................
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60.7. troubleshooting ssh ........................................................................................ 627
60.8. sshd ............................................................................................................ 628
60.9. sshd keys ..................................................................................................... 628
60.10. ssh-agent ................................................................................................... 628
60.11. practice: ssh ............................................................................................... 629
60.12. solution: ssh ............................................................................................... 630
61. introduction to nfs ................................................................................................. 632
61.1. nfs protocol versions ..................................................................................... 633
61.2. rpcinfo ........................................................................................................ 633
61.3. server configuration ....................................................................................... 634
61.4. /etc/exports ................................................................................................... 634
61.5. exportfs ....................................................................................................... 634
61.6. client configuration ....................................................................................... 635
61.7. practice: introduction to nfs ............................................................................ 636
62. introduction to networking ..................................................................................... 637
62.1. introduction to iptables .................................................................................. 638
62.2. practice : iptables .......................................................................................... 639
62.3. solution : iptables .......................................................................................... 640
62.4. xinetd and inetd ............................................................................................ 641
62.5. practice : inetd and xinetd ............................................................................... 643
62.6. network file system ....................................................................................... 644
62.7. practice : network file system .......................................................................... 646
XV. kernel management ........................................................................................................ 647
63. the Linux kernel .................................................................................................... 649
63.1. about the Linux kernel ................................................................................... 650
63.2. Linux kernel source ....................................................................................... 653
63.3. kernel boot files ............................................................................................ 657
63.4. Linux kernel modules .................................................................................... 659
63.5. compiling a kernel ........................................................................................ 664
63.6. compiling one module ................................................................................... 667
64. library management ............................................................................................... 669
64.1. introduction ................................................................................................. 670
64.2. /lib and /usr/lib ............................................................................................. 670
64.3. ldd ............................................................................................................. 670
64.4. ltrace .......................................................................................................... 671
64.5. dpkg -S and debsums .................................................................................... 671
64.6. rpm -qf and rpm -V ...................................................................................... 672
64.7. tracing with strace ......................................................................................... 673
XVI. backup management ...................................................................................................... 674
65. backup .................................................................................................................. 676
65.1. About tape devices ........................................................................................ 676
65.2. Compression ................................................................................................ 677
65.3. tar .............................................................................................................. 677
65.4. Backup Types .............................................................................................. 679
65.5. dump and restore .......................................................................................... 680
65.6. cpio ............................................................................................................ 680
65.7. dd .............................................................................................................. 681
65.8. split ............................................................................................................ 682
65.9. practice: backup ............................................................................................ 682
XVII. apache and squid ......................................................................................................... 684
66. apache web server ................................................................................................. 686
66.1. introduction to apache .................................................................................... 687
66.2. port virtual hosts on Debian ............................................................................ 694
66.3. named virtual hosts on Debian ........................................................................ 698
66.4. password protected website on Debian .............................................................. 700
66.5. port virtual hosts on CentOS ........................................................................... 701
66.6. named virtual hosts on CentOS ....................................................................... 705
66.7. password protected website on CentOS ............................................................. 707
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66.8. troubleshooting apache ...................................................................................
66.9. virtual hosts example .....................................................................................
66.10. aliases and redirects .....................................................................................
66.11. more on .htaccess ........................................................................................
66.12. traffic ........................................................................................................
66.13. self signed cert on Debian ............................................................................
66.14. self signed cert on RHEL/CentOS ..................................................................
66.15. practice: apache ..........................................................................................
67. introduction to squid ..............................................................................................
67.1. about proxy servers .......................................................................................
67.2. installing squid .............................................................................................
67.3. port 3128 .....................................................................................................
67.4. starting and stopping .....................................................................................
67.5. client proxy settings ......................................................................................
67.6. upside down images ......................................................................................
67.7. /var/log/squid ................................................................................................
67.8. access control ...............................................................................................
67.9. testing squid ................................................................................................
67.10. name resolution ..........................................................................................
XVIII. mysql database ...........................................................................................................
68. introduction to sql using mysql ...............................................................................
68.1. installing mysql ............................................................................................
68.2. accessing mysql ............................................................................................
68.3. mysql databases ............................................................................................
68.4. mysql tables .................................................................................................
68.5. mysql records ...............................................................................................
68.6. joining two tables .........................................................................................
68.7. mysql triggers ..............................................................................................
XIX. dns server ....................................................................................................................
69. introduction to DNS ...............................................................................................
69.1. about dns .....................................................................................................
69.2. dns namespace .............................................................................................
69.3. caching only servers ......................................................................................
69.4. authoritative dns servers .................................................................................
69.5. primary and secondary ...................................................................................
69.6. zone transfers ...............................................................................................
69.7. master and slave ...........................................................................................
69.8. SOA record .................................................................................................
69.9. full or incremental zone transfers .....................................................................
69.10. DNS cache .................................................................................................
69.11. forward lookup zone example ........................................................................
69.12. example: caching only DNS server .................................................................
69.13. example: caching only with forwarder .............................................................
69.14. example: primary authoritative server ..............................................................
69.15. example: a DNS slave server .........................................................................
69.16. practice: dns ...............................................................................................
69.17. solution: dns ...............................................................................................
70. advanced DNS .......................................................................................................
70.1. example: DNS round robin .............................................................................
70.2. DNS delegation ............................................................................................
70.3. example: DNS delegation ...............................................................................
70.4. example: split-horizon dns ..............................................................................
70.5. old dns topics ...............................................................................................
XX. dhcp server ...................................................................................................................
71. introduction to dhcp ..............................................................................................
71.1. four broadcasts .............................................................................................
71.2. picturing dhcp ..............................................................................................
71.3. installing a dhcp server ..................................................................................
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71.4. dhcp server on Red Hat .................................................................................
71.5. dhcp options ................................................................................................
71.6. client reservations .........................................................................................
71.7. example config files ......................................................................................
71.8. older example config files ..............................................................................
71.9. advanced dhcp ..............................................................................................
71.10. Practice: dhcp .............................................................................................
XXI. iptables firewall ............................................................................................................
72. introduction to routers ...........................................................................................
72.1. router or firewall ..........................................................................................
72.2. packet forwarding .........................................................................................
72.3. packet filtering .............................................................................................
72.4. stateful ........................................................................................................
72.5. nat (network address translation) ......................................................................
72.6. pat (port address translation) ...........................................................................
72.7. snat (source nat) ...........................................................................................
72.8. masquerading ...............................................................................................
72.9. dnat (destination nat) .....................................................................................
72.10. port forwarding ...........................................................................................
72.11. /proc/sys/net/ipv4/ip_forward .........................................................................
72.12. /etc/sysctl.conf ............................................................................................
72.13. sysctl ........................................................................................................
72.14. practice: packet forwarding ...........................................................................
72.15. solution: packet forwarding ...........................................................................
73. iptables firewall .....................................................................................................
73.1. iptables tables ..............................................................................................
73.2. starting and stopping iptables ..........................................................................
73.3. the filter table ..............................................................................................
73.4. practice: packet filtering .................................................................................
73.5. solution: packet filtering .................................................................................
73.6. network address translation .............................................................................
XXII. selinux .......................................................................................................................
74. introduction to SELinux .........................................................................................
74.1. selinux modes ..............................................................................................
74.2. logging ........................................................................................................
74.3. activating selinux ..........................................................................................
74.4. getenforce ....................................................................................................
74.5. setenforce ....................................................................................................
74.6. sestatus .......................................................................................................
74.7. policy .........................................................................................................
74.8. /etc/selinux/config .........................................................................................
74.9. DAC or MAC ..............................................................................................
74.10. ls -Z .........................................................................................................
74.11. -Z .............................................................................................................
74.12. /selinux ......................................................................................................
74.13. identity ......................................................................................................
74.14. role ...........................................................................................................
74.15. type (or domain) .........................................................................................
74.16. security context ...........................................................................................
74.17. transition ...................................................................................................
74.18. extended attributes .......................................................................................
74.19. process security context ................................................................................
74.20. chcon ........................................................................................................
74.21. an example ................................................................................................
74.22. setroubleshoot .............................................................................................
74.23. booleans ....................................................................................................
XXIII. Introduction to Samba .................................................................................................
75. introduction to samba ............................................................................................
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76.
77.
78.
79.
80.
81.
75.1. verify installed version ................................................................................... 836
75.2. installing samba ............................................................................................ 837
75.3. documentation .............................................................................................. 838
75.4. starting and stopping samba ............................................................................ 839
75.5. samba daemons ............................................................................................ 840
75.6. the SMB protocol ......................................................................................... 841
75.7. practice: introduction to samba ........................................................................ 842
getting started with samba ...................................................................................... 843
76.1. /etc/samba/smb.conf ....................................................................................... 844
76.2. /usr/bin/testparm ............................................................................................ 845
76.3. /usr/bin/smbclient .......................................................................................... 846
76.4. /usr/bin/smbtree ............................................................................................ 848
76.5. server string ................................................................................................. 849
76.6. Samba Web Administration Tool (SWAT) ......................................................... 850
76.7. practice: getting started with samba .................................................................. 851
76.8. solution: getting started with samba .................................................................. 852
a read only file server ............................................................................................ 854
77.1. Setting up a directory to share ......................................................................... 855
77.2. configure the share ........................................................................................ 855
77.3. restart the server ........................................................................................... 856
77.4. verify the share ............................................................................................ 856
77.5. a note on netcat ............................................................................................ 858
77.6. practice: read only file server .......................................................................... 859
77.7. solution: read only file server .......................................................................... 860
a writable file server .............................................................................................. 861
78.1. set up a directory to share .............................................................................. 862
78.2. share section in smb.conf ............................................................................... 862
78.3. configure the share ........................................................................................ 862
78.4. test connection with windows .......................................................................... 862
78.5. test writing with windows .............................................................................. 863
78.6. How is this possible ? .................................................................................... 863
78.7. practice: writable file server ............................................................................ 864
78.8. solution: writable file server ............................................................................ 865
samba first user account ......................................................................................... 866
79.1. creating a samba user .................................................................................... 867
79.2. ownership of files ......................................................................................... 867
79.3. /usr/bin/smbpasswd ........................................................................................ 867
79.4. /etc/samba/smbpasswd .................................................................................... 867
79.5. passdb backend ............................................................................................ 868
79.6. forcing this user ............................................................................................ 868
79.7. practice: first samba user account ..................................................................... 869
79.8. solution: first samba user account .................................................................... 870
samba authentication ............................................................................................. 871
80.1. creating the users on Linux ............................................................................. 872
80.2. creating the users on samba ............................................................................ 872
80.3. security = user .............................................................................................. 872
80.4. configuring the share ..................................................................................... 873
80.5. testing access with net use .............................................................................. 873
80.6. testing access with smbclient .......................................................................... 873
80.7. verify ownership ........................................................................................... 874
80.8. common problems ......................................................................................... 874
80.9. practice : samba authentication ........................................................................ 876
80.10. solution: samba authentication ....................................................................... 877
samba securing shares ............................................................................................ 878
81.1. security based on user name ........................................................................... 879
81.2. security based on ip-address ............................................................................ 879
81.3. security through obscurity .............................................................................. 880
81.4. file system security ....................................................................................... 880
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81.5. practice: securing shares .................................................................................
81.6. solution: securing shares ................................................................................
82. samba domain member ..........................................................................................
82.1. changes in smb.conf ......................................................................................
82.2. joining an Active Directory domain ..................................................................
82.3. winbind .......................................................................................................
82.4. wbinfo ........................................................................................................
82.5. getent ..........................................................................................................
82.6. file ownership ..............................................................................................
82.7. practice : samba domain member .....................................................................
83. samba domain controller ........................................................................................
83.1. about Domain Controllers ...............................................................................
83.2. About security modes ....................................................................................
83.3. About password backends ..............................................................................
83.4. [global] section in smb.conf ............................................................................
83.5. netlogon share ..............................................................................................
83.6. other [share] sections .....................................................................................
83.7. Users and Groups .........................................................................................
83.8. tdbsam ........................................................................................................
83.9. about computer accounts ................................................................................
83.10. local or roaming profiles ..............................................................................
83.11. Groups in NTFS acls ...................................................................................
83.12. logon scripts ...............................................................................................
83.13. practice: samba domain controller ..................................................................
84. a brief look at samba 4 ..........................................................................................
84.1. Samba 4 alpha 6 ...........................................................................................
XXIV. introducing git ...........................................................................................................
85. git ........................................................................................................................
85.1. git ..............................................................................................................
85.2. installing git .................................................................................................
85.3. starting a project ...........................................................................................
85.4. git branches .................................................................................................
85.5. to be continued... ..........................................................................................
85.6. github.com ...................................................................................................
85.7. add your public key to github .........................................................................
85.8. practice: git ..................................................................................................
XXV. ipv6 ..........................................................................................................................
86. Introduction to ipv6 ...............................................................................................
86.1. about ipv6 ...................................................................................................
86.2. network id and host id ...................................................................................
86.3. host part generation .......................................................................................
86.4. ipv4 mapped ipv6 address ..............................................................................
86.5. link local addresses .......................................................................................
86.6. unique local addresses ...................................................................................
86.7. globally unique unicast addresses .....................................................................
86.8. 6to4 ............................................................................................................
86.9. ISP .............................................................................................................
86.10. non routable addresses .................................................................................
86.11. ping6 ........................................................................................................
86.12. Belgium and ipv6 ........................................................................................
86.13. other websites .............................................................................................
86.14. 6to4 gateways .............................................................................................
86.15. ping6 and dns .............................................................................................
86.16. ipv6 and tcp/http .........................................................................................
86.17. ipv6 PTR record .........................................................................................
86.18. 6to4 setup on Linux .....................................................................................
XXVI. Appendices ................................................................................................................
A. certifications ...........................................................................................................
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A.1. Certification ..................................................................................................
B. Raspberry Pi Linux ................................................................................................
B.1. buying the hardware .......................................................................................
B.2. downloading the images ..................................................................................
B.3. writing the SD cards .......................................................................................
B.4. adjusting network settings ................................................................................
B.5. connecting the hardware ..................................................................................
B.6. connecting with ssh ........................................................................................
B.7. camera module ..............................................................................................
B.8. ...................................................................................................................
B.9. building a stack of pi's ....................................................................................
B.10. sending mail from the pi ................................................................................
B.11. disable ipv6 .................................................................................................
B.12. .................................................................................................................
B.13. .................................................................................................................
B.14. .................................................................................................................
B.15. raspi camera ................................................................................................
C. keyboard settings ....................................................................................................
C.1. about keyboard layout .....................................................................................
C.2. X Keyboard Layout ........................................................................................
C.3. shell keyboard layout ......................................................................................
D. hardware ...............................................................................................................
D.1. buses ...........................................................................................................
D.2. interrupts ......................................................................................................
D.3. io ports ........................................................................................................
D.4. dma .............................................................................................................
E. installing Linux .......................................................................................................
E.1. about ............................................................................................................
E.2. legacy content ...............................................................................................
F. disk quotas .............................................................................................................
F.1. About Disk Quotas .........................................................................................
F.2. Practice Disk quotas .......................................................................................
G. introduction to vnc .................................................................................................
G.1. About VNC ..................................................................................................
G.2. VNC Server ..................................................................................................
G.3. VNC Client ..................................................................................................
G.4. Practice VNC ................................................................................................
H. cloning ...................................................................................................................
H.1. About cloning ...............................................................................................
H.2. About offline cloning .....................................................................................
H.3. Offline cloning example ..................................................................................
I. License ...................................................................................................................
Index ..................................................................................................................................
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955
955
955
955
956
957
957
957
957
959
966
List of Tables
2.1. choosing a Linux distro ...................................................................................................... 7
4.1. Debian releases ................................................................................................................ 15
22.1. getting to command mode .............................................................................................. 222
22.2. switch to insert mode .................................................................................................... 222
22.3. replace and delete ......................................................................................................... 223
22.4. undo and repeat ............................................................................................................ 223
22.5. cut, copy and paste a line ............................................................................................... 223
22.6. cut, copy and paste lines ................................................................................................ 224
22.7. start and end of line ...................................................................................................... 224
22.8. join two lines ............................................................................................................... 224
22.9. words .......................................................................................................................... 225
22.10. save and exit vi .......................................................................................................... 225
22.11. searching ................................................................................................................... 225
22.12. replace ...................................................................................................................... 226
22.13. read files and input ...................................................................................................... 226
22.14. text buffers ................................................................................................................ 226
22.15. multiple files .............................................................................................................. 227
22.16. abbreviations .............................................................................................................. 227
30.1. Debian User Environment .............................................................................................. 294
30.2. Red Hat User Environment ............................................................................................. 294
32.1. Unix special files .......................................................................................................... 309
32.2. standard Unix file permissions ........................................................................................ 310
32.3. Unix file permissions position ......................................................................................... 310
32.4. Octal permissions .......................................................................................................... 313
39.1. ide device naming ......................................................................................................... 368
39.2. scsi device naming ........................................................................................................ 368
40.1. primary, extended and logical partitions ............................................................................ 380
40.2. Partition naming ........................................................................................................... 380
50.1. systemd power management ........................................................................................... 512
69.1. the first top level domains .............................................................................................. 748
69.2. new general purpose tld's ............................................................................................... 748
72.1. Packet Forwarding Exercise ............................................................................................ 799
72.2. Packet Forwarding Solution ............................................................................................ 801
xx
Part I. introduction to Linux
Table of Contents
1. Linux history ..................................................................................................................... 3
1.1. Linux history ............................................................................................................ 4
2. distributions ....................................................................................................................... 5
2.1. Red Hat ................................................................................................................... 6
2.2. Ubuntu ..................................................................................................................... 6
2.3. Debian ..................................................................................................................... 6
2.4. Other ....................................................................................................................... 6
2.5. Which to choose ? ..................................................................................................... 7
3. licensing ............................................................................................................................. 8
3.1. about software licenses ............................................................................................... 9
3.2. public domain software and freeware ............................................................................ 9
3.3. Free Software or Open Source Software ......................................................................... 9
3.4. GNU General Public License ..................................................................................... 10
3.5. using GPLv3 software ............................................................................................... 10
3.6. BSD license ............................................................................................................ 11
3.7. other licenses .......................................................................................................... 11
3.8. combination of software licenses ................................................................................. 11
2
Chapter 1. Linux history
This chapter briefly tells the history of Unix and where Linux fits in.
If you are eager to start working with Linux without this blah, blah, blah over history,
distributions, and licensing then jump straight to Part II - Chapter 6. Working with
Directories page 28.
3
Linux history
1.1. Linux history
All modern operating systems have their roots in 1969 when Dennis Ritchie and Ken
Thompson developed the C language and the Unix operating system at AT&T Bell Labs.
They shared their source code (yes, there was open source back in the Seventies) with the
rest of the world, including the hippies in Berkeley California. By 1975, when AT&T started
selling Unix commercially, about half of the source code was written by others. The hippies
were not happy that a commercial company sold software that they had written; the resulting
(legal) battle ended in there being two versions of Unix in the Seventies : the official AT&T
Unix, and the free BSD Unix.
In the Eighties many companies started developing their own Unix: IBM created AIX, Sun
SunOS (later Solaris), HP HP-UX and about a dozen other companies did the same. The
result was a mess of Unix dialects and a dozen different ways to do the same thing. And
here is the first real root of Linux, when Richard Stallman aimed to end this era of Unix
separation and everybody re-inventing the wheel by starting the GNU project (GNU is Not
Unix). His goal was to make an operating system that was freely available to everyone, and
where everyone could work together (like in the Seventies). Many of the command line tools
that you use today on Linux or Solaris are GNU tools.
The Nineties started with Linus Torvalds, a Swedish speaking Finnish student, buying a
386 computer and writing a brand new POSIX compliant kernel. He put the source code
online, thinking it would never support anything but 386 hardware. Many people embraced
the combination of this kernel with the GNU tools, and the rest, as they say, is history.
Today more than 90 percent of supercomputers (including the complete top 10), more than
half of all smartphones, many millions of desktop computers, around 70 percent of all web
servers, a large chunk of tablet computers, and several appliances (dvd-players, washing
machines, dsl modems, routers, ...) run Linux. It is by far the most commonly used operating
system in the world.
Linux kernel version 3.2 was released in January 2012. Its source code grew by almost
two hundred thousand lines (compared to version 3.1) thanks to contributions of over 4000
developers paid by about 200 commercial companies including Red Hat, Intel, Broadcom,
Texas Instruments, IBM, Novell, Qualcomm, Samsung, Nokia, Oracle, Google and even
Microsoft.
http://en.wikipedia.org/wiki/Dennis_Ritchie
http://en.wikipedia.org/wiki/Richard_Stallman
http://en.wikipedia.org/wiki/Linus_Torvalds
http://kernel.org
http://lwn.net/Articles/472852/
http://www.linuxfoundation.org/
http://en.wikipedia.org/wiki/Linux
http://www.levenez.com/unix/ (a huge Unix history poster)
4
Chapter 2. distributions
This chapter gives a short overview of current Linux distributions.
A Linux distribution is a collection of (usually open source) software on top of a Linux
kernel. A distribution (or short, distro) can bundle server software, system management
tools, documentation and many desktop applications in a central secure software
repository. A distro aims to provide a common look and feel, secure and easy software
management and often a specific operational purpose.
Let's take a look at some popular distributions.
5
distributions
2.1. Red Hat
Red Hat is a billion dollar commercial Linux company that puts a lot of effort in developing
Linux. They have hundreds of Linux specialists and are known for their excellent support.
They give their products (Red Hat Enterprise Linux and Fedora) away for free. While Red
Hat Enterprise Linux (RHEL) is well tested before release and supported for up to seven
years after release, Fedora is a distro with faster updates but without support.
2.2. Ubuntu
Canonical started sending out free compact discs with Ubuntu Linux in 2004 and quickly
became popular for home users (many switching from Microsoft Windows). Canonical
wants Ubuntu to be an easy to use graphical Linux desktop without need to ever see a
command line. Of course they also want to make a profit by selling support for Ubuntu.
2.3. Debian
There is no company behind Debian. Instead there are thousands of well organised
developers that elect a Debian Project Leader every two years. Debian is seen as one of
the most stable Linux distributions. It is also the basis of every release of Ubuntu. Debian
comes in three versions: stable, testing and unstable. Every Debian release is named after
a character in the movie Toy Story.
2.4. Other
Distributions like CentOS, Oracle Enterprise Linux and Scientific Linux are based on
Red Hat Enterprise Linux and share many of the same principles, directories and
system administration techniques. Linux Mint, Edubuntu and many other *buntu named
distributions are based on Ubuntu and thus share a lot with Debian. There are hundreds of
other Linux distributions.
6
distributions
2.5. Which to choose ?
Below are some very personal opinions on some of the most popular Linux Distributions.
Keep in mind that any of the below Linux distributions can be a stable server and a nice
graphical desktop client.
Table 2.1. choosing a Linux distro
distribution name
reason(s) for using
Red Hat Enterprise (RHEL) You are a manager and you want a good support contract.
CentOS
You want Red Hat without the support contract from Red Hat.
Fedora
You want Red Hat on your laptop/desktop.
Linux Mint
You want a personal graphical desktop to play movies, music and games.
Debian
My personal favorite for servers, laptops, and any other device.
Ubuntu
Very popular, based on Debian, not my favorite.
Kali
others
You want a pointy-clicky hacking interface.
Advanced users may prefer Arch, Gentoo, OpenSUSE, Scientific, ...
When you are new to Linux in 2015, go for the latest Mint or Fedora. If you only want to
practice the Linux command line then install one Debian server and/or one CentOS server
(without graphical interface).
Here are some links to help you choose:
distrowatch.com
redhat.com
centos.org
debian.org
www.linuxmint.com
ubuntu.com
7
Chapter 3. licensing
This chapter briefly explains the different licenses used for distributing operating systems
software.
Many thanks go to Ywein Van den Brande for writing most of this chapter.
Ywein is an attorney at law, co-author of The International FOSS Law Book and author
of Praktijkboek Informaticarecht (in Dutch).
http://ifosslawbook.org
http://www.crealaw.eu
8
licensing
3.1. about software licenses
There are two predominant software paradigms: Free and Open Source Software (FOSS)
and proprietary software. The criteria for differentiation between these two approaches is
based on control over the software. With proprietary software, control tends to lie more
with the vendor, while with Free and Open Source Software it tends to be more weighted
towards the end user. But even though the paradigms differ, they use the same copyright
laws to reach and enforce their goals. From a legal perspective, Free and Open Source
Software can be considered as software to which users generally receive more rights via
their license agreement than they would have with a proprietary software license, yet the
underlying license mechanisms are the same.
Legal theory states that the author of FOSS, contrary to the author of public domain
software, has in no way whatsoever given up his rights on his work. FOSS supports on the
rights of the author (the copyright) to impose FOSS license conditions. The FOSS license
conditions need to be respected by the user in the same way as proprietary license conditions.
Always check your license carefully before you use third party software.
Examples of proprietary software are AIX from IBM, HP-UX from HP and Oracle
Database 11g. You are not authorised to install or use this software without paying a
licensing fee. You are not authorised to distribute copies and you are not authorised to modify
the closed source code.
3.2. public domain software and freeware
Software that is original in the sense that it is an intellectual creation of the author benefits
copyright protection. Non-original software does not come into consideration for copyright
protection and can, in principle, be used freely.
Public domain software is considered as software to which the author has given up all rights
and on which nobody is able to enforce any rights. This software can be used, reproduced or
executed freely, without permission or the payment of a fee. Public domain software can in
certain cases even be presented by third parties as own work, and by modifying the original
work, third parties can take certain versions of the public domain software out of the public
domain again.
Freeware is not public domain software or FOSS. It is proprietary software that you can use
without paying a license cost. However, the often strict license terms need to be respected.
Examples of freeware are Adobe Reader, Skype and Command and Conquer: Tiberian
Sun (this game was sold as proprietary in 1999 and is since 2011 available as freeware).
3.3. Free Software or Open Source Software
Both the Free Software (translates to vrije software in Dutch and to Logiciel Libre in
French) and the Open Source Software movement largely pursue similar goals and endorse
similar software licenses. But historically, there has been some perception of differentiation
due to different emphases. Where the Free Software movement focuses on the rights (the
9
licensing
four freedoms) which Free Software provides to its users, the Open Source Software
movement points to its Open Source Definition and the advantages of peer-to-peer software
development.
Recently, the term free and open source software or FOSS has arisen as a neutral alternative.
A lesser-used variant is free/libre/open source software (FLOSS), which uses libre to clarify
the meaning of free as in freedom rather than as in at no charge.
Examples of free software are gcc, MySQL and gimp.
Detailed information about the four freedoms can be found here:
http://www.gnu.org/philosophy/free-sw.html
The open source definition can be found at:
http://www.opensource.org/docs/osd
The above definition is based on the Debian Free Software Guidelines available here:
http://www.debian.org/social_contract#guidelines
3.4. GNU General Public License
More and more software is being released under the GNU GPL (in 2006 Java was released
under the GPL). This license (v2 and v3) is the main license endorsed by the Free Software
Foundation. It’s main characteristic is the copyleft principle. This means that everyone in the
chain of consecutive users, in return for the right of use that is assigned, needs to distribute
the improvements he makes to the software and his derivative works under the same
conditions to other users, if he chooses to distribute such improvements or derivative works.
In other words, software which incorporates GNU GPL software, needs to be distributed
in turn as GNU GPL software (or compatible, see below). It is not possible to incorporate
copyright protected parts of GNU GPL software in a proprietary licensed work. The GPL
has been upheld in court.
3.5. using GPLv3 software
You can use GPLv3 software almost without any conditions. If you solely run the software
you even don’t have to accept the terms of the GPLv3. However, any other use - such as
modifying or distributing the software - implies acceptance.
In case you use the software internally (including over a network), you may modify the
software without being obliged to distribute your modification. You may hire third parties
to work on the software exclusively for you and under your direction and control. But if you
modify the software and use it otherwise than merely internally, this will be considered as
distribution. You must distribute your modifications under GPLv3 (the copyleft principle).
Several more obligations apply if you distribute GPLv3 software. Check the GPLv3 license
carefully.
You create output with GPLv3 software: The GPLv3 does not automatically apply to the
output.
10
licensing
3.6. BSD license
There are several versions of the original Berkeley Distribution License. The most common
one is the 3-clause license ("New BSD License" or "Modified BSD License").
This is a permissive free software license. The license places minimal restrictions on how
the software can be redistributed. This is in contrast to copyleft licenses such as the GPLv.
3 discussed above, which have a copyleft mechanism.
This difference is of less importance when you merely use the software, but kicks in when
you start redistributing verbatim copies of the software or your own modified versions.
3.7. other licenses
FOSS or not, there are many kind of licenses on software. You should read and understand
them before using any software.
3.8. combination of software licenses
When you use several sources or wishes to redistribute your software under a different
license, you need to verify whether all licenses are compatible. Some FOSS licenses (such
as BSD) are compatible with proprietary licenses, but most are not. If you detect a license
incompatibility, you must contact the author to negotiate different license conditions or
refrain from using the incompatible software.
11
Part II. installing Linux
Table of Contents
4. installing Debian 8 ............................................................................................................
4.1. Debian ...................................................................................................................
4.2. Downloading ...........................................................................................................
4.3. virtualbox networking ...............................................................................................
4.4. setting the hostname .................................................................................................
4.5. adding a static ip address ...........................................................................................
4.6. Debian package management .....................................................................................
5. installing CentOS 7 ...........................................................................................................
5.1. download a CentOS 7 image ......................................................................................
5.2. Virtualbox ...............................................................................................................
5.3. CentOS 7 installing ..................................................................................................
5.4. CentOS 7 first logon .................................................................................................
5.5. Virtualbox network interface ......................................................................................
5.6. configuring the network ............................................................................................
5.7. adding one static ip address .......................................................................................
5.8. package management ................................................................................................
5.9. logon from Linux and MacOSX ..................................................................................
5.10. logon from MS Windows .........................................................................................
6. getting Linux at home .......................................................................................................
6.1. download a Linux CD image .....................................................................................
6.2. download Virtualbox ................................................................................................
6.3. create a virtual machine ............................................................................................
6.4. attach the CD image .................................................................................................
6.5. install Linux ............................................................................................................
13
14
15
15
31
33
33
34
35
36
38
43
51
52
53
53
54
55
55
57
58
58
59
64
67
Chapter 4. installing Debian 8
This module is a step by step demonstration of an actual installation of Debian 8 (also known
as Jessie).
We start by downloading an image from the internet and install Debian 8 as a virtual machine
in Virtualbox. We will also do some basic configuration of this new machine like setting
an ip address and fixing a hostname.
This procedure should be very similar for other versions of Debian, and also for distributions
like Linux Mint, xubuntu/ubuntu/kubuntu or Mepis. This procedure can also be helpful
if you are using another virtualization solution.
Go to the next chapter if you want to install CentOS, Fedora, Red Hat Enterprise
Linux, ....
14
installing Debian 8
4.1. Debian
Debian is one of the oldest Linux distributions. I use Debian myself on almost every
computer that I own (including raspbian on the Raspberry Pi).
Debian comes in releases named after characters in the movie Toy Story. The Jessie release
contains about 36000 packages.
Table 4.1. Debian releases
name
number
year
Woody
3.0
2002
Sarge
3.1
2005
Etch
4.0
2007
Lenny
5.0
2009
Squeeze
6.0
2011
Wheezy
7
2013
Jessie
8
2015
There is never a fixed date for the next Debian release. The next version is released when
it is ready.
4.2. Downloading
All these screenshots were made in November 2014, which means Debian 8 was still in
'testing' (but in 'freeze', so there will be no major changes when it is released).
Download Debian here:
15
installing Debian 8
After a couple of clicks on that website, I ended up downloading Debian 8 (testing) here. It
should be only one click once Debian 8 is released (somewhere in 2015).
You have many other options to download and install Debian. We will discuss them much
later.
This small screenshot shows the downloading of a netinst .iso file. Most of the software will
be downloaded during the installation. This also means that you will have the most recent
version of all packages when the install is finished.
I already have Debian 8 installed on my laptop (hence the paul@debian8 prompt). Anyway,
this is the downloaded file just before starting the installation.
paul@debian8:~$ ls -hl debian-testing-amd64-netinst.iso
-rw-r--r-- 1 paul paul 231M Nov 10 17:59 debian-testing-amd64-netinst.iso
16
installing Debian 8
Create a new virtualbox machine (I already have five, you might have zero for now). Click
the New button to start a wizard that will help you create a virtual machine.
The machine needs a name, this screenshot shows that I named it server42.
17
installing Debian 8
Most of the defaults in Virtualbox are ok.
512MB of RAM is enough to practice all the topics in this book.
We do not care about the virtual disk format.
18
installing Debian 8
Choosing dynamically allocated will save you some disk space (for a small performance
hit).
8GB should be plenty for learning about Linux servers.
This finishes the wizard. You virtual machine is almost ready to begin the installation.
19
installing Debian 8
First, make sure that you attach the downloaded .iso image to the virtual CD drive. (by
opening Settings, Storage followed by a mouse click on the round CD icon)
Personally I also disable sound and usb, because I never use these features. I also remove
the floppy disk and use a PS/2 mouse pointer. This is probably not very important, but I like
the idea that it saves some resources.
Now boot the virtual machine and begin the actual installation. After a couple of seconds
you should see a screen similar to this. Choose Install to begin the installation of Debian.
20
installing Debian 8
First select the language you want to use.
Choose your country. This information will be used to suggest a download mirror.
21
installing Debian 8
Choose the correct keyboard. On servers this is of no importance since most servers are
remotely managed via ssh.
Enter a hostname (with fqdn to set a dnsdomainname).
22
installing Debian 8
Give the root user a password. Remember this password (or use hunter2).
It is adviced to also create a normal user account. I don't give my full name, Debian 8 accepts
an identical username and full name paul.
23
installing Debian 8
The use entire disk refers to the virtual disk that you created before in Virtualbox..
Again the default is probably what you want. Only change partitioning if you really know
what you are doing.
24
installing Debian 8
Accept the partition layout (again only change if you really know what you are doing).
This is the point of no return, the magical moment where pressing yes will forever erase
data on the (virtual) computer.
25
installing Debian 8
Software is downloaded from a mirror repository, preferably choose one that is close by (as
in the same country).
This setup was done in Belgium.
26
installing Debian 8
Leave the proxy field empty (unless you are sure that you are behind a proxy server).
Choose whether you want to send anonymous statistics to the Debian project (it gathers data
about installed packages). You can view the statistics here http://popcon.debian.org/.
27
installing Debian 8
Choose what software to install, we do not need any graphical stuff for this training.
The latest versions are being downloaded.
28
installing Debian 8
Say yes to install the bootloader on the virtual machine.
Booting for the first time shows the grub screen
29
installing Debian 8
A couple seconds later you should see a lot of text scrolling of the screen (dmesg). After
which you are presented with this getty and are allowed your first logon.
You should now be able to log on to your virtual machine with the root account. Do you
remember the password ? Was it hunter2 ?
The screenshots in this book will look like this from now on. You can just type those
commands in the terminal (after you logged on).
root@server42:~# who am i
root
tty1
2014-11-10 18:21
root@server42:~# hostname
server42
root@server42:~# date
Mon Nov 10 18:21:56 CET 2014
30
installing Debian 8
4.3. virtualbox networking
You can also log on from remote (or from your Windows/Mac/Linux host computer) using
ssh or putty. Change the network settings in the virtual machine to bridge. This will enable
your virtual machine to receive an ip address from your local dhcp server.
The default virtualbox networking is to attach virtual network cards to nat. This screenshiot
shows the ip address 10.0.2.15 when on nat:
root@server42:~# ifconfig
eth0
Link encap:Ethernet HWaddr 08:00:27:f5:74:cf
inet addr:10.0.2.15 Bcast:10.0.2.255 Mask:255.255.255.0
inet6 addr: fe80::a00:27ff:fef5:74cf/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:11 errors:0 dropped:0 overruns:0 frame:0
TX packets:19 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:2352 (2.2 KiB) TX bytes:1988 (1.9 KiB)
lo
Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:65536 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)
By shutting down the network interface and enabling it again, we force Debian to renew an
ip address from the bridged network.
root@server42:~# # do not run ifdown while connected over ssh!
root@server42:~# ifdown eth0
Killed old client process
Internet Systems Consortium DHCP Client 4.3.1
Copyright 2004-2014 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/
Listening on LPF/eth0/08:00:27:f5:74:cf
Sending on
LPF/eth0/08:00:27:f5:74:cf
31
installing Debian 8
Sending on
Socket/fallback
DHCPRELEASE on eth0 to 10.0.2.2 port 67
root@server42:~# # now enable bridge in virtualbox settings
root@server42:~# ifup eth0
Internet Systems Consortium DHCP Client 4.3.1
Copyright 2004-2014 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/
Listening on LPF/eth0/08:00:27:f5:74:cf
Sending on
LPF/eth0/08:00:27:f5:74:cf
Sending on
Socket/fallback
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 8
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 8
DHCPREQUEST on eth0 to 255.255.255.255 port 67
DHCPOFFER from 192.168.1.42
DHCPACK from 192.168.1.42
bound to 192.168.1.111 -- renewal in 2938 seconds.
root@server42:~# ifconfig eth0
eth0
Link encap:Ethernet HWaddr 08:00:27:f5:74:cf
inet addr:192.168.1.111 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::a00:27ff:fef5:74cf/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:15 errors:0 dropped:0 overruns:0 frame:0
TX packets:31 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:3156 (3.0 KiB) TX bytes:3722 (3.6 KiB)
root@server42:~#
Here is an example of ssh to this freshly installed computer. Note that Debian 8 has disabled
remote root access, so i need to use the normal user account.
paul@debian8:~$ ssh [email protected]
[email protected]'s password:
The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.
Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
paul@server42:~$
paul@server42:~$ su Password:
root@server42:~#
TODO: putty screenshot here...
32
installing Debian 8
4.4. setting the hostname
The hostname of the server is asked during installation, so there is no need to configure this
manually.
root@server42:~# hostname
server42
root@server42:~# cat /etc/hostname
server42
root@server42:~# dnsdomainname
paul.local
root@server42:~# grep server42 /etc/hosts
127.0.1.1
server42.paul.local
server42
root@server42:~#
4.5. adding a static ip address
This example shows how to add a static ip address to your server.
You can use ifconfig to set a static address that is active until the next reboot (or until the
next ifdown).
a
root@server42:~# ifconfig eth0:0 10.104.33.39
Adding a couple of lines to the /etc/network/interfaces file to enable an extra ip address
forever.
root@server42:~# vi /etc/network/interfaces
root@server42:~# tail -4 /etc/network/interfaces
auto eth0:0
iface eth0:0 inet static
address 10.104.33.39
netmask 255.255.0.0
root@server42:~# ifconfig
eth0
Link encap:Ethernet HWaddr 08:00:27:f5:74:cf
inet addr:192.168.1.111 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::a00:27ff:fef5:74cf/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:528 errors:0 dropped:0 overruns:0 frame:0
TX packets:333 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:45429 (44.3 KiB) TX bytes:48763 (47.6 KiB)
eth0:0
Link encap:Ethernet HWaddr 08:00:27:f5:74:cf
inet addr:10.104.33.39 Bcast:10.255.255.255 Mask:255.0.0.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
lo
Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:65536 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)
root@server42:~#
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installing Debian 8
4.6. Debian package management
To get all information about the newest packages form the online repository:
root@server42:~# aptitude update
Get: 1 http://ftp.be.debian.org jessie InRelease [191 kB]
Get: 2 http://security.debian.org jessie/updates InRelease [84.1 kB]
Get: 3 http://ftp.be.debian.org jessie-updates InRelease [117 kB]
Get: 4 http://ftp.be.debian.org jessie-backports InRelease [118 kB]
Get: 5 http://security.debian.org jessie/updates/main Sources [14 B]
Get: 6 http://ftp.be.debian.org jessie/main Sources/DiffIndex [7,876 B]
... (output truncated)
To download and apply all updates for all installed packages:
root@server42:~# aptitude upgrade
Resolving dependencies...
The following NEW packages will be installed:
firmware-linux-free{a} irqbalance{a} libnuma1{a} linux-image-3.16.0-4-amd64{a}
The following packages will be upgraded:
busybox file libc-bin libc6 libexpat1 libmagic1 libpaper-utils libpaper1 libsqlite3-0
linux-image-amd64 locales multiarch-support
12 packages upgraded, 4 newly installed, 0 to remove and 0 not upgraded.
Need to get 44.9 MB of archives. After unpacking 161 MB will be used.
Do you want to continue? [Y/n/?]
... (output truncated)
To install new software (vim and tmux in this example):
root@server42:~# aptitude install vim tmux
The following NEW packages will be installed:
tmux vim vim-runtime{a}
0 packages upgraded, 3 newly installed, 0 to remove and 0 not upgraded.
Need to get 6,243 kB of archives. After unpacking 29.0 MB will be used.
Do you want to continue? [Y/n/?]
Get: 1 http://ftp.be.debian.org/debian/ jessie/main tmux amd64 1.9-6 [245 kB]
Get: 2 http://ftp.be.debian.org/debian/ jessie/main vim-runtime all 2:7.4.488-1 [5,046 kB]
Get: 3 http://ftp.be.debian.org/debian/ jessie/main vim amd64 2:7.4.488-1 [952 kB]
Refer to the package management chapter in LinuxAdm.pdf for more information.
34
Chapter 5. installing CentOS 7
This module is a step by step demonstration of an actual installation of CentOS 7.
We start by downloading an image from the internet and install CentOS 7 as a virtual
machine in Virtualbox. We will also do some basic configuration of this new machine like
setting an ip address and fixing a hostname.
This procedure should be very similar for other versions of CentOS, and also for
distributions like RHEL (Red Hat Enterprise Linux) or Fedora. This procedure can also be
helpful if you are using another virtualization solution.
35
installing CentOS 7
5.1. download a CentOS 7 image
This demonstration uses a laptop computer with Virtualbox to install CentOS 7 as a virtual
machine. The first task is to download an .iso image of CentOS 7.
The CentOS 7 website looks like this today (November 2014). They change the look
regularly, so it may look different when you visit it.
You can download a full DVD, which allows for an off line installation of a graphical
CentOS 7 desktop. You can select this because it should be easy and complete, and should
get you started with a working CentOS 7 virtual machine.
36
installing CentOS 7
But I clicked instead on 'alternative downloads', selected CentOS 7 and x86_64 and ended
up on a mirror list. Each mirror is a server that contains copies of CentOS 7 media. I
selected a Belgian mirror because I currently am in Belgium.
There is again the option for full DVD's and more. This demonstration will use the minimal
.iso file, because it is much smaller in size. The download takes a couple of minutes.
Verify the size of the file after download to make sure it is complete. Probably a right click
on the file and selecting 'properties' (if you use Windows or Mac OSX).
I use Linux on the laptop already:
paul@debian8:~$ ls -lh CentOS-7.0-1406-x86_64-Minimal.iso
-rw-r--r-- 1 paul paul 566M Nov 1 14:45 CentOS-7.0-1406-x86_64-Minimal.iso
Do not worry if you do no understand the above command. Just try to make sure that the
size of this file is the same as the size that is mentioned on the CentOS 7 website.
37
installing CentOS 7
5.2. Virtualbox
This screenshot shows up when I start Virtualbox. I already have four virtual machines, you
might have none.
Below are the steps for creating a new virtual machine. Start by clicking New and give your
machine a name (I chose server33). Click Next.
38
installing CentOS 7
A Linux computer without graphical interface will run fine on half a gigabyte of RAM.
A Linux virtual machine will need a virtual hard drive.
39
installing CentOS 7
Any format will do for our purpose, so I left the default vdi.
The default dynamically allocated type will save disk space (until we fill the virtual disk
up to 100 percent). It makes the virtual machine a bit slower than fixed size, but the fixed
size speed improvement is not worth it for our purpose.
40
installing CentOS 7
The name of the virtual disk file on the host computer will be server33.vdi in my case (I left
it default and it uses the vm name). Also 16 GB should be enough to practice Linux. The
file will stay much smaller than 16GB, unless you copy a lot of files to the virtual machine.
You should now be back to the start screen of Virtualbox. If all went well, then you should
see the machine you just created in the list.
41
installing CentOS 7
After finishing the setup, we go into the Settings of our virtual machine and attach the .iso
file we downloaded before. Below is the default screenshot.
This is a screenshot with the .iso file properly attached.
42
installing CentOS 7
5.3. CentOS 7 installing
The screenshots below will show every step from starting the virtual machine for the first
time (with the .iso file attached) until the first logon.
You should see this when booting, otherwise verify the attachment of the .iso file form the
previous steps. Select Test this media and install CentOS 7.
43
installing CentOS 7
Carefully select the language in which you want your CentOS. I always install operating
systems in English, even though my native language is not English.
Also select the right keyboard, mine is a US qwerty, but yours may be different.
You should arrive at a summary page (with one or more warnings).
44
installing CentOS 7
Start by configuring the network. During this demonstration I had a DHCP server running
at 192.168.1.42, yours is probably different. Ask someone (a network administator ?) for
help if this step fails.
Select your time zone, and activate ntp.
45
installing CentOS 7
Choose a mirror that is close to you. If you can't find a local mirror, then you can copy the
one from this screenshot (it is a general CentOS mirror).
It can take a couple of seconds before the mirror is verified.
46
installing CentOS 7
I did not select any software here (because I want to show it all in this training).
After configuring network, location, software and all, you should be back on this page. Make
sure there are no warnings anymore (and that you made the correct choice everywhere).
47
installing CentOS 7
You can enter a root password and create a user account while the installation is
downloading from the internet. This is the longest step, it can take several minutes (or up to
an hour if you have a slow internet connection).
If you see this, then the installation was successful.
Time to reboot the computer and start CentOS 7 for the first time.
48
installing CentOS 7
This screen will appear briefly when the virtual machines starts. You don't have to do
anything.
After a couple of seconds, you should see a logon screen. This is called a tty or a getty. Here
you can type root as username. The login process will then ask your password (nothing will
appear on screen when you type your password).
49
installing CentOS 7
And this is what it looks like after logon. You are logged on to your own Linux machine,
very good.
All subsequent screenshots will be text only, no images anymore.
For example this screenshot shows three commands being typed on my new CentOS 7
install.
[root@localhost ~]# who am i
root
pts/0
2014-11-01 22:14
[root@localhost ~]# hostname
localhost.localdomain
[root@localhost ~]# date
Sat Nov 1 22:14:37 CET 2014
When using ssh the same commands will give this screenshot:
[root@localhost ~]# who am i
root
pts/0
2014-11-01 21:00 (192.168.1.35)
[root@localhost ~]# hostname
localhost.localdomain
[root@localhost ~]# date
Sat Nov 1 22:10:04 CET 2014
[root@localhost ~]#
If the last part is a bit too fast, take a look at the next topic CentOS 7 first logon.
50
installing CentOS 7
5.4. CentOS 7 first logon
All you have to log on, after finishing the installation, is this screen in Virtualbox.
This is workable to learn Linux, and you will be able to practice a lot. But there are more
ways to access your virtual machine, the next chapters discuss some of these and will also
introduce some basic system configuration.
5.4.1. setting the hostname
Setting the hostname is a simple as changing the /etc/hostname file. As you can see here,
it is set to localhost.localdomain by default.
[root@localhost ~]# cat /etc/hostname
localhost.localdomain
You could do echo server33.netsec.local > /etc/hostname followed by a reboot. But there
is also the new CentOS 7 way of setting a new hostname.
[root@localhost ~]# nmtui
The above command will give you a menu to choose from with a set system hostname
option. Using this nmtui option will edit the /etc/hostname file for you.
[root@localhost ~]# cat /etc/hostname
server33.netsec.local
[root@localhost ~]# hostname
server33.netsec.local
[root@localhost ~]# dnsdomainname
netsec.local
For some reason the documentation on the centos.org and docs.redhat.com websites tell
you to also execute this command:
[root@localhost ~]# systemctl restart systemd-hostnamed
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installing CentOS 7
5.5. Virtualbox network interface
By default Virtualbox will connect your virtual machine over a nat interface. This will
show up as a 10.0.2.15 (or similar).
[root@server33 ~]# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: enp0s3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast s\
tate UP qlen 1000
link/ether 08:00:27:1c:f5:ab brd ff:ff:ff:ff:ff:ff
inet 10.0.2.15/24 brd 10.0.2.255 scope global dynamic enp0s3
valid_lft 86399sec preferred_lft 86399sec
inet6 fe80::a00:27ff:fe1c:f5ab/64 scope link
valid_lft forever preferred_lft forever
You can change this to bridge (over your wi-fi or over the ethernet cable) and thus make it
appear as if your virtual machine is directly on your local network (receiving an ip address
from your real dhcp server).
You can make this change while the vm is running, provided that you execute this command:
[root@server33 ~]# systemctl restart network
[root@server33 ~]# ip a s dev enp0s3
2: enp0s3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast s\
tate UP qlen 1000
link/ether 08:00:27:1c:f5:ab brd ff:ff:ff:ff:ff:ff
inet 192.168.1.110/24 brd 192.168.1.255 scope global dynamic enp0s3
valid_lft 7199sec preferred_lft 7199sec
inet6 fe80::a00:27ff:fe1c:f5ab/64 scope link
valid_lft forever preferred_lft forever
[root@server33 ~]#
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installing CentOS 7
5.6. configuring the network
The new way of changing network configuration is through the nmtui tool. If you want to
manually play with the files in /etc/sysconfig/network-scripts then you will first need to
verify (and disable) NetworkManager on that interface.
Verify whether an interface is controlled by NetworkManager using the nmcli command
(connected means managed bu NM).
[root@server33 ~]# nmcli dev
DEVICE TYPE
STATE
enp0s3 ethernet connected
lo
loopback unmanaged
status
CONNECTION
enp0s3
--
Disable NetworkManager on an interface (enp0s3 in this case):
echo 'NM_CONTROLLED=no' >> /etc/sysconfig/network-scripts/ifcfg-enp0s3
You can restart the network without a reboot like this:
[root@server33 ~]# systemctl restart network
Also, forget ifconfig and instead use ip a.
[root@server33 ~]# ip a s dev enp0s3 | grep inet
inet 192.168.1.110/24 brd 192.168.1.255 scope global dynamic enp0s3
inet6 fe80::a00:27ff:fe1c:f5ab/64 scope link
[root@server33 ~]#
5.7. adding one static ip address
This example shows how to add one static ip address to your computer.
[root@server33 ~]# nmtui edit enp0s3
In this interface leave the IPv4 configuration to automatic, and add an ip address just below.
IPv4 CONFIGURATION <Automatic>
Addresses 10.104.33.32/16__________ <Remove>
<Hide>
Execute this command after exiting nmtui.
[root@server33 ~]# systemctl restart network
And verify with ip (not with ifconfig):
[root@server33 ~]# ip a s dev enp0s3 | grep
inet 192.168.1.110/24 brd 192.168.1.255
inet 10.104.33.32/16 brd 10.104.255.255
inet6 fe80::a00:27ff:fe1c:f5ab/64 scope
[root@server33 ~]#
53
inet
scope global dynamic enp0s3
scope global enp0s3
link
installing CentOS 7
5.8. package management
Even with a network install, CentOS 7 did not install the latest version of some packages.
Luckily there is only one command to run (as root). This can take a while.
[root@server33 ~]# yum update
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
* base: centos.weepeetelecom.be
* extras: centos.weepeetelecom.be
* updates: centos.weepeetelecom.be
Resolving Dependencies
--> Running transaction check
---> Package NetworkManager.x86_64 1:0.9.9.1-13.git20140326.4dba720.el7 \
will be updated
... (output truncated)
You can also use yum to install one or more packages. Do not forget to run yum update
from time to time.
[root@server33 ~]# yum update -y && yum install vim -y
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
* base: centos.weepeetelecom.be
... (output truncated)
Refer to the package management chapter for more information on installing and removing
packages.
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installing CentOS 7
5.9. logon from Linux and MacOSX
You can now open a terminal on Linux or MacOSX and use ssh to log on to your virtual
machine.
paul@debian8:~$ ssh [email protected]
[email protected]'s password:
Last login: Sun Nov 2 11:53:57 2014
[root@server33 ~]# hostname
server33.netsec.local
[root@server33 ~]#
5.10. logon from MS Windows
There is no ssh installed on MS Windows, but you can download putty.exe from http://
www.chiark.greenend.org.uk/~sgtatham/putty/download.html (just Google it).
Use putty.exe as shown in this screenshot (I saved the ip address by giving it a name
'server33' and presing the 'save' button).
55
installing CentOS 7
The first time you will get a message about keys, accept this (this is explained in the ssh
chapter).
Enter your userid (or root) and the correct password (nothing will appear on the screen when
typing a password).
56
Chapter 6. getting Linux at home
This chapter shows a Ubuntu install in Virtualbox. Consider it legacy and use CentOS7
or Debian8 instead (each have their own chapter now).
This book assumes you have access to a working Linux computer. Most companies have
one or more Linux servers, if you have already logged on to it, then you 're all set (skip this
chapter and go to the next).
Another option is to insert a Ubuntu Linux CD in a computer with (or without) Microsoft
Windows and follow the installation. Ubuntu will resize (or create) partitions and setup a
menu at boot time to choose Windows or Linux.
If you do not have access to a Linux computer at the moment, and if you are unable or unsure
about installing Linux on your computer, then this chapter proposes a third option: installing
Linux in a virtual machine.
Installation in a virtual machine (provided by Virtualbox) is easy and safe. Even when you
make mistakes and crash everything on the virtual Linux machine, then nothing on the real
computer is touched.
This chapter gives easy steps and screenshots to get a working Ubuntu server in a Virtualbox
virtual machine. The steps are very similar to installing Fedora or CentOS or even Debian,
and if you like you can also use VMWare instead of Virtualbox.
57
getting Linux at home
6.1. download a Linux CD image
Start by downloading a Linux CD image (an .ISO file) from the distribution of your choice
from the Internet. Take care selecting the correct cpu architecture of your computer; choose
i386 if unsure. Choosing the wrong cpu type (like x86_64 when you have an old Pentium)
will almost immediately fail to boot the CD.
6.2. download Virtualbox
Step two (when the .ISO file has finished downloading) is to download Virtualbox. If you are
currently running Microsoft Windows, then download and install Virtualbox for Windows!
58
getting Linux at home
6.3. create a virtual machine
Now start Virtualbox. Contrary to the screenshot below, your left pane should be empty.
Click New to create a new virtual machine. We will walk together through the wizard. The
screenshots below are taken on Mac OSX; they will be slightly different if you are running
Microsoft Windows.
59
getting Linux at home
Name your virtual machine (and maybe select 32-bit or 64-bit).
Give the virtual machine some memory (512MB if you have 2GB or more, otherwise select
256MB).
60
getting Linux at home
Select to create a new disk (remember, this will be a virtual disk).
If you get the question below, choose vdi.
61
getting Linux at home
Choose dynamically allocated (fixed size is only useful in production or on really old, slow
hardware).
Choose between 10GB and 16GB as the disk size.
62
getting Linux at home
Click create to create the virtual disk.
Click create to create the virtual machine.
63
getting Linux at home
6.4. attach the CD image
Before we start the virtual computer, let us take a look at some settings (click Settings).
Do not worry if your screen looks different, just find the button named storage.
64
getting Linux at home
Remember the .ISO file you downloaded? Connect this .ISO file to this virtual machine by
clicking on the CD icon next to Empty.
Now click on the other CD icon and attach your ISO file to this virtual CD drive.
65
getting Linux at home
Verify that your download is accepted. If Virtualbox complains at this point, then you
probably did not finish the download of the CD (try downloading it again).
It could be useful to set the network adapter to bridge instead of NAT. Bridged usually will
connect your virtual computer to the Internet.
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getting Linux at home
6.5. install Linux
The virtual machine is now ready to start. When given a choice at boot, select install and
follow the instructions on the screen. When the installation is finished, you can log on to
the machine and start practising Linux!
67
Part III. first steps on
the command line
Table of Contents
7. man pages ........................................................................................................................ 70
7.1. man $command ....................................................................................................... 71
7.2. man $configfile ........................................................................................................ 71
7.3. man $daemon .......................................................................................................... 71
7.4. man -k (apropos) ...................................................................................................... 71
7.5. whatis .................................................................................................................... 71
7.6. whereis ................................................................................................................... 71
7.7. man sections ........................................................................................................... 72
7.8. man $section $file .................................................................................................... 72
7.9. man man ................................................................................................................ 72
7.10. mandb .................................................................................................................. 72
8. working with directories .................................................................................................... 73
8.1. pwd ....................................................................................................................... 74
8.2. cd .......................................................................................................................... 74
8.3. absolute and relative paths ......................................................................................... 75
8.4. path completion ....................................................................................................... 76
8.5. ls ........................................................................................................................... 76
8.6. mkdir ..................................................................................................................... 78
8.7. rmdir ...................................................................................................................... 78
8.8. practice: working with directories ............................................................................... 80
8.9. solution: working with directories ............................................................................... 81
9. working with files ............................................................................................................. 83
9.1. all files are case sensitive .......................................................................................... 84
9.2. everything is a file ................................................................................................... 84
9.3. file ........................................................................................................................ 84
9.4. touch ...................................................................................................................... 85
9.5. rm ......................................................................................................................... 86
9.6. cp .......................................................................................................................... 87
9.7. mv ......................................................................................................................... 88
9.8. rename ................................................................................................................... 89
9.9. practice: working with files ........................................................................................ 90
9.10. solution: working with files ...................................................................................... 91
10. working with file contents ................................................................................................ 93
10.1. head ..................................................................................................................... 94
10.2. tail ....................................................................................................................... 94
10.3. cat ....................................................................................................................... 95
10.4. tac ....................................................................................................................... 96
10.5. more and less ........................................................................................................ 97
10.6. strings .................................................................................................................. 97
10.7. practice: file contents .............................................................................................. 98
10.8. solution: file contents .............................................................................................. 99
11. the Linux file tree .......................................................................................................... 100
11.1. filesystem hierarchy standard .................................................................................. 101
11.2. man hier .............................................................................................................. 101
11.3. the root directory / ................................................................................................ 101
11.4. binary directories .................................................................................................. 102
11.5. configuration directories ......................................................................................... 104
11.6. data directories ..................................................................................................... 106
11.7. in memory directories ............................................................................................ 108
11.8. /usr Unix System Resources .................................................................................... 113
11.9. /var variable data .................................................................................................. 115
11.10. practice: file system tree ....................................................................................... 117
11.11. solution: file system tree ....................................................................................... 119
69
Chapter 7. man pages
This chapter will explain the use of man pages (also called manual pages) on your Unix
or Linux computer.
You will learn the man command together with related commands like whereis, whatis
and mandb.
Most Unix files and commands have pretty good man pages to explain their use. Man
pages also come in handy when you are using multiple flavours of Unix or several Linux
distributions since options and parameters sometimes vary.
70
man pages
7.1. man $command
Type man followed by a command (for which you want help) and start reading. Press q to
quit the manpage. Some man pages contain examples (near the end).
paul@laika:~$ man whois
Reformatting whois(1), please wait...
7.2. man $configfile
Most configuration files have their own manual.
paul@laika:~$ man syslog.conf
Reformatting syslog.conf(5), please wait...
7.3. man $daemon
This is also true for most daemons (background programs) on your system..
paul@laika:~$ man syslogd
Reformatting syslogd(8), please wait...
7.4. man -k (apropos)
man -k (or apropos) shows a list of man pages containing a string.
paul@laika:~$ man -k syslog
lm-syslog-setup (8) - configure laptop mode to switch syslog.conf ...
logger (1)
- a shell command interface to the syslog(3) ...
syslog-facility (8) - Setup and remove LOCALx facility for sysklogd
syslog.conf (5)
- syslogd(8) configuration file
syslogd (8)
- Linux system logging utilities.
syslogd-listfiles (8) - list system logfiles
7.5. whatis
To see just the description of a manual page, use whatis followed by a string.
paul@u810:~$ whatis route
route (8)
- show / manipulate the IP routing table
7.6. whereis
The location of a manpage can be revealed with whereis.
paul@laika:~$ whereis -m whois
whois: /usr/share/man/man1/whois.1.gz
This file is directly readable by man.
paul@laika:~$ man /usr/share/man/man1/whois.1.gz
71
man pages
7.7. man sections
By now you will have noticed the numbers between the round brackets. man man will
explain to you that these are section numbers. Executable programs and shell commands
reside in section one.
1
2
3
4
5
6
7
8
9
Executable programs or shell commands
System calls (functions provided by the kernel)
Library calls (functions within program libraries)
Special files (usually found in /dev)
File formats and conventions eg /etc/passwd
Games
Miscellaneous (including macro packages and conventions), e.g. man(7)
System administration commands (usually only for root)
Kernel routines [Non standard]
7.8. man $section $file
Therefor, when referring to the man page of the passwd command, you will see it written
as passwd(1); when referring to the passwd file, you will see it written as passwd(5). The
screenshot explains how to open the man page in the correct section.
[paul@RHEL52 ~]$ man passwd
[paul@RHEL52 ~]$ man 5 passwd
# opens the first manual found
# opens a page from section 5
7.9. man man
If you want to know more about man, then Read The Fantastic Manual (RTFM).
Unfortunately, manual pages do not have the answer to everything...
paul@laika:~$ man woman
No manual entry for woman
7.10. mandb
Should you be convinced that a man page exists, but you can't access it, then try running
mandb on Debian/Mint.
root@laika:~# mandb
0 man subdirectories contained newer manual pages.
0 manual pages were added.
0 stray cats were added.
0 old database entries were purged.
Or run makewhatis on CentOS/Redhat.
[root@centos65 ~]# apropos scsi
scsi: nothing appropriate
[root@centos65 ~]# makewhatis
[root@centos65 ~]# apropos scsi
hpsa
(4) - HP Smart Array SCSI driver
lsscsi
(8) - list SCSI devices (or hosts) and their attributes
sd
(4) - Driver for SCSI Disk Drives
st
(4) - SCSI tape device
72
Chapter 8. working with directories
This module is a brief overview of the most common commands to work with directories:
pwd, cd, ls, mkdir and rmdir. These commands are available on any Linux (or Unix)
system.
This module also discusses absolute and relative paths and path completion in the bash
shell.
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working with directories
8.1. pwd
The you are here sign can be displayed with the pwd command (Print Working Directory).
Go ahead, try it: Open a command line interface (also called a terminal, console or xterm)
and type pwd. The tool displays your current directory.
paul@debian8:~$ pwd
/home/paul
8.2. cd
You can change your current directory with the cd command (Change Directory).
paul@debian8$
paul@debian8$
/etc
paul@debian8$
paul@debian8$
/bin
paul@debian8$
paul@debian8$
/home/paul
cd /etc
pwd
cd /bin
pwd
cd /home/paul/
pwd
8.2.1. cd ~
The cd is also a shortcut to get back into your home directory. Just typing cd without a target
directory, will put you in your home directory. Typing cd ~ has the same effect.
paul@debian8$
paul@debian8$
/etc
paul@debian8$
paul@debian8$
/home/paul
paul@debian8$
paul@debian8$
/home/paul
cd /etc
pwd
cd
pwd
cd ~
pwd
8.2.2. cd ..
To go to the parent directory (the one just above your current directory in the directory
tree), type cd .. .
paul@debian8$ pwd
/usr/share/games
paul@debian8$ cd ..
paul@debian8$ pwd
/usr/share
To stay in the current directory, type cd . ;-) We will see useful use of the . character
representing the current directory later.
74
working with directories
8.2.3. cd Another useful shortcut with cd is to just type cd - to go to the previous directory.
paul@debian8$
/home/paul
paul@debian8$
paul@debian8$
/etc
paul@debian8$
/home/paul
paul@debian8$
/etc
pwd
cd /etc
pwd
cd cd -
8.3. absolute and relative paths
You should be aware of absolute and relative paths in the file tree. When you type a path
starting with a slash (/), then the root of the file tree is assumed. If you don't start your path
with a slash, then the current directory is the assumed starting point.
The screenshot below first shows the current directory /home/paul. From within this
directory, you have to type cd /home instead of cd home to go to the /home directory.
paul@debian8$ pwd
/home/paul
paul@debian8$ cd home
bash: cd: home: No such file or directory
paul@debian8$ cd /home
paul@debian8$ pwd
/home
When inside /home, you have to type cd paul instead of cd /paul to enter the subdirectory
paul of the current directory /home.
paul@debian8$ pwd
/home
paul@debian8$ cd /paul
bash: cd: /paul: No such file or directory
paul@debian8$ cd paul
paul@debian8$ pwd
/home/paul
In case your current directory is the root directory /, then both cd /home and cd home will
get you in the /home directory.
paul@debian8$
/
paul@debian8$
paul@debian8$
/home
paul@debian8$
paul@debian8$
paul@debian8$
/home
pwd
cd home
pwd
cd /
cd /home
pwd
This was the last screenshot with pwd statements. From now on, the current directory will
often be displayed in the prompt. Later in this book we will explain how the shell variable
$PS1 can be configured to show this.
75
working with directories
8.4. path completion
The tab key can help you in typing a path without errors. Typing cd /et followed by the tab
key will expand the command line to cd /etc/. When typing cd /Et followed by the tab key,
nothing will happen because you typed the wrong path (upper case E).
You will need fewer key strokes when using the tab key, and you will be sure your typed
path is correct!
8.5. ls
You can list the contents of a directory with ls.
paul@debian8:~$ ls
allfiles.txt dmesg.txt
paul@debian8:~$
services
stuff
summer.txt
8.5.1. ls -a
A frequently used option with ls is -a to show all files. Showing all files means including
the hidden files. When a file name on a Linux file system starts with a dot, it is considered
a hidden file and it doesn't show up in regular file listings.
paul@debian8:~$ ls
allfiles.txt dmesg.txt services stuff summer.txt
paul@debian8:~$ ls -a
.
allfiles.txt
.bash_profile dmesg.txt
.lesshst
.. .bash_history .bashrc
services
.ssh
paul@debian8:~$
stuff
summer.txt
8.5.2. ls -l
Many times you will be using options with ls to display the contents of the directory in
different formats or to display different parts of the directory. Typing just ls gives you a
list of files in the directory. Typing ls -l (that is a letter L, not the number 1) gives you a
long listing.
paul@debian8:~$ ls -l
total 17296
-rw-r--r-- 1 paul paul 17584442 Sep 17 00:03 allfiles.txt
-rw-r--r-- 1 paul paul
96650 Sep 17 00:03 dmesg.txt
-rw-r--r-- 1 paul paul
19558 Sep 17 00:04 services
drwxr-xr-x 2 paul paul
4096 Sep 17 00:04 stuff
-rw-r--r-- 1 paul paul
0 Sep 17 00:04 summer.txt
76
working with directories
8.5.3. ls -lh
Another frequently used ls option is -h. It shows the numbers (file sizes) in a more human
readable format. Also shown below is some variation in the way you can give the options
to ls. We will explain the details of the output later in this book.
Note that we use the letter L as an option in this screenshot, not the number 1.
paul@debian8:~$ ls -l -h
total 17M
-rw-r--r-- 1 paul paul 17M
-rw-r--r-- 1 paul paul 95K
-rw-r--r-- 1 paul paul 20K
drwxr-xr-x 2 paul paul 4.0K
-rw-r--r-- 1 paul paul
0
paul@debian8:~$ ls -lh
total 17M
-rw-r--r-- 1 paul paul 17M
-rw-r--r-- 1 paul paul 95K
-rw-r--r-- 1 paul paul 20K
drwxr-xr-x 2 paul paul 4.0K
-rw-r--r-- 1 paul paul
0
paul@debian8:~$ ls -hl
total 17M
-rw-r--r-- 1 paul paul 17M
-rw-r--r-- 1 paul paul 95K
-rw-r--r-- 1 paul paul 20K
drwxr-xr-x 2 paul paul 4.0K
-rw-r--r-- 1 paul paul
0
paul@debian8:~$ ls -h -l
total 17M
-rw-r--r-- 1 paul paul 17M
-rw-r--r-- 1 paul paul 95K
-rw-r--r-- 1 paul paul 20K
drwxr-xr-x 2 paul paul 4.0K
-rw-r--r-- 1 paul paul
0
paul@debian8:~$
Sep
Sep
Sep
Sep
Sep
17
17
17
17
17
00:03
00:03
00:04
00:04
00:04
allfiles.txt
dmesg.txt
services
stuff
summer.txt
Sep
Sep
Sep
Sep
Sep
17
17
17
17
17
00:03
00:03
00:04
00:04
00:04
allfiles.txt
dmesg.txt
services
stuff
summer.txt
Sep
Sep
Sep
Sep
Sep
17
17
17
17
17
00:03
00:03
00:04
00:04
00:04
allfiles.txt
dmesg.txt
services
stuff
summer.txt
Sep
Sep
Sep
Sep
Sep
17
17
17
17
17
00:03
00:03
00:04
00:04
00:04
allfiles.txt
dmesg.txt
services
stuff
summer.txt
77
working with directories
8.6. mkdir
Walking around the Unix file tree is fun, but it is even more fun to create your own directories
with mkdir. You have to give at least one parameter to mkdir, the name of the new directory
to be created. Think before you type a leading / .
paul@debian8:~$ mkdir mydir
paul@debian8:~$ cd mydir
paul@debian8:~/mydir$ ls -al
total 8
drwxr-xr-x 2 paul paul 4096 Sep 17 00:07 .
drwxr-xr-x 48 paul paul 4096 Sep 17 00:07 ..
paul@debian8:~/mydir$ mkdir stuff
paul@debian8:~/mydir$ mkdir otherstuff
paul@debian8:~/mydir$ ls -l
total 8
drwxr-xr-x 2 paul paul 4096 Sep 17 00:08 otherstuff
drwxr-xr-x 2 paul paul 4096 Sep 17 00:08 stuff
paul@debian8:~/mydir$
8.6.1. mkdir -p
The following command will fail, because the parent directory of threedirsdeep does not
exist.
paul@debian8:~$ mkdir mydir2/mysubdir2/threedirsdeep
mkdir: cannot create directory ‘mydir2/mysubdir2/threedirsdeep’: No such fi\
le or directory
When given the option -p, then mkdir will create parent directories as needed.
paul@debian8:~$ mkdir -p mydir2/mysubdir2/threedirsdeep
paul@debian8:~$ cd mydir2
paul@debian8:~/mydir2$ ls -l
total 4
drwxr-xr-x 3 paul paul 4096 Sep 17 00:11 mysubdir2
paul@debian8:~/mydir2$ cd mysubdir2
paul@debian8:~/mydir2/mysubdir2$ ls -l
total 4
drwxr-xr-x 2 paul paul 4096 Sep 17 00:11 threedirsdeep
paul@debian8:~/mydir2/mysubdir2$ cd threedirsdeep/
paul@debian8:~/mydir2/mysubdir2/threedirsdeep$ pwd
/home/paul/mydir2/mysubdir2/threedirsdeep
8.7. rmdir
When a directory is empty, you can use rmdir to remove the directory.
paul@debian8:~/mydir$ ls -l
total 8
drwxr-xr-x 2 paul paul 4096 Sep 17 00:08 otherstuff
drwxr-xr-x 2 paul paul 4096 Sep 17 00:08 stuff
paul@debian8:~/mydir$ rmdir otherstuff
paul@debian8:~/mydir$ cd ..
paul@debian8:~$ rmdir mydir
rmdir: failed to remove ‘mydir’: Directory not empty
paul@debian8:~$ rmdir mydir/stuff
paul@debian8:~$ rmdir mydir
paul@debian8:~$
78
working with directories
8.7.1. rmdir -p
And similar to the mkdir -p option, you can also use rmdir to recursively remove
directories.
paul@debian8:~$ mkdir -p test42/subdir
paul@debian8:~$ rmdir -p test42/subdir
paul@debian8:~$
79
working with directories
8.8. practice: working with directories
1. Display your current directory.
2. Change to the /etc directory.
3. Now change to your home directory using only three key presses.
4. Change to the /boot/grub directory using only eleven key presses.
5. Go to the parent directory of the current directory.
6. Go to the root directory.
7. List the contents of the root directory.
8. List a long listing of the root directory.
9. Stay where you are, and list the contents of /etc.
10. Stay where you are, and list the contents of /bin and /sbin.
11. Stay where you are, and list the contents of ~.
12. List all the files (including hidden files) in your home directory.
13. List the files in /boot in a human readable format.
14. Create a directory testdir in your home directory.
15. Change to the /etc directory, stay here and create a directory newdir in your home
directory.
16. Create in one command the directories ~/dir1/dir2/dir3 (dir3 is a subdirectory from dir2,
and dir2 is a subdirectory from dir1 ).
17. Remove the directory testdir.
18. If time permits (or if you are waiting for other students to finish this practice), use and
understand pushd and popd. Use the man page of bash to find information about these
commands.
80
working with directories
8.9. solution: working with directories
1. Display your current directory.
pwd
2. Change to the /etc directory.
cd /etc
3. Now change to your home directory using only three key presses.
cd (and the enter key)
4. Change to the /boot/grub directory using only eleven key presses.
cd /boot/grub (use the tab key)
5. Go to the parent directory of the current directory.
cd .. (with space between cd and ..)
6. Go to the root directory.
cd /
7. List the contents of the root directory.
ls
8. List a long listing of the root directory.
ls -l
9. Stay where you are, and list the contents of /etc.
ls /etc
10. Stay where you are, and list the contents of /bin and /sbin.
ls /bin /sbin
11. Stay where you are, and list the contents of ~.
ls ~
12. List all the files (including hidden files) in your home directory.
ls -al ~
13. List the files in /boot in a human readable format.
ls -lh /boot
14. Create a directory testdir in your home directory.
mkdir ~/testdir
15. Change to the /etc directory, stay here and create a directory newdir in your home
directory.
81
working with directories
cd /etc ; mkdir ~/newdir
16. Create in one command the directories ~/dir1/dir2/dir3 (dir3 is a subdirectory from dir2,
and dir2 is a subdirectory from dir1 ).
mkdir -p ~/dir1/dir2/dir3
17. Remove the directory testdir.
rmdir testdir
18. If time permits (or if you are waiting for other students to finish this practice), use and
understand pushd and popd. Use the man page of bash to find information about these
commands.
man bash
/pushd
n
# opens the manual
# searches for pushd
# next (do this two/three times)
The Bash shell has two built-in commands called pushd and popd. Both commands work
with a common stack of previous directories. Pushd adds a directory to the stack and changes
to a new current directory, popd removes a directory from the stack and sets the current
directory.
paul@debian7:/etc$ cd /bin
paul@debian7:/bin$ pushd /lib
/lib /bin
paul@debian7:/lib$ pushd /proc
/proc /lib /bin
paul@debian7:/proc$ popd
/lib /bin
paul@debian7:/lib$ popd
/bin
82
Chapter 9. working with files
In this chapter we learn how to recognise, create, remove, copy and move files using
commands like file, touch, rm, cp, mv and rename.
83
working with files
9.1. all files are case sensitive
Files on Linux (or any Unix) are case sensitive. This means that FILE1 is different from
file1, and /etc/hosts is different from /etc/Hosts (the latter one does not exist on a typical
Linux computer).
This screenshot shows the difference between two files, one with upper case W, the other
with lower case w.
paul@laika:~/Linux$ ls
winter.txt Winter.txt
paul@laika:~/Linux$ cat winter.txt
It is cold.
paul@laika:~/Linux$ cat Winter.txt
It is very cold!
9.2. everything is a file
A directory is a special kind of file, but it is still a (case sensitive!) file. Each terminal
window (for example /dev/pts/4), any hard disk or partition (for example /dev/sdb1) and
any process are all represented somewhere in the file system as a file. It will become clear
throughout this course that everything on Linux is a file.
9.3. file
The file utility determines the file type. Linux does not use extensions to determine the
file type. The command line does not care whether a file ends in .txt or .pdf. As a system
administrator, you should use the file command to determine the file type. Here are some
examples on a typical Linux system.
paul@laika:~$ file pic33.png
pic33.png: PNG image data, 3840 x 1200, 8-bit/color RGBA, non-interlaced
paul@laika:~$ file /etc/passwd
/etc/passwd: ASCII text
paul@laika:~$ file HelloWorld.c
HelloWorld.c: ASCII C program text
The file command uses a magic file that contains patterns to recognise file types. The magic
file is located in /usr/share/file/magic. Type man 5 magic for more information.
It is interesting to point out file -s for special files like those in /dev and /proc.
root@debian6~# file /dev/sda
/dev/sda: block special
root@debian6~# file -s /dev/sda
/dev/sda: x86 boot sector; partition 1: ID=0x83, active, starthead...
root@debian6~# file /proc/cpuinfo
/proc/cpuinfo: empty
root@debian6~# file -s /proc/cpuinfo
/proc/cpuinfo: ASCII C++ program text
84
working with files
9.4. touch
9.4.1. create an empty file
One easy way to create an empty file is with touch. (We will see many other ways for
creating files later in this book.)
This screenshot starts with an empty directory, creates two files with touch and the lists
those files.
paul@debian7:~$ ls -l
total 0
paul@debian7:~$ touch file42
paul@debian7:~$ touch file33
paul@debian7:~$ ls -l
total 0
-rw-r--r-- 1 paul paul 0 Oct 15 08:57 file33
-rw-r--r-- 1 paul paul 0 Oct 15 08:56 file42
paul@debian7:~$
9.4.2. touch -t
The touch command can set some properties while creating empty files. Can you determine
what is set by looking at the next screenshot? If not, check the manual for touch.
paul@debian7:~$ touch -t
paul@debian7:~$ touch -t
paul@debian7:~$ ls -l
total 0
-rw-r--r-- 1 paul paul 0
-rw-r--r-- 1 paul paul 0
-rw-r--r-- 1 paul paul 0
-rw-r--r-- 1 paul paul 0
paul@debian7:~$
200505050000 SinkoDeMayo
130207111630 BigBattle.txt
Jul 11 1302 BigBattle.txt
Oct 15 08:57 file33
Oct 15 08:56 file42
May 5 2005 SinkoDeMayo
85
working with files
9.5. rm
9.5.1. remove forever
When you no longer need a file, use rm to remove it. Unlike some graphical user interfaces,
the command line in general does not have a waste bin or trash can to recover files. When
you use rm to remove a file, the file is gone. Therefore, be careful when removing files!
paul@debian7:~$ ls
BigBattle.txt file33 file42 SinkoDeMayo
paul@debian7:~$ rm BigBattle.txt
paul@debian7:~$ ls
file33 file42 SinkoDeMayo
paul@debian7:~$
9.5.2. rm -i
To prevent yourself from accidentally removing a file, you can type rm -i.
paul@debian7:~$ ls
file33 file42 SinkoDeMayo
paul@debian7:~$ rm -i file33
rm: remove regular empty file `file33'? yes
paul@debian7:~$ rm -i SinkoDeMayo
rm: remove regular empty file `SinkoDeMayo'? n
paul@debian7:~$ ls
file42 SinkoDeMayo
paul@debian7:~$
9.5.3. rm -rf
By default, rm -r will not remove non-empty directories. However rm accepts several
options that will allow you to remove any directory. The rm -rf statement is famous because
it will erase anything (providing that you have the permissions to do so). When you are
logged on as root, be very careful with rm -rf (the f means force and the r means recursive)
since being root implies that permissions don't apply to you. You can literally erase your
entire file system by accident.
paul@debian7:~$ mkdir test
paul@debian7:~$ rm test
rm: cannot remove `test': Is a directory
paul@debian7:~$ rm -rf test
paul@debian7:~$ ls test
ls: cannot access test: No such file or directory
paul@debian7:~$
86
working with files
9.6. cp
9.6.1. copy one file
To copy a file, use cp with a source and a target argument.
paul@debian7:~$ ls
file42 SinkoDeMayo
paul@debian7:~$ cp file42 file42.copy
paul@debian7:~$ ls
file42 file42.copy SinkoDeMayo
9.6.2. copy to another directory
If the target is a directory, then the source files are copied to that target directory.
paul@debian7:~$ mkdir dir42
paul@debian7:~$ cp SinkoDeMayo dir42
paul@debian7:~$ ls dir42/
SinkoDeMayo
9.6.3. cp -r
To copy complete directories, use cp -r (the -r option forces recursive copying of all files
in all subdirectories).
paul@debian7:~$ ls
dir42 file42 file42.copy SinkoDeMayo
paul@debian7:~$ cp -r dir42/ dir33
paul@debian7:~$ ls
dir33 dir42 file42 file42.copy SinkoDeMayo
paul@debian7:~$ ls dir33/
SinkoDeMayo
9.6.4. copy multiple files to directory
You can also use cp to copy multiple files into a directory. In this case, the last argument
(a.k.a. the target) must be a directory.
paul@debian7:~$ cp file42 file42.copy SinkoDeMayo dir42/
paul@debian7:~$ ls dir42/
file42 file42.copy SinkoDeMayo
9.6.5. cp -i
To prevent cp from overwriting existing files, use the -i (for interactive) option.
paul@debian7:~$ cp SinkoDeMayo file42
paul@debian7:~$ cp SinkoDeMayo file42
paul@debian7:~$ cp -i SinkoDeMayo file42
cp: overwrite `file42'? n
paul@debian7:~$
87
working with files
9.7. mv
9.7.1. rename files with mv
Use mv to rename a file or to move the file to another directory.
paul@debian7:~$ ls
dir33 dir42 file42 file42.copy
paul@debian7:~$ mv file42 file33
paul@debian7:~$ ls
dir33 dir42 file33 file42.copy
paul@debian7:~$
SinkoDeMayo
SinkoDeMayo
When you need to rename only one file then mv is the preferred command to use.
9.7.2. rename directories with mv
The same mv command can be used to rename directories.
paul@debian7:~$ ls -l
total 8
drwxr-xr-x 2 paul paul 4096 Oct
drwxr-xr-x 2 paul paul 4096 Oct
-rw-r--r-- 1 paul paul
0 Oct
-rw-r--r-- 1 paul paul
0 Oct
-rw-r--r-- 1 paul paul
0 May
paul@debian7:~$ mv dir33 backup
paul@debian7:~$ ls -l
total 8
drwxr-xr-x 2 paul paul 4096 Oct
drwxr-xr-x 2 paul paul 4096 Oct
-rw-r--r-- 1 paul paul
0 Oct
-rw-r--r-- 1 paul paul
0 Oct
-rw-r--r-- 1 paul paul
0 May
paul@debian7:~$
15
15
15
15
5
09:36
09:36
09:38
09:16
2005
dir33
dir42
file33
file42.copy
SinkoDeMayo
15
15
15
15
5
09:36
09:36
09:38
09:16
2005
backup
dir42
file33
file42.copy
SinkoDeMayo
9.7.3. mv -i
The mv also has a -i switch similar to cp and rm.
this screenshot shows that mv -i will ask permission to overwrite an existing file.
paul@debian7:~$ mv -i file33 SinkoDeMayo
mv: overwrite `SinkoDeMayo'? no
paul@debian7:~$
88
working with files
9.8. rename
9.8.1. about rename
The rename command is one of the rare occasions where the Linux Fundamentals book
has to make a distinction between Linux distributions. Almost every command in the
Fundamentals part of this book works on almost every Linux computer. But rename is
different.
Try to use mv whenever you need to rename only a couple of files.
9.8.2. rename on Debian/Ubuntu
The rename command on Debian uses regular expressions (regular expression or shor regex
are explained in a later chapter) to rename many files at once.
Below a rename example that switches all occurrences of txt to png for all file names ending
in .txt.
paul@debian7:~/test42$ ls
abc.txt file33.txt file42.txt
paul@debian7:~/test42$ rename 's/\.txt/\.png/' *.txt
paul@debian7:~/test42$ ls
abc.png file33.png file42.png
This second example switches all (first) occurrences of file into document for all file names
ending in .png.
paul@debian7:~/test42$ ls
abc.png file33.png file42.png
paul@debian7:~/test42$ rename 's/file/document/' *.png
paul@debian7:~/test42$ ls
abc.png document33.png document42.png
paul@debian7:~/test42$
9.8.3. rename on CentOS/RHEL/Fedora
On Red Hat Enterprise Linux, the syntax of rename is a bit different. The first example
below renames all *.conf files replacing any occurrence of .conf with .backup.
[paul@centos7 ~]$ touch one.conf two.conf three.conf
[paul@centos7 ~]$ rename .conf .backup *.conf
[paul@centos7 ~]$ ls
one.backup three.backup two.backup
[paul@centos7 ~]$
The second example renames all (*) files replacing one with ONE.
[paul@centos7 ~]$ ls
one.backup three.backup two.backup
[paul@centos7 ~]$ rename one ONE *
[paul@centos7 ~]$ ls
ONE.backup three.backup two.backup
[paul@centos7 ~]$
89
working with files
9.9. practice: working with files
1. List the files in the /bin directory
2. Display the type of file of /bin/cat, /etc/passwd and /usr/bin/passwd.
3a. Download wolf.jpg and LinuxFun.pdf from http://linux-training.be (wget http://
linux-training.be/files/studentfiles/wolf.jpg and wget http://linux-training.be/files/books/
LinuxFun.pdf)
wget http://linux-training.be/files/studentfiles/wolf.jpg
wget http://linux-training.be/files/studentfiles/wolf.png
wget http://linux-training.be/files/books/LinuxFun.pdf
3b. Display the type of file of wolf.jpg and LinuxFun.pdf
3c. Rename wolf.jpg to wolf.pdf (use mv).
3d. Display the type of file of wolf.pdf and LinuxFun.pdf.
4. Create a directory ~/touched and enter it.
5. Create the files today.txt and yesterday.txt in touched.
6. Change the date on yesterday.txt to match yesterday's date.
7. Copy yesterday.txt to copy.yesterday.txt
8. Rename copy.yesterday.txt to kim
9. Create a directory called ~/testbackup and copy all files from ~/touched into it.
10. Use one command to remove the directory ~/testbackup and all files into it.
11. Create a directory ~/etcbackup and copy all *.conf files from /etc into it. Did you include
all subdirectories of /etc ?
12. Use rename to rename all *.conf files to *.backup . (if you have more than one distro
available, try it on all!)
90
working with files
9.10. solution: working with files
1. List the files in the /bin directory
ls /bin
2. Display the type of file of /bin/cat, /etc/passwd and /usr/bin/passwd.
file /bin/cat /etc/passwd /usr/bin/passwd
3a. Download wolf.jpg and LinuxFun.pdf from http://linux-training.be (wget http://
linux-training.be/files/studentfiles/wolf.jpg and wget http://linux-training.be/files/books/
LinuxFun.pdf)
wget http://linux-training.be/files/studentfiles/wolf.jpg
wget http://linux-training.be/files/studentfiles/wolf.png
wget http://linux-training.be/files/books/LinuxFun.pdf
3b. Display the type of file of wolf.jpg and LinuxFun.pdf
file wolf.jpg LinuxFun.pdf
3c. Rename wolf.jpg to wolf.pdf (use mv).
mv wolf.jpg wolf.pdf
3d. Display the type of file of wolf.pdf and LinuxFun.pdf.
file wolf.pdf LinuxFun.pdf
4. Create a directory ~/touched and enter it.
mkdir ~/touched ; cd ~/touched
5. Create the files today.txt and yesterday.txt in touched.
touch today.txt yesterday.txt
6. Change the date on yesterday.txt to match yesterday's date.
touch -t 200810251405 yesterday.txt (substitute 20081025 with yesterday)
7. Copy yesterday.txt to copy.yesterday.txt
cp yesterday.txt copy.yesterday.txt
8. Rename copy.yesterday.txt to kim
mv copy.yesterday.txt kim
9. Create a directory called ~/testbackup and copy all files from ~/touched into it.
mkdir ~/testbackup ; cp -r ~/touched ~/testbackup/
10. Use one command to remove the directory ~/testbackup and all files into it.
rm -rf ~/testbackup
11. Create a directory ~/etcbackup and copy all *.conf files from /etc into it. Did you include
all subdirectories of /etc ?
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working with files
cp -r /etc/*.conf ~/etcbackup
Only *.conf files that are directly in /etc/ are copied.
12. Use rename to rename all *.conf files to *.backup . (if you have more than one distro
available, try it on all!)
On RHEL: touch 1.conf 2.conf ; rename conf backup *.conf
On Debian: touch 1.conf 2.conf ; rename 's/conf/backup/' *.conf
92
Chapter 10. working with file contents
In this chapter we will look at the contents of text files with head, tail, cat, tac, more, less
and strings.
We will also get a glimpse of the possibilities of tools like cat on the command line.
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working with file contents
10.1. head
You can use head to display the first ten lines of a file.
paul@debian7~$ head /etc/passwd
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
lp:x:7:7:lp:/var/spool/lpd:/bin/sh
mail:x:8:8:mail:/var/mail:/bin/sh
news:x:9:9:news:/var/spool/news:/bin/sh
root@debian7~#
The head command can also display the first n lines of a file.
paul@debian7~$ head -4 /etc/passwd
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
paul@debian7~$
And head can also display the first n bytes.
paul@debian7~$ head -c14 /etc/passwd
root:x:0:0:roopaul@debian7~$
10.2. tail
Similar to head, the tail command will display the last ten lines of a file.
paul@debian7~$ tail /etc/services
vboxd
20012/udp
binkp
24554/tcp
asp
27374/tcp
asp
27374/udp
csync2
30865/tcp
dircproxy
57000/tcp
tfido
60177/tcp
fido
60179/tcp
# binkp fidonet protocol
# Address Search Protocol
#
#
#
#
cluster synchronization tool
Detachable IRC Proxy
fidonet EMSI over telnet
fidonet EMSI over TCP
# Local services
paul@debian7~$
You can give tail the number of lines you want to see.
paul@debian7~$ tail -3 /etc/services
fido
60179/tcp
# fidonet EMSI over TCP
# Local services
paul@debian7~$
The tail command has other useful options, some of which we will use during this course.
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working with file contents
10.3. cat
The cat command is one of the most universal tools, yet all it does is copy standard input to
standard output. In combination with the shell this can be very powerful and diverse. Some
examples will give a glimpse into the possibilities. The first example is simple, you can use
cat to display a file on the screen. If the file is longer than the screen, it will scroll to the end.
paul@debian8:~$ cat /etc/resolv.conf
domain linux-training.be
search linux-training.be
nameserver 192.168.1.42
10.3.1. concatenate
cat is short for concatenate. One of the basic uses of cat is to concatenate files into a bigger
(or complete) file.
paul@debian8:~$
paul@debian8:~$
paul@debian8:~$
paul@debian8:~$
one
paul@debian8:~$
two
paul@debian8:~$
three
paul@debian8:~$
one
two
three
paul@debian8:~$
paul@debian8:~$
one
two
three
paul@debian8:~$
echo one >part1
echo two >part2
echo three >part3
cat part1
cat part2
cat part3
cat part1 part2 part3
cat part1 part2 part3 >all
cat all
10.3.2. create files
You can use cat to create flat text files. Type the cat > winter.txt command as shown in the
screenshot below. Then type one or more lines, finishing each line with the enter key. After
the last line, type and hold the Control (Ctrl) key and press d.
paul@debian8:~$
It is very cold
paul@debian8:~$
It is very cold
paul@debian8:~$
cat > winter.txt
today!
cat winter.txt
today!
The Ctrl d key combination will send an EOF (End of File) to the running process ending
the cat command.
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working with file contents
10.3.3. custom end marker
You can choose an end marker for cat with << as is shown in this screenshot. This
construction is called a here directive and will end the cat command.
paul@debian8:~$ cat > hot.txt <<stop
> It is hot today!
> Yes it is summer.
> stop
paul@debian8:~$ cat hot.txt
It is hot today!
Yes it is summer.
paul@debian8:~$
10.3.4. copy files
In the third example you will see that cat can be used to copy files. We will explain in detail
what happens here in the bash shell chapter.
paul@debian8:~$
It is very cold
paul@debian8:~$
paul@debian8:~$
It is very cold
paul@debian8:~$
cat winter.txt
today!
cat winter.txt > cold.txt
cat cold.txt
today!
10.4. tac
Just one example will show you the purpose of tac (cat backwards).
paul@debian8:~$ cat count
one
two
three
four
paul@debian8:~$ tac count
four
three
two
one
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working with file contents
10.5. more and less
The more command is useful for displaying files that take up more than one screen. More
will allow you to see the contents of the file page by page. Use the space bar to see the next
page, or q to quit. Some people prefer the less command to more.
10.6. strings
With the strings command you can display readable ascii strings found in (binary) files.
This example locates the ls binary then displays readable strings in the binary file (output
is truncated).
paul@laika:~$ which ls
/bin/ls
paul@laika:~$ strings /bin/ls
/lib/ld-linux.so.2
librt.so.1
__gmon_start__
_Jv_RegisterClasses
clock_gettime
libacl.so.1
...
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working with file contents
10.7. practice: file contents
1. Display the first 12 lines of /etc/services.
2. Display the last line of /etc/passwd.
3. Use cat to create a file named count.txt that looks like this:
One
Two
Three
Four
Five
4. Use cp to make a backup of this file to cnt.txt.
5. Use cat to make a backup of this file to catcnt.txt.
6. Display catcnt.txt, but with all lines in reverse order (the last line first).
7. Use more to display /etc/services.
8. Display the readable character strings from the /usr/bin/passwd command.
9. Use ls to find the biggest file in /etc.
10. Open two terminal windows (or tabs) and make sure you are in the same directory in
both. Type echo this is the first line > tailing.txt in the first terminal, then issue tail -f
tailing.txt in the second terminal. Now go back to the first terminal and type echo This is
another line >> tailing.txt (note the double >>), verify that the tail -f in the second terminal
shows both lines. Stop the tail -f with Ctrl-C.
11. Use cat to create a file named tailing.txt that contains the contents of tailing.txt followed
by the contents of /etc/passwd.
12. Use cat to create a file named tailing.txt that contains the contents of tailing.txt preceded
by the contents of /etc/passwd.
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working with file contents
10.8. solution: file contents
1. Display the first 12 lines of /etc/services.
head -12 /etc/services
2. Display the last line of /etc/passwd.
tail -1 /etc/passwd
3. Use cat to create a file named count.txt that looks like this:
cat > count.txt
One
Two
Three
Four
Five (followed by Ctrl-d)
4. Use cp to make a backup of this file to cnt.txt.
cp count.txt cnt.txt
5. Use cat to make a backup of this file to catcnt.txt.
cat count.txt > catcnt.txt
6. Display catcnt.txt, but with all lines in reverse order (the last line first).
tac catcnt.txt
7. Use more to display /etc/services.
more /etc/services
8. Display the readable character strings from the /usr/bin/passwd command.
strings /usr/bin/passwd
9. Use ls to find the biggest file in /etc.
ls -lrS /etc
10. Open two terminal windows (or tabs) and make sure you are in the same directory in
both. Type echo this is the first line > tailing.txt in the first terminal, then issue tail -f
tailing.txt in the second terminal. Now go back to the first terminal and type echo This is
another line >> tailing.txt (note the double >>), verify that the tail -f in the second terminal
shows both lines. Stop the tail -f with Ctrl-C.
11. Use cat to create a file named tailing.txt that contains the contents of tailing.txt followed
by the contents of /etc/passwd.
cat /etc/passwd >> tailing.txt
12. Use cat to create a file named tailing.txt that contains the contents of tailing.txt preceded
by the contents of /etc/passwd.
mv tailing.txt tmp.txt ; cat /etc/passwd tmp.txt > tailing.txt
99
Chapter 11. the Linux file tree
This chapter takes a look at the most common directories in the Linux file tree. It also shows
that on Unix everything is a file.
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the Linux file tree
11.1. filesystem hierarchy standard
Many Linux distributions partially follow the Filesystem Hierarchy Standard. The FHS
may help make more Unix/Linux file system trees conform better in the future. The FHS
is available online at http://www.pathname.com/fhs/ where we read: "The filesystem
hierarchy standard has been designed to be used by Unix distribution developers, package
developers, and system implementers. However, it is primarily intended to be a reference
and is not a tutorial on how to manage a Unix filesystem or directory hierarchy."
11.2. man hier
There are some differences in the filesystems between Linux distributions. For help about
your machine, enter man hier to find information about the file system hierarchy. This
manual will explain the directory structure on your computer.
11.3. the root directory /
All Linux systems have a directory structure that starts at the root directory. The root
directory is represented by a forward slash, like this: /. Everything that exists on your Linux
system can be found below this root directory. Let's take a brief look at the contents of the
root directory.
[paul@RHELv4u3 ~]$ ls /
bin
dev home media mnt
boot etc lib
misc
opt
proc
root
sbin
selinux
101
srv
sys
tftpboot
tmp
usr
var
the Linux file tree
11.4. binary directories
Binaries are files that contain compiled source code (or machine code). Binaries can be
executed on the computer. Sometimes binaries are called executables.
11.4.1. /bin
The /bin directory contains binaries for use by all users. According to the FHS the /bin
directory should contain /bin/cat and /bin/date (among others).
In the screenshot below you see common Unix/Linux commands like cat, cp, cpio, date, dd,
echo, grep, and so on. Many of these will be covered in this book.
paul@laika:~$ ls /bin
archdetect
egrep
autopartition
false
bash
fgconsole
bunzip2
fgrep
bzcat
fuser
bzcmp
fusermount
bzdiff
get_mountoptions
bzegrep
grep
bzexe
gunzip
bzfgrep
gzexe
bzgrep
gzip
bzip2
hostname
bzip2recover
hw-detect
bzless
ip
bzmore
kbd_mode
cat
kill
...
mt
mt-gnu
mv
nano
nc
nc.traditional
netcat
netstat
ntfs-3g
ntfs-3g.probe
parted_devices
parted_server
partman
partman-commit
perform_recipe
pidof
setupcon
sh
sh.distrib
sleep
stralign
stty
su
sync
sysfs
tailf
tar
tempfile
touch
true
ulockmgr
umount
11.4.2. other /bin directories
You can find a /bin subdirectory in many other directories. A user named serena could put
her own programs in /home/serena/bin.
Some applications, often when installed directly from source will put themselves in /opt. A
samba server installation can use /opt/samba/bin to store its binaries.
11.4.3. /sbin
/sbin contains binaries to configure the operating system. Many of the system binaries
require root privilege to perform certain tasks.
Below a screenshot containing system binaries to change the ip address, partition a disk
and create an ext4 file system.
paul@ubu1010:~$ ls -l /sbin/ifconfig /sbin/fdisk /sbin/mkfs.ext4
-rwxr-xr-x 1 root root 97172 2011-02-02 09:56 /sbin/fdisk
-rwxr-xr-x 1 root root 65708 2010-07-02 09:27 /sbin/ifconfig
-rwxr-xr-x 5 root root 55140 2010-08-18 18:01 /sbin/mkfs.ext4
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the Linux file tree
11.4.4. /lib
Binaries found in /bin and /sbin often use shared libraries located in /lib. Below is a
screenshot of the partial contents of /lib.
paul@laika:~$ ls /lib/libc*
/lib/libc-2.5.so
/lib/libcfont.so.0.0.0
/lib/libcap.so.1
/lib/libcidn-2.5.so
/lib/libcap.so.1.10 /lib/libcidn.so.1
/lib/libcfont.so.0
/lib/libcom_err.so.2
/lib/libcom_err.so.2.1
/lib/libconsole.so.0
/lib/libconsole.so.0.0.0
/lib/libcrypt-2.5.so
/lib/modules
Typically, the Linux kernel loads kernel modules from /lib/modules/$kernel-version/.
This directory is discussed in detail in the Linux kernel chapter.
/lib32 and /lib64
We currently are in a transition between 32-bit and 64-bit systems. Therefore, you may
encounter directories named /lib32 and /lib64 which clarify the register size used during
compilation time of the libraries. A 64-bit computer may have some 32-bit binaries and
libraries for compatibility with legacy applications. This screenshot uses the file utility to
demonstrate the difference.
paul@laika:~$ file /lib32/libc-2.5.so
/lib32/libc-2.5.so: ELF 32-bit LSB shared object, Intel 80386, \
version 1 (SYSV), for GNU/Linux 2.6.0, stripped
paul@laika:~$ file /lib64/libcap.so.1.10
/lib64/libcap.so.1.10: ELF 64-bit LSB shared object, AMD x86-64, \
version 1 (SYSV), stripped
The ELF (Executable and Linkable Format) is used in almost every Unix-like operating
system since System V.
11.4.5. /opt
The purpose of /opt is to store optional software. In many cases this is software from outside
the distribution repository. You may find an empty /opt directory on many systems.
A large package can install all its files in /bin, /lib, /etc subdirectories within /opt/
$packagename/. If for example the package is called wp, then it installs in /opt/wp, putting
binaries in /opt/wp/bin and manpages in /opt/wp/man.
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the Linux file tree
11.5. configuration directories
11.5.1. /boot
The /boot directory contains all files needed to boot the computer. These files don't change
very often. On Linux systems you typically find the /boot/grub directory here. /boot/grub
contains /boot/grub/grub.cfg (older systems may still have /boot/grub/grub.conf) which
defines the boot menu that is displayed before the kernel starts.
11.5.2. /etc
All of the machine-specific configuration files should be located in /etc. Historically /etc
stood for etcetera, today people often use the Editable Text Configuration backronym.
Many times the name of a configuration files is the same as the application, daemon, or
protocol with .conf added as the extension.
paul@laika:~$ ls /etc/*.conf
/etc/adduser.conf
/etc/ld.so.conf
/etc/brltty.conf
/etc/lftp.conf
/etc/ccertificates.conf /etc/libao.conf
/etc/cvs-cron.conf
/etc/logrotate.conf
/etc/ddclient.conf
/etc/ltrace.conf
/etc/debconf.conf
/etc/mke2fs.conf
/etc/deluser.conf
/etc/netscsid.conf
/etc/fdmount.conf
/etc/nsswitch.conf
/etc/hdparm.conf
/etc/pam.conf
/etc/host.conf
/etc/pnm2ppa.conf
/etc/inetd.conf
/etc/povray.conf
/etc/kernel-img.conf
/etc/resolv.conf
paul@laika:~$
/etc/scrollkeeper.conf
/etc/sysctl.conf
/etc/syslog.conf
/etc/ucf.conf
/etc/uniconf.conf
/etc/updatedb.conf
/etc/usplash.conf
/etc/uswsusp.conf
/etc/vnc.conf
/etc/wodim.conf
/etc/wvdial.conf
There is much more to be found in /etc.
/etc/init.d/
A lot of Unix/Linux distributions have an /etc/init.d directory that contains scripts to start
and stop daemons. This directory could disappear as Linux migrates to systems that replace
the old init way of starting all daemons.
/etc/X11/
The graphical display (aka X Window System or just X) is driven by software from the
X.org foundation. The configuration file for your graphical display is /etc/X11/xorg.conf.
/etc/skel/
The skeleton directory /etc/skel is copied to the home directory of a newly created user. It
usually contains hidden files like a .bashrc script.
/etc/sysconfig/
This directory, which is not mentioned in the FHS, contains a lot of Red Hat Enterprise
Linux configuration files. We will discuss some of them in greater detail. The screenshot
below is the /etc/sysconfig directory from RHELv4u4 with everything installed.
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the Linux file tree
paul@RHELv4u4:~$ ls /etc/sysconfig/
apmd
firstboot
irda
apm-scripts grub
irqbalance
authconfig
hidd
keyboard
autofs
httpd
kudzu
bluetooth
hwconf
lm_sensors
clock
i18n
mouse
console
init
mouse.B
crond
installinfo
named
desktop
ipmi
netdump
diskdump
iptables
netdump_id_dsa
dund
iptables-cfg netdump_id_dsa.p
paul@RHELv4u4:~$
network
networking
ntpd
openib.conf
pand
pcmcia
pgsql
prelink
rawdevices
rhn
samba
saslauthd
selinux
spamassassin
squid
syslog
sys-config-sec
sys-config-users
sys-logviewer
tux
vncservers
xinetd
The file /etc/sysconfig/firstboot tells the Red Hat Setup Agent not to run at boot time. If
you want to run the Red Hat Setup Agent at the next reboot, then simply remove this file,
and run chkconfig --level 5 firstboot on. The Red Hat Setup Agent allows you to install
the latest updates, create a user account, join the Red Hat Network and more. It will then
create the /etc/sysconfig/firstboot file again.
paul@RHELv4u4:~$ cat /etc/sysconfig/firstboot
RUN_FIRSTBOOT=NO
The /etc/sysconfig/harddisks file contains some parameters to tune the hard disks. The file
explains itself.
You can see hardware detected by kudzu in /etc/sysconfig/hwconf. Kudzu is software from
Red Hat for automatic discovery and configuration of hardware.
The keyboard type and keymap table are set in the /etc/sysconfig/keyboard file. For more
console keyboard information, check the manual pages of keymaps(5), dumpkeys(1),
loadkeys(1) and the directory /lib/kbd/keymaps/.
root@RHELv4u4:/etc/sysconfig# cat keyboard
KEYBOARDTYPE="pc"
KEYTABLE="us"
We will discuss networking files in this directory in the networking chapter.
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the Linux file tree
11.6. data directories
11.6.1. /home
Users can store personal or project data under /home. It is common (but not mandatory by
the fhs) practice to name the users home directory after the user name in the format /home/
$USERNAME. For example:
paul@ubu606:~$ ls /home
geert annik sandra paul
tom
Besides giving every user (or every project or group) a location to store personal files, the
home directory of a user also serves as a location to store the user profile. A typical Unix
user profile contains many hidden files (files whose file name starts with a dot). The hidden
files of the Unix user profiles contain settings specific for that user.
paul@ubu606:~$ ls -d /home/paul/.*
/home/paul/.
/home/paul/.bash_profile
/home/paul/..
/home/paul/.bashrc
/home/paul/.bash_history /home/paul/.lesshst
/home/paul/.ssh
/home/paul/.viminfo
11.6.2. /root
On many systems /root is the default location for personal data and profile of the root user.
If it does not exist by default, then some administrators create it.
11.6.3. /srv
You may use /srv for data that is served by your system. The FHS allows locating cvs,
rsync, ftp and www data in this location. The FHS also approves administrative naming in /
srv, like /srv/project55/ftp and /srv/sales/www.
On Sun Solaris (or Oracle Solaris) /export is used for this purpose.
11.6.4. /media
The /media directory serves as a mount point for removable media devices such as CDROM's, digital cameras, and various usb-attached devices. Since /media is rather new in the
Unix world, you could very well encounter systems running without this directory. Solaris
9 does not have it, Solaris 10 does. Most Linux distributions today mount all removable
media in /media.
paul@debian5:~$ ls /media/
cdrom cdrom0 usbdisk
11.6.5. /mnt
The /mnt directory should be empty and should only be used for temporary mount points
(according to the FHS).
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the Linux file tree
Unix and Linux administrators used to create many directories here, like /mnt/something/.
You likely will encounter many systems with more than one directory created and/or
mounted inside /mnt to be used for various local and remote filesystems.
11.6.6. /tmp
Applications and users should use /tmp to store temporary data when needed. Data stored
in /tmp may use either disk space or RAM. Both of which are managed by the operating
system. Never use /tmp to store data that is important or which you wish to archive.
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the Linux file tree
11.7. in memory directories
11.7.1. /dev
Device files in /dev appear to be ordinary files, but are not actually located on the hard disk.
The /dev directory is populated with files as the kernel is recognising hardware.
common physical devices
Common hardware such as hard disk devices are represented by device files in /dev. Below
a screenshot of SATA device files on a laptop and then IDE attached drives on a desktop.
(The detailed meaning of these devices will be discussed later.)
#
# SATA or SCSI or USB
#
paul@laika:~$ ls /dev/sd*
/dev/sda /dev/sda1 /dev/sda2
/dev/sda3
/dev/sdb
/dev/sdb1
/dev/sdb2
#
# IDE or ATAPI
#
paul@barry:~$ ls /dev/hd*
/dev/hda /dev/hda1 /dev/hda2
/dev/hdb
/dev/hdb1
/dev/hdb2
/dev/hdc
Besides representing physical hardware, some device files are special. These special devices
can be very useful.
/dev/tty and /dev/pts
For example, /dev/tty1 represents a terminal or console attached to the system. (Don't
break your head on the exact terminology of 'terminal' or 'console', what we mean here is
a command line interface.) When typing commands in a terminal that is part of a graphical
interface like Gnome or KDE, then your terminal will be represented as /dev/pts/1 (1 can
be another number).
/dev/null
On Linux you will find other special devices such as /dev/null which can be considered
a black hole; it has unlimited storage, but nothing can be retrieved from it. Technically
speaking, anything written to /dev/null will be discarded. /dev/null can be useful to discard
unwanted output from commands. /dev/null is not a good location to store your backups ;-).
11.7.2. /proc conversation with the kernel
/proc is another special directory, appearing to be ordinary files, but not taking up disk
space. It is actually a view of the kernel, or better, what the kernel manages, and is a means
to interact with it directly. /proc is a proc filesystem.
paul@RHELv4u4:~$ mount -t proc
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the Linux file tree
none on /proc type proc (rw)
When listing the /proc directory you will see many numbers (on any Unix) and some
interesting files (on Linux)
mul@laika:~$ ls /proc
1
2339
4724 5418
10175 2523
4729 5421
10211 2783
4741 5658
10239 2975
4873 5661
141
29775 4874 5665
15045 29792 4878 5927
1519
2997
4879 6
1548
3
4881 6032
1551
30228 4882 6033
1554
3069
5
6145
1557
31422 5073 6298
1606
3149
5147 6414
180
31507 5203 6418
181
3189
5206 6419
182
3193
5228 6420
18898 3246
5272 6421
19799 3248
5291 6422
19803 3253
5294 6423
19804 3372
5356 6424
1987
4
5370 6425
1989
42
5379 6426
2
45
5380 6430
20845 4542
5412 6450
221
46
5414 6551
2338
4704
5416 6568
6587
6596
6599
6638
6652
6719
6736
6737
6755
6762
6774
6816
6991
6993
6996
7157
7163
7164
7171
7175
7188
7189
7191
7192
7199
7201
7204
7206
7214
7216
7218
7223
7224
7227
7260
7267
7275
7282
7298
7319
7330
7345
7513
7525
7529
9964
acpi
asound
buddyinfo
bus
cmdline
cpuinfo
crypto
devices
diskstats
dma
driver
execdomains
fb
filesystems
fs
ide
interrupts
iomem
ioports
irq
kallsyms
kcore
key-users
kmsg
loadavg
locks
meminfo
misc
modules
mounts
mtrr
net
pagetypeinfo
partitions
sched_debug
scsi
self
slabinfo
stat
swaps
sys
sysrq-trigger
sysvipc
timer_list
timer_stats
tty
uptime
version
version_signature
vmcore
vmnet
vmstat
zoneinfo
Let's investigate the file properties inside /proc. Looking at the date and time will display
the current date and time showing the files are constantly updated (a view on the kernel).
paul@RHELv4u4:~$ date
Mon Jan 29 18:06:32 EST 2007
paul@RHELv4u4:~$ ls -al /proc/cpuinfo
-r--r--r-- 1 root root 0 Jan 29 18:06 /proc/cpuinfo
paul@RHELv4u4:~$
paul@RHELv4u4:~$ ...time passes...
paul@RHELv4u4:~$
paul@RHELv4u4:~$ date
Mon Jan 29 18:10:00 EST 2007
paul@RHELv4u4:~$ ls -al /proc/cpuinfo
-r--r--r-- 1 root root 0 Jan 29 18:10 /proc/cpuinfo
Most files in /proc are 0 bytes, yet they contain data--sometimes a lot of data. You can see
this by executing cat on files like /proc/cpuinfo, which contains information about the CPU.
paul@RHELv4u4:~$ file /proc/cpuinfo
/proc/cpuinfo: empty
paul@RHELv4u4:~$ cat /proc/cpuinfo
processor
: 0
vendor_id
: AuthenticAMD
cpu family
: 15
model
: 43
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the Linux file tree
model name
stepping
cpu MHz
cache size
fdiv_bug
hlt_bug
f00f_bug
coma_bug
fpu
fpu_exception
cpuid level
wp
flags
bogomips
:
:
:
:
:
:
:
:
:
:
:
:
:
:
AMD Athlon(tm) 64 X2 Dual Core Processor 4600+
1
2398.628
512 KB
no
no
no
no
yes
yes
1
yes
fpu vme de pse tsc msr pae mce cx8 apic mtrr pge...
4803.54
Just for fun, here is /proc/cpuinfo on a Sun Sunblade 1000...
paul@pasha:~$ cat /proc/cpuinfo
cpu : TI UltraSparc III (Cheetah)
fpu : UltraSparc III integrated FPU
promlib : Version 3 Revision 2
prom : 4.2.2
type : sun4u
ncpus probed : 2
ncpus active : 2
Cpu0Bogo : 498.68
Cpu0ClkTck : 000000002cb41780
Cpu1Bogo : 498.68
Cpu1ClkTck : 000000002cb41780
MMU Type : Cheetah
State:
CPU0: online
CPU1: online
Most of the files in /proc are read only, some require root privileges, some files are writable,
and many files in /proc/sys are writable. Let's discuss some of the files in /proc.
110
the Linux file tree
/proc/interrupts
On the x86 architecture, /proc/interrupts displays the interrupts.
paul@RHELv4u4:~$ cat /proc/interrupts
CPU0
0:
13876877
IO-APIC-edge timer
1:
15
IO-APIC-edge i8042
8:
1
IO-APIC-edge rtc
9:
0
IO-APIC-level acpi
12:
67
IO-APIC-edge i8042
14:
128
IO-APIC-edge ide0
15:
124320
IO-APIC-edge ide1
169:
111993
IO-APIC-level ioc0
177:
2428
IO-APIC-level eth0
NMI:
0
LOC:
13878037
ERR:
0
MIS:
0
On a machine with two CPU's, the file looks like this.
paul@laika:~$ cat /proc/interrupts
CPU0
CPU1
0:
860013
0 IO-APIC-edge
1:
4533
0 IO-APIC-edge
7:
0
0 IO-APIC-edge
8:
6588227
0 IO-APIC-edge
10:
2314
0 IO-APIC-fasteoi
12:
133
0 IO-APIC-edge
14:
0
0 IO-APIC-edge
15:
72269
0 IO-APIC-edge
18:
1
0 IO-APIC-fasteoi
19:
115036
0 IO-APIC-fasteoi
20:
126871
0 IO-APIC-fasteoi
21:
30204
0 IO-APIC-fasteoi
22:
1334
0 IO-APIC-fasteoi
24:
234739
0 IO-APIC-fasteoi
NMI:
72
42
LOC:
860000
859994
ERR:
0
timer
i8042
parport0
rtc
acpi
i8042
libata
libata
yenta
eth0
libata, ohci1394
ehci_hcd:usb1, uhci_hcd:usb2
saa7133[0], saa7133[0]
nvidia
/proc/kcore
The physical memory is represented in /proc/kcore. Do not try to cat this file, instead use a
debugger. The size of /proc/kcore is the same as your physical memory, plus four bytes.
paul@laika:~$ ls -lh /proc/kcore
-r-------- 1 root root 2.0G 2007-01-30 08:57 /proc/kcore
paul@laika:~$
111
the Linux file tree
11.7.3. /sys Linux 2.6 hot plugging
The /sys directory was created for the Linux 2.6 kernel. Since 2.6, Linux uses sysfs
to support usb and IEEE 1394 (FireWire) hot plug devices. See the manual pages
of udev(8) (the successor of devfs) and hotplug(8) for more info (or visit http://linuxhotplug.sourceforge.net/ ).
Basically the /sys directory contains kernel information about hardware.
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the Linux file tree
11.8. /usr Unix System Resources
Although /usr is pronounced like user, remember that it stands for Unix System Resources.
The /usr hierarchy should contain shareable, read only data. Some people choose to mount
/usr as read only. This can be done from its own partition or from a read only NFS share
(NFS is discussed later).
11.8.1. /usr/bin
The /usr/bin directory contains a lot of commands.
paul@deb508:~$ ls /usr/bin | wc -l
1395
(On Solaris the /bin directory is a symbolic link to /usr/bin.)
11.8.2. /usr/include
The /usr/include directory contains general use include files for C.
paul@ubu1010:~$ ls /usr/include/
aalib.h
expat_config.h
af_vfs.h
expat_external.h
aio.h
expat.h
AL
fcntl.h
aliases.h
features.h
...
math.h
mcheck.h
memory.h
menu.h
mntent.h
search.h
semaphore.h
setjmp.h
sgtty.h
shadow.h
11.8.3. /usr/lib
The /usr/lib directory contains libraries that are not directly executed by users or scripts.
paul@deb508:~$ ls /usr/lib | head -7
4Suite
ao
apt
arj
aspell
avahi
bonobo
11.8.4. /usr/local
The /usr/local directory can be used by an administrator to install software locally.
paul@deb508:~$ ls /usr/local/
bin etc games include lib man
paul@deb508:~$ du -sh /usr/local/
128K /usr/local/
sbin
share
src
11.8.5. /usr/share
The /usr/share directory contains architecture independent data. As you can see, this is a
fairly large directory.
paul@deb508:~$ ls /usr/share/ | wc -l
113
the Linux file tree
263
paul@deb508:~$ du -sh /usr/share/
1.3G /usr/share/
This directory typically contains /usr/share/man for manual pages.
paul@deb508:~$ ls /usr/share/man
cs fr
hu it.UTF-8 man2 man6 pl.ISO8859-2 sv
de fr.ISO8859-1 id ja
man3 man7 pl.UTF-8
tr
es fr.UTF-8
it ko
man4 man8 pt_BR
zh_CN
fi gl
it.ISO8859-1 man1
man5 pl
ru
zh_TW
And it contains /usr/share/games for all static game data (so no high-scores or play logs).
paul@ubu1010:~$ ls /usr/share/games/
openttd wesnoth
11.8.6. /usr/src
The /usr/src directory is the recommended location for kernel source files.
paul@deb508:~$ ls -l /usr/src/
total 12
drwxr-xr-x 4 root root 4096 2011-02-01 14:43 linux-headers-2.6.26-2-686
drwxr-xr-x 18 root root 4096 2011-02-01 14:43 linux-headers-2.6.26-2-common
drwxr-xr-x 3 root root 4096 2009-10-28 16:01 linux-kbuild-2.6.26
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the Linux file tree
11.9. /var variable data
Files that are unpredictable in size, such as log, cache and spool files, should be located in
/var.
11.9.1. /var/log
The /var/log directory serves as a central point to contain all log files.
[paul@RHEL4b ~]$ ls /var/log
acpid
cron.2
maillog.2
amanda
cron.3
maillog.3
anaconda.log
cron.4
maillog.4
anaconda.syslog cups
mailman
anaconda.xlog
dmesg
messages
audit
exim
messages.1
boot.log
gdm
messages.2
boot.log.1
httpd
messages.3
boot.log.2
iiim
messages.4
boot.log.3
iptraf
mysqld.log
boot.log.4
lastlog
news
canna
mail
pgsql
cron
maillog
ppp
cron.1
maillog.1 prelink.log
quagga
radius
rpmpkgs
rpmpkgs.1
rpmpkgs.2
rpmpkgs.3
rpmpkgs.4
sa
samba
scrollkeeper.log
secure
secure.1
secure.2
secure.3
secure.4
spooler
spooler.1
spooler.2
spooler.3
spooler.4
squid
uucp
vbox
vmware-tools-guestd
wtmp
wtmp.1
Xorg.0.log
Xorg.0.log.old
11.9.2. /var/log/messages
A typical first file to check when troubleshooting on Red Hat (and derivatives) is the /var/
log/messages file. By default this file will contain information on what just happened to the
system. The file is called /var/log/syslog on Debian and Ubuntu.
[root@RHEL4b ~]# tail /var/log/messages
Jul 30 05:13:56 anacron: anacron startup succeeded
Jul 30 05:13:56 atd: atd startup succeeded
Jul 30 05:13:57 messagebus: messagebus startup succeeded
Jul 30 05:13:57 cups-config-daemon: cups-config-daemon startup succeeded
Jul 30 05:13:58 haldaemon: haldaemon startup succeeded
Jul 30 05:14:00 fstab-sync[3560]: removed all generated mount points
Jul 30 05:14:01 fstab-sync[3628]: added mount point /media/cdrom for...
Jul 30 05:14:01 fstab-sync[3646]: added mount point /media/floppy for...
Jul 30 05:16:46 sshd(pam_unix)[3662]: session opened for user paul by...
Jul 30 06:06:37 su(pam_unix)[3904]: session opened for user root by paul
11.9.3. /var/cache
The /var/cache directory can contain cache data for several applications.
paul@ubu1010:~$ ls /var/cache/
apt
dictionaries-common
binfmts flashplugin-installer
cups
fontconfig
debconf fonts
gdm
hald
jockey
ldconfig
man
pm-utils
pppconfig
samba
software-center
11.9.4. /var/spool
The /var/spool directory typically contains spool directories for mail and cron, but also
serves as a parent directory for other spool files (for example print spool files).
115
the Linux file tree
11.9.5. /var/lib
The /var/lib directory contains application state information.
Red Hat Enterprise Linux for example keeps files pertaining to rpm in /var/lib/rpm/.
11.9.6. /var/...
/var also contains Process ID files in /var/run (soon to be replaced with /run) and temporary
files that survive a reboot in /var/tmp and information about file locks in /var/lock. There
will be more examples of /var usage further in this book.
116
the Linux file tree
11.10. practice: file system tree
1. Does the file /bin/cat exist ? What about /bin/dd and /bin/echo. What is the type of these
files ?
2. What is the size of the Linux kernel file(s) (vmlinu*) in /boot ?
3. Create a directory ~/test. Then issue the following commands:
cd ~/test
dd if=/dev/zero of=zeroes.txt count=1 bs=100
od zeroes.txt
dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file /dev/zero
to ~/test/zeroes.txt. Can you describe the functionality of /dev/zero ?
4. Now issue the following command:
dd if=/dev/random of=random.txt count=1 bs=100 ; od random.txt
dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file /dev/
random to ~/test/random.txt. Can you describe the functionality of /dev/random ?
5. Issue the following two commands, and look at the first character of each output line.
ls -l /dev/sd* /dev/hd*
ls -l /dev/tty* /dev/input/mou*
The first ls will show block(b) devices, the second ls shows character(c) devices. Can you
tell the difference between block and character devices ?
6. Use cat to display /etc/hosts and /etc/resolv.conf. What is your idea about the purpose
of these files ?
7. Are there any files in /etc/skel/ ? Check also for hidden files.
8. Display /proc/cpuinfo. On what architecture is your Linux running ?
9. Display /proc/interrupts. What is the size of this file ? Where is this file stored ?
10. Can you enter the /root directory ? Are there (hidden) files ?
11. Are ifconfig, fdisk, parted, shutdown and grub-install present in /sbin ? Why are these
binaries in /sbin and not in /bin ?
12. Is /var/log a file or a directory ? What about /var/spool ?
13. Open two command prompts (Ctrl-Shift-T in gnome-terminal) or terminals (Ctrl-Alt-F1,
Ctrl-Alt-F2, ...) and issue the who am i in both. Then try to echo a word from one terminal
to the other.
117
the Linux file tree
14. Read the man page of random and explain the difference between /dev/random and /
dev/urandom.
118
the Linux file tree
11.11. solution: file system tree
1. Does the file /bin/cat exist ? What about /bin/dd and /bin/echo. What is the type of these
files ?
ls /bin/cat ; file /bin/cat
ls /bin/dd ; file /bin/dd
ls /bin/echo ; file /bin/echo
2. What is the size of the Linux kernel file(s) (vmlinu*) in /boot ?
ls -lh /boot/vm*
3. Create a directory ~/test. Then issue the following commands:
cd ~/test
dd if=/dev/zero of=zeroes.txt count=1 bs=100
od zeroes.txt
dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file /dev/zero
to ~/test/zeroes.txt. Can you describe the functionality of /dev/zero ?
/dev/zero is a Linux special device. It can be considered a source of zeroes. You cannot send
something to /dev/zero, but you can read zeroes from it.
4. Now issue the following command:
dd if=/dev/random of=random.txt count=1 bs=100 ; od random.txt
dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file /dev/
random to ~/test/random.txt. Can you describe the functionality of /dev/random ?
/dev/random acts as a random number generator on your Linux machine.
5. Issue the following two commands, and look at the first character of each output line.
ls -l /dev/sd* /dev/hd*
ls -l /dev/tty* /dev/input/mou*
The first ls will show block(b) devices, the second ls shows character(c) devices. Can you
tell the difference between block and character devices ?
Block devices are always written to (or read from) in blocks. For hard disks, blocks of 512
bytes are common. Character devices act as a stream of characters (or bytes). Mouse and
keyboard are typical character devices.
6. Use cat to display /etc/hosts and /etc/resolv.conf. What is your idea about the purpose
of these files ?
/etc/hosts contains hostnames with their ip address
/etc/resolv.conf should contain the ip address of a DNS name server.
119
the Linux file tree
7. Are there any files in /etc/skel/ ? Check also for hidden files.
Issue "ls -al /etc/skel/". Yes, there should be hidden files there.
8. Display /proc/cpuinfo. On what architecture is your Linux running ?
The file should contain at least one line with Intel or other cpu.
9. Display /proc/interrupts. What is the size of this file ? Where is this file stored ?
The size is zero, yet the file contains data. It is not stored anywhere because /proc is a
virtual file system that allows you to talk with the kernel. (If you answered "stored in RAMmemory, that is also correct...).
10. Can you enter the /root directory ? Are there (hidden) files ?
Try "cd /root". The /root directory is not accessible for normal users on most modern Linux sys
11. Are ifconfig, fdisk, parted, shutdown and grub-install present in /sbin ? Why are these
binaries in /sbin and not in /bin ?
Because those files are only meant for system administrators.
12. Is /var/log a file or a directory ? What about /var/spool ?
Both are directories.
13. Open two command prompts (Ctrl-Shift-T in gnome-terminal) or terminals (Ctrl-Alt-F1,
Ctrl-Alt-F2, ...) and issue the who am i in both. Then try to echo a word from one terminal
to the other.
tty-terminal: echo Hello > /dev/tty1
pts-terminal: echo Hello > /dev/pts/1
14. Read the man page of random and explain the difference between /dev/random and /
dev/urandom.
man 4 random
120
Part IV. shell expansion
Table of Contents
12. commands and arguments ..............................................................................................
12.1. arguments ............................................................................................................
12.2. white space removal ..............................................................................................
12.3. single quotes ........................................................................................................
12.4. double quotes .......................................................................................................
12.5. echo and quotes ....................................................................................................
12.6. commands ...........................................................................................................
12.7. aliases .................................................................................................................
12.8. displaying shell expansion ......................................................................................
12.9. practice: commands and arguments ..........................................................................
12.10. solution: commands and arguments ........................................................................
13. control operators ...........................................................................................................
13.1. ; semicolon ..........................................................................................................
13.2. & ampersand ........................................................................................................
13.3. $? dollar question mark .........................................................................................
13.4. && double ampersand ...........................................................................................
13.5. || double vertical bar ..............................................................................................
13.6. combining && and || .............................................................................................
13.7. # pound sign ........................................................................................................
13.8. \ escaping special characters ...................................................................................
13.9. practice: control operators ......................................................................................
13.10. solution: control operators .....................................................................................
14. shell variables ................................................................................................................
14.1. $ dollar sign ........................................................................................................
14.2. case sensitive .......................................................................................................
14.3. creating variables ..................................................................................................
14.4. quotes .................................................................................................................
14.5. set ......................................................................................................................
14.6. unset ...................................................................................................................
14.7. $PS1 ...................................................................................................................
14.8. $PATH ...............................................................................................................
14.9. env .....................................................................................................................
14.10. export ...............................................................................................................
14.11. delineate variables ...............................................................................................
14.12. unbound variables ...............................................................................................
14.13. practice: shell variables ........................................................................................
14.14. solution: shell variables ........................................................................................
15. shell embedding and options ...........................................................................................
15.1. shell embedding ....................................................................................................
15.2. shell options ........................................................................................................
15.3. practice: shell embedding .......................................................................................
15.4. solution: shell embedding .......................................................................................
16. shell history ...................................................................................................................
16.1. repeating the last command ....................................................................................
16.2. repeating other commands ......................................................................................
16.3. history ................................................................................................................
16.4. !n .......................................................................................................................
16.5. Ctrl-r ..................................................................................................................
16.6. $HISTSIZE ..........................................................................................................
16.7. $HISTFILE ..........................................................................................................
16.8. $HISTFILESIZE ...................................................................................................
16.9. prevent recording a command .................................................................................
16.10. (optional)regular expressions .................................................................................
16.11. (optional) Korn shell history .................................................................................
16.12. practice: shell history ...........................................................................................
122
124
125
125
126
126
126
127
128
129
130
132
134
135
135
135
136
136
136
137
137
138
139
140
141
141
141
142
142
142
143
144
145
145
146
146
147
148
149
150
151
152
153
154
155
155
155
155
156
156
156
156
157
157
157
158
shell expansion
16.13. solution: shell history ...........................................................................................
17. file globbing ..................................................................................................................
17.1. * asterisk .............................................................................................................
17.2. ? question mark ....................................................................................................
17.3. [] square brackets ..................................................................................................
17.4. a-z and 0-9 ranges ................................................................................................
17.5. $LANG and square brackets ...................................................................................
17.6. preventing file globbing .........................................................................................
17.7. practice: shell globbing ..........................................................................................
17.8. solution: shell globbing ..........................................................................................
123
159
160
161
161
162
163
163
164
165
166
Chapter 12. commands and
arguments
This chapter introduces you to shell expansion by taking a close look at commands and
arguments. Knowing shell expansion is important because many commands on your
Linux system are processed and most likely changed by the shell before they are executed.
The command line interface or shell used on most Linux systems is called bash, which
stands for Bourne again shell. The bash shell incorporates features from sh (the original
Bourne shell), csh (the C shell), and ksh (the Korn shell).
This chapter frequently uses the echo command to demonstrate shell features. The echo
command is very simple: it echoes the input that it receives.
paul@laika:~$ echo Burtonville
Burtonville
paul@laika:~$ echo Smurfs are blue
Smurfs are blue
124
commands and arguments
12.1. arguments
One of the primary features of a shell is to perform a command line scan. When you enter
a command at the shell's command prompt and press the enter key, then the shell will start
scanning that line, cutting it up in arguments. While scanning the line, the shell may make
many changes to the arguments you typed.
This process is called shell expansion. When the shell has finished scanning and modifying
that line, then it will be executed.
12.2. white space removal
Parts that are separated by one or more consecutive white spaces (or tabs) are considered
separate arguments, any white space is removed. The first argument is the command to be
executed, the other arguments are given to the command. The shell effectively cuts your
command into one or more arguments.
This explains why the following four different command lines are the same after shell
expansion.
[paul@RHELv4u3
Hello World
[paul@RHELv4u3
Hello World
[paul@RHELv4u3
Hello World
[paul@RHELv4u3
Hello World
~]$ echo Hello World
~]$ echo Hello
~]$ echo
~]$
echo
Hello
World
World
Hello
World
The echo command will display each argument it receives from the shell. The echo
command will also add a new white space between the arguments it received.
125
commands and arguments
12.3. single quotes
You can prevent the removal of white spaces by quoting the spaces. The contents of the
quoted string are considered as one argument. In the screenshot below the echo receives
only one argument.
[paul@RHEL4b ~]$ echo 'A line with
A line with
single
quotes
[paul@RHEL4b ~]$
single
quotes'
12.4. double quotes
You can also prevent the removal of white spaces by double quoting the spaces. Same as
above, echo only receives one argument.
[paul@RHEL4b ~]$ echo "A line with
A line with
double
quotes
[paul@RHEL4b ~]$
double
quotes"
Later in this book, when discussing variables we will see important differences between
single and double quotes.
12.5. echo and quotes
Quoted lines can include special escaped characters recognised by the echo command (when
using echo -e). The screenshot below shows how to use \n for a newline and \t for a tab
(usually eight white spaces).
[paul@RHEL4b
A line with
a newline
[paul@RHEL4b
A line with
a newline
[paul@RHEL4b
A line with
[paul@RHEL4b
A line with
[paul@RHEL4b
~]$ echo -e "A line with \na newline"
~]$ echo -e 'A line with \na newline'
~]$ echo -e "A line with \ta tab"
a tab
~]$ echo -e 'A line with \ta tab'
a tab
~]$
The echo command can generate more than white spaces, tabs and newlines. Look in the
man page for a list of options.
126
commands and arguments
12.6. commands
12.6.1. external or builtin commands ?
Not all commands are external to the shell, some are builtin. External commands are
programs that have their own binary and reside somewhere in the file system. Many external
commands are located in /bin or /sbin. Builtin commands are an integral part of the shell
program itself.
12.6.2. type
To find out whether a command given to the shell will be executed as an external command
or as a builtin command, use the type command.
paul@laika:~$ type cd
cd is a shell builtin
paul@laika:~$ type cat
cat is /bin/cat
As you can see, the cd command is builtin and the cat command is external.
You can also use this command to show you whether the command is aliased or not.
paul@laika:~$ type ls
ls is aliased to `ls --color=auto'
12.6.3. running external commands
Some commands have both builtin and external versions. When one of these commands is
executed, the builtin version takes priority. To run the external version, you must enter the
full path to the command.
paul@laika:~$ type -a echo
echo is a shell builtin
echo is /bin/echo
paul@laika:~$ /bin/echo Running the external echo command...
Running the external echo command...
12.6.4. which
The which command will search for binaries in the $PATH environment variable (variables
will be explained later). In the screenshot below, it is determined that cd is builtin, and ls,
cp, rm, mv, mkdir, pwd, and which are external commands.
[root@RHEL4b ~]# which cp ls cd mkdir pwd
/bin/cp
/bin/ls
/usr/bin/which: no cd in (/usr/kerberos/sbin:/usr/kerberos/bin:...
/bin/mkdir
/bin/pwd
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commands and arguments
12.7. aliases
12.7.1. create an alias
The shell allows you to create aliases. Aliases are often used to create an easier to remember
name for an existing command or to easily supply parameters.
[paul@RHELv4u3 ~]$ cat count.txt
one
two
three
[paul@RHELv4u3 ~]$ alias dog=tac
[paul@RHELv4u3 ~]$ dog count.txt
three
two
one
12.7.2. abbreviate commands
An alias can also be useful to abbreviate an existing command.
paul@laika:~$ alias ll='ls -lh --color=auto'
paul@laika:~$ alias c='clear'
paul@laika:~$
12.7.3. default options
Aliases can be used to supply commands with default options. The example below shows
how to set the -i option default when typing rm.
[paul@RHELv4u3 ~]$
rm: remove regular
[paul@RHELv4u3 ~]$
[paul@RHELv4u3 ~]$
ls: winter.txt: No
[paul@RHELv4u3 ~]$
[paul@RHELv4u3 ~]$
[paul@RHELv4u3 ~]$
rm: remove regular
[paul@RHELv4u3 ~]$
rm -i winter.txt
file `winter.txt'? no
rm winter.txt
ls winter.txt
such file or directory
touch winter.txt
alias rm='rm -i'
rm winter.txt
empty file `winter.txt'? no
Some distributions enable default aliases to protect users from accidentally erasing files ('rm
-i', 'mv -i', 'cp -i')
12.7.4. viewing aliases
You can provide one or more aliases as arguments to the alias command to get their
definitions. Providing no arguments gives a complete list of current aliases.
paul@laika:~$ alias c ll
alias c='clear'
alias ll='ls -lh --color=auto'
128
commands and arguments
12.7.5. unalias
You can undo an alias with the unalias command.
[paul@RHEL4b ~]$
/bin/rm
[paul@RHEL4b ~]$
[paul@RHEL4b ~]$
alias rm='rm -i'
/bin/rm
[paul@RHEL4b ~]$
[paul@RHEL4b ~]$
/bin/rm
[paul@RHEL4b ~]$
which rm
alias rm='rm -i'
which rm
unalias rm
which rm
12.8. displaying shell expansion
You can display shell expansion with set -x, and stop displaying it with set +x. You might
want to use this further on in this course, or when in doubt about exactly what the shell is
doing with your command.
[paul@RHELv4u3 ~]$ set -x
++ echo -ne '\033]0;paul@RHELv4u3:~\007'
[paul@RHELv4u3 ~]$ echo $USER
+ echo paul
paul
++ echo -ne '\033]0;paul@RHELv4u3:~\007'
[paul@RHELv4u3 ~]$ echo \$USER
+ echo '$USER'
$USER
++ echo -ne '\033]0;paul@RHELv4u3:~\007'
[paul@RHELv4u3 ~]$ set +x
+ set +x
[paul@RHELv4u3 ~]$ echo $USER
paul
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commands and arguments
12.9. practice: commands and arguments
1. How many arguments are in this line (not counting the command itself).
touch '/etc/cron/cron.allow' 'file 42.txt' "file 33.txt"
2. Is tac a shell builtin command ?
3. Is there an existing alias for rm ?
4. Read the man page of rm, make sure you understand the -i option of rm. Create and
remove a file to test the -i option.
5. Execute: alias rm='rm -i' . Test your alias with a test file. Does this work as expected ?
6. List all current aliases.
7a. Create an alias called 'city' that echoes your hometown.
7b. Use your alias to test that it works.
8. Execute set -x to display shell expansion for every command.
9. Test the functionality of set -x by executing your city and rm aliases.
10 Execute set +x to stop displaying shell expansion.
11. Remove your city alias.
12. What is the location of the cat and the passwd commands ?
13. Explain the difference between the following commands:
echo
/bin/echo
14. Explain the difference between the following commands:
echo Hello
echo -n Hello
15. Display A B C with two spaces between B and C.
(optional)16. Complete the following command (do not use spaces) to display exactly the
following output:
4+4
10+14
=8
=24
17. Use echo to display the following exactly:
??\\
130
commands and arguments
Find two solutions with single quotes, two with double quotes and one without quotes (and
say thank you to René and Darioush from Google for this extra).
18. Use one echo command to display three words on three lines.
131
commands and arguments
12.10. solution: commands and arguments
1. How many arguments are in this line (not counting the command itself).
touch '/etc/cron/cron.allow' 'file 42.txt' "file 33.txt"
answer: three
2. Is tac a shell builtin command ?
type tac
3. Is there an existing alias for rm ?
alias rm
4. Read the man page of rm, make sure you understand the -i option of rm. Create and
remove a file to test the -i option.
man rm
touch testfile
rm -i testfile
5. Execute: alias rm='rm -i' . Test your alias with a test file. Does this work as expected ?
touch testfile
rm testfile (should ask for confirmation)
6. List all current aliases.
alias
7a. Create an alias called 'city' that echoes your hometown.
alias city='echo Antwerp'
7b. Use your alias to test that it works.
city (it should display Antwerp)
8. Execute set -x to display shell expansion for every command.
set -x
9. Test the functionality of set -x by executing your city and rm aliases.
shell should display the resolved aliases and then execute the command:
paul@deb503:~$ set -x
paul@deb503:~$ city
+ echo antwerp
antwerp
10 Execute set +x to stop displaying shell expansion.
set +x
11. Remove your city alias.
132
commands and arguments
unalias city
12. What is the location of the cat and the passwd commands ?
which cat (probably /bin/cat)
which passwd (probably /usr/bin/passwd)
13. Explain the difference between the following commands:
echo
/bin/echo
The echo command will be interpreted by the shell as the built-in echo command. The /bin/
echo command will make the shell execute the echo binary located in the /bin directory.
14. Explain the difference between the following commands:
echo Hello
echo -n Hello
The -n option of the echo command will prevent echo from echoing a trailing newline. echo
Hello will echo six characters in total, echo -n hello only echoes five characters.
(The -n option might not work in the Korn shell.)
15. Display A B C with two spaces between B and C.
echo "A B
C"
16. Complete the following command (do not use spaces) to display exactly the following
output:
4+4
10+14
=8
=24
The solution is to use tabs with \t.
echo -e "4+4\t=8" ; echo -e "10+14\t=24"
17. Use echo to display the following exactly:
??\\
echo
echo
echo
echo
echo
'??\\'
-e '??\\\\'
"??\\\\"
-e "??\\\\\\"
??\\\\
Find two solutions with single quotes, two with double quotes and one without quotes (and
say thank you to René and Darioush from Google for this extra).
18. Use one echo command to display three words on three lines.
echo -e "one \ntwo \nthree"
133
Chapter 13. control operators
In this chapter we put more than one command on the command line using control
operators. We also briefly discuss related parameters ($?) and similar special characters(&).
134
control operators
13.1. ; semicolon
You can put two or more commands on the same line separated by a semicolon ; . The shell
will scan the line until it reaches the semicolon. All the arguments before this semicolon
will be considered a separate command from all the arguments after the semicolon. Both
series will be executed sequentially with the shell waiting for each command to finish before
starting the next one.
[paul@RHELv4u3
Hello
[paul@RHELv4u3
World
[paul@RHELv4u3
Hello
World
[paul@RHELv4u3
~]$ echo Hello
~]$ echo World
~]$ echo Hello ; echo World
~]$
13.2. & ampersand
When a line ends with an ampersand &, the shell will not wait for the command to finish.
You will get your shell prompt back, and the command is executed in background. You will
get a message when this command has finished executing in background.
[paul@RHELv4u3 ~]$ sleep 20 &
[1] 7925
[paul@RHELv4u3 ~]$
...wait 20 seconds...
[paul@RHELv4u3 ~]$
[1]+ Done
sleep 20
The technical explanation of what happens in this case is explained in the chapter about
processes.
13.3. $? dollar question mark
The exit code of the previous command is stored in the shell variable $?. Actually $? is a
shell parameter and not a variable, since you cannot assign a value to $?.
paul@debian5:~/test$ touch file1
paul@debian5:~/test$ echo $?
0
paul@debian5:~/test$ rm file1
paul@debian5:~/test$ echo $?
0
paul@debian5:~/test$ rm file1
rm: cannot remove `file1': No such file or directory
paul@debian5:~/test$ echo $?
1
paul@debian5:~/test$
135
control operators
13.4. && double ampersand
The shell will interpret && as a logical AND. When using && the second command is
executed only if the first one succeeds (returns a zero exit status).
paul@barry:~$ echo first && echo second
first
second
paul@barry:~$ zecho first && echo second
-bash: zecho: command not found
Another example of the same logical AND principle. This example starts with a working cd
followed by ls, then a non-working cd which is not followed by ls.
[paul@RHELv4u3 ~]$ cd gen && ls
file1 file3 File55 fileab FileAB
fileabc
file2 File4 FileA
Fileab fileab2
[paul@RHELv4u3 gen]$ cd gen && ls
-bash: cd: gen: No such file or directory
13.5. || double vertical bar
The || represents a logical OR. The second command is executed only when the first
command fails (returns a non-zero exit status).
paul@barry:~$ echo first || echo second ; echo third
first
third
paul@barry:~$ zecho first || echo second ; echo third
-bash: zecho: command not found
second
third
paul@barry:~$
Another example of the same logical OR principle.
[paul@RHELv4u3 ~]$ cd gen || ls
[paul@RHELv4u3 gen]$ cd gen || ls
-bash: cd: gen: No such file or directory
file1 file3 File55 fileab FileAB
fileabc
file2 File4 FileA
Fileab fileab2
13.6. combining && and ||
You can use this logical AND and logical OR to write an if-then-else structure on the
command line. This example uses echo to display whether the rm command was successful.
paul@laika:~/test$ rm file1 && echo It worked! || echo It failed!
It worked!
paul@laika:~/test$ rm file1 && echo It worked! || echo It failed!
rm: cannot remove `file1': No such file or directory
It failed!
paul@laika:~/test$
136
control operators
13.7. # pound sign
Everything written after a pound sign (#) is ignored by the shell. This is useful to write a
shell comment, but has no influence on the command execution or shell expansion.
paul@debian4:~$ mkdir test
paul@debian4:~$ cd test
paul@debian4:~/test$ ls
paul@debian4:~/test$
# we create a directory
#### we enter the directory
# is it empty ?
13.8. \ escaping special characters
The backslash \ character enables the use of control characters, but without the shell
interpreting it, this is called escaping characters.
[paul@RHELv4u3
hello ; world
[paul@RHELv4u3
hello
world
[paul@RHELv4u3
escaping \ # &
[paul@RHELv4u3
escaping \?*"'
~]$ echo hello \; world
~]$ echo hello\ \ \ world
~]$ echo escaping \\\ \#\ \&\ \"\ \'
" '
~]$ echo escaping \\\?\*\"\'
13.8.1. end of line backslash
Lines ending in a backslash are continued on the next line. The shell does not interpret the
newline character and will wait on shell expansion and execution of the command line until
a newline without backslash is encountered.
[paul@RHEL4b ~]$ echo This command line \
> is split in three \
> parts
This command line is split in three parts
[paul@RHEL4b ~]$
137
control operators
13.9. practice: control operators
0. Each question can be answered by one command line!
1. When you type passwd, which file is executed ?
2. What kind of file is that ?
3. Execute the pwd command twice. (remember 0.)
4. Execute ls after cd /etc, but only if cd /etc did not error.
5. Execute cd /etc after cd etc, but only if cd etc fails.
6. Echo it worked when touch test42 works, and echo it failed when the touch failed. All
on one command line as a normal user (not root). Test this line in your home directory and
in /bin/ .
7. Execute sleep 6, what is this command doing ?
8. Execute sleep 200 in background (do not wait for it to finish).
9. Write a command line that executes rm file55. Your command line should print 'success'
if file55 is removed, and print 'failed' if there was a problem.
(optional)10. Use echo to display "Hello World with strange' characters \ * [ } ~ \
\ ." (including all quotes)
138
control operators
13.10. solution: control operators
0. Each question can be answered by one command line!
1. When you type passwd, which file is executed ?
which passwd
2. What kind of file is that ?
file /usr/bin/passwd
3. Execute the pwd command twice. (remember 0.)
pwd ; pwd
4. Execute ls after cd /etc, but only if cd /etc did not error.
cd /etc && ls
5. Execute cd /etc after cd etc, but only if cd etc fails.
cd etc || cd /etc
6. Echo it worked when touch test42 works, and echo it failed when the touch failed. All
on one command line as a normal user (not root). Test this line in your home directory and
in /bin/ .
paul@deb503:~$ cd ; touch test42 && echo it worked || echo it failed
it worked
paul@deb503:~$ cd /bin; touch test42 && echo it worked || echo it failed
touch: cannot touch `test42': Permission denied
it failed
7. Execute sleep 6, what is this command doing ?
pausing for six seconds
8. Execute sleep 200 in background (do not wait for it to finish).
sleep 200 &
9. Write a command line that executes rm file55. Your command line should print 'success'
if file55 is removed, and print 'failed' if there was a problem.
rm file55 && echo success || echo failed
(optional)10. Use echo to display "Hello World with strange' characters \ * [ } ~ \
\ ." (including all quotes)
echo \"Hello World with strange\' characters \\ \* \[ \} \~ \\\\ \. \"
or
echo \""Hello World with strange' characters \ * [ } ~ \\ . "\"
139
Chapter 14. shell variables
In this chapter we learn to manage environment variables in the shell. These variables are
often needed by applications.
140
shell variables
14.1. $ dollar sign
Another important character interpreted by the shell is the dollar sign $. The shell will look
for an environment variable named like the string following the dollar sign and replace it
with the value of the variable (or with nothing if the variable does not exist).
These are some examples using $HOSTNAME, $USER, $UID, $SHELL, and $HOME.
[paul@RHELv4u3 ~]$ echo This is the $SHELL shell
This is the /bin/bash shell
[paul@RHELv4u3 ~]$ echo This is $SHELL on computer $HOSTNAME
This is /bin/bash on computer RHELv4u3.localdomain
[paul@RHELv4u3 ~]$ echo The userid of $USER is $UID
The userid of paul is 500
[paul@RHELv4u3 ~]$ echo My homedir is $HOME
My homedir is /home/paul
14.2. case sensitive
This example shows that shell variables are case sensitive!
[paul@RHELv4u3 ~]$ echo Hello $USER
Hello paul
[paul@RHELv4u3 ~]$ echo Hello $user
Hello
14.3. creating variables
This example creates the variable $MyVar and sets its value. It then uses echo to verify
the value.
[paul@RHELv4u3 gen]$ MyVar=555
[paul@RHELv4u3 gen]$ echo $MyVar
555
[paul@RHELv4u3 gen]$
141
shell variables
14.4. quotes
Notice that double quotes still allow the parsing of variables, whereas single quotes prevent
this.
[paul@RHELv4u3
[paul@RHELv4u3
555
[paul@RHELv4u3
555
[paul@RHELv4u3
$MyVar
~]$ MyVar=555
~]$ echo $MyVar
~]$ echo "$MyVar"
~]$ echo '$MyVar'
The bash shell will replace variables with their value in double quoted lines, but not in single
quoted lines.
paul@laika:~$ city=Burtonville
paul@laika:~$ echo "We are in $city today."
We are in Burtonville today.
paul@laika:~$ echo 'We are in $city today.'
We are in $city today.
14.5. set
You can use the set command to display a list of environment variables. On Ubuntu and
Debian systems, the set command will also list shell functions after the shell variables. Use
set | more to see the variables then.
14.6. unset
Use the unset command to remove a variable from your shell environment.
[paul@RHEL4b
[paul@RHEL4b
8472
[paul@RHEL4b
[paul@RHEL4b
~]$ MyVar=8472
~]$ echo $MyVar
~]$ unset MyVar
~]$ echo $MyVar
[paul@RHEL4b ~]$
142
shell variables
14.7. $PS1
The $PS1 variable determines your shell prompt. You can use backslash escaped special
characters like \u for the username or \w for the working directory. The bash manual has
a complete reference.
In this example we change the value of $PS1 a couple of times.
paul@deb503:~$ PS1=prompt
prompt
promptPS1='prompt '
prompt
prompt PS1='> '
>
> PS1='\u@\h$ '
paul@deb503$
paul@deb503$ PS1='\u@\h:\W$'
paul@deb503:~$
To avoid unrecoverable mistakes, you can set normal user prompts to green and the root
prompt to red. Add the following to your .bashrc for a green user prompt:
# color prompt by paul
RED='\[\033[01;31m\]'
WHITE='\[\033[01;00m\]'
GREEN='\[\033[01;32m\]'
BLUE='\[\033[01;34m\]'
export PS1="${debian_chroot:+($debian_chroot)}$GREEN\u$WHITE@$BLUE\h$WHITE\w\$ "
143
shell variables
14.8. $PATH
The $PATH variable is determines where the shell is looking for commands to execute
(unless the command is builtin or aliased). This variable contains a list of directories,
separated by colons.
[[paul@RHEL4b ~]$ echo $PATH
/usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin:
The shell will not look in the current directory for commands to execute! (Looking for
executables in the current directory provided an easy way to hack PC-DOS computers). If
you want the shell to look in the current directory, then add a . at the end of your $PATH.
[paul@RHEL4b ~]$ PATH=$PATH:.
[paul@RHEL4b ~]$ echo $PATH
/usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin:.
[paul@RHEL4b ~]$
Your path might be different when using su instead of su - because the latter will take on
the environment of the target user. The root user typically has /sbin directories added to the
$PATH variable.
[paul@RHEL3 ~]$ su
Password:
[root@RHEL3 paul]# echo $PATH
/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin
[root@RHEL3 paul]# exit
[paul@RHEL3 ~]$ su Password:
[root@RHEL3 ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:
[root@RHEL3 ~]#
144
shell variables
14.9. env
The env command without options will display a list of exported variables. The difference
with set with options is that set lists all variables, including those not exported to child shells.
But env can also be used to start a clean shell (a shell without any inherited environment).
The env -i command clears the environment for the subshell.
Notice in this screenshot that bash will set the $SHELL variable on startup.
[paul@RHEL4b ~]$ bash -c 'echo $SHELL $HOME $USER'
/bin/bash /home/paul paul
[paul@RHEL4b ~]$ env -i bash -c 'echo $SHELL $HOME $USER'
/bin/bash
[paul@RHEL4b ~]$
You can use the env command to set the $LANG, or any other, variable for just one instance
of bash with one command. The example below uses this to show the influence of the
$LANG variable on file globbing (see the chapter on file globbing).
[paul@RHEL4b test]$ env LANG=C bash -c 'ls File[a-z]'
Filea Fileb
[paul@RHEL4b test]$ env LANG=en_US.UTF-8 bash -c 'ls File[a-z]'
Filea FileA Fileb FileB
[paul@RHEL4b test]$
14.10. export
You can export shell variables to other shells with the export command. This will export
the variable to child shells.
[paul@RHEL4b
[paul@RHEL4b
[paul@RHEL4b
[paul@RHEL4b
three four
[paul@RHEL4b
[paul@RHEL4b
four
~]$
~]$
~]$
~]$
var3=three
var4=four
export var4
echo $var3 $var4
~]$ bash
~]$ echo $var3 $var4
But it will not export to the parent shell (previous screenshot continued).
[paul@RHEL4b
[paul@RHEL4b
four five
[paul@RHEL4b
exit
[paul@RHEL4b
three four
[paul@RHEL4b
~]$ export var5=five
~]$ echo $var3 $var4 $var5
~]$ exit
~]$ echo $var3 $var4 $var5
~]$
145
shell variables
14.11. delineate variables
Until now, we have seen that bash interprets a variable starting from a dollar sign, continuing
until the first occurrence of a non-alphanumeric character that is not an underscore. In some
situations, this can be a problem. This issue can be resolved with curly braces like in this
example.
[paul@RHEL4b ~]$ prefix=Super
[paul@RHEL4b ~]$ echo Hello $prefixman and $prefixgirl
Hello and
[paul@RHEL4b ~]$ echo Hello ${prefix}man and ${prefix}girl
Hello Superman and Supergirl
[paul@RHEL4b ~]$
14.12. unbound variables
The example below tries to display the value of the $MyVar variable, but it fails because the
variable does not exist. By default the shell will display nothing when a variable is unbound
(does not exist).
[paul@RHELv4u3 gen]$ echo $MyVar
[paul@RHELv4u3 gen]$
There is, however, the nounset shell option that you can use to generate an error when a
variable does not exist.
paul@laika:~$ set -u
paul@laika:~$ echo $Myvar
bash: Myvar: unbound variable
paul@laika:~$ set +u
paul@laika:~$ echo $Myvar
paul@laika:~$
In the bash shell set -u is identical to set -o nounset and likewise set +u is identical to set
+o nounset.
146
shell variables
14.13. practice: shell variables
1. Use echo to display Hello followed by your username. (use a bash variable!)
2. Create a variable answer with a value of 42.
3. Copy the value of $LANG to $MyLANG.
4. List all current shell variables.
5. List all exported shell variables.
6. Do the env and set commands display your variable ?
6. Destroy your answer variable.
7. Create two variables, and export one of them.
8. Display the exported variable in an interactive child shell.
9. Create a variable, give it the value 'Dumb', create another variable with value 'do'. Use
echo and the two variables to echo Dumbledore.
10. Find the list of backslash escaped characters in the manual of bash. Add the time to your
PS1 prompt.
147
shell variables
14.14. solution: shell variables
1. Use echo to display Hello followed by your username. (use a bash variable!)
echo Hello $USER
2. Create a variable answer with a value of 42.
answer=42
3. Copy the value of $LANG to $MyLANG.
MyLANG=$LANG
4. List all current shell variables.
set
set|more on Ubuntu/Debian
5. List all exported shell variables.
env
6. Do the env and set commands display your variable ?
env | more
set | more
6. Destroy your answer variable.
unset answer
7. Create two variables, and export one of them.
var1=1; export var2=2
8. Display the exported variable in an interactive child shell.
bash
echo $var2
9. Create a variable, give it the value 'Dumb', create another variable with value 'do'. Use
echo and the two variables to echo Dumbledore.
varx=Dumb; vary=do
echo ${varx}le${vary}re
solution by Yves from Dexia : echo $varx'le'$vary're'
solution by Erwin from Telenet : echo "$varx"le"$vary"re
10. Find the list of backslash escaped characters in the manual of bash. Add the time to your
PS1 prompt.
PS1='\t \u@\h \W$ '
148
Chapter 15. shell embedding and
options
This chapter takes a brief look at child shells, embedded shells and shell options.
149
shell embedding and options
15.1. shell embedding
Shells can be embedded on the command line, or in other words, the command line scan
can spawn new processes containing a fork of the current shell. You can use variables to
prove that new shells are created. In the screenshot below, the variable $var1 only exists in
the (temporary) sub shell.
[paul@RHELv4u3 gen]$ echo $var1
[paul@RHELv4u3 gen]$ echo $(var1=5;echo $var1)
5
[paul@RHELv4u3 gen]$ echo $var1
[paul@RHELv4u3 gen]$
You can embed a shell in an embedded shell, this is called nested embedding of shells.
This screenshot shows an embedded shell inside an embedded shell.
paul@deb503:~$ A=shell
paul@deb503:~$ echo $C$B$A $(B=sub;echo $C$B$A; echo $(C=sub;echo $C$B$A))
shell subshell subsubshell
15.1.1. backticks
Single embedding can be useful to avoid changing your current directory. The screenshot
below uses backticks instead of dollar-bracket to embed.
[paul@RHELv4u3 ~]$ echo `cd /etc; ls -d * | grep pass`
passwd passwd- passwd.OLD
[paul@RHELv4u3 ~]$
You can only use the $() notation to nest embedded shells, backticks cannot do this.
15.1.2. backticks or single quotes
Placing the embedding between backticks uses one character less than the dollar and
parenthesis combo. Be careful however, backticks are often confused with single quotes.
The technical difference between ' and ` is significant!
[paul@RHELv4u3 gen]$ echo `var1=5;echo $var1`
5
[paul@RHELv4u3 gen]$ echo 'var1=5;echo $var1'
var1=5;echo $var1
[paul@RHELv4u3 gen]$
150
shell embedding and options
15.2. shell options
Both set and unset are builtin shell commands. They can be used to set options of the bash
shell itself. The next example will clarify this. By default, the shell will treat unset variables
as a variable having no value. By setting the -u option, the shell will treat any reference to
unset variables as an error. See the man page of bash for more information.
[paul@RHEL4b ~]$ echo $var123
[paul@RHEL4b ~]$ set -u
[paul@RHEL4b ~]$ echo $var123
-bash: var123: unbound variable
[paul@RHEL4b ~]$ set +u
[paul@RHEL4b ~]$ echo $var123
[paul@RHEL4b ~]$
To list all the set options for your shell, use echo $-. The noclobber (or -C) option will be
explained later in this book (in the I/O redirection chapter).
[paul@RHEL4b
himBH
[paul@RHEL4b
[paul@RHEL4b
himuBCH
[paul@RHEL4b
[paul@RHEL4b
himBH
[paul@RHEL4b
~]$ echo $~]$ set -C ; set -u
~]$ echo $~]$ set +C ; set +u
~]$ echo $~]$
When typing set without options, you get a list of all variables without function when the
shell is on posix mode. You can set bash in posix mode typing set -o posix.
151
shell embedding and options
15.3. practice: shell embedding
1. Find the list of shell options in the man page of bash. What is the difference between set
-u and set -o nounset?
2. Activate nounset in your shell. Test that it shows an error message when using nonexisting variables.
3. Deactivate nounset.
4. Execute cd /var and ls in an embedded shell.
The echo command is only needed to show the result of the ls command. Omitting will result
in the shell trying to execute the first file as a command.
5. Create the variable embvar in an embedded shell and echo it. Does the variable exist in
your current shell now ?
6. Explain what "set -x" does. Can this be useful ?
(optional)7. Given the following screenshot, add exactly four characters to that command
line so that the total output is FirstMiddleLast.
[paul@RHEL4b ~]$ echo
First; echo
Middle; echo
Last
8. Display a long listing (ls -l) of the passwd command using the which command inside
an embedded shell.
152
shell embedding and options
15.4. solution: shell embedding
1. Find the list of shell options in the man page of bash. What is the difference between set
-u and set -o nounset?
read the manual of bash (man bash), search for nounset -- both mean the same thing.
2. Activate nounset in your shell. Test that it shows an error message when using nonexisting variables.
set -u
OR
set -o nounset
Both these lines have the same effect.
3. Deactivate nounset.
set +u
OR
set +o nounset
4. Execute cd /var and ls in an embedded shell.
echo $(cd /var ; ls)
The echo command is only needed to show the result of the ls command. Omitting will result
in the shell trying to execute the first file as a command.
5. Create the variable embvar in an embedded shell and echo it. Does the variable exist in
your current shell now ?
echo $(embvar=emb;echo $embvar) ; echo $embvar #the last echo fails
$embvar does not exist in your current shell
6. Explain what "set -x" does. Can this be useful ?
It displays shell expansion for troubleshooting your command.
(optional)7. Given the following screenshot, add exactly four characters to that command
line so that the total output is FirstMiddleLast.
[paul@RHEL4b ~]$ echo
First; echo
Middle; echo
Last
echo -n First; echo -n Middle; echo Last
8. Display a long listing (ls -l) of the passwd command using the which command inside
an embedded shell.
ls -l $(which passwd)
153
Chapter 16. shell history
The shell makes it easy for us to repeat commands, this chapter explains how.
154
shell history
16.1. repeating the last command
To repeat the last command in bash, type !!. This is pronounced as bang bang.
paul@debian5:~/test42$ echo this will be repeated > file42.txt
paul@debian5:~/test42$ !!
echo this will be repeated > file42.txt
paul@debian5:~/test42$
16.2. repeating other commands
You can repeat other commands using one bang followed by one or more characters. The
shell will repeat the last command that started with those characters.
paul@debian5:~/test42$ touch file42
paul@debian5:~/test42$ cat file42
paul@debian5:~/test42$ !to
touch file42
paul@debian5:~/test42$
16.3. history
To see older commands, use history to display the shell command history (or use history
n to see the last n commands).
paul@debian5:~/test$ history 10
38 mkdir test
39 cd test
40 touch file1
41 echo hello > file2
42 echo It is very cold today > winter.txt
43 ls
44 ls -l
45 cp winter.txt summer.txt
46 ls -l
47 history 10
16.4. !n
When typing ! followed by the number preceding the command you want repeated, then the
shell will echo the command and execute it.
paul@debian5:~/test$ !43
ls
file1 file2 summer.txt
winter.txt
155
shell history
16.5. Ctrl-r
Another option is to use ctrl-r to search in the history. In the screenshot below i only typed
ctrl-r followed by four characters apti and it finds the last command containing these four
consecutive characters.
paul@debian5:~$
(reverse-i-search)`apti': sudo aptitude install screen
16.6. $HISTSIZE
The $HISTSIZE variable determines the number of commands that will be remembered in
your current environment. Most distributions default this variable to 500 or 1000.
paul@debian5:~$ echo $HISTSIZE
500
You can change it to any value you like.
paul@debian5:~$ HISTSIZE=15000
paul@debian5:~$ echo $HISTSIZE
15000
16.7. $HISTFILE
The $HISTFILE variable points to the file that contains your history. The bash shell defaults
this value to ~/.bash_history.
paul@debian5:~$ echo $HISTFILE
/home/paul/.bash_history
A session history is saved to this file when you exit the session!
Closing a gnome-terminal with the mouse, or typing reboot as root will NOT save your
terminal's history.
16.8. $HISTFILESIZE
The number of commands kept in your history file can be set using $HISTFILESIZE.
paul@debian5:~$ echo $HISTFILESIZE
15000
156
shell history
16.9. prevent recording a command
You can prevent a command from being recorded in history using a space prefix.
paul@debian8:~/github$ echo abc
abc
paul@debian8:~/github$ echo def
def
paul@debian8:~/github$ echo ghi
ghi
paul@debian8:~/github$ history 3
9501 echo abc
9502 echo ghi
9503 history 3
16.10. (optional)regular expressions
It is possible to use regular expressions when using the bang to repeat commands. The
screenshot below switches 1 into 2.
paul@debian5:~/test$ cat file1
paul@debian5:~/test$ !c:s/1/2
cat file2
hello
paul@debian5:~/test$
16.11. (optional) Korn shell history
Repeating a command in the Korn shell is very similar. The Korn shell also has the history
command, but uses the letter r to recall lines from history.
This screenshot shows the history command. Note the different meaning of the parameter.
$ history 17
17 clear
18 echo hoi
19 history 12
20 echo world
21 history 17
Repeating with r can be combined with the line numbers given by the history command, or
with the first few letters of the command.
$ r e
echo world
world
$ cd /etc
$ r
cd /etc
$
157
shell history
16.12. practice: shell history
1. Issue the command echo The answer to the meaning of life, the universe and
everything is 42.
2. Repeat the previous command using only two characters (there are two solutions!)
3. Display the last 5 commands you typed.
4. Issue the long echo from question 1 again, using the line numbers you received from the
command in question 3.
5. How many commands can be kept in memory for your current shell session ?
6. Where are these commands stored when exiting the shell ?
7. How many commands can be written to the history file when exiting your current shell
session ?
8. Make sure your current bash shell remembers the next 5000 commands you type.
9. Open more than one console (by press Ctrl-shift-t in gnome-terminal, or by opening an
extra putty.exe in MS Windows) with the same user account. When is command history
written to the history file ?
158
shell history
16.13. solution: shell history
1. Issue the command echo The answer to the meaning of life, the universe and
everything is 42.
echo The answer to the meaning of life, the universe and everything is 42
2. Repeat the previous command using only two characters (there are two solutions!)
!!
OR
!e
3. Display the last 5 commands you typed.
paul@ubu1010:~$ history 5
52 ls -l
53 ls
54 df -h | grep sda
55 echo The answer to the meaning of life, the universe and everything is 42
56 history 5
You will receive different line numbers.
4. Issue the long echo from question 1 again, using the line numbers you received from the
command in question 3.
paul@ubu1010:~$ !55
echo The answer to the meaning of life, the universe and everything is 42
The answer to the meaning of life, the universe and everything is 42
5. How many commands can be kept in memory for your current shell session ?
echo $HISTSIZE
6. Where are these commands stored when exiting the shell ?
echo $HISTFILE
7. How many commands can be written to the history file when exiting your current shell
session ?
echo $HISTFILESIZE
8. Make sure your current bash shell remembers the next 5000 commands you type.
HISTSIZE=5000
9. Open more than one console (by press Ctrl-shift-t in gnome-terminal, or by opening an
extra putty.exe in MS Windows) with the same user account. When is command history
written to the history file ?
when you type exit
159
Chapter 17. file globbing
The shell is also responsible for file globbing (or dynamic filename generation). This chapter
will explain file globbing.
160
file globbing
17.1. * asterisk
The asterisk * is interpreted by the shell as a sign to generate filenames, matching the asterisk
to any combination of characters (even none). When no path is given, the shell will use
filenames in the current directory. See the man page of glob(7) for more information. (This
is part of LPI topic 1.103.3.)
[paul@RHELv4u3 gen]$
file1 file2 file3
[paul@RHELv4u3 gen]$
File4 File55 FileA
[paul@RHELv4u3 gen]$
file1 file2 file3
[paul@RHELv4u3 gen]$
File55
[paul@RHELv4u3 gen]$
File55
[paul@RHELv4u3 gen]$
File55
[paul@RHELv4u3 gen]$
ls
File4 File55 FileA
ls File*
Fileab FileAB
ls file*
fileab fileabc
ls *ile55
fileab
Fileab
FileAB
fileabc
ls F*ile55
ls F*55
17.2. ? question mark
Similar to the asterisk, the question mark ? is interpreted by the shell as a sign to generate
filenames, matching the question mark with exactly one character.
[paul@RHELv4u3 gen]$ ls
file1 file2 file3 File4 File55
[paul@RHELv4u3 gen]$ ls File?
File4 FileA
[paul@RHELv4u3 gen]$ ls Fil?4
File4
[paul@RHELv4u3 gen]$ ls Fil??
File4 FileA
[paul@RHELv4u3 gen]$ ls File??
File55 Fileab FileAB
[paul@RHELv4u3 gen]$
FileA
161
fileab
Fileab
FileAB
fileabc
file globbing
17.3. [] square brackets
The square bracket [ is interpreted by the shell as a sign to generate filenames, matching
any of the characters between [ and the first subsequent ]. The order in this list between the
brackets is not important. Each pair of brackets is replaced by exactly one character.
[paul@RHELv4u3 gen]$ ls
file1 file2 file3 File4 File55 FileA fileab Fileab FileAB
[paul@RHELv4u3 gen]$ ls File[5A]
FileA
[paul@RHELv4u3 gen]$ ls File[A5]
FileA
[paul@RHELv4u3 gen]$ ls File[A5][5b]
File55
[paul@RHELv4u3 gen]$ ls File[a5][5b]
File55 Fileab
[paul@RHELv4u3 gen]$ ls File[a5][5b][abcdefghijklm]
ls: File[a5][5b][abcdefghijklm]: No such file or directory
[paul@RHELv4u3 gen]$ ls file[a5][5b][abcdefghijklm]
fileabc
[paul@RHELv4u3 gen]$
fileabc
You can also exclude characters from a list between square brackets with the exclamation
mark !. And you are allowed to make combinations of these wild cards.
[paul@RHELv4u3 gen]$
file1 file2 file3
[paul@RHELv4u3 gen]$
fileab
[paul@RHELv4u3 gen]$
file1 file2 file3
[paul@RHELv4u3 gen]$
fileab
[paul@RHELv4u3 gen]$
ls
File4 File55 FileA
ls file[a5][!Z]
ls file[!5]*
fileab fileabc
ls file[!5]?
162
fileab
Fileab
FileAB
fileabc
file globbing
17.4. a-z and 0-9 ranges
The bash shell will also understand ranges of characters between brackets.
[paul@RHELv4u3 gen]$ ls
file1 file3 File55 fileab FileAB
fileabc
file2 File4 FileA
Fileab fileab2
[paul@RHELv4u3 gen]$ ls file[a-z]*
fileab fileab2 fileabc
[paul@RHELv4u3 gen]$ ls file[0-9]
file1 file2 file3
[paul@RHELv4u3 gen]$ ls file[a-z][a-z][0-9]*
fileab2
[paul@RHELv4u3 gen]$
17.5. $LANG and square brackets
But, don't forget the influence of the LANG variable. Some languages include lower case
letters in an upper case range (and vice versa).
paul@RHELv4u4:~/test$ ls [A-Z]ile?
file1 file2 file3 File4
paul@RHELv4u4:~/test$ ls [a-z]ile?
file1 file2 file3 File4
paul@RHELv4u4:~/test$ echo $LANG
en_US.UTF-8
paul@RHELv4u4:~/test$ LANG=C
paul@RHELv4u4:~/test$ echo $LANG
C
paul@RHELv4u4:~/test$ ls [a-z]ile?
file1 file2 file3
paul@RHELv4u4:~/test$ ls [A-Z]ile?
File4
paul@RHELv4u4:~/test$
If $LC_ALL is set, then this will also need to be reset to prevent file globbing.
163
file globbing
17.6. preventing file globbing
The screenshot below should be no surprise. The echo * will echo a * when in an empty
directory. And it will echo the names of all files when the directory is not empty.
paul@ubu1010:~$ mkdir test42
paul@ubu1010:~$ cd test42
paul@ubu1010:~/test42$ echo *
*
paul@ubu1010:~/test42$ touch file42 file33
paul@ubu1010:~/test42$ echo *
file33 file42
Globbing can be prevented using quotes or by escaping the special characters, as shown in
this screenshot.
paul@ubu1010:~/test42$
file33 file42
paul@ubu1010:~/test42$
*
paul@ubu1010:~/test42$
*
paul@ubu1010:~/test42$
*
echo *
echo \*
echo '*'
echo "*"
164
file globbing
17.7. practice: shell globbing
1. Create a test directory and enter it.
2. Create the following files :
file1
file10
file11
file2
File2
File3
file33
fileAB
filea
fileA
fileAAA
file(
file 2
(the last one has 6 characters including a space)
3. List (with ls) all files starting with file
4. List (with ls) all files starting with File
5. List (with ls) all files starting with file and ending in a number.
6. List (with ls) all files starting with file and ending with a letter
7. List (with ls) all files starting with File and having a digit as fifth character.
8. List (with ls) all files starting with File and having a digit as fifth character and nothing
else.
9. List (with ls) all files starting with a letter and ending in a number.
10. List (with ls) all files that have exactly five characters.
11. List (with ls) all files that start with f or F and end with 3 or A.
12. List (with ls) all files that start with f have i or R as second character and end in a number.
13. List all files that do not start with the letter F.
14. Copy the value of $LANG to $MyLANG.
15. Show the influence of $LANG in listing A-Z or a-z ranges.
16. You receive information that one of your servers was cracked, the cracker probably
replaced the ls command. You know that the echo command is safe to use. Can echo replace
ls ? How can you list the files in the current directory with echo ?
17. Is there another command besides cd to change directories ?
165
file globbing
17.8. solution: shell globbing
1. Create a test directory and enter it.
mkdir testdir; cd testdir
2. Create the following files :
file1
file10
file11
file2
File2
File3
file33
fileAB
filea
fileA
fileAAA
file(
file 2
(the last one has 6 characters including a space)
touch
touch
touch
touch
file1 file10 file11 file2 File2 File3
file33 fileAB filea fileA fileAAA
"file("
"file 2"
3. List (with ls) all files starting with file
ls file*
4. List (with ls) all files starting with File
ls File*
5. List (with ls) all files starting with file and ending in a number.
ls file*[0-9]
6. List (with ls) all files starting with file and ending with a letter
ls file*[a-z]
7. List (with ls) all files starting with File and having a digit as fifth character.
ls File[0-9]*
8. List (with ls) all files starting with File and having a digit as fifth character and nothing
else.
ls File[0-9]
9. List (with ls) all files starting with a letter and ending in a number.
ls [a-z]*[0-9]
10. List (with ls) all files that have exactly five characters.
166
file globbing
ls ?????
11. List (with ls) all files that start with f or F and end with 3 or A.
ls [fF]*[3A]
12. List (with ls) all files that start with f have i or R as second character and end in a number.
ls f[iR]*[0-9]
13. List all files that do not start with the letter F.
ls [!F]*
14. Copy the value of $LANG to $MyLANG.
MyLANG=$LANG
15. Show the influence of $LANG in listing A-Z or a-z ranges.
see example in book
16. You receive information that one of your servers was cracked, the cracker probably
replaced the ls command. You know that the echo command is safe to use. Can echo replace
ls ? How can you list the files in the current directory with echo ?
echo *
17. Is there another command besides cd to change directories ?
pushd popd
167
Part V. pipes and commands
Table of Contents
18. I/O redirection ...............................................................................................................
18.1. stdin, stdout, and stderr ..........................................................................................
18.2. output redirection ..................................................................................................
18.3. error redirection ....................................................................................................
18.4. output redirection and pipes ....................................................................................
18.5. joining stdout and stderr .........................................................................................
18.6. input redirection ...................................................................................................
18.7. confusing redirection .............................................................................................
18.8. quick file clear .....................................................................................................
18.9. practice: input/output redirection ..............................................................................
18.10. solution: input/output redirection ............................................................................
19. filters ............................................................................................................................
19.1. cat ......................................................................................................................
19.2. tee ......................................................................................................................
19.3. grep ....................................................................................................................
19.4. cut ......................................................................................................................
19.5. tr ........................................................................................................................
19.6. wc ......................................................................................................................
19.7. sort .....................................................................................................................
19.8. uniq ....................................................................................................................
19.9. comm .................................................................................................................
19.10. od .....................................................................................................................
19.11. sed ....................................................................................................................
19.12. pipe examples .....................................................................................................
19.13. practice: filters ....................................................................................................
19.14. solution: filters ....................................................................................................
20. basic Unix tools .............................................................................................................
20.1. find ....................................................................................................................
20.2. locate ..................................................................................................................
20.3. date ....................................................................................................................
20.4. cal ......................................................................................................................
20.5. sleep ...................................................................................................................
20.6. time ....................................................................................................................
20.7. gzip - gunzip ........................................................................................................
20.8. zcat - zmore .........................................................................................................
20.9. bzip2 - bunzip2 ....................................................................................................
20.10. bzcat - bzmore ....................................................................................................
20.11. practice: basic Unix tools .....................................................................................
20.12. solution: basic Unix tools .....................................................................................
21. regular expressions ........................................................................................................
21.1. regex versions ......................................................................................................
21.2. grep ....................................................................................................................
21.3. rename ................................................................................................................
21.4. sed .....................................................................................................................
21.5. bash history .........................................................................................................
169
170
171
172
174
175
175
176
177
177
178
179
180
181
181
181
183
183
184
185
186
187
188
189
190
191
192
194
195
196
196
197
197
198
199
199
200
200
201
202
204
205
206
211
214
218
Chapter 18. I/O redirection
One of the powers of the Unix command line is the use of input/output redirection and
pipes.
This chapter explains redirection of input, output and error streams.
170
I/O redirection
18.1. stdin, stdout, and stderr
The bash shell has three basic streams; it takes input from stdin (stream 0), it sends output
to stdout (stream 1) and it sends error messages to stderr (stream 2) .
The drawing below has a graphical interpretation of these three streams.
The keyboard often serves as stdin, whereas stdout and stderr both go to the display. This
can be confusing to new Linux users because there is no obvious way to recognize stdout
from stderr. Experienced users know that separating output from errors can be very useful.
The next sections will explain how to redirect these streams.
171
I/O redirection
18.2. output redirection
18.2.1. > stdout
stdout can be redirected with a greater than sign. While scanning the line, the shell will
see the > sign and will clear the file.
The > notation is in fact the abbreviation of 1> (stdout being referred to as stream 1).
[paul@RHELv4u3 ~]$ echo It is cold today!
It is cold today!
[paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt
[paul@RHELv4u3 ~]$ cat winter.txt
It is cold today!
[paul@RHELv4u3 ~]$
Note that the bash shell effectively removes the redirection from the command line before
argument 0 is executed. This means that in the case of this command:
echo hello > greetings.txt
the shell only counts two arguments (echo = argument 0, hello = argument 1). The redirection
is removed before the argument counting takes place.
18.2.2. output file is erased
While scanning the line, the shell will see the > sign and will clear the file! Since this
happens before resolving argument 0, this means that even when the command fails, the
file will have been cleared!
[paul@RHELv4u3 ~]$ cat winter.txt
It is cold today!
[paul@RHELv4u3 ~]$ zcho It is cold today! > winter.txt
-bash: zcho: command not found
[paul@RHELv4u3 ~]$ cat winter.txt
[paul@RHELv4u3 ~]$
172
I/O redirection
18.2.3. noclobber
Erasing a file while using > can be prevented by setting the noclobber option.
[paul@RHELv4u3 ~]$
It is cold today!
[paul@RHELv4u3 ~]$
[paul@RHELv4u3 ~]$
-bash: winter.txt:
[paul@RHELv4u3 ~]$
[paul@RHELv4u3 ~]$
cat winter.txt
set -o noclobber
echo It is cold today! > winter.txt
cannot overwrite existing file
set +o noclobber
18.2.4. overruling noclobber
The noclobber can be overruled with >|.
[paul@RHELv4u3 ~]$ set -o noclobber
[paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt
-bash: winter.txt: cannot overwrite existing file
[paul@RHELv4u3 ~]$ echo It is very cold today! >| winter.txt
[paul@RHELv4u3 ~]$ cat winter.txt
It is very cold today!
[paul@RHELv4u3 ~]$
18.2.5. >> append
Use >> to append output to a file.
[paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt
[paul@RHELv4u3 ~]$ cat winter.txt
It is cold today!
[paul@RHELv4u3 ~]$ echo Where is the summer ? >> winter.txt
[paul@RHELv4u3 ~]$ cat winter.txt
It is cold today!
Where is the summer ?
[paul@RHELv4u3 ~]$
173
I/O redirection
18.3. error redirection
18.3.1. 2> stderr
Redirecting stderr is done with 2>. This can be very useful to prevent error messages from
cluttering your screen.
The screenshot below shows redirection of stdout to a file, and stderr to /dev/null. Writing
1> is the same as >.
[paul@RHELv4u3 ~]$ find / > allfiles.txt 2> /dev/null
[paul@RHELv4u3 ~]$
18.3.2. 2>&1
To redirect both stdout and stderr to the same file, use 2>&1.
[paul@RHELv4u3 ~]$ find / > allfiles_and_errors.txt 2>&1
[paul@RHELv4u3 ~]$
Note that the order of redirections is significant. For example, the command
ls > dirlist 2>&1
directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the
file dirlist, while the command
ls 2>&1 > dirlist
directs only the standard output to file dirlist, because the standard error made a copy of the
standard output before the standard output was redirected to dirlist.
174
I/O redirection
18.4. output redirection and pipes
By default you cannot grep inside stderr when using pipes on the command line, because
only stdout is passed.
paul@debian7:~$ rm file42 file33 file1201 | grep file42
rm: cannot remove ‘file42’: No such file or directory
rm: cannot remove ‘file33’: No such file or directory
rm: cannot remove ‘file1201’: No such file or directory
With 2>&1 you can force stderr to go to stdout. This enables the next command in the
pipe to act on both streams.
paul@debian7:~$ rm file42 file33 file1201 2>&1 | grep file42
rm: cannot remove ‘file42’: No such file or directory
You cannot use both 1>&2 and 2>&1 to switch stdout and stderr.
paul@debian7:~$ rm file42 file33 file1201 2>&1 1>&2 | grep file42
rm: cannot remove ‘file42’: No such file or directory
paul@debian7:~$ echo file42 2>&1 1>&2 | sed 's/file42/FILE42/'
FILE42
You need a third stream to switch stdout and stderr after a pipe symbol.
paul@debian7:~$ echo file42 3>&1 1>&2 2>&3 | sed 's/file42/FILE42/'
file42
paul@debian7:~$ rm file42 3>&1 1>&2 2>&3 | sed 's/file42/FILE42/'
rm: cannot remove ‘FILE42’: No such file or directory
18.5. joining stdout and stderr
The &> construction will put both stdout and stderr in one stream (to a file).
paul@debian7:~$ rm file42 &> out_and_err
paul@debian7:~$ cat out_and_err
rm: cannot remove ‘file42’: No such file or directory
paul@debian7:~$ echo file42 &> out_and_err
paul@debian7:~$ cat out_and_err
file42
paul@debian7:~$
175
I/O redirection
18.6. input redirection
18.6.1. < stdin
Redirecting stdin is done with < (short for 0<).
[paul@RHEL4b ~]$ cat < text.txt
one
two
[paul@RHEL4b ~]$ tr 'onetw' 'ONEZZ' < text.txt
ONE
ZZO
[paul@RHEL4b ~]$
18.6.2. << here document
The here document (sometimes called here-is-document) is a way to append input until a
certain sequence (usually EOF) is encountered. The EOF marker can be typed literally or
can be called with Ctrl-D.
[paul@RHEL4b
> one
> two
> EOF
[paul@RHEL4b
one
two
[paul@RHEL4b
> brel
> brol
[paul@RHEL4b
brel
[paul@RHEL4b
~]$ cat <<EOF > text.txt
~]$ cat text.txt
~]$ cat <<brol > text.txt
~]$ cat text.txt
~]$
18.6.3. <<< here string
The here string can be used to directly pass strings to a command. The result is the same
as using echo string | command (but you have one less process running).
paul@ubu1110~$ base64 <<< linux-training.be
bGludXgtdHJhaW5pbmcuYmUK
paul@ubu1110~$ base64 -d <<< bGludXgtdHJhaW5pbmcuYmUK
linux-training.be
See rfc 3548 for more information about base64.
176
I/O redirection
18.7. confusing redirection
The shell will scan the whole line before applying redirection. The following command line
is very readable and is correct.
cat winter.txt > snow.txt 2> errors.txt
But this one is also correct, but less readable.
2> errors.txt cat winter.txt > snow.txt
Even this will be understood perfectly by the shell.
< winter.txt > snow.txt 2> errors.txt cat
18.8. quick file clear
So what is the quickest way to clear a file ?
>foo
And what is the quickest way to clear a file when the noclobber option is set ?
>|bar
177
I/O redirection
18.9. practice: input/output redirection
1. Activate the noclobber shell option.
2. Verify that noclobber is active by repeating an ls on /etc/ with redirected output to a file.
3. When listing all shell options, which character represents the noclobber option ?
4. Deactivate the noclobber option.
5. Make sure you have two shells open on the same computer. Create an empty tailing.txt
file. Then type tail -f tailing.txt. Use the second shell to append a line of text to that file.
Verify that the first shell displays this line.
6. Create a file that contains the names of five people. Use cat and output redirection to
create the file and use a here document to end the input.
178
I/O redirection
18.10. solution: input/output redirection
1. Activate the noclobber shell option.
set -o noclobber
set -C
2. Verify that noclobber is active by repeating an ls on /etc/ with redirected output to a file.
ls /etc > etc.txt
ls /etc > etc.txt (should not work)
4. When listing all shell options, which character represents the noclobber option ?
echo $- (noclobber is visible as C)
5. Deactivate the noclobber option.
set +o noclobber
6. Make sure you have two shells open on the same computer. Create an empty tailing.txt
file. Then type tail -f tailing.txt. Use the second shell to append a line of text to that file.
Verify that the first shell displays this line.
paul@deb503:~$ > tailing.txt
paul@deb503:~$ tail -f tailing.txt
hello
world
in the other shell:
paul@deb503:~$ echo hello >> tailing.txt
paul@deb503:~$ echo world >> tailing.txt
7. Create a file that contains the names of five people. Use cat and output redirection to
create the file and use a here document to end the input.
paul@deb503:~$ cat > tennis.txt << ace
> Justine Henin
> Venus Williams
> Serena Williams
> Martina Hingis
> Kim Clijsters
> ace
paul@deb503:~$ cat tennis.txt
Justine Henin
Venus Williams
Serena Williams
Martina Hingis
Kim Clijsters
paul@deb503:~$
179
Chapter 19. filters
Commands that are created to be used with a pipe are often called filters. These filters
are very small programs that do one specific thing very efficiently. They can be used as
building blocks.
This chapter will introduce you to the most common filters. The combination of simple
commands and filters in a long pipe allows you to design elegant solutions.
180
filters
19.1. cat
When between two pipes, the cat command does nothing (except putting stdin on stdout).
[paul@RHEL4b pipes]$ tac count.txt | cat | cat | cat | cat | cat
five
four
three
two
one
[paul@RHEL4b pipes]$
19.2. tee
Writing long pipes in Unix is fun, but sometimes you may want intermediate results. This
is were tee comes in handy. The tee filter puts stdin on stdout and also into a file. So tee is
almost the same as cat, except that it has two identical outputs.
[paul@RHEL4b pipes]$ tac count.txt | tee temp.txt | tac
one
two
three
four
five
[paul@RHEL4b pipes]$ cat temp.txt
five
four
three
two
one
[paul@RHEL4b pipes]$
19.3. grep
The grep filter is famous among Unix users. The most common use of grep is to filter lines
of text containing (or not containing) a certain string.
[paul@RHEL4b pipes]$ cat tennis.txt
Amelie Mauresmo, Fra
Kim Clijsters, BEL
Justine Henin, Bel
Serena Williams, usa
Venus Williams, USA
[paul@RHEL4b pipes]$ cat tennis.txt | grep Williams
Serena Williams, usa
Venus Williams, USA
You can write this without the cat.
[paul@RHEL4b pipes]$ grep Williams tennis.txt
Serena Williams, usa
Venus Williams, USA
One of the most useful options of grep is grep -i which filters in a case insensitive way.
[paul@RHEL4b pipes]$ grep Bel tennis.txt
Justine Henin, Bel
[paul@RHEL4b pipes]$ grep -i Bel tennis.txt
181
filters
Kim Clijsters, BEL
Justine Henin, Bel
[paul@RHEL4b pipes]$
Another very useful option is grep -v which outputs lines not matching the string.
[paul@RHEL4b pipes]$ grep -v Fra tennis.txt
Kim Clijsters, BEL
Justine Henin, Bel
Serena Williams, usa
Venus Williams, USA
[paul@RHEL4b pipes]$
And of course, both options can be combined to filter all lines not containing a case
insensitive string.
[paul@RHEL4b pipes]$ grep -vi usa tennis.txt
Amelie Mauresmo, Fra
Kim Clijsters, BEL
Justine Henin, Bel
[paul@RHEL4b pipes]$
With grep -A1 one line after the result is also displayed.
paul@debian5:~/pipes$ grep -A1 Henin tennis.txt
Justine Henin, Bel
Serena Williams, usa
With grep -B1 one line before the result is also displayed.
paul@debian5:~/pipes$ grep -B1 Henin tennis.txt
Kim Clijsters, BEL
Justine Henin, Bel
With grep -C1 (context) one line before and one after are also displayed. All three options
(A,B, and C) can display any number of lines (using e.g. A2, B4 or C20).
paul@debian5:~/pipes$ grep -C1 Henin tennis.txt
Kim Clijsters, BEL
Justine Henin, Bel
Serena Williams, usa
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filters
19.4. cut
The cut filter can select columns from files, depending on a delimiter or a count of bytes.
The screenshot below uses cut to filter for the username and userid in the /etc/passwd file.
It uses the colon as a delimiter, and selects fields 1 and 3.
[[paul@RHEL4b pipes]$ cut -d: -f1,3 /etc/passwd | tail -4
Figo:510
Pfaff:511
Harry:516
Hermione:517
[paul@RHEL4b pipes]$
When using a space as the delimiter for cut, you have to quote the space.
[paul@RHEL4b pipes]$ cut -d" " -f1 tennis.txt
Amelie
Kim
Justine
Serena
Venus
[paul@RHEL4b pipes]$
This example uses cut to display the second to the seventh character of /etc/passwd.
[paul@RHEL4b pipes]$ cut -c2-7 /etc/passwd | tail -4
igo:x:
faff:x
arry:x
ermion
[paul@RHEL4b pipes]$
19.5. tr
You can translate characters with tr. The screenshot shows the translation of all occurrences
of e to E.
[paul@RHEL4b pipes]$ cat tennis.txt | tr 'e' 'E'
AmEliE MaurEsmo, Fra
Kim ClijstErs, BEL
JustinE HEnin, BEl
SErEna Williams, usa
VEnus Williams, USA
Here we set all letters to uppercase by defining two ranges.
[paul@RHEL4b pipes]$ cat tennis.txt | tr 'a-z' 'A-Z'
AMELIE MAURESMO, FRA
KIM CLIJSTERS, BEL
JUSTINE HENIN, BEL
SERENA WILLIAMS, USA
VENUS WILLIAMS, USA
[paul@RHEL4b pipes]$
Here we translate all newlines to spaces.
[paul@RHEL4b pipes]$ cat count.txt
one
two
183
filters
three
four
five
[paul@RHEL4b pipes]$ cat count.txt | tr '\n' ' '
one two three four five [paul@RHEL4b pipes]$
The tr -s filter can also be used to squeeze multiple occurrences of a character to one.
[paul@RHEL4b pipes]$ cat spaces.txt
one
two
three
four
five six
[paul@RHEL4b pipes]$ cat spaces.txt | tr -s ' '
one two three
four five six
[paul@RHEL4b pipes]$
You can also use tr to 'encrypt' texts with rot13.
[paul@RHEL4b pipes]$ cat count.txt | tr 'a-z' 'nopqrstuvwxyzabcdefghijklm'
bar
gjb
guerr
sbhe
svir
[paul@RHEL4b pipes]$ cat count.txt | tr 'a-z' 'n-za-m'
bar
gjb
guerr
sbhe
svir
[paul@RHEL4b pipes]$
This last example uses tr -d to delete characters.
paul@debian5:~/pipes$ cat tennis.txt | tr -d e
Amli Maursmo, Fra
Kim Clijstrs, BEL
Justin Hnin, Bl
Srna Williams, usa
Vnus Williams, USA
19.6. wc
Counting words, lines and characters is easy with wc.
[paul@RHEL4b pipes]$ wc
5 15 100 tennis.txt
[paul@RHEL4b pipes]$ wc
5 tennis.txt
[paul@RHEL4b pipes]$ wc
15 tennis.txt
[paul@RHEL4b pipes]$ wc
100 tennis.txt
[paul@RHEL4b pipes]$
tennis.txt
-l tennis.txt
-w tennis.txt
-c tennis.txt
184
filters
19.7. sort
The sort filter will default to an alphabetical sort.
paul@debian5:~/pipes$ cat music.txt
Queen
Brel
Led Zeppelin
Abba
paul@debian5:~/pipes$ sort music.txt
Abba
Brel
Led Zeppelin
Queen
But the sort filter has many options to tweak its usage. This example shows sorting different
columns (column 1 or column 2).
[paul@RHEL4b pipes]$ sort -k1 country.txt
Belgium, Brussels, 10
France, Paris, 60
Germany, Berlin, 100
Iran, Teheran, 70
Italy, Rome, 50
[paul@RHEL4b pipes]$ sort -k2 country.txt
Germany, Berlin, 100
Belgium, Brussels, 10
France, Paris, 60
Italy, Rome, 50
Iran, Teheran, 70
The screenshot below shows the difference between an alphabetical sort and a numerical
sort (both on the third column).
[paul@RHEL4b pipes]$ sort -k3 country.txt
Belgium, Brussels, 10
Germany, Berlin, 100
Italy, Rome, 50
France, Paris, 60
Iran, Teheran, 70
[paul@RHEL4b pipes]$ sort -n -k3 country.txt
Belgium, Brussels, 10
Italy, Rome, 50
France, Paris, 60
Iran, Teheran, 70
Germany, Berlin, 100
185
filters
19.8. uniq
With uniq you can remove duplicates from a sorted list.
paul@debian5:~/pipes$ cat music.txt
Queen
Brel
Queen
Abba
paul@debian5:~/pipes$ sort music.txt
Abba
Brel
Queen
Queen
paul@debian5:~/pipes$ sort music.txt |uniq
Abba
Brel
Queen
uniq can also count occurrences with the -c option.
paul@debian5:~/pipes$ sort music.txt |uniq -c
1 Abba
1 Brel
2 Queen
186
filters
19.9. comm
Comparing streams (or files) can be done with the comm. By default comm will output
three columns. In this example, Abba, Cure and Queen are in both lists, Bowie and Sweet
are only in the first file, Turner is only in the second.
paul@debian5:~/pipes$ cat > list1.txt
Abba
Bowie
Cure
Queen
Sweet
paul@debian5:~/pipes$ cat > list2.txt
Abba
Cure
Queen
Turner
paul@debian5:~/pipes$ comm list1.txt list2.txt
Abba
Bowie
Cure
Queen
Sweet
Turner
The output of comm can be easier to read when outputting only a single column. The digits
point out which output columns should not be displayed.
paul@debian5:~/pipes$ comm -12 list1.txt list2.txt
Abba
Cure
Queen
paul@debian5:~/pipes$ comm -13 list1.txt list2.txt
Turner
paul@debian5:~/pipes$ comm -23 list1.txt list2.txt
Bowie
Sweet
187
filters
19.10. od
European humans like to work with ascii characters, but computers store files in bytes. The
example below creates a simple file, and then uses od to show the contents of the file in
hexadecimal bytes
paul@laika:~/test$ cat > text.txt
abcdefg
1234567
paul@laika:~/test$ od -t x1 text.txt
0000000 61 62 63 64 65 66 67 0a 31 32 33 34 35 36 37 0a
0000020
The same file can also be displayed in octal bytes.
paul@laika:~/test$ od -b text.txt
0000000 141 142 143 144 145 146 147 012 061 062 063 064 065 066 067 012
0000020
And here is the file in ascii (or backslashed) characters.
paul@laika:~/test$ od -c text.txt
0000000
a
b
c
d
e
f
g
0000020
\n
188
1
2
3
4
5
6
7
\n
filters
19.11. sed
The stream editor sed can perform editing functions in the stream, using regular
expressions.
paul@debian5:~/pipes$ echo level5 | sed 's/5/42/'
level42
paul@debian5:~/pipes$ echo level5 | sed 's/level/jump/'
jump5
Add g for global replacements (all occurrences of the string per line).
paul@debian5:~/pipes$ echo level5 level7 | sed 's/level/jump/'
jump5 level7
paul@debian5:~/pipes$ echo level5 level7 | sed 's/level/jump/g'
jump5 jump7
With d you can remove lines from a stream containing a character.
paul@debian5:~/test42$ cat tennis.txt
Venus Williams, USA
Martina Hingis, SUI
Justine Henin, BE
Serena williams, USA
Kim Clijsters, BE
Yanina Wickmayer, BE
paul@debian5:~/test42$ cat tennis.txt | sed '/BE/d'
Venus Williams, USA
Martina Hingis, SUI
Serena williams, USA
189
filters
19.12. pipe examples
19.12.1. who | wc
How many users are logged on to this system ?
[paul@RHEL4b pipes]$ who
root
tty1
Jul 25 10:50
paul
pts/0
Jul 25 09:29 (laika)
Harry
pts/1
Jul 25 12:26 (barry)
paul
pts/2
Jul 25 12:26 (pasha)
[paul@RHEL4b pipes]$ who | wc -l
4
19.12.2. who | cut | sort
Display a sorted list of logged on users.
[paul@RHEL4b pipes]$ who | cut -d' ' -f1 | sort
Harry
paul
paul
root
Display a sorted list of logged on users, but every user only once .
[paul@RHEL4b pipes]$ who | cut -d' ' -f1 | sort | uniq
Harry
paul
root
19.12.3. grep | cut
Display a list of all bash user accounts on this computer. Users accounts are explained in
detail later.
paul@debian5:~$ grep bash /etc/passwd
root:x:0:0:root:/root:/bin/bash
paul:x:1000:1000:paul,,,:/home/paul:/bin/bash
serena:x:1001:1001::/home/serena:/bin/bash
paul@debian5:~$ grep bash /etc/passwd | cut -d: -f1
root
paul
serena
190
filters
19.13. practice: filters
1. Put a sorted list of all bash users in bashusers.txt.
2. Put a sorted list of all logged on users in onlineusers.txt.
3. Make a list of all filenames in /etc that contain the string conf in their filename.
4. Make a sorted list of all files in /etc that contain the case insensitive string conf in their
filename.
5. Look at the output of /sbin/ifconfig. Write a line that displays only ip address and the
subnet mask.
6. Write a line that removes all non-letters from a stream.
7. Write a line that receives a text file, and outputs all words on a separate line.
8. Write a spell checker on the command line. (There may be a dictionary in /usr/share/
dict/ .)
191
filters
19.14. solution: filters
1. Put a sorted list of all bash users in bashusers.txt.
grep bash /etc/passwd | cut -d: -f1 | sort > bashusers.txt
2. Put a sorted list of all logged on users in onlineusers.txt.
who | cut -d' ' -f1 | sort > onlineusers.txt
3. Make a list of all filenames in /etc that contain the string conf in their filename.
ls /etc | grep conf
4. Make a sorted list of all files in /etc that contain the case insensitive string conf in their
filename.
ls /etc | grep -i conf | sort
5. Look at the output of /sbin/ifconfig. Write a line that displays only ip address and the
subnet mask.
/sbin/ifconfig | head -2 | grep 'inet ' | tr -s ' ' | cut -d' ' -f3,5
6. Write a line that removes all non-letters from a stream.
paul@deb503:~$ cat text
This is, yes really! , a text with ?&* too many str$ange# characters ;-)
paul@deb503:~$ cat text | tr -d ',!$?.*&^%#@;()-'
This is yes really a text with too many strange characters
7. Write a line that receives a text file, and outputs all words on a separate line.
paul@deb503:~$ cat text2
it is very cold today without the sun
paul@deb503:~$ cat text2 | tr ' ' '\n'
it
is
very
cold
today
without
the
sun
8. Write a spell checker on the command line. (There may be a dictionary in /usr/share/
dict/ .)
paul@rhel ~$ echo "The zun is shining today" > text
paul@rhel ~$ cat > DICT
is
shining
sun
the
192
filters
today
paul@rhel ~$ cat text | tr 'A-Z ' 'a-z\n' | sort | uniq | comm -23 - DICT
zun
You could also add the solution from question number 6 to remove non-letters, and tr -s '
' to remove redundant spaces.
193
Chapter 20. basic Unix tools
This chapter introduces commands to find or locate files and to compress files, together
with other common tools that were not discussed before. While the tools discussed here are
technically not considered filters, they can be used in pipes.
194
basic Unix tools
20.1. find
The find command can be very useful at the start of a pipe to search for files. Here are some
examples. You might want to add 2>/dev/null to the command lines to avoid cluttering your
screen with error messages.
Find all files in /etc and put the list in etcfiles.txt
find /etc > etcfiles.txt
Find all files of the entire system and put the list in allfiles.txt
find / > allfiles.txt
Find files that end in .conf in the current directory (and all subdirs).
find . -name "*.conf"
Find files of type file (not directory, pipe or etc.) that end in .conf.
find . -type f -name "*.conf"
Find files of type directory that end in .bak .
find /data -type d -name "*.bak"
Find files that are newer than file42.txt
find . -newer file42.txt
Find can also execute another command on every file found. This example will look for
*.odf files and copy them to /backup/.
find /data -name "*.odf" -exec cp {} /backup/ \;
Find can also execute, after your confirmation, another command on every file found. This
example will remove *.odf files if you approve of it for every file found.
find /data -name "*.odf" -ok rm {} \;
195
basic Unix tools
20.2. locate
The locate tool is very different from find in that it uses an index to locate files. This is a
lot faster than traversing all the directories, but it also means that it is always outdated. If
the index does not exist yet, then you have to create it (as root on Red Hat Enterprise Linux)
with the updatedb command.
[paul@RHEL4b ~]$ locate Samba
warning: locate: could not open database: /var/lib/slocate/slocate.db:...
warning: You need to run the 'updatedb' command (as root) to create th...
Please have a look at /etc/updatedb.conf to enable the daily cron job.
[paul@RHEL4b ~]$ updatedb
fatal error: updatedb: You are not authorized to create a default sloc...
[paul@RHEL4b ~]$ su Password:
[root@RHEL4b ~]# updatedb
[root@RHEL4b ~]#
Most Linux distributions will schedule the updatedb to run once every day.
20.3. date
The date command can display the date, time, time zone and more.
paul@rhel55 ~$ date
Sat Apr 17 12:44:30 CEST 2010
A date string can be customised to display the format of your choice. Check the man page
for more options.
paul@rhel55 ~$ date +'%A %d-%m-%Y'
Saturday 17-04-2010
Time on any Unix is calculated in number of seconds since 1969 (the first second being the
first second of the first of January 1970). Use date +%s to display Unix time in seconds.
paul@rhel55 ~$ date +%s
1271501080
When will this seconds counter reach two thousand million ?
paul@rhel55 ~$ date -d '1970-01-01 + 2000000000 seconds'
Wed May 18 04:33:20 CEST 2033
196
basic Unix tools
20.4. cal
The cal command displays the current month, with the current day highlighted.
paul@rhel55 ~$ cal
April 2010
Su Mo Tu We Th Fr Sa
1 2 3
4 5 6 7 8 9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24
25 26 27 28 29 30
You can select any month in the past or the future.
paul@rhel55
February
Su Mo Tu We
1 2 3 4
8 9 10 11
15 16 17 18
22 23 24 25
~$ cal 2 1970
1970
Th Fr Sa
5 6 7
12 13 14
19 20 21
26 27 28
20.5. sleep
The sleep command is sometimes used in scripts to wait a number of seconds. This example
shows a five second sleep.
paul@rhel55 ~$ sleep 5
paul@rhel55 ~$
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basic Unix tools
20.6. time
The time command can display how long it takes to execute a command. The date command
takes only a little time.
paul@rhel55 ~$ time date
Sat Apr 17 13:08:27 CEST 2010
real
user
sys
0m0.014s
0m0.008s
0m0.006s
The sleep 5 command takes five real seconds to execute, but consumes little cpu time.
paul@rhel55 ~$ time sleep 5
real
user
sys
0m5.018s
0m0.005s
0m0.011s
This bzip2 command compresses a file and uses a lot of cpu time.
paul@rhel55 ~$ time bzip2 text.txt
real
user
sys
0m2.368s
0m0.847s
0m0.539s
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basic Unix tools
20.7. gzip - gunzip
Users never have enough disk space, so compression comes in handy. The gzip command
can make files take up less space.
paul@rhel55 ~$ ls -lh text.txt
-rw-rw-r-- 1 paul paul 6.4M Apr 17 13:11 text.txt
paul@rhel55 ~$ gzip text.txt
paul@rhel55 ~$ ls -lh text.txt.gz
-rw-rw-r-- 1 paul paul 760K Apr 17 13:11 text.txt.gz
You can get the original back with gunzip.
paul@rhel55 ~$ gunzip text.txt.gz
paul@rhel55 ~$ ls -lh text.txt
-rw-rw-r-- 1 paul paul 6.4M Apr 17 13:11 text.txt
20.8. zcat - zmore
Text files that are compressed with gzip can be viewed with zcat and zmore.
paul@rhel55 ~$ head -4 text.txt
/
/opt
/opt/VBoxGuestAdditions-3.1.6
/opt/VBoxGuestAdditions-3.1.6/routines.sh
paul@rhel55 ~$ gzip text.txt
paul@rhel55 ~$ zcat text.txt.gz | head -4
/
/opt
/opt/VBoxGuestAdditions-3.1.6
/opt/VBoxGuestAdditions-3.1.6/routines.sh
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basic Unix tools
20.9. bzip2 - bunzip2
Files can also be compressed with bzip2 which takes a little more time than gzip, but
compresses better.
paul@rhel55 ~$ bzip2 text.txt
paul@rhel55 ~$ ls -lh text.txt.bz2
-rw-rw-r-- 1 paul paul 569K Apr 17 13:11 text.txt.bz2
Files can be uncompressed again with bunzip2.
paul@rhel55 ~$ bunzip2 text.txt.bz2
paul@rhel55 ~$ ls -lh text.txt
-rw-rw-r-- 1 paul paul 6.4M Apr 17 13:11 text.txt
20.10. bzcat - bzmore
And in the same way bzcat and bzmore can display files compressed with bzip2.
paul@rhel55 ~$ bzip2 text.txt
paul@rhel55 ~$ bzcat text.txt.bz2 | head -4
/
/opt
/opt/VBoxGuestAdditions-3.1.6
/opt/VBoxGuestAdditions-3.1.6/routines.sh
200
basic Unix tools
20.11. practice: basic Unix tools
1. Explain the difference between these two commands. This question is very important. If
you don't know the answer, then look back at the shell chapter.
find /data -name "*.txt"
find /data -name *.txt
2. Explain the difference between these two statements. Will they both work when there are
200 .odf files in /data ? How about when there are 2 million .odf files ?
find /data -name "*.odf" > data_odf.txt
find /data/*.odf > data_odf.txt
3. Write a find command that finds all files created after January 30th 2010.
4. Write a find command that finds all *.odf files created in September 2009.
5. Count the number of *.conf files in /etc and all its subdirs.
6. Here are two commands that do the same thing: copy *.odf files to /backup/ . What would
be a reason to replace the first command with the second ? Again, this is an important
question.
cp -r /data/*.odf /backup/
find /data -name "*.odf" -exec cp {} /backup/ \;
7. Create a file called loctest.txt. Can you find this file with locate ? Why not ? How do
you make locate find this file ?
8. Use find and -exec to rename all .htm files to .html.
9. Issue the date command. Now display the date in YYYY/MM/DD format.
10. Issue the cal command. Display a calendar of 1582 and 1752. Notice anything special ?
201
basic Unix tools
20.12. solution: basic Unix tools
1. Explain the difference between these two commands. This question is very important. If
you don't know the answer, then look back at the shell chapter.
find /data -name "*.txt"
find /data -name *.txt
When *.txt is quoted then the shell will not touch it. The find tool will look in the /data
for all files ending in .txt.
When *.txt is not quoted then the shell might expand this (when one or more files that ends
in .txt exist in the current directory). The find might show a different result, or can result
in a syntax error.
2. Explain the difference between these two statements. Will they both work when there are
200 .odf files in /data ? How about when there are 2 million .odf files ?
find /data -name "*.odf" > data_odf.txt
find /data/*.odf > data_odf.txt
The first find will output all .odf filenames in /data and all subdirectories. The shell will
redirect this to a file.
The second find will output all files named .odf in /data and will also output all files that
exist in directories named *.odf (in /data).
With two million files the command line would be expanded beyond the maximum that the
shell can accept. The last part of the command line would be lost.
3. Write a find command that finds all files created after January 30th 2010.
touch -t 201001302359 marker_date
find . -type f -newer marker_date
There is another solution :
find . -type f -newerat "20100130 23:59:59"
4. Write a find command that finds all *.odf files created in September 2009.
touch -t 200908312359 marker_start
touch -t 200910010000 marker_end
find . -type f -name "*.odf" -newer marker_start ! -newer marker_end
The exclamation mark ! -newer can be read as not newer.
5. Count the number of *.conf files in /etc and all its subdirs.
find /etc -type f -name '*.conf' | wc -l
6. Here are two commands that do the same thing: copy *.odf files to /backup/ . What would
be a reason to replace the first command with the second ? Again, this is an important
question.
cp -r /data/*.odf /backup/
202
basic Unix tools
find /data -name "*.odf" -exec cp {} /backup/ \;
The first might fail when there are too many files to fit on one command line.
7. Create a file called loctest.txt. Can you find this file with locate ? Why not ? How do
you make locate find this file ?
You cannot locate this with locate because it is not yet in the index.
updatedb
8. Use find and -exec to rename all .htm files to .html.
paul@rhel55 ~$ find . -name '*.htm'
./one.htm
./two.htm
paul@rhel55 ~$ find . -name '*.htm' -exec mv {} {}l \;
paul@rhel55 ~$ find . -name '*.htm*'
./one.html
./two.html
9. Issue the date command. Now display the date in YYYY/MM/DD format.
date +%Y/%m/%d
10. Issue the cal command. Display a calendar of 1582 and 1752. Notice anything special ?
cal 1582
The calendars are different depending on the country. Check http://linux-training.be/files/
studentfiles/dates.txt
203
Chapter 21. regular expressions
Regular expressions are a very powerful tool in Linux. They can be used with a variety of
programs like bash, vi, rename, grep, sed, and more.
This chapter introduces you to the basics of regular expressions.
204
regular expressions
21.1. regex versions
There are three different versions of regular expression syntax:
BRE: Basic Regular Expressions
ERE: Extended Regular Expressions
PRCE: Perl Regular Expressions
Depending on the tool being used, one or more of these syntaxes can be used.
For example the grep tool has the -E option to force a string to be read as ERE while -G
forces BRE and -P forces PRCE.
Note that grep also has -F to force the string to be read literally.
The sed tool also has options to choose a regex syntax.
Read the manual of the tools you use!
205
regular expressions
21.2. grep
21.2.1. print lines matching a pattern
grep is a popular Linux tool to search for lines that match a certain pattern. Below are some
examples of the simplest regular expressions.
This is the contents of the test file. This file contains three lines (or three newline characters).
paul@rhel65:~$ cat names
Tania
Laura
Valentina
When grepping for a single character, only the lines containing that character are returned.
paul@rhel65:~$ grep u names
Laura
paul@rhel65:~$ grep e names
Valentina
paul@rhel65:~$ grep i names
Tania
Valentina
The pattern matching in this example should be very straightforward; if the given character
occurs on a line, then grep will return that line.
21.2.2. concatenating characters
Two concatenated characters will have to be concatenated in the same way to have a match.
This example demonstrates that ia will match Tania but not Valentina and in will match
Valentina but not Tania.
paul@rhel65:~$ grep a names
Tania
Laura
Valentina
paul@rhel65:~$ grep ia names
Tania
paul@rhel65:~$ grep in names
Valentina
paul@rhel65:~$
206
regular expressions
21.2.3. one or the other
PRCE and ERE both use the pipe symbol to signify OR. In this example we grep for lines
containing the letter i or the letter a.
paul@debian7:~$ cat list
Tania
Laura
paul@debian7:~$ grep -E 'i|a' list
Tania
Laura
Note that we use the -E switch of grep to force interpretion of our string as an ERE.
We need to escape the pipe symbol in a BRE to get the same logical OR.
paul@debian7:~$ grep -G 'i|a' list
paul@debian7:~$ grep -G 'i\|a' list
Tania
Laura
21.2.4. one or more
The * signifies zero, one or more occurences of the previous and the + signifies one or more
of the previous.
paul@debian7:~$ cat list2
ll
lol
lool
loool
paul@debian7:~$ grep -E 'o*' list2
ll
lol
lool
loool
paul@debian7:~$ grep -E 'o+' list2
lol
lool
loool
paul@debian7:~$
207
regular expressions
21.2.5. match the end of a string
For the following examples, we will use this file.
paul@debian7:~$ cat names
Tania
Laura
Valentina
Fleur
Floor
The two examples below show how to use the dollar character to match the end of a string.
paul@debian7:~$ grep a$ names
Tania
Laura
Valentina
paul@debian7:~$ grep r$ names
Fleur
Floor
21.2.6. match the start of a string
The caret character (^) will match a string at the start (or the beginning) of a line.
Given the same file as above, here are two examples.
paul@debian7:~$ grep ^Val names
Valentina
paul@debian7:~$ grep ^F names
Fleur
Floor
Both the dollar sign and the little hat are called anchors in a regex.
208
regular expressions
21.2.7. separating words
Regular expressions use a \b sequence to reference a word separator. Take for example this
file:
paul@debian7:~$ cat text
The governer is governing.
The winter is over.
Can you get over there?
Simply grepping for over will give too many results.
paul@debian7:~$ grep over text
The governer is governing.
The winter is over.
Can you get over there?
Surrounding the searched word with spaces is not a good solution (because other characters
can be word separators). This screenshot below show how to use \b to find only the searched
word:
paul@debian7:~$ grep '\bover\b' text
The winter is over.
Can you get over there?
paul@debian7:~$
Note that grep also has a -w option to grep for words.
paul@debian7:~$ cat text
The governer is governing.
The winter is over.
Can you get over there?
paul@debian7:~$ grep -w over text
The winter is over.
Can you get over there?
paul@debian7:~$
209
regular expressions
21.2.8. grep features
Sometimes it is easier to combine a simple regex with grep options, than it is to write a more
complex regex. These options where discussed before:
grep
grep
grep
grep
grep
grep
-i
-v
-w
-A5
-B5
-C5
21.2.9. preventing shell expansion of a regex
The dollar sign is a special character, both for the regex and also for the shell (remember
variables and embedded shells). Therefore it is advised to always quote the regex, this
prevents shell expansion.
paul@debian7:~$ grep 'r$' names
Fleur
Floor
210
regular expressions
21.3. rename
21.3.1. the rename command
On Debian Linux the /usr/bin/rename command is a link to /usr/bin/prename installed by
the perl package.
paul@pi ~ $ dpkg -S $(readlink -f $(which rename))
perl: /usr/bin/prename
Red Hat derived systems do not install the same rename command, so this section does not
describe rename on Red Hat (unless you copy the perl script manually).
There is often confusion on the internet about the rename command because solutions
that work fine in Debian (and Ubuntu, xubuntu, Mint, ...) cannot be used in Red Hat
(and CentOS, Fedora, ...).
21.3.2. perl
The rename command is actually a perl script that uses perl regular expressions. The
complete manual for these can be found by typing perldoc perlrequick (after installing
perldoc).
root@pi:~# aptitude install perl-doc
The following NEW packages will be installed:
perl-doc
0 packages upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 8,170 kB of archives. After unpacking 13.2 MB will be used.
Get: 1 http://mirrordirector.raspbian.org/raspbian/ wheezy/main perl-do...
Fetched 8,170 kB in 19s (412 kB/s)
Selecting previously unselected package perl-doc.
(Reading database ... 67121 files and directories currently installed.)
Unpacking perl-doc (from .../perl-doc_5.14.2-21+rpi2_all.deb) ...
Adding 'diversion of /usr/bin/perldoc to /usr/bin/perldoc.stub by perl-doc'
Processing triggers for man-db ...
Setting up perl-doc (5.14.2-21+rpi2) ...
root@pi:~# perldoc perlrequick
211
regular expressions
21.3.3. well known syntax
The most common use of the rename is to search for filenames matching a certain string
and replacing this string with an other string.
This is often presented as s/string/other string/ as seen in this example:
paul@pi ~
abc
abc.conf
paul@pi ~
paul@pi ~
abc
abc.conf
$ ls
allfiles.TXT bllfiles.TXT Scratch
tennis2.TXT
backup
cllfiles.TXT temp.TXT tennis.TXT
$ rename 's/TXT/text/' *
$ ls
allfiles.text bllfiles.text Scratch
tennis2.text
backup
cllfiles.text temp.text tennis.text
And here is another example that uses rename with the well know syntax to change the
extensions of the same files once more:
paul@pi ~
abc
abc.conf
paul@pi ~
paul@pi ~
abc
abc.conf
paul@pi ~
$ ls
allfiles.text bllfiles.text Scratch
tennis2.text
backup
cllfiles.text temp.text tennis.text
$ rename 's/text/txt/' *.text
$ ls
allfiles.txt bllfiles.txt Scratch
tennis2.txt
backup
cllfiles.txt temp.txt tennis.txt
$
These two examples appear to work because the strings we used only exist at the end of the
filename. Remember that file extensions have no meaning in the bash shell.
The next example shows what can go wrong with this syntax.
paul@pi ~
paul@pi ~
paul@pi ~
abc
abc.conf
paul@pi ~
$ touch atxt.txt
$ rename 's/txt/problem/' atxt.txt
$ ls
allfiles.txt backup
cllfiles.txt
aproblem.txt bllfiles.txt Scratch
$
temp.txt
tennis2.txt
Only the first occurrence of the searched string is replaced.
212
tennis.txt
regular expressions
21.3.4. a global replace
The syntax used in the previous example can be described as s/regex/replacement/. This
is simple and straightforward, you enter a regex between the first two slashes and a
replacement string between the last two.
This example expands this syntax only a little, by adding a modifier.
paul@pi ~ $ rename -n 's/TXT/txt/g' aTXT.TXT
aTXT.TXT renamed as atxt.txt
paul@pi ~ $
The syntax we use now can be described as s/regex/replacement/g where s signifies switch
and g stands for global.
Note that this example used the -n switch to show what is being done (instead of actually
renaming the file).
21.3.5. case insensitive replace
Another modifier that can be useful is i. this example shows how to replace a case insensitive
string with another string.
paul@debian7:~/files$ ls
file1.text file2.TEXT file3.txt
paul@debian7:~/files$ rename 's/.text/.txt/i' *
paul@debian7:~/files$ ls
file1.txt file2.txt file3.txt
paul@debian7:~/files$
21.3.6. renaming extensions
Command line Linux has no knowledge of MS-DOS like extensions, but many end users
and graphical application do use them.
Here is an example on how to use rename to only rename the file extension. It uses the
dollar sign to mark the ending of the filename.
paul@pi ~ $ ls *.txt
allfiles.txt bllfiles.txt cllfiles.txt really.txt.txt temp.txt
paul@pi ~ $ rename 's/.txt$/.TXT/' *.txt
paul@pi ~ $ ls *.TXT
allfiles.TXT bllfiles.TXT
cllfiles.TXT
really.txt.TXT
temp.TXT
tennis.TXT
paul@pi ~ $
tennis.txt
Note that the dollar sign in the regex means at the end. Without the dollar sign this
command would fail on the really.txt.txt file.
213
regular expressions
21.4. sed
21.4.1. stream editor
The stream editor or short sed uses regex for stream editing.
In this example sed is used to replace a string.
echo Sunday | sed 's/Sun/Mon/'
Monday
The slashes can be replaced by a couple of other characters, which can be handy in some
cases to improve readability.
echo Sunday | sed 's:Sun:Mon:'
Monday
echo Sunday | sed 's_Sun_Mon_'
Monday
echo Sunday | sed 's|Sun|Mon|'
Monday
21.4.2. interactive editor
While sed is meant to be used in a stream, it can also be used interactively on a file.
paul@debian7:~/files$
paul@debian7:~/files$
Sunday
paul@debian7:~/files$
paul@debian7:~/files$
Monday
echo Sunday > today
cat today
sed -i 's/Sun/Mon/' today
cat today
214
regular expressions
21.4.3. simple back referencing
The ampersand character can be used to reference the searched (and found) string.
In this example the ampersand is used to double the occurence of the found string.
echo Sunday | sed 's/Sun/&&/'
SunSunday
echo Sunday | sed 's/day/&&/'
Sundayday
21.4.4. back referencing
Parentheses (often called round brackets) are used to group sections of the regex so they
can leter be referenced.
Consider this simple example:
paul@debian7:~$ echo Sunday | sed 's_\(Sun\)_\1ny_'
Sunnyday
paul@debian7:~$ echo Sunday | sed 's_\(Sun\)_\1ny \1_'
Sunny Sunday
21.4.5. a dot for any character
In a regex a simple dot can signify any character.
paul@debian7:~$ echo 2014-04-01 | sed 's/....-..-../YYYY-MM-DD/'
YYYY-MM-DD
paul@debian7:~$ echo abcd-ef-gh | sed 's/....-..-../YYYY-MM-DD/'
YYYY-MM-DD
21.4.6. multiple back referencing
When more than one pair of parentheses is used, each of them can be referenced separately
by consecutive numbers.
paul@debian7:~$ echo 2014-04-01 | sed 's/\(....\)-\(..\)-\(..\)/\1+\2+\3/'
2014+04+01
paul@debian7:~$ echo 2014-04-01 | sed 's/\(....\)-\(..\)-\(..\)/\3:\2:\1/'
01:04:2014
This feature is called grouping.
215
regular expressions
21.4.7. white space
The \s can refer to white space such as a space or a tab.
This example looks for white spaces (\s) globally and replaces them with 1 space.
paul@debian7:~$ echo -e 'today\tis\twarm'
today
is
warm
paul@debian7:~$ echo -e 'today\tis\twarm' | sed 's_\s_ _g'
today is warm
21.4.8. optional occurrence
A question mark signifies that the previous is optional.
The example below searches for three consecutive letter o, but the third o is optional.
paul@debian7:~$ cat list2
ll
lol
lool
loool
paul@debian7:~$ grep -E 'ooo?' list2
lool
loool
paul@debian7:~$ cat list2 | sed 's/ooo\?/A/'
ll
lol
lAl
lAl
216
regular expressions
21.4.9. exactly n times
You can demand an exact number of times the oprevious has to occur.
This example wants exactly three o's.
paul@debian7:~$ cat list2
ll
lol
lool
loool
paul@debian7:~$ grep -E 'o{3}' list2
loool
paul@debian7:~$ cat list2 | sed 's/o\{3\}/A/'
ll
lol
lool
lAl
paul@debian7:~$
21.4.10. between n and m times
And here we demand exactly from minimum 2 to maximum 3 times.
paul@debian7:~$
ll
lol
lool
loool
paul@debian7:~$
lool
loool
paul@debian7:~$
lool
loool
paul@debian7:~$
ll
lol
lAl
lAl
paul@debian7:~$
cat list2
grep -E 'o{2,3}' list2
grep 'o\{2,3\}' list2
cat list2 | sed 's/o\{2,3\}/A/'
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regular expressions
21.5. bash history
The bash shell can also interprete some regular expressions.
This example shows how to manipulate the exclamation mask history feature of the bash
shell.
paul@debian7:~$ mkdir hist
paul@debian7:~$ cd hist/
paul@debian7:~/hist$ touch file1 file2 file3
paul@debian7:~/hist$ ls -l file1
-rw-r--r-- 1 paul paul 0 Apr 15 22:07 file1
paul@debian7:~/hist$ !l
ls -l file1
-rw-r--r-- 1 paul paul 0 Apr 15 22:07 file1
paul@debian7:~/hist$ !l:s/1/3
ls -l file3
-rw-r--r-- 1 paul paul 0 Apr 15 22:07 file3
paul@debian7:~/hist$
This also works with the history numbers in bash.
paul@debian7:~/hist$ history 6
2089 mkdir hist
2090 cd hist/
2091 touch file1 file2 file3
2092 ls -l file1
2093 ls -l file3
2094 history 6
paul@debian7:~/hist$ !2092
ls -l file1
-rw-r--r-- 1 paul paul 0 Apr 15 22:07 file1
paul@debian7:~/hist$ !2092:s/1/2
ls -l file2
-rw-r--r-- 1 paul paul 0 Apr 15 22:07 file2
paul@debian7:~/hist$
218
Part VI. vi
Table of Contents
22. Introduction to vi .......................................................................................................... 221
22.1. command mode and insert mode .............................................................................. 222
22.2. start typing (a A i I o O) ........................................................................................ 222
22.3. replace and delete a character (r x X) ....................................................................... 223
22.4. undo and repeat (u .) ............................................................................................. 223
22.5. cut, copy and paste a line (dd yy p P) ....................................................................... 223
22.6. cut, copy and paste lines (3dd 2yy) .......................................................................... 224
22.7. start and end of a line (0 or ^ and $) ........................................................................ 224
22.8. join two lines (J) and more ..................................................................................... 224
22.9. words (w b) ......................................................................................................... 225
22.10. save (or not) and exit (:w :q :q! ) ............................................................................ 225
22.11. Searching (/ ?) .................................................................................................... 225
22.12. replace all ( :1,$ s/foo/bar/g ) ................................................................................. 226
22.13. reading files (:r :r !cmd) ....................................................................................... 226
22.14. text buffers ........................................................................................................ 226
22.15. multiple files ...................................................................................................... 226
22.16. abbreviations ...................................................................................................... 227
22.17. key mappings ..................................................................................................... 228
22.18. setting options .................................................................................................... 228
22.19. practice: vi(m) .................................................................................................... 229
22.20. solution: vi(m) .................................................................................................... 230
220
Chapter 22. Introduction to vi
The vi editor is installed on almost every Unix. Linux will very often install vim (vi
improved) which is similar. Every system administrator should know vi(m), because it is
an easy tool to solve problems.
The vi editor is not intuitive, but once you get to know it, vi becomes a very powerful
application. Most Linux distributions will include the vimtutor which is a 45 minute lesson
in vi(m).
221
Introduction to vi
22.1. command mode and insert mode
The vi editor starts in command mode. In command mode, you can type commands. Some
commands will bring you into insert mode. In insert mode, you can type text. The escape
key will return you to command mode.
Table 22.1. getting to command mode
key
action
Esc
set vi(m) in command mode.
22.2. start typing (a A i I o O)
The difference between a A i I o and O is the location where you can start typing. a will
append after the current character and A will append at the end of the line. i will insert before
the current character and I will insert at the beginning of the line. o will put you in a new
line after the current line and O will put you in a new line before the current line.
Table 22.2. switch to insert mode
command
action
a
start typing after the current character
A
start typing at the end of the current line
i
start typing before the current character
I
start typing at the start of the current line
o
start typing on a new line after the current line
O
start typing on a new line before the current line
222
Introduction to vi
22.3. replace and delete a character (r x X)
When in command mode (it doesn't hurt to hit the escape key more than once) you can use
the x key to delete the current character. The big X key (or shift x) will delete the character
left of the cursor. Also when in command mode, you can use the r key to replace one single
character. The r key will bring you in insert mode for just one key press, and will return you
immediately to command mode.
Table 22.3. replace and delete
command
action
x
delete the character below the cursor
X
delete the character before the cursor
r
replace the character below the cursor
p
paste after the cursor (here the last deleted character)
xp
switch two characters
22.4. undo and repeat (u .)
When in command mode, you can undo your mistakes with u. You can do your mistakes
twice with . (in other words, the . will repeat your last command).
Table 22.4. undo and repeat
command
action
u
undo the last action
.
repeat the last action
22.5. cut, copy and paste a line (dd yy p P)
When in command mode, dd will cut the current line. yy will copy the current line. You can
paste the last copied or cut line after (p) or before (P) the current line.
Table 22.5. cut, copy and paste a line
command
action
dd
cut the current line
yy
(yank yank) copy the current line
p
paste after the current line
P
paste before the current line
223
Introduction to vi
22.6. cut, copy and paste lines (3dd 2yy)
When in command mode, before typing dd or yy, you can type a number to repeat the
command a number of times. Thus, 5dd will cut 5 lines and 4yy will copy (yank) 4 lines.
That last one will be noted by vi in the bottom left corner as "4 line yanked".
Table 22.6. cut, copy and paste lines
command
action
3dd
cut three lines
4yy
copy four lines
22.7. start and end of a line (0 or ^ and $)
When in command mode, the 0 and the caret ^ will bring you to the start of the current line,
whereas the $ will put the cursor at the end of the current line. You can add 0 and $ to the d
command, d0 will delete every character between the current character and the start of the
line. Likewise d$ will delete everything from the current character till the end of the line.
Similarly y0 and y$ will yank till start and end of the current line.
Table 22.7. start and end of line
command
action
0
jump to start of current line
^
jump to start of current line
$
jump to end of current line
d0
delete until start of line
d$
delete until end of line
22.8. join two lines (J) and more
When in command mode, pressing J will append the next line to the current line. With yyp
you duplicate a line and with ddp you switch two lines.
Table 22.8. join two lines
command
J
action
join two lines
yyp
duplicate a line
ddp
switch two lines
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Introduction to vi
22.9. words (w b)
When in command mode, w will jump to the next word and b will move to the previous
word. w and b can also be combined with d and y to copy and cut words (dw db yw yb).
Table 22.9. words
command
action
w
forward one word
b
back one word
3w
forward three words
dw
delete one word
yw
yank (copy) one word
5yb
yank five words back
7dw
delete seven words
22.10. save (or not) and exit (:w :q :q! )
Pressing the colon : will allow you to give instructions to vi (technically speaking, typing
the colon will open the ex editor). :w will write (save) the file, :q will quit an unchanged
file without saving, and :q! will quit vi discarding any changes. :wq will save and quit and
is the same as typing ZZ in command mode.
Table 22.10. save and exit vi
command
:w
:w fname
:q
action
save (write)
save as fname
quit
:wq
save and quit
ZZ
save and quit
:q!
quit (discarding your changes)
:w!
save (and write to non-writable file!)
The last one is a bit special. With :w! vi will try to chmod the file to get write permission
(this works when you are the owner) and will chmod it back when the write succeeds. This
should always work when you are root (and the file system is writable).
22.11. Searching (/ ?)
When in command mode typing / will allow you to search in vi for strings (can be a regular
expression). Typing /foo will do a forward search for the string foo and typing ?bar will do
a backward search for bar.
Table 22.11. searching
command
/string
action
forward search for string
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Introduction to vi
command
action
?string
backward search for string
n
go to next occurrence of search string
/^string
forward search string at beginning of line
/string$
forward search string at end of line
/br[aeio]l
search for bral brel bril and brol
/\<he\>
search for the word he (and not for here or the)
22.12. replace all ( :1,$ s/foo/bar/g )
To replace all occurrences of the string foo with bar, first switch to ex mode with : . Then
tell vi which lines to use, for example 1,$ will do the replace all from the first to the last
line. You can write 1,5 to only process the first five lines. The s/foo/bar/g will replace all
occurrences of foo with bar.
Table 22.12. replace
command
action
:4,8 s/foo/bar/g
replace foo with bar on lines 4 to 8
:1,$ s/foo/bar/g
replace foo with bar on all lines
22.13. reading files (:r :r !cmd)
When in command mode, :r foo will read the file named foo, :r !foo will execute the
command foo. The result will be put at the current location. Thus :r !ls will put a listing of
the current directory in your text file.
Table 22.13. read files and input
command
action
:r fname
(read) file fname and paste contents
:r !cmd
execute cmd and paste its output
22.14. text buffers
There are 36 buffers in vi to store text. You can use them with the " character.
Table 22.14. text buffers
command
action
"add
delete current line and put text in buffer a
"g7yy
copy seven lines into buffer g
"ap
paste from buffer a
22.15. multiple files
You can edit multiple files with vi. Here are some tips.
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Introduction to vi
Table 22.15. multiple files
command
vi file1 file2 file3
:args
action
start editing three files
lists files and marks active file
:n
start editing the next file
:e
toggle with last edited file
:rew
rewind file pointer to first file
22.16. abbreviations
With :ab you can put abbreviations in vi. Use :una to undo the abbreviation.
Table 22.16. abbreviations
command
:ab str long string
:una str
action
abbreviate str to be 'long string'
un-abbreviate str
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Introduction to vi
22.17. key mappings
Similarly to their abbreviations, you can use mappings with :map for command mode and
:map! for insert mode.
This example shows how to set the F6 function key to toggle between set number and set
nonumber. The <bar> separates the two commands, set number! toggles the state and set
number? reports the current state.
:map <F6> :set number!<bar>set number?<CR>
22.18. setting options
Some options that you can set in vim.
:set number ( also try :se nu )
:set nonumber
:syntax on
:syntax off
:set all (list all options)
:set tabstop=8
:set tx
(CR/LF style endings)
:set notx
You can set these options (and much more) in ~/.vimrc for vim or in ~/.exrc for standard vi.
paul@barry:~$ cat ~/.vimrc
set number
set tabstop=8
set textwidth=78
map <F6> :set number!<bar>set number?<CR>
paul@barry:~$
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Introduction to vi
22.19. practice: vi(m)
1. Start the vimtutor and do some or all of the exercises. You might need to run aptitude
install vim on xubuntu.
2. What 3 key sequence in command mode will duplicate the current line.
3. What 3 key sequence in command mode will switch two lines' place (line five becomes
line six and line six becomes line five).
4. What 2 key sequence in command mode will switch a character's place with the next one.
5. vi can understand macro's. A macro can be recorded with q followed by the name of
the macro. So qa will record the macro named a. Pressing q again will end the recording.
You can recall the macro with @ followed by the name of the macro. Try this example: i 1
'Escape Key' qa yyp 'Ctrl a' q 5@a (Ctrl a will increase the number with one).
6. Copy /etc/passwd to your ~/passwd. Open the last one in vi and press Ctrl v. Use the arrow
keys to select a Visual Block, you can copy this with y or delete it with d. Try pasting it.
7. What does dwwP do when you are at the beginning of a word in a sentence ?
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Introduction to vi
22.20. solution: vi(m)
1. Start the vimtutor and do some or all of the exercises. You might need to run aptitude
install vim on xubuntu.
vimtutor
2. What 3 key sequence in command mode will duplicate the current line.
yyp
3. What 3 key sequence in command mode will switch two lines' place (line five becomes
line six and line six becomes line five).
ddp
4. What 2 key sequence in command mode will switch a character's place with the next one.
xp
5. vi can understand macro's. A macro can be recorded with q followed by the name of
the macro. So qa will record the macro named a. Pressing q again will end the recording.
You can recall the macro with @ followed by the name of the macro. Try this example: i 1
'Escape Key' qa yyp 'Ctrl a' q 5@a (Ctrl a will increase the number with one).
6. Copy /etc/passwd to your ~/passwd. Open the last one in vi and press Ctrl v. Use the arrow
keys to select a Visual Block, you can copy this with y or delete it with d. Try pasting it.
cp /etc/passwd ~
vi passwd
(press Ctrl-V)
7. What does dwwP do when you are at the beginning of a word in a sentence ?
dwwP can switch the current word with the next word.
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Part VII. scripting
Table of Contents
23. scripting introduction .....................................................................................................
23.1. prerequisites .........................................................................................................
23.2. hello world ..........................................................................................................
23.3. she-bang ..............................................................................................................
23.4. comment .............................................................................................................
23.5. variables ..............................................................................................................
23.6. sourcing a script ...................................................................................................
23.7. troubleshooting a script ..........................................................................................
23.8. prevent setuid root spoofing ....................................................................................
23.9. practice: introduction to scripting .............................................................................
23.10. solution: introduction to scripting ...........................................................................
24. scripting loops ...............................................................................................................
24.1. test [ ] .................................................................................................................
24.2. if then else ..........................................................................................................
24.3. if then elif ...........................................................................................................
24.4. for loop ...............................................................................................................
24.5. while loop ...........................................................................................................
24.6. until loop ............................................................................................................
24.7. practice: scripting tests and loops .............................................................................
24.8. solution: scripting tests and loops ............................................................................
25. scripting parameters ......................................................................................................
25.1. script parameters ...................................................................................................
25.2. shift through parameters .........................................................................................
25.3. runtime input .......................................................................................................
25.4. sourcing a config file .............................................................................................
25.5. get script options with getopts .................................................................................
25.6. get shell options with shopt ....................................................................................
25.7. practice: parameters and options ..............................................................................
25.8. solution: parameters and options ..............................................................................
26. more scripting ...............................................................................................................
26.1. eval ....................................................................................................................
26.2. (( )) ....................................................................................................................
26.3. let ......................................................................................................................
26.4. case ....................................................................................................................
26.5. shell functions ......................................................................................................
26.6. practice : more scripting .........................................................................................
26.7. solution : more scripting .........................................................................................
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235
235
236
236
237
238
239
240
241
241
241
242
242
243
244
246
247
248
248
249
250
251
252
253
254
255
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Chapter 23. scripting introduction
Shells like bash and Korn have support for programming constructs that can be saved as
scripts. These scripts in turn then become more shell commands. Many Linux commands
are scripts. User profile scripts are run when a user logs on and init scripts are run when
a daemon is stopped or started.
This means that system administrators also need basic knowledge of scripting to understand
how their servers and their applications are started, updated, upgraded, patched, maintained,
configured and removed, and also to understand how a user environment is built.
The goal of this chapter is to give you enough information to be able to read and understand
scripts. Not to become a writer of complex scripts.
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scripting introduction
23.1. prerequisites
You should have read and understood part III shell expansion and part IV pipes and
commands before starting this chapter.
23.2. hello world
Just like in every programming course, we start with a simple hello_world script. The
following script will output Hello World.
echo Hello World
After creating this simple script in vi or with echo, you'll have to chmod +x hello_world
to make it executable. And unless you add the scripts directory to your path, you'll have to
type the path to the script for the shell to be able to find it.
[paul@RHEL4a
[paul@RHEL4a
[paul@RHEL4a
Hello World
[paul@RHEL4a
~]$ echo echo Hello World > hello_world
~]$ chmod +x hello_world
~]$ ./hello_world
~]$
23.3. she-bang
Let's expand our example a little further by putting #!/bin/bash on the first line of the script.
The #! is called a she-bang (sometimes called sha-bang), where the she-bang is the first
two characters of the script.
#!/bin/bash
echo Hello World
You can never be sure which shell a user is running. A script that works flawlessly in bash
might not work in ksh, csh, or dash. To instruct a shell to run your script in a certain shell,
you can start your script with a she-bang followed by the shell it is supposed to run in. This
script will run in a bash shell.
#!/bin/bash
echo -n hello
echo A bash subshell `echo -n hello`
This script will run in a Korn shell (unless /bin/ksh is a hard link to /bin/bash). The /etc/
shells file contains a list of shells on your system.
#!/bin/ksh
echo -n hello
echo a Korn subshell `echo -n hello`
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scripting introduction
23.4. comment
Let's expand our example a little further by adding comment lines.
#!/bin/bash
#
# Hello World Script
#
echo Hello World
23.5. variables
Here is a simple example of a variable inside a script.
#!/bin/bash
#
# simple variable in script
#
var1=4
echo var1 = $var1
Scripts can contain variables, but since scripts are run in their own shell, the variables do
not survive the end of the script.
[paul@RHEL4a ~]$ echo $var1
[paul@RHEL4a ~]$ ./vars
var1 = 4
[paul@RHEL4a ~]$ echo $var1
[paul@RHEL4a ~]$
23.6. sourcing a script
Luckily, you can force a script to run in the same shell; this is called sourcing a script.
[paul@RHEL4a ~]$ source ./vars
var1 = 4
[paul@RHEL4a ~]$ echo $var1
4
[paul@RHEL4a ~]$
The above is identical to the below.
[paul@RHEL4a ~]$ . ./vars
var1 = 4
[paul@RHEL4a ~]$ echo $var1
4
[paul@RHEL4a ~]$
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scripting introduction
23.7. troubleshooting a script
Another way to run a script in a separate shell is by typing bash with the name of the script
as a parameter.
paul@debian6~/test$ bash runme
42
Expanding this to bash -x allows you to see the commands that the shell is executing (after
shell expansion).
paul@debian6~/test$ bash -x runme
+ var4=42
+ echo 42
42
paul@debian6~/test$ cat runme
# the runme script
var4=42
echo $var4
paul@debian6~/test$
Notice the absence of the commented (#) line, and the replacement of the variable before
execution of echo.
23.8. prevent setuid root spoofing
Some user may try to perform setuid based script root spoofing. This is a rare but possible
attack. To improve script security and to avoid interpreter spoofing, you need to add -- after
the #!/bin/bash, which disables further option processing so the shell will not accept any
options.
#!/bin/bash or
#!/bin/bash --
Any arguments after the -- are treated as filenames and arguments. An argument of - is
equivalent to --.
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scripting introduction
23.9. practice: introduction to scripting
0. Give each script a different name, keep them for later!
1. Write a script that outputs the name of a city.
2. Make sure the script runs in the bash shell.
3. Make sure the script runs in the Korn shell.
4. Create a script that defines two variables, and outputs their value.
5. The previous script does not influence your current shell (the variables do not exist outside
of the script). Now run the script so that it influences your current shell.
6. Is there a shorter way to source the script ?
7. Comment your scripts so that you know what they are doing.
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scripting introduction
23.10. solution: introduction to scripting
0. Give each script a different name, keep them for later!
1. Write a script that outputs the name of a city.
$ echo 'echo Antwerp' > first.bash
$ chmod +x first.bash
$ ./first.bash
Antwerp
2. Make sure the script runs in the bash shell.
$ cat first.bash
#!/bin/bash
echo Antwerp
3. Make sure the script runs in the Korn shell.
$ cat first.bash
#!/bin/ksh
echo Antwerp
Note that while first.bash will technically work as a Korn shell script, the name ending
in .bash is confusing.
4. Create a script that defines two variables, and outputs their value.
$ cat second.bash
#!/bin/bash
var33=300
var42=400
echo $var33 $var42
5. The previous script does not influence your current shell (the variables do not exist outside
of the script). Now run the script so that it influences your current shell.
source second.bash
6. Is there a shorter way to source the script ?
. ./second.bash
7. Comment your scripts so that you know what they are doing.
$ cat second.bash
#!/bin/bash
# script to test variables and sourcing
# define two variables
var33=300
var42=400
# output the value of these variables
echo $var33 $var42
238
Chapter 24. scripting loops
239
scripting loops
24.1. test [ ]
The test command can test whether something is true or false. Let's start by testing whether
10 is greater than 55.
[paul@RHEL4b ~]$ test 10 -gt 55 ; echo $?
1
[paul@RHEL4b ~]$
The test command returns 1 if the test fails. And as you see in the next screenshot, test returns
0 when a test succeeds.
[paul@RHEL4b ~]$ test 56 -gt 55 ; echo $?
0
[paul@RHEL4b ~]$
If you prefer true and false, then write the test like this.
[paul@RHEL4b ~]$ test 56 -gt 55 && echo true || echo false
true
[paul@RHEL4b ~]$ test 6 -gt 55 && echo true || echo false
false
The test command can also be written as square brackets, the screenshot below is identical
to the one above.
[paul@RHEL4b ~]$ [ 56 -gt 55 ] && echo true || echo false
true
[paul@RHEL4b ~]$ [ 6 -gt 55 ] && echo true || echo false
false
Below are some example tests. Take a look at man test to see more options for tests.
[
[
[
[
[
[
[
[
[
[
[
-d foo ]
-e bar ]
'/etc' = $PWD ]
$1 != 'secret' ]
55 -lt $bar ]
$foo -ge 1000 ]
"abc" < $bar ]
-f foo ]
-r bar ]
foo -nt bar ]
-o nounset ]
Does the directory foo exist ?
Does the file bar exist ?
Is the string /etc equal to the variable $PWD ?
Is the first parameter different from secret ?
Is 55 less than the value of $bar ?
Is the value of $foo greater or equal to 1000 ?
Does abc sort before the value of $bar ?
Is foo a regular file ?
Is bar a readable file ?
Is file foo newer than file bar ?
Is the shell option nounset set ?
Tests can be combined with logical AND and OR.
paul@RHEL4b:~$ [ 66 -gt 55 -a 66 -lt 500 ] && echo true || echo false
true
paul@RHEL4b:~$ [ 66 -gt 55 -a 660 -lt 500 ] && echo true || echo false
false
paul@RHEL4b:~$ [ 66 -gt 55 -o 660 -lt 500 ] && echo true || echo false
true
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scripting loops
24.2. if then else
The if then else construction is about choice. If a certain condition is met, then execute
something, else execute something else. The example below tests whether a file exists, and
if the file exists then a proper message is echoed.
#!/bin/bash
if [ -f isit.txt ]
then echo isit.txt exists!
else echo isit.txt not found!
fi
If we name the above script 'choice', then it executes like this.
[paul@RHEL4a scripts]$ ./choice
isit.txt not found!
[paul@RHEL4a scripts]$ touch isit.txt
[paul@RHEL4a scripts]$ ./choice
isit.txt exists!
[paul@RHEL4a scripts]$
24.3. if then elif
You can nest a new if inside an else with elif. This is a simple example.
#!/bin/bash
count=42
if [ $count -eq 42 ]
then
echo "42 is correct."
elif [ $count -gt 42 ]
then
echo "Too much."
else
echo "Not enough."
fi
24.4. for loop
The example below shows the syntax of a classical for loop in bash.
for i in 1 2 4
do
echo $i
done
An example of a for loop combined with an embedded shell.
#!/bin/ksh
for counter in `seq 1 20`
do
echo counting from 1 to 20, now at $counter
sleep 1
done
The same example as above can be written without the embedded shell using the bash
{from..to} shorthand.
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scripting loops
#!/bin/bash
for counter in {1..20}
do
echo counting from 1 to 20, now at $counter
sleep 1
done
This for loop uses file globbing (from the shell expansion). Putting the instruction on the
command line has identical functionality.
kahlan@solexp11$ ls
count.ksh go.ksh
kahlan@solexp11$ for file in *.ksh ; do cp $file $file.backup ; done
kahlan@solexp11$ ls
count.ksh count.ksh.backup go.ksh go.ksh.backup
24.5. while loop
Below a simple example of a while loop.
i=100;
while [ $i -ge 0 ] ;
do
echo Counting down, from 100 to 0, now at $i;
let i--;
done
Endless loops can be made with while true or while : , where the colon is the equivalent
of no operation in the Korn and bash shells.
#!/bin/ksh
# endless loop
while :
do
echo hello
sleep 1
done
24.6. until loop
Below a simple example of an until loop.
let i=100;
until [ $i -le 0 ] ;
do
echo Counting down, from 100 to 1, now at $i;
let i--;
done
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scripting loops
24.7. practice: scripting tests and loops
1. Write a script that uses a for loop to count from 3 to 7.
2. Write a script that uses a for loop to count from 1 to 17000.
3. Write a script that uses a while loop to count from 3 to 7.
4. Write a script that uses an until loop to count down from 8 to 4.
5. Write a script that counts the number of files ending in .txt in the current directory.
6. Wrap an if statement around the script so it is also correct when there are zero files ending
in .txt.
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scripting loops
24.8. solution: scripting tests and loops
1. Write a script that uses a for loop to count from 3 to 7.
#!/bin/bash
for i in 3 4 5 6 7
do
echo Counting from 3 to 7, now at $i
done
2. Write a script that uses a for loop to count from 1 to 17000.
#!/bin/bash
for i in `seq 1 17000`
do
echo Counting from 1 to 17000, now at $i
done
3. Write a script that uses a while loop to count from 3 to 7.
#!/bin/bash
i=3
while [ $i -le 7 ]
do
echo Counting from 3 to 7, now at $i
let i=i+1
done
4. Write a script that uses an until loop to count down from 8 to 4.
#!/bin/bash
i=8
until [ $i -lt 4 ]
do
echo Counting down from 8 to 4, now at $i
let i=i-1
done
5. Write a script that counts the number of files ending in .txt in the current directory.
#!/bin/bash
let i=0
for file in *.txt
do
let i++
done
echo "There are $i files ending in .txt"
6. Wrap an if statement around the script so it is also correct when there are zero files ending
in .txt.
#!/bin/bash
ls *.txt > /dev/null 2>&1
if [ $? -ne 0 ]
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scripting loops
then echo "There are 0 files ending in .txt"
else
let i=0
for file in *.txt
do
let i++
done
echo "There are $i files ending in .txt"
fi
245
Chapter 25. scripting parameters
246
scripting parameters
25.1. script parameters
A bash shell script can have parameters. The numbering you see in the script below
continues if you have more parameters. You also have special parameters containing the
number of parameters, a string of all of them, and also the process id, and the last return
code. The man page of bash has a full list.
#!/bin/bash
echo The first argument is $1
echo The second argument is $2
echo The third argument is $3
echo
echo
echo
echo
\$
\#
\?
\*
$$
$#
$?
$*
PID of the script
count arguments
last return code
all the arguments
Below is the output of the script above in action.
[paul@RHEL4a scripts]$ ./pars one two three
The first argument is one
The second argument is two
The third argument is three
$ 5610 PID of the script
# 3 count arguments
? 0 last return code
* one two three all the arguments
Once more the same script, but with only two parameters.
[paul@RHEL4a scripts]$ ./pars 1 2
The first argument is 1
The second argument is 2
The third argument is
$ 5612 PID of the script
# 2 count arguments
? 0 last return code
* 1 2 all the arguments
[paul@RHEL4a scripts]$
Here is another example, where we use $0. The $0 parameter contains the name of the script.
paul@debian6~$ cat myname
echo this script is called $0
paul@debian6~$ ./myname
this script is called ./myname
paul@debian6~$ mv myname test42
paul@debian6~$ ./test42
this script is called ./test42
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scripting parameters
25.2. shift through parameters
The shift statement can parse all parameters one by one. This is a sample script.
kahlan@solexp11$ cat shift.ksh
#!/bin/ksh
if [ "$#" == "0" ]
then
echo You have to give at least one parameter.
exit 1
fi
while (( $# ))
do
echo You gave me $1
shift
done
Below is some sample output of the script above.
kahlan@solexp11$ ./shift.ksh one
You gave me one
kahlan@solexp11$ ./shift.ksh one two three 1201 "33 42"
You gave me one
You gave me two
You gave me three
You gave me 1201
You gave me 33 42
kahlan@solexp11$ ./shift.ksh
You have to give at least one parameter.
25.3. runtime input
You can ask the user for input with the read command in a script.
#!/bin/bash
echo -n Enter a number:
read number
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scripting parameters
25.4. sourcing a config file
The source (as seen in the shell chapters) can be used to source a configuration file.
Below a sample configuration file for an application.
[paul@RHEL4a scripts]$ cat myApp.conf
# The config file of myApp
# Enter the path here
myAppPath=/var/myApp
# Enter the number of quines here
quines=5
And here an application that uses this file.
[paul@RHEL4a scripts]$ cat myApp.bash
#!/bin/bash
#
# Welcome to the myApp application
#
. ./myApp.conf
echo There are $quines quines
The running application can use the values inside the sourced configuration file.
[paul@RHEL4a scripts]$ ./myApp.bash
There are 5 quines
[paul@RHEL4a scripts]$
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scripting parameters
25.5. get script options with getopts
The getopts function allows you to parse options given to a command. The following script
allows for any combination of the options a, f and z.
kahlan@solexp11$ cat options.ksh
#!/bin/ksh
while getopts ":afz" option;
do
case $option in
a)
echo received -a
;;
f)
echo received -f
;;
z)
echo received -z
;;
*)
echo "invalid option -$OPTARG"
;;
esac
done
This is sample output from the script above. First we use correct options, then we enter twice
an invalid option.
kahlan@solexp11$ ./options.ksh
kahlan@solexp11$ ./options.ksh -af
received -a
received -f
kahlan@solexp11$ ./options.ksh -zfg
received -z
received -f
invalid option -g
kahlan@solexp11$ ./options.ksh -a -b -z
received -a
invalid option -b
received -z
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scripting parameters
You can also check for options that need an argument, as this example shows.
kahlan@solexp11$ cat argoptions.ksh
#!/bin/ksh
while getopts ":af:z" option;
do
case $option in
a)
echo received -a
;;
f)
echo received -f with $OPTARG
;;
z)
echo received -z
;;
:)
echo "option -$OPTARG needs an argument"
;;
*)
echo "invalid option -$OPTARG"
;;
esac
done
This is sample output from the script above.
kahlan@solexp11$ ./argoptions.ksh -a -f hello -z
received -a
received -f with hello
received -z
kahlan@solexp11$ ./argoptions.ksh -zaf 42
received -z
received -a
received -f with 42
kahlan@solexp11$ ./argoptions.ksh -zf
received -z
option -f needs an argument
25.6. get shell options with shopt
You can toggle the values of variables controlling optional shell behaviour with the shopt
built-in shell command. The example below first verifies whether the cdspell option is set;
it is not. The next shopt command sets the value, and the third shopt command verifies that
the option really is set. You can now use minor spelling mistakes in the cd command. The
man page of bash has a complete list of options.
paul@laika:~$
1
paul@laika:~$
paul@laika:~$
0
paul@laika:~$
/etc
shopt -q cdspell ; echo $?
shopt -s cdspell
shopt -q cdspell ; echo $?
cd /Etc
251
scripting parameters
25.7. practice: parameters and options
1. Write a script that receives four parameters, and outputs them in reverse order.
2. Write a script that receives two parameters (two filenames) and outputs whether those
files exist.
3. Write a script that asks for a filename. Verify existence of the file, then verify that you
own the file, and whether it is writable. If not, then make it writable.
4. Make a configuration file for the previous script. Put a logging switch in the config file,
logging means writing detailed output of everything the script does to a log file in /tmp.
252
scripting parameters
25.8. solution: parameters and options
1. Write a script that receives four parameters, and outputs them in reverse order.
echo $4 $3 $2 $1
2. Write a script that receives two parameters (two filenames) and outputs whether those
files exist.
#!/bin/bash
if [ -f $1 ]
then echo $1 exists!
else echo $1 not found!
fi
if [ -f $2 ]
then echo $2 exists!
else echo $2 not found!
fi
3. Write a script that asks for a filename. Verify existence of the file, then verify that you
own the file, and whether it is writable. If not, then make it writable.
4. Make a configuration file for the previous script. Put a logging switch in the config file,
logging means writing detailed output of everything the script does to a log file in /tmp.
253
Chapter 26. more scripting
254
more scripting
26.1. eval
eval reads arguments as input to the shell (the resulting commands are executed). This allows
using the value of a variable as a variable.
paul@deb503:~/test42$ answer=42
paul@deb503:~/test42$ word=answer
paul@deb503:~/test42$ eval x=\$$word ; echo $x
42
Both in bash and Korn the arguments can be quoted.
kahlan@solexp11$ answer=42
kahlan@solexp11$ word=answer
kahlan@solexp11$ eval "y=\$$word" ; echo $y
42
Sometimes the eval is needed to have correct parsing of arguments. Consider this example
where the date command receives one parameter 1 week ago.
paul@debian6~$ date --date="1 week ago"
Thu Mar 8 21:36:25 CET 2012
When we set this command in a variable, then executing that variable fails unless we use
eval.
paul@debian6~$ lastweek='date --date="1 week ago"'
paul@debian6~$ $lastweek
date: extra operand `ago"'
Try `date --help' for more information.
paul@debian6~$ eval $lastweek
Thu Mar 8 21:36:39 CET 2012
26.2. (( ))
The (( )) allows for evaluation of numerical expressions.
paul@deb503:~/test42$
true
paul@deb503:~/test42$
false
paul@deb503:~/test42$
paul@deb503:~/test42$
true
paul@deb503:~/test42$
true
paul@deb503:~/test42$
paul@deb503:~/test42$
false
(( 42 > 33 )) && echo true || echo false
(( 42 > 1201 )) && echo true || echo false
var42=42
(( 42 == var42 )) && echo true || echo false
(( 42 == $var42 )) && echo true || echo false
var42=33
(( 42 == var42 )) && echo true || echo false
255
more scripting
26.3. let
The let built-in shell function instructs the shell to perform an evaluation of arithmetic
expressions. It will return 0 unless the last arithmetic expression evaluates to 0.
[paul@RHEL4b
7
[paul@RHEL4b
20
[paul@RHEL4b
18
[paul@RHEL4b
30
~]$ let x="3 + 4" ; echo $x
~]$ let x="10 + 100/10" ; echo $x
~]$ let x="10-2+100/10" ; echo $x
~]$ let x="10*2+100/10" ; echo $x
The shell can also convert between different bases.
[paul@RHEL4b
255
[paul@RHEL4b
192
[paul@RHEL4b
168
[paul@RHEL4b
56
[paul@RHEL4b
63
[paul@RHEL4b
192
~]$ let x="0xFF" ; echo $x
~]$ let x="0xC0" ; echo $x
~]$ let x="0xA8" ; echo $x
~]$ let x="8#70" ; echo $x
~]$ let x="8#77" ; echo $x
~]$ let x="16#c0" ; echo $x
There is a difference between assigning a variable directly, or using let to evaluate the
arithmetic expressions (even if it is just assigning a value).
kahlan@solexp11$
kahlan@solexp11$
15 017 0x0f
kahlan@solexp11$
kahlan@solexp11$
15 15 15
dec=15 ; oct=017 ; hex=0x0f
echo $dec $oct $hex
let dec=15 ; let oct=017 ; let hex=0x0f
echo $dec $oct $hex
256
more scripting
26.4. case
You can sometimes simplify nested if statements with a case construct.
[paul@RHEL4b ~]$ ./help
What animal did you see ? lion
You better start running fast!
[paul@RHEL4b ~]$ ./help
What animal did you see ? dog
Don't worry, give it a cookie.
[paul@RHEL4b ~]$ cat help
#!/bin/bash
#
# Wild Animals Helpdesk Advice
#
echo -n "What animal did you see ? "
read animal
case $animal in
"lion" | "tiger")
echo "You better start running fast!"
;;
"cat")
echo "Let that mouse go..."
;;
"dog")
echo "Don't worry, give it a cookie."
;;
"chicken" | "goose" | "duck" )
echo "Eggs for breakfast!"
;;
"liger")
echo "Approach and say 'Ah you big fluffy kitty...'."
;;
"babelfish")
echo "Did it fall out your ear ?"
;;
*)
echo "You discovered an unknown animal, name it!"
;;
esac
[paul@RHEL4b ~]$
257
more scripting
26.5. shell functions
Shell functions can be used to group commands in a logical way.
kahlan@solexp11$ cat funcs.ksh
#!/bin/ksh
function greetings {
echo Hello World!
echo and hello to $USER to!
}
echo We will now call a function
greetings
echo The end
This is sample output from this script with a function.
kahlan@solexp11$ ./funcs.ksh
We will now call a function
Hello World!
and hello to kahlan to!
The end
A shell function can also receive parameters.
kahlan@solexp11$ cat addfunc.ksh
#!/bin/ksh
function plus {
let result="$1 + $2"
echo $1 + $2 = $result
}
plus 3 10
plus 20 13
plus 20 22
This script produces the following output.
kahlan@solexp11$ ./addfunc.ksh
3 + 10 = 13
20 + 13 = 33
20 + 22 = 42
258
more scripting
26.6. practice : more scripting
1. Write a script that asks for two numbers, and outputs the sum and product (as shown here).
Enter a number: 5
Enter another number: 2
Sum:
Product:
5 + 2 = 7
5 x 2 = 10
2. Improve the previous script to test that the numbers are between 1 and 100, exit with an
error if necessary.
3. Improve the previous script to congratulate the user if the sum equals the product.
4. Write a script with a case insensitive case statement, using the shopt nocasematch option.
The nocasematch option is reset to the value it had before the scripts started.
5. If time permits (or if you are waiting for other students to finish this practice), take a look
at Linux system scripts in /etc/init.d and /etc/rc.d and try to understand them. Where does
execution of a script start in /etc/init.d/samba ? There are also some hidden scripts in ~, we
will discuss them later.
259
more scripting
26.7. solution : more scripting
1. Write a script that asks for two numbers, and outputs the sum and product (as shown here).
Enter a number: 5
Enter another number: 2
Sum:
Product:
5 + 2 = 7
5 x 2 = 10
#!/bin/bash
echo -n "Enter a number : "
read n1
echo -n "Enter another number : "
read n2
let sum="$n1+$n2"
let pro="$n1*$n2"
echo -e "Sum\t: $n1 + $n2 = $sum"
echo -e "Product\t: $n1 * $n2 = $pro"
2. Improve the previous script to test that the numbers are between 1 and 100, exit with an
error if necessary.
echo -n "Enter a number between 1 and 100 : "
read n1
if [ $n1 -lt 1 -o $n1 -gt 100 ]
then
echo Wrong number...
exit 1
fi
3. Improve the previous script to congratulate the user if the sum equals the product.
if [ $sum -eq $pro ]
then echo Congratulations $sum == $pro
fi
4. Write a script with a case insensitive case statement, using the shopt nocasematch option.
The nocasematch option is reset to the value it had before the scripts started.
#!/bin/bash
#
# Wild Animals Case Insensitive Helpdesk Advice
#
if shopt -q nocasematch; then
nocase=yes;
else
nocase=no;
shopt -s nocasematch;
fi
echo -n "What animal did you see ? "
read animal
case $animal in
260
more scripting
"lion" | "tiger")
echo "You better start running fast!"
;;
"cat")
echo "Let that mouse go..."
;;
"dog")
echo "Don't worry, give it a cookie."
;;
"chicken" | "goose" | "duck" )
echo "Eggs for breakfast!"
;;
"liger")
echo "Approach and say 'Ah you big fluffy kitty.'"
;;
"babelfish")
echo "Did it fall out your ear ?"
;;
*)
echo "You discovered an unknown animal, name it!"
;;
esac
if [ nocase = yes ] ; then
shopt -s nocasematch;
else
shopt -u nocasematch;
fi
5. If time permits (or if you are waiting for other students to finish this practice), take a look
at Linux system scripts in /etc/init.d and /etc/rc.d and try to understand them. Where does
execution of a script start in /etc/init.d/samba ? There are also some hidden scripts in ~, we
will discuss them later.
261
Part VIII. local user management
Table of Contents
27. introduction to users ...................................................................................................... 265
27.1. whoami ............................................................................................................... 266
27.2. who .................................................................................................................... 266
27.3. who am i ............................................................................................................. 266
27.4. w ....................................................................................................................... 266
27.5. id ....................................................................................................................... 266
27.6. su to another user ................................................................................................. 267
27.7. su to root ............................................................................................................ 267
27.8. su as root ............................................................................................................ 267
27.9. su - $username ..................................................................................................... 267
27.10. su - ................................................................................................................... 267
27.11. run a program as another user ............................................................................... 268
27.12. visudo ............................................................................................................... 268
27.13. sudo su - ........................................................................................................... 269
27.14. sudo logging ...................................................................................................... 269
27.15. practice: introduction to users ................................................................................ 270
27.16. solution: introduction to users ................................................................................ 271
28. user management ........................................................................................................... 273
28.1. user management .................................................................................................. 274
28.2. /etc/passwd ........................................................................................................... 274
28.3. root .................................................................................................................... 274
28.4. useradd ............................................................................................................... 275
28.5. /etc/default/useradd ................................................................................................ 275
28.6. userdel ................................................................................................................ 275
28.7. usermod .............................................................................................................. 275
28.8. creating home directories ....................................................................................... 276
28.9. /etc/skel/ .............................................................................................................. 276
28.10. deleting home directories ...................................................................................... 276
28.11. login shell .......................................................................................................... 277
28.12. chsh .................................................................................................................. 277
28.13. practice: user management .................................................................................... 278
28.14. solution: user management .................................................................................... 279
29. user passwords .............................................................................................................. 281
29.1. passwd ................................................................................................................ 282
29.2. shadow file .......................................................................................................... 282
29.3. encryption with passwd .......................................................................................... 283
29.4. encryption with openssl ......................................................................................... 283
29.5. encryption with crypt ............................................................................................. 284
29.6. /etc/login.defs ....................................................................................................... 285
29.7. chage .................................................................................................................. 285
29.8. disabling a password ............................................................................................. 286
29.9. editing local files .................................................................................................. 286
29.10. practice: user passwords ....................................................................................... 287
29.11. solution: user passwords ....................................................................................... 288
30. user profiles .................................................................................................................. 290
30.1. system profile ...................................................................................................... 291
30.2. ~/.bash_profile ...................................................................................................... 291
30.3. ~/.bash_login ........................................................................................................ 292
30.4. ~/.profile ............................................................................................................. 292
30.5. ~/.bashrc ............................................................................................................. 292
30.6. ~/.bash_logout ...................................................................................................... 293
30.7. Debian overview ................................................................................................... 294
30.8. RHEL5 overview .................................................................................................. 294
30.9. practice: user profiles ............................................................................................ 295
30.10. solution: user profiles ........................................................................................... 296
263
local user management
31. groups ..........................................................................................................................
31.1. groupadd .............................................................................................................
31.2. group file ............................................................................................................
31.3. groups .................................................................................................................
31.4. usermod ..............................................................................................................
31.5. groupmod ............................................................................................................
31.6. groupdel ..............................................................................................................
31.7. gpasswd ..............................................................................................................
31.8. newgrp ................................................................................................................
31.9. vigr ....................................................................................................................
31.10. practice: groups ..................................................................................................
31.11. solution: groups ..................................................................................................
264
297
298
298
298
299
299
299
300
301
301
302
303
Chapter 27. introduction to users
This little chapter will teach you how to identify your user account on a Unix computer using
commands like who am i, id, and more.
In a second part you will learn how to become another user with the su command.
And you will learn how to run a program as another user with sudo.
265
introduction to users
27.1. whoami
The whoami command tells you your username.
[paul@centos7 ~]$ whoami
paul
[paul@centos7 ~]$
27.2. who
The who command will give you information about who is logged on the system.
[paul@centos7 ~]$ who
root
pts/0
2014-10-10 23:07 (10.104.33.101)
paul
pts/1
2014-10-10 23:30 (10.104.33.101)
laura
pts/2
2014-10-10 23:34 (10.104.33.96)
tania
pts/3
2014-10-10 23:39 (10.104.33.91)
[paul@centos7 ~]$
27.3. who am i
With who am i the who command will display only the line pointing to your current session.
[paul@centos7 ~]$ who am i
paul
pts/1
2014-10-10 23:30 (10.104.33.101)
[paul@centos7 ~]$
27.4. w
The w command shows you who is logged on and what they are doing.
[paul@centos7 ~]$ w
23:34:07 up 31 min, 2 users, load average: 0.00, 0.01, 0.02
USER
TTY
LOGIN@
IDLE
JCPU
PCPU WHAT
root
pts/0
23:07
15.00s 0.01s 0.01s top
paul
pts/1
23:30
7.00s 0.00s 0.00s w
[paul@centos7 ~]$
27.5. id
The id command will give you your user id, primary group id, and a list of the groups that
you belong to.
paul@debian7:~$ id
uid=1000(paul) gid=1000(paul) groups=1000(paul)
On RHEL/CentOS you will also get SELinux context information with this command.
[root@centos7 ~]# id
uid=0(root) gid=0(root) groups=0(root) context=unconfined_u:unconfined_r\
:unconfined_t:s0-s0:c0.c1023
266
introduction to users
27.6. su to another user
The su command allows a user to run a shell as another user.
laura@debian7:~$ su tania
Password:
tania@debian7:/home/laura$
27.7. su to root
Yes you can also su to become root, when you know the root password.
laura@debian7:~$ su root
Password:
root@debian7:/home/laura#
27.8. su as root
You need to know the password of the user you want to substitute to, unless your are logged
in as root. The root user can become any existing user without knowing that user's password.
root@debian7:~# id
uid=0(root) gid=0(root) groups=0(root)
root@debian7:~# su - valentina
valentina@debian7:~$
27.9. su - $username
By default, the su command maintains the same shell environment. To become another user
and also get the target user's environment, issue the su - command followed by the target
username.
root@debian7:~# su laura
laura@debian7:/root$ exit
exit
root@debian7:~# su - laura
laura@debian7:~$ pwd
/home/laura
27.10. su When no username is provided to su or su -, the command will assume root is the target.
tania@debian7:~$ su Password:
root@debian7:~#
267
introduction to users
27.11. run a program as another user
The sudo program allows a user to start a program with the credentials of another user.
Before this works, the system administrator has to set up the /etc/sudoers file. This can be
useful to delegate administrative tasks to another user (without giving the root password).
The screenshot below shows the usage of sudo. User paul received the right to run useradd
with the credentials of root. This allows paul to create new users on the system without
becoming root and without knowing the root password.
First the command fails for paul.
paul@debian7:~$ /usr/sbin/useradd -m valentina
useradd: Permission denied.
useradd: cannot lock /etc/passwd; try again later.
But with sudo it works.
paul@debian7:~$ sudo /usr/sbin/useradd -m valentina
[sudo] password for paul:
paul@debian7:~$
27.12. visudo
Check the man page of visudo before playing with the /etc/sudoers file. Editing the sudoers
is out of scope for this fundamentals book.
paul@rhel65:~$ apropos visudo
visudo
(8) - edit the sudoers file
paul@rhel65:~$
268
introduction to users
27.13. sudo su On some Linux systems like Ubuntu and Xubuntu, the root user does not have a password
set. This means that it is not possible to login as root (extra security). To perform tasks as
root, the first user is given all sudo rights via the /etc/sudoers. In fact all users that are
members of the admin group can use sudo to run all commands as root.
root@laika:~# grep admin /etc/sudoers
# Members of the admin group may gain root privileges
%admin ALL=(ALL) ALL
The end result of this is that the user can type sudo su - and become root without having to
enter the root password. The sudo command does require you to enter your own password.
Thus the password prompt in the screenshot below is for sudo, not for su.
paul@laika:~$ sudo su Password:
root@laika:~#
27.14. sudo logging
Using sudo without authorization will result in a severe warning:
paul@rhel65:~$ sudo su We trust you have received the usual lecture from the local System
Administrator. It usually boils down to these three things:
#1) Respect the privacy of others.
#2) Think before you type.
#3) With great power comes great responsibility.
[sudo] password for paul:
paul is not in the sudoers file.
paul@rhel65:~$
This incident will be reported.
The root user can see this in the /var/log/secure on Red Hat and in /var/log/auth.log on
Debian).
root@rhel65:~# tail /var/log/secure | grep sudo | tr -s ' '
Apr 13 16:03:42 rhel65 sudo: paul : user NOT in sudoers ; TTY=pts/0 ; PWD=\
/home/paul ; USER=root ; COMMAND=/bin/su root@rhel65:~#
269
introduction to users
27.15. practice: introduction to users
1. Run a command that displays only your currently logged on user name.
2. Display a list of all logged on users.
3. Display a list of all logged on users including the command they are running at this very
moment.
4. Display your user name and your unique user identification (userid).
5. Use su to switch to another user account (unless you are root, you will need the password
of the other account). And get back to the previous account.
6. Now use su - to switch to another user and notice the difference.
Note that su - gets you into the home directory of Tania.
7. Try to create a new user account (when using your normal user account). this should fail.
(Details on adding user accounts are explained in the next chapter.)
8. Now try the same, but with sudo before your command.
270
introduction to users
27.16. solution: introduction to users
1. Run a command that displays only your currently logged on user name.
laura@debian7:~$ whoami
laura
laura@debian7:~$ echo $USER
laura
2. Display a list of all logged on users.
laura@debian7:~$ who
laura
pts/0
laura@debian7:~$
2014-10-13 07:22 (10.104.33.101)
3. Display a list of all logged on users including the command they are running at this very
moment.
laura@debian7:~$ w
07:47:02 up 16 min, 2 users, load average: 0.00, 0.00,
USER
TTY
FROM
LOGIN@
IDLE
JCPU
root
pts/0
10.104.33.101
07:30
6.00s 0.04s
root
pts/1
10.104.33.101
07:46
6.00s 0.01s
laura@debian7:~$
0.00
PCPU WHAT
0.00s w
0.00s sleep 42
4. Display your user name and your unique user identification (userid).
laura@debian7:~$ id
uid=1005(laura) gid=1007(laura) groups=1007(laura)
laura@debian7:~$
5. Use su to switch to another user account (unless you are root, you will need the password
of the other account). And get back to the previous account.
laura@debian7:~$ su tania
Password:
tania@debian7:/home/laura$ id
uid=1006(tania) gid=1008(tania) groups=1008(tania)
tania@debian7:/home/laura$ exit
laura@debian7:~$
6. Now use su - to switch to another user and notice the difference.
laura@debian7:~$ su - tania
Password:
tania@debian7:~$ pwd
/home/tania
tania@debian7:~$ logout
laura@debian7:~$
Note that su - gets you into the home directory of Tania.
271
introduction to users
7. Try to create a new user account (when using your normal user account). this should fail.
(Details on adding user accounts are explained in the next chapter.)
laura@debian7:~$ useradd valentina
-su: useradd: command not found
laura@debian7:~$ /usr/sbin/useradd valentina
useradd: Permission denied.
useradd: cannot lock /etc/passwd; try again later.
It is possible that useradd is located in /sbin/useradd on your computer.
8. Now try the same, but with sudo before your command.
laura@debian7:~$ sudo /usr/sbin/useradd valentina
[sudo] password for laura:
laura is not in the sudoers file. This incident will be reported.
laura@debian7:~$
Notice that laura has no permission to use the sudo on this system.
272
Chapter 28. user management
This chapter will teach you how to use useradd, usermod and userdel to create, modify
and remove user accounts.
You will need root access on a Linux computer to complete this chapter.
273
user management
28.1. user management
User management on Linux can be done in three complimentary ways. You can use the
graphical tools provided by your distribution. These tools have a look and feel that depends
on the distribution. If you are a novice Linux user on your home system, then use the
graphical tool that is provided by your distribution. This will make sure that you do not run
into problems.
Another option is to use command line tools like useradd, usermod, gpasswd, passwd and
others. Server administrators are likely to use these tools, since they are familiar and very
similar across many different distributions. This chapter will focus on these command line
tools.
A third and rather extremist way is to edit the local configuration files directly using vi (or
vipw/vigr). Do not attempt this as a novice on production systems!
28.2. /etc/passwd
The local user database on Linux (and on most Unixes) is /etc/passwd.
[root@RHEL5 ~]# tail /etc/passwd
inge:x:518:524:art dealer:/home/inge:/bin/ksh
ann:x:519:525:flute player:/home/ann:/bin/bash
frederik:x:520:526:rubius poet:/home/frederik:/bin/bash
steven:x:521:527:roman emperor:/home/steven:/bin/bash
pascale:x:522:528:artist:/home/pascale:/bin/ksh
geert:x:524:530:kernel developer:/home/geert:/bin/bash
wim:x:525:531:master damuti:/home/wim:/bin/bash
sandra:x:526:532:radish stresser:/home/sandra:/bin/bash
annelies:x:527:533:sword fighter:/home/annelies:/bin/bash
laura:x:528:534:art dealer:/home/laura:/bin/ksh
As you can see, this file contains seven columns separated by a colon. The columns contain
the username, an x, the user id, the primary group id, a description, the name of the home
directory, and the login shell.
More information can be found by typing man 5 passwd.
[root@RHEL5 ~]# man 5 passwd
28.3. root
The root user also called the superuser is the most powerful account on your Linux system.
This user can do almost anything, including the creation of other users. The root user always
has userid 0 (regardless of the name of the account).
[root@RHEL5 ~]# head -1 /etc/passwd
root:x:0:0:root:/root:/bin/bash
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28.4. useradd
You can add users with the useradd command. The example below shows how to add a
user named yanina (last parameter) and at the same time forcing the creation of the home
directory (-m), setting the name of the home directory (-d), and setting a description (-c).
[root@RHEL5 ~]# useradd -m -d /home/yanina -c "yanina wickmayer" yanina
[root@RHEL5 ~]# tail -1 /etc/passwd
yanina:x:529:529:yanina wickmayer:/home/yanina:/bin/bash
The user named yanina received userid 529 and primary group id 529.
28.5. /etc/default/useradd
Both Red Hat Enterprise Linux and Debian/Ubuntu have a file called /etc/default/useradd
that contains some default user options. Besides using cat to display this file, you can also
use useradd -D.
[root@RHEL4 ~]# useradd -D
GROUP=100
HOME=/home
INACTIVE=-1
EXPIRE=
SHELL=/bin/bash
SKEL=/etc/skel
28.6. userdel
You can delete the user yanina with userdel. The -r option of userdel will also remove the
home directory.
[root@RHEL5 ~]# userdel -r yanina
28.7. usermod
You can modify the properties of a user with the usermod command. This example uses
usermod to change the description of the user harry.
[root@RHEL4 ~]# tail -1 /etc/passwd
harry:x:516:520:harry potter:/home/harry:/bin/bash
[root@RHEL4 ~]# usermod -c 'wizard' harry
[root@RHEL4 ~]# tail -1 /etc/passwd
harry:x:516:520:wizard:/home/harry:/bin/bash
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28.8. creating home directories
The easiest way to create a home directory is to supply the -m option with useradd (it is
likely set as a default option on Linux).
A less easy way is to create a home directory manually with mkdir which also requires
setting the owner and the permissions on the directory with chmod and chown (both
commands are discussed in detail in another chapter).
[root@RHEL5 ~]# mkdir /home/laura
[root@RHEL5 ~]# chown laura:laura /home/laura
[root@RHEL5 ~]# chmod 700 /home/laura
[root@RHEL5 ~]# ls -ld /home/laura/
drwx------ 2 laura laura 4096 Jun 24 15:17 /home/laura/
28.9. /etc/skel/
When using useradd the -m option, the /etc/skel/ directory is copied to the newly created
home directory. The /etc/skel/ directory contains some (usually hidden) files that contain
profile settings and default values for applications. In this way /etc/skel/ serves as a default
home directory and as a default user profile.
[root@RHEL5 ~]# ls
total 48
drwxr-xr-x 2 root
drwxr-xr-x 97 root
-rw-r--r-- 1 root
-rw-r--r-- 1 root
-rw-r--r-- 1 root
-la /etc/skel/
root 4096 Apr 1 00:11 .
root 12288 Jun 24 15:36 ..
root
24 Jul 12 2006 .bash_logout
root
176 Jul 12 2006 .bash_profile
root
124 Jul 12 2006 .bashrc
28.10. deleting home directories
The -r option of userdel will make sure that the home directory is deleted together with the
user account.
[root@RHEL5 ~]# ls -ld /home/wim/
drwx------ 2 wim wim 4096 Jun 24 15:19 /home/wim/
[root@RHEL5 ~]# userdel -r wim
[root@RHEL5 ~]# ls -ld /home/wim/
ls: /home/wim/: No such file or directory
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28.11. login shell
The /etc/passwd file specifies the login shell for the user. In the screenshot below you can
see that user annelies will log in with the /bin/bash shell, and user laura with the /bin/ksh
shell.
[root@RHEL5 ~]# tail -2 /etc/passwd
annelies:x:527:533:sword fighter:/home/annelies:/bin/bash
laura:x:528:534:art dealer:/home/laura:/bin/ksh
You can use the usermod command to change the shell for a user.
[root@RHEL5 ~]# usermod -s /bin/bash laura
[root@RHEL5 ~]# tail -1 /etc/passwd
laura:x:528:534:art dealer:/home/laura:/bin/bash
28.12. chsh
Users can change their login shell with the chsh command. First, user harry obtains a list of
available shells (he could also have done a cat /etc/shells) and then changes his login shell
to the Korn shell (/bin/ksh). At the next login, harry will default into ksh instead of bash.
[laura@centos7 ~]$ chsh -l
/bin/sh
/bin/bash
/sbin/nologin
/usr/bin/sh
/usr/bin/bash
/usr/sbin/nologin
/bin/ksh
/bin/tcsh
/bin/csh
[laura@centos7 ~]$
Note that the -l option does not exist on Debian and that the above screenshot assumes that
ksh and csh shells are installed.
The screenshot below shows how laura can change her default shell (active on next login).
[laura@centos7 ~]$ chsh -s /bin/ksh
Changing shell for laura.
Password:
Shell changed.
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28.13. practice: user management
1. Create a user account named serena, including a home directory and a description (or
comment) that reads Serena Williams. Do all this in one single command.
2. Create a user named venus, including home directory, bash shell, a description that reads
Venus Williams all in one single command.
3. Verify that both users have correct entries in /etc/passwd, /etc/shadow and /etc/group.
4. Verify that their home directory was created.
5. Create a user named einstime with /bin/date as his default logon shell.
7. What happens when you log on with the einstime user ? Can you think of a useful real
world example for changing a user's login shell to an application ?
8. Create a file named welcome.txt and make sure every new user will see this file in their
home directory.
9. Verify this setup by creating (and deleting) a test user account.
10. Change the default login shell for the serena user to /bin/bash. Verify before and after
you make this change.
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28.14. solution: user management
1. Create a user account named serena, including a home directory and a description (or
comment) that reads Serena Williams. Do all this in one single command.
root@debian7:~# useradd -m -c 'Serena Williams' serena
2. Create a user named venus, including home directory, bash shell, a description that reads
Venus Williams all in one single command.
root@debian7:~# useradd -m -c "Venus Williams" -s /bin/bash venus
3. Verify that both users have correct entries in /etc/passwd, /etc/shadow and /etc/group.
root@debian7:~# tail -2 /etc/passwd
serena:x:1008:1010:Serena Williams:/home/serena:/bin/sh
venus:x:1009:1011:Venus Williams:/home/venus:/bin/bash
root@debian7:~# tail -2 /etc/shadow
serena:!:16358:0:99999:7:::
venus:!:16358:0:99999:7:::
root@debian7:~# tail -2 /etc/group
serena:x:1010:
venus:x:1011:
4. Verify that their home directory was created.
root@debian7:~# ls -lrt /home | tail -2
drwxr-xr-x 2 serena
serena
4096 Oct 15 10:50 serena
drwxr-xr-x 2 venus
venus
4096 Oct 15 10:59 venus
root@debian7:~#
5. Create a user named einstime with /bin/date as his default logon shell.
root@debian7:~# useradd -s /bin/date einstime
Or even better:
root@debian7:~# useradd -s $(which date) einstime
7. What happens when you log on with the einstime user ? Can you think of a useful real
world example for changing a user's login shell to an application ?
root@debian7:~# su - einstime
Wed Oct 15 11:05:56 UTC 2014 # You get the output of the date command
root@debian7:~#
It can be useful when users need to access only one application on the server. Just logging
in opens the application for them, and closing the application automatically logs them out.
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user management
8. Create a file named welcome.txt and make sure every new user will see this file in their
home directory.
root@debian7:~# echo Hello > /etc/skel/welcome.txt
9. Verify this setup by creating (and deleting) a test user account.
root@debian7:~# useradd -m test
root@debian7:~# ls -l /home/test
total 4
-rw-r--r-- 1 test test 6 Oct 15 11:16 welcome.txt
root@debian7:~# userdel -r test
root@debian7:~#
10. Change the default login shell for the serena user to /bin/bash. Verify before and after
you make this change.
root@debian7:~# grep serena /etc/passwd
serena:x:1008:1010:Serena Williams:/home/serena:/bin/sh
root@debian7:~# usermod -s /bin/bash serena
root@debian7:~# grep serena /etc/passwd
serena:x:1008:1010:Serena Williams:/home/serena:/bin/bash
root@debian7:~#
280
Chapter 29. user passwords
This chapter will tell you more about passwords for local users.
Three methods for setting passwords are explained; using the passwd command, using
openssel passwd, and using the crypt function in a C program.
The chapter will also discuss password settings and disabling, suspending or locking
accounts.
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user passwords
29.1. passwd
Passwords of users can be set with the passwd command. Users will have to provide their
old password before twice entering the new one.
[tania@centos7 ~]$ passwd
Changing password for user tania.
Changing password for tania.
(current) UNIX password:
New password:
BAD PASSWORD: The password is shorter than 8 characters
New password:
BAD PASSWORD: The password is a palindrome
New password:
BAD PASSWORD: The password is too similar to the old one
passwd: Have exhausted maximum number of retries for service
As you can see, the passwd tool will do some basic verification to prevent users from using
too simple passwords. The root user does not have to follow these rules (there will be
a warning though). The root user also does not have to provide the old password before
entering the new password twice.
root@debian7:~# passwd tania
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
29.2. shadow file
User passwords are encrypted and kept in /etc/shadow. The /etc/shadow file is read only
and can only be read by root. We will see in the file permissions section how it is possible
for users to change their password. For now, you will have to know that users can change
their password with the /usr/bin/passwd command.
[root@centos7 ~]# tail -4 /etc/shadow
paul:$6$ikp2Xta5BT.Tml.p$2TZjNnOYNNQKpwLJqoGJbVsZG5/Fti8ovBRd.VzRbiDSl7TEq\
IaSMH.TeBKnTS/SjlMruW8qffC0JNORW.BTW1:16338:0:99999:7:::
tania:$6$8Z/zovxj$9qvoqT8i9KIrmN.k4EQwAF5ryz5yzNwEvYjAa9L5XVXQu.z4DlpvMREH\
eQpQzvRnqFdKkVj17H5ST.c79HDZw0:16356:0:99999:7:::
laura:$6$glDuTY5e$/NYYWLxfHgZFWeoujaXSMcR.Mz.lGOxtcxFocFVJNb98nbTPhWFXfKWG\
SyYh1WCv6763Wq54.w24Yr3uAZBOm/:16356:0:99999:7:::
valentina:$6$jrZa6PVI$1uQgqR6En9mZB6mKJ3LXRB4CnFko6LRhbh.v4iqUk9MVreui1lv7\
GxHOUDSKA0N55ZRNhGHa6T2ouFnVno/0o1:16356:0:99999:7:::
[root@centos7 ~]#
The /etc/shadow file contains nine colon separated columns. The nine fields contain (from
left to right) the user name, the encrypted password (note that only inge and laura have an
encrypted password), the day the password was last changed (day 1 is January 1, 1970),
number of days the password must be left unchanged, password expiry day, warning number
of days before password expiry, number of days after expiry before disabling the account,
and the day the account was disabled (again, since 1970). The last field has no meaning yet.
All the passwords in the screenshot above are hashes of hunter2.
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user passwords
29.3. encryption with passwd
Passwords are stored in an encrypted format. This encryption is done by the crypt function.
The easiest (and recommended) way to add a user with a password to the system is to add
the user with the useradd -m user command, and then set the user's password with passwd.
[root@RHEL4 ~]# useradd -m xavier
[root@RHEL4 ~]# passwd xavier
Changing password for user xavier.
New UNIX password:
Retype new UNIX password:
passwd: all authentication tokens updated successfully.
[root@RHEL4 ~]#
29.4. encryption with openssl
Another way to create users with a password is to use the -p option of useradd, but that
option requires an encrypted password. You can generate this encrypted password with the
openssl passwd command.
The openssl passwd command will generate several distinct hashes for the same password,
for this it uses a salt.
paul@rhel65:~$ openssl passwd hunter2
86jcUNlnGDFpY
paul@rhel65:~$ openssl passwd hunter2
Yj7mDO9OAnvq6
paul@rhel65:~$ openssl passwd hunter2
YqDcJeGoDbzKA
paul@rhel65:~$
This salt can be chosen and is visible as the first two characters of the hash.
paul@rhel65:~$ openssl passwd -salt 42 hunter2
42ZrbtP1Ze8G.
paul@rhel65:~$ openssl passwd -salt 42 hunter2
42ZrbtP1Ze8G.
paul@rhel65:~$ openssl passwd -salt 42 hunter2
42ZrbtP1Ze8G.
paul@rhel65:~$
This example shows how to create a user with password.
root@rhel65:~# useradd -m -p $(openssl passwd hunter2) mohamed
Note that this command puts the password in your command history!
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user passwords
29.5. encryption with crypt
A third option is to create your own C program using the crypt function, and compile this
into a command.
paul@rhel65:~$ cat MyCrypt.c
#include <stdio.h>
#define __USE_XOPEN
#include <unistd.h>
int main(int argc, char** argv)
{
if(argc==3)
{
printf("%s\n", crypt(argv[1],argv[2]));
}
else
{
printf("Usage: MyCrypt $password $salt\n" );
}
return 0;
}
This little program can be compiled with gcc like this.
paul@rhel65:~$ gcc MyCrypt.c -o MyCrypt -lcrypt
To use it, we need to give two parameters to MyCrypt. The first is the unencrypted password,
the second is the salt. The salt is used to perturb the encryption algorithm in one of 4096
different ways. This variation prevents two users with the same password from having the
same entry in /etc/shadow.
paul@rhel65:~$ ./MyCrypt hunter2 42
42ZrbtP1Ze8G.
paul@rhel65:~$ ./MyCrypt hunter2 33
33d6taYSiEUXI
Did you notice that the first two characters of the password are the salt?
The standard output of the crypt function is using the DES algorithm which is old and can
be cracked in minutes. A better method is to use md5 passwords which can be recognized
by a salt starting with $1$.
paul@rhel65:~$ ./MyCrypt hunter2 '$1$42'
$1$42$7l6Y3xT5282XmZrtDOF9f0
paul@rhel65:~$ ./MyCrypt hunter2 '$6$42'
$6$42$OqFFAVnI3gTSYG0yI9TZWX9cpyQzwIop7HwpG1LLEsNBiMr4w6OvLX1KDa./UpwXfrFk1i...
The md5 salt can be up to eight characters long. The salt is displayed in /etc/shadow between
the second and third $, so never use the password as the salt!
paul@rhel65:~$ ./MyCrypt hunter2 '$1$hunter2'
$1$hunter2$YVxrxDmidq7Xf8Gdt6qM2.
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user passwords
29.6. /etc/login.defs
The /etc/login.defs file contains some default settings for user passwords like password
aging and length settings. (You will also find the numerical limits of user ids and group ids
and whether or not a home directory should be created by default).
root@rhel65:~# grep ^PASS /etc/login.defs
PASS_MAX_DAYS
99999
PASS_MIN_DAYS
0
PASS_MIN_LEN
5
PASS_WARN_AGE
7
Debian also has this file.
root@debian7:~# grep PASS /etc/login.defs
# PASS_MAX_DAYS
Maximum number of days a password may be used.
# PASS_MIN_DAYS
Minimum number of days allowed between password changes.
# PASS_WARN_AGE
Number of days warning given before a password expires.
PASS_MAX_DAYS
99999
PASS_MIN_DAYS
0
PASS_WARN_AGE
7
#PASS_CHANGE_TRIES
#PASS_ALWAYS_WARN
#PASS_MIN_LEN
#PASS_MAX_LEN
# NO_PASSWORD_CONSOLE
root@debian7:~#
29.7. chage
The chage command can be used to set an expiration date for a user account (-E), set a
minimum (-m) and maximum (-M) password age, a password expiration date, and set the
number of warning days before the password expiration date. Much of this functionality is
also available from the passwd command. The -l option of chage will list these settings for
a user.
root@rhel65:~# chage -l paul
Last password change
Password expires
Password inactive
Account expires
Minimum number of days between password change
Maximum number of days between password change
Number of days of warning before password expires
root@rhel65:~#
285
:
:
:
:
:
:
:
Mar 27, 2014
never
never
never
0
99999
7
user passwords
29.8. disabling a password
Passwords in /etc/shadow cannot begin with an exclamation mark. When the second field
in /etc/passwd starts with an exclamation mark, then the password can not be used.
Using this feature is often called locking, disabling, or suspending a user account. Besides
vi (or vipw) you can also accomplish this with usermod.
The first command in the next screenshot will show the hashed password of laura in /etc/
shadow. The next command disables the password of laura, making it impossible for Laura
to authenticate using this password.
root@debian7:~# grep laura /etc/shadow | cut -c1-70
laura:$6$JYj4JZqp$stwwWACp3OtE1R2aZuE87j.nbW.puDkNUYVk7mCHfCVMa3CoDUJV
root@debian7:~# usermod -L laura
As you can see below, the password hash is simply preceded with an exclamation mark.
root@debian7:~# grep laura /etc/shadow | cut -c1-70
laura:!$6$JYj4JZqp$stwwWACp3OtE1R2aZuE87j.nbW.puDkNUYVk7mCHfCVMa3CoDUJ
root@debian7:~#
The root user (and users with sudo rights on su) still will be able to su into the laura account
(because the password is not needed here). Also note that laura will still be able to login
if she has set up passwordless ssh!
root@debian7:~# su - laura
laura@debian7:~$
You can unlock the account again with usermod -U.
root@debian7:~# usermod -U laura
root@debian7:~# grep laura /etc/shadow | cut -c1-70
laura:$6$JYj4JZqp$stwwWACp3OtE1R2aZuE87j.nbW.puDkNUYVk7mCHfCVMa3CoDUJV
Watch out for tiny differences in the command line options of passwd, usermod, and
useradd on different Linux distributions. Verify the local files when using features like
"disabling, suspending, or locking" on user accounts and their passwords.
29.9. editing local files
If you still want to manually edit the /etc/passwd or /etc/shadow, after knowing these
commands for password management, then use vipw instead of vi(m) directly. The vipw
tool will do proper locking of the file.
[root@RHEL5 ~]# vipw /etc/passwd
vipw: the password file is busy (/etc/ptmp present)
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user passwords
29.10. practice: user passwords
1. Set the password for serena to hunter2.
2. Also set a password for venus and then lock the venus user account with usermod. Verify
the locking in /etc/shadow before and after you lock it.
3. Use passwd -d to disable the serena password. Verify the serena line in /etc/shadow
before and after disabling.
4. What is the difference between locking a user account and disabling a user account's
password like we just did with usermod -L and passwd -d?
5. Try changing the password of serena to serena as serena.
6. Make sure serena has to change her password in 10 days.
7. Make sure every new user needs to change their password every 10 days.
8. Take a backup as root of /etc/shadow. Use vi to copy an encrypted hunter2 hash from
venus to serena. Can serena now log on with hunter2 as a password ?
9. Why use vipw instead of vi ? What could be the problem when using vi or vim ?
10. Use chsh to list all shells (only works on RHEL/CentOS/Fedora), and compare to cat /
etc/shells.
11. Which useradd option allows you to name a home directory ?
12. How can you see whether the password of user serena is locked or unlocked ? Give a
solution with grep and a solution with passwd.
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user passwords
29.11. solution: user passwords
1. Set the password for serena to hunter2.
root@debian7:~# passwd serena
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
2. Also set a password for venus and then lock the venus user account with usermod. Verify
the locking in /etc/shadow before and after you lock it.
root@debian7:~# passwd venus
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
root@debian7:~# grep venus /etc/shadow | cut -c1-70
venus:$6$gswzXICW$uSnKFV1kFKZmTPaMVS4AvNA/KO27OxN0v5LHdV9ed0gTyXrjUeM/
root@debian7:~# usermod -L venus
root@debian7:~# grep venus /etc/shadow | cut -c1-70
venus:!$6$gswzXICW$uSnKFV1kFKZmTPaMVS4AvNA/KO27OxN0v5LHdV9ed0gTyXrjUeM
Note that usermod -L precedes the password hash with an exclamation mark (!).
3. Use passwd -d to disable the serena password. Verify the serena line in /etc/shadow
before and after disabling.
root@debian7:~# grep serena /etc/shadow | cut -c1-70
serena:$6$Es/omrPE$F2Ypu8kpLrfKdW0v/UIwA5jrYyBD2nwZ/dt.i/IypRgiPZSdB/B
root@debian7:~# passwd -d serena
passwd: password expiry information changed.
root@debian7:~# grep serena /etc/shadow
serena::16358:0:99999:7:::
root@debian7:~#
4. What is the difference between locking a user account and disabling a user account's
password like we just did with usermod -L and passwd -d?
Locking will prevent the user from logging on to the system with his password by putting
a ! in front of the password in /etc/shadow.
Disabling with passwd will erase the password from /etc/shadow.
5. Try changing the password of serena to serena as serena.
log on as serena, then execute: passwd serena... it should fail!
6. Make sure serena has to change her password in 10 days.
chage -M 10 serena
7. Make sure every new user needs to change their password every 10 days.
vi /etc/login.defs (and change PASS_MAX_DAYS to 10)
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user passwords
8. Take a backup as root of /etc/shadow. Use vi to copy an encrypted hunter2 hash from
venus to serena. Can serena now log on with hunter2 as a password ?
Yes.
9. Why use vipw instead of vi ? What could be the problem when using vi or vim ?
vipw will give a warning when someone else is already using that file (with vipw).
10. Use chsh to list all shells (only works on RHEL/CentOS/Fedora), and compare to cat /
etc/shells.
chsh -l
cat /etc/shells
11. Which useradd option allows you to name a home directory ?
-d
12. How can you see whether the password of user serena is locked or unlocked ? Give a
solution with grep and a solution with passwd.
grep serena /etc/shadow
passwd -S serena
289
Chapter 30. user profiles
Logged on users have a number of preset (and customized) aliases, variables, and functions,
but where do they come from ? The shell uses a number of startup files that are executed
(or rather sourced) whenever the shell is invoked. What follows is an overview of startup
scripts.
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user profiles
30.1. system profile
Both the bash and the ksh shell will verify the existence of /etc/profile and source it if it
exists.
When reading this script, you will notice (both on Debian and on Red Hat Enterprise Linux)
that it builds the PATH environment variable (among others). The script might also change
the PS1 variable, set the HOSTNAME and execute even more scripts like /etc/inputrc
This screenshot uses grep to show PATH manipulation in /etc/profile on Debian.
root@debian7:~# grep PATH /etc/profile
PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
PATH="/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games"
export PATH
root@debian7:~#
This screenshot uses grep to show PATH manipulation in /etc/profile on RHEL7/CentOS7.
[root@centos7 ~]# grep PATH /etc/profile
case ":${PATH}:" in
PATH=$PATH:$1
PATH=$1:$PATH
export PATH USER LOGNAME MAIL HOSTNAME HISTSIZE HISTCONTROL
[root@centos7 ~]#
The root user can use this script to set aliases, functions, and variables for every user on
the system.
30.2. ~/.bash_profile
When this file exists in the home directory, then bash will source it. On Debian Linux 5/6/7
this file does not exist by default.
RHEL7/CentOS7 uses a small ~/.bash_profile where it checks for the existence of
~/.bashrc and then sources it. It also adds $HOME/bin to the $PATH variable.
[root@rhel7 ~]# cat /home/paul/.bash_profile
# .bash_profile
# Get the aliases and functions
if [ -f ~/.bashrc ]; then
. ~/.bashrc
fi
# User specific environment and startup programs
PATH=$PATH:$HOME/.local/bin:$HOME/bin
export PATH
[root@rhel7 ~]#
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user profiles
30.3. ~/.bash_login
When .bash_profile does not exist, then bash will check for ~/.bash_login and source it.
Neither Debian nor Red Hat have this file by default.
30.4. ~/.profile
When neither ~/.bash_profile and ~/.bash_login exist, then bash will verify the existence
of ~/.profile and execute it. This file does not exist by default on Red Hat.
On Debian this script can execute ~/.bashrc and will add $HOME/bin to the $PATH
variable.
root@debian7:~# tail -11 /home/paul/.profile
if [ -n "$BASH_VERSION" ]; then
# include .bashrc if it exists
if [ -f "$HOME/.bashrc" ]; then
. "$HOME/.bashrc"
fi
fi
# set PATH so it includes user's private bin if it exists
if [ -d "$HOME/bin" ] ; then
PATH="$HOME/bin:$PATH"
fi
RHEL/CentOS does not have this file by default.
30.5. ~/.bashrc
The ~/.bashrc script is often sourced by other scripts. Let us take a look at what it does
by default.
Red Hat uses a very simple ~/.bashrc, checking for /etc/bashrc and sourcing it. It also leaves
room for custom aliases and functions.
[root@rhel7 ~]# cat /home/paul/.bashrc
# .bashrc
# Source global definitions
if [ -f /etc/bashrc ]; then
. /etc/bashrc
fi
# Uncomment the following line if you don't like systemctl's auto-paging feature:
# export SYSTEMD_PAGER=
# User specific aliases and functions
On Debian this script is quite a bit longer and configures $PS1, some history variables and
a number af active and inactive aliases.
root@debian7:~# wc -l /home/paul/.bashrc
110 /home/paul/.bashrc
292
user profiles
30.6. ~/.bash_logout
When exiting bash, it can execute ~/.bash_logout.
Debian use this opportunity to clear the console screen.
serena@deb503:~$ cat .bash_logout
# ~/.bash_logout: executed by bash(1) when login shell exits.
# when leaving the console clear the screen to increase privacy
if [ "$SHLVL" = 1 ]; then
[ -x /usr/bin/clear_console ] && /usr/bin/clear_console -q
fi
Red Hat Enterprise Linux 5 will simple call the /usr/bin/clear command in this script.
[serena@rhel53 ~]$ cat .bash_logout
# ~/.bash_logout
/usr/bin/clear
Red Hat Enterprise Linux 6 and 7 create this file, but leave it empty (except for a comment).
paul@rhel65:~$ cat .bash_logout
# ~/.bash_logout
293
user profiles
30.7. Debian overview
Below is a table overview of when Debian is running any of these bash startup scripts.
Table 30.1. Debian User Environment
script
su
su -
ssh
gdm
~./bashrc
no
yes
yes
yes
~/.profile
no
yes
yes
yes
/etc/profile
no
yes
yes
yes
/etc/bash.bashrc
yes
no
no
yes
30.8. RHEL5 overview
Below is a table overview of when Red Hat Enterprise Linux 5 is running any of these bash
startup scripts.
Table 30.2. Red Hat User Environment
script
su
su -
ssh
gdm
~./bashrc
yes
yes
yes
yes
~/.bash_profile
no
yes
yes
yes
/etc/profile
no
yes
yes
yes
/etc/bashrc
yes
yes
yes
yes
294
user profiles
30.9. practice: user profiles
1. Make a list of all the profile files on your system.
2. Read the contents of each of these, often they source extra scripts.
3. Put a unique variable, alias and function in each of those files.
4. Try several different ways to obtain a shell (su, su -, ssh, tmux, gnome-terminal, Ctrlalt-F1, ...) and verify which of your custom variables, aliases and function are present in
your environment.
5. Do you also know the order in which they are executed?
6. When an application depends on a setting in $HOME/.profile, does it matter whether
$HOME/.bash_profile exists or not ?
295
user profiles
30.10. solution: user profiles
1. Make a list of all the profile files on your system.
ls -a ~ ; ls -l /etc/pro* /etc/bash*
2. Read the contents of each of these, often they source extra scripts.
3. Put a unique variable, alias and function in each of those files.
4. Try several different ways to obtain a shell (su, su -, ssh, tmux, gnome-terminal, Ctrlalt-F1, ...) and verify which of your custom variables, aliases and function are present in
your environment.
5. Do you also know the order in which they are executed?
same name aliases, functions and variables will overwrite each other
6. When an application depends on a setting in $HOME/.profile, does it matter whether
$HOME/.bash_profile exists or not ?
Yes it does matter. (man bash /INVOCATION)
296
Chapter 31. groups
Users can be listed in groups. Groups allow you to set permissions on the group level instead
of having to set permissions for every individual user.
Every Unix or Linux distribution will have a graphical tool to manage groups. Novice users
are advised to use this graphical tool. More experienced users can use command line tools to
manage users, but be careful: Some distributions do not allow the mixed use of GUI and CLI
tools to manage groups (YaST in Novell Suse). Senior administrators can edit the relevant
files directly with vi or vigr.
297
groups
31.1. groupadd
Groups can be created with the groupadd command. The example below shows the creation
of five (empty) groups.
root@laika:~#
root@laika:~#
root@laika:~#
root@laika:~#
root@laika:~#
groupadd
groupadd
groupadd
groupadd
groupadd
tennis
football
snooker
formula1
salsa
31.2. group file
Users can be a member of several groups. Group membership is defined by the /etc/group
file.
root@laika:~# tail -5 /etc/group
tennis:x:1006:
football:x:1007:
snooker:x:1008:
formula1:x:1009:
salsa:x:1010:
root@laika:~#
The first field is the group's name. The second field is the group's (encrypted) password (can
be empty). The third field is the group identification or GID. The fourth field is the list of
members, these groups have no members.
31.3. groups
A user can type the groups command to see a list of groups where the user belongs to.
[harry@RHEL4b ~]$ groups
harry sports
[harry@RHEL4b ~]$
298
groups
31.4. usermod
Group membership can be modified with the useradd or usermod command.
root@laika:~# usermod -a -G tennis inge
root@laika:~# usermod -a -G tennis katrien
root@laika:~# usermod -a -G salsa katrien
root@laika:~# usermod -a -G snooker sandra
root@laika:~# usermod -a -G formula1 annelies
root@laika:~# tail -5 /etc/group
tennis:x:1006:inge,katrien
football:x:1007:
snooker:x:1008:sandra
formula1:x:1009:annelies
salsa:x:1010:katrien
root@laika:~#
Be careful when using usermod to add users to groups. By default, the usermod command
will remove the user from every group of which he is a member if the group is not listed in
the command! Using the -a (append) switch prevents this behaviour.
31.5. groupmod
You can change the group name with the groupmod command.
root@laika:~# groupmod -n darts snooker
root@laika:~# tail -5 /etc/group
tennis:x:1006:inge,katrien
football:x:1007:
formula1:x:1009:annelies
salsa:x:1010:katrien
darts:x:1008:sandra
31.6. groupdel
You can permanently remove a group with the groupdel command.
root@laika:~# groupdel tennis
root@laika:~#
299
groups
31.7. gpasswd
You can delegate control of group membership to another user with the gpasswd command.
In the example below we delegate permissions to add and remove group members to serena
for the sports group. Then we su to serena and add harry to the sports group.
[root@RHEL4b ~]# gpasswd -A serena sports
[root@RHEL4b ~]# su - serena
[serena@RHEL4b ~]$ id harry
uid=516(harry) gid=520(harry) groups=520(harry)
[serena@RHEL4b ~]$ gpasswd -a harry sports
Adding user harry to group sports
[serena@RHEL4b ~]$ id harry
uid=516(harry) gid=520(harry) groups=520(harry),522(sports)
[serena@RHEL4b ~]$ tail -1 /etc/group
sports:x:522:serena,venus,harry
[serena@RHEL4b ~]$
Group administrators do not have to be a member of the group. They can remove themselves
from a group, but this does not influence their ability to add or remove members.
[serena@RHEL4b ~]$ gpasswd -d serena sports
Removing user serena from group sports
[serena@RHEL4b ~]$ exit
Information about group administrators is kept in the /etc/gshadow file.
[root@RHEL4b ~]# tail -1 /etc/gshadow
sports:!:serena:venus,harry
[root@RHEL4b ~]#
To remove all group administrators from a group, use the gpasswd command to set an empty
administrators list.
[root@RHEL4b ~]# gpasswd -A "" sports
300
groups
31.8. newgrp
You can start a child shell with a new temporary primary group using the newgrp
command.
root@rhel65:~# mkdir prigroup
root@rhel65:~# cd prigroup/
root@rhel65:~/prigroup# touch standard.txt
root@rhel65:~/prigroup# ls -l
total 0
-rw-r--r--. 1 root root 0 Apr 13 17:49 standard.txt
root@rhel65:~/prigroup# echo $SHLVL
1
root@rhel65:~/prigroup# newgrp tennis
root@rhel65:~/prigroup# echo $SHLVL
2
root@rhel65:~/prigroup# touch newgrp.txt
root@rhel65:~/prigroup# ls -l
total 0
-rw-r--r--. 1 root tennis 0 Apr 13 17:49 newgrp.txt
-rw-r--r--. 1 root root
0 Apr 13 17:49 standard.txt
root@rhel65:~/prigroup# exit
exit
root@rhel65:~/prigroup#
31.9. vigr
Similar to vipw, the vigr command can be used to manually edit the /etc/group file, since
it will do proper locking of the file. Only experienced senior administrators should use vi
or vigr to manage groups.
301
groups
31.10. practice: groups
1. Create the groups tennis, football and sports.
2. In one command, make venus a member of tennis and sports.
3. Rename the football group to foot.
4. Use vi to add serena to the tennis group.
5. Use the id command to verify that serena is a member of tennis.
6. Make someone responsible for managing group membership of foot and sports. Test that
it works.
302
groups
31.11. solution: groups
1. Create the groups tennis, football and sports.
groupadd tennis ; groupadd football ; groupadd sports
2. In one command, make venus a member of tennis and sports.
usermod -a -G tennis,sports venus
3. Rename the football group to foot.
groupmod -n foot football
4. Use vi to add serena to the tennis group.
vi /etc/group
5. Use the id command to verify that serena is a member of tennis.
id (and after logoff logon serena should be member)
6. Make someone responsible for managing group membership of foot and sports. Test that
it works.
gpasswd -A (to make manager)
gpasswd -a (to add member)
303
Part IX. file security
Table of Contents
32. standard file permissions ................................................................................................
32.1. file ownership ......................................................................................................
32.2. list of special files .................................................................................................
32.3. permissions ..........................................................................................................
32.4. practice: standard file permissions ............................................................................
32.5. solution: standard file permissions ...........................................................................
33. advanced file permissions ...............................................................................................
33.1. sticky bit on directory ............................................................................................
33.2. setgid bit on directory ............................................................................................
33.3. setgid and setuid on regular files .............................................................................
33.4. setuid on sudo ......................................................................................................
33.5. practice: sticky, setuid and setgid bits .......................................................................
33.6. solution: sticky, setuid and setgid bits .......................................................................
34. access control lists ..........................................................................................................
34.1. acl in /etc/fstab .....................................................................................................
34.2. getfacl .................................................................................................................
34.3. setfacl .................................................................................................................
34.4. remove an acl entry ...............................................................................................
34.5. remove the complete acl ........................................................................................
34.6. the acl mask ........................................................................................................
34.7. eiciel ..................................................................................................................
35. file links ........................................................................................................................
35.1. inodes .................................................................................................................
35.2. about directories ...................................................................................................
35.3. hard links ............................................................................................................
35.4. symbolic links ......................................................................................................
35.5. removing links .....................................................................................................
35.6. practice : links ......................................................................................................
35.7. solution : links ......................................................................................................
305
306
307
309
310
315
316
318
319
319
320
320
321
322
324
325
325
325
326
326
326
327
328
329
330
331
332
332
333
334
Chapter 32. standard file permissions
This chapter contains details about basic file security through file ownership and file
permissions.
306
standard file permissions
32.1. file ownership
32.1.1. user owner and group owner
The users and groups of a system can be locally managed in /etc/passwd and /etc/group,
or they can be in a NIS, LDAP, or Samba domain. These users and groups can own files.
Actually, every file has a user owner and a group owner, as can be seen in the following
screenshot.
paul@rhel65:~/owners$ ls -lh
total 636K
-rw-r--r--. 1 paul snooker 1.1K
-rw-r--r--. 1 paul paul
626K
-rw-r--r--. 1 root tennis
185
-rw-rw-r--. 1 root root
0
paul@rhel65:~/owners$
Apr
Apr
Apr
Apr
8
8
8
8
18:47
18:46
18:46
18:47
data.odt
file1
file2
stuff.txt
User paul owns three files; file1 has paul as user owner and has the group paul as group
owner, data.odt is group owned by the group snooker, file2 by the group tennis.
The last file is called stuff.txt and is owned by the root user and the root group.
32.1.2. listing user accounts
You can use the following command to list all local user accounts.
paul@debian7~$ cut -d: -f1 /etc/passwd | column
root
ntp
sam
bert
daemon
mysql
tom
rino
bin
paul
wouter
antonio
sys
maarten
robrecht
simon
sync
kevin
bilal
sven
games
yuri
dimitri
wouter2
man
william
ahmed
tarik
lp
yves
dylan
jan
mail
kris
robin
ian
news
hamid
matthias
ivan
uucp
vladimir
ben
azeddine
proxy
abiy
mike
eric
www-data
david
kevin2
kamel
backup
chahid
kenzo
ischa
list
stef
aaron
bart
irc
joeri
lorenzo
omer
gnats
glenn
jens
kurt
nobody
yannick
ruben
steve
libuuid
christof
jelle
constantin
Debian-exim
george
stefaan
sam2
statd
joost
marc
bjorn
sshd
arno
thomas
ronald
307
naomi
matthias2
bram
fabrice
chimene
messagebus
roger
frank
toon
rinus
eddy
bram2
keith
jesse
frederick
hans
dries
steve2
tomas
johan
tom2
standard file permissions
32.1.3. chgrp
You can change the group owner of a file using the chgrp command.
root@rhel65:/home/paul/owners# ls -l file2
-rw-r--r--. 1 root tennis 185 Apr 8 18:46 file2
root@rhel65:/home/paul/owners# chgrp snooker file2
root@rhel65:/home/paul/owners# ls -l file2
-rw-r--r--. 1 root snooker 185 Apr 8 18:46 file2
root@rhel65:/home/paul/owners#
32.1.4. chown
The user owner of a file can be changed with chown command.
root@laika:/home/paul#
-rw-r--r-- 1 root paul
root@laika:/home/paul#
root@laika:/home/paul#
-rw-r--r-- 1 paul paul
ls -l FileForPaul
0 2008-08-06 14:11 FileForPaul
chown paul FileForPaul
ls -l FileForPaul
0 2008-08-06 14:11 FileForPaul
You can also use chown to change both the user owner and the group owner.
root@laika:/home/paul# ls -l FileForPaul
-rw-r--r-- 1 paul paul 0 2008-08-06 14:11 FileForPaul
root@laika:/home/paul# chown root:project42 FileForPaul
root@laika:/home/paul# ls -l FileForPaul
-rw-r--r-- 1 root project42 0 2008-08-06 14:11 FileForPaul
308
standard file permissions
32.2. list of special files
When you use ls -l, for each file you can see ten characters before the user and group owner.
The first character tells us the type of file. Regular files get a -, directories get a d, symbolic
links are shown with an l, pipes get a p, character devices a c, block devices a b, and sockets
an s.
Table 32.1. Unix special files
first character
file type
-
normal file
d
directory
l
symbolic link
p
named pipe
b
block device
c
character device
s
socket
Below a screenshot of a character device (the console) and a block device (the hard disk).
paul@debian6lt~$ ls -ld /dev/console /dev/sda
crw------1 root root 5, 1 Mar 15 12:45 /dev/console
brw-rw---1 root disk 8, 0 Mar 15 12:45 /dev/sda
And here you can see a directory, a regular file and a symbolic link.
paul@debian6lt~$ ls
drwxr-xr-x 128 root
-rw-r--r-1 root
lrwxrwxrwx
1 root
-ld /etc /etc/hosts /etc/motd
root 12288 Mar 15 18:34 /etc
root
372 Dec 10 17:36 /etc/hosts
root
13 Dec 5 10:36 /etc/motd -> /var/run/motd
309
standard file permissions
32.3. permissions
32.3.1. rwx
The nine characters following the file type denote the permissions in three triplets. A
permission can be r for read access, w for write access, and x for execute. You need the r
permission to list (ls) the contents of a directory. You need the x permission to enter (cd) a
directory. You need the w permission to create files in or remove files from a directory.
Table 32.2. standard Unix file permissions
permission
on a file
on a directory
r (read)
read file contents (cat)
read directory contents (ls)
w (write)
change file contents (vi)
create files in (touch)
x (execute)
execute the file
enter the directory (cd)
32.3.2. three sets of rwx
We already know that the output of ls -l starts with ten characters for each file. This
screenshot shows a regular file (because the first character is a - ).
paul@RHELv4u4:~/test$ ls -l proc42.bash
-rwxr-xr-- 1 paul proj 984 Feb 6 12:01 proc42.bash
Below is a table describing the function of all ten characters.
Table 32.3. Unix file permissions position
position
characters
function
1
-
this is a regular file
2-4
rwx
permissions for the user owner
5-7
r-x
permissions for the group owner
8-10
r--
permissions for others
When you are the user owner of a file, then the user owner permissions apply to you. The
rest of the permissions have no influence on your access to the file.
When you belong to the group that is the group owner of a file, then the group owner
permissions apply to you. The rest of the permissions have no influence on your access to
the file.
When you are not the user owner of a file and you do not belong to the group owner, then
the others permissions apply to you. The rest of the permissions have no influence on your
access to the file.
310
standard file permissions
32.3.3. permission examples
Some example combinations on files and directories are seen in this screenshot. The name
of the file explains the permissions.
paul@laika:~/perms$ ls
total 12K
drwxr-xr-x 2 paul paul
-rwxrwxrwx 1 paul paul
-r--r----- 1 paul paul
-rwxrwx--- 1 paul paul
dr-xr-x--- 2 paul paul
dr-x------ 2 paul paul
paul@laika:~/perms$
-lh
4.0K
0
0
0
4.0K
4.0K
2007-02-07
2007-02-07
2007-02-07
2007-02-07
2007-02-07
2007-02-07
22:26
22:21
22:21
22:21
22:25
22:25
AllEnter_UserCreateDelete
EveryoneFullControl.txt
OnlyOwnersRead.txt
OwnersAll_RestNothing.txt
UserAndGroupEnter
OnlyUserEnter
To summarise, the first rwx triplet represents the permissions for the user owner. The
second triplet corresponds to the group owner; it specifies permissions for all members
of that group. The third triplet defines permissions for all other users that are not the user
owner and are not a member of the group owner.
311
standard file permissions
32.3.4. setting permissions (chmod)
Permissions can be changed with chmod. The first example gives the user owner execute
permissions.
paul@laika:~/perms$ ls -l permissions.txt
-rw-r--r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
paul@laika:~/perms$ chmod u+x permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwxr--r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
This example removes the group owners read permission.
paul@laika:~/perms$ chmod g-r permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwx---r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
This example removes the others read permission.
paul@laika:~/perms$ chmod o-r permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwx------ 1 paul paul 0 2007-02-07 22:34 permissions.txt
This example gives all of them the write permission.
paul@laika:~/perms$ chmod a+w permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwx-w--w- 1 paul paul 0 2007-02-07 22:34 permissions.txt
You don't even have to type the a.
paul@laika:~/perms$ chmod +x permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwx-wx-wx 1 paul paul 0 2007-02-07 22:34 permissions.txt
You can also set explicit permissions.
paul@laika:~/perms$ chmod u=rw permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rw--wx-wx 1 paul paul 0 2007-02-07 22:34 permissions.txt
Feel free to make any kind of combination.
paul@laika:~/perms$ chmod u=rw,g=rw,o=r permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rw-rw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
Even fishy combinations are accepted by chmod.
paul@laika:~/perms$ chmod u=rwx,ug+rw,o=r permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwxrw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
312
standard file permissions
32.3.5. setting octal permissions
Most Unix administrators will use the old school octal system to talk about and set
permissions. Look at the triplet bitwise, equating r to 4, w to 2, and x to 1.
Table 32.4. Octal permissions
binary
octal
permission
000
0
---
001
1
--x
010
2
-w-
011
3
-wx
100
4
r--
101
5
r-x
110
6
rw-
111
7
rwx
This makes 777 equal to rwxrwxrwx and by the same logic, 654 mean rw-r-xr-- . The chmod
command will accept these numbers.
paul@laika:~/perms$ chmod 777 permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwxrwxrwx 1 paul paul 0 2007-02-07 22:34 permissions.txt
paul@laika:~/perms$ chmod 664 permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rw-rw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
paul@laika:~/perms$ chmod 750 permissions.txt
paul@laika:~/perms$ ls -l permissions.txt
-rwxr-x--- 1 paul paul 0 2007-02-07 22:34 permissions.txt
313
standard file permissions
32.3.6. umask
When creating a file or directory, a set of default permissions are applied. These default
permissions are determined by the umask. The umask specifies permissions that you do
not want set on by default. You can display the umask with the umask command.
[Harry@RHEL4b
0002
[Harry@RHEL4b
[Harry@RHEL4b
-rw-rw-r-- 1
[Harry@RHEL4b
~]$ umask
~]$ touch test
~]$ ls -l test
Harry Harry 0 Jul 24 06:03 test
~]$
As you can also see, the file is also not executable by default. This is a general security
feature among Unixes; newly created files are never executable by default. You have to
explicitly do a chmod +x to make a file executable. This also means that the 1 bit in the
umask has no meaning--a umask of 0022 is the same as 0033.
32.3.7. mkdir -m
When creating directories with mkdir you can use the -m option to set the mode. This
screenshot explains.
paul@debian5~$ mkdir -m 700 MyDir
paul@debian5~$ mkdir -m 777 Public
paul@debian5~$ ls -dl MyDir/ Public/
drwx------ 2 paul paul 4096 2011-10-16 19:16 MyDir/
drwxrwxrwx 2 paul paul 4096 2011-10-16 19:16 Public/
32.3.8. cp -p
To preserve permissions and time stamps from source files, use cp -p.
paul@laika:~/perms$ cp file* cp
paul@laika:~/perms$ cp -p file* cpp
paul@laika:~/perms$ ll *
-rwx------ 1 paul paul
0 2008-08-25 13:26 file33
-rwxr-x--- 1 paul paul
0 2008-08-25 13:26 file42
cp:
total 0
-rwx------ 1 paul paul 0 2008-08-25 13:34 file33
-rwxr-x--- 1 paul paul 0 2008-08-25 13:34 file42
cpp:
total 0
-rwx------ 1 paul paul 0 2008-08-25 13:26 file33
-rwxr-x--- 1 paul paul 0 2008-08-25 13:26 file42
314
standard file permissions
32.4. practice: standard file permissions
1. As normal user, create a directory ~/permissions. Create a file owned by yourself in there.
2. Copy a file owned by root from /etc/ to your permissions dir, who owns this file now ?
3. As root, create a file in the users ~/permissions directory.
4. As normal user, look at who owns this file created by root.
5. Change the ownership of all files in ~/permissions to yourself.
6. Make sure you have all rights to these files, and others can only read.
7. With chmod, is 770 the same as rwxrwx--- ?
8. With chmod, is 664 the same as r-xr-xr-- ?
9. With chmod, is 400 the same as r-------- ?
10. With chmod, is 734 the same as rwxr-xr-- ?
11a. Display the umask in octal and in symbolic form.
11b. Set the umask to 077, but use the symbolic format to set it. Verify that this works.
12. Create a file as root, give only read to others. Can a normal user read this file ? Test
writing to this file with vi.
13a. Create a file as normal user, give only read to others. Can another normal user read this
file ? Test writing to this file with vi.
13b. Can root read this file ? Can root write to this file with vi ?
14. Create a directory that belongs to a group, where every member of that group can read
and write to files, and create files. Make sure that people can only delete their own files.
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standard file permissions
32.5. solution: standard file permissions
1. As normal user, create a directory ~/permissions. Create a file owned by yourself in there.
mkdir ~/permissions ; touch ~/permissions/myfile.txt
2. Copy a file owned by root from /etc/ to your permissions dir, who owns this file now ?
cp /etc/hosts ~/permissions/
The copy is owned by you.
3. As root, create a file in the users ~/permissions directory.
(become root)# touch /home/username/permissions/rootfile
4. As normal user, look at who owns this file created by root.
ls -l ~/permissions
The file created by root is owned by root.
5. Change the ownership of all files in ~/permissions to yourself.
chown user ~/permissions/*
You cannot become owner of the file that belongs to root.
6. Make sure you have all rights to these files, and others can only read.
chmod 644 (on files)
chmod 755 (on directories)
7. With chmod, is 770 the same as rwxrwx--- ?
yes
8. With chmod, is 664 the same as r-xr-xr-- ?
No
9. With chmod, is 400 the same as r-------- ?
yes
10. With chmod, is 734 the same as rwxr-xr-- ?
no
11a. Display the umask in octal and in symbolic form.
umask ; umask -S
11b. Set the umask to 077, but use the symbolic format to set it. Verify that this works.
umask -S u=rwx,go=
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standard file permissions
12. Create a file as root, give only read to others. Can a normal user read this file ? Test
writing to this file with vi.
(become root)
# echo hello > /home/username/root.txt
# chmod 744 /home/username/root.txt
(become user)
vi ~/root.txt
13a. Create a file as normal user, give only read to others. Can another normal user read this
file ? Test writing to this file with vi.
echo hello > file ; chmod 744 file
Yes, others can read this file
13b. Can root read this file ? Can root write to this file with vi ?
Yes, root can read and write to this file. Permissions do not apply to root.
14. Create a directory that belongs to a group, where every member of that group can read
and write to files, and create files. Make sure that people can only delete their own files.
mkdir /home/project42 ; groupadd project42
chgrp project42 /home/project42 ; chmod 775 /home/project42
You can not yet do the last part of this exercise...
317
Chapter 33. advanced file
permissions
318
advanced file permissions
33.1. sticky bit on directory
You can set the sticky bit on a directory to prevent users from removing files that they do
not own as a user owner. The sticky bit is displayed at the same location as the x permission
for others. The sticky bit is represented by a t (meaning x is also there) or a T (when there
is no x for others).
root@RHELv4u4:~# mkdir /project55
root@RHELv4u4:~# ls -ld /project55
drwxr-xr-x 2 root root 4096 Feb 7 17:38 /project55
root@RHELv4u4:~# chmod +t /project55/
root@RHELv4u4:~# ls -ld /project55
drwxr-xr-t 2 root root 4096 Feb 7 17:38 /project55
root@RHELv4u4:~#
The sticky bit can also be set with octal permissions, it is binary 1 in the first of four triplets.
root@RHELv4u4:~# chmod 1775 /project55/
root@RHELv4u4:~# ls -ld /project55
drwxrwxr-t 2 root root 4096 Feb 7 17:38 /project55
root@RHELv4u4:~#
You will typically find the sticky bit on the /tmp directory.
root@barry:~# ls -ld /tmp
drwxrwxrwt 6 root root 4096 2009-06-04 19:02 /tmp
33.2. setgid bit on directory
setgid can be used on directories to make sure that all files inside the directory are owned
by the group owner of the directory. The setgid bit is displayed at the same location as the x
permission for group owner. The setgid bit is represented by an s (meaning x is also there)
or a S (when there is no x for the group owner). As this example shows, even though root
does not belong to the group proj55, the files created by root in /project55 will belong to
proj55 since the setgid is set.
root@RHELv4u4:~# groupadd proj55
root@RHELv4u4:~# chown root:proj55 /project55/
root@RHELv4u4:~# chmod 2775 /project55/
root@RHELv4u4:~# touch /project55/fromroot.txt
root@RHELv4u4:~# ls -ld /project55/
drwxrwsr-x 2 root proj55 4096 Feb 7 17:45 /project55/
root@RHELv4u4:~# ls -l /project55/
total 4
-rw-r--r-- 1 root proj55 0 Feb 7 17:45 fromroot.txt
root@RHELv4u4:~#
You can use the find command to find all setgid directories.
paul@laika:~$ find / -type d -perm -2000 2> /dev/null
/var/log/mysql
/var/log/news
/var/local
...
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advanced file permissions
33.3. setgid and setuid on regular files
These two permissions cause an executable file to be executed with the permissions of the
file owner instead of the executing owner. This means that if any user executes a program
that belongs to the root user, and the setuid bit is set on that program, then the program
runs as root. This can be dangerous, but sometimes this is good for security.
Take the example of passwords; they are stored in /etc/shadow which is only readable by
root. (The root user never needs permissions anyway.)
root@RHELv4u4:~# ls -l /etc/shadow
-r-------- 1 root root 1260 Jan 21 07:49 /etc/shadow
Changing your password requires an update of this file, so how can normal non-root users
do this? Let's take a look at the permissions on the /usr/bin/passwd.
root@RHELv4u4:~# ls -l /usr/bin/passwd
-r-s--x--x 1 root root 21200 Jun 17 2005 /usr/bin/passwd
When running the passwd program, you are executing it with root credentials.
You can use the find command to find all setuid programs.
paul@laika:~$ find /usr/bin -type f -perm -04000
/usr/bin/arping
/usr/bin/kgrantpty
/usr/bin/newgrp
/usr/bin/chfn
/usr/bin/sudo
/usr/bin/fping6
/usr/bin/passwd
/usr/bin/gpasswd
...
In most cases, setting the setuid bit on executables is sufficient. Setting the setgid bit will
result in these programs to run with the credentials of their group owner.
33.4. setuid on sudo
The sudo binary has the setuid bit set, so any user can run it with the effective userid of root.
paul@rhel65:~$ ls -l $(which sudo)
---s--x--x. 1 root root 123832 Oct
paul@rhel65:~$
7
2013 /usr/bin/sudo
320
advanced file permissions
33.5. practice: sticky, setuid and setgid bits
1a. Set up a directory, owned by the group sports.
1b. Members of the sports group should be able to create files in this directory.
1c. All files created in this directory should be group-owned by the sports group.
1d. Users should be able to delete only their own user-owned files.
1e. Test that this works!
2. Verify the permissions on /usr/bin/passwd. Remove the setuid, then try changing your
password as a normal user. Reset the permissions back and try again.
3. If time permits (or if you are waiting for other students to finish this practice), read about
file attributes in the man page of chattr and lsattr. Try setting the i attribute on a file and
test that it works.
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advanced file permissions
33.6. solution: sticky, setuid and setgid bits
1a. Set up a directory, owned by the group sports.
groupadd sports
mkdir /home/sports
chown root:sports /home/sports
1b. Members of the sports group should be able to create files in this directory.
chmod 770 /home/sports
1c. All files created in this directory should be group-owned by the sports group.
chmod 2770 /home/sports
1d. Users should be able to delete only their own user-owned files.
chmod +t /home/sports
1e. Test that this works!
Log in with different users (group members and others and root), create files and watch the
permissions. Try changing and deleting files...
2. Verify the permissions on /usr/bin/passwd. Remove the setuid, then try changing your
password as a normal user. Reset the permissions back and try again.
root@deb503:~# ls -l /usr/bin/passwd
-rwsr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd
root@deb503:~# chmod 755 /usr/bin/passwd
root@deb503:~# ls -l /usr/bin/passwd
-rwxr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd
A normal user cannot change password now.
root@deb503:~# chmod 4755 /usr/bin/passwd
root@deb503:~# ls -l /usr/bin/passwd
-rwsr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd
3. If time permits (or if you are waiting for other students to finish this practice), read about
file attributes in the man page of chattr and lsattr. Try setting the i attribute on a file and
test that it works.
paul@laika:~$ sudo su [sudo] password for paul:
root@laika:~# mkdir attr
root@laika:~# cd attr/
root@laika:~/attr# touch file42
root@laika:~/attr# lsattr
------------------ ./file42
root@laika:~/attr# chattr +i file42
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advanced file permissions
root@laika:~/attr# lsattr
----i------------- ./file42
root@laika:~/attr# rm -rf file42
rm: cannot remove `file42': Operation not permitted
root@laika:~/attr# chattr -i file42
root@laika:~/attr# rm -rf file42
root@laika:~/attr#
323
Chapter 34. access control lists
Standard Unix permissions might not be enough for some organisations. This chapter
introduces access control lists or acl's to further protect files and directories.
324
access control lists
34.1. acl in /etc/fstab
File systems that support access control lists, or acls, have to be mounted with the acl
option listed in /etc/fstab. In the example below, you can see that the root file system has
acl support, whereas /home/data does not.
root@laika:~# tail -4 /etc/fstab
/dev/sda1
/
ext3
/dev/sdb2
/home/data
auto
pasha:/home/r
/home/pasha
nfs
wolf:/srv/data
/home/wolf
nfs
acl,relatime
noacl,defaults
defaults
defaults
0
0
0
0
1
0
0
0
34.2. getfacl
Reading acls can be done with /usr/bin/getfacl. This screenshot shows how to read the acl
of file33 with getfacl.
paul@laika:~/test$ getfacl file33
# file: file33
# owner: paul
# group: paul
user::rwgroup::r-mask::rwx
other::r--
34.3. setfacl
Writing or changing acls can be done with /usr/bin/setfacl. These screenshots show how
to change the acl of file33 with setfacl.
First we add user sandra with octal permission 7 to the acl.
paul@laika:~/test$ setfacl -m u:sandra:7 file33
Then we add the group tennis with octal permission 6 to the acl of the same file.
paul@laika:~/test$ setfacl -m g:tennis:6 file33
The result is visible with getfacl.
paul@laika:~/test$ getfacl file33
# file: file33
# owner: paul
# group: paul
user::rwuser:sandra:rwx
group::r-group:tennis:rwmask::rwx
other::r--
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access control lists
34.4. remove an acl entry
The -x option of the setfacl command will remove an acl entry from the targeted file.
paul@laika:~/test$
paul@laika:~/test$
user:sandra:rwx
paul@laika:~/test$
paul@laika:~/test$
setfacl -m u:sandra:7 file33
getfacl file33 | grep sandra
setfacl -x sandra file33
getfacl file33 | grep sandra
Note that omitting the u or g when defining the acl for an account will default it to a user
account.
34.5. remove the complete acl
The -b option of the setfacl command will remove the acl from the targeted file.
paul@laika:~/test$ setfacl -b file33
paul@laika:~/test$ getfacl file33
# file: file33
# owner: paul
# group: paul
user::rwgroup::r-other::r--
34.6. the acl mask
The acl mask defines the maximum effective permissions for any entry in the acl. This
mask is calculated every time you execute the setfacl or chmod commands.
You can prevent the calculation by using the --no-mask switch.
paul@laika:~/test$ setfacl --no-mask -m u:sandra:7 file33
paul@laika:~/test$ getfacl file33
# file: file33
# owner: paul
# group: paul
user::rwuser:sandra:rwx
#effective:rwgroup::r-mask::rwother::r--
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access control lists
34.7. eiciel
Desktop users might want to use eiciel to manage acls with a graphical tool.
You will need to install eiciel and nautilus-actions to have an extra tab in nautilus to
manage acls.
paul@laika:~$ sudo aptitude install eiciel nautilus-actions
327
Chapter 35. file links
An average computer using Linux has a file system with many hard links and symbolic
links.
To understand links in a file system, you first have to understand what an inode is.
328
file links
35.1. inodes
35.1.1. inode contents
An inode is a data structure that contains metadata about a file. When the file system stores
a new file on the hard disk, it stores not only the contents (data) of the file, but also extra
properties like the name of the file, the creation date, its permissions, the owner of the file,
and more. All this information (except the name of the file and the contents of the file) is
stored in the inode of the file.
The ls -l command will display some of the inode contents, as seen in this screenshot.
root@rhel53 ~# ls -ld /home/project42/
drwxr-xr-x 4 root pro42 4.0K Mar 27 14:29 /home/project42/
35.1.2. inode table
The inode table contains all of the inodes and is created when you create the file system
(with mkfs). You can use the df -i command to see how many inodes are used and free on
mounted file systems.
root@rhel53 ~# df -i
Filesystem
Inodes
IUsed
IFree IUse% Mounted on
/dev/mapper/VolGroup00-LogVol00
4947968 115326 4832642
3% /
/dev/hda1
26104
45
26059
1% /boot
tmpfs
64417
1
64416
1% /dev/shm
/dev/sda1
262144
2207 259937
1% /home/project42
/dev/sdb1
74400
5519
68881
8% /home/project33
/dev/sdb5
0
0
0
- /home/sales
/dev/sdb6
100744
11 100733
1% /home/research
In the df -i screenshot above you can see the inode usage for several mounted file systems.
You don't see numbers for /dev/sdb5 because it is a fat file system.
35.1.3. inode number
Each inode has a unique number (the inode number). You can see the inode numbers with
the ls -li command.
paul@RHELv4u4:~/test$ touch file1
paul@RHELv4u4:~/test$ touch file2
paul@RHELv4u4:~/test$ touch file3
paul@RHELv4u4:~/test$ ls -li
total 12
817266 -rw-rw-r-- 1 paul paul 0 Feb
817267 -rw-rw-r-- 1 paul paul 0 Feb
817268 -rw-rw-r-- 1 paul paul 0 Feb
paul@RHELv4u4:~/test$
5 15:38 file1
5 15:38 file2
5 15:38 file3
These three files were created one after the other and got three different inodes (the first
column). All the information you see with this ls command resides in the inode, except for
the filename (which is contained in the directory).
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file links
35.1.4. inode and file contents
Let's put some data in one of the files.
paul@RHELv4u4:~/test$ ls -li
total 16
817266 -rw-rw-r-- 1 paul paul 0 Feb
817270 -rw-rw-r-- 1 paul paul 92 Feb
817268 -rw-rw-r-- 1 paul paul 0 Feb
paul@RHELv4u4:~/test$ cat file2
It is winter now and it is very cold.
We do not like the cold, we prefer hot
paul@RHELv4u4:~/test$
5 15:38 file1
5 15:42 file2
5 15:38 file3
summer nights.
The data that is displayed by the cat command is not in the inode, but somewhere else on
the disk. The inode contains a pointer to that data.
35.2. about directories
35.2.1. a directory is a table
A directory is a special kind of file that contains a table which maps filenames to inodes.
Listing our current directory with ls -ali will display the contents of the directory file.
paul@RHELv4u4:~/test$
total 32
817262 drwxrwxr-x
2
800768 drwx------ 16
817266 -rw-rw-r-1
817270 -rw-rw-r-1
817268 -rw-rw-r-1
paul@RHELv4u4:~/test$
ls -ali
paul
paul
paul
paul
paul
paul 4096 Feb
paul 4096 Feb
paul
0 Feb
paul
92 Feb
paul
0 Feb
5
5
5
5
5
15:42
15:42
15:38
15:42
15:38
.
..
file1
file2
file3
35.2.2. . and ..
You can see five names, and the mapping to their five inodes. The dot . is a mapping to itself,
and the dotdot .. is a mapping to the parent directory. The three other names are mappings
to different inodes.
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file links
35.3. hard links
35.3.1. creating hard links
When we create a hard link to a file with ln, an extra entry is added in the directory. A new
file name is mapped to an existing inode.
paul@RHELv4u4:~/test$ ln file2 hardlink_to_file2
paul@RHELv4u4:~/test$ ls -li
total 24
817266 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file1
817270 -rw-rw-r-- 2 paul paul 92 Feb 5 15:42 file2
817268 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file3
817270 -rw-rw-r-- 2 paul paul 92 Feb 5 15:42 hardlink_to_file2
paul@RHELv4u4:~/test$
Both files have the same inode, so they will always have the same permissions and the same
owner. Both files will have the same content. Actually, both files are equal now, meaning
you can safely remove the original file, the hardlinked file will remain. The inode contains
a counter, counting the number of hard links to itself. When the counter drops to zero, then
the inode is emptied.
35.3.2. finding hard links
You can use the find command to look for files with a certain inode. The screenshot below
shows how to search for all filenames that point to inode 817270. Remember that an inode
number is unique to its partition.
paul@RHELv4u4:~/test$ find / -inum 817270 2> /dev/null
/home/paul/test/file2
/home/paul/test/hardlink_to_file2
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file links
35.4. symbolic links
Symbolic links (sometimes called soft links) do not link to inodes, but create a name to
name mapping. Symbolic links are created with ln -s. As you can see below, the symbolic
link gets an inode of its own.
paul@RHELv4u4:~/test$ ln -s file2 symlink_to_file2
paul@RHELv4u4:~/test$ ls -li
total 32
817273 -rw-rw-r-- 1 paul paul 13 Feb 5 17:06 file1
817270 -rw-rw-r-- 2 paul paul 106 Feb 5 17:04 file2
817268 -rw-rw-r-- 1 paul paul
0 Feb 5 15:38 file3
817270 -rw-rw-r-- 2 paul paul 106 Feb 5 17:04 hardlink_to_file2
817267 lrwxrwxrwx 1 paul paul
5 Feb 5 16:55 symlink_to_file2 -> file2
paul@RHELv4u4:~/test$
Permissions on a symbolic link have no meaning, since the permissions of the target apply.
Hard links are limited to their own partition (because they point to an inode), symbolic links
can link anywhere (other file systems, even networked).
35.5. removing links
Links can be removed with rm.
paul@laika:~$
paul@laika:~$
paul@laika:~$
paul@laika:~$
paul@laika:~$
touch data.txt
ln -s data.txt sl_data.txt
ln data.txt hl_data.txt
rm sl_data.txt
rm hl_data.txt
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file links
35.6. practice : links
1. Create two files named winter.txt and summer.txt, put some text in them.
2. Create a hard link to winter.txt named hlwinter.txt.
3. Display the inode numbers of these three files, the hard links should have the same inode.
4. Use the find command to list the two hardlinked files
5. Everything about a file is in the inode, except two things : name them!
6. Create a symbolic link to summer.txt called slsummer.txt.
7. Find all files with inode number 2. What does this information tell you ?
8. Look at the directories /etc/init.d/ /etc/rc.d/ /etc/rc3.d/ ... do you see the links ?
9. Look in /lib with ls -l...
10. Use find to look in your home directory for regular files that do not(!) have one hard link.
333
file links
35.7. solution : links
1. Create two files named winter.txt and summer.txt, put some text in them.
echo cold > winter.txt ; echo hot > summer.txt
2. Create a hard link to winter.txt named hlwinter.txt.
ln winter.txt hlwinter.txt
3. Display the inode numbers of these three files, the hard links should have the same inode.
ls -li winter.txt summer.txt hlwinter.txt
4. Use the find command to list the two hardlinked files
find . -inum xyz
5. Everything about a file is in the inode, except two things : name them!
The name of the file is in a directory, and the contents is somewhere on the disk.
6. Create a symbolic link to summer.txt called slsummer.txt.
ln -s summer.txt slsummer.txt
7. Find all files with inode number 2. What does this information tell you ?
It tells you there is more than one inode table (one for every formatted partition + virtual
file systems)
8. Look at the directories /etc/init.d/ /etc/rc.d/ /etc/rc3.d/ ... do you see the links ?
ls -l /etc/init.d
ls -l /etc/rc.d
ls -l /etc/rc3.d
9. Look in /lib with ls -l...
ls -l /lib
10. Use find to look in your home directory for regular files that do not(!) have one hard link.
find ~ ! -links 1 -type f
334
Part X. process management
Table of Contents
36. introduction to processes ................................................................................................
36.1. terminology .........................................................................................................
36.2. basic process management ......................................................................................
36.3. signalling processes ...............................................................................................
36.4. practice : basic process management .........................................................................
36.5. solution : basic process management ........................................................................
37. process priorities ...........................................................................................................
37.1. priority and nice values ..........................................................................................
37.2. practice : process priorities .....................................................................................
37.3. solution : process priorities .....................................................................................
38. background jobs ............................................................................................................
38.1. background processes ............................................................................................
38.2. practice : background processes ...............................................................................
38.3. solution : background processes ...............................................................................
336
337
338
339
343
346
347
349
350
353
354
356
357
359
360
Chapter 36. introduction to processes
337
introduction to processes
36.1. terminology
36.1.1. process
A process is compiled source code that is currently running on the system.
36.1.2. PID
All processes have a process id or PID.
36.1.3. PPID
Every process has a parent process (with a PPID). The child process is often started by the
parent process.
36.1.4. init
The init process always has process ID 1. The init process is started by the kernel itself so
technically it does not have a parent process. init serves as a foster parent for orphaned
processes.
36.1.5. kill
When a process stops running, the process dies, when you want a process to die, you kill it.
36.1.6. daemon
Processes that start at system startup and keep running forever are called daemon processes
or daemons. These daemons never die.
36.1.7. zombie
When a process is killed, but it still shows up on the system, then the process is referred to
as zombie. You cannot kill zombies, because they are already dead.
338
introduction to processes
36.2. basic process management
36.2.1. $$ and $PPID
Some shell environment variables contain information about processes. The $$ variable will
hold your current process ID, and $PPID contains the parent PID. Actually $$ is a shell
parameter and not a variable, you cannot assign a value to it.
Below we use echo to display the values of $$ and $PPID.
[paul@RHEL4b ~]$ echo $$ $PPID
4224 4223
36.2.2. pidof
You can find all process id's by name using the pidof command.
root@rhel53 ~# pidof mingetty
2819 2798 2797 2796 2795 2794
36.2.3. parent and child
Processes have a parent-child relationship. Every process has a parent process.
When starting a new bash you can use echo to verify that the pid from before is the ppid
of the new shell. The child process from above is now the parent process.
[paul@RHEL4b ~]$ bash
[paul@RHEL4b ~]$ echo $$ $PPID
4812 4224
Typing exit will end the current process and brings us back to our original values for $$
and $PPID.
[paul@RHEL4b
4812 4224
[paul@RHEL4b
exit
[paul@RHEL4b
4224 4223
[paul@RHEL4b
~]$ echo $$ $PPID
~]$ exit
~]$ echo $$ $PPID
~]$
339
introduction to processes
36.2.4. fork and exec
A process starts another process in two phases. First the process creates a fork of itself, an
identical copy. Then the forked process executes an exec to replace the forked process with
the target child process.
[paul@RHEL4b
4224
[paul@RHEL4b
[paul@RHEL4b
5310 4224
[paul@RHEL4b
~]$ echo $$
~]$ bash
~]$ echo $$ $PPID
~]$
36.2.5. exec
With the exec command, you can execute a process without forking a new process. In the
following screenshot a Korn shell (ksh) is started and is being replaced with a bash shell
using the exec command. The pid of the bash shell is the same as the pid of the Korn
shell. Exiting the child bash shell will get me back to the parent bash, not to the Korn shell
(which does not exist anymore).
[paul@RHEL4b ~]$
4224
[paul@RHEL4b ~]$
$ echo $$ $PPID
5343 4224
$ exec bash
[paul@RHEL4b ~]$
5343 4224
[paul@RHEL4b ~]$
exit
[paul@RHEL4b ~]$
4224
echo $$
# PID of bash
ksh
# PID of ksh and bash
echo $$ $PPID
# PID of bash and bash
exit
echo $$
340
introduction to processes
36.2.6. ps
One of the most common tools on Linux to look at processes is ps. The following screenshot
shows the parent child relationship between three bash processes.
[paul@RHEL4b
4224 4223
[paul@RHEL4b
[paul@RHEL4b
4866 4224
[paul@RHEL4b
[paul@RHEL4b
4884 4866
[paul@RHEL4b
PID TTY
4223 ?
4224 pts/0
4866 pts/0
4884 pts/0
4902 pts/0
[paul@RHEL4b
exit
[paul@RHEL4b
PID TTY
4223 ?
4224 pts/0
4866 pts/0
4903 pts/0
[paul@RHEL4b
exit
[paul@RHEL4b
PID TTY
4223 ?
4224 pts/0
4904 pts/0
[paul@RHEL4b
~]$ echo $$ $PPID
~]$ bash
~]$ echo $$ $PPID
~]$ bash
~]$ echo $$ $PPID
~]$ ps fx
STAT
TIME COMMAND
S
0:01 sshd: paul@pts/0
Ss
0:00 \_ -bash
S
0:00
\_ bash
S
0:00
\_ bash
R+
0:00
\_ ps fx
~]$ exit
~]$ ps fx
STAT
TIME COMMAND
S
0:01 sshd: paul@pts/0
Ss
0:00 \_ -bash
S
0:00
\_ bash
R+
0:00
\_ ps fx
~]$ exit
~]$ ps fx
STAT
TIME COMMAND
S
0:01 sshd: paul@pts/0
Ss
0:00 \_ -bash
R+
0:00
\_ ps fx
~]$
On Linux, ps fax is often used. On Solaris ps -ef (which also works on Linux) is common.
Here is a partial output from ps fax.
[paul@RHEL4a ~]$ ps fax
PID TTY
STAT
TIME COMMAND
1 ?
S
0:00 init [5]
...
3713
5042
5044
5045
5077
?
?
?
pts/1
pts/1
Ss
Ss
S
Ss
R+
0:00 /usr/sbin/sshd
0:00 \_ sshd: paul [priv]
0:00
\_ sshd: paul@pts/1
0:00
\_ -bash
0:00
\_ ps fax
341
introduction to processes
36.2.7. pgrep
Similar to the ps -C, you can also use pgrep to search for a process by its command name.
[paul@RHEL5 ~]$ sleep 1000 &
[1] 32558
[paul@RHEL5 ~]$ pgrep sleep
32558
[paul@RHEL5 ~]$ ps -C sleep
PID TTY
TIME CMD
32558 pts/3
00:00:00 sleep
You can also list the command name of the process with pgrep.
paul@laika:~$ pgrep -l sleep
9661 sleep
36.2.8. top
Another popular tool on Linux is top. The top tool can order processes according to cpu
usage or other properties. You can also kill processes from within top. Press h inside top
for help.
In case of trouble, top is often the first tool to fire up, since it also provides you memory
and swap space information.
342
introduction to processes
36.3. signalling processes
36.3.1. kill
The kill command will kill (or stop) a process. The screenshot shows how to use a standard
kill to stop the process with pid 1942.
paul@ubuntu910:~$ kill 1942
paul@ubuntu910:~$
By using the kill we are sending a signal to the process.
36.3.2. list signals
Running processes can receive signals from each other or from the users. You can have a
list of signals by typing kill -l, that is a letter l, not the number 1.
[paul@RHEL4a ~]$ kill -l
1) SIGHUP
2) SIGINT
3) SIGQUIT
4) SIGILL
5) SIGTRAP
6) SIGABRT
7) SIGBUS
8) SIGFPE
9) SIGKILL
10) SIGUSR1
11) SIGSEGV
12) SIGUSR2
13) SIGPIPE
14) SIGALRM
15) SIGTERM
17) SIGCHLD
18) SIGCONT
19) SIGSTOP
20) SIGTSTP
21) SIGTTIN
22) SIGTTOU
23) SIGURG
24) SIGXCPU
25) SIGXFSZ
26) SIGVTALRM
27) SIGPROF
28) SIGWINCH
29) SIGIO
30) SIGPWR
31) SIGSYS
34) SIGRTMIN
35) SIGRTMIN+1
36) SIGRTMIN+2 37) SIGRTMIN+3 38) SIGRTMIN+4 39) SIGRTMIN+5
40) SIGRTMIN+6 41) SIGRTMIN+7 42) SIGRTMIN+8 43) SIGRTMIN+9
44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13
48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13
52) SIGRTMAX-12 53) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9
56) SIGRTMAX-8 57) SIGRTMAX-7 58) SIGRTMAX-6 59) SIGRTMAX-5
60) SIGRTMAX-4 61) SIGRTMAX-3 62) SIGRTMAX-2 63) SIGRTMAX-1
64) SIGRTMAX
[paul@RHEL4a ~]$
36.3.3. kill -1 (SIGHUP)
It is common on Linux to use the first signal SIGHUP (or HUP or 1) to tell a process that
it should re-read its configuration file. Thus, the kill -1 1 command forces the init process
(init always runs with pid 1) to re-read its configuration file.
root@deb503:~# kill -1 1
root@deb503:~#
It is up to the developer of the process to decide whether the process can do this running,
or whether it needs to stop and start. It is up to the user to read the documentation of the
program.
343
introduction to processes
36.3.4. kill -15 (SIGTERM)
The SIGTERM signal is also called a standard kill. Whenever kill is executed without
specifying the signal, a kill -15 is assumed.
Both commands in the screenshot below are identical.
paul@ubuntu910:~$ kill 1942
paul@ubuntu910:~$ kill -15 1942
36.3.5. kill -9 (SIGKILL)
The SIGKILL is different from most other signals in that it is not being sent to the process,
but to the Linux kernel. A kill -9 is also called a sure kill. The kernel will shoot down the
process. As a developer you have no means to intercept a kill -9 signal.
root@rhel53 ~# kill -9 3342
36.3.6. SIGSTOP and SIGCONT
A running process can be suspended when it receives a SIGSTOP signal. This is the same
as kill -19 on Linux, but might have a different number in other Unix systems.
A suspended process does not use any cpu cycles, but it stays in memory and can be reanimated with a SIGCONT signal (kill -18 on Linux).
Both signals will be used in the section about background processes.
344
introduction to processes
36.3.7. pkill
You can use the pkill command to kill a process by its command name.
[paul@RHEL5 ~]$ sleep 1000 &
[1] 30203
[paul@RHEL5 ~]$ pkill sleep
[1]+ Terminated
[paul@RHEL5 ~]$
sleep 1000
36.3.8. killall
The killall command will send a signal 15 to all processes with a certain name.
paul@rhel65:~$ sleep 8472 &
[1] 18780
paul@rhel65:~$ sleep 1201 &
[2] 18781
paul@rhel65:~$ jobs
[1]- Running
[2]+ Running
paul@rhel65:~$ killall sleep
[1]- Terminated
[2]+ Terminated
paul@rhel65:~$ jobs
paul@rhel65:~$
sleep 8472 &
sleep 1201 &
sleep 8472
sleep 1201
36.3.9. killall5
Its SysV counterpart killall5 can by used when shutting down the system. This screenshot
shows how Red Hat Enterprise Linux 5.3 uses killall5 when halting the system.
root@rhel53 ~# grep killall /etc/init.d/halt
action $"Sending all processes the TERM signal..." /sbin/killall5 -15
action $"Sending all processes the KILL signal..." /sbin/killall5 -9
36.3.10. top
Inside top the k key allows you to select a signal and pid to kill. Below is a partial screenshot
of the line just below the summary in top after pressing k.
PID to kill: 1932
Kill PID 1932 with signal [15]: 9
345
introduction to processes
36.4. practice : basic process management
1. Use ps to search for the init process by name.
2. What is the process id of the init process ?
3. Use the who am i command to determine your terminal name.
4. Using your terminal name from above, use ps to find all processes associated with your
terminal.
5. What is the process id of your shell ?
6. What is the parent process id of your shell ?
7. Start two instances of the sleep 3342 in background.
8. Locate the process id of all sleep commands.
9. Display only those two sleep processes in top. Then quit top.
10. Use a standard kill to kill one of the sleep processes.
11. Use one command to kill all sleep processes.
346
introduction to processes
36.5. solution : basic process management
1. Use ps to search for the init process by name.
root@rhel53 ~# ps -C init
PID TTY
TIME CMD
1 ?
00:00:04 init
2. What is the process id of the init process ?
1
3. Use the who am i command to determine your terminal name.
root@rhel53 ~# who am i
paul
pts/0
2010-04-12 17:44 (192.168.1.38)
4. Using your terminal name from above, use ps to find all processes associated with your
terminal.
oot@rhel53 ~# ps fax | grep pts/0
2941 ?
S
0:00
\_ sshd: paul@pts/0
2942 pts/0
Ss
0:00
\_ -bash
2972 pts/0
S
0:00
\_ su 2973 pts/0
S
0:00
\_ -bash
3808 pts/0
R+
0:00
\_ ps fax
3809 pts/0
R+
0:00
\_ grep pts/0
or also
root@rhel53 ~# ps -ef
paul
2941 2939
paul
2942 2941
root
2972 2942
root
2973 2972
root
3816 2973
root
3817 2973
|
0
0
0
0
0
0
grep pts/0
17:44 ?
17:44 pts/0
17:45 pts/0
17:45 pts/0
21:25 pts/0
21:25 pts/0
00:00:00
00:00:00
00:00:00
00:00:00
00:00:00
00:00:00
sshd: paul@pts/0
-bash
su -bash
ps -ef
grep pts/0
5. What is the process id of your shell ?
2973 in the screenshot above, probably different for you
echo $$ should display same number as the one you found
6. What is the parent process id of your shell ?
2972 in the screenshot above, probably different for you
in this example the PPID is from the su - command, but when inside gnome then for example
gnome-terminal can be the parent process
7. Start two instances of the sleep 3342 in background.
347
introduction to processes
sleep 3342 &
sleep 3342 &
8. Locate the process id of all sleep commands.
pidof sleep
9. Display only those two sleep processes in top. Then quit top.
top -p pidx,pidy (replace pidx pidy with the actual numbers)
10. Use a standard kill to kill one of the sleep processes.
kill pidx
11. Use one command to kill all sleep processes.
pkill sleep
348
Chapter 37. process priorities
349
process priorities
37.1. priority and nice values
37.1.1. introduction
All processes have a priority and a nice value. Higher priority processes will get more
cpu time than lower priority processes. You can influence this with the nice and renice
commands.
37.1.2. pipes (mkfifo)
Processes can communicate with each other via pipes. These pipes can be created with the
mkfifo command.
The screenshots shows the creation of four distinct pipes (in a new directory).
paul@ubuntu910:~$ mkdir procs
paul@ubuntu910:~$ cd procs/
paul@ubuntu910:~/procs$ mkfifo pipe33a pipe33b pipe42a pipe42b
paul@ubuntu910:~/procs$ ls -l
total 0
prw-r--r-- 1 paul paul 0 2010-04-12 13:21 pipe33a
prw-r--r-- 1 paul paul 0 2010-04-12 13:21 pipe33b
prw-r--r-- 1 paul paul 0 2010-04-12 13:21 pipe42a
prw-r--r-- 1 paul paul 0 2010-04-12 13:21 pipe42b
paul@ubuntu910:~/procs$
37.1.3. some fun with cat
To demonstrate the use of the top and renice commands we will make the cat command
use the previously created pipes to generate a full load on the cpu.
The cat is copied with a distinct name to the current directory. (This enables us to easily
recognize the processes within top. You could do the same exercise without copying the cat
command, but using different users. Or you could just look at the pid of each process.)
paul@ubuntu910:~/procs$
paul@ubuntu910:~/procs$
paul@ubuntu910:~/procs$
[1] 1670
paul@ubuntu910:~/procs$
[2] 1671
paul@ubuntu910:~/procs$
[3] 1673
paul@ubuntu910:~/procs$
[4] 1674
cp /bin/cat proj33
cp /bin/cat proj42
echo -n x | ./proj33 - pipe33a > pipe33b &
./proj33 <pipe33b >pipe33a &
echo -n z | ./proj42 - pipe42a > pipe42b &
./proj42 <pipe42b >pipe42a &
The commands you see above will create two proj33 processes that use cat to bounce the x
character between pipe33a and pipe33b. And ditto for the z character and proj42.
350
process priorities
37.1.4. top
Just running top without options or arguments will display all processes and an overview of
innformation. The top of the top screen might look something like this.
top - 13:59:29 up 48 min, 4 users, load average: 1.06, 0.25, 0.14
Tasks: 139 total,
3 running, 136 sleeping,
0 stopped,
0 zombie
Cpu(s): 0.3%us, 99.7%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem:
509352k total,
460040k used,
49312k free,
66752k buffers
Swap:
746980k total,
0k used,
746980k free,
247324k cached
Notice the cpu idle time (0.0%id) is zero. This is because our cat processes are consuming
the whole cpu. Results can vary on systems with four or more cpu cores.
37.1.5. top -p
The top -p 1670,1671,1673,1674 screenshot below shows four processes, all of then using
approximately 25 percent of the cpu.
paul@ubuntu910:~$ top -p 1670,1671,1673,1674
PID
1674
1670
1671
1673
USER
paul
paul
paul
paul
PR
20
20
20
20
NI
0
0
0
0
VIRT
2972
2972
2972
2972
RES
616
616
616
620
SHR
524
524
524
524
S
S
R
S
R
%CPU %MEM
26.6 0.1
25.0 0.1
24.6 0.1
23.0 0.1
TIME+
0:11.92
0:23.16
0:23.07
0:11.48
COMMAND
proj42
proj33
proj33
proj42
All four processes have an equal priority (PR), and are battling for cpu time. On some
systems the Linux kernel might attribute slightly varying priority values, but the result
will still be four processes fighting for cpu time.
37.1.6. renice
Since the processes are already running, we need to use the renice command to change their
nice value (NI).
The screenshot shows how to use renice on both the proj33 processes.
paul@ubuntu910:~$ renice +8 1670
1670: old priority 0, new priority 8
paul@ubuntu910:~$ renice +8 1671
1671: old priority 0, new priority 8
Normal users can attribute a nice value from zero to 20 to processes they own. Only the
root user can use negative nice values. Be very careful with negative nice values, since they
can make it impossible to use the keyboard or ssh to a system.
351
process priorities
37.1.7. impact of nice values
The impact of a nice value on running processes can vary. The screenshot below shows the
result of our renice +8 command. Look at the %CPU values.
PID
1674
1673
1671
1670
USER
paul
paul
paul
paul
PR
20
20
28
28
NI
0
0
8
8
VIRT
2972
2972
2972
2972
RES
616
620
616
616
SHR
524
524
524
524
S %CPU %MEM
S 46.6 0.1
R 42.6 0.1
S 5.7 0.1
R 4.7 0.1
TIME+
0:22.37
0:21.65
0:29.65
0:29.82
COMMAND
proj42
proj42
proj33
proj33
Important to remember is to always make less important processes nice to more important
processes. Using negative nice values can have a severe impact on a system's usability.
37.1.8. nice
The nice works identical to the renice but it is used when starting a command.
The screenshot shows how to start a script with a nice value of five.
paul@ubuntu910:~$ nice -5 ./backup.sh
352
process priorities
37.2. practice : process priorities
1. Create a new directory and create six pipes in that directory.
2. Bounce a character between two pipes.
3. Use top and ps to display information (pid, ppid, priority, nice value, ...) about these two
cat processes.
4. Bounce another character between two other pipes, but this time start the commands nice.
Verify that all cat processes are battling for the cpu. (Feel free to fire up two more cats with
the remaining pipes).
5. Use ps to verify that the two new cat processes have a nice value. Use the -o and -C
options of ps for this.
6. Use renice te increase the nice value from 10 to 15. Notice the difference with the usual
commands.
353
process priorities
37.3. solution : process priorities
1. Create a new directory and create six pipes in that directory.
[paul@rhel53
[paul@rhel53
[paul@rhel53
total 0
prw-rw-r-- 1
prw-rw-r-- 1
prw-rw-r-- 1
prw-rw-r-- 1
prw-rw-r-- 1
prw-rw-r-- 1
~]$ mkdir pipes ; cd pipes
pipes]$ mkfifo p1 p2 p3 p4 p5 p6
pipes]$ ls -l
paul
paul
paul
paul
paul
paul
paul
paul
paul
paul
paul
paul
0
0
0
0
0
0
Apr
Apr
Apr
Apr
Apr
Apr
12
12
12
12
12
12
22:15
22:15
22:15
22:15
22:15
22:15
p1
p2
p3
p4
p5
p6
2. Bounce a character between two pipes.
[paul@rhel53 pipes]$ echo -n x | cat - p1 > p2 &
[1] 4013
[paul@rhel53 pipes]$ cat <p2 >p1 &
[2] 4016
3. Use top and ps to display information (pid, ppid, priority, nice value, ...) about these two
cat processes.
top (probably the top two lines)
[paul@rhel53 pipes]$ ps
PID TTY
TIME
4013 pts/0
00:03:38
4016 pts/0
00:01:07
-C cat
CMD
cat
cat
[paul@rhel53 pipes]$ ps fax
4013 pts/0
R
4:00
4016 pts/0
S
1:13
4044 pts/0
S+
0:00
| grep cat
|
|
|
\_ cat - p1
\_ cat
\_ grep cat
4. Bounce another character between two other pipes, but this time start the commands nice.
Verify that all cat processes are battling for the cpu. (Feel free to fire up two more cats with
the remaining pipes).
echo -n y | nice cat - p3 > p4 &
nice cat <p4 >p3 &
5. Use ps to verify that the two new cat processes have a nice value. Use the -o and -C
options of ps for this.
[paul@rhel53 pipes]$ ps -C cat -o pid,ppid,pri,ni,comm
PID PPID PRI NI COMMAND
4013 3947 14
0 cat
4016 3947 21
0 cat
4025 3947 13 10 cat
4026 3947 13 10 cat
6. Use renice te increase the nice value from 10 to 15. Notice the difference with the usual
commands.
[paul@rhel53 pipes]$ renice +15 4025
4025: old priority 10, new priority 15
[paul@rhel53 pipes]$ renice +15 4026
354
process priorities
4026: old priority 10, new priority 15
[paul@rhel53 pipes]$ ps -C cat -o pid,ppid,pri,ni,comm
PID PPID PRI NI COMMAND
4013 3947 14
0 cat
4016 3947 21
0 cat
4025 3947
9 15 cat
4026 3947
8 15 cat
355
Chapter 38. background jobs
356
background jobs
38.1. background processes
38.1.1. jobs
Stuff that runs in background of your current shell can be displayed with the jobs command.
By default you will not have any jobs running in background.
root@rhel53 ~# jobs
root@rhel53 ~#
This jobs command will be used several times in this section.
38.1.2. control-Z
Some processes can be suspended with the Ctrl-Z key combination. This sends a SIGSTOP
signal to the Linux kernel, effectively freezing the operation of the process.
When doing this in vi(m), then vi(m) goes to the background. The background vi(m) can
be seen with the jobs command.
[paul@RHEL4a ~]$ vi procdemo.txt
[5]+ Stopped
[paul@RHEL4a ~]$ jobs
[5]+ Stopped
vim procdemo.txt
vim procdemo.txt
38.1.3. & ampersand
Processes that are started in background using the & character at the end of the command
line are also visible with the jobs command.
[paul@RHEL4a ~]$ find / > allfiles.txt 2> /dev/null &
[6] 5230
[paul@RHEL4a ~]$ jobs
[5]+ Stopped
vim procdemo.txt
[6]- Running
find / >allfiles.txt 2>/dev/null &
[paul@RHEL4a ~]$
38.1.4. jobs -p
An interesting option is jobs -p to see the process id of background processes.
[paul@RHEL4b
[1] 4902
[paul@RHEL4b
[2] 4903
[paul@RHEL4b
4902
4903
[paul@RHEL4b
~]$ sleep 500 &
~]$ sleep 400 &
~]$ jobs -p
~]$ ps `jobs -p`
357
background jobs
PID TTY
STAT
4902 pts/0
S
4903 pts/0
S
[paul@RHEL4b ~]$
TIME COMMAND
0:00 sleep 500
0:00 sleep 400
38.1.5. fg
Running the fg command will bring a background job to the foreground. The number of the
background job to bring forward is the parameter of fg.
[paul@RHEL5 ~]$ jobs
[1]
Running
[2]- Running
[3]+ Running
[paul@RHEL5 ~]$ fg 3
sleep 2000
sleep 1000 &
sleep 1000 &
sleep 2000 &
38.1.6. bg
Jobs that are suspended in background can be started in background with bg. The bg will
send a SIGCONT signal.
Below an example of the sleep command (suspended with Ctrl-Z) being reactivated in
background with bg.
[paul@RHEL5 ~]$ jobs
[paul@RHEL5 ~]$ sleep 5000 &
[1] 6702
[paul@RHEL5 ~]$ sleep 3000
[2]+ Stopped
[paul@RHEL5 ~]$
[1]- Running
[2]+ Stopped
[paul@RHEL5 ~]$
[2]+ sleep 3000
[paul@RHEL5 ~]$
[1]- Running
[2]+ Running
[paul@RHEL5 ~]$
sleep 3000
jobs
sleep 5000 &
sleep 3000
bg 2
&
jobs
sleep 5000 &
sleep 3000 &
358
background jobs
38.2. practice : background processes
1. Use the jobs command to verify whether you have any processes running in background.
2. Use vi to create a little text file. Suspend vi in background.
3. Verify with jobs that vi is suspended in background.
4. Start find / > allfiles.txt 2>/dev/null in foreground. Suspend it in background before it
finishes.
5. Start two long sleep processes in background.
6. Display all jobs in background.
7. Use the kill command to suspend the last sleep process.
8. Continue the find process in background (make sure it runs again).
9. Put one of the sleep commands back in foreground.
10. (if time permits, a general review question...) Explain in detail where the numbers come
from in the next screenshot. When are the variables replaced by their value ? By which shell ?
[paul@RHEL4b ~]$ echo $$ $PPID
4224 4223
[paul@RHEL4b ~]$ bash -c "echo $$ $PPID"
4224 4223
[paul@RHEL4b ~]$ bash -c 'echo $$ $PPID'
5059 4224
[paul@RHEL4b ~]$ bash -c `echo $$ $PPID`
4223: 4224: command not found
359
background jobs
38.3. solution : background processes
1. Use the jobs command to verify whether you have any processes running in background.
jobs (maybe the catfun is still running?)
2. Use vi to create a little text file. Suspend vi in background.
vi text.txt
(inside vi press ctrl-z)
3. Verify with jobs that vi is suspended in background.
[paul@rhel53 ~]$ jobs
[1]+ Stopped
vim text.txt
4. Start find / > allfiles.txt 2>/dev/null in foreground. Suspend it in background before it
finishes.
[paul@rhel53 ~]$ find / > allfiles.txt 2>/dev/null
(press ctrl-z)
[2]+ Stopped
find / > allfiles.txt 2> /dev/null
5. Start two long sleep processes in background.
sleep 4000 & ; sleep 5000 &
6. Display all jobs in background.
[paul@rhel53 ~]$ jobs
[1]- Stopped
[2]+ Stopped
[3]
Running
[4]
Running
vim text.txt
find / > allfiles.txt 2> /dev/null
sleep 4000 &
sleep 5000 &
7. Use the kill command to suspend the last sleep process.
[paul@rhel53 ~]$ kill -SIGSTOP 4519
[paul@rhel53 ~]$ jobs
[1]
Stopped
vim text.txt
[2]- Stopped
find / > allfiles.txt 2> /dev/null
[3]
Running
sleep 4000 &
[4]+ Stopped
sleep 5000
8. Continue the find process in background (make sure it runs again).
bg 2 (verify the job-id in your jobs list)
9. Put one of the sleep commands back in foreground.
fg 3 (again verify your job-id)
10. (if time permits, a general review question...) Explain in detail where the numbers come
from in the next screenshot. When are the variables replaced by their value ? By which shell ?
[paul@RHEL4b ~]$ echo $$ $PPID
4224 4223
[paul@RHEL4b ~]$ bash -c "echo $$ $PPID"
360
background jobs
4224 4223
[paul@RHEL4b ~]$ bash -c 'echo $$ $PPID'
5059 4224
[paul@RHEL4b ~]$ bash -c `echo $$ $PPID`
4223: 4224: command not found
The current bash shell will replace the $$ and $PPID while scanning the line, and before
executing the echo command.
[paul@RHEL4b ~]$ echo $$ $PPID
4224 4223
The variables are now double quoted, but the current bash shell will replace $$ and $PPID
while scanning the line, and before executing the bach -c command.
[paul@RHEL4b ~]$ bash -c "echo $$ $PPID"
4224 4223
The variables are now single quoted. The current bash shell will not replace the $$ and
the $PPID. The bash -c command will be executed before the variables replaced with their
value. This latter bash is the one replacing the $$ and $PPID with their value.
[paul@RHEL4b ~]$ bash -c 'echo $$ $PPID'
5059 4224
With backticks the shell will still replace both variable before the embedded echo is
executed. The result of this echo is the two process id's. These are given as commands to
bash -c. But two numbers are not commands!
[paul@RHEL4b ~]$ bash -c `echo $$ $PPID`
4223: 4224: command not found
361
Part XI. disk management
Table of Contents
39. disk devices ...................................................................................................................
39.1. terminology .........................................................................................................
39.2. device naming ......................................................................................................
39.3. discovering disk devices .........................................................................................
39.4. erasing a hard disk ................................................................................................
39.5. advanced hard disk settings ....................................................................................
39.6. practice: hard disk devices ......................................................................................
39.7. solution: hard disk devices .....................................................................................
40. disk partitions ...............................................................................................................
40.1. about partitions .....................................................................................................
40.2. discovering partitions .............................................................................................
40.3. partitioning new disks ............................................................................................
40.4. about the partition table .........................................................................................
40.5. practice: partitions .................................................................................................
40.6. solution: partitions ................................................................................................
41. file systems ....................................................................................................................
41.1. about file systems .................................................................................................
41.2. common file systems .............................................................................................
41.3. putting a file system on a partition ...........................................................................
41.4. tuning a file system ...............................................................................................
41.5. checking a file system ...........................................................................................
41.6. practice: file systems .............................................................................................
41.7. solution: file systems .............................................................................................
42. mounting ......................................................................................................................
42.1. mounting local file systems ....................................................................................
42.2. displaying mounted file systems ..............................................................................
42.3. from start to finish ................................................................................................
42.4. permanent mounts .................................................................................................
42.5. securing mounts ....................................................................................................
42.6. mounting remote file systems ..................................................................................
42.7. practice: mounting file systems ...............................................................................
42.8. solution: mounting file systems ...............................................................................
43. troubleshooting tools ......................................................................................................
43.1. lsof .....................................................................................................................
43.2. fuser ...................................................................................................................
43.3. chroot .................................................................................................................
43.4. iostat ..................................................................................................................
43.5. iotop ...................................................................................................................
43.6. vmstat .................................................................................................................
43.7. practice: troubleshooting tools .................................................................................
43.8. solution: troubleshooting tools .................................................................................
44. introduction to uuid's .....................................................................................................
44.1. about unique objects ..............................................................................................
44.2. tune2fs ................................................................................................................
44.3. uuid ....................................................................................................................
44.4. uuid in /etc/fstab ...................................................................................................
44.5. uuid as a boot device .............................................................................................
44.6. practice: uuid and filesystems .................................................................................
44.7. solution: uuid and filesystems .................................................................................
45. introduction to raid ........................................................................................................
45.1. hardware or software .............................................................................................
45.2. raid levels ............................................................................................................
45.3. building a software raid5 array ................................................................................
45.4. practice: raid ........................................................................................................
45.5. solution: raid ........................................................................................................
363
365
366
368
369
374
375
376
377
379
380
381
383
385
386
387
388
389
390
393
394
395
396
397
398
399
400
402
403
404
405
406
407
409
410
411
412
413
414
415
416
417
418
419
419
419
420
421
422
423
424
424
425
427
430
431
disk management
46. logical volume management ............................................................................................
46.1. introduction to lvm ...............................................................................................
46.2. lvm terminology ...................................................................................................
46.3. example: using lvm ...............................................................................................
46.4. example: extend a logical volume ............................................................................
46.5. example: resize a physical Volume ..........................................................................
46.6. example: mirror a logical volume ............................................................................
46.7. example: snapshot a logical volume .........................................................................
46.8. verifying existing physical volumes ..........................................................................
46.9. verifying existing volume groups .............................................................................
46.10. verifying existing logical volumes ..........................................................................
46.11. manage physical volumes .....................................................................................
46.12. manage volume groups .........................................................................................
46.13. manage logical volumes .......................................................................................
46.14. practice : lvm .....................................................................................................
46.15. solution : lvm .....................................................................................................
47. iSCSI devices .................................................................................................................
47.1. iSCSI terminology ................................................................................................
47.2. iSCSI Target in RHEL/CentOS ...............................................................................
47.3. iSCSI Initiator in RHEL/CentOS .............................................................................
47.4. iSCSI target on Debian ..........................................................................................
47.5. iSCSI target setup with dd files ...............................................................................
47.6. ISCSI initiator on ubuntu .......................................................................................
47.7. using iSCSI devices ..............................................................................................
47.8. practice: iSCSI devices ..........................................................................................
47.9. solution: iSCSI devices ..........................................................................................
48. introduction to multipathing ...........................................................................................
48.1. install multipath ....................................................................................................
48.2. configure multipath ...............................................................................................
48.3. network ...............................................................................................................
48.4. start multipathd and iscsi ........................................................................................
48.5. multipath list ........................................................................................................
48.6. using the device ....................................................................................................
48.7. practice: multipathing ............................................................................................
48.8. solution: multipathing ............................................................................................
364
432
433
434
435
437
439
441
442
443
445
446
447
449
451
453
454
458
459
459
461
463
464
466
468
469
470
471
472
472
473
473
475
476
477
478
Chapter 39. disk devices
This chapter teaches you how to locate and recognise hard disk devices. This prepares you
for the next chapter, where we put partitions on these devices.
365
disk devices
39.1. terminology
39.1.1. platter, head, track, cylinder, sector
Data is commonly stored on magnetic or optical disk platters. The platters are rotated (at
high speeds). Data is read by heads, which are very close to the surface of the platter, without
touching it! The heads are mounted on an arm (sometimes called a comb or a fork).
Data is written in concentric circles called tracks. Track zero is (usually) on the outside.
The time it takes to position the head over a certain track is called the seek time. Often
the platters are stacked on top of each other, hence the set of tracks accessible at a certain
position of the comb forms a cylinder. Tracks are divided into 512 byte sectors, with more
unused space (gap) between the sectors on the outside of the platter.
When you break down the advertised access time of a hard drive, you will notice that most
of that time is taken by movement of the heads (about 65%) and rotational latency (about
30%).
39.1.2. ide or scsi
Actually, the title should be ata or scsi, since ide is an ata compatible device. Most desktops
use ata devices, most servers use scsi.
39.1.3. ata
An ata controller allows two devices per bus, one master and one slave. Unless your
controller and devices support cable select, you have to set this manually with jumpers.
With the introduction of sata (serial ata), the original ata was renamed to parallel ata.
Optical drives often use atapi, which is an ATA interface using the SCSI communication
protocol.
39.1.4. scsi
A scsi controller allows more than two devices. When using SCSI (small computer system
interface), each device gets a unique scsi id. The scsi controller also needs a scsi id, do not
use this id for a scsi-attached device.
Older 8-bit SCSI is now called narrow, whereas 16-bit is wide. When the bus speeds was
doubled to 10Mhz, this was known as fast SCSI. Doubling to 20Mhz made it ultra SCSI.
Take a look at http://en.wikipedia.org/wiki/SCSI for more SCSI standards.
366
disk devices
39.1.5. block device
Random access hard disk devices have an abstraction layer called block device to enable
formatting in fixed-size (usually 512 bytes) blocks. Blocks can be accessed independent of
access to other blocks.
[root@centos65 ~]# lsblk
NAME
MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda
8:0
0
40G 0 disk
--sda1
8:1
0 500M 0 part /boot
--sda2
8:2
0 39.5G 0 part
--VolGroup-lv_root (dm-0) 253:0
0 38.6G 0 lvm /
--VolGroup-lv_swap (dm-1) 253:1
0 928M 0 lvm [SWAP]
sdb
8:16
0
72G 0 disk
sdc
8:32
0 144G 0 disk
A block device has the letter b to denote the file type in the output of ls -l.
[root@centos65 ~]#
brw-rw----. 1 root
brw-rw----. 1 root
brw-rw----. 1 root
brw-rw----. 1 root
brw-rw----. 1 root
ls -l /dev/sd*
disk 8, 0 Apr
disk 8, 1 Apr
disk 8, 2 Apr
disk 8, 16 Apr
disk 8, 32 Apr
19
19
19
19
19
10:12
10:12
10:12
10:12
10:12
/dev/sda
/dev/sda1
/dev/sda2
/dev/sdb
/dev/sdc
Note that a character device is a constant stream of characters, being denoted by a c in ls l. Note also that the ISO 9660 standard for cdrom uses a 2048 byte block size.
Old hard disks (and floppy disks) use cylinder-head-sector addressing to access a sector
on the disk. Most current disks use LBA (Logical Block Addressing).
39.1.6. solid state drive
A solid state drive or ssd is a block device without moving parts. It is comparable to flash
memory. An ssd is more expensive than a hard disk, but it typically has a much faster access
time.
In this book we will use the following pictograms for spindle disks (in brown) and solid
state disks (in blue).
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disk devices
39.2. device naming
39.2.1. ata (ide) device naming
All ata drives on your system will start with /dev/hd followed by a unit letter. The master
hdd on the first ata controller is /dev/hda, the slave is /dev/hdb. For the second controller,
the names of the devices are /dev/hdc and /dev/hdd.
Table 39.1. ide device naming
controller
ide0
ide1
connection
device name
master
/dev/hda
slave
/dev/hdb
master
/dev/hdc
slave
/dev/hdd
It is possible to have only /dev/hda and /dev/hdd. The first one is a single ata hard disk, the
second one is the cdrom (by default configured as slave).
39.2.2. scsi device naming
scsi drives follow a similar scheme, but all start with /dev/sd. When you run out of letters
(after /dev/sdz), you can continue with /dev/sdaa and /dev/sdab and so on. (We will see later
on that lvm volumes are commonly seen as /dev/md0, /dev/md1 etc.)
Below a sample of how scsi devices on a Linux can be named. Adding a scsi disk or raid
controller with a lower scsi address will change the naming scheme (shifting the higher scsi
addresses one letter further in the alphabet).
Table 39.2. scsi device naming
device
scsi id
device name
disk 0
0
/dev/sda
disk 1
1
/dev/sdb
raid controller 0
5
/dev/sdc
raid controller 1
6
/dev/sdd
A modern Linux system will use /dev/sd* for scsi and sata devices, and also for sd-cards,
usb-sticks, (legacy) ATA/IDE devices and solid state drives.
368
disk devices
39.3. discovering disk devices
39.3.1. fdisk
You can start by using /sbin/fdisk to find out what kind of disks are seen by the kernel.
Below the result on old Debian desktop, with two ata-ide disks present.
root@barry:~# fdisk -l | grep Disk
Disk /dev/hda: 60.0 GB, 60022480896 bytes
Disk /dev/hdb: 81.9 GB, 81964302336 bytes
And here an example of sata and scsi disks on a server with CentOS. Remember that sata
disks are also presented to you with the scsi /dev/sd* notation.
[root@centos65
Disk /dev/sda:
Disk /dev/sdb:
Disk /dev/sdc:
Disk /dev/sdd:
~]# fdisk -l | grep 'Disk /dev/sd'
42.9 GB, 42949672960 bytes
77.3 GB, 77309411328 bytes
154.6 GB, 154618822656 bytes
154.6 GB, 154618822656 bytes
Here is an overview of disks on a RHEL4u3 server with two real 72GB scsi disks. This
server is attached to a NAS with four NAS disks of half a terabyte. On the NAS disks, four
LVM (/dev/mdx) software RAID devices are configured.
[root@tsvtl1 ~]# fdisk -l | grep Disk
Disk /dev/sda: 73.4 GB, 73407488000 bytes
Disk /dev/sdb: 73.4 GB, 73407488000 bytes
Disk /dev/sdc: 499.0 GB, 499036192768 bytes
Disk /dev/sdd: 499.0 GB, 499036192768 bytes
Disk /dev/sde: 499.0 GB, 499036192768 bytes
Disk /dev/sdf: 499.0 GB, 499036192768 bytes
Disk /dev/md0: 271 MB, 271319040 bytes
Disk /dev/md2: 21.4 GB, 21476081664 bytes
Disk /dev/md3: 21.4 GB, 21467889664 bytes
Disk /dev/md1: 21.4 GB, 21476081664 bytes
You can also use fdisk to obtain information about one specific hard disk device.
[root@centos65 ~]# fdisk -l /dev/sdc
Disk /dev/sdc: 154.6 GB, 154618822656 bytes
255 heads, 63 sectors/track, 18798 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000
Later we will use fdisk to do dangerous stuff like creating and deleting partitions.
369
disk devices
39.3.2. dmesg
Kernel boot messages can be seen after boot with dmesg. Since hard disk devices are
detected by the kernel during boot, you can also use dmesg to find information about disk
devices.
[root@centos65 ~]# dmesg | grep 'sd[a-z]' | head
sd 0:0:0:0: [sda] 83886080 512-byte logical blocks: (42.9 GB/40.0 GiB)
sd 0:0:0:0: [sda] Write Protect is off
sd 0:0:0:0: [sda] Mode Sense: 00 3a 00 00
sd 0:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support \
DPO or FUA
sda: sda1 sda2
sd 0:0:0:0: [sda] Attached SCSI disk
sd 3:0:0:0: [sdb] 150994944 512-byte logical blocks: (77.3 GB/72.0 GiB)
sd 3:0:0:0: [sdb] Write Protect is off
sd 3:0:0:0: [sdb] Mode Sense: 00 3a 00 00
sd 3:0:0:0: [sdb] Write cache: enabled, read cache: enabled, doesn't support \
DPO or FUA
Here is another example of dmesg on a computer with a 200GB ata disk.
paul@barry:~$ dmesg
[
2.624149] hda:
[
2.904150] hdb:
[
3.472148] hdd:
| grep -i "ata disk"
ST360021A, ATA DISK drive
Maxtor 6Y080L0, ATA DISK drive
WDC WD2000BB-98DWA0, ATA DISK drive
Third and last example of dmesg running on RHEL5.3.
root@rhel53
sd 0:0:2:0:
sd 0:0:3:0:
sd 0:0:6:0:
~# dmesg
Attached
Attached
Attached
| grep -i
scsi disk
scsi disk
scsi disk
"scsi disk"
sda
sdb
sdc
370
disk devices
39.3.3. /sbin/lshw
The lshw tool will list hardware. With the right options lshw can show a lot of information
about disks (and partitions).
Below a truncated screenshot on Debian 6:
root@debian6~# lshw -class volume | grep -A1 -B2 scsi
description: Linux raid autodetect partition
physical id: 1
bus info: scsi@1:0.0.0,1
logical name: /dev/sdb1
-description: Linux raid autodetect partition
physical id: 1
bus info: scsi@2:0.0.0,1
logical name: /dev/sdc1
-description: Linux raid autodetect partition
physical id: 1
bus info: scsi@3:0.0.0,1
logical name: /dev/sdd1
-description: Linux raid autodetect partition
physical id: 1
bus info: scsi@4:0.0.0,1
logical name: /dev/sde1
-vendor: Linux
physical id: 1
bus info: scsi@0:0.0.0,1
logical name: /dev/sda1
-vendor: Linux
physical id: 2
bus info: scsi@0:0.0.0,2
logical name: /dev/sda2
-description: Extended partition
physical id: 3
bus info: scsi@0:0.0.0,3
logical name: /dev/sda3
Redhat and CentOS do not have this tool (unless you add a repository).
371
disk devices
39.3.4. /sbin/lsscsi
The lsscsi command provides a nice readable output of all scsi (and scsi emulated devices).
This first screenshot shows lsscsi on a SPARC system.
root@shaka:~# lsscsi
[0:0:0:0]
disk
Adaptec
[1:0:0:0]
disk
SEAGATE
root@shaka:~#
RAID5
ST336605FSUN36G
V1.0
0438
/dev/sda
/dev/sdb
Below a screenshot of lsscsi on a QNAP NAS (which has four 750GB disks and boots from
a usb stick).
lroot@debian6~# lsscsi
[0:0:0:0]
disk
SanDisk
[1:0:0:0]
disk
ATA
[2:0:0:0]
disk
ATA
[3:0:0:0]
disk
ATA
[4:0:0:0]
disk
ATA
Cruzer Edge
ST3750330AS
ST3750330AS
ST3750330AS
ST3750330AS
1.19
SD04
SD04
SD04
SD04
/dev/sda
/dev/sdb
/dev/sdc
/dev/sdd
/dev/sde
This screenshot shows the classic output of lsscsi.
root@debian6~# lsscsi -c
Attached devices:
Host: scsi0 Channel: 00 Target: 00 Lun:
Vendor: SanDisk Model: Cruzer Edge
Type:
Direct-Access
Host: scsi1 Channel: 00 Target: 00 Lun:
Vendor: ATA
Model: ST3750330AS
Type:
Direct-Access
Host: scsi2 Channel: 00 Target: 00 Lun:
Vendor: ATA
Model: ST3750330AS
Type:
Direct-Access
Host: scsi3 Channel: 00 Target: 00 Lun:
Vendor: ATA
Model: ST3750330AS
Type:
Direct-Access
Host: scsi4 Channel: 00 Target: 00 Lun:
Vendor: ATA
Model: ST3750330AS
Type:
Direct-Access
00
Rev: 1.19
ANSI SCSI revision: 02
00
Rev: SD04
ANSI SCSI revision: 05
00
Rev: SD04
ANSI SCSI revision: 05
00
Rev: SD04
ANSI SCSI revision: 05
00
372
Rev: SD04
ANSI SCSI revision: 05
disk devices
39.3.5. /proc/scsi/scsi
Another way to locate scsi (or sd) devices is via /proc/scsi/scsi.
This screenshot is from a sparc computer with adaptec RAID5.
root@shaka:~# cat /proc/scsi/scsi
Attached devices:
Host: scsi0 Channel: 00 Id: 00 Lun: 00
Vendor: Adaptec Model: RAID5
Type:
Direct-Access
Host: scsi1 Channel: 00 Id: 00 Lun: 00
Vendor: SEAGATE Model: ST336605FSUN36G
Type:
Direct-Access
root@shaka:~#
Rev: V1.0
ANSI SCSI revision: 02
Rev: 0438
ANSI SCSI revision: 03
Here we run cat /proc/scsi/scsi on the QNAP from above (with Debian Linux).
root@debian6~# cat /proc/scsi/scsi
Attached devices:
Host: scsi0 Channel: 00 Id: 00 Lun: 00
Vendor: SanDisk Model: Cruzer Edge
Type:
Direct-Access
Host: scsi1 Channel: 00 Id: 00 Lun: 00
Vendor: ATA
Model: ST3750330AS
Type:
Direct-Access
Host: scsi2 Channel: 00 Id: 00 Lun: 00
Vendor: ATA
Model: ST3750330AS
Type:
Direct-Access
Host: scsi3 Channel: 00 Id: 00 Lun: 00
Vendor: ATA
Model: ST3750330AS
Type:
Direct-Access
Host: scsi4 Channel: 00 Id: 00 Lun: 00
Vendor: ATA
Model: ST3750330AS
Type:
Direct-Access
Rev: 1.19
ANSI SCSI revision: 02
Rev: SD04
ANSI SCSI revision: 05
Rev: SD04
ANSI SCSI revision: 05
Rev: SD04
ANSI SCSI revision: 05
Rev: SD04
ANSI SCSI revision: 05
Note that some recent versions of Debian have this disabled in the kernel. You can enable
it (after a kernel compile) using this entry:
# CONFIG_SCSI_PROC_FS is not set
Redhat and CentOS have this by default (if there are scsi devices present).
[root@centos65 ~]# cat /proc/scsi/scsi
Attached devices:
Host: scsi0 Channel: 00 Id: 00 Lun: 00
Vendor: ATA
Model: VBOX HARDDISK
Type:
Direct-Access
Host: scsi3 Channel: 00 Id: 00 Lun: 00
Vendor: ATA
Model: VBOX HARDDISK
Type:
Direct-Access
Host: scsi4 Channel: 00 Id: 00 Lun: 00
Vendor: ATA
Model: VBOX HARDDISK
Type:
Direct-Access
373
Rev: 1.0
ANSI SCSI revision: 05
Rev: 1.0
ANSI SCSI revision: 05
Rev: 1.0
ANSI SCSI revision: 05
disk devices
39.4. erasing a hard disk
Before selling your old hard disk on the internet, it may be a good idea to erase it. By simply
repartitioning, or by using the Microsoft Windows format utility, or even after an mkfs
command, some people will still be able to read most of the data on the disk.
root@debian6~# aptitude search foremost autopsy sleuthkit | tr -s ' '
p autopsy - graphical interface to SleuthKit
p foremost - Forensics application to recover data
p sleuthkit - collection of tools for forensics analysis
Although technically the /sbin/badblocks tool is meant to look for bad blocks, you can use
it to completely erase all data from a disk. Since this is really writing to every sector of the
disk, it can take a long time!
root@RHELv4u2:~# badblocks -ws /dev/sdb
Testing with pattern 0xaa: done
Reading and comparing: done
Testing with pattern 0x55: done
Reading and comparing: done
Testing with pattern 0xff: done
Reading and comparing: done
Testing with pattern 0x00: done
Reading and comparing: done
The previous screenshot overwrites every sector of the disk four times. Erasing once with
a tool like dd is enough to destroy all data.
Warning, this screenshot shows how to permanently destroy all data on a block device.
[root@rhel65 ~]# dd if=/dev/zero of=/dev/sdb
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disk devices
39.5. advanced hard disk settings
Tweaking of hard disk settings (dma, gap, ...) are not covered in this course. Several tools
exists, hdparm and sdparm are two of them.
hdparm can be used to display or set information and parameters about an ATA (or SATA)
hard disk device. The -i and -I options will give you even more information about the
physical properties of the device.
root@laika:~# hdparm /dev/sdb
/dev/sdb:
IO_support
readonly
readahead
geometry
= 0 (default 16-bit)
= 0 (off)
= 256 (on)
= 12161/255/63, sectors = 195371568, start = 0
Below hdparm info about a 200GB IDE disk.
root@barry:~# hdparm /dev/hdd
/dev/hdd:
multcount
IO_support
unmaskirq
using_dma
keepsettings
readonly
readahead
geometry
= 0 (off)
= 0 (default)
= 0 (off)
= 1 (on)
= 0 (off)
= 0 (off)
= 256 (on)
= 24321/255/63, sectors = 390721968, start = 0
Here a screenshot of sdparm on Ubuntu 10.10.
root@ubu1010:~# aptitude install sdparm
...
root@ubu1010:~# sdparm /dev/sda | head -1
/dev/sda: ATA
FUJITSU MJA2160B 0081
root@ubu1010:~# man sdparm
Use hdparm and sdparm with care.
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disk devices
39.6. practice: hard disk devices
About this lab: To practice working with hard disks, you will need some hard disks. When
there are no physical hard disk available, you can use virtual disks in vmware or VirtualBox.
The teacher will help you in attaching a couple of ATA and/or SCSI disks to a virtual
machine. The results of this lab can be used in the next three labs (partitions, file systems,
mounting).
It is adviced to attach three 1GB disks and three 2GB disks to the virtual machine. This will
allow for some freedom in the practices of this chapter as well as the next chapters (raid,
lvm, iSCSI).
1. Use dmesg to make a list of hard disk devices detected at boot-up.
2. Use fdisk to find the total size of all hard disk devices on your system.
3. Stop a virtual machine, add three virtual 1 gigabyte scsi hard disk devices and one virtual
400 megabyte ide hard disk device. If possible, also add another virtual 400 megabyte ide
disk.
4. Use dmesg to verify that all the new disks are properly detected at boot-up.
5. Verify that you can see the disk devices in /dev.
6. Use fdisk (with grep and /dev/null) to display the total size of the new disks.
7. Use badblocks to completely erase one of the smaller hard disks.
8. Look at /proc/scsi/scsi.
9. If possible, install lsscsi, lshw and use them to list the disks.
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disk devices
39.7. solution: hard disk devices
1. Use dmesg to make a list of hard disk devices detected at boot-up.
Some possible answers...
dmesg | grep -i disk
Looking for ATA disks: dmesg | grep hd[abcd]
Looking for ATA disks: dmesg | grep -i "ata disk"
Looking for SCSI disks: dmesg | grep sd[a-f]
Looking for SCSI disks: dmesg | grep -i "scsi disk"
2. Use fdisk to find the total size of all hard disk devices on your system.
fdisk -l
3. Stop a virtual machine, add three virtual 1 gigabyte scsi hard disk devices and one virtual
400 megabyte ide hard disk device. If possible, also add another virtual 400 megabyte ide
disk.
This exercise happens in the settings of vmware or VirtualBox.
4. Use dmesg to verify that all the new disks are properly detected at boot-up.
See 1.
5. Verify that you can see the disk devices in /dev.
SCSI+SATA: ls -l /dev/sd*
ATA: ls -l /dev/hd*
6. Use fdisk (with grep and /dev/null) to display the total size of the new disks.
root@rhel53 ~#
Disk /dev/hda:
Disk /dev/hdb:
Disk /dev/sda:
Disk /dev/sdb:
Disk /dev/sdc:
fdisk -l
21.4 GB,
1073 MB,
2147 MB,
2147 MB,
2147 MB,
2>/dev/null | grep [MGT]B
21474836480 bytes
1073741824 bytes
2147483648 bytes
2147483648 bytes
2147483648 bytes
7. Use badblocks to completely erase one of the smaller hard disks.
#Verify the device (/dev/sdc??) you want to erase before typing this.
#
root@rhel53 ~# badblocks -ws /dev/sdc
Testing with pattern 0xaa: done
Reading and comparing: done
Testing with pattern 0x55: done
Reading and comparing: done
Testing with pattern 0xff: done
Reading and comparing: done
Testing with pattern 0x00: done
Reading and comparing: done
8. Look at /proc/scsi/scsi.
root@rhel53 ~# cat /proc/scsi/scsi
377
disk devices
Attached devices:
Host: scsi0 Channel: 00 Id: 02 Lun: 00
Vendor: VBOX
Model: HARDDISK
Type:
Direct-Access
Host: scsi0 Channel: 00 Id: 03 Lun: 00
Vendor: VBOX
Model: HARDDISK
Type:
Direct-Access
Host: scsi0 Channel: 00 Id: 06 Lun: 00
Vendor: VBOX
Model: HARDDISK
Type:
Direct-Access
Rev: 1.0
ANSI SCSI revision: 05
Rev: 1.0
ANSI SCSI revision: 05
Rev: 1.0
ANSI SCSI revision: 05
9. If possible, install lsscsi, lshw and use them to list the disks.
Debian,Ubuntu: aptitude install lsscsi lshw
Fedora: yum install lsscsi lshw
root@rhel53 ~# lsscsi
[0:0:2:0]
disk
VBOX
[0:0:3:0]
disk
VBOX
[0:0:6:0]
disk
VBOX
HARDDISK
HARDDISK
HARDDISK
378
1.0
1.0
1.0
/dev/sda
/dev/sdb
/dev/sdc
Chapter 40. disk partitions
This chapter continues on the hard disk devices from the previous one. Here we will put
partitions on those devices.
This chapter prepares you for the next chapter, where we put file systems on our partitions.
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disk partitions
40.1. about partitions
40.1.1. primary, extended and logical
Linux requires you to create one or more partitions. The next paragraphs will explain how
to create and use partitions.
A partition's geometry and size is usually defined by a starting and ending cylinder
(sometimes by sector). Partitions can be of type primary (maximum four), extended
(maximum one) or logical (contained within the extended partition). Each partition has a
type field that contains a code. This determines the computers operating system or the
partitions file system.
Table 40.1. primary, extended and logical partitions
Partition Type
naming
Primary (max 4)
1-4
Extended (max 1)
1-4
Logical
5-
40.1.2. partition naming
We saw before that hard disk devices are named /dev/hdx or /dev/sdx with x depending on
the hardware configuration. Next is the partition number, starting the count at 1. Hence the
four (possible) primary partitions are numbered 1 to 4. Logical partition counting always
starts at 5. Thus /dev/hda2 is the second partition on the first ATA hard disk device, and /
dev/hdb5 is the first logical partition on the second ATA hard disk device. Same for SCSI, /
dev/sdb3 is the third partition on the second SCSI disk.
Table 40.2. Partition naming
partition
device
/dev/hda1
first primary partition on /dev/hda
/dev/hda2
second primary or extended partition on /dev/hda
/dev/sda5
first logical drive on /dev/sda
/dev/sdb6
second logical on /dev/sdb
The picture below shows two (spindle) disks with partitions. Note that an extended partition
is a container holding logical drives.
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disk partitions
40.2. discovering partitions
40.2.1. fdisk -l
In the fdisk -l example below you can see that two partitions exist on /dev/sdb. The first
partition spans 31 cylinders and contains a Linux swap partition. The second partition is
much bigger.
root@laika:~# fdisk -l /dev/sdb
Disk /dev/sdb: 100.0 GB, 100030242816 bytes
255 heads, 63 sectors/track, 12161 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot
/dev/sdb1
/dev/sdb2
root@laika:~#
Start
1
32
End
31
12161
Blocks
248976
97434225
Id
82
83
System
Linux swap / Solaris
Linux
40.2.2. /proc/partitions
The /proc/partitions file contains a table with major and minor number of partitioned
devices, their number of blocks and the device name in /dev. Verify with /proc/devices to
link the major number to the proper device.
paul@RHELv4u4:~$ cat /proc/partitions
major minor #blocks name
3
3
8
8
8
8
8
8
253
253
0
64
0
1
2
16
32
48
0
1
524288
734003
8388608
104391
8281507
1048576
1048576
1048576
7176192
1048576
hda
hdb
sda
sda1
sda2
sdb
sdc
sdd
dm-0
dm-1
The major number corresponds to the device type (or driver) and can be found in /proc/
devices. In this case 3 corresponds to ide and 8 to sd. The major number determines the
device driver to be used with this device.
The minor number is a unique identification of an instance of this device type. The
devices.txt file in the kernel tree contains a full list of major and minor numbers.
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disk partitions
40.2.3. parted and others
You may be interested in alternatives to fdisk like parted, cfdisk, sfdisk and gparted. This
course mainly uses fdisk to partition hard disks.
parted is recommended by some Linux distributions for handling storage with gpt instead
of mbr.
Below a screenshot of parted on CentOS.
[root@centos65 ~]# rpm -q parted
parted-2.1-21.el6.x86_64
[root@centos65 ~]# parted /dev/sda
GNU Parted 2.1
Using /dev/sda
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) print
Model: ATA VBOX HARDDISK (scsi)
Disk /dev/sda: 42.9GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Number
1
2
Start
1049kB
525MB
End
525MB
42.9GB
Size
524MB
42.4GB
Type
primary
primary
(parted)
382
File system
ext4
Flags
boot
lvm
disk partitions
40.3. partitioning new disks
In the example below, we bought a new disk for our system. After the new hardware is
properly attached, you can use fdisk and parted to create the necessary partition(s). This
example uses fdisk, but there is nothing wrong with using parted.
40.3.1. recognising the disk
First, we check with fdisk -l whether Linux can see the new disk. Yes it does, the new disk
is seen as /dev/sdb, but it does not have any partitions yet.
root@RHELv4u2:~# fdisk -l
Disk /dev/sda: 12.8 GB, 12884901888 bytes
255 heads, 63 sectors/track, 1566 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot
/dev/sda1
*
/dev/sda2
Start
1
14
End
13
1566
Blocks
104391
12474472+
Id
83
8e
System
Linux
Linux LVM
Disk /dev/sdb: 1073 MB, 1073741824 bytes
255 heads, 63 sectors/track, 130 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk /dev/sdb doesn't contain a valid partition table
40.3.2. opening the disk with fdisk
Then we create a partition with fdisk on /dev/sdb. First we start the fdisk tool with /dev/sdb
as argument. Be very very careful not to partition the wrong disk!!
root@RHELv4u2:~# fdisk /dev/sdb
Device contains neither a valid DOS partition table, nor Sun, SGI...
Building a new DOS disklabel. Changes will remain in memory only,
until you decide to write them. After that, of course, the previous
content won't be recoverable.
Warning: invalid flag 0x0000 of partition table 4 will be corrected...
40.3.3. empty partition table
Inside the fdisk tool, we can issue the p command to see the current disks partition table.
Command (m for help): p
Disk /dev/sdb: 1073 MB, 1073741824 bytes
255 heads, 63 sectors/track, 130 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot
Start
End
Blocks
383
Id
System
disk partitions
40.3.4. create a new partition
No partitions exist yet, so we issue n to create a new partition. We choose p for primary, 1
for the partition number, 1 for the start cylinder and 14 for the end cylinder.
Command (m for help): n
Command action
e
extended
p
primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-130, default 1):
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-130, default 130): 14
We can now issue p again to verify our changes, but they are not yet written to disk. This
means we can still cancel this operation! But it looks good, so we use w to write the changes
to disk, and then quit the fdisk tool.
Command (m for help): p
Disk /dev/sdb: 1073 MB, 1073741824 bytes
255 heads, 63 sectors/track, 130 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot
/dev/sdb1
Start
End
1
14
Blocks
Id System
112423+ 83 Linux
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
Syncing disks.
root@RHELv4u2:~#
40.3.5. display the new partition
Let's verify again with fdisk -l to make sure reality fits our dreams. Indeed, the screenshot
below now shows a partition on /dev/sdb.
root@RHELv4u2:~# fdisk -l
Disk /dev/sda: 12.8 GB, 12884901888 bytes
255 heads, 63 sectors/track, 1566 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot
/dev/sda1
*
/dev/sda2
Start
1
14
End
13
1566
Blocks
104391
12474472+
Id
83
8e
System
Linux
Linux LVM
Disk /dev/sdb: 1073 MB, 1073741824 bytes
255 heads, 63 sectors/track, 130 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot
Start
/dev/sdb1
root@RHELv4u2:~#
End
1
14
Blocks
Id System
112423+ 83 Linux
384
disk partitions
40.4. about the partition table
40.4.1. master boot record
The partition table information (primary and extended partitions) is written in the master
boot record or mbr. You can use dd to copy the mbr to a file.
This example copies the master boot record from the first SCSI hard disk.
dd if=/dev/sda of=/SCSIdisk.mbr bs=512 count=1
The same tool can also be used to wipe out all information about partitions on a disk. This
example writes zeroes over the master boot record.
dd if=/dev/zero of=/dev/sda bs=512 count=1
Or to wipe out the whole partition or disk.
dd if=/dev/zero of=/dev/sda
40.4.2. partprobe
Don't forget that after restoring a master boot record with dd, that you need to force the
kernel to reread the partition table with partprobe. After running partprobe, the partitions
can be used again.
[root@RHEL5 ~]# partprobe
[root@RHEL5 ~]#
40.4.3. logical drives
The partition table does not contain information about logical drives. So the dd backup
of the mbr only works for primary and extended partitions. To backup the partition table
including the logical drives, you can use sfdisk.
This example shows how to backup all partition and logical drive information to a file.
sfdisk -d /dev/sda > parttable.sda.sfdisk
The following example copies the mbr and all logical drive info from /dev/sda to /dev/sdb.
sfdisk -d /dev/sda | sfdisk /dev/sdb
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disk partitions
40.5. practice: partitions
1. Use fdisk -l to display existing partitions and sizes.
2. Use df -h to display existing partitions and sizes.
3. Compare the output of fdisk and df.
4. Create a 200MB primary partition on a small disk.
5. Create a 400MB primary partition and two 300MB logical drives on a big disk.
6. Use df -h and fdisk -l to verify your work.
7. Compare the output again of fdisk and df. Do both commands display the new partitions ?
8. Create a backup with dd of the mbr that contains your 200MB primary partition.
9. Take a backup of the partition table containing your 400MB primary and 300MB logical
drives. Make sure the logical drives are in the backup.
10. (optional) Remove all your partitions with fdisk. Then restore your backups.
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disk partitions
40.6. solution: partitions
1. Use fdisk -l to display existing partitions and sizes.
as root: # fdisk -l
2. Use df -h to display existing partitions and sizes.
df -h
3. Compare the output of fdisk and df.
Some partitions will be listed in both outputs (maybe /dev/sda1 or /dev/hda1).
4. Create a 200MB primary partition on a small disk.
Choose one of the disks you added (this example uses /dev/sdc).
root@rhel53 ~# fdisk /dev/sdc
...
Command (m for help): n
Command action
e
extended
p
primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-261, default 1): 1
Last cylinder or +size or +sizeM or +sizeK (1-261, default 261): +200m
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
Syncing disks.
5. Create a 400MB primary partition and two 300MB logical drives on a big disk.
Choose one of the disks you added (this example uses /dev/sdb)
fdisk /dev/sdb
inside fdisk : n p 1 +400m enter --- n e 2 enter enter --- n l +300m (twice)
6. Use df -h and fdisk -l to verify your work.
fdisk -l ; df -h
7. Compare the output again of fdisk and df. Do both commands display the new partitions ?
The newly created partitions are visible with fdisk.
But they are not displayed by df.
8. Create a backup with dd of the mbr that contains your 200MB primary partition.
dd if=/dev/sdc of=bootsector.sdc.dd count=1 bs=512
9. Take a backup of the partition table containing your 400MB primary and 300MB logical
drives. Make sure the logical drives are in the backup.
sfdisk -d /dev/sdb > parttable.sdb.sfdisk
387
Chapter 41. file systems
When you are finished partitioning the hard disk, you can put a file system on each partition.
This chapter builds on the partitions from the previous chapter, and prepares you for the
next one where we will mount the filesystems.
388
file systems
41.1. about file systems
A file system is a way of organizing files on your partition. Besides file-based storage, file
systems usually include directories and access control, and contain meta information about
files like access times, modification times and file ownership.
The properties (length, character set, ...) of filenames are determined by the file system you
choose. Directories are usually implemented as files, you will have to learn how this is
implemented! Access control in file systems is tracked by user ownership (and group ownerand membership) in combination with one or more access control lists.
41.1.1. man fs
The manual page about filesystems is accessed by typing man fs.
[root@rhel65 ~]# man fs
41.1.2. /proc/filesystems
The Linux kernel will inform you about currently loaded file system drivers in /proc/
filesystems.
root@rhel53 ~# cat /proc/filesystems
ext2
iso9660
ext3
| grep -v nodev
41.1.3. /etc/filesystems
The /etc/filesystems file contains a list of autodetected filesystems (in case the mount
command is used without the -t option.
Help for this file is provided by man mount.
[root@rhel65 ~]# man mount
389
file systems
41.2. common file systems
41.2.1. ext2 and ext3
Once the most common Linux file systems is the ext2 (the second extended) file system. A
disadvantage is that file system checks on ext2 can take a long time.
ext2 was being replaced by ext3 on most Linux machines. They are essentially the same,
except for the journaling which is only present in ext3.
Journaling means that changes are first written to a journal on the disk. The journal is
flushed regularly, writing the changes in the file system. Journaling keeps the file system
in a consistent state, so you don't need a file system check after an unclean shutdown or
power failure.
41.2.2. creating ext2 and ext3
You can create these file systems with the /sbin/mkfs or /sbin/mke2fs commands. Use
mke2fs -j to create an ext3 file system.
You can convert an ext2 to ext3 with tune2fs -j. You can mount an ext3 file system as ext2,
but then you lose the journaling. Do not forget to run mkinitrd if you are booting from this
device.
41.2.3. ext4
The newest incarnation of the ext file system is named ext4 and is available in the Linux
kernel since 2008. ext4 supports larger files (up to 16 terabyte) and larger file systems than
ext3 (and many more features).
Development started by making ext3 fully capable for 64-bit. When it turned out the changes
were significant, the developers decided to name it ext4.
41.2.4. xfs
Redhat Enterprise Linux 7 will have XFS as the default file system. This is a highly scalable
high-performance file system.
xfs was created for Irix and for a couple of years it was also used in FreeBSD. It is supported
by the Linux kernel, but rarely used in dsitributions outside of the Redhat/CentOS realm.
390
file systems
41.2.5. vfat
The vfat file system exists in a couple of forms : fat12 for floppy disks, fat16 on ms-dos, and
fat32 for larger disks. The Linux vfat implementation supports all of these, but vfat lacks a
lot of features like security and links. fat disks can be read by every operating system, and
are used a lot for digital cameras, usb sticks and to exchange data between different OS'ses
on a home user's computer.
41.2.6. iso 9660
iso 9660 is the standard format for cdroms. Chances are you will encounter this file system
also on your hard disk in the form of images of cdroms (often with the .iso extension). The
iso 9660 standard limits filenames to the 8.3 format. The Unix world didn't like this, and thus
added the rock ridge extensions, which allows for filenames up to 255 characters and Unixstyle file-modes, ownership and symbolic links. Another extensions to iso 9660 is joliet,
which adds 64 unicode characters to the filename. The el torito standard extends iso 9660
to be able to boot from CD-ROM's.
41.2.7. udf
Most optical media today (including cd's and dvd's) use udf, the Universal Disk Format.
41.2.8. swap
All things considered, swap is not a file system. But to use a partition as a swap partition
it must be formatted and mounted as swap space.
41.2.9. gfs
Linux clusters often use a dedicated cluster filesystem like GFS, GFS2, ClusterFS, ...
41.2.10. and more...
You may encounter reiserfs on older Linux systems. Maybe you will see Sun's zfs or the
open source btrfs. This last one requires a chapter on itself.
391
file systems
41.2.11. /proc/filesystems
The /proc/filesystems file displays a list of supported file systems. When you mount a file
system without explicitly defining one, then mount will first try to probe /etc/filesystems
and then probe /proc/filesystems for all the filesystems without the nodev label. If /etc/
filesystems ends with a line containing only an asterisk (*) then both files are probed.
paul@RHELv4u4:~$ cat /proc/filesystems
nodev
sysfs
nodev
rootfs
nodev
bdev
nodev
proc
nodev
sockfs
nodev
binfmt_misc
nodev
usbfs
nodev
usbdevfs
nodev
futexfs
nodev
tmpfs
nodev
pipefs
nodev
eventpollfs
nodev
devpts
ext2
nodev
ramfs
nodev
hugetlbfs
iso9660
nodev
relayfs
nodev
mqueue
nodev
selinuxfs
ext3
nodev
rpc_pipefs
nodev
vmware-hgfs
nodev
autofs
paul@RHELv4u4:~$
392
file systems
41.3. putting a file system on a partition
We now have a fresh partition. The system binaries to make file systems can be found with ls.
[root@RHEL4b ~]# ls -lS
-rwxr-xr-x 3 root root
-rwxr-xr-x 3 root root
-rwxr-xr-x 3 root root
-rwxr-xr-x 3 root root
-rwxr-xr-x 3 root root
-rwxr-xr-x 3 root root
-rwxr-xr-x 1 root root
-rwxr-x--- 1 root root
-rwxr-xr-x 1 root root
-rwxr-xr-x 1 root root
-rwxr-xr-x 1 root root
-rwxr-xr-x 1 root root
[root@RHEL4b ~]#
/sbin/mk*
34832 Apr
34832 Apr
34832 Apr
28484 Oct
28484 Oct
28484 Oct
20313 Apr
15444 Oct
15300 May
13036 May
6912 May
5905 Aug
24
24
24
13
13
13
10
5
24
24
24
3
2006
2006
2006
2004
2004
2004
2006
2004
2006
2006
2006
2004
/sbin/mke2fs
/sbin/mkfs.ext2
/sbin/mkfs.ext3
/sbin/mkdosfs
/sbin/mkfs.msdos
/sbin/mkfs.vfat
/sbin/mkinitrd
/sbin/mkzonedb
/sbin/mkfs.cramfs
/sbin/mkswap
/sbin/mkfs
/sbin/mkbootdisk
It is time for you to read the manual pages of mkfs and mke2fs. In the example below,
you see the creation of an ext2 file system on /dev/sdb1. In real life, you might want to use
options like -m0 and -j.
root@RHELv4u2:~# mke2fs /dev/sdb1
mke2fs 1.35 (28-Feb-2004)
Filesystem label=
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
28112 inodes, 112420 blocks
5621 blocks (5.00%) reserved for the super user
First data block=1
Maximum filesystem blocks=67371008
14 block groups
8192 blocks per group, 8192 fragments per group
2008 inodes per group
Superblock backups stored on blocks:
8193, 24577, 40961, 57345, 73729
Writing inode tables: done
Writing superblocks and filesystem accounting information: done
This filesystem will be automatically checked every 37 mounts or
180 days, whichever comes first. Use tune2fs -c or -i to override.
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file systems
41.4. tuning a file system
You can use tune2fs to list and set file system settings. The first screenshot lists the reserved
space for root (which is set at five percent).
[root@rhel4 ~]# tune2fs -l /dev/sda1 | grep -i "block count"
Block count:
104388
Reserved block count:
5219
[root@rhel4 ~]#
This example changes this value to ten percent. You can use tune2fs while the file system
is active, even if it is the root file system (as in this example).
[root@rhel4 ~]# tune2fs -m10 /dev/sda1
tune2fs 1.35 (28-Feb-2004)
Setting reserved blocks percentage to 10 (10430 blocks)
[root@rhel4 ~]# tune2fs -l /dev/sda1 | grep -i "block count"
Block count:
104388
Reserved block count:
10430
[root@rhel4 ~]#
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file systems
41.5. checking a file system
The fsck command is a front end tool used to check a file system for errors.
[root@RHEL4b ~]# ls /sbin/*fsck*
/sbin/dosfsck /sbin/fsck
/sbin/e2fsck
/sbin/fsck.cramfs
[root@RHEL4b ~]#
/sbin/fsck.ext2
/sbin/fsck.ext3
/sbin/fsck.msdos
/sbin/fsck.vfat
The last column in /etc/fstab is used to determine whether a file system should be checked
at boot-up.
[paul@RHEL4b ~]$ grep ext /etc/fstab
/dev/VolGroup00/LogVol00
/
LABEL=/boot
/boot
[paul@RHEL4b ~]$
ext3
ext3
defaults
defaults
1 1
1 2
Manually checking a mounted file system results in a warning from fsck.
[root@RHEL4b ~]# fsck /boot
fsck 1.35 (28-Feb-2004)
e2fsck 1.35 (28-Feb-2004)
/dev/sda1 is mounted.
WARNING!!! Running e2fsck on a mounted filesystem may cause
SEVERE filesystem damage.
Do you really want to continue (y/n)? no
check aborted.
But after unmounting fsck and e2fsck can be used to check an ext2 file system.
[root@RHEL4b ~]# fsck /boot
fsck 1.35 (28-Feb-2004)
e2fsck 1.35 (28-Feb-2004)
/boot: clean, 44/26104 files, 17598/104388 blocks
[root@RHEL4b ~]# fsck -p /boot
fsck 1.35 (28-Feb-2004)
/boot: clean, 44/26104 files, 17598/104388 blocks
[root@RHEL4b ~]# e2fsck -p /dev/sda1
/boot: clean, 44/26104 files, 17598/104388 blocks
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file systems
41.6. practice: file systems
1. List the filesystems that are known by your system.
2. Create an ext2 filesystem on the 200MB partition.
3. Create an ext3 filesystem on one of the 300MB logical drives.
4. Create an ext4 on the 400MB partition.
5. Set the reserved space for root on the ext3 filesystem to 0 percent.
6. Verify your work with fdisk and df.
7. Perform a file system check on all the new file systems.
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file systems
41.7. solution: file systems
1. List the filesystems that are known by your system.
man fs
cat /proc/filesystems
cat /etc/filesystems (not on all Linux distributions)
2. Create an ext2 filesystem on the 200MB partition.
mke2fs /dev/sdc1 (replace sdc1 with the correct partition)
3. Create an ext3 filesystem on one of the 300MB logical drives.
mke2fs -j /dev/sdb5 (replace sdb5 with the correct partition)
4. Create an ext4 on the 400MB partition.
mkfs.ext4 /dev/sdb1 (replace sdb1 with the correct partition)
5. Set the reserved space for root on the ext3 filesystem to 0 percent.
tune2fs -m 0 /dev/sdb5
6. Verify your work with fdisk and df.
mkfs (mke2fs) makes no difference in the output of these commands
The big change is in the next topic: mounting
7. Perform a file system check on all the new file systems.
fsck /dev/sdb1
fsck /dev/sdc1
fsck /dev/sdb5
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Chapter 42. mounting
Once you've put a file system on a partition, you can mount it. Mounting a file system
makes it available for use, usually as a directory. We say mounting a file system instead
of mounting a partition because we will see later that we can also mount file systems that
do not exists on partitions.
On all Unix systems, every file and every directory is part of one big file tree. To access
a file, you need to know the full path starting from the root directory. When adding a file
system to your computer, you need to make it available somewhere in the file tree. The
directory where you make a file system available is called a mount point.
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mounting
42.1. mounting local file systems
42.1.1. mkdir
This example shows how to create a new mount point with mkdir.
root@RHELv4u2:~# mkdir /home/project42
42.1.2. mount
When the mount point is created, and a file system is present on the partition, then mount
can mount the file system on the mount point directory.
root@RHELv4u2:~# mount -t ext2 /dev/sdb1 /home/project42/
Once mounted, the new file system is accessible to users.
42.1.3. /etc/filesystems
Actually the explicit -t ext2 option to set the file system is not always necessary. The mount
command is able to automatically detect a lot of file systems.
When mounting a file system without specifying explicitly the file system, then mount will
first probe /etc/filesystems. Mount will skip lines with the nodev directive.
paul@RHELv4u4:~$ cat /etc/filesystems
ext3
ext2
nodev proc
nodev devpts
iso9660
vfat
hfs
42.1.4. /proc/filesystems
When /etc/filesystems does not exist, or ends with a single * on the last line, then mount
will read /proc/filesystems.
[root@RHEL52 ~]# cat /proc/filesystems | grep -v ^nodev
ext2
iso9660
ext3
42.1.5. umount
You can unmount a mounted file system using the umount command.
root@pasha:~# umount /home/reet
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42.2. displaying mounted file systems
To display all mounted file systems, issue the mount command. Or look at the files /proc/
mounts and /etc/mtab.
42.2.1. mount
The simplest and most common way to view all mounts is by issuing the mount command
without any arguments.
root@RHELv4u2:~# mount | grep /dev/sdb
/dev/sdb1 on /home/project42 type ext2 (rw)
42.2.2. /proc/mounts
The kernel provides the info in /proc/mounts in file form, but /proc/mounts does not exist
as a file on any hard disk. Looking at /proc/mounts is looking at information that comes
directly from the kernel.
root@RHELv4u2:~# cat /proc/mounts | grep /dev/sdb
/dev/sdb1 /home/project42 ext2 rw 0 0
42.2.3. /etc/mtab
The /etc/mtab file is not updated by the kernel, but is maintained by the mount command.
Do not edit /etc/mtab manually.
root@RHELv4u2:~# cat /etc/mtab | grep /dev/sdb
/dev/sdb1 /home/project42 ext2 rw 0 0
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mounting
42.2.4. df
A more user friendly way to look at mounted file systems is df. The df (diskfree) command
has the added benefit of showing you the free space on each mounted disk. Like a lot of
Linux commands, df supports the -h switch to make the output more human readable.
root@RHELv4u2:~# df
Filesystem
1K-blocks
Used Available Use% Mounted on
/dev/mapper/VolGroup00-LogVol00
11707972
6366996
4746240 58% /
/dev/sda1
101086
9300
86567 10% /boot
none
127988
0
127988
0% /dev/shm
/dev/sdb1
108865
1550
101694
2% /home/project42
root@RHELv4u2:~# df -h
Filesystem
Size Used Avail Use% Mounted on
/dev/mapper/VolGroup00-LogVol00
12G 6.1G 4.6G 58% /
/dev/sda1
99M 9.1M
85M 10% /boot
none
125M
0 125M
0% /dev/shm
/dev/sdb1
107M 1.6M 100M
2% /home/project42
42.2.5. df -h
In the df -h example below you can see the size, free space, used gigabytes and percentage
and mount point of a partition.
root@laika:~# df -h | egrep -e "(sdb2|File)"
Filesystem
Size Used Avail Use% Mounted on
/dev/sdb2
92G
83G 8.6G 91% /media/sdb2
42.2.6. du
The du command can summarize disk usage for files and directories. By using du on a
mount point you effectively get the disk space used on a file system.
While du can go display each subdirectory recursively, the -s option will give you a total
summary for the parent directory. This option is often used together with -h. This means du
-sh on a mount point gives the total amount used by the file system in that partition.
root@debian6~# du -sh /boot /srv/wolf
6.2M /boot
1.1T /srv/wolf
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mounting
42.3. from start to finish
Below is a screenshot that show a summary roadmap starting with detection of the hardware
(/dev/sdb) up until mounting on /mnt.
[root@centos65 ~]# dmesg | grep '\[sdb\]'
sd 3:0:0:0: [sdb] 150994944 512-byte logical blocks: (77.3 GB/72.0 GiB)
sd 3:0:0:0: [sdb] Write Protect is off
sd 3:0:0:0: [sdb] Mode Sense: 00 3a 00 00
sd 3:0:0:0: [sdb] Write cache: enabled, read cache: enabled, doesn't support \
DPO or FUA
sd 3:0:0:0: [sdb] Attached SCSI disk
[root@centos65 ~]# parted /dev/sdb
(parted) mklabel msdos
(parted) mkpart primary ext4 1 77000
(parted) print
Model: ATA VBOX HARDDISK (scsi)
Disk /dev/sdb: 77.3GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Number
1
Start
1049kB
End
77.0GB
Size
77.0GB
Type
primary
File system
(parted) quit
[root@centos65 ~]# mkfs.ext4 /dev/sdb1
mke2fs 1.41.12 (17-May-2010)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
4702208 inodes, 18798592 blocks
939929 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=4294967296
574 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
( output truncated )
...
[root@centos65 ~]# mount /dev/sdb1 /mnt
[root@centos65 ~]# mount | grep mnt
/dev/sdb1 on /mnt type ext4 (rw)
[root@centos65 ~]# df -h | grep mnt
/dev/sdb1
71G 180M
67G
1% /mnt
[root@centos65 ~]# du -sh /mnt
20K
/mnt
[root@centos65 ~]# umount /mnt
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mounting
42.4. permanent mounts
Until now, we performed all mounts manually. This works nice, until the next reboot.
Luckily there is a way to tell your computer to automatically mount certain file systems
during boot.
42.4.1. /etc/fstab
The file system table located in /etc/fstab contains a list of file systems, with an option to
automtically mount each of them at boot time.
Below is a sample /etc/fstab file.
root@RHELv4u2:~# cat /etc/fstab
/dev/VolGroup00/LogVol00 /
LABEL=/boot
/boot
none
/dev/pts
none
/dev/shm
none
/proc
none
/sys
/dev/VolGroup00/LogVol01 swap
ext3
ext3
devpts
tmpfs
proc
sysfs
swap
defaults
defaults
gid=5,mode=620
defaults
defaults
defaults
defaults
1
1
0
0
0
0
0
1
2
0
0
0
0
0
By adding the following line, we can automate the mounting of a file system.
/dev/sdb1
/home/project42
ext2
defaults
0 0
42.4.2. mount /mountpoint
Adding an entry to /etc/fstab has the added advantage that you can simplify the mount
command. The command in the screenshot below forces mount to look for the partition
info in /etc/fstab.
root@rhel65:~# mount /home/project42
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mounting
42.5. securing mounts
File systems can be secured with several mount options. Here are some examples.
42.5.1. ro
The ro option will mount a file system as read only, preventing anyone from writing.
root@rhel53 ~# mount -t ext2 -o ro /dev/hdb1 /home/project42
root@rhel53 ~# touch /home/project42/testwrite
touch: cannot touch `/home/project42/testwrite': Read-only file system
42.5.2. noexec
The noexec option will prevent the execution of binaries and scripts on the mounted file
system.
root@rhel53 ~# mount -t ext2 -o noexec /dev/hdb1 /home/project42
root@rhel53 ~# cp /bin/cat /home/project42
root@rhel53 ~# /home/project42/cat /etc/hosts
-bash: /home/project42/cat: Permission denied
root@rhel53 ~# echo echo hello > /home/project42/helloscript
root@rhel53 ~# chmod +x /home/project42/helloscript
root@rhel53 ~# /home/project42/helloscript
-bash: /home/project42/helloscript: Permission denied
42.5.3. nosuid
The nosuid option will ignore setuid bit set binaries on the mounted file system.
Note that you can still set the setuid bit on files.
root@rhel53 ~# mount -o nosuid /dev/hdb1 /home/project42
root@rhel53 ~# cp /bin/sleep /home/project42/
root@rhel53 ~# chmod 4555 /home/project42/sleep
root@rhel53 ~# ls -l /home/project42/sleep
-r-sr-xr-x 1 root root 19564 Jun 24 17:57 /home/project42/sleep
But users cannot exploit the setuid feature.
root@rhel53 ~# su - paul
[paul@rhel53 ~]$ /home/project42/sleep 500 &
[1] 2876
[paul@rhel53 ~]$ ps -f 2876
UID
PID PPID C STIME TTY
STAT
paul
2876 2853 0 17:58 pts/0
S
[paul@rhel53 ~]$
TIME CMD
0:00 /home/project42/sleep 500
42.5.4. noacl
To prevent cluttering permissions with acl's, use the noacl option.
root@rhel53 ~# mount -o noacl /dev/hdb1 /home/project42
More mount options can be found in the manual page of mount.
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mounting
42.6. mounting remote file systems
42.6.1. smb/cifs
The Samba team (samba.org) has a Unix/Linux service that is compatible with the SMB/
CIFS protocol. This protocol is mainly used by networked Microsoft Windows computers.
Connecting to a Samba server (or to a Microsoft computer) is also done with the mount
command.
This example shows how to connect to the 10.0.0.42 server, to a share named data2.
[root@centos65 ~]# mount -t cifs -o user=paul //10.0.0.42/data2 /home/data2
Password:
[root@centos65 ~]# mount | grep cifs
//10.0.0.42/data2 on /home/data2 type cifs (rw)
The above requires yum install cifs-client.
42.6.2. nfs
Unix servers often use nfs (aka the network file system) to share directories over the network.
Setting up an nfs server is discussed later. Connecting as a client to an nfs server is done
with mount, and is very similar to connecting to local storage.
This command shows how to connect to the nfs server named server42, which is sharing
the directory /srv/data. The mount point at the end of the command (/home/data) must
already exist.
[root@centos65 ~]# mount -t nfs server42:/srv/data /home/data
[root@centos65 ~]#
If this server42 has ip-address 10.0.0.42 then you can also write:
[root@centos65 ~]# mount -t nfs 10.0.0.42:/srv/data /home/data
[root@centos65 ~]# mount | grep data
10.0.0.42:/srv/data on /home/data type nfs (rw,vers=4,addr=10.0.0.42,clienta\
ddr=10.0.0.33)
42.6.3. nfs specific mount options
bg If mount fails, retry in background.
fg (default)If mount fails, retry in foreground.
soft Stop trying to mount after X attempts.
hard (default)Continue trying to mount.
The soft+bg options combined guarantee the fastest client boot if there are NFS problems.
retrans=X Try X times to connect (over udp).
tcp Force tcp (default and supported)
udp Force udp (unsupported)
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mounting
42.7. practice: mounting file systems
1. Mount the small 200MB partition on /home/project22.
2. Mount the big 400MB primary partition on /mnt, the copy some files to it (everything in /
etc). Then umount, and mount the file system as read only on /srv/nfs/salesnumbers. Where
are the files you copied ?
3. Verify your work with fdisk, df and mount. Also look in /etc/mtab and /proc/mounts.
4. Make both mounts permanent, test that it works.
5. What happens when you mount a file system on a directory that contains some files ?
6. What happens when you mount two file systems on the same mount point ?
7. (optional) Describe the difference between these commands: find, locate, updatedb,
makewhatis, whereis, apropos, which and type.
8. (optional) Perform a file system check on the partition mounted at /srv/nfs/salesnumbers.
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mounting
42.8. solution: mounting file systems
1. Mount the small 200MB partition on /home/project22.
mkdir /home/project22
mount /dev/sdc1 /home/project22
2. Mount the big 400MB primary partition on /mnt, the copy some files to it (everything in /
etc). Then umount, and mount the file system as read only on /srv/nfs/salesnumbers. Where
are the files you copied ?
mount /dev/sdb1 /mnt
cp -r /etc /mnt
ls -l /mnt
umount /mnt
ls -l /mnt
mkdir -p /srv/nfs/salesnumbers
mount /dev/sdb1 /srv/nfs/salesnumbers
You see the files in /srv/nfs/salenumbers now...
But physically they are on ext3 on partition /dev/sdb1
3. Verify your work with fdisk, df and mount. Also look in /etc/mtab and /proc/mounts.
fdisk -l
df -h
mount
All three the above commands should show your mounted partitions.
grep project22 /etc/mtab
grep project22 /proc/mounts
4. Make both mounts permanent, test that it works.
add the following lines to /etc/fstab
/dev/sdc1 /home/project22 auto defaults 0 0
/dev/sdb1 /srv/nfs/salesnumbers auto defaults 0 0
5. What happens when you mount a file system on a directory that contains some files ?
The files are hidden until umount.
6. What happens when you mount two file systems on the same mount point ?
Only the last mounted fs is visible.
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7. (optional) Describe the difference between these commands: find, locate, updatedb,
makewhatis, whereis, apropos, which and type.
man find
man locate
...
8. (optional) Perform a file system check on the partition mounted at /srv/nfs/salesnumbers.
# umount /srv/nfs/salesnumbers (optional but recommended)
# fsck /dev/sdb1
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Chapter 43. troubleshooting tools
This chapter introduces some tools that go beyond df -h and du -sh. Tools that will enable
you to troubleshoot a variety of issues with file systems and storage.
409
troubleshooting tools
43.1. lsof
List open files with lsof.
When invoked without options, lsof will list all open files. You can see the command (init in
this case), its PID (1) and the user (root) has openend the root directory and /sbin/init. The
FD (file descriptor) columns shows that / is both the root directory (rtd) and current working
directory (cwd) for the /sbin/init command. The FD column displays rtd for root directory,
cwd for current directory and txt for text (both including data and code).
root@debian7:~# lsof | head -4
COMMAND PID TID
USER
FD
init
1
root cwd
init
1
root rtd
init
1
root txt
TYPE
DIR
DIR
REG
DEVICE SIZE/OFF
254,0
4096
254,0
4096
254,0
36992
NODE
2
2
130856
NAME
/
/
/sbin/init
Other options in the FD column besides w for writing, are r for reading and u for both reading
and writing. You can look at open files for a process id by typing lsof -p PID. For init this
would look like this:
lsof -p 1
The screenshot below shows basic use of lsof to prove that vi keeps a .swp file open (even
when stopped in background) on our freshly mounted file system.
[root@RHEL65 ~]# df -h | grep sdb
/dev/sdb1
541M
17M 497M
4% /srv/project33
[root@RHEL65 ~]# vi /srv/project33/busyfile.txt
[1]+ Stopped
vi /srv/project33/busyfile.txt
[root@RHEL65 ~]# lsof /srv/*
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
vi
3243 root
3u REG
8,17
4096
12 /srv/project33/.busyfile.txt.swp
Here we see that rsyslog has a couple of log files open for writing (the FD column).
root@debian7:~# lsof /var/log/*
COMMAND
PID USER
FD
TYPE DEVICE SIZE/OFF
NODE NAME
rsyslogd 2013 root
1w
REG 254,0
454297 1308187 /var/log/syslog
rsyslogd 2013 root
2w
REG 254,0
419328 1308189 /var/log/kern.log
rsyslogd 2013 root
5w
REG 254,0
116725 1308200 /var/log/debug
rsyslogd 2013 root
6w
REG 254,0
309847 1308201 /var/log/messages
rsyslogd 2013 root
7w
REG 254,0
17591 1308188 /var/log/daemon.log
rsyslogd 2013 root
8w
REG 254,0
101768 1308186 /var/log/auth.log
You can specify a specific user with lsof -u. This example shows the current working
directory for a couple of command line programs.
[paul@RHEL65
bash
3302
lsof
3329
grep
3330
lsof
3331
~]$ lsof -u paul | grep home
paul cwd
DIR 253,0
4096
paul cwd
DIR 253,0
4096
paul cwd
DIR 253,0
4096
paul cwd
DIR 253,0
4096
788024
788024
788024
788024
/home/paul
/home/paul
/home/paul
/home/paul
The -u switch of lsof also supports the ^ character meaning 'not'. To see all open files, but
not those open by root:
lsof -u^root
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troubleshooting tools
43.2. fuser
The fuser command will display the 'user' of a file system.
In this example we still have a vi process in background and we use fuser to find the process
id of the process using this file system.
[root@RHEL65 ~]# jobs
[1]+ Stopped
vi /srv/project33/busyfile.txt
[root@RHEL65 ~]# fuser -m /srv/project33/
/srv/project33/:
3243
Adding the -u switch will also display the user name.
[root@RHEL65 ~]# fuser -m -u /srv/project33/
/srv/project33/:
3243(root)
You can quickly kill all processes that are using a specific file (or directory) with the -k
switch.
[root@RHEL65 ~]# fuser -m -k -u /srv/project33/
/srv/project33/:
3243(root)
[1]+ Killed
vi /srv/project33/busyfile.txt
[root@RHEL65 ~]# fuser -m -u /srv/project33/
[root@RHEL65 ~]#
This example shows all processes that are using the current directory (bash and vi in this
case).
root@debian7:~/test42# vi file42
[1]+ Stopped
vi file42
root@debian7:~/test42# fuser -v .
USER
PID ACCESS COMMAND
/root/test42:
root
2909 ..c.. bash
root
3113 ..c.. vi
This example shows that the vi command actually accesses /usr/bin/vim.basic as an
executable file.
root@debian7:~/test42# fuser -v $(which vi)
USER
PID ACCESS COMMAND
/usr/bin/vim.basic: root
3113 ...e. vi
The last example shows how to find the process that is accessing a specific file.
[root@RHEL65 ~]# vi /srv/project33/busyfile.txt
[1]+ Stopped
vi /srv/project33/busyfile.txt
[root@RHEL65 ~]# fuser -v -m /srv/project33/busyfile.txt
USER
PID ACCESS COMMAND
/srv/project33/busyfile.txt:
root
13938 F.... vi
[root@RHEL65 ~]# ps -fp 13938
UID
PID PPID C STIME TTY
TIME CMD
root
13938 3110 0 15:47 pts/0
00:00:00 vi /srv/project33/busyfile.txt
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troubleshooting tools
43.3. chroot
The chroot command creates a shell with an alternate root directory. It effectively hides
anything outside of this directory.
In the example below we assume that our system refuses to start (maybe because there is a
problem with /etc/fstab or the mounting of the root file system).
We start a live system (booted from cd/dvd/usb) to troubleshoot our server. The live system
will not use our main hard disk as root device
root@livecd:~# df -h | grep root
rootfs
186M
11M 175M
6% /
/dev/loop0
807M 807M
0 100% /lib/live/mount/rootfs/filesystem.squashfs
root@livecd:~# mount | grep root
/dev/loop0 on /lib/live/mount/rootfs/filesystem.squashfs type squashfs (ro)
We create some test file on the current rootfs.
root@livecd:~# touch /file42
root@livecd:~# mkdir /dir42
root@livecd:~# ls /
bin
dir42
home
lib64
boot etc
initrd.img media
dev
file42 lib
mnt
opt
proc
root
run
sbin
selinux
srv
sys
tmp
usr
var
vmlinuz
First we mount the root file system from the disk (which is on lvm so we use /dev/mapper
instead of /dev/sda5).
root@livecd:~# mount /dev/mapper/packer--debian--7-root /mnt
We are now ready to chroot into the rootfs on disk.
root@livecd:~# cd /mnt
root@livecd:/mnt# chroot /mnt
root@livecd:/# ls /
bin
dev
initrd.img lost+found
boot etc
lib
media
data home lib64
mnt
opt
proc
root
run
sbin
selinux
srv
sys
tmp
usr
vagrant
var
vmlinuz
Our test files (file42 and dir42) are not visible because they are out of the chrooted
environment.
Note that the hostname of the chrooted environment is identical to the existing hostname.
To exit the chrooted file system:
root@livecd:/# exit
exit
root@livecd:~# ls /
bin
dir42
home
boot etc
initrd.img
dev
file42 lib
lib64
media
mnt
opt
proc
root
run
sbin
selinux
412
srv
sys
tmp
usr
var
vmlinuz
troubleshooting tools
43.4. iostat
iostat reports IO statitics every given period of time. It also includes a small cpu usage
summary. This example shows iostat running every ten seconds with /dev/sdc and /dev/sde
showing a lot of write activity.
[root@RHEL65 ~]# iostat 10 3
Linux 2.6.32-431.el6.x86_64 (RHEL65)
avg-cpu:
%user
5.81
Device:
sda
sdb
sdc
sdd
sde
sdf
sdg
dm-0
dm-1
avg-cpu:
Device:
sda
sdb
sdc
sdd
sde
sdf
sdg
dm-0
dm-1
avg-cpu:
%nice %system %iowait
0.00
3.15
0.18
tps
42.08
1.20
0.92
0.91
1.04
0.70
0.69
191.68
49.26
%user
56.11
Device:
sda
sdb
sdc
sdd
sde
sdf
sdg
dm-0
dm-1
Blk_read/s
1204.10
7.69
5.30
5.29
6.28
3.40
3.40
1045.78
150.54
Blk_read/s
10185.97
0.00
1.60
0.00
1.60
0.00
0.00
10185.97
0.00
Blk_read/s
26961.09
0.00
0.90
0.00
0.00
0.00
0.00
26938.46
22.62
%steal
0.00
Blk_wrtn/s
76.95
0.00
2953.11
0.00
4813.63
0.00
0.00
76.95
0.00
%nice %system %iowait
0.00
31.11
0.11
tps
466.86
0.00
31.45
0.00
0.34
0.00
0.00
503.62
2.83
%steal
0.00
Blk_wrtn/s
1634.88
45.78
45.82
45.78
91.49
91.46
91.46
1362.30
243.55
%nice %system %iowait
0.00
16.83
0.10
tps
257.01
0.00
3.81
0.00
4.91
0.00
0.00
283.77
0.00
%user
67.65
06/16/2014
%steal
0.00
Blk_wrtn/s
178.28
0.00
24997.29
0.00
5.43
0.00
0.00
178.28
0.00
_x86_64_
(1 CPU)
%idle
90.85
Blk_read
1743708
11134
7672
7656
9100
4918
4918
1514434
218000
Blk_wrtn
2367530
66292
66348
66292
132496
132440
132440
1972808
352696
%idle
26.95
Blk_read
101656
0
16
0
16
0
0
101656
0
Blk_wrtn
768
0
29472
0
48040
0
0
768
0
%idle
1.13
Blk_read
238336
0
8
0
0
0
0
238136
200
Blk_wrtn
1576
0
220976
0
48
0
0
1576
0
[root@RHEL65 ~]#
Other options are to specify the disks you want to monitor (every 5 seconds here):
iostat sdd sde sdf 5
Or to show statistics per partition:
iostat -p sde -p sdf 5
413
troubleshooting tools
43.5. iotop
iotop works like the top command but orders processes by input/output instead of by CPU.
By default iotop will show all processes. This example uses iotop -o to only display
processes with actual I/O.
[root@RHEL65 ~]# iotop -o
Total
TID
15000
25000
24988
25003
25004
DISK READ: 8.63 M/s | Total DISK WRITE: 0.00 B/s
PRIO USER DISK READ DISK WRITE SWAPIN
IO>
be/4 root
2.43 M/s
0.00 B/s 0.00 % 14.60 %
be/4 root
6.20 M/s
0.00 B/s 0.00 % 6.15 %
be/4 root
0.00 B/s
7.21 M/s 0.00 % 0.00 %
be/4 root
0.00 B/s 1591.19 K/s 0.00 % 0.00 %
be/4 root
0.00 B/s 193.51 K/s 0.00 % 0.00 %
COMMAND
tar cjf /srv/di...
tar czf /srv/di...
gzip
gzip
bzip2
Use the -b switch to create a log of iotop output (instead of the default interactive view).
[root@RHEL65 ~]# iotop -bod 10
Total DISK READ: 12.82 M/s | Total DISK WRITE: 5.69 M/s
TID PRIO USER DISK READ DISK WRITE SWAPIN
IO
25153 be/4 root
2.05 M/s
0.00 B/s 0.00 % 7.81 %
25152 be/4 root
10.77 M/s
0.00 B/s 0.00 % 2.94 %
25144 be/4 root
408.54 B/s
0.00 B/s 0.00 % 0.05 %
12516 be/3 root
0.00 B/s 1491.33 K/s 0.00 % 0.04 %
12522 be/3 root
0.00 B/s
45.48 K/s 0.00 % 0.01 %
25158 be/4 root
0.00 B/s
0.00 B/s 0.00 % 0.00 %
25155 be/4 root
0.00 B/s 493.12 K/s 0.00 % 0.00 %
25156 be/4 root
0.00 B/s
2.81 M/s 0.00 % 0.00 %
25159 be/4 root
0.00 B/s 528.63 K/s 0.00 % 0.00 %
COMMAND
tar cjf /srv/di...
tar czf /srv/di...
python /usr/sbi...
[jbd2/sdc1-8]
[jbd2/sde1-8]
[flush-8:64]
bzip2
gzip
[flush-8:32]
This is an example of iotop to track disk I/O every ten seconds for one user named vagrant
(and only one process of this user, but this can be omitted). The -a switch accumulates I/
O over time.
[root@RHEL65 ~]# iotop -q -a -u vagrant -b -p 5216 -d 10 -n
Total DISK READ: 0.00 B/s | Total DISK WRITE: 0.00 B/s
TID PRIO USER
DISK READ DISK WRITE SWAPIN
IO
5216 be/4 vagrant
0.00 B
0.00 B 0.00 % 0.00 %
Total DISK READ: 818.22 B/s | Total DISK WRITE: 20.78 M/s
5216 be/4 vagrant
0.00 B
213.89 M 0.00 % 0.00 %
Total DISK READ: 2045.95 B/s | Total DISK WRITE: 23.16 M/s
5216 be/4 vagrant
0.00 B
430.70 M 0.00 % 0.00 %
Total DISK READ: 1227.50 B/s | Total DISK WRITE: 22.37 M/s
5216 be/4 vagrant
0.00 B
642.02 M 0.00 % 0.00 %
Total DISK READ: 818.35 B/s | Total DISK WRITE: 16.44 M/s
5216 be/4 vagrant
0.00 B
834.09 M 0.00 % 0.00 %
Total DISK READ: 6.95 M/s | Total DISK WRITE: 8.74 M/s
5216 be/4 vagrant
0.00 B
920.69 M 0.00 % 0.00 %
Total DISK READ: 21.71 M/s | Total DISK WRITE: 11.99 M/s
414
10
COMMAND
gzip
gzip
gzip
gzip
gzip
gzip
troubleshooting tools
43.6. vmstat
While vmstat is mainly a memory monitoring tool, it is worth mentioning here for its
reporting on summary I/O data for block devices and swap space.
This example shows some disk activity (underneath the -----io---- column), without
swapping.
[root@RHEL65 ~]# vmstat 5 10
procs ----------memory---------- ---swap-- -----io---- --system-- -----cpu----r b swpd
free
buff cache
si
so
bi
bo
in
cs us sy id wa st
0 0 5420
9092 14020 340876
7
12
235
252
77 100 2 1 98 0 0
2 0 5420
6104 13840 338176
0
0 7401 7812 747 1887 38 12 50 0 0
2 0 5420 10136 13696 336012
0
0 11334
14 1725 4036 76 24 0 0 0
0 0 5420 14160 13404 341552
0
0 10161 9914 1174 1924 67 15 18 0 0
0 0 5420 14300 13420 341564
0
0
0
16
28
18 0 0 100 0 0
0 0 5420 14300 13420 341564
0
0
0
0
22
16 0 0 100 0 0
...
[root@RHEL65 ~]#
You can benefit from vmstat's ability to display memory in kilobytes, megabytes or even
kibibytes and mebibytes using -S (followed by k K m or M).
[root@RHEL65 ~]# vmstat -SM 5 10
procs ----------memory---------- ---swap-- -----io---r b swpd
free
buff cache
si
so
bi
bo
0 0
5
14
11
334
0
0
259
255
0 0
5
14
11
334
0
0
0
2
0 0
5
15
11
334
0
0
6
0
2 0
5
6
11
336
0
0 17100 7814
2 0
5
6
11
336
0
0 13193
14
2 0
5
13
11
330
0
0 11656 9781
2 0
5
9
11
334
0
0 10705 2716
1 0
5
14
11
336
0
0 6467 3788
0 0
5
14
11
336
0
0
0
13
0 0
5
14
11
336
0
0
0
0
[root@RHEL65 ~]#
vmstat is also discussed in other chapters.
415
--system-- -----cpu----in
cs us sy id wa st
79 107 2 1 97 0 0
21
18 0 0 100 0 0
35
31 0 0 100 0 0
1378 2945 48 21 31 0 0
1662 3343 78 22 0 0 0
1419 2642 82 18 0 0 0
1504 2657 81 19 0 0 0
765 1384 43 9 48 0 0
28
24 0 0 100 0 0
20
15 0 0 100 0 0
troubleshooting tools
43.7. practice: troubleshooting tools
0. It is imperative that you practice these tools before trouble arises. It will help you get
familiar with the tools and allow you to create a base line of normal behaviour for your
systems.
1. Read the theory on fuser and explore its man page. Use this command to find files that
you open yourself.
2. Read the theory on lsof and explore its man page. Use this command to find files that
you open yourself.
3. Boot a live image on an existing computer (virtual or real) and chroot into to it.
4. Start one or more disk intensive jobs and monitor them with iostat and iotop (compare
to vmstat).
416
troubleshooting tools
43.8. solution: troubleshooting tools
0. It is imperative that you practice these tools before trouble arises. It will help you get
familiar with the tools and allow you to create a base line of normal behaviour for your
systems.
1. Read the theory on fuser and explore its man page. Use this command to find files that
you open yourself.
2. Read the theory on lsof and explore its man page. Use this command to find files that
you open yourself.
3. Boot a live image on an existing computer (virtual or real) and chroot into to it.
4. Start one or more disk intensive jobs and monitor them with iostat and iotop (compare
to vmstat).
417
Chapter 44. introduction to uuid's
A uuid or universally unique identifier is used to uniquely identify objects. This 128bit
standard allows anyone to create a unique uuid.
This chapter takes a brief look at uuid's.
418
introduction to uuid's
44.1. about unique objects
Older versions of Linux have a vol_id utility to display the uuid of a file system.
root@debian5:~# vol_id --uuid /dev/sda1
193c3c9b-2c40-9290-8b71-4264ee4d4c82
Red Hat Enterprise Linux 5 puts vol_id in /lib/udev/vol_id, which is not in the $PATH. The
syntax is also a bit different from Debian/Ubuntu/Mint.
root@rhel53 ~# /lib/udev/vol_id -u /dev/hda1
48a6a316-9ca9-4214-b5c6-e7b33a77e860
This utility is not available in standard installations of RHEL6 or Debian6.
44.2. tune2fs
Use tune2fs to find the uuid of a file system.
[root@RHEL5 ~]# tune2fs -l /dev/sda1 | grep UUID
Filesystem UUID:
11cfc8bc-07c0-4c3f-9f64-78422ef1dd5c
[root@RHEL5 ~]# /lib/udev/vol_id -u /dev/sda1
11cfc8bc-07c0-4c3f-9f64-78422ef1dd5c
44.3. uuid
There is more information in the manual of uuid, a tool that can generate uuid's.
[root@rhel65 ~]# yum install uuid
(output truncated)
[root@rhel65 ~]# man uuid
419
introduction to uuid's
44.4. uuid in /etc/fstab
You can use the uuid to make sure that a volume is universally uniquely identified in /etc/
fstab. The device name can change depending on the disk devices that are present at boot
time, but a uuid never changes.
First we use tune2fs to find the uuid.
[root@RHEL5 ~]# tune2fs -l /dev/sdc1 | grep UUID
Filesystem UUID:
7626d73a-2bb6-4937-90ca-e451025d64e8
Then we check that it is properly added to /etc/fstab, the uuid replaces the variable
devicename /dev/sdc1.
[root@RHEL5 ~]# grep UUID /etc/fstab
UUID=7626d73a-2bb6-4937-90ca-e451025d64e8 /home/pro42 ext3 defaults 0 0
Now we can mount the volume using the mount point defined in /etc/fstab.
[root@RHEL5 ~]# mount /home/pro42
[root@RHEL5 ~]# df -h | grep 42
/dev/sdc1
397M
11M 366M
3% /home/pro42
The real test now, is to remove /dev/sdb from the system, reboot the machine and see what
happens. After the reboot, the disk previously known as /dev/sdc is now /dev/sdb.
[root@RHEL5 ~]# tune2fs -l /dev/sdb1 | grep UUID
Filesystem UUID:
7626d73a-2bb6-4937-90ca-e451025d64e8
And thanks to the uuid in /etc/fstab, the mountpoint is mounted on the same disk as before.
[root@RHEL5 ~]# df -h | grep sdb
/dev/sdb1
397M
11M
366M
420
3% /home/pro42
introduction to uuid's
44.5. uuid as a boot device
Recent Linux distributions (Debian, Ubuntu, ...) use grub with a uuid to identify the root
file system.
This example shows how a root=/dev/sda1 is replaced with a uuid.
title
Ubuntu 9.10, kernel 2.6.31-19-generic
uuid
f001ba5d-9077-422a-9634-8d23d57e782a
kernel
/boot/vmlinuz-2.6.31-19-generic \
root=UUID=f001ba5d-9077-422a-9634-8d23d57e782a ro quiet splash
initrd
/boot/initrd.img-2.6.31-19-generic
The screenshot above contains only four lines. The line starting with root= is the
continuation of the kernel line.
RHEL and CentOS boot from LVM after a default install.
421
introduction to uuid's
44.6. practice: uuid and filesystems
1. Find the uuid of one of your ext3 partitions with tune2fs ( and vol_id if you are on
RHEL5).
2. Use this uuid in /etc/fstab and test that it works with a simple mount.
3. (optional) Test it also by removing a disk (so the device name is changed). You can edit
settings in vmware/Virtualbox to remove a hard disk.
4. Display the root= directive in /boot/grub/menu.lst. (We see later in the course how to
maintain this file.)
5. (optional on ubuntu) Replace the /dev/xxx in /boot/grub/menu.lst with a uuid (use an
extra stanza for this). Test that it works.
422
introduction to uuid's
44.7. solution: uuid and filesystems
1. Find the uuid of one of your ext3 partitions with tune2fs ( and vol_id if you are on
RHEL5).
root@rhel55:~# /lib/udev/vol_id -u /dev/hda1
60926898-2c78-49b4-a71d-c1d6310c87cc
root@ubu1004:~# tune2fs -l /dev/sda2 | grep UUID
Filesystem UUID:
3007b743-1dce-2d62-9a59-cf25f85191b7
2. Use this uuid in /etc/fstab and test that it works with a simple mount.
tail -1 /etc/fstab
UUID=60926898-2c78-49b4-a71d-c1d6310c87cc /home/pro42 ext3 defaults 0 0
3. (optional) Test it also by removing a disk (so the device name is changed). You can edit
settings in vmware/Virtualbox to remove a hard disk.
4. Display the root= directive in /boot/grub/menu.lst. (We see later in the course how to
maintain this file.)
paul@deb503:~$ grep ^[^#] /boot/grub/menu.lst | grep root=
kernel
/boot/vmlinuz-2.6.26-2-686 root=/dev/hda1 ro selinux=1 quiet
kernel
/boot/vmlinuz-2.6.26-2-686 root=/dev/hda1 ro selinux=1 single
5. (optional on ubuntu) Replace the /dev/xxx in /boot/grub/menu.lst with a uuid (use an
extra stanza for this). Test that it works.
423
Chapter 45. introduction to raid
45.1. hardware or software
Redundant Array of Independent (originally Inexpensive) Disks or RAID can be set up using
hardware or software. Hardware RAID is more expensive, but offers better performance.
Software RAID is cheaper and easier to manage, but it uses your CPU and your memory.
Where ten years ago nobody was arguing about the best choice being hardware RAID, this
has changed since technologies like mdadm, lvm and even zfs focus more on managability.
The workload on the cpu for software RAID used to be high, but cpu's have gotten a lot
faster.
424
introduction to raid
45.2. raid levels
45.2.1. raid 0
raid 0 uses two or more disks, and is often called striping (or stripe set, or striped volume).
Data is divided in chunks, those chunks are evenly spread across every disk in the array.
The main advantage of raid 0 is that you can create larger drives. raid 0 is the only raid
without redundancy.
45.2.2. jbod
jbod uses two or more disks, and is often called concatenating (spanning, spanned set, or
spanned volume). Data is written to the first disk, until it is full. Then data is written to the
second disk... The main advantage of jbod (Just a Bunch of Disks) is that you can create
larger drives. JBOD offers no redundancy.
45.2.3. raid 1
raid 1 uses exactly two disks, and is often called mirroring (or mirror set, or mirrored
volume). All data written to the array is written on each disk. The main advantage of raid 1
is redundancy. The main disadvantage is that you lose at least half of your available disk
space (in other words, you at least double the cost).
45.2.4. raid 2, 3 and 4 ?
raid 2 uses bit level striping, raid 3 byte level, and raid 4 is the same as raid 5, but with a
dedicated parity disk. This is actually slower than raid 5, because every write would have
to write parity to this one (bottleneck) disk. It is unlikely that you will ever see these raid
levels in production.
45.2.5. raid 5
raid 5 uses three or more disks, each divided into chunks. Every time chunks are written
to the array, one of the disks will receive a parity chunk. Unlike raid 4, the parity chunk
will alternate between all disks. The main advantage of this is that raid 5 will allow for full
data recovery in case of one hard disk failure.
45.2.6. raid 6
raid 6 is very similar to raid 5, but uses two parity chunks. raid 6 protects against two hard
disk failures. Oracle Solaris zfs calls this raidz2 (and also had raidz3 with triple parity).
425
introduction to raid
45.2.7. raid 0+1
raid 0+1 is a mirror(1) of stripes(0). This means you first create two raid 0 stripe sets, and
then you set them up as a mirror set. For example, when you have six 100GB disks, then
the stripe sets are each 300GB. Combined in a mirror, this makes 300GB total. raid 0+1
will survive one disk failure. It will only survive the second disk failure if this disk is in the
same stripe set as the previous failed disk.
45.2.8. raid 1+0
raid 1+0 is a stripe(0) of mirrors(1). For example, when you have six 100GB disks, then
you first create three mirrors of 100GB each. You then stripe them together into a 300GB
drive. In this example, as long as not all disks in the same mirror fail, it can survive up to
three hard disk failures.
45.2.9. raid 50
raid 5+0 is a stripe(0) of raid 5 arrays. Suppose you have nine disks of 100GB, then you
can create three raid 5 arrays of 200GB each. You can then combine them into one large
stripe set.
45.2.10. many others
There are many other nested raid combinations, like raid 30, 51, 60, 100, 150, ...
426
introduction to raid
45.3. building a software raid5 array
45.3.1. do we have three disks?
First, you have to attach some disks to your computer. In this scenario, three brand new disks
of eight gigabyte each are added. Check with fdisk -l that they are connected.
[root@rhel6c ~]# fdisk -l 2> /dev/null | grep MB
Disk /dev/sdb: 8589 MB, 8589934592 bytes
Disk /dev/sdc: 8589 MB, 8589934592 bytes
Disk /dev/sdd: 8589 MB, 8589934592 bytes
45.3.2. fd partition type
The next step is to create a partition of type fd on every disk. The fd type is to set the partition
as Linux RAID autodetect. See this (truncated) screenshot:
[root@rhel6c ~]# fdisk /dev/sdd
...
Command (m for help): n
Command action
e
extended
p
primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-1044, default 1):
Using default value 1
Last cylinder, +cylinders or +size{K,M,G} (1-1044, default 1044):
Using default value 1044
Command (m for help): t
Selected partition 1
Hex code (type L to list codes): fd
Changed system type of partition 1 to fd (Linux raid autodetect)
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
Syncing disks.
45.3.3. verify all three partitions
Now all three disks are ready for raid 5, so we have to tell the system what to do with these
disks.
[root@rhel6c ~]# fdisk -l 2> /dev/null | grep raid
/dev/sdb1
1
1044
8385898+ fd Linux raid autodetect
/dev/sdc1
1
1044
8385898+ fd Linux raid autodetect
/dev/sdd1
1
1044
8385898+ fd Linux raid autodetect
427
introduction to raid
45.3.4. create the raid5
The next step used to be create the raid table in /etc/raidtab. Nowadays, you can just issue
the command mdadm with the correct parameters.
The command below is split on two lines to fit this print, but you should type it on one line,
without the backslash (\).
[root@rhel6c ~]# mdadm --create /dev/md0 --chunk=64 --level=5 --raid-\
devices=3 /dev/sdb1 /dev/sdc1 /dev/sdd1
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.
Below a partial screenshot how fdisk -l sees the raid 5.
[root@rhel6c ~]# fdisk -l /dev/md0
Disk /dev/md0: 17.2 GB, 17172135936 bytes
2 heads, 4 sectors/track, 4192416 cylinders
Units = cylinders of 8 * 512 = 4096 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 65536 bytes / 131072 bytes
Disk identifier: 0x00000000
Disk /dev/md0 doesn't contain a valid partition table
We could use this software raid 5 array in the next topic: lvm.
45.3.5. /proc/mdstat
The status of the raid devices can be seen in /proc/mdstat. This example shows a raid 5
in the process of rebuilding.
[root@rhel6c ~]# cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4]
md0 : active raid5 sdd1[3] sdc1[1] sdb1[0]
16769664 blocks super 1.2 level 5, 64k chunk, algorithm 2 [3/2] [UU_]
[============>........] recovery = 62.8% (5266176/8384832) finish=0\
.3min speed=139200K/sec
This example shows an active software raid 5.
[root@rhel6c ~]# cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4]
md0 : active raid5 sdd1[3] sdc1[1] sdb1[0]
16769664 blocks super 1.2 level 5, 64k chunk, algorithm 2 [3/3] [UUU]
428
introduction to raid
45.3.6. mdadm --detail
Use mdadm --detail to get information on a raid device.
[root@rhel6c ~]# mdadm --detail /dev/md0
/dev/md0:
Version : 1.2
Creation Time : Sun Jul 17 13:48:41 2011
Raid Level : raid5
Array Size : 16769664 (15.99 GiB 17.17 GB)
Used Dev Size : 8384832 (8.00 GiB 8.59 GB)
Raid Devices : 3
Total Devices : 3
Persistence : Superblock is persistent
Update Time
State
Active Devices
Working Devices
Failed Devices
Spare Devices
:
:
:
:
:
:
Sun Jul 17 13:49:43 2011
clean
3
3
0
0
Layout : left-symmetric
Chunk Size : 64K
Name : rhel6c:0 (local to host rhel6c)
UUID : c10fd9c3:08f9a25f:be913027:999c8e1f
Events : 18
Number
0
1
3
Major
8
8
8
Minor
17
33
49
RaidDevice
0
1
2
State
active sync
active sync
active sync
/dev/sdb1
/dev/sdc1
/dev/sdd1
45.3.7. removing a software raid
The software raid is visible in /proc/mdstat when active. To remove the raid completely so
you can use the disks for other purposes, you stop (de-activate) it with mdadm.
[root@rhel6c ~]# mdadm --stop /dev/md0
mdadm: stopped /dev/md0
The disks can now be repartitioned.
45.3.8. further reading
Take a look at the man page of mdadm for more information. Below an example command
to add a new partition while removing a faulty one.
mdadm /dev/md0 --add /dev/sdd1 --fail /dev/sdb1 --remove /dev/sdb1
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introduction to raid
45.4. practice: raid
1. Add three virtual disks of 1GB each to a virtual machine.
2. Create a software raid 5 on the three disks. (It is not necessary to put a filesystem on it)
3. Verify with fdisk and in /proc that the raid 5 exists.
4. Stop and remove the raid 5.
5. Create a raid 1 to mirror two disks.
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introduction to raid
45.5. solution: raid
1. Add three virtual disks of 1GB each to a virtual machine.
2. Create a software raid 5 on the three disks. (It is not necessary to put a filesystem on it)
3. Verify with fdisk and in /proc that the raid 5 exists.
4. Stop and remove the raid 5.
5. Create a raid 1 to mirror two disks.
[root@rhel6c ~]# mdadm --create /dev/md0 --level=1 --raid-devices=2 \
/dev/sdb1 /dev/sdc1
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.
[root@rhel6c ~]# cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4] [raid1]
md0 : active raid1 sdc1[1] sdb1[0]
8384862 blocks super 1.2 [2/2] [UU]
[====>................] resync = 20.8% (1745152/8384862) \
finish=0.5min speed=218144K/sec
431
Chapter 46. logical volume
management
Most lvm implementations support physical storage grouping, logical volume resizing
and data migration.
Physical storage grouping is a fancy name for grouping multiple block devices (hard disks,
but also iSCSI etc) into a logical mass storage device. To enlarge this physical group, block
devices (including partitions) can be added at a later time.
The size of lvm volumes on this physical group is independent of the individual size of the
components. The total size of the group is the limit.
One of the nice features of lvm is the logical volume resizing. You can increase the size of
an lvm volume, sometimes even without any downtime. Additionally, you can migrate data
away from a failing hard disk device, create mirrors and create snapshots.
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logical volume management
46.1. introduction to lvm
46.1.1. problems with standard partitions
There are some problems when working with hard disks and standard partitions. Consider
a system with a small and a large hard disk device, partitioned like this. The first disk (/
dev/sda) is partitioned in two, the second disk (/dev/sdb) has two partitions and some empty
space.
In the example above, consider the options when you want to enlarge the space available
for /srv/project42. What can you do ? The solution will always force you to unmount the
file system, take a backup of the data, remove and recreate partitions, and then restore the
data and remount the file system.
46.1.2. solution with lvm
Using lvm will create a virtual layer between the mounted file systems and the hardware
devices. This virtual layer will allow for an administrator to enlarge a mounted file system in
use. When lvm is properly used, then there is no need to unmount the file system to enlarge it.
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logical volume management
46.2. lvm terminology
46.2.1. physical volume (pv)
A physical volume is any block device (a disk, a partition, a RAID device or even an iSCSI
device). All these devices can become a member of a volume group.
The commands used to manage a physical volume start with pv.
[root@centos65 ~]# pv
pvchange
pvck
pvcreate
pvresize
pvs
pvscan
pvdisplay
pvmove
pvremove
46.2.2. volume group (vg)
A volume group is an abstraction layer between block devices and logical volumes.
The commands used to manage a volume group start with vg.
[root@centos65
vgcfgbackup
vgcfgrestore
vgchange
vgck
~]# vg
vgconvert
vgcreate
vgdisplay
vgexport
vgextend
vgimport
vgimportclone
vgmerge
vgmknodes
vgreduce
vgremove
vgrename
vgs
vgscan
vgsplit
46.2.3. logical volume (lv)
A logical volume is created in a volume group. Logical volumes that contain a file system
can be mounted. The use of logical volumes is similar to the use of partitions and is
accomplished with the same standard commands (mkfs, mount, fsck, df, ...).
The commands used to manage a logical volume start with lv.
[root@centos65 ~]# lv
lvchange
lvextend
lvconvert
lvm
lvcreate
lvmchange
lvdisplay
lvmconf
lvmdiskscan
lvmdump
lvmetad
lvmsadc
lvmsar
lvreduce
lvremove
lvrename
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lvresize
lvs
lvscan
logical volume management
46.3. example: using lvm
This example shows how you can use a device (in this case /dev/sdc, but it could have been /
dev/sdb or any other disk or partition) with lvm, how to create a volume group (vg) and how
to create and use a logical volume (vg/lvol0).
First thing to do, is create physical volumes that can join the volume group with pvcreate.
This command makes a disk or partition available for use in Volume Groups. The screenshot
shows how to present the SCSI Disk device to LVM.
root@RHEL4:~# pvcreate /dev/sdc
Physical volume "/dev/sdc" successfully created
Note: lvm will work fine when using the complete device, but another operating system on the
same computer (or on the same SAN) will not recognize lvm and will mark the block device
as being empty! You can avoid this by creating a partition that spans the whole device, then
run pvcreate on the partition instead of the disk.
Then vgcreate creates a volume group using one device. Note that more devices could be
added to the volume group.
root@RHEL4:~# vgcreate vg /dev/sdc
Volume group "vg" successfully created
The last step lvcreate creates a logical volume.
root@RHEL4:~# lvcreate --size 500m vg
Logical volume "lvol0" created
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logical volume management
The logical volume /dev/vg/lvol0 can now be formatted with ext3, and mounted for normal
use.
root@RHELv4u2:~# mke2fs -m0 -j /dev/vg/lvol0
mke2fs 1.35 (28-Feb-2004)
Filesystem label=
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
128016 inodes, 512000 blocks
0 blocks (0.00%) reserved for the super user
First data block=1
Maximum filesystem blocks=67633152
63 block groups
8192 blocks per group, 8192 fragments per group
2032 inodes per group
Superblock backups stored on blocks:
8193, 24577, 40961, 57345, 73729, 204801, 221185, 401409
Writing inode tables: done
Creating journal (8192 blocks): done
Writing superblocks and filesystem accounting information: done
This filesystem will be automatically checked every 37 mounts or
180 days, whichever comes first. Use tune2fs -c or -i to override.
root@RHELv4u2:~# mkdir /home/project10
root@RHELv4u2:~# mount /dev/vg/lvol0 /home/project10/
root@RHELv4u2:~# df -h | grep proj
/dev/mapper/vg-lvol0 485M
11M 474M
3% /home/project10
A logical volume is very similar to a partition, it can be formatted with a file system, and
can be mounted so users can access it.
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logical volume management
46.4. example: extend a logical volume
A logical volume can be extended without unmounting the file system. Whether or not a
volume can be extended depends on the file system it uses. Volumes that are mounted as
vfat or ext2 cannot be extended, so in the example here we use the ext3 file system.
The fdisk command shows us newly added scsi-disks that will serve our lvm volume. This
volume will then be extended. First, take a look at these disks.
[root@RHEL5 ~]# fdisk -l | grep sd[bc]
Disk /dev/sdb doesn't contain a valid partition table
Disk /dev/sdc doesn't contain a valid partition table
Disk /dev/sdb: 1181 MB, 1181115904 bytes
Disk /dev/sdc: 429 MB, 429496320 bytes
You already know how to partition a disk, below the first disk is partitioned (in one big
primary partition), the second disk is left untouched.
[root@RHEL5 ~]# fdisk -l | grep sd[bc]
Disk /dev/sdc doesn't contain a valid partition table
Disk /dev/sdb: 1181 MB, 1181115904 bytes
/dev/sdb1
1
143
1148616
83
Disk /dev/sdc: 429 MB, 429496320 bytes
Linux
You also know how to prepare disks for lvm with pvcreate, and how to create a volume
group with vgcreate. This example adds both the partitioned disk and the untouched disk
to the volume group named vg2.
[root@RHEL5 ~]# pvcreate /dev/sdb1
Physical volume "/dev/sdb1" successfully created
[root@RHEL5 ~]# pvcreate /dev/sdc
Physical volume "/dev/sdc" successfully created
[root@RHEL5 ~]# vgcreate vg2 /dev/sdb1 /dev/sdc
Volume group "vg2" successfully created
You can use pvdisplay to verify that both the disk and the partition belong to the volume
group.
[root@RHEL5 ~]# pvdisplay | grep -B1 vg2
PV Name
/dev/sdb1
VG Name
vg2
-PV Name
/dev/sdc
VG Name
vg2
And you are familiar both with the lvcreate command to create a small logical volume and
the mke2fs command to put ext3 on it.
[root@RHEL5 ~]# lvcreate --size 200m vg2
Logical volume "lvol0" created
[root@RHEL5 ~]# mke2fs -m20 -j /dev/vg2/lvol0
...
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logical volume management
As you see, we end up with a mounted logical volume that according to df is almost 200
megabyte in size.
[root@RHEL5 ~]# mkdir /home/resizetest
[root@RHEL5 ~]# mount /dev/vg2/lvol0 /home/resizetest/
[root@RHEL5 ~]# df -h | grep resizetest
194M 5.6M 149M
4% /home/resizetest
Extending the volume is easy with lvextend.
[root@RHEL5 ~]# lvextend -L +100 /dev/vg2/lvol0
Extending logical volume lvol0 to 300.00 MB
Logical volume lvol0 successfully resized
But as you can see, there is a small problem: it appears that df is not able to display the
extended volume in its full size. This is because the filesystem is only set for the size of the
volume before the extension was added.
[root@RHEL5 ~]# df -h | grep resizetest
194M 5.6M 149M
4% /home/resizetest
With lvdisplay however we can see that the volume is indeed extended.
[root@RHEL5 ~]# lvdisplay /dev/vg2/lvol0 | grep Size
LV Size
300.00 MB
To finish the extension, you need resize2fs to span the filesystem over the full size of the
logical volume.
[root@RHEL5 ~]# resize2fs /dev/vg2/lvol0
resize2fs 1.39 (29-May-2006)
Filesystem at /dev/vg2/lvol0 is mounted on /home/resizetest; on-line re\
sizing required
Performing an on-line resize of /dev/vg2/lvol0 to 307200 (1k) blocks.
The filesystem on /dev/vg2/lvol0 is now 307200 blocks long.
Congratulations, you just successfully expanded a logical volume.
[root@RHEL5 ~]# df -h | grep resizetest
291M 6.1M 225M
[root@RHEL5 ~]#
438
3% /home/resizetest
logical volume management
46.5. example: resize a physical Volume
This is a humble demonstration of how to resize a physical Volume with lvm (after you
resize it with fdisk). The demonstration starts with a 100MB partition named /dev/sde1. We
used fdisk to create it, and to verify the size.
[root@RHEL5 ~]# fdisk -l 2>/dev/null | grep sde1
/dev/sde1
1
100
102384
[root@RHEL5 ~]#
83
Linux
Now we can use pvcreate to create the Physical Volume, followed by pvs to verify the
creation.
[root@RHEL5 ~]# pvcreate /dev/sde1
Physical volume "/dev/sde1" successfully created
[root@RHEL5 ~]# pvs | grep sde1
/dev/sde1
lvm2 -99.98M 99.98M
[root@RHEL5 ~]#
The next step is to use fdisk to enlarge the partition (actually deleting it and then recreating /
dev/sde1 with more cylinders).
[root@RHEL5 ~]# fdisk /dev/sde
Command (m for help): p
Disk /dev/sde: 858 MB, 858993152 bytes
64 heads, 32 sectors/track, 819 cylinders
Units = cylinders of 2048 * 512 = 1048576 bytes
Device Boot
/dev/sde1
Start
1
End
100
Blocks
102384
Id
83
System
Linux
Command (m for help): d
Selected partition 1
Command (m for help): n
Command action
e
extended
p
primary partition (1-4)
p
Partition number (1-4):
Value out of range.
Partition number (1-4): 1
First cylinder (1-819, default 1):
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-819, default 819): 200
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
Syncing disks.
[root@RHEL5 ~]#
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logical volume management
When we now use fdisk and pvs to verify the size of the partition and the Physical Volume,
then there is a size difference. LVM is still using the old size.
[root@RHEL5 ~]# fdisk -l 2>/dev/null | grep sde1
/dev/sde1
1
200
204784
[root@RHEL5 ~]# pvs | grep sde1
/dev/sde1
lvm2 -99.98M 99.98M
[root@RHEL5 ~]#
83
Linux
Executing pvresize on the Physical Volume will make lvm aware of the size change of the
partition. The correct size can be displayed with pvs.
[root@RHEL5 ~]# pvresize /dev/sde1
Physical volume "/dev/sde1" changed
1 physical volume(s) resized / 0 physical volume(s) not resized
[root@RHEL5 ~]# pvs | grep sde1
/dev/sde1
lvm2 -199.98M 199.98M
[root@RHEL5 ~]#
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logical volume management
46.6. example: mirror a logical volume
We start by creating three physical volumes for lvm. Then we verify the creation and the
size with pvs. Three physical disks because lvm uses two disks for the mirror and a third
disk for the mirror log!
[root@RHEL5 ~]# pvcreate /dev/sdb /dev/sdc /dev/sdd
Physical volume "/dev/sdb" successfully created
Physical volume "/dev/sdc" successfully created
Physical volume "/dev/sdd" successfully created
[root@RHEL5 ~]# pvs
PV
VG
Fmt Attr PSize
PFree
/dev/sdb
lvm2 -409.60M 409.60M
/dev/sdc
lvm2 -409.60M 409.60M
/dev/sdd
lvm2 -409.60M 409.60M
Then we create the Volume Group and verify again with pvs. Notice how the three physical
volumes now belong to vg33, and how the size is rounded down (in steps of the extent size,
here 4MB).
[root@RHEL5 ~]# vgcreate vg33 /dev/sdb /dev/sdc /dev/sdd
Volume group "vg33" successfully created
[root@RHEL5 ~]# pvs
PV
VG
Fmt Attr PSize
PFree
/dev/sda2 VolGroup00 lvm2 a15.88G
0
/dev/sdb
vg33
lvm2 a408.00M 408.00M
/dev/sdc
vg33
lvm2 a408.00M 408.00M
/dev/sdd
vg33
lvm2 a408.00M 408.00M
[root@RHEL5 ~]#
The last step is to create the Logical Volume with lvcreate. Notice the -m 1 switch to create
one mirror. Notice also the change in free space in all three Physical Volumes!
[root@RHEL5 ~]# lvcreate --size 300m -n lvmir -m 1 vg33
Logical volume "lvmir" created
[root@RHEL5 ~]# pvs
PV
VG
Fmt Attr PSize
PFree
/dev/sda2 VolGroup00 lvm2 a15.88G
0
/dev/sdb
vg33
lvm2 a408.00M 108.00M
/dev/sdc
vg33
lvm2 a408.00M 108.00M
/dev/sdd
vg33
lvm2 a408.00M 404.00M
You can see the copy status of the mirror with lvs. It currently shows a 100 percent copy.
[root@RHEL5 ~]# lvs vg33/lvmir
LV
VG
Attr
LSize
Origin Snap%
lvmir vg33 mwi-ao 300.00M
441
Move Log
Copy%
lvmir_mlog 100.00
logical volume management
46.7. example: snapshot a logical volume
A snapshot is a virtual copy of all the data at a point in time on a volume. A snapshot Logical
Volume will retain a copy of all changed files of the snapshotted Logical Volume.
The example below creates a snapshot of the bigLV Logical Volume.
[root@RHEL5 ~]# lvcreate -L100M -s -n snapLV vg42/bigLV
Logical volume "snapLV" created
[root@RHEL5 ~]#
You can see with lvs that the snapshot snapLV is indeed a snapshot of bigLV. Moments
after taking the snapshot, there are few changes to bigLV (0.02 percent).
[root@RHEL5 ~]# lvs
LV
VG
bigLV
vg42
snapLV
vg42
[root@RHEL5 ~]#
Attr
LSize
Origin Snap% Move Log Copy%
owi-a- 200.00M
swi-a- 100.00M bigLV
0.02
But after using bigLV for a while, more changes are done. This means the snapshot volume
has to keep more original data (10.22 percent).
[root@RHEL5 ~]# lvs | grep vg42
bigLV
vg42
owi-ao 200.00M
snapLV
vg42
swi-a- 100.00M bigLV
[root@RHEL5 ~]#
10.22
You can now use regular backup tools (dump, tar, cpio, ...) to take a backup of the snapshot
Logical Volume. This backup will contain all data as it existed on bigLV at the time the
snapshot was taken. When the backup is done, you can remove the snapshot.
[root@RHEL5 ~]# lvremove vg42/snapLV
Do you really want to remove active logical volume "snapLV"? [y/n]: y
Logical volume "snapLV" successfully removed
[root@RHEL5 ~]#
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logical volume management
46.8. verifying existing physical volumes
46.8.1. lvmdiskscan
To get a list of block devices that can be used with LVM, use lvmdiskscan. The example
below uses grep to limit the result to SCSI devices.
[root@RHEL5 ~]# lvmdiskscan | grep sd
/dev/sda1
[
101.94 MB]
/dev/sda2
[
15.90 GB] LVM physical volume
/dev/sdb
[
409.60 MB]
/dev/sdc
[
409.60 MB]
/dev/sdd
[
409.60 MB] LVM physical volume
/dev/sde1
[
95.98 MB]
/dev/sde5
[
191.98 MB]
/dev/sdf
[
819.20 MB] LVM physical volume
/dev/sdg1
[
818.98 MB]
[root@RHEL5 ~]#
46.8.2. pvs
The easiest way to verify whether devices are known to lvm is with the pvs command. The
screenshot below shows that only /dev/sda2 is currently known for use with LVM. It shows
that /dev/sda2 is part of Volgroup00 and is almost 16GB in size. It also shows /dev/sdc and /
dev/sdd as part of vg33. The device /dev/sdb is knwon to lvm, but not linked to any Volume
Group.
[root@RHEL5 ~]# pvs
PV
VG
/dev/sda2 VolGroup00
/dev/sdb
/dev/sdc
vg33
/dev/sdd
vg33
[root@RHEL5 ~]#
Fmt
lvm2
lvm2
lvm2
lvm2
Attr
a-aa-
PSize
15.88G
409.60M
408.00M
408.00M
PFree
0
409.60M
408.00M
408.00M
46.8.3. pvscan
The pvscan command will scan all disks for existing Physical Volumes. The information is
similar to pvs, plus you get a line with total sizes.
[root@RHEL5 ~]# pvscan
PV /dev/sdc
VG vg33
lvm2 [408.00 MB / 408.00 MB free]
PV /dev/sdd
VG vg33
lvm2 [408.00 MB / 408.00 MB free]
PV /dev/sda2
VG VolGroup00
lvm2 [15.88 GB / 0
free]
PV /dev/sdb
lvm2 [409.60 MB]
Total: 4 [17.07 GB] / in use: 3 [16.67 GB] / in no VG: 1 [409.60 MB]
[root@RHEL5 ~]#
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logical volume management
46.8.4. pvdisplay
Use pvdisplay to get more information about physical volumes. You can also use pvdisplay
without an argument to display information about all physical (lvm) volumes.
[root@RHEL5 ~]# pvdisplay /dev/sda2
--- Physical volume --PV Name
/dev/sda2
VG Name
VolGroup00
PV Size
15.90 GB / not usable 20.79 MB
Allocatable
yes (but full)
PE Size (KByte)
32768
Total PE
508
Free PE
0
Allocated PE
508
PV UUID
TobYfp-Ggg0-Rf8r-xtLd-5XgN-RSPc-8vkTHD
[root@RHEL5 ~]#
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logical volume management
46.9. verifying existing volume groups
46.9.1. vgs
Similar to pvs is the use of vgs to display a quick overview of all volume groups. There
is only one volume group in the screenshot below, it is named VolGroup00 and is almost
16GB in size.
[root@RHEL5 ~]# vgs
VG
#PV #LV #SN Attr
VSize VFree
VolGroup00
1
2
0 wz--n- 15.88G
0
[root@RHEL5 ~]#
46.9.2. vgscan
The vgscan command will scan all disks for existing Volume Groups. It will also update the
/etc/lvm/.cache file. This file contains a list of all current lvm devices.
[root@RHEL5 ~]# vgscan
Reading all physical volumes. This may take a while...
Found volume group "VolGroup00" using metadata type lvm2
[root@RHEL5 ~]#
LVM will run the vgscan automatically at boot-up, so if you add hot swap devices, then you
will need to run vgscan to update /etc/lvm/.cache with the new devices.
46.9.3. vgdisplay
The vgdisplay command will give you more detailed information about a volume group (or
about all volume groups if you omit the argument).
[root@RHEL5 ~]# vgdisplay VolGroup00
--- Volume group --VG Name
VolGroup00
System ID
Format
lvm2
Metadata Areas
1
Metadata Sequence No 3
VG Access
read/write
VG Status
resizable
MAX LV
0
Cur LV
2
Open LV
2
Max PV
0
Cur PV
1
Act PV
1
VG Size
15.88 GB
PE Size
32.00 MB
Total PE
508
Alloc PE / Size
508 / 15.88 GB
Free PE / Size
0 / 0
VG UUID
qsXvJb-71qV-9l7U-ishX-FobM-qptE-VXmKIg
[root@RHEL5 ~]#
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logical volume management
46.10. verifying existing logical volumes
46.10.1. lvs
Use lvs for a quick look at all existing logical volumes. Below you can see two logical
volumes named LogVol00 and LogVol01.
[root@RHEL5 ~]# lvs
LV
VG
Attr
LSize Origin Snap%
LogVol00 VolGroup00 -wi-ao 14.88G
LogVol01 VolGroup00 -wi-ao 1.00G
[root@RHEL5 ~]#
Move Log Copy%
46.10.2. lvscan
The lvscan command will scan all disks for existing Logical Volumes.
[root@RHEL5 ~]# lvscan
ACTIVE
'/dev/VolGroup00/LogVol00' [14.88 GB] inherit
ACTIVE
'/dev/VolGroup00/LogVol01' [1.00 GB] inherit
[root@RHEL5 ~]#
46.10.3. lvdisplay
More detailed information about logical volumes is available through the lvdisplay(1)
command.
[root@RHEL5 ~]# lvdisplay VolGroup00/LogVol01
--- Logical volume --LV Name
/dev/VolGroup00/LogVol01
VG Name
VolGroup00
LV UUID
RnTGK6-xWsi-t530-ksJx-7cax-co5c-A1KlDp
LV Write Access
read/write
LV Status
available
# open
1
LV Size
1.00 GB
Current LE
32
Segments
1
Allocation
inherit
Read ahead sectors
0
Block device
253:1
[root@RHEL5 ~]#
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logical volume management
46.11. manage physical volumes
46.11.1. pvcreate
Use the pvcreate command to add devices to lvm. This example shows how to add a disk
(or hardware RAID device) to lvm.
[root@RHEL5 ~]# pvcreate /dev/sdb
Physical volume "/dev/sdb" successfully created
[root@RHEL5 ~]#
This example shows how to add a partition to lvm.
[root@RHEL5 ~]# pvcreate /dev/sdc1
Physical volume "/dev/sdc1" successfully created
[root@RHEL5 ~]#
You can also add multiple disks or partitions as target to pvcreate. This example adds three
disks to lvm.
[root@RHEL5 ~]# pvcreate /dev/sde /dev/sdf /dev/sdg
Physical volume "/dev/sde" successfully created
Physical volume "/dev/sdf" successfully created
Physical volume "/dev/sdg" successfully created
[root@RHEL5 ~]#
46.11.2. pvremove
Use the pvremove command to remove physical volumes from lvm. The devices may not
be in use.
[root@RHEL5
Labels on
Labels on
Labels on
[root@RHEL5
~]# pvremove /dev/sde /dev/sdf /dev/sdg
physical volume "/dev/sde" successfully wiped
physical volume "/dev/sdf" successfully wiped
physical volume "/dev/sdg" successfully wiped
~]#
46.11.3. pvresize
When you used fdisk to resize a partition on a disk, then you must use pvresize to make lvm
recognize the new size of the physical volume that represents this partition.
[root@RHEL5 ~]# pvresize /dev/sde1
Physical volume "/dev/sde1" changed
1 physical volume(s) resized / 0 physical volume(s) not resized
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logical volume management
46.11.4. pvchange
With pvchange you can prevent the allocation of a Physical Volume in a new Volume Group
or Logical Volume. This can be useful if you plan to remove a Physical Volume.
[root@RHEL5 ~]# pvchange -xn /dev/sdd
Physical volume "/dev/sdd" changed
1 physical volume changed / 0 physical volumes not changed
[root@RHEL5 ~]#
To revert your previous decision, this example shows you how te re-enable the Physical
Volume to allow allocation.
[root@RHEL5 ~]# pvchange -xy /dev/sdd
Physical volume "/dev/sdd" changed
1 physical volume changed / 0 physical volumes not changed
[root@RHEL5 ~]#
46.11.5. pvmove
With pvmove you can move Logical Volumes from within a Volume Group to another
Physical Volume. This must be done before removing a Physical Volume.
[root@RHEL5
/dev/sdf
/dev/sdg
[root@RHEL5
/dev/sdf:
/dev/sdf:
[root@RHEL5
/dev/sdf
/dev/sdg
~]# pvs | grep vg1
vg1
lvm2 avg1
lvm2 a~]# pvmove /dev/sdf
Moved: 70.1%
Moved: 100.0%
~]# pvs | grep vg1
vg1
lvm2 avg1
lvm2 a-
816.00M
0
816.00M 816.00M
816.00M 816.00M
816.00M
0
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logical volume management
46.12. manage volume groups
46.12.1. vgcreate
Use the vgcreate command to create a volume group. You can immediately name all the
physical volumes that span the volume group.
[root@RHEL5 ~]# vgcreate vg42 /dev/sde /dev/sdf
Volume group "vg42" successfully created
[root@RHEL5 ~]#
46.12.2. vgextend
Use the vgextend command to extend an existing volume group with a physical volume.
[root@RHEL5 ~]# vgextend vg42 /dev/sdg
Volume group "vg42" successfully extended
[root@RHEL5 ~]#
46.12.3. vgremove
Use the vgremove command to remove volume groups from lvm. The volume groups may
not be in use.
[root@RHEL5 ~]# vgremove vg42
Volume group "vg42" successfully removed
[root@RHEL5 ~]#
46.12.4. vgreduce
Use the vgreduce command to remove a Physical Volume from the Volume Group.
The following example adds Physical Volume /dev/sdg to the vg1 Volume Group using
vgextend. And then removes it again using vgreduce.
[root@RHEL5 ~]# pvs | grep sdg
/dev/sdg
lvm2 -819.20M 819.20M
[root@RHEL5 ~]# vgextend vg1 /dev/sdg
Volume group "vg1" successfully extended
[root@RHEL5 ~]# pvs | grep sdg
/dev/sdg
vg1
lvm2 a816.00M 816.00M
[root@RHEL5 ~]# vgreduce vg1 /dev/sdg
Removed "/dev/sdg" from volume group "vg1"
[root@RHEL5 ~]# pvs | grep sdg
/dev/sdg
lvm2 -819.20M 819.20M
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logical volume management
46.12.5. vgchange
Use the vgchange command to change parameters of a Volume Group.
This example shows how to prevent Physical Volumes from being added or removed to the
Volume Group vg1.
[root@RHEL5 ~]# vgchange -xn vg1
Volume group "vg1" successfully changed
[root@RHEL5 ~]# vgextend vg1 /dev/sdg
Volume group vg1 is not resizable.
You can also use vgchange to change most other properties of a Volume Group. This
example changes the maximum number of Logical Volumes and maximum number of
Physical Volumes that vg1 can serve.
[root@RHEL5 ~]# vgdisplay vg1 | grep -i max
MAX LV
0
Max PV
0
[root@RHEL5 ~]# vgchange -l16 vg1
Volume group "vg1" successfully changed
[root@RHEL5 ~]# vgchange -p8 vg1
Volume group "vg1" successfully changed
[root@RHEL5 ~]# vgdisplay vg1 | grep -i max
MAX LV
16
Max PV
8
46.12.6. vgmerge
Merging two Volume Groups into one is done with vgmerge. The following example merges
vg2 into vg1, keeping all the properties of vg1.
[root@RHEL5 ~]# vgmerge vg1 vg2
Volume group "vg2" successfully merged into "vg1"
[root@RHEL5 ~]#
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logical volume management
46.13. manage logical volumes
46.13.1. lvcreate
Use the lvcreate command to create Logical Volumes in a Volume Group. This example
creates an 8GB Logical Volume in Volume Group vg42.
[root@RHEL5 ~]# lvcreate -L5G vg42
Logical volume "lvol0" created
[root@RHEL5 ~]#
As you can see, lvm automatically names the Logical Volume lvol0. The next example
creates a 200MB Logical Volume named MyLV in Volume Group vg42.
[root@RHEL5 ~]# lvcreate -L200M -nMyLV vg42
Logical volume "MyLV" created
[root@RHEL5 ~]#
The next example does the same thing, but with different syntax.
[root@RHEL5 ~]# lvcreate --size 200M -n MyLV vg42
Logical volume "MyLV" created
[root@RHEL5 ~]#
This example creates a Logical Volume that occupies 10 percent of the Volume Group.
[root@RHEL5 ~]# lvcreate -l 10%VG -n MyLV2 vg42
Logical volume "MyLV2" created
[root@RHEL5 ~]#
This example creates a Logical Volume that occupies 30 percent of the remaining free space
in the Volume Group.
[root@RHEL5 ~]# lvcreate -l 30%FREE -n MyLV3 vg42
Logical volume "MyLV3" created
[root@RHEL5 ~]#
46.13.2. lvremove
Use the lvremove command to remove Logical Volumes from a Volume Group. Removing
a Logical Volume requires the name of the Volume Group.
[root@RHEL5 ~]# lvremove vg42/MyLV
Do you really want to remove active logical volume "MyLV"? [y/n]: y
Logical volume "MyLV" successfully removed
[root@RHEL5 ~]#
Removing multiple Logical Volumes will request confirmation for each individual volume.
[root@RHEL5 ~]# lvremove vg42/MyLV vg42/MyLV2 vg42/MyLV3
Do you really want to remove active logical volume "MyLV"? [y/n]: y
Logical volume "MyLV" successfully removed
Do you really want to remove active logical volume "MyLV2"? [y/n]: y
Logical volume "MyLV2" successfully removed
Do you really want to remove active logical volume "MyLV3"? [y/n]: y
Logical volume "MyLV3" successfully removed
[root@RHEL5 ~]#
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logical volume management
46.13.3. lvextend
Extending the volume is easy with lvextend. This example extends a 200MB Logical
Volume with 100 MB.
[root@RHEL5 ~]# lvdisplay /dev/vg2/lvol0 | grep Size
LV Size
200.00 MB
[root@RHEL5 ~]# lvextend -L +100 /dev/vg2/lvol0
Extending logical volume lvol0 to 300.00 MB
Logical volume lvol0 successfully resized
[root@RHEL5 ~]# lvdisplay /dev/vg2/lvol0 | grep Size
LV Size
300.00 MB
The next example creates a 100MB Logical Volume, and then extends it to 500MB.
[root@RHEL5 ~]# lvcreate --size 100M -n extLV vg42
Logical volume "extLV" created
[root@RHEL5 ~]# lvextend -L 500M vg42/extLV
Extending logical volume extLV to 500.00 MB
Logical volume extLV successfully resized
[root@RHEL5 ~]#
This example doubles the size of a Logical Volume.
[root@RHEL5 ~]# lvextend -l+100%LV vg42/extLV
Extending logical volume extLV to 1000.00 MB
Logical volume extLV successfully resized
[root@RHEL5 ~]#
46.13.4. lvrename
Renaming a Logical Volume is done with lvrename. This example renames extLV to bigLV
in the vg42 Volume Group.
[root@RHEL5 ~]# lvrename vg42/extLV vg42/bigLV
Renamed "extLV" to "bigLV" in volume group "vg42"
[root@RHEL5 ~]#
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logical volume management
46.14. practice : lvm
1. Create a volume group that contains a complete disk and a partition on another disk.
2. Create two logical volumes (a small one and a bigger one) in this volumegroup. Format
them wih ext3, mount them and copy some files to them.
3. Verify usage with fdisk, mount, pvs, vgs, lvs, pvdisplay, vgdisplay, lvdisplay and df. Does
fdisk give you any information about lvm?
4. Enlarge the small logical volume by 50 percent, and verify your work!
5. Take a look at other commands that start with vg* , pv* or lv*.
6. Create a mirror and a striped Logical Volume.
7. Convert a linear logical volume to a mirror.
8. Convert a mirror logical volume to a linear.
9. Create a snapshot of a Logical Volume, take a backup of the snapshot. Then delete some
files on the Logical Volume, then restore your backup.
10. Move your volume group to another disk (keep the Logical Volumes mounted).
11. If time permits, split a Volume Group with vgsplit, then merge it again with vgmerge.
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logical volume management
46.15. solution : lvm
1. Create a volume group that contains a complete disk and a partition on another disk.
step 1: select disks:
root@rhel65:~# fdisk -l | grep Disk
Disk /dev/sda: 8589 MB, 8589934592 bytes
Disk identifier: 0x00055ca0
Disk /dev/sdb: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
Disk /dev/sdc: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
...
I choose /dev/sdb and /dev/sdc for now.
step 2: partition /dev/sdc
root@rhel65:~# fdisk /dev/sdc
Device contains neither a valid DOS partition table, nor Sun, SGI or OSF disk\
label
Building a new DOS disklabel with disk identifier 0x94c0e5d5.
Changes will remain in memory only, until you decide to write them.
After that, of course, the previous content won't be recoverable.
Warning: invalid flag 0x0000 of partition table 4 will be corrected by w(rite)
WARNING: DOS-compatible mode is deprecated. It's strongly recommended to
switch off the mode (command 'c') and change display units to
sectors (command 'u').
Command (m for help): n
Command action
e
extended
p
primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-130, default 1):
Using default value 1
Last cylinder, +cylinders or +size{K,M,G} (1-130, default 130):
Using default value 130
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
Syncing disks.
step 3: pvcreate and vgcreate
root@rhel65:~# pvcreate /dev/sdb /dev/sdc1
Physical volume "/dev/sdb" successfully created
Physical volume "/dev/sdc1" successfully created
root@rhel65:~# vgcreate VG42 /dev/sdb /dev/sdc1
Volume group "VG42" successfully created
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logical volume management
2. Create two logical volumes (a small one and a bigger one) in this volumegroup. Format
them wih ext3, mount them and copy some files to them.
root@rhel65:~# lvcreate --size 200m --name LVsmall VG42
Logical volume "LVsmall" created
root@rhel65:~# lvcreate --size 600m --name LVbig VG42
Logical volume "LVbig" created
root@rhel65:~# ls -l /dev/mapper/VG42-LVsmall
lrwxrwxrwx. 1 root root 7 Apr 20 20:41 /dev/mapper/VG42-LVsmall -> ../dm-2
root@rhel65:~# ls -l /dev/VG42/LVsmall
lrwxrwxrwx. 1 root root 7 Apr 20 20:41 /dev/VG42/LVsmall -> ../dm-2
root@rhel65:~# ls -l /dev/dm-2
brw-rw----. 1 root disk 253, 2 Apr 20 20:41 /dev/dm-2
root@rhel65:~# mkfs.ext3 /dev/mapper/VG42-LVsmall
mke2fs 1.41.12 (17-May-2010)
Filesystem label=
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
Stride=0 blocks, Stripe width=0 blocks
51200 inodes, 204800 blocks
10240 blocks (5.00%) reserved for the super user
First data block=1
Maximum filesystem blocks=67371008
25 block groups
8192 blocks per group, 8192 fragments per group
2048 inodes per group
Superblock backups stored on blocks:
8193, 24577, 40961, 57345, 73729
Writing inode tables: done
Creating journal (4096 blocks): done
Writing superblocks and filesystem accounting information: done
This filesystem will be automatically checked every 39 mounts or
180 days, whichever comes first. Use tune2fs -c or -i to override.
root@rhel65:~# mkfs.ext3 /dev/VG42/LVbig
mke2fs 1.41.12 (17-May-2010)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
38400 inodes, 153600 blocks
7680 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=159383552
5 block groups
32768 blocks per group, 32768 fragments per group
7680 inodes per group
Superblock backups stored on blocks:
32768, 98304
Writing inode tables: done
Creating journal (4096 blocks): done
Writing superblocks and filesystem accounting information: done
This filesystem will be automatically checked every 25 mounts or
180 days, whichever comes first. Use tune2fs -c or -i to override.
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logical volume management
The mounting and copying of files.
root@rhel65:~#
root@rhel65:~#
root@rhel65:~#
root@rhel65:~#
root@rhel65:~#
root@rhel65:~#
mkdir
mkdir
mount
mount
cp -r
cp -r
/srv/LVsmall
/srv/LVbig
/dev/mapper/VG42-LVsmall /srv/LVsmall
/dev/VG42/LVbig /srv/LVbig
/etc /srv/LVsmall/
/var/log /srv/LVbig/
3. Verify usage with fdisk, mount, pvs, vgs, lvs, pvdisplay, vgdisplay, lvdisplay and df. Does
fdisk give you any information about lvm?
Run all those commands (only two are shown below), then answer 'no'.
root@rhel65:~# df -h
Filesystem
Size Used Avail Use% Mounted on
/dev/mapper/VolGroup-lv_root
6.7G 1.4G 5.0G 21% /
tmpfs
246M
0 246M
0% /dev/shm
/dev/sda1
485M
77M 383M 17% /boot
/dev/mapper/VG42-LVsmall
194M
30M 154M 17% /srv/LVsmall
/dev/mapper/VG42-LVbig
591M
20M 541M
4% /srv/LVbig
root@rhel65:~# mount | grep VG42
/dev/mapper/VG42-LVsmall on /srv/LVsmall type ext3 (rw)
/dev/mapper/VG42-LVbig on /srv/LVbig type ext3 (rw)
4. Enlarge the small logical volume by 50 percent, and verify your work!
root@rhel65:~# lvextend VG42/LVsmall -l+50%LV
Extending logical volume LVsmall to 300.00 MiB
Logical volume LVsmall successfully resized
root@rhel65:~# resize2fs /dev/mapper/VG42-LVsmall
resize2fs 1.41.12 (17-May-2010)
Filesystem at /dev/mapper/VG42-LVsmall is mounted on /srv/LVsmall; on-line res\
izing required
old desc_blocks = 1, new_desc_blocks = 2
Performing an on-line resize of /dev/mapper/VG42-LVsmall to 307200 (1k) blocks.
The filesystem on /dev/mapper/VG42-LVsmall is now 307200 blocks long.
root@rhel65:~# df -h | grep small
/dev/mapper/VG42-LVsmall
291M
31M 246M
root@rhel65:~#
12% /srv/LVsmall
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logical volume management
5. Take a look at other commands that start with vg* , pv* or lv*.
6. Create a mirror and a striped Logical Volume.
7. Convert a linear logical volume to a mirror.
8. Convert a mirror logical volume to a linear.
9. Create a snapshot of a Logical Volume, take a backup of the snapshot. Then delete some
files on the Logical Volume, then restore your backup.
10. Move your volume group to another disk (keep the Logical Volumes mounted).
11. If time permits, split a Volume Group with vgsplit, then merge it again with vgmerge.
457
Chapter 47. iSCSI devices
This chapter teaches you how to setup an iSCSI target server and an iSCSI initiator client.
458
iSCSI devices
47.1. iSCSI terminology
iSCSI is a protocol that enables SCSI over IP. This means that you can have local SCSI
devices (like /dev/sdb) without having the storage hardware in the local computer.
The computer holding the physical storage hardware is called the iSCSI Target. Each
individual addressable iSCSI device on the target server will get a LUN number.
The iSCSI client computer that is connecting to the Target server is called an Initiator. An
initiator will send SCSI commands over IP instead of directly to the hardware. The Initiator
will connect to the Target.
47.2. iSCSI Target in RHEL/CentOS
This section will describe iSCSI Target setup on RHEL6, RHEL7 and CentOS.
Start with installing the iSCSI Target package.
yum install scsi-target-utils
We configure three local disks in /etc/tgt/targets.conf to become three LUN's.
<target iqn.2008-09.com.example:server.target2>
direct-store /dev/sdb
direct-store /dev/sdc
direct-store /dev/sdd
incominguser paul hunter2
</target>
Restart the service.
[root@centos65 ~]# service tgtd start
Starting SCSI target daemon:
[
OK
]
The standard local port for iSCSI Target is 3260, in case of doubt you can verify this with
netstat.
[root@server1 tgt]# netstat -ntpl | grep tgt
tcp
0
0 0.0.0.0:3260
0.0.0.0:*
tcp
0
0 :::3260
:::*
459
LISTEN
LISTEN
1670/tgtd
1670/tgtd
iSCSI devices
The tgt-admin -s command should now give you a nice overview of the three LUN's (and
also LUN 0 for the controller).
[root@server1 tgt]# tgt-admin -s
Target 1: iqn.2014-04.be.linux-training:server1.target1
System information:
Driver: iscsi
State: ready
I_T nexus information:
LUN information:
LUN: 0
Type: controller
SCSI ID: IET
00010000
SCSI SN: beaf10
Size: 0 MB, Block size: 1
Online: Yes
Removable media: No
Prevent removal: No
Readonly: No
Backing store type: null
Backing store path: None
Backing store flags:
LUN: 1
Type: disk
SCSI ID: IET
00010001
SCSI SN: VB9f23197b-af6cfb60
Size: 1074 MB, Block size: 512
Online: Yes
Removable media: No
Prevent removal: No
Readonly: No
Backing store type: rdwr
Backing store path: /dev/sdb
Backing store flags:
LUN: 2
Type: disk
SCSI ID: IET
00010002
SCSI SN: VB8f554351-a1410828
Size: 1074 MB, Block size: 512
Online: Yes
Removable media: No
Prevent removal: No
Readonly: No
Backing store type: rdwr
Backing store path: /dev/sdc
Backing store flags:
LUN: 3
Type: disk
SCSI ID: IET
00010003
SCSI SN: VB1035d2f0-7ae90b49
Size: 1074 MB, Block size: 512
Online: Yes
Removable media: No
Prevent removal: No
Readonly: No
Backing store type: rdwr
Backing store path: /dev/sdd
Backing store flags:
Account information:
ACL information:
ALL
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iSCSI devices
47.3. iSCSI Initiator in RHEL/CentOS
This section will describe iSCSI Initiator setup on RHEL6, RHEL7 and CentOS.
Start with installing the iSCSI Initiator package.
[root@server2 ~]# yum install iscsi-initiator-utils
Then ask the iSCSI target server to send you the target names.
[root@server2 ~]# iscsiadm -m discovery -t sendtargets -p 192.168.1.95:3260
Starting iscsid:
[ OK ]
192.168.1.95:3260,1 iqn.2014-04.be.linux-training:centos65.target1
We received iqn.2014-04.be.linux-training:centos65.target1.
We use this iqn to configure the username and the password (paul and hunter2) that we set
on the target server.
[root@server2 iscsi]# iscsiadm -m node --targetname
ining:centos65.target1 --portal "192.168.1.95:3260"
session.auth.username --value=paul
[root@server2 iscsi]# iscsiadm -m node --targetname
ining:centos65.target1 --portal "192.168.1.95:3260"
session.auth.password --value=hunter2
[root@server2 iscsi]# iscsiadm -m node --targetname
ining:centos65.target1 --portal "192.168.1.95:3260"
session.auth.authmethod --value=CHAP
iqn.2014-04.be.linux-tra\
--op=update --name node.\
iqn.2014-04.be.linux-tra\
--op=update --name node.\
iqn.2014-04.be.linux-tra\
--op=update --name node.\
RHEL and CentOS will store these in /var/lib/iscsi/nodes/.
[root@server2 iscsi]# grep auth /var/lib/iscsi/nodes/iqn.2014-04.be.linux-tr\
aining\:centos65.target1/192.168.1.95\,3260\,1/default
node.session.auth.authmethod = CHAP
node.session.auth.username = paul
node.session.auth.password = hunter2
node.conn[0].timeo.auth_timeout = 45
[root@server2 iscsi]#
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iSCSI devices
A restart of the iscsi service will add three new devices to our system.
[root@server2 iscsi]# fdisk -l | grep Disk
Disk /dev/sda: 42.9 GB, 42949672960 bytes
Disk identifier: 0x0004f229
Disk /dev/sdb: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
Disk /dev/sdc: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
Disk /dev/sdd: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
Disk /dev/sde: 2147 MB, 2147483648 bytes
Disk identifier: 0x00000000
Disk /dev/sdf: 2147 MB, 2147483648 bytes
Disk identifier: 0x00000000
Disk /dev/sdg: 2147 MB, 2147483648 bytes
Disk identifier: 0x00000000
Disk /dev/mapper/VolGroup-lv_root: 41.4 GB, 41448112128 bytes
Disk identifier: 0x00000000
Disk /dev/mapper/VolGroup-lv_swap: 973 MB, 973078528 bytes
Disk identifier: 0x00000000
[root@server2 iscsi]# service iscsi restart
Stopping iscsi:
[ OK
Starting iscsi:
[ OK
[root@server2 iscsi]# fdisk -l | grep Disk
Disk /dev/sda: 42.9 GB, 42949672960 bytes
Disk identifier: 0x0004f229
Disk /dev/sdb: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
Disk /dev/sdc: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
Disk /dev/sdd: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
Disk /dev/sde: 2147 MB, 2147483648 bytes
Disk identifier: 0x00000000
Disk /dev/sdf: 2147 MB, 2147483648 bytes
Disk identifier: 0x00000000
Disk /dev/sdg: 2147 MB, 2147483648 bytes
Disk identifier: 0x00000000
Disk /dev/mapper/VolGroup-lv_root: 41.4 GB, 41448112128 bytes
Disk identifier: 0x00000000
Disk /dev/mapper/VolGroup-lv_swap: 973 MB, 973078528 bytes
Disk identifier: 0x00000000
Disk /dev/sdh: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
Disk /dev/sdi: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
Disk /dev/sdj: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
You can verify iscsi status with:
service iscsi status
462
]
]
iSCSI devices
47.4. iSCSI target on Debian
Installing the software for the target server requires iscsitarget on Ubuntu and Debian, and
an extra iscsitarget-dkms for the kernel modules only on Debian.
root@debby6:~# aptitude install iscsitarget
The following NEW packages will be installed:
iscsitarget
0 packages upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 69.4 kB of archives. After unpacking 262 kB will be used.
Get:1 http://ftp.belnet.be/debian/ squeeze/main iscsitarget i386 1.4.20.2-1\
[69.4 kB]
Fetched 69.4 kB in 0s (415 kB/s)
Selecting previously deselected package iscsitarget.
(Reading database ... 36441 files and directories currently installed.)
Unpacking iscsitarget (from .../iscsitarget_1.4.20.2-1_i386.deb) ...
Processing triggers for man-db ...
Setting up iscsitarget (1.4.20.2-1) ...
iscsitarget not enabled in "/etc/default/iscsitarget", not starting...(warning).
On Debian 6 you will also need aptitude install iscsitarget-dkms for the kernel modules,
on Debian 5 this is aptitude install iscsitarget-modules-`uname -a`. Ubuntu includes the
kernel modules in the main package.
The iSCSI target server is disabled by default, so we enable it.
root@debby6:~# cat /etc/default/iscsitarget
ISCSITARGET_ENABLE=false
root@debby6:~# vi /etc/default/iscsitarget
root@debby6:~# cat /etc/default/iscsitarget
ISCSITARGET_ENABLE=true
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iSCSI devices
47.5. iSCSI target setup with dd files
You can use LVM volumes (/dev/md0/lvol0), physical partitions (/dev/sda) ,raid devices (/
dev/md0) or just plain files for storage. In this demo, we use files created with dd.
This screenshot shows how to create three small files (100MB, 200MB and 300MB).
root@debby6:~# mkdir /iscsi
root@debby6:~# dd if=/dev/zero of=/iscsi/lun1.img bs=1M count=100
100+0 records in
100+0 records out
104857600 bytes (105 MB) copied, 0.315825 s, 332 MB/s
root@debby6:~# dd if=/dev/zero of=/iscsi/lun2.img bs=1M count=200
200+0 records in
200+0 records out
209715200 bytes (210 MB) copied, 1.08342 s, 194 MB/s
root@debby6:~# dd if=/dev/zero of=/iscsi/lun3.img bs=1M count=300
300+0 records in
300+0 records out
314572800 bytes (315 MB) copied, 1.36209 s, 231 MB/s
We need to declare these three files as iSCSI targets in /etc/iet/ietd.conf (used to be /etc/
ietd.conf).
root@debby6:/etc/iet# cp ietd.conf ietd.conf.original
root@debby6:/etc/iet# > ietd.conf
root@debby6:/etc/iet# vi ietd.conf
root@debby6:/etc/iet# cat ietd.conf
Target iqn.2010-02.be.linux-training:storage.lun1
IncomingUser isuser hunter2
OutgoingUser
Lun 0 Path=/iscsi/lun1.img,Type=fileio
Alias LUN1
Target iqn.2010-02.be.linux-training:storage.lun2
IncomingUser isuser hunter2
OutgoingUser
Lun 0 Path=/iscsi/lun2.img,Type=fileio
Alias LUN2
Target iqn.2010-02.be.linux-training:storage.lun3
IncomingUser isuser hunter2
OutgoingUser
Lun 0 Path=/iscsi/lun3.img,Type=fileio
Alias LUN3
We also need to add our devices to the /etc/initiators.allow file.
root@debby6:/etc/iet# cp initiators.allow initiators.allow.original
root@debby6:/etc/iet# >initiators.allow
root@debby6:/etc/iet# vi initiators.allow
root@debby6:/etc/iet# cat initiators.allow
iqn.2010-02.be.linux-training:storage.lun1
iqn.2010-02.be.linux-training:storage.lun2
iqn.2010-02.be.linux-training:storage.lun3
464
iSCSI devices
Time to start the server now:
root@debby6:/etc/iet# /etc/init.d/iscsitarget start
Starting iSCSI enterprise target service:.
.
root@debby6:/etc/iet#
Verify activation of the storage devices in /proc/net/iet:
root@debby6:/etc/iet# cat /proc/net/iet/volume
tid:3 name:iqn.2010-02.be.linux-training:storage.lun3
lun:0 state:0 iotype:fileio iomode:wt blocks:614400 blocksize:\
512 path:/iscsi/lun3.img
tid:2 name:iqn.2010-02.be.linux-training:storage.lun2
lun:0 state:0 iotype:fileio iomode:wt blocks:409600 blocksize:\
512 path:/iscsi/lun2.img
tid:1 name:iqn.2010-02.be.linux-training:storage.lun1
lun:0 state:0 iotype:fileio iomode:wt blocks:204800 blocksize:\
512 path:/iscsi/lun1.img
root@debby6:/etc/iet# cat /proc/net/iet/session
tid:3 name:iqn.2010-02.be.linux-training:storage.lun3
tid:2 name:iqn.2010-02.be.linux-training:storage.lun2
tid:1 name:iqn.2010-02.be.linux-training:storage.lun1
465
iSCSI devices
47.6. ISCSI initiator on ubuntu
First we install the iSCSi client software (on another computer than the target).
root@ubu1104:~# aptitude install open-iscsi
Reading package lists... Done
Building dependency tree
Reading state information... Done
Reading extended state information
Initializing package states... Done
The following NEW packages will be installed:
open-iscsi open-iscsi-utils{a}
Then we set the iSCSI client to start automatically.
root@ubu1104:/etc/iscsi# cp iscsid.conf iscsid.conf.original
root@ubu1104:/etc/iscsi# vi iscsid.conf
root@ubu1104:/etc/iscsi# grep ^node.startup iscsid.conf
node.startup = automatic
Or you could start it manually.
root@ubu1104:/etc/iscsi/nodes# /etc/init.d/open-iscsi start
* Starting iSCSI initiator service iscsid
* Setting up iSCSI targets
root@ubu1104:/etc/iscsi/nodes#
[ OK ]
[ OK ]
Now we can connect to the Target server and use iscsiadm to discover the devices it offers:
root@ubu1104:/etc/iscsi# iscsiadm -m discovery -t st -p 192.168.1.31
192.168.1.31:3260,1 iqn.2010-02.be.linux-training:storage.lun2
192.168.1.31:3260,1 iqn.2010-02.be.linux-training:storage.lun1
192.168.1.31:3260,1 iqn.2010-02.be.linux-training:storage.lun3
We can use the same iscsiadm to edit the files in /etc/iscsi/nodes/.
root@ubu1104:/etc/iscsi# iscsiadm -m node --targetname "iqn.2010-02.be.linu\
x-training:storage.lun1" --portal "192.168.1.31:3260" --op=update --name no\
de.session.auth.authmethod --value=CHAP
root@ubu1104:/etc/iscsi# iscsiadm -m node --targetname "iqn.2010-02.be.linu\
x-training:storage.lun1" --portal "192.168.1.31:3260" --op=update --name no\
de.session.auth.username --value=isuser
root@ubu1104:/etc/iscsi# iscsiadm -m node --targetname "iqn.2010-02.be.linu\
x-training:storage.lun1" --portal "192.168.1.31:3260" --op=update --name no\
de.session.auth.password --value=hunter2
Repeat the above for the other two devices.
466
iSCSI devices
Restart the initiator service to log in to the target.
root@ubu1104:/etc/iscsi/nodes# /etc/init.d/open-iscsi restart
* Disconnecting iSCSI targets
* Stopping iSCSI initiator service
* Starting iSCSI initiator service iscsid
* Setting up iSCSI targets
[ OK ]
[ OK ]
[ OK ]
Use fdisk -l to enjoy three new iSCSI devices.
root@ubu1104:/etc/iscsi/nodes# fdisk -l 2> /dev/null | grep Disk
Disk /dev/sda: 17.2 GB, 17179869184 bytes
Disk identifier: 0x0001983f
Disk /dev/sdb: 209 MB, 209715200 bytes
Disk identifier: 0x00000000
Disk /dev/sdd: 314 MB, 314572800 bytes
Disk identifier: 0x00000000
Disk /dev/sdc: 104 MB, 104857600 bytes
Disk identifier: 0x00000000
The Target (the server) now shows active sessions.
root@debby6:/etc/iet# cat /proc/net/iet/session
tid:3 name:iqn.2010-02.be.linux-training:storage.lun3
sid:5348024611832320 initiator:iqn.1993-08.org.debian:01:8983ed2d770
cid:0 ip:192.168.1.35 state:active hd:none dd:none
tid:2 name:iqn.2010-02.be.linux-training:storage.lun2
sid:4785074624856576 initiator:iqn.1993-08.org.debian:01:8983ed2d770
cid:0 ip:192.168.1.35 state:active hd:none dd:none
tid:1 name:iqn.2010-02.be.linux-training:storage.lun1
sid:5066549618344448 initiator:iqn.1993-08.org.debian:01:8983ed2d770
cid:0 ip:192.168.1.35 state:active hd:none dd:none
root@debby6:/etc/iet#
467
iSCSI devices
47.7. using iSCSI devices
There is no difference between using SCSI or iSCSI devices once they are connected :
partition, make filesystem, mount.
root@ubu1104:/etc/iscsi/nodes# history | tail -13
94 fdisk /dev/sdc
95 fdisk /dev/sdd
96 fdisk /dev/sdb
97 mke2fs /dev/sdb1
98 mke2fs -j /dev/sdc1
99 mkfs.ext4 /dev/sdd1
100 mkdir /mnt/is1
101 mkdir /mnt/is2
102 mkdir /mnt/is3
103 mount /dev/sdb1 /mnt/is1
104 mount /dev/sdc1 /mnt/is2
105 mount /dev/sdd1 /mnt/is3
106 history | tail -13
root@ubu1104:/etc/iscsi/nodes# mount | grep is
/dev/sdb1 on /mnt/is1 type ext2 (rw)
/dev/sdc1 on /mnt/is2 type ext3 (rw)
/dev/sdd1 on /mnt/is3 type ext4 (rw)
468
iSCSI devices
47.8. practice: iSCSI devices
1. Set up a target (using an LVM and a SCSI device) and an initiator that connects to both.
469
iSCSI devices
47.9. solution: iSCSI devices
1. Set up a target (using an LVM and a SCSI device) and an initiator that connects to both.
This solution was done on Debian/ubuntu/Mint. For RHEL/CentOS check the theory.
Decide (with a partner) on a computer to be the Target and another computer to be the
Initiator.
On the Target computer:
First install iscsitarget using the standard tools for installing software in your distribution.
Then use your knowledge from the previous chapter to setup a logical volume (/dev/vg/
lvol0) and use the RAID chapter to setup /dev/md0. Then perform the following step:
vi /etc/default/iscsitarget (set enable to true)
Add your devices to /etc/iet/ietf.conf
root@debby6:/etc/iet# cat ietd.conf
Target iqn.2010-02.be.linux-training:storage.lun1
IncomingUser isuser hunter2
OutgoingUser
Lun 0 Path=/dev/vg/lvol0,Type=fileio
Alias LUN1
Target iqn.2010-02.be.linux-training:storage.lun2
IncomingUser isuser hunter2
OutgoingUser
Lun 0 Path=/dev/md0,Type=fileio
Alias LUN2
Add both devices to /etc/iet/initiators.allow
root@debby6:/etc/iet# cat initiators.allow
iqn.2010-02.be.linux-training:storage.lun1
iqn.2010-02.be.linux-training:storage.lun2
Now start the iscsitarget daemon and move over to the Initiator.
On the Initiator computer:
Install open-iscsi and start the daemon.
Then use iscsiadm -m discovery -t st -p 'target-ip' to see the iscsi devices on the Target.
Edit the files /etc/iscsi/nodes/ as shown in the book. Then restart the iSCSI daemon and
rund fdisk -l to see the iSCSI devices.
470
Chapter 48. introduction to
multipathing
471
introduction to multipathing
48.1. install multipath
RHEL and CentOS need the device-mapper-multipath package.
yum install device-mapper-multipath
This will create a sample
multipath-0.4.9/multipath.conf.
multipath.conf
in
/usr/share/doc/device-mapper-
There is no /etc/multipath.conf until you initialize it with mpathconf.
[root@server2 ~]# mpathconf --enable --with_multipathd y
Starting multipathd daemon:
[root@server2 ~]# wc -l /etc/multipath.conf
99 /etc/multipath.conf
[
OK
]
48.2. configure multipath
You can now choose to either edit /etc/multipath.conf or use mpathconf to change this
file for you.
[root@server2 ~]# grep user_friendly_names /etc/multipath.conf
user_friendly_names yes
# user_friendly_names yes
[root@server2 ~]# mpathconf --enable --user_friendly_names n
[root@server2 ~]# grep user_friendly_names /etc/multipath.conf
user_friendly_names no
# user_friendly_names yes
[root@server2 ~]# mpathconf --enable --user_friendly_names y
[root@server2 ~]# grep user_friendly_names /etc/multipath.conf
user_friendly_names yes
# user_friendly_names yes
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introduction to multipathing
48.3. network
This example uses three networks, make sure the iSCSI Target is connected to all three
networks.
[root@server1 tgt]# ifconfig | grep -B1 192.168
eth1
Link encap:Ethernet HWaddr 08:00:27:4E:AB:8E
inet addr:192.168.1.98 Bcast:192.168.1.255 Mask:255.255.255.0
-eth2
Link encap:Ethernet HWaddr 08:00:27:3F:A9:D1
inet addr:192.168.2.98 Bcast:192.168.2.255 Mask:255.255.255.0
-eth3
Link encap:Ethernet HWaddr 08:00:27:94:52:26
inet addr:192.168.3.98 Bcast:192.168.3.255 Mask:255.255.255.0
The same must be true for the multipath Initiator:
[root@server2 ~]# ifconfig | grep -B1 192.168
eth1
Link encap:Ethernet HWaddr 08:00:27:A1:43:41
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
-eth2
Link encap:Ethernet HWaddr 08:00:27:12:A8:70
inet addr:192.168.2.99 Bcast:192.168.2.255 Mask:255.255.255.0
-eth3
Link encap:Ethernet HWaddr 08:00:27:6E:99:9B
inet addr:192.168.3.99 Bcast:192.168.3.255 Mask:255.255.255.0
48.4. start multipathd and iscsi
Time to start (or restart) both the multipathd and iscsi services:
[root@server2 ~]# service multipathd restart
Stopping multipathd daemon:
Starting multipathd daemon:
[root@server2 ~]# service iscsi restart
Stopping iscsi:
Starting iscsi:
473
[
[
OK
OK
]
]
[
[
OK
OK
]
]
introduction to multipathing
This shows fdisk output when leaving the default friendly_names option to yes. The bottom
three are the multipath devices to use.
[root@server2 ~]# fdisk -l | grep Disk
Disk /dev/sda: 42.9 GB, 42949672960 bytes
Disk identifier: 0x0004f229
Disk /dev/sdb: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
Disk /dev/sdc: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
Disk /dev/sdd: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
Disk /dev/sde: 2147 MB, 2147483648 bytes
Disk identifier: 0x00000000
Disk /dev/sdf: 2147 MB, 2147483648 bytes
Disk identifier: 0x00000000
Disk /dev/sdg: 2147 MB, 2147483648 bytes
Disk identifier: 0x00000000
Disk /dev/mapper/VolGroup-lv_root: 41.4 GB, 41448112128 bytes
Disk identifier: 0x00000000
Disk /dev/mapper/VolGroup-lv_swap: 973 MB, 973078528 bytes
Disk identifier: 0x00000000
Disk /dev/sdh: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
Disk /dev/sdi: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
Disk /dev/sdj: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
Disk /dev/sdl: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
Disk /dev/sdn: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
Disk /dev/sdk: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
Disk /dev/sdm: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
Disk /dev/sdp: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
Disk /dev/sdo: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
Disk /dev/mapper/mpathh: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
Disk /dev/mapper/mpathi: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
Disk /dev/mapper/mpathj: 1073 MB, 1073741824 bytes
Disk identifier: 0x00000000
[root@server2 ~]#
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introduction to multipathing
48.5. multipath list
You can list the multipath connections and devices with multipath -ll.
[root@server2 ~]# multipath -ll
mpathj (1IET
00010001) dm-4 Reddy,VBOX HARDDISK
size=1.0G features='0' hwhandler='0' wp=rw
|-+- policy='round-robin 0' prio=1 status=active
| `- 13:0:0:1 sdh 8:112 active ready running
|-+- policy='round-robin 0' prio=1 status=enabled
| `- 12:0:0:1 sdi 8:128 active ready running
`-+- policy='round-robin 0' prio=1 status=enabled
`- 14:0:0:1 sdm 8:192 active ready running
mpathi (1IET
00010003) dm-3 Reddy,VBOX HARDDISK
size=1.0G features='0' hwhandler='0' wp=rw
|-+- policy='round-robin 0' prio=1 status=active
| `- 13:0:0:3 sdk 8:160 active ready running
|-+- policy='round-robin 0' prio=1 status=enabled
| `- 12:0:0:3 sdn 8:208 active ready running
`-+- policy='round-robin 0' prio=1 status=enabled
`- 14:0:0:3 sdp 8:240 active ready running
mpathh (1IET
00010002) dm-2 Reddy,VBOX HARDDISK
size=1.0G features='0' hwhandler='0' wp=rw
|-+- policy='round-robin 0' prio=1 status=active
| `- 12:0:0:2 sdl 8:176 active ready running
|-+- policy='round-robin 0' prio=1 status=enabled
| `- 13:0:0:2 sdj 8:144 active ready running
`-+- policy='round-robin 0' prio=1 status=enabled
`- 14:0:0:2 sdo 8:224 active ready running
[root@server2 ~]#
The IET (iSCSI Enterprise Target) ID should match the ones you see on the Target server.
[root@server1 ~]# tgt-admin -s | grep -e LUN -e IET -e dev
LUN information:
LUN: 0
SCSI ID: IET
00010000
LUN: 1
SCSI ID: IET
00010001
Backing store path: /dev/sdb
LUN: 2
SCSI ID: IET
00010002
Backing store path: /dev/sdc
LUN: 3
SCSI ID: IET
00010003
Backing store path: /dev/sdd
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introduction to multipathing
48.6. using the device
The rest is standard mkfs, mkdir, mount:
[root@server2 ~]# mkfs.ext4 /dev/mapper/mpathi
mke2fs 1.41.12 (17-May-2010)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
65536 inodes, 262144 blocks
13107 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=268435456
8 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks:
32768, 98304, 163840, 229376
Writing inode tables: done
Creating journal (8192 blocks): done
Writing superblocks and filesystem accounting information: done
This filesystem will be automatically checked every 38 mounts or
180 days, whichever comes first. Use tune2fs -c or -i to override.
[root@server2 ~]# mkdir /srv/multipath
[root@server2 ~]# mount /dev/mapper/mpathi /srv/multipath/
[root@server2 ~]# df -h /srv/multipath/
Filesystem
Size Used Avail Use% Mounted on
/dev/mapper/mpathi 1008M
34M 924M
4% /srv/multipath
476
introduction to multipathing
48.7. practice: multipathing
1. Find a partner and decide who will be iSCSI Target and who will be iSCSI Initiator and
Multipather. Set up Multipath as we did in the theory.
2. Uncomment the big 'defaults' section in /etc/multipath.conf and disable friendly names.
Verify that multipath can work. You may need to check the manual for /lib/dev/scsi_id and
for multipath.conf.
477
introduction to multipathing
48.8. solution: multipathing
1. Find a partner and decide who will be iSCSI Target and who will be iSCSI Initiator and
Multipather. Set up Multipath as we did in the theory.
Look in the theory...
2. Uncomment the big 'defaults' section in /etc/multipath.conf and disable friendly names.
Verify that multipath can work. You may need to check the manual for /lib/dev/scsi_id and
for multipath.conf.
vi multipath.conf
remove # for the big defaults section
add # for the very small one with friendly_names active
add the --replace-whitespace option to scsi_id.
defaults {
udev_dir
polling_interval
path_selector
path_grouping_policy
getuid_callout
-whitespace --device=/dev/%n"
prio
path_checker
rr_min_io
max_fds
rr_weight
failback
no_path_retry
user_friendly_names
}
/dev
10
"round-robin 0"
multibus
"/lib/udev/scsi_id --whitelisted --replace\
const
readsector0
100
8192
priorities
immediate
fail
no
The names now (after service restart) look like:
root@server2 etc]# multipath -ll
1IET_00010001 dm-8 Reddy,VBOX HARDDISK
size=1.0G features='0' hwhandler='0' wp=rw
`-+- policy='round-robin 0' prio=1 status=active
|- 17:0:0:1 sdh 8:112 active ready running
|- 16:0:0:1 sdi 8:128 active ready running
`- 15:0:0:1 sdn 8:208 active ready running
1IET_00010003 dm-10 Reddy,VBOX HARDDISK
size=1.0G features='0' hwhandler='0' wp=rw
`-+- policy='round-robin 0' prio=1 status=active
|- 17:0:0:3 sdl 8:176 active ready running
|- 16:0:0:3 sdm 8:192 active ready running
`- 15:0:0:3 sdp 8:240 active ready running
1IET_00010002 dm-9 Reddy,VBOX HARDDISK
size=1.0G features='0' hwhandler='0' wp=rw
`-+- policy='round-robin 0' prio=1 status=active
|- 17:0:0:2 sdj 8:144 active ready running
|- 16:0:0:2 sdk 8:160 active ready running
`- 15:0:0:2 sdo 8:224 active ready running
478
introduction to multipathing
Did you blacklist your own devices ?
vi multipath.conf
--> search for blacklist:
add
devnode "^sd[a-g]"
479
Part XII. boot management
Table of Contents
49. bootloader .....................................................................................................................
49.1. boot terminology ..................................................................................................
49.2. grub ....................................................................................................................
49.3. grub2 ..................................................................................................................
49.4. lilo .....................................................................................................................
49.5. practice: bootloader ...............................................................................................
49.6. solution: bootloader ...............................................................................................
50. init and runlevels ...........................................................................................................
50.1. system init(ialization) ............................................................................................
50.2. daemon or demon ? ...............................................................................................
50.3. starting and stopping daemons .................................................................................
50.4. chkconfig ............................................................................................................
50.5. update-rc.d ...........................................................................................................
50.6. bum ....................................................................................................................
50.7. runlevels .............................................................................................................
50.8. systemd ...............................................................................................................
50.9. practice: init .........................................................................................................
50.10. solution : init ......................................................................................................
481
482
483
486
491
492
493
494
495
496
501
501
502
504
505
506
508
514
515
Chapter 49. bootloader
This chapter briefly discusses the boot sequence of an (Intel 32-bit or 64-bit) Linux
computer.
Systems booting with lilo are rare nowadays, so this section is brief.
The most common bootloader on Linux systems today is grub, yet this is not a Linux project.
Distributions like FreeBSD and Solaris also use grub.
Likewise, grub is not limited to Intel architecture. It can also load kernels on PowerPC.
Note that grub, while still the default in Debian, is slowly being replaced in most
distributions with grub2.
482
bootloader
49.1. boot terminology
The exact order of things that happen when starting a computer system, depends on the
hardware architecture (Intel x86 is different from Sun Sparc etc), on the boot loader (grub
is different from lilo) and on the operating system (Linux, Solaris, BSD etc). Most of this
chapter is focused on booting Linux on Intel x86 with grub.
49.1.1. post
A computer starts booting the moment you turn on the power (no kidding). This first process
is called post or power on self test. If all goes well then this leads to the bios. If all goes
not so well, then you might hear nothing, or hear beeping, or see an error message on the
screen, or maybe see smoke coming out of the computer (burning hardware smells bad!).
49.1.2. bios
All Intel x86 computers will have a basic input/output system or bios to detect, identify
and initialize hardware. The bios then goes looking for a boot device. This can be a floppy,
hard disk, cdrom, network card or usb drive.
During the bios you can see a message on the screen telling you which key (often Del or
F2) to press to enter the bios setup.
483
bootloader
49.1.3. openboot
Sun sparc systems start with openboot to test the hardware and to boot the operating system.
Bill Callkins explains openboot in his Solaris System Administration books. The details of
openboot are not the focus of this course.
49.1.4. boot password
The bios allows you to set a password. Do not forget this password, or you will have to
open up the hardware to reset it. You can sometimes set a password to boot the system, and
another password to protect the bios from being modified.
49.1.5. boot device
The bios will look for a boot device in the order configured in the bios setup. Usually an
operating system on a production server boots of a hard disk.
484
bootloader
49.1.6. master boot record
The master boot record or mbr is the first sector of a hard disk. The partitioning of a disk
in primary partitions, and the active partition are defined in the mbr.
The mbr is 512 bytes long and can be copied with dd.
dd if=/dev/sda of=bootsect.mbr count=1 bs=512
49.1.7. bootloader
The mbr is executed by the bios and contains either (a small) bootloader or code to load
a bootloader.
Looking at the mbr with od can reveal information about the bootloader.
paul@laika:~$ sudo dd if=/dev/sda count=1 bs=16 skip=24 2>/dev/null|od -c
0000000 376
G
R
U
B
\0
G
e
o
m \0
H
a
r
d
0000020
There are a variety of bootloaders available, most common on Intel architecture is grub,
which is replacing lilo in many places. When installing Linux on sparc architecture, you
can choose silo, Itanium systems can use elilo, IBM S/390 and zSeries use z/IPL, Alpha
uses milo and PowerPC architectures use yaboot (yet another boot loader).
Bootable cd's and dvd's often use syslinux.
49.1.8. kernel
The goal of all this is to load an operating system, or rather the kernel of an operating system.
A typical bootloader like grub will copy a kernel from hard disk to memory, and will then
hand control of the computer to the kernel (execute the kernel).
Once the Linux kernel is loaded, the bootloader turns control over to it. From that moment
on, the kernel is in control of the system. After discussing bootloaders, we continue with the
init system that starts all the daemons.
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bootloader
49.2. grub
49.2.1. /boot/grub/grub.cfg
Debian switched to grub2, which will be discussed in the next section. The main boot menu
configuration file for grub2 is grub.cfg.
root@debian7:~# ls -l /boot/grub/grub.cfg
-r--r--r-- 1 root root 2453 May 13 17:22 /boot/grub/grub.cfg
root@debian7:~#
49.2.2. /boot/grub/grub.conf
Distributions like Red Hat Enterprise Linux 6 use grub.conf and provide a symbolic link
from /boot/grub/menu.lst and from /etc/grub.conf to this file.
[root@centos65 ~]#
lrwxrwxrwx. 1 root
[root@centos65 ~]#
-rw-------. 1 root
[root@centos65 ~]#
ls -l /boot/grub/menu.lst
root 11 Mar 7 11:53 /boot/grub/menu.lst -> ./grub.conf
ls -l /boot/grub/grub.conf
root 1189 May 5 11:47 /boot/grub/grub.conf
The file currently (RHEL 6.5) looks like this:
[root@centos65 ~]# more /boot/grub/grub.conf
# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making changes to this file
# NOTICE: You have a /boot partition. This means that
#
all kernel and initrd paths are relative to /boot/, eg.
#
root (hd0,0)
#
kernel /vmlinuz-version ro root=/dev/mapper/VolGroup-lv_root
#
initrd /initrd-[generic-]version.img
#boot=/dev/sda
default=0
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title CentOS (2.6.32-431.11.2.el6.x86_64)
root (hd0,0)
kernel /vmlinuz-2.6.32-431.11.2.el6.x86_64 ro root=/dev/mapper/VolGr\
oup-lv_root rd_NO_LUKS LANG=en_US.UTF-8 rd_NO_MD rd_LVM_LV=VolGroup/lv_swap \
SYSFONT=latarcyrheb-sun16 crashkernel=auto rd_LVM_LV=VolGroup/lv_root KEYBO\
ARDTYPE=pc KEYTABLE=us rd_NO_DM rhgb quiet
initrd /initramfs-2.6.32-431.11.2.el6.x86_64.img
title CentOS (2.6.32-431.el6.x86_64)
root (hd0,0)
kernel /vmlinuz-2.6.32-431.el6.x86_64 ro root=/dev/mapper/VolGroup-l\
v_root rd_NO_LUKS LANG=en_US.UTF-8 rd_NO_MD rd_LVM_LV=VolGroup/lv_swap SYSFO\
NT=latarcyrheb-sun16 crashkernel=auto rd_LVM_LV=VolGroup/lv_root KEYBOARDTY\
PE=pc KEYTABLE=us rd_NO_DM rhgb quiet
initrd /initramfs-2.6.32-431.el6.x86_64.img
[root@centos65 ~]#
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bootloader
49.2.3. menu commands
The menu commands must be at the top of grub's configuration file.
default
The default command sets a default entry to start. The first entry has number 0.
default=0
Each entry or stanza starts with a title directive.
fallback
In case the default does not boot, use the fallback entry instead.
fallback=1
timeout
The timeout will wait a number of seconds before booting the default entry.
timeout=5
hiddenmenu
The hiddenmenu will hide the grub menu unless the user presses Esc before the timeout
expires.
hiddenmenu
title
With title we can start a new entry or stanza.
title CentOS (2.6.32-431.11.2.el6.x86_64)
password
You can add a password to prevent interactive selection of a boot environment while grub
is running.
password --md5 $1$Ec.id/$T2C2ahI/EG3WRRsmmu/HN/
Use the grub interactive shell to create the password hash.
grub> md5crypt
Password: ********
Encrypted: $1$Ec.id/$T2C2ahI/EG3WRRsmmu/HN/
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bootloader
49.2.4. stanza commands
Every operating system or kernel that you want to boot with grub will have a stanza aka
an entry of a couple of lines. Listed here are some of the common stanza commands.
boot
Technically the boot command is only mandatory when running the grub command line.
This command does not have any parameters and can only be set as the last command of
a stanza.
boot
kernel
The kernel command points to the location of the kernel. To boot Linux this means booting
a gzip compressed zImage or bzip2 compressed bzImage.
This screenshot shows a kernel command used to load a Debian kernel.
kernel
/boot/vmlinuz-2.6.17-2-686 root=/dev/hda1 ro
And this is how RHEL 5 uses the kernel command.
kernel /vmlinuz-2.6.18-128.el5 ro root=/dev/VolGroup00/LogVol00 rhgb quiet
All parameters in the kernel line can be read by the kernel itself or by any other program
(which are started later) by reading /proc/cmdline
initrd
Many Linux installations will need an initial ramdisk at boot time. This can be set in grub
with the initrd command.
Here a screenshot of Debian 4.0
initrd /boot/initrd.img-2.6.17-2-686
And the same for Red Hat Enterprise Linux 5
initrd /initrd-2.6.18-128.el5.img
root
The root command accepts the root device as a parameter.
The root command will point to the hard disk and partition to use, with hd0 as the first
hard disk device and hd1 as the second hard disk device. The same numbering is used for
partitions, so hd0,0 is the first partition on the first disk and hd0,1 is the second partition
on that disk.
root (hd0,0)
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bootloader
savedefault
The savedefault command can be used together with default saved as a menu command.
This combination will set the currently booted stanza as the next default stanza to boot.
default saved
timeout 10
title Linux
root (hd0,0)
kernel /boot/vmlinuz
savedefault
title DOS
root (hd0,1)
makeactive
chainloader +1
savedefault
49.2.5. chainloading
With grub booting, there are two choices: loading an operating system or chainloading
another bootloader. The chainloading feature of grub loads the bootsector of a partition
(that contains an operating system).
Some older operating systems require a primary partition that is set as active. Only one
partition can be set active so grub can do this on the fly just before chainloading.
This screenshot shows how to set the first primary partition active with grub.
root (hd0,0)
makeactive
Chainloading refers to grub loading another operating system's bootloader. The chainloader
switch receives one option: the number of sectors to read and boot. For DOS and OS/2 one
sector is enough. Note that DOS requires the boot/root partition to be active!
Here is a complete example to chainload an old operating system.
title MS-DOS 6.22
root (hd0,1)
makeactive
chainloader +1
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bootloader
49.2.6. simple stanza examples
This is a screenshot of a Debian 4 stanza.
title
root
kernel
initrd
Debian GNU/Linux, kernel 2.6.17-2-686
(hd0,0)
/boot/vmlinuz-2.6.17-2-686 root=/dev/hda1 ro
/boot/initrd.img-2.6.17-2-686
Here a screenshot of a Red Hat Enterprise Linux 5 stanza.
title Red Hat Enterprise Linux Server (2.6.18-128.el5)
root (hd0,0)
kernel /vmlinuz-2.6.18-98.el5 ro root=/dev/VolGroup00/LogVol00 rhgb quiet
initrd /initrd-2.6.18-98.el5.img
49.2.7. editing grub at boot time
At boot time, when the grub menu is displayed, you can type e to edit the current stanza.
This enables you to add parameters to the kernel.
One such parameter, useful when you lost the root password, is single. This will boot the
kernel in single user mode (although some distributions will still require you to type the
root password.
kernel
/boot/vmlinuz-2.6.17-2-686 root=/dev/hda1 ro single
Another option to reset a root password is to use an init=/bin/bash parameter.
kernel
/boot/vmlinuz-2.6.17-2-686 root=/dev/hda1 ro init=/bin/bash
Note that some distributions will disable this option at kernel compile time.
49.2.8. installing grub
Run the grub-install command to install grub. The command requires a destination for
overwriting the boot sector or mbr.
# grub-install /dev/hda
You will rarely have to do this manually, since grub is installed when installing the operating
system and does not need any re-install when changing configuration (as is the case for lilo).
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bootloader
49.3. grub2
49.3.1. grub 2.0 ?
The main configuration file is now /boot/grub/grub.cfg. And while this file may look
familiar, one should never edit this file directly (because it is generated!).
root@debian7:~# ls -l /boot/grub/grub.cfg
-r--r--r-- 1 root root 2453 May 13 17:22 /boot/grub/grub.cfg
root@debian7:~# head -3 /boot/grub/grub.cfg
#
# DO NOT EDIT THIS FILE
#
49.3.2. /etc/grub.d/40_custom
The /etc/grub.d/40_custom file can be changed to include custom entries. These entries are
automatically added to grub.
root@debian7:~# ls -l /etc/grub.d/40_custom
-rwxr-xr-x 1 root root 214 Jul 3 2013 /etc/grub.d/40_custom
root@debian7:~# cat /etc/grub.d/40_custom
#!/bin/sh
exec tail -n +3 $0
# This file provides an easy way to add custom menu entries. Simply type the
# menu entries you want to add after this comment. Be careful not to change
# the 'exec tail' line above.
49.3.3. /etc/default/grub
The new configuration file for changing grub is now /etc/default/grub.
root@debian7:~# head /etc/default/grub
# If you change this file, run 'update-grub' afterwards to update
# /boot/grub/grub.cfg.
# For full documentation of the options in this file, see:
#
info -f grub -n 'Simple configuration'
GRUB_DEFAULT=0
GRUB_TIMEOUT=5
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="quiet"
GRUB_CMDLINE_LINUX="debian-installer=en_US"
49.3.4. update-grub
Whenever the /etc/default/grub file is changed, you will need to run update-grub to apply
the changes.
root@debian7:~# vi /etc/default/grub
root@debian7:~# update-grub
Generating grub.cfg ...
Found linux image: /boot/vmlinuz-3.2.0-4-amd64
Found initrd image: /boot/initrd.img-3.2.0-4-amd64
done
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bootloader
49.4. lilo
49.4.1. Linux loader
lilo used to be the most used Linux bootloader, but is steadily being replaced with grub and
recently grub2.
49.4.2. lilo.conf
Here is an example of a lilo.conf file. The delay switch receives a number in tenths of a
second. So the delay below is three seconds, not thirty!
boot = /dev/hda
delay = 30
image = /boot/vmlinuz
root = /dev/hda1
label = Red Hat 5.2
image = /boot/vmlinuz
root = /dev/hda2
label = S.U.S.E. 8.0
other = /dev/hda4
table = /dev/hda
label = MS-DOS 6.22
The configration file shows three example stanzas. The first one boots Red Hat from the first
partition on the first disk (hda1). The second stanza boots Suse 8.0 from the next partition.
The last one loads MS-DOS.
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bootloader
49.5. practice: bootloader
0. Find out whether your system is using lilo, grub or grub2. Only do the practices that are
appropriate for your system.
1. Make a copy of the kernel, initrd and System.map files in /boot. Put the copies also in /
boot but replace 2.x or 3.x with 4.0 (just imagine that Linux 4.0 is out.).
2. Add a stanza in grub for the 4.0 files. Make sure the title is different.
3. Set the boot menu timeout to 30 seconds.
4. Reboot and test the new stanza.
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bootloader
49.6. solution: bootloader
0. Find out whether your system is using lilo, grub or grub2. Only do the practices that are
appropriate for your system.
1. Make a copy of the kernel, initrd and System.map files in /boot. Put the copies also in /
boot but replace 2.x or 3.x with 4.0 (just imagine that Linux 4.0 is out.).
[root@centos65 boot]# uname -r
2.6.32-431.11.2.el6.x86_64
[root@centos65 boot]# cp System.map-2.6.32-431.11.2.el6.x86_64 System.map-4.0
[root@centos65 boot]# cp vmlinuz-2.6.32-431.11.2.el6.x86_64 vmlinuz-4.0
[root@centos65 boot]# cp initramfs-2.6.32-431.11.2.el6.x86_64.img initramfs-4.0\
.img
Do not forget that the initrd (or initramfs) file ends in .img .
2. Add a stanza in grub for the 4.0 files. Make sure the title is different.
[root@centos65 grub]# cut -c1-70 menu.lst | tail -12
title CentOS (4.0)
root (hd0,0)
kernel /vmlinuz-4.0 ro root=/dev/mapper/VolGroup-lv_root rd_NO_LUKS L
initrd /initramfs-4.0.img
title CentOS (2.6.32-431.11.2.el6.x86_64)
root (hd0,0)
kernel /vmlinuz-2.6.32-431.11.2.el6.x86_64 ro root=/dev/mapper/VolGro
initrd /initramfs-2.6.32-431.11.2.el6.x86_64.img
title CentOS (2.6.32-431.el6.x86_64)
root (hd0,0)
kernel /vmlinuz-2.6.32-431.el6.x86_64 ro root=/dev/mapper/VolGroup-lv
initrd /initramfs-2.6.32-431.el6.x86_64.img
[root@centos65 grub]#
3. Set the boot menu timeout to 30 seconds.
[root@centos65 grub]# vi menu.lst
[root@centos65 grub]# grep timeout /boot/grub/grub.conf
timeout=30
4. Reboot and test the new stanza.
[root@centos65 grub]# reboot
Select your stanza and if it boots then you did it correct.
494
Chapter 50. init and runlevels
Many Unix and Linux distributions use init scripts to start daemons in the same way that
Unix System V did. This chapter will explain in detail how that works.
Init starts daemons by using scripts, where each script starts one daemon, and where each
script waits for the previous script to finish. This serial process of starting daemons is slow,
and although slow booting is not a problem on servers where uptime is measured in years,
the recent uptake of Linux on the desktop results in user complaints.
To improve Linux startup speed, Canonical has developed upstart, which was first used
in Ubuntu. Solaris also used init up to Solaris 9, for Solaris 10 Sun developed Service
Management Facility. Both systems start daemons in parallel and can replace the SysV init
scripts. There is also an ongoing effort to create initng (init next generation).
In 2014 the systemd initiative has taken a lead when after Fedora, RHEL7 and CentOS7
also Debian has chosen this to be the prefered replacement for init. The end of this module
contains an introduction to systemd.
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init and runlevels
50.1. system init(ialization)
50.1.1. process id 1
The kernel receives system control from the bootloader. After a while the kernel starts the
init daemon. The init daemon (/sbin/init) is the first daemon that is started and receives
process id 1 (PID 1). Init never dies.
50.1.2. configuration in /etc/inittab
When /sbin/init is started, it will first read its configuration file /etc/inittab. In that file, it
will look for the value of initdefault (3 in the screenshot below).
[paul@rhel4 ~]$ grep ^id /etc/inittab
id:3:initdefault:
50.1.3. initdefault
The value found in initdefault indicates the default runlevel. Some Linux distributions have
a brief description of runlevels in /etc/inittab, like here on Red Hat Enterprise Linux 4.
# Default runlevel. The runlevels used by RHS are:
#
0 - halt (Do NOT set initdefault to this)
#
1 - Single user mode
#
2 - Multiuser, without NFS (The same as 3, if you don't have network)
#
3 - Full multiuser mode
#
4 - unused
#
5 - X11
#
6 - reboot (Do NOT set initdefault to this)
Runlevel 0 means the system is shutting down. Runlevel 1 is used for troubleshooting, only
the root user can log on, and only at the console. Runlevel 3 is typical for servers, whereas
runlevel 5 is typical for desktops (graphical logon). Besides runlevels 0, 1 and 6, the use may
vary depending on the distribution. Debian and derived Linux systems have full network
and GUI logon on runlevels 2 to 5. So always verify the proper meaning of runlevels on
your system.
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init and runlevels
50.1.4. sysinit script
/etc/rc.d/rc.sysinit
The next line in /etc/inittab in Red Hat and derivatives is the following.
si::sysinit:/etc/rc.d/rc.sysinit
This means that independent of the selected runlevel, init will run the /etc/rc.d/rc.sysinit
script. This script initializes hardware, sets some basic environment, populates /etc/mtab
while mounting file systems, starts swap and more.
[paul@rhel ~]$ egrep -e"^# Ini" -e"^# Sta" -e"^# Che" /etc/rc.d/rc.sysinit
# Check SELinux status
# Initialize hardware
# Start the graphical boot, if necessary; /usr may not be mounted yet...
# Initialiaze ACPI bits
# Check filesystems
# Start the graphical boot, if necessary and not done yet.
# Check to see if SELinux requires a relabel
# Initialize pseudo-random number generator
# Start up swapping.
# Initialize the serial ports.
That egrep command could also have been written with grep like this :
grep "^# \(Ini\|Sta\|Che\)".
/etc/init.d/rcS
Debian has the following line after initdefault.
si::sysinit:/etc/init.d/rcS
The /etc/init.d/rcS script will always run on Debian (independent of the selected runlevel).
The script is actually running all scripts in the /etc/rcS.d/ directory in alphabetical order.
root@barry:~# cat /etc/init.d/rcS
#! /bin/sh
#
# rcS
#
# Call all S??* scripts in /etc/rcS.d/ in numerical/alphabetical order
#
exec /etc/init.d/rc S
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init and runlevels
50.1.5. rc scripts
Init will continue to read /etc/inittab and meets this section on Debian Linux.
l0:0:wait:/etc/init.d/rc
l1:1:wait:/etc/init.d/rc
l2:2:wait:/etc/init.d/rc
l3:3:wait:/etc/init.d/rc
l4:4:wait:/etc/init.d/rc
l5:5:wait:/etc/init.d/rc
l6:6:wait:/etc/init.d/rc
0
1
2
3
4
5
6
On Red Hat Enterprise Linux it is identical except init.d is rc.d.
l0:0:wait:/etc/rc.d/rc
l1:1:wait:/etc/rc.d/rc
l2:2:wait:/etc/rc.d/rc
l3:3:wait:/etc/rc.d/rc
l4:4:wait:/etc/rc.d/rc
l5:5:wait:/etc/rc.d/rc
l6:6:wait:/etc/rc.d/rc
0
1
2
3
4
5
6
In both cases, this means that init will start the rc script with the runlevel as the only
parameter. Actually /etc/inittab has fields seperated by colons. The second field determines
the runlevel in which this line should be executed. So in both cases, only one line of the
seven will be executed, depending on the runlevel set by initdefault.
50.1.6. rc directories
When you take a look any of the /etc/rcX.d/ directories, then you will see a lot of (links to)
scripts who's name start with either uppercase K or uppercase S.
[root@RHEL52
lrwxrwxrwx 1
lrwxrwxrwx 1
lrwxrwxrwx 1
lrwxrwxrwx 1
rc3.d]# ls -l | tail -4
root root 19 Oct 11 2008
root root 19 Oct 11 2008
root root 11 Jan 21 04:16
root root 16 Jan 21 04:17
S98haldaemon -> ../init.d/haldaemon
S99firstboot -> ../init.d/firstboot
S99local -> ../rc.local
S99smartd -> ../init.d/smartd
The /etc/rcX.d/ directories only contain links to scripts in /etc/init.d/. Links allow for the
script to have a different name. When entering a runlevel, all scripts that start with uppercase
K or uppercase S will be started in alphabetical order. Those that start with K will be started
first, with stop as the only parameter. The remaining scripts with S will be started with start
as the only parameter.
All this is done by the /etc/rc.d/rc script on Red Hat and by the /etc/init.d/rc script on
Debian.
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init and runlevels
50.1.7. mingetty
mingetty in /etc/inittab
Almost at the end of /etc/inittab there is a section to start and respawn several mingetty
daemons.
[root@RHEL4b ~]# grep getty /etc/inittab
# Run gettys in standard runlevels
1:2345:respawn:/sbin/mingetty tty1
2:2345:respawn:/sbin/mingetty tty2
3:2345:respawn:/sbin/mingetty tty3
4:2345:respawn:/sbin/mingetty tty4
5:2345:respawn:/sbin/mingetty tty5
6:2345:respawn:/sbin/mingetty tty6
mingetty and /bin/login
This /sbin/mingetty will display a message on a virtual console and allow you to type a
userid. Then it executes the /bin/login command with that userid. The /bin/login program
will verify whether that user exists in /etc/passwd and prompt for (and verify) a password.
If the password is correct, /bin/login passes control to the shell listed in /etc/passwd.
respawning mingetty
The mingetty daemons are started by init and watched until they die (user exits the shell and
is logged out). When this happens, the init daemon will respawn a new mingetty. So even
if you kill a mingetty daemon, it will be restarted automatically.
This example shows that init respawns mingetty daemons. Look at the PID's of the last two
mingetty processes.
[root@RHEL52 ~]# ps -C mingetty
PID TTY
TIME CMD
2407 tty1
00:00:00 mingetty
2408 tty2
00:00:00 mingetty
2409 tty3
00:00:00 mingetty
2410 tty4
00:00:00 mingetty
2411 tty5
00:00:00 mingetty
2412 tty6
00:00:00 mingetty
When we kill the last two mingettys, then init will notice this and start them again (with
a different PID).
[root@RHEL52 ~]# kill 2411 2412
[root@RHEL52 ~]# ps -C mingetty
PID TTY
TIME CMD
2407 tty1
00:00:00 mingetty
2408 tty2
00:00:00 mingetty
2409 tty3
00:00:00 mingetty
2410 tty4
00:00:00 mingetty
2821 tty5
00:00:00 mingetty
2824 tty6
00:00:00 mingetty
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init and runlevels
disabling a mingetty
You can disable a mingetty for a certain tty by removing the runlevel from the second field
in its line in /etc/inittab. Don't forget to tell init about the change of its configuration file
with kill -1 1.
The example below shows how to disable mingetty on tty3 to tty6 in runlevels 4 and 5.
[root@RHEL52 ~]# grep getty /etc/inittab
# Run gettys in standard runlevels
1:2345:respawn:/sbin/mingetty tty1
2:2345:respawn:/sbin/mingetty tty2
3:23:respawn:/sbin/mingetty tty3
4:23:respawn:/sbin/mingetty tty4
5:23:respawn:/sbin/mingetty tty5
6:23:respawn:/sbin/mingetty tty6
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init and runlevels
50.2. daemon or demon ?
A daemon is a process that runs in background, without a link to a GUI or terminal. Daemons
are usually started at system boot, and stay alive until the system shuts down. In more recent
technical writings, daemons are often refered to as services.
Unix daemons are not to be confused with demons. Evi Nemeth, co-author of the UNIX
System Administration Handbook has the following to say about daemons:
Many people equate the word "daemon" with the word "demon", implying some
kind of satanic connection between UNIX and the underworld. This is an egregious
misunderstanding. "Daemon" is actually a much older form of "demon"; daemons have no
particular bias towards good or evil, but rather serve to help define a person's character or
personality. The ancient Greeks' concept of a "personal daemon" was similar to the modern
concept of a "guardian angel" ....
50.3. starting and stopping daemons
The K and S scripts are links to the real scripts in /etc/init.d/. These can also be used when the
system is running to start and stop daemons (or services). Most of them accept the following
parameters: start, stop, restart, status.
For example in this screenshot we restart the samba daemon.
root@laika:~# /etc/init.d/samba restart
* Stopping Samba daemons...
* Starting Samba daemons...
[ OK ]
[ OK ]
You can achieve the same result on RHEL/Fedora with the service command.
[root@RHEL4b ~]# service smb restart
Shutting down SMB services:
Shutting down NMB services:
Starting SMB services:
Starting NMB services:
[
[
[
[
You might also want to take a look at chkconfig, update-rc.d.
501
OK
OK
OK
OK
]
]
]
]
init and runlevels
50.4. chkconfig
The purpose of chkconfig is to relieve system administrators of manually managing all the
links and scripts in /etc/init.d and /etc/rcX.d/.
50.4.1. chkconfig --list
Here we use chkconfig to list the status of a service in the different runlevels. You can see
that the crond daemon (or service) is only activated in runlevels 2 to 5.
[root@RHEL52 ~]# chkconfig --list crond
crond
0:off 1:off 2:on 3:on 4:on 5:on 6:off
When you compare the screenshot above with the one below, you can see that off equals to
a K link to the script, whereas on equals to an S link.
[root@RHEL52 etc]# find ./rc?.d/ -name \*crond -exec ls -l {} \;|cut -b40./rc0.d/K60crond -> ../init.d/crond
./rc1.d/K60crond -> ../init.d/crond
./rc2.d/S90crond -> ../init.d/crond
./rc3.d/S90crond -> ../init.d/crond
./rc4.d/S90crond -> ../init.d/crond
./rc5.d/S90crond -> ../init.d/crond
./rc6.d/K60crond -> ../init.d/crond
50.4.2. runlevel configuration
Here you see how to use chkconfig to disable (or enable) a service in a certain runlevel.
This screenshot shows how to disable crond in runlevel 3.
[root@RHEL52 ~]# chkconfig --level 3 crond off
[root@RHEL52 ~]# chkconfig --list crond
crond
0:off 1:off 2:on 3:off 4:on 5:on 6:off
This screenshot shows how to enable crond in runlevels 3 and 4.
[root@RHEL52 ~]# chkconfig --level 34 crond on
[root@RHEL52 ~]# chkconfig --list crond
crond
0:off 1:off 2:on 3:on 4:on 5:on 6:off
502
init and runlevels
50.4.3. chkconfig configuration
Every script in /etc/init.d/ can have (comment) lines to tell chkconfig what to do with the
service. The line with # chkconfig: contains the runlevels in which the service should be
started (2345), followed by the priority for start (90) and stop (60).
[root@RHEL52 ~]# head -9 /etc/init.d/crond | tail -5
# chkconfig: 2345 90 60
# description: cron is a standard UNIX program that runs user-specified
#
programs at periodic scheduled times. vixie cron adds a
#
number of features to the basic UNIX cron, including better
#
security and more powerful configuration options.
50.4.4. enable and disable services
Services can be enabled or disabled in all runlevels with one command. Runlevels 0, 1 and
6 are always stopping services (or calling the scripts with stop) even when their name starts
with uppercase S.
[root@RHEL52
[root@RHEL52
crond
[root@RHEL52
[root@RHEL52
crond
~]# chkconfig crond off
~]# chkconfig --list crond
0:off
1:off
2:off
3:off
~]# chkconfig crond on
~]# chkconfig --list crond
0:off
1:off
2:on
3:on
503
4:off
5:off
6:off
4:on
5:on
6:off
init and runlevels
50.5. update-rc.d
50.5.1. about update-rc.d
The Debian equivalent of chkconfig is called update-rc.d. This tool is designed for use in
scripts, if you prefer a graphical tool then look at bum.
When there are existing links in /etc/rcX.d/ then update-rc.d does not do anything. This is
to avoid that post installation scripts using update-rc.d are overwriting changes made by
a system administrator.
root@barry:~# update-rc.d cron remove
update-rc.d: /etc/init.d/cron exists during rc.d purge (use -f to force)
As you can see in the next screenshot, nothing changed for the cron daemon.
root@barry:~# find
/etc/rc0.d/K11cron
/etc/rc1.d/K11cron
/etc/rc2.d/S89cron
/etc/rc3.d/S89cron
/etc/rc4.d/S89cron
/etc/rc5.d/S89cron
/etc/rc6.d/K11cron
/etc/rc?.d/ -name '*cron' -exec ls -l {} \;|cut -b44-> ../init.d/cron
-> ../init.d/cron
-> ../init.d/cron
-> ../init.d/cron
-> ../init.d/cron
-> ../init.d/cron
-> ../init.d/cron
50.5.2. removing a service
Here we remove cron from all runlevels. Remember that the proper way to disable a service
is to put K scripts oin all runlevels!
root@barry:~# update-rc.d -f cron remove
Removing any system startup links for /etc/init.d/cron ...
/etc/rc0.d/K11cron
/etc/rc1.d/K11cron
/etc/rc2.d/S89cron
/etc/rc3.d/S89cron
/etc/rc4.d/S89cron
/etc/rc5.d/S89cron
/etc/rc6.d/K11cron
root@barry:~# find /etc/rc?.d/ -name '*cron' -exec ls -l {} \;|cut -b44root@barry:~#
50.5.3. enable a service
This screenshot shows how to use update-rc.d to enable a service in runlevels 2, 3, 4 and
5 and disable the service in runlevels 0, 1 and 6.
root@barry:~# update-rc.d cron defaults
Adding system startup for /etc/init.d/cron ...
/etc/rc0.d/K20cron -> ../init.d/cron
/etc/rc1.d/K20cron -> ../init.d/cron
/etc/rc6.d/K20cron -> ../init.d/cron
/etc/rc2.d/S20cron -> ../init.d/cron
/etc/rc3.d/S20cron -> ../init.d/cron
/etc/rc4.d/S20cron -> ../init.d/cron
/etc/rc5.d/S20cron -> ../init.d/cron
504
init and runlevels
50.5.4. customize a service
And here is an example on how to set your custom configuration for the cron daemon.
root@barry:~# update-rc.d -n cron start 11 2 3 4 5 . stop 89 0 1 6 .
Adding system startup for /etc/init.d/cron ...
/etc/rc0.d/K89cron -> ../init.d/cron
/etc/rc1.d/K89cron -> ../init.d/cron
/etc/rc6.d/K89cron -> ../init.d/cron
/etc/rc2.d/S11cron -> ../init.d/cron
/etc/rc3.d/S11cron -> ../init.d/cron
/etc/rc4.d/S11cron -> ../init.d/cron
/etc/rc5.d/S11cron -> ../init.d/cron
50.6. bum
This screenshot shows bum in advanced mode.
505
init and runlevels
50.7. runlevels
50.7.1. display the runlevel
You can see your current runlevel with the runlevel or who -r commands.
The runlevel command is typical Linux and will output the previous and the current runlevel.
If there was no previous runlevel, then it will mark it with the letter N.
[root@RHEL4b ~]# runlevel
N 3
The history of who -r dates back to Seventies Unix, it still works on Linux.
[root@RHEL4b ~]# who -r
run-level 3 Jul 28 09:15
last=S
50.7.2. changing the runlevel
You can switch to another runlevel with the telinit command. On Linux /sbin/telinit is
usually a (hard) link to /sbin/init.
This screenshot shows how to switch from runlevel 2 to runlevel 3 without reboot.
root@barry:~# runlevel
N 2
root@barry:~# init 3
root@barry:~# runlevel
2 3
50.7.3. /sbin/shutdown
The shutdown command is used to properly shut down a system.
Common switches used with shutdown are -a, -t, -h and -r.
The -a switch forces /sbin/shutdown to use /etc/shutdown.allow. The -t switch is used
to define the number of seconds between the sending of the TERM signal and the KILL
signal. The -h switch halts the system instead of changing to runlevel 1. The -r switch tells
/sbin/shutdown to reboot after shutting down.
This screenshot shows how to use shutdown with five seconds between TERM and KILL
signals.
root@barry:~# shutdown -t5 -h now
The now is the time argument. This can be +m for the number of minutes to wait before
shutting down (with now as an alias for +0. The command will also accept hh:mm instead
of +m.
50.7.4. halt, reboot and poweroff
The binary /sbin/reboot is the same as /sbin/halt and /sbin/poweroff. Depending on the
name we use to call the command, it can behave differently.
506
init and runlevels
When in runlevel 0 or 6 halt, reboot and poweroff will tell the kernel to halt, reboot or
poweroff the system.
When not in runlevel 0 or 6, typing reboot as root actually calls the shutdown command
with the -r switch and typing poweroff will switch off the power when halting the system.
50.7.5. /var/log/wtmp
halt, reboot and poweroff all write to /var/log/wtmp. To look at /var/log/wtmp, we need
to use th last.
[root@RHEL52 ~]# last
reboot
system boot
reboot
system boot
reboot
system boot
reboot
system boot
| grep reboot
2.6.18-128.el5
2.6.18-128.el5
2.6.18-128.el5
2.6.18-128.el5
Fri
Wed
Mon
Mon
May 29 11:44
May 27 12:10
May 25 19:34
Feb 9 13:20
(192+05:01)
(06:49)
(1+15:59)
(106+21:13)
50.7.6. Ctrl-Alt-Del
When rc is finished starting all those scripts, init will continue to read /etc/inittab. The next
line is about what to do when the user hits Ctrl-Alt-Delete on the keyboard.
Here is what Debian 4.0 does.
root@barry:~# grep -i ctrl /etc/inittab
# What to do when CTRL-ALT-DEL is pressed.
ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now
Which is very similar to the default Red Hat Enterprise Linux 5.2 action.
[root@RHEL52 ~]# grep -i ctrl /etc/inittab
# Trap CTRL-ALT-DELETE
ca::ctrlaltdel:/sbin/shutdown -t3 -r now
One noticable difference is that Debian forces shutdown to use /etc/shutdown.allow, where
Red Hat allows everyone to invoke shutdown pressing Ctrl-Alt-Delete.
50.7.7. UPS and loss of power
[root@RHEL52 ~]# grep ^p /etc/inittab
pf::powerfail:/sbin/shutdown -f -h +2 "Power Failure; System Shutting Down"
pr:12345:powerokwait:/sbin/shutdown -c "Power Restored; Shutdown Cancelled"
It will read commands on what to execute in case of powerfailure, powerok and Ctrl-AltDelete. The init process never stops keeping an eye on power failures and that triple key
combo.
root@barry:~# grep ^p /etc/inittab
pf::powerwait:/etc/init.d/powerfail start
pn::powerfailnow:/etc/init.d/powerfail now
po::powerokwait:/etc/init.d/powerfail stop
507
init and runlevels
50.8. systemd
It is likely that systemd will replace all the standard init/runlevel/rc functionality. Both Red
Hat and Debian have decided in 2014 that systemd will be replacing init in future releases
(RHEL7/CentOS7 and Debian 8).
The screenshot below shows systemd running as pid 1 on RHEL7.
[root@rhel7 ~]# ps fax | grep systemd | cut -c1-76
1 ?
Ss
0:01 /usr/lib/systemd/systemd --switched-root --system
505 ?
Ss
0:00 /usr/lib/systemd/systemd-journald
545 ?
Ss
0:00 /usr/lib/systemd/systemd-udevd
670 ?
Ss
0:00 /usr/lib/systemd/systemd-logind
677 ?
Ssl
0:00 /bin/dbus-daemon --system --address=systemd: --no
2662 pts/1
S+
0:00
\_ grep --color=auto systemd
[root@rhel7 ~]#
Debian 8 (not yet released in September 2014) uses parts of systemd, but still has init as
pid 1.
root@debian8:~# ps fax | grep systemd
2042 ?
S
0:00 /sbin/cgmanager --daemon -m name=systemd
10127 pts/4
S+
0:00
|
\_ grep systemd
2777 ?
S
0:00 /lib/systemd/systemd-logind
root@debian8:~#
508
init and runlevels
50.8.1. systemd targets
The first command to learn is systemctl list-units --type=target (or the shorter version
systemctl -t target). It will show you the different targets on the system.
[root@rhel7 ~]# systemctl list-units --type=target
UNIT
LOAD
ACTIVE SUB
DESCRIPTION
basic.target
loaded active active Basic System
cryptsetup.target
loaded active active Encrypted Volumes
getty.target
loaded active active Login Prompts
graphical.target
loaded active active Graphical Interface
local-fs-pre.target loaded active active Local File Systems (Pre)
local-fs.target
loaded active active Local File Systems
multi-user.target
loaded active active Multi-User System
network.target
loaded active active Network
nfs.target
loaded active active Network File System Server
paths.target
loaded active active Paths
remote-fs.target
loaded active active Remote File Systems
slices.target
loaded active active Slices
sockets.target
loaded active active Sockets
swap.target
loaded active active Swap
sysinit.target
loaded active active System Initialization
timers.target
loaded active active Timers
LOAD
= Reflects whether the unit definition was properly loaded.
ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
SUB
= The low-level unit activation state, values depend on unit type.
16 loaded units listed. Pass --all to see loaded but inactive units, too.
To show all installed unit files use 'systemctl list-unit-files'.
[root@rhel7 ~]#
Targets are the replacement of runlevels and define specific points to reach when booting
the system. For example the graphical.target is reached when you get a graphical interface,
and the nfs.target requires a running nfs server.
To switch to a target (for example multi-user.target), we now use systemctl isolate (instead
of the equivalent init 3 to change the runlevel).
[root@rhel7
169
[root@rhel7
[root@rhel7
129
[root@rhel7
~]# ps fax | wc -l
~]# systemctl isolate multi-user.target
~]# ps fax | wc -l
~]#
To change the default target, we again use this systemctl command (instead of editing the
/etc/inittab file).
[root@rhel7 ~]# systemctl enable multi-user.target --force
rm '/etc/systemd/system/default.target'
ln -s '/usr/lib/systemd/system/multi-user.target' '/etc/systemd/system/default\
.target'
[root@rhel7 ~]#
This command removed the file /etc/systemd/system/default.target and replaced it with a
symbolic link to the multi-user-.target target.
509
init and runlevels
50.8.2. systemd dependencies
Dependencies are no longer defined by alfabetical order of running scripts, but by
configuration in /etc/systemd/system/. For example here are the required services for the
multi-user.target on Red Hat Enterprise 7.
[root@rhel7 ~]# ls /etc/systemd/system/multi-user.target.wants/
abrt-ccpp.service
hypervkvpd.service
postfix.service
abrtd.service
hypervvssd.service
remote-fs.target
abrt-oops.service
irqbalance.service
rhsmcertd.service
abrt-vmcore.service
ksm.service
rngd.service
abrt-xorg.service
ksmtuned.service
rpcbind.service
atd.service
libstoragemgmt.service rsyslog.service
auditd.service
libvirtd.service
smartd.service
avahi-daemon.service mdmonitor.service
sshd.service
chronyd.service
ModemManager.service
sysstat.service
crond.service
NetworkManager.service tuned.service
cups.path
nfs.target
vmtoolsd.service
[root@rhel7 ~]#
Debian8 is not fully migrated yet.
root@debian8:~# ls /etc/systemd/system/multi-user.target.wants/
anacron.service
binfmt-support.service pppd-dns.service ssh.service
atd.service
fancontrol.service
remote-fs.target
avahi-daemon.service lm-sensors.service
rsyslog.service
Typical rc scripts are replaced with services. Issue the systemctl list-units -t service --all
(or systemctl -at service) to get a list of all services on your system.
[root@rhel7 ~]# systemctl -at service | head -5
UNIT
LOAD
ACTIVE
SUB
abrt-ccpp.service
loaded active
exited
abrt-oops.service
loaded active
running
abrt-vmcore.service loaded inactive dead
abrt-xorg.service
loaded active
running
[root@rhel7 ~]#
| column -t | cut -c1-78
DESCRIPTION
Install
ABRT
coredump
ABRT
kernel
log
Harvest
vmcores for
ABRT
Xorg
log
And here an example on how to see the status of the sshd service.
[root@rhel7 ~]# systemctl status sshd.service
sshd.service - OpenSSH server daemon
Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled)
Active: active (running) since Wed 2014-09-10 13:42:21 CEST; 55min ago
Main PID: 1400 (sshd)
CGroup: /system.slice/sshd.service
--1400 /usr/sbin/sshd -D
Sep 10 13:42:21 rhel7 systemd[1]: Started OpenSSH server daemon.
Sep 10 13:42:21 rhel7 sshd[1400]: Server listening on 0.0.0.0 port 22.
Sep 10 13:42:21 rhel7 sshd[1400]: Server listening on :: port 22.
[root@rhel7 ~]#
510
init and runlevels
50.8.3. systemd services
The chkconfig and service commands are considered 'legacy'. They are replaced with
systemctl.
This screenshot shows the new way to start and stop a service.
[root@rhel7 ~]# systemctl
[root@rhel7 ~]# systemctl
LoadState=loaded
ActiveState=active
SubState=running
UnitFileState=enabled
[root@rhel7 ~]# systemctl
[root@rhel7 ~]# systemctl
LoadState=loaded
ActiveState=inactive
SubState=dead
UnitFileState=enabled
[root@rhel7 ~]#
start crond.service
show crond.service | grep State
stop crond.service
show crond.service | grep State
And here is the new way to stop and disable a service.
[root@rhel7 ~]# systemctl stop crond.service
[root@rhel7 ~]# systemctl disable crond.service
rm '/etc/systemd/system/multi-user.target.wants/crond.service'
[root@rhel7 ~]# systemctl show crond.service | grep State
LoadState=loaded
ActiveState=inactive
SubState=dead
UnitFileState=disabled
[root@rhel7 ~]#
This screenshot shows how to enable and start the service again.
[root@rhel7 ~]# systemctl enable crond.service
ln -s '/usr/lib/systemd/system/crond.service' '/etc/systemd/system/multi-user.\
target.wants/crond.service'
[root@rhel7 ~]# systemctl start crond.service
[root@rhel7 ~]# systemctl show crond.service | grep State
LoadState=loaded
ActiveState=active
SubState=running
UnitFileState=enabled
[root@rhel7 ~]#
511
init and runlevels
50.8.4. systemd signalling
You can also use systemd to kill problematic services.
[root@rhel7 ~]# systemctl show crond.service | grep State
LoadState=loaded
ActiveState=active
SubState=running
UnitFileState=enabled
[root@rhel7 ~]# systemctl kill -s SIGKILL crond.service
[root@rhel7 ~]# systemctl show crond.service | grep State
LoadState=loaded
ActiveState=failed
SubState=failed
UnitFileState=enabled
[root@rhel7 ~]#
50.8.5. systemd shutdown
The poweroff, halt and reboot commands are considered legacy now and are handeld by
systemctl. The table below shows the legacy commands on the left and their new systemd
equivalent on the right.
Table 50.1. systemd power management
legacy command
systemd command
poweroff
systemctl poweroff
reboot
systemctl reboot
halt
systemctl halt
pm-suspend
systemctl suspend
pm-hibernate
systemctl hibernate
50.8.6. remote systemd
The systemctl utility has a buil-in remote control providing there is an ssh daemon running
on the remote system.
This screenshot shows how to use systemctl to verify a service on an other RHEL server.
[root@rhel7 ~]# systemctl -H [email protected] status sshd
[email protected]'s password:
sshd.service - OpenSSH server daemon
Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled)
Active: active (running) since Thu 2014-09-11 13:04:10 CEST; 16min ago
Process: 1328 ExecStartPre=/usr/sbin/sshd-keygen (code=exited, status=0/SUCCE\
SS)
Main PID: 1363 (sshd)
CGroup: /system.slice/sshd.service
[root@rhel7 ~]#
512
init and runlevels
50.8.7. there is more systemd
There are other tools...
systemd-analyze
systemd-ask-password
systemd-cat
systemd-cgls
systemd-cgtop
systemd-coredumpctl
systemd-delta
systemd-detect-virt
systemd-inhibit
systemd-loginctl
systemd-machine-id-setup
systemd-notify
systemd-nspawn
systemd-run
systemd-stdio-bridge
systemd-sysv-convert
systemd-tmpfiles
systemd-tty-ask-password-agent
For example systemd-analyze blame will give you an overview of the time it took for each
service to boot.
[root@rhel7 ~]# systemd-analyze blame | head
1.977s firewalld.service
1.096s tuned.service
993ms postfix.service
939ms iprinit.service
925ms vboxadd-x11.service
880ms firstboot-graphical.service
839ms accounts-daemon.service
829ms network.service
822ms iprupdate.service
795ms boot.mount
[root@rhel7 ~]#
513
init and runlevels
50.9. practice: init
1. Change /etc/inittab so that only two mingetty's are respawned. Kill the other mingetty's
and verify that they don't come back.
2. Use the Red Hat Enterprise Linux virtual machine. Go to runlevel 5, display the current
and previous runlevel, then go back to runlevel 3.
3. Is the sysinit script on your computers setting or changing the PATH environment
variable ?
4. List all init.d scripts that are started in runlevel 2.
5. Write a script that acts like a daemon script in /etc/init.d/. It should have a case statement
to act on start/stop/restart and status. Test the script!
6. Use chkconfig to setup your script to start in runlevels 3,4 and 5, and to stop in any other
runlevel.
514
init and runlevels
50.10. solution : init
1. Change /etc/inittab so that only two mingetty's are respawned. Kill the other mingetty's
and verify that they don't come back.
Killing the mingetty's will result in init respawning them. You can edit /etc/inittab so it
looks like the screenshot below. Don't forget to also run kill -1 1.
[root@RHEL5 ~]# grep tty /etc/inittab
# Run gettys in standard runlevels
1:2345:respawn:/sbin/mingetty tty1
2:2345:respawn:/sbin/mingetty tty2
3:2:respawn:/sbin/mingetty tty3
4:2:respawn:/sbin/mingetty tty4
5:2:respawn:/sbin/mingetty tty5
6:2:respawn:/sbin/mingetty tty6
[root@RHEL5 ~]#
2. Use the Red Hat Enterprise Linux virtual machine. Go to runlevel 5, display the current
and previous runlevel, then go back to runlevel 3.
init 5 (watch the console for the change taking place)
runlevel
init 3 (again you can follow this on the console)
3. Is the sysinit script on your computers setting or changing the PATH environment
variable ?
On Red Hat, grep for PATH in /etc/rc.sysinit, on Debian/Ubuntu check /etc/rc.local and /
etc/ini.t/rc.local. The answer is probably no, but on RHEL5 the rc.sysinit script does set
the HOSTNAME variable.
[root@RHEL5 etc]# grep HOSTNAME rc.sysinit
4. List all init.d scripts that are started in runlevel 2.
root@RHEL5 ~# chkconfig --list | grep '2:on'
5. Write a script that acts like a daemon script in /etc/init.d/. It should have a case statement
to act on start/stop/restart and status. Test the script!
The script could look something like this.
#!/bin/bash
#
# chkconfig: 345 99 01
# description: pold demo script
#
# /etc/init.d/pold
515
init and runlevels
#
case "$1" in
start)
echo -n "Starting pold..."
sleep 1;
touch /var/lock/subsys/pold
echo "done."
echo pold started >> /var/log/messages
;;
stop)
echo -n "Stopping pold..."
sleep 1;
rm -rf /var/lock/subsys/pold
echo "done."
echo pold stopped >> /var/log/messages
;;
*)
echo "Usage: /etc/init.d/pold {start|stop}"
exit 1
;;
esac
exit 0
The touch /var/lock/subsys/pold is mandatory and must be the same filename as the script
name, if you want the stop sequence (the K01pold link) to be run.
6. Use chkconfig to setup your script to start in runlevels 3,4 and 5, and to stop in any other
runlevel.
chkconfig --add pold
The command above will only work when the # chkconfig: and # description: lines in the
pold script are there.
516
Part XIII. system management
Table of Contents
51. scheduling .....................................................................................................................
51.1. one time jobs with at .............................................................................................
51.2. cron ....................................................................................................................
51.3. practice : scheduling ..............................................................................................
51.4. solution : scheduling ..............................................................................................
52. logging ..........................................................................................................................
52.1. login logging ........................................................................................................
52.2. syslogd ...............................................................................................................
52.3. logger .................................................................................................................
52.4. watching logs .......................................................................................................
52.5. rotating logs .........................................................................................................
52.6. practice : logging ..................................................................................................
52.7. solution : logging ..................................................................................................
53. memory management .....................................................................................................
53.1. displaying memory and cache .................................................................................
53.2. managing swap space ............................................................................................
53.3. monitoring memory with vmstat ..............................................................................
53.4. practice : memory .................................................................................................
53.5. solution : memory .................................................................................................
54. resource monitoring .......................................................................................................
54.1. four basic resources ...............................................................................................
54.2. top .....................................................................................................................
54.3. free ....................................................................................................................
54.4. watch ..................................................................................................................
54.5. vmstat .................................................................................................................
54.6. iostat ..................................................................................................................
54.7. mpstat .................................................................................................................
54.8. sadc and sar .........................................................................................................
54.9. ntop ....................................................................................................................
54.10. iftop ..................................................................................................................
54.11. iptraf .................................................................................................................
54.12. nmon ................................................................................................................
54.13. htop ..................................................................................................................
55. package management .....................................................................................................
55.1. package terminology ..............................................................................................
55.2. deb package management .......................................................................................
55.3. apt-get ................................................................................................................
55.4. aptitude ...............................................................................................................
55.5. apt ......................................................................................................................
55.6. rpm ....................................................................................................................
55.7. yum ....................................................................................................................
55.8. alien ...................................................................................................................
55.9. downloading software outside the repository ..............................................................
55.10. compiling software ..............................................................................................
55.11. practice: package management ...............................................................................
55.12. solution: package management ...............................................................................
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527
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533
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534
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Chapter 51. scheduling
Linux administrators use the at to schedule one time jobs. Recurring jobs are better
scheduled with cron. The next two sections will discuss both tools.
519
scheduling
51.1. one time jobs with at
51.1.1. at
Simple scheduling can be done with the at command. This screenshot shows the scheduling
of the date command at 22:01 and the sleep command at 22:03.
root@laika:~# at 22:01
at> date
at> <EOT>
job 1 at Wed Aug 1 22:01:00 2007
root@laika:~# at 22:03
at> sleep 10
at> <EOT>
job 2 at Wed Aug 1 22:03:00 2007
root@laika:~#
In real life you will hopefully be scheduling more useful commands ;-)
51.1.2. atq
It is easy to check when jobs are scheduled with the atq or at -l commands.
root@laika:~# atq
1
Wed Aug 1 22:01:00
2
Wed Aug 1 22:03:00
root@laika:~# at -l
1
Wed Aug 1 22:01:00
2
Wed Aug 1 22:03:00
root@laika:~#
2007 a root
2007 a root
2007 a root
2007 a root
The at command understands English words like tomorrow and teatime to schedule
commands the next day and at four in the afternoon.
root@laika:~# at 10:05 tomorrow
at> sleep 100
at> <EOT>
job 5 at Thu Aug 2 10:05:00 2007
root@laika:~# at teatime tomorrow
at> tea
at> <EOT>
job 6 at Thu Aug 2 16:00:00 2007
root@laika:~# atq
6
Thu Aug 2 16:00:00 2007 a root
5
Thu Aug 2 10:05:00 2007 a root
root@laika:~#
520
scheduling
51.1.3. atrm
Jobs in the at queue can be removed with atrm.
root@laika:~# atq
6
Thu Aug 2
5
Thu Aug 2
root@laika:~# atrm
root@laika:~# atq
6
Thu Aug 2
root@laika:~#
16:00:00 2007 a root
10:05:00 2007 a root
5
16:00:00 2007 a root
51.1.4. at.allow and at.deny
You can also use the /etc/at.allow and /etc/at.deny files to manage who can schedule jobs
with at.
The /etc/at.allow file can contain a list of users that are allowed to schedule at jobs. When
/etc/at.allow does not exist, then everyone can use at unless their username is listed in /
etc/at.deny.
If none of these files exist, then everyone can use at.
521
scheduling
51.2. cron
51.2.1. crontab file
The crontab(1) command can be used to maintain the crontab(5) file. Each user can have
their own crontab file to schedule jobs at a specific time. This time can be specified with
five fields in this order: minute, hour, day of the month, month and day of the week. If a
field contains an asterisk (*), then this means all values of that field.
The following example means : run script42 eight minutes after two, every day of the month,
every month and every day of the week.
8 14 * * * script42
Run script8472 every month on the first of the month at 25 past midnight.
25 0 1 * * script8472
Run this script33 every two minutes on Sunday (both 0 and 7 refer to Sunday).
*/2 * * * 0
Instead of these five fields, you can also type one of these: @reboot, @yearly or @annually,
@monthly, @weekly, @daily or @midnight, and @hourly.
51.2.2. crontab command
Users should not edit the crontab file directly, instead they should type crontab -e which
will use the editor defined in the EDITOR or VISUAL environment variable. Users can
display their cron table with crontab -l.
51.2.3. cron.allow and cron.deny
The cron daemon crond is reading the cron tables, taking into account the /etc/cron.allow
and /etc/cron.deny files.
These files work in the same way as at.allow and at.deny. When the cron.allow file exists,
then your username has to be in it, otherwise you cannot use cron. When the cron.allow file
does not exists, then your username cannot be in the cron.deny file if you want to use cron.
522
scheduling
51.2.4. /etc/crontab
The /etc/crontab file contains entries for when to run hourly/daily/weekly/monthly tasks.
It will look similar to this output.
SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
20 3 * * *
40 3 * * 7
55 3 1 * *
root
root
root
run-parts --report /etc/cron.daily
run-parts --report /etc/cron.weekly
run-parts --report /etc/cron.monthly
51.2.5. /etc/cron.*
The directories shown in the next screenshot contain the tasks that are run at the times
scheduled in /etc/crontab. The /etc/cron.d directory is for special cases, to schedule jobs
that require finer control than hourly/daily/weekly/monthly.
paul@laika:~$ ls -ld /etc/cron.*
drwxr-xr-x 2 root root 4096 2008-04-11
drwxr-xr-x 2 root root 4096 2008-04-19
drwxr-xr-x 2 root root 4096 2008-04-11
drwxr-xr-x 2 root root 4096 2008-04-11
drwxr-xr-x 2 root root 4096 2008-04-11
09:14
15:04
09:14
09:14
09:14
/etc/cron.d
/etc/cron.daily
/etc/cron.hourly
/etc/cron.monthly
/etc/cron.weekly
51.2.6. /etc/cron.*
Note that Red Hat uses anacron to schedule daily, weekly and monthly cron jobs.
root@rhel65:/etc# cat anacrontab
# /etc/anacrontab: configuration file for anacron
# See anacron(8) and anacrontab(5) for details.
SHELL=/bin/sh
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
# the maximal random delay added to the base delay of the jobs
RANDOM_DELAY=45
# the jobs will be started during the following hours only
START_HOURS_RANGE=3-22
#period in days
delay in minutes
1
5
cron.daily
7
25
cron.weekly
@monthly 45
cron.monthly
root@rhel65:/etc#
job-identifier
command
nice run-parts /etc/cron.daily
nice run-parts /etc/cron.weekly
nice run-parts /etc/cron.monthly
523
scheduling
51.3. practice : scheduling
1. Schedule two jobs with at, display the at queue and remove a job.
2. As normal user, use crontab -e to schedule a script to run every four minutes.
3. As root, display the crontab file of your normal user.
4. As the normal user again, remove your crontab file.
5. Take a look at the cron files and directories in /etc and understand them. What is the runparts command doing ?
524
scheduling
51.4. solution : scheduling
1. Schedule two jobs with at, display the at queue and remove a job.
root@rhel55 ~# at 9pm today
at> echo go to bed >> /root/todo.txt
at> <EOT>
job 1 at 2010-11-14 21:00
root@rhel55 ~# at 17h31 today
at> echo go to lunch >> /root/todo.txt
at> <EOT>
job 2 at 2010-11-14 17:31
root@rhel55 ~# atq
2 2010-11-14 17:31 a root
1 2010-11-14 21:00 a root
root@rhel55 ~# atrm 1
root@rhel55 ~# atq
2 2010-11-14 17:31 a root
root@rhel55 ~# date
Sun Nov 14 17:31:01 CET 2010
root@rhel55 ~# cat /root/todo.txt
go to lunch
2. As normal user, use crontab -e to schedule a script to run every four minutes.
paul@rhel55 ~$ crontab -e
no crontab for paul - using an empty one
crontab: installing new crontab
3. As root, display the crontab file of your normal user.
root@rhel55 ~# crontab -l -u paul
*/4 * * * * echo `date` >> /home/paul/crontest.txt
4. As the normal user again, remove your crontab file.
paul@rhel55 ~$ crontab -r
paul@rhel55 ~$ crontab -l
no crontab for paul
5. Take a look at the cron files and directories in /etc and understand them. What is the runparts command doing ?
run-parts runs a script in a directory
525
Chapter 52. logging
This chapter has three distinct subjects.
First we look at login logging ; how can we find out who is logging in to the system, when
and from where. And who is not logging in, who fails at su or ssh.
Second we discuss how to configure the syslog daemon, and how to test it with logger.
The last part is mostly about rotating logs and mentions the tail -f and watch commands
for watching logs.
526
logging
52.1. login logging
To keep track of who is logging into the system, Linux can maintain the /var/log/wtmp, /
var/log/btmp, /var/run/utmp and /var/log/lastlog files.
52.1.1. /var/run/utmp (who)
Use the who command to see the /var/run/utmp file. This command is showing you all the
currently logged in users. Notice that the utmp file is in /var/run and not in /var/log .
[root@rhel4 ~]# who
paul
pts/1
sandra
pts/2
inge
pts/3
els
pts/4
Feb
Feb
Feb
Feb
14
14
14
14
18:21
18:11
12:01
14:33
(192.168.1.45)
(192.168.1.42)
(192.168.1.33)
(192.168.1.19)
52.1.2. /var/log/wtmp (last)
The /var/log/wtmp file is updated by the login program. Use last to see the /var/run/wtmp
file.
[root@rhel4a ~]# last | head
paul
pts/1
192.168.1.45
reboot
system boot 2.6.9-42.0.8.ELs
nicolas pts/5
pc-dss.telematic
stefaan pts/3
pc-sde.telematic
nicolas pts/3
pc-nae.telematic
nicolas pts/3
pc-nae.telematic
dirk
pts/5
pc-dss.telematic
nicolas pts/3
pc-nae.telematic
dimitri pts/5
rhel4
stefaan pts/4
pc-sde.telematic
[root@rhel4a ~]#
Wed
Wed
Wed
Wed
Wed
Wed
Wed
Wed
Wed
Wed
Feb
Feb
Feb
Feb
Feb
Feb
Feb
Feb
Feb
Feb
14
14
14
14
14
14
14
14
14
14
18:39
18:21
12:32
12:28
11:36
11:34
10:03
09:45
07:57
07:16
-
still logged in
(01:15)
13:06 (00:33)
12:40 (00:12)
12:21 (00:45)
11:36 (00:01)
12:31 (02:28)
11:34 (01:48)
08:38 (00:40)
down
(05:50)
The last command can also be used to get a list of last reboots.
[paul@rekkie ~]$ last reboot
reboot
system boot 2.6.16-rekkie
Mon Jul 30 05:13
wtmp begins Tue May 30 23:11:45 2006
[paul@rekkie ~]
527
(370+08:42)
logging
52.1.3. /var/log/lastlog (lastlog)
Use lastlog to see the /var/log/lastlog file.
[root@rhel4a ~]# lastlog | tail
tim
pts/5 10.170.1.122
rm
pts/6 rhel4
henk
stefaan
pts/3 pc-sde.telematic
dirk
pts/5 pc-dss.telematic
arsene
nicolas
pts/5 pc-dss.telematic
dimitri
pts/5 rhel4
bashuserrm
pts/7 rhel4
kornuserrm
pts/5 rhel4
[root@rhel4a ~]#
Tue Feb
Tue Feb
**Never
Wed Feb
Wed Feb
**Never
Wed Feb
Wed Feb
Tue Feb
Tue Feb
13 09:36:54
13 10:06:56
logged in**
14 12:28:38
14 10:03:11
logged in**
14 12:32:18
14 07:57:19
13 10:35:40
13 10:06:17
+0100 2007
+0100 2007
+0100 2007
+0100 2007
+0100
+0100
+0100
+0100
2007
2007
2007
2007
52.1.4. /var/log/btmp (lastb)
There is also the lastb command to display the /var/log/btmp file. This file is updated by
the login program when entering the wrong password, so it contains failed login attempts.
Many computers will not have this file, resulting in no logging of failed login attempts.
[root@RHEL4b ~]# lastb
lastb: /var/log/btmp: No such file or directory
Perhaps this file was removed by the operator to prevent logging lastb\
info.
[root@RHEL4b ~]#
The reason given for this is that users sometimes type their password by mistake instead
of their login, so this world readable file poses a security risk. You can enable bad login
logging by simply creating the file. Doing a chmod o-r /var/log/btmp improves security.
[root@RHEL4b ~]# touch /var/log/btmp
[root@RHEL4b ~]# ll /var/log/btmp
-rw-r--r-- 1 root root 0 Jul 30 06:12 /var/log/btmp
[root@RHEL4b ~]# chmod o-r /var/log/btmp
[root@RHEL4b ~]# lastb
btmp begins Mon Jul 30 06:12:19 2007
[root@RHEL4b ~]#
Failed logins via ssh, rlogin or su are not registered in /var/log/btmp. Failed logins via tty are.
[root@RHEL4b ~]# lastb
HalvarFl tty3
Maria
tty1
Roberto tty1
Mon Jul 30 07:10 - 07:10
Mon Jul 30 07:09 - 07:09
Mon Jul 30 07:09 - 07:09
btmp begins Mon Jul 30 07:09:32 2007
[root@RHEL4b ~]#
528
(00:00)
(00:00)
(00:00)
logging
52.1.5. su and ssh logins
Depending on the distribution, you may also have the /var/log/secure file being filled with
messages from the auth and/or authpriv syslog facilities. This log will include su and/or
ssh failed login attempts. Some distributions put this in /var/log/auth.log, verify the syslog
configuration.
[root@RHEL4b ~]# cat /var/log/secure
Jul 30 07:09:03 sshd[4387]: Accepted publickey for paul from ::ffff:19\
2.168.1.52 port 33188 ssh2
Jul 30 05:09:03 sshd[4388]: Accepted publickey for paul from ::ffff:19\
2.168.1.52 port 33188 ssh2
Jul 30 07:22:27 sshd[4655]: Failed password for Hermione from ::ffff:1\
92.168.1.52 port 38752 ssh2
Jul 30 05:22:27 sshd[4656]: Failed password for Hermione from ::ffff:1\
92.168.1.52 port 38752 ssh2
Jul 30 07:22:30 sshd[4655]: Failed password for Hermione from ::ffff:1\
92.168.1.52 port 38752 ssh2
Jul 30 05:22:30 sshd[4656]: Failed password for Hermione from ::ffff:1\
92.168.1.52 port 38752 ssh2
Jul 30 07:22:33 sshd[4655]: Failed password for Hermione from ::ffff:1\
92.168.1.52 port 38752 ssh2
Jul 30 05:22:33 sshd[4656]: Failed password for Hermione from ::ffff:1\
92.168.1.52 port 38752 ssh2
Jul 30 08:27:33 sshd[5018]: Invalid user roberto from ::ffff:192.168.1\
.52
Jul 30 06:27:33 sshd[5019]: input_userauth_request: invalid user rober\
to
Jul 30 06:27:33 sshd[5019]: Failed none for invalid user roberto from \
::ffff:192.168.1.52 port 41064 ssh2
Jul 30 06:27:33 sshd[5019]: Failed publickey for invalid user roberto \
from ::ffff:192.168.1.52 port 41064 ssh2
Jul 30 08:27:36 sshd[5018]: Failed password for invalid user roberto f\
rom ::ffff:192.168.1.52 port 41064 ssh2
Jul 30 06:27:36 sshd[5019]: Failed password for invalid user roberto f\
rom ::ffff:192.168.1.52 port 41064 ssh2
[root@RHEL4b ~]#
You can enable this yourself, with a custom log file by adding the following line tot
syslog.conf.
auth.*,authpriv.*
/var/log/customsec.log
529
logging
52.2. syslogd
52.2.1. about syslog
The standard method of logging on Linux was through the syslogd daemon. Syslog was
developed by Eric Allman for sendmail, but quickly became a standard among many
Unix applications and was much later written as rfc 3164. The syslog daemon can receive
messages on udp port 514 from many applications (and appliances), and can append to log
files, print, display messages on terminals and forward logs to other syslogd daemons on
other machines. The syslogd daemon is configured in /etc/syslog.conf.
52.2.2. about rsyslog
The new method is called reliable and extended syslogd and uses the rsyslogd daemon
and the /etc/rsyslogd.conf configuration file. The syntax is backwards compatible.
Each line in the configuration file uses a facility to determine where the message is coming
from. It also contains a priority for the severity of the message, and an action to decide on
what to do with the message.
52.2.3. modules
The new rsyslog has many more features that can be expanded by using modules. Modules
allow for example exporting of syslog logging to a database.
Se the manuals for more information (when you are done with this chapter).
root@rhel65:/etc# man rsyslog.conf
root@rhel65:/etc# man rsyslogd
root@rhel65:/etc#
530
logging
52.2.4. facilities
The man rsyslog.conf command will explain the different default facilities for certain
daemons, such as mail, lpr, news and kern(el) messages. The local0 to local7 facility can
be used for appliances (or any networked device that supports syslog). Here is a list of all
facilities for rsyslog.conf version 1.3. The security keyword is deprecated.
auth (security)
authpriv
cron
daemon
ftp
kern
lpr mail
mark (internal use only)
news
syslog
user
uucp
local0-7
52.2.5. priorities
The worst severity a message can have is emerg followed by alert and crit. Lowest priority
should go to info and debug messages. Specifying a severity will also log all messages with
a higher severity. You can prefix the severity with = to obtain only messages that match that
severity. You can also specify .none to prevent a specific action from any message from
a certain facility.
Here is a list of all priorities, in ascending order. The keywords warn, error and panic are
deprecated.
debug
info
notice
warning (warn)
err (error)
crit
alert
emerg (panic)
531
logging
52.2.6. actions
The default action is to send a message to the username listed as action. When the action is
prefixed with a / then rsyslog will send the message to the file (which can be a regular file,
but also a printer or terminal). The @ sign prefix will send the message on to another syslog
server. Here is a list of all possible actions.
root,user1
*
/
-/
|
@
list of users, separated by comma's
message to all logged on users
file (can be a printer, a console, a tty, ...)
file, but don't sync after every write
named pipe
other syslog hostname
In addition, you can prefix actions with a - to omit syncing the file after every logging.
52.2.7. configuration
Below a sample configuration of custom local4 messages in /etc/rsyslog.conf.
local4.crit
local4.=crit
local4.*
/var/log/critandabove
/var/log/onlycrit
/var/log/alllocal4
52.2.8. restarting rsyslogd
Don't forget to restart the server after changing its configuration.
root@rhel65:/etc# service rsyslog restart
Shutting down system logger:
Starting system logger:
root@rhel65:/etc#
532
[
[
OK
OK
]
]
logging
52.3. logger
The logger command can be used to generate syslog test messages. You can aslo use it in
scripts. An example of testing syslogd with the logger tool.
[root@rhel4a
[root@rhel4a
[root@rhel4a
[root@rhel4a
~]# logger -p local4.debug "l4 debug"
~]# logger -p local4.crit "l4 crit"
~]# logger -p local4.emerg "l4 emerg"
~]#
The results of the tests with logger.
[root@rhel4a ~]# cat /var/log/critandabove
Feb 14 19:55:19 rhel4a paul: l4 crit
Feb 14 19:55:28 rhel4a paul: l4 emerg
[root@rhel4a ~]# cat /var/log/onlycrit
Feb 14 19:55:19 rhel4a paul: l4 crit
[root@rhel4a ~]# cat /var/log/alllocal4
Feb 14 19:55:11 rhel4a paul: l4 debug
Feb 14 19:55:19 rhel4a paul: l4 crit
Feb 14 19:55:28 rhel4a paul: l4 emerg
[root@rhel4a ~]#
52.4. watching logs
You might want to use the tail -f command to look at the last lines of a log file. The -f option
will dynamically display lines that are appended to the log.
paul@ubu1010:~$ tail -f /var/log/udev
SEQNUM=1741
SOUND_INITIALIZED=1
ID_VENDOR_FROM_DATABASE=nVidia Corporation
ID_MODEL_FROM_DATABASE=MCP79 High Definition Audio
ID_BUS=pci
ID_VENDOR_ID=0x10de
ID_MODEL_ID=0x0ac0
ID_PATH=pci-0000:00:08.0
SOUND_FORM_FACTOR=internal
You can automatically repeat commands by preceding them with the watch command.
When executing the following:
[root@rhel6 ~]# watch who
Something similar to this, repeating the output of the who command every two seconds,
will appear on the screen.
Every 2.0s: who
root
paul
paul
tty1
pts/0
pts/1
Sun Jul 17 15:31:03 2011
2011-07-17 13:28
2011-07-17 13:31 (192.168.1.30)
2011-07-17 15:19 (192.168.1.30)
533
logging
52.5. rotating logs
A lot of log files are always growing in size. To keep this within bounds, you may
want to use logrotate to rotate, compress, remove and mail log files. More info on the
logrotate command in /etc/logrotate.conf.. Individual configurations can be found in the /
etc/logrotate.d/ directory.
Below a screenshot of the default Red Hat logrotate.conf file.
root@rhel65:/etc# cat logrotate.conf
# see "man logrotate" for details
# rotate log files weekly
weekly
# keep 4 weeks worth of backlogs
rotate 4
# create new (empty) log files after rotating old ones
create
# use date as a suffix of the rotated file
dateext
# uncomment this if you want your log files compressed
#compress
# RPM packages drop log rotation information into this directory
include /etc/logrotate.d
# no packages own wtmp and btmp -- we'll rotate them here
/var/log/wtmp {
monthly
create 0664 root utmp
minsize 1M
rotate 1
}
/var/log/btmp {
missingok
monthly
create 0600 root utmp
rotate 1
}
# system-specific logs may be also be configured here.
root@rhel65:/etc#
534
logging
52.6. practice : logging
1. Display the /var/run/utmp file with the proper command (not with cat or vi).
2. Display the /var/log/wtmp file.
3. Use the lastlog and lastb commands, understand the difference.
4. Examine syslog to find the location of the log file containing ssh failed logins.
5. Configure syslog to put local4.error and above messages in /var/log/l4e.log and local4.info
only .info in /var/log/l4i.log. Test that it works with the logger tool!
6. Configure /var/log/Mysu.log, all the su to root messages should go in that log. Test that
it works!
7. Send the local5 messages to the syslog server of your neighbour. Test that it works.
8. Write a script that executes logger to local4 every 15 seconds (different message). Use
tail -f and watch on your local4 log files.
535
logging
52.7. solution : logging
1. Display the /var/run/utmp file.
who
2. Display the /var/log/wtmp file.
last
3. Use the lastlog and lastb commands, understand the difference.
lastlog : when users last logged on
lastb: failed (bad) login attempts
4. Examine syslog to find the location of the log file containing ssh failed logins.
root@rhel53 ~# grep authpriv /etc/syslog.conf
authpriv.*
/var/log/secure
Debian/Ubuntu: /var/log/auth.log
Ubuntu 9.10 and Debian Lenny have switched to using rsyslog.
root@ubuntu910:~# grep authpriv /etc/rsyslog.d/50-default.conf
auth,authpriv.*
/var/log/auth.log
root@deb503:~# grep authpriv /etc/rsyslog.conf
auth,authpriv.*
/var/log/auth.log
5. Configure syslog to put local4.error and above messages in /var/log/l4e.log and local4.info
only .info in /var/log/l4i.log. Test that it works with the logger tool!
echo local4.error /var/log/l4e.log >> /etc/syslog.conf
echo local4.=info /var/log/l4i.log >> /etc/syslog.conf
/etc/init.d/syslog restart
logger -p local4.error "l4 error test"
logger -p local4.alert "l4 alert test"
logger -p local4.info "l4 info test"
cat /var/log/l4e.log
cat /var/log/l4i.log
6. Configure /var/log/Mysu.log, all the su to root messages should go in that log. Test that
it works!
echo authpriv.*
/var/log/Mysu.log >> /etc/syslog.conf
This will log more than just the su usage.
536
logging
7. Send the local5 messages to the syslog server of your neighbour. Test that it works.
On RHEL5, edit /etc/sysconfig/syslog to enable remote listening on the server.
On RHEL7, uncomment these two lines in /etc/rsyslog.conf to enable 'UDP syslog
reception'.
# Provides UDP syslog reception
$ModLoad imudp
$UDPServerRun 514
On Debian/Ubuntu edit /etc/default/syslog or /etc/default/rsyslog.
on the client: logger -p local5.info "test local5 to neighbour"
8. Write a script that executes logger to local4 every 15 seconds (different message). Use
tail -f and watch on your local4 log files.
root@rhel53 scripts# cat logloop
#!/bin/bash
for i in `seq 1 10`
do
logger -p local4.info "local4.info test number $i"
sleep 15
done
root@rhel53 scripts# chmod +x logloop
root@rhel53 scripts# ./logloop &
[1] 8264
root@rhel53 scripts# tail -f /var/log/local4.all.log
Mar 28 13:13:36 rhel53 root: local4.info test number 1
Mar 28 13:13:51 rhel53 root: local4.info test number 2
...
537
Chapter 53. memory management
This chapter will tell you how to manage RAM memory and cache.
We start with some simple tools to display information about memory: free -om, top and
cat /proc/meminfo.
We continue with managing swap space, using terms like swapping, paging and virtual
memory.
The last part is about using vmstat to monitor swap usage.
538
memory management
53.1. displaying memory and cache
53.1.1. /proc/meminfo
Displaying /proc/meminfo will tell you a lot about the memory on your Linux computer.
paul@ubu1010:~$ cat /proc/meminfo
MemTotal:
3830176 kB
MemFree:
244060 kB
Buffers:
41020 kB
Cached:
2035292 kB
SwapCached:
9892 kB
...
The first line contains the total amount of physical RAM, the second line is the unused RAM.
Buffers is RAM used for buffering files, cached is the amount of RAM used as cache and
SwapCached is the amount of swap used as cache. The file gives us much more information
outside of the scope of this course.
53.1.2. free
The free tool can display the information provided by /proc/meminfo in a more readable
format. The example below displays brief memory information in megabytes.
paul@ubu1010:~$ free -om
total
used
Mem:
3740
3519
Swap:
6234
82
free
221
6152
shared
0
buffers
42
cached
1994
53.1.3. top
The top tool is often used to look at processes consuming most of the cpu, but it also displays
memory information on line four and five (which can be toggled by pressing m).
Below a screenshot of top on the same ubu1010 from above.
top - 10:44:34 up 16 days, 9:56, 6 users, load average: 0.13, 0.09, 0.12
Tasks: 166 total,
1 running, 165 sleeping,
0 stopped,
0 zombie
Cpu(s): 5.1%us, 4.6%sy, 0.6%ni, 88.7%id, 0.8%wa, 0.0%hi, 0.3%si, 0.0%st
Mem:
3830176k total, 3613720k used,
216456k free,
45452k buffers
Swap: 6384636k total,
84988k used, 6299648k free, 2050948k cached
539
memory management
53.2. managing swap space
53.2.1. about swap space
When the operating system needs more memory than physically present in RAM, it can use
swap space. Swap space is located on slower but cheaper memory. Notice that, although
hard disks are commonly used for swap space, their access times are one hundred thousand
times slower.
The swap space can be a file, a partition, or a combination of files and partitions. You can
see the swap space with the free command, or with cat /proc/swaps.
paul@ubu1010:~$ free -o | grep -v Mem
total
used
free
Swap:
6384636
84988
6299648
paul@ubu1010:~$ cat /proc/swaps
Filename
Type
/dev/sda3
partition
shared
Size
6384636
buffers
Used
84988
cached
Priority
-1
The amount of swap space that you need depends heavily on the services that the computer
provides.
53.2.2. creating a swap partition
You can activate or deactivate swap space with the swapon and swapoff commands. New
swap space can be created with the mkswap command. The screenshot below shows the
creation and activation of a swap partition.
root@RHELv4u4:~# fdisk -l 2> /dev/null | grep hda
Disk /dev/hda: 536 MB, 536870912 bytes
/dev/hda1
1
1040
524128+
root@RHELv4u4:~# mkswap /dev/hda1
Setting up swapspace version 1, size = 536702 kB
root@RHELv4u4:~# swapon /dev/hda1
83
Linux
Now you can see that /proc/swaps displays all swap spaces separately, whereas the free om command only makes a human readable summary.
root@RHELv4u4:~# cat /proc/swaps
Filename
/dev/mapper/VolGroup00-LogVol01
/dev/hda1
root@RHELv4u4:~# free -om
total
used
free
Mem:
249
245
4
Swap:
1535
0
1535
Type
partition
partition
shared
0
540
Size
Used
1048568 0
524120 0
buffers
125
cached
54
Priority
-1
-2
memory management
53.2.3. creating a swap file
Here is one more example showing you how to create a swap file. On Solaris you can use
mkfile instead of dd.
root@RHELv4u4:~# dd if=/dev/zero of=/smallswapfile bs=1024 count=4096
4096+0 records in
4096+0 records out
root@RHELv4u4:~# mkswap /smallswapfile
Setting up swapspace version 1, size = 4190 kB
root@RHELv4u4:~# swapon /smallswapfile
root@RHELv4u4:~# cat /proc/swaps
Filename
Type
Size
Used
Priority
/dev/mapper/VolGroup00-LogVol01
partition
1048568 0
-1
/dev/hda1
partition
524120 0
-2
/smallswapfile
file
4088
0
-3
53.2.4. swap space in /etc/fstab
If you like these swaps to be permanent, then don't forget to add them to /etc/fstab. The
lines in /etc/fstab will be similar to the following.
/dev/hda1
/smallswapfile
swap
swap
swap
swap
defaults
defaults
541
0 0
0 0
memory management
53.3. monitoring memory with vmstat
You can find information about swap usage using vmstat.
Below a simple vmstat displaying information in megabytes.
paul@ubu1010:~$ vmstat -S m
procs ---------memory-------- ---swap-- -----io---- -system- ----cpu---r b swpd free buff cache si
so
bi
bo
in
cs us sy id wa
0 0
87
225
46 2097
0
0
2
5
14
8 6 5 89 1
Below a sample vmstat when (in another terminal) root launches a find /. It generates a lot
of disk i/o (bi and bo are disk blocks in and out). There is no need for swapping here.
paul@ubu1010:~$ vmstat 2 100
procs ----------memory---------- ---swap-- -----io---- -system-- ----cpu---r b
swpd
free buff cache
si
so
bi
bo
in
cs us sy id wa
0 0 84984 1999436 53416 269536
0
0
2
5
2
10 6 5 89 1
0 0 84984 1999428 53416 269564
0
0
0
0 1713 2748 4 4 92 0
0 0 84984 1999552 53416 269564
0
0
0
0 1672 1838 4 6 90 0
0 0 84984 1999552 53424 269560
0
0
0
14 1587 2526 5 7 87 2
0 0 84984 1999180 53424 269580
0
0
0
100 1748 2193 4 6 91 0
1 0 84984 1997800 54508 269760
0
0
610
0 1836 3890 17 10 68 4
1 0 84984 1994620 55040 269748
0
0
250
168 1724 4365 19 17 56 9
0 1 84984 1978508 55292 269704
0
0
126
0 1957 2897 19 18 58 4
0 0 84984 1974608 58964 269784
0
0 1826
478 2605 4355 7 7 44 41
0 2 84984 1971260 62268 269728
0
0 1634
756 2257 3865 7 7 47 39
Below a sample vmstat when executing (on RHEL6) a simple memory leaking program.
Now you see a lot of memory being swapped (si is 'swapped in').
[paul@rhel6c ~]$ vmstat 2 100
procs
r b
0 3
0 2
1 3
1 4
0 4
3 5
----------memory-------swpd free buff cache
245208 5280
232 1916
263372 4800
72
908
350672 4792
56
992
449584 4788
56 1024
471968 4828
56 1140
505960 4764
56 1136
---swap-- ----io---- --system-- -----cpu----si
so
bi
bo
in
cs us sy id wa st
261
0
0
42
27
21 0 1 98 1 0
143840 128 0 1138 462 191 2 10 0 88 0
169280 256 0 1092 360 142 1 13 0 86 0
95880
64
0
606 471 191 2 13 0 85 0
44832
80
0
390 235
90 2 12 0 87 0
68008
16
0
538 286 109 1 12 0 87 0
The code below was used to simulate a memory leak (and force swapping). This code was
found on wikipedia without author.
paul@mac:~$ cat memleak.c
#include <stdlib.h>
int main(void)
{
while (malloc(50));
return 0;
}
542
memory management
53.4. practice : memory
1. Use dmesg to find the total amount of memory in your computer.
2. Use free to display memory usage in kilobytes (then in megabytes).
3. On a virtual machine, create a swap partition (you might need an extra virtual disk for this).
4. Add a 20 megabyte swap file to the system.
5. Put all swap spaces in /etc/fstab and activate them. Test with a reboot that they are
mounted.
6. Use free to verify usage of current swap.
7. (optional) Display the usage of swap with vmstat and free -s during a memory leak.
543
memory management
53.5. solution : memory
1. Use dmesg to find the total amount of memory in your computer.
dmesg | grep Memory
2. Use free to display memory usage in kilobytes (then in megabytes).
free ; free -m
3. On a virtual machine, create a swap partition (you might need an extra virtual disk for this).
mkswap /dev/sdd1 ; swapon /dev/sdd1
4. Add a 20 megabyte swap file to the system.
dd if=/dev/zero of=/swapfile20mb bs=1024 count=20000
mkswap /swapfile20mb
swapon /swapfile20mb
5. Put all swap spaces in /etc/fstab and activate them. Test with a reboot that they are
mounted.
root@computer# tail -2 /etc/fstab
/dev/sdd1
swap swap defaults 0 0
/swapfile20mb swap swap defaults 0 0
6. Use free to verify usage of current swap.
free -om
7. (optional) Display the usage of swap with vmstat and free -s during a memory leak.
544
Chapter 54. resource monitoring
Monitoring is the process of obtaining information about the utilization of memory, cpu,
bandwidth and storage. You should start monitoring your system as soon as possible, to be
able to create a baseline. Make sure that you get to know your system! This baseline is
important because it allows you to see a steady or sudden growth in resource utilization
and likewise steady (or sudden) decline in resource availability. It will allow you to plan
for scaling up or scaling out.
Let us look at some tools that go beyond ps fax, df -h, free -om and du -sh.
545
resource monitoring
54.1. four basic resources
The four basic resources to monitor are:
•
•
•
•
cpu
network
ram memory
storage
54.2. top
To start monitoring, you can use top. This tool will monitor ram memory, cpu and swap. Top
will automatically refresh. Inside top you can use many commands, like k to kill processes,
or t and m to toggle displaying task and memory information, or the number 1 to have one
line per cpu, or one summary line for all cpu's.
top - 12:23:16 up 2 days, 4:01, 2 users, load average: 0.00, 0.00, 0.00
Tasks: 61 total,
1 running, 60 sleeping,
0 stopped,
0 zombie
Cpu(s): 0.3% us, 0.5% sy, 0.0% ni, 98.9% id, 0.2% wa, 0.0% hi, 0.0% si
Mem:
255972k total,
240952k used,
15020k free,
59024k buffers
Swap:
524280k total,
144k used,
524136k free,
112356k cached
PID USER
1 root
2 root
3 root
4 root
5 root
16 root
26 root
...
PR NI VIRT RES SHR S
16
0 2816 560 480 S
34 19
0
0
0 S
5 -10
0
0
0 S
5 -10
0
0
0 S
15 -10
0
0
0 S
5 -10
0
0
0 S
15
0
0
0
0 S
%CPU
0.0
0.0
0.0
0.0
0.0
0.0
0.0
%MEM
0.2
0.0
0.0
0.0
0.0
0.0
0.0
TIME+
0:00.91
0:00.01
0:00.57
0:00.00
0:00.00
0:00.08
0:02.86
COMMAND
init
ksoftirqd/0
events/0
khelper
kacpid
kblockd/0
pdflush
You can customize top to display the columns of your choice, or to display only the processes
that you find interesting.
[paul@RHELv4u3 ~]$ top p 3456 p 8732 p 9654
54.3. free
The free command is common on Linux to monitor free memory. You can use free to display
information every x seconds, but the output is not ideal.
[paul@RHELv4u3 gen]$ free -om -s 10
total
used
free
shared
Mem:
249
222
27
Swap:
511
0
511
total
Mem:
Swap:
used
249
511
free
222
0
shared
27
511
[paul@RHELv4u3 gen]$
546
buffers
0
cached
50
109
buffers
0
cached
50
109
resource monitoring
54.4. watch
It might be more interesting to combine free with the watch program. This program can run
commands with a delay, and can highlight changes (with the -d switch).
[paul@RHELv4u3 ~]$ watch -d -n 3 free -om
...
Every 3.0s: free -om
total
Mem:
Swap:
used
249
511
free
230
0
shared
19
511
Sat Jan 27 12:13:03 2007
buffers
cached
0
56
109
54.5. vmstat
To monitor CPU, disk and memory statistics in one line there is vmstat. The screenshot
below shows vmstat running every two seconds 100 times (or until the Ctrl-C). Below the
r, you see the number of processes waiting for the CPU, sleeping processes go below b.
Swap usage (swpd) stayed constant at 144 kilobytes, free memory dropped from 16.7MB
to 12.9MB. See man vmstat for the rest.
[paul@RHELv4u3 ~]$ vmstat 2 100
procs ----------memory--------- --swap-- ---io--- --system-- ---cpu---r b swpd
free
buff cache si so bi
bo
in
cs us sy id wa
0 0
144 16708 58212 111612
0
0
3
4
75
62 0 1 99 0
0 0
144 16708 58212 111612
0
0
0
0 976
22 0 0 100 0
0 0
144 16708 58212 111612
0
0
0
0 958
14 0 1 99 0
1 0
144 16528 58212 111612
0
0
0
18 1432 7417 1 32 66 0
1 0
144 16468 58212 111612
0
0
0
0 2910 20048 4 95 1 0
1 0
144 16408 58212 111612
0
0
0
0 3210 19509 4 97 0 0
1 0
144 15568 58816 111612
0
0 300 1632 2423 10189 2 62 0 36
0 1
144 13648 60324 111612
0
0 754
0 1910 2843 1 27 0 72
0 0
144 12928 60948 111612
0
0 312 418 1346 1258 0 14 57 29
0 0
144 12928 60948 111612
0
0
0
0 977
19 0 0 100 0
0 0
144 12988 60948 111612
0
0
0
0 977
15 0 0 100 0
0 0
144 12988 60948 111612
0
0
0
0 978
18 0 0 100 0
[paul@RHELv4u3 ~]$
547
resource monitoring
54.6. iostat
The iostat tool can display disk and cpu statistics. The -d switch below makes iostat only
display disk information (500 times every two seconds). The first block displays statistics
since the last reboot.
[paul@RHELv4u3 ~]$ iostat -d 2 500
Linux 2.6.9-34.EL (RHELv4u3.localdomain)
01/27/2007
Device:
hdc
sda
sda1
sda2
dm-0
dm-1
tps
0.00
0.52
0.00
1.13
1.13
0.00
Blk_read/s
0.01
5.07
0.01
5.06
5.05
0.00
Blk_wrtn/s
0.00
7.78
0.00
7.78
7.77
0.00
Blk_read
1080
941798
968
939862
939034
360
Blk_wrtn
0
1445148
4
1445144
1444856
288
Device:
hdc
sda
sda1
sda2
dm-0
dm-1
...
[paul@RHELv4u3
tps
0.00
0.00
0.00
0.00
0.00
0.00
Blk_read/s
0.00
0.00
0.00
0.00
0.00
0.00
Blk_wrtn/s
0.00
0.00
0.00
0.00
0.00
0.00
Blk_read
0
0
0
0
0
0
Blk_wrtn
0
0
0
0
0
0
~]$
You can have more statistics using iostat -d -x, or display only cpu statistics with iostat -c.
[paul@RHELv4u3 ~]$ iostat -c 5 500
Linux 2.6.9-34.EL (RHELv4u3.localdomain)
01/27/2007
avg-cpu: %user
%nice
%sys %iowait
0.31
0.02
0.52
0.23
98.92
%idle
avg-cpu: %user
%nice
%sys %iowait
0.62
0.00
52.16
47.23
0.00
%idle
avg-cpu: %user
%nice
%sys %iowait
2.92
0.00
36.95
60.13
0.00
%idle
avg-cpu: %user
%nice
%sys %iowait
0.63
0.00
36.63
62.32
0.42
%idle
avg-cpu: %user
%nice
%sys %iowait
0.00
0.00
0.20
0.20
99.59
%idle
[paul@RHELv4u3 ~]$
548
resource monitoring
54.7. mpstat
On multi-processor machines, mpstat can display statistics for all, or for a selected cpu.
paul@laika:~$ mpstat -P ALL
Linux 2.6.20-3-generic (laika)
CPU %user %nice
all 1.77
0.03
0 1.73
0.02
1 1.81
0.03
paul@laika:~$
%sys %iowait
1.37
1.03
1.47
1.93
1.27
0.13
02/09/2007
%irq
0.02
0.04
0.00
%soft
0.39
0.77
0.00
%steal
0.00
0.00
0.00
%idle
95.40
94.04
96.76
intr/s
1304.91
1304.91
0.00
54.8. sadc and sar
The sadc tool writes system utilization data to /var/log/sa/sa??, where ?? is replaced with
the current day of the month. By default, cron runs the sal script every 10 minutes, the sal
script runs sadc for one second. Just before midnight every day, cron runs the sa2 script,
which in turn invokes sar. The sar tool will read the daily data generated by sadc and put it
in /var/log/sa/sar??. These sar reports contain a lot of statistics.
You can also use sar to display a portion of the statistics that were gathered. Like this example
for cpu statistics.
[paul@RHELv4u3 sa]$ sar -u | head
Linux 2.6.9-34.EL (RHELv4u3.localdomain)
12:00:01 AM
CPU
12:10:01 AM
all
12:20:01 AM
all
12:30:01 AM
all
12:40:02 AM
all
12:50:01 AM
all
01:00:01 AM
all
01:10:01 AM
all
[paul@RHELv4u3 sa]$
%user
0.48
0.49
0.49
0.44
0.42
0.47
0.45
%nice
0.01
0.01
0.01
0.01
0.01
0.01
0.01
01/27/2007
%system
0.60
0.60
0.64
0.62
0.60
0.65
0.68
%iowait
0.04
0.06
0.25
0.07
0.10
0.08
0.08
%idle
98.87
98.84
98.62
98.86
98.87
98.80
98.78
There are other useful sar options, like sar -I PROC to display interrupt activity per interrupt
and per CPU, or sar -r for memory related statistics. Check the manual page of sar for more.
549
resource monitoring
54.9. ntop
The ntop tool is not present in default Red Hat installs. Once run, it will generate a very
extensive analysis of network traffic in html on http://localhost:3000 .
54.10. iftop
The iftop tool will display bandwidth by socket statistics for a specific network device. Not
available on default Red Hat servers.
1.91Mb
3.81Mb
5.72Mb
7.63Mb
9.54Mb
--------------|-------------|--------------|-------------|--------|---laika.local
=> barry
4.94Kb 6.65Kb 69.9Kb
<=
7.41Kb 16.4Kb
766Kb
laika.local
=> ik-in-f19.google.com
0b
1.58Kb 14.4Kb
<=
0b
292b
41.0Kb
laika.local
=> ik-in-f99.google.com
0b
83b
4.01Kb
<=
0b
83b
39.8Kb
laika.local
=> ug-in-f189.google.com
0b
42b
664b
<=
0b
42b
406b
laika.local
=> 10.0.0.138
0b
0b
149b
<=
0b
0b
256b
laika.local
=> 224.0.0.251
0b
0b
86b
<=
0b
0b
0b
laika.local
=> ik-in-f83.google.com
0b
0b
39b
<=
0b
0b
21b
54.11. iptraf
Use iptraf for a colourful display of ip traffic over the network cards.
[root@centos65 ~]# iptraf
[root@centos65 ~]# iptraf -i eth0
550
resource monitoring
54.12. nmon
Another popular and all round tool is nmon.
54.13. htop
You can use htop instead of top.
551
Chapter 55. package management
Most Linux distributions have a package management system with online repositories
containing thousands of packages. This makes it very easy to install and remove applications,
operating system components, documentation and much more.
We first discuss the Debian package format .deb and its tools dpkg, apt-get and aptitude.
This should be similar on Debian, Ubuntu, Mint and all derived distributions.
Then we look at the Red Hat package format .rpm and its tools rpm and yum. This should
be similar on Red Hat, Fedora, CentOS and all derived distributions.
552
package management
55.1. package terminology
55.1.1. repository
A lot of software and documentation for your Linux distribution is available as packages
in one or more centrally distributed repositories. These packages in such a repository are
tested and very easy to install (or remove) with a graphical or command line installer.
55.1.2. .deb packages
Debian, Ubuntu, Mint and all derivatives from Debian and Ubuntu use .deb packages. To
manage software on these systems, you can use aptitude or apt-get, both these tools are
a front end for dpkg.
55.1.3. .rpm packages
Red Hat, Fedora, CentOS, OpenSUSE, Mandriva, Red Flag and others use .rpm packages.
The tools to manage software packages on these systems are yum and rpm.
55.1.4. dependency
Some packages need other packages to function. Tools like apt-get, aptitude and yum will
install all dependencies you need. When using dpkg or rpm, or when building from source,
you will need to install dependencies yourself.
55.1.5. open source
These repositories contain a lot of independent open source software. Often the source code
is customized to integrate better with your distribution. Most distributions also offer this
modified source code as a package in one or more source repositories.
You are free to go to the project website itself (samba.org, apache.org, github.com, ...) an
download the vanilla (= without the custom distribution changes) source code.
553
package management
55.1.6. GUI software management
End users have several graphical applications available via the desktop (look for 'add/remove
software' or something similar).
Below a screenshot of Ubuntu Software Center running on Ubuntu 12.04. Graphical tools
are not discussed in this book.
554
package management
55.2. deb package management
55.2.1. about deb
Most people use aptitude or apt-get to manage their Debian/Ubuntu family of Linux
distributions. Both are a front end for dpkg and are themselves a back end for synaptic and
other graphical tools.
55.2.2. dpkg -l
The low level tool to work with .deb packages is dpkg. Here you see how to obtain a list
of all installed packages on a Debian server.
root@debian6:~# dpkg -l | wc -l
265
Compare this to the same list on a Ubuntu Desktop computer.
root@ubu1204~# dpkg -l | wc -l
2527
55.2.3. dpkg -l $package
Here is an example on how to get information on an individual package. The ii at the
beginning means the package is installed.
root@debian6:~# dpkg -l rsync | tail -1 | tr -s ' '
ii rsync 3.0.7-2 fast remote file copy program (like rcp)
55.2.4. dpkg -S
You can find the package that installed a certain file on your computer with dpkg -S. This
example shows how to find the package for three files on a typical Debian server.
root@debian6:~# dpkg -S /usr/share/doc/tmux/ /etc/ssh/ssh_config /sbin/ifconfig
tmux: /usr/share/doc/tmux/
openssh-client: /etc/ssh/ssh_config
net-tools: /sbin/ifconfig
55.2.5. dpkg -L
You can also get a list of all files that are installed by a certain program. Below is the list
for the tmux package.
root@debian6:~# dpkg -L tmux
/.
/etc
/etc/init.d
/etc/init.d/tmux-cleanup
/usr
/usr/share
/usr/share/lintian
/usr/share/lintian/overrides
/usr/share/lintian/overrides/tmux
/usr/share/doc
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package management
/usr/share/doc/tmux
/usr/share/doc/tmux/TODO.gz
/usr/share/doc/tmux/FAQ.gz
/usr/share/doc/tmux/changelog.Debian.gz
/usr/share/doc/tmux/NEWS.Debian.gz
/usr/share/doc/tmux/changelog.gz
/usr/share/doc/tmux/copyright
/usr/share/doc/tmux/examples
/usr/share/doc/tmux/examples/tmux.vim.gz
/usr/share/doc/tmux/examples/h-boetes.conf
/usr/share/doc/tmux/examples/n-marriott.conf
/usr/share/doc/tmux/examples/screen-keys.conf
/usr/share/doc/tmux/examples/t-williams.conf
/usr/share/doc/tmux/examples/vim-keys.conf
/usr/share/doc/tmux/NOTES
/usr/share/man
/usr/share/man/man1
/usr/share/man/man1/tmux.1.gz
/usr/bin
/usr/bin/tmux
55.2.6. dpkg
You could use dpkg -i to install a package and dpkg -r to remove a package, but you'd have
to manually keep track of dependencies. Using apt-get or aptitude is much easier.
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package management
55.3. apt-get
Debian has been using apt-get to manage packages since 1998. Today Debian and
many Debian-based distributions still actively support apt-get, though some experts claim
aptitude is better at handling dependencies than apt-get.
Both commands use the same configuration files and can be used alternately; whenever you
see apt-get in documentation, feel free to type aptitude.
We will start with apt-get and discuss aptitude in the next section.
55.3.1. apt-get update
When typing apt-get update you are downloading the names, versions and short description
of all packages available on all configured repositories for your system.
In the example below you can see some repositories at the url be.archive.ubuntu.com
because this computer was installed in Belgium. This url can be different for you.
root@ubu1204~# apt-get update
Ign http://be.archive.ubuntu.com precise InRelease
Ign http://extras.ubuntu.com precise InRelease
Ign http://security.ubuntu.com precise-security InRelease
Ign http://archive.canonical.com precise InRelease
Ign http://be.archive.ubuntu.com precise-updates InRelease
...
Hit http://be.archive.ubuntu.com precise-backports/main Translation-en
Hit http://be.archive.ubuntu.com precise-backports/multiverse Translation-en
Hit http://be.archive.ubuntu.com precise-backports/restricted Translation-en
Hit http://be.archive.ubuntu.com precise-backports/universe Translation-en
Fetched 13.7 MB in 8s (1682 kB/s)
Reading package lists... Done
root@mac~#
Run apt-get update every time before performing other package operations.
55.3.2. apt-get upgrade
One of the nicest features of apt-get is that it allows for a secure update of all software
currently installed on your computer with just one command.
root@debian6:~# apt-get upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
root@debian6:~#
The above screenshot shows that all software is updated to the latest version available for
my distribution.
55.3.3. apt-get clean
apt-get keeps a copy of downloaded packages in /var/cache/apt/archives, as can be seen
in this screenshot.
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package management
root@ubu1204~# ls /var/cache/apt/archives/ | head
accountsservice_0.6.15-2ubuntu9.4_i386.deb
apport_2.0.1-0ubuntu14_all.deb
apport-gtk_2.0.1-0ubuntu14_all.deb
apt_0.8.16~exp12ubuntu10.3_i386.deb
apt-transport-https_0.8.16~exp12ubuntu10.3_i386.deb
apt-utils_0.8.16~exp12ubuntu10.3_i386.deb
bind9-host_1%3a9.8.1.dfsg.P1-4ubuntu0.4_i386.deb
chromium-browser_20.0.1132.47~r144678-0ubuntu0.12.04.1_i386.deb
chromium-browser-l10n_20.0.1132.47~r144678-0ubuntu0.12.04.1_all.deb
chromium-codecs-ffmpeg_20.0.1132.47~r144678-0ubuntu0.12.04.1_i386.deb
Running apt-get clean removes all .deb files from that directory.
root@ubu1204~# apt-get clean
root@ubu1204~# ls /var/cache/apt/archives/*.deb
ls: cannot access /var/cache/apt/archives/*.deb: No such file or directory
55.3.4. apt-cache search
Use apt-cache search to search for availability of a package. Here we look for rsync.
root@ubu1204~# apt-cache search rsync | grep ^rsync
rsync - fast, versatile, remote (and local) file-copying tool
rsyncrypto - rsync friendly encryption
55.3.5. apt-get install
You can install one or more applications by appending their name behind apt-get install.
The screenshot shows how to install the rsync package.
root@ubu1204~# apt-get install rsync
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following NEW packages will be installed:
rsync
0 upgraded, 1 newly installed, 0 to remove and 8 not upgraded.
Need to get 299 kB of archives.
After this operation, 634 kB of additional disk space will be used.
Get:1 http://be.archive.ubuntu.com/ubuntu/ precise/main rsync i386 3.0.9-1ubuntu1 [299 kB]
Fetched 299 kB in 0s (740 kB/s)
Selecting previously unselected package rsync.
(Reading database ... 323649 files and directories currently installed.)
Unpacking rsync (from .../rsync_3.0.9-1ubuntu1_i386.deb) ...
Processing triggers for man-db ...
Processing triggers for ureadahead ...
Setting up rsync (3.0.9-1ubuntu1) ...
Removing any system startup links for /etc/init.d/rsync ...
root@ubu1204~#
55.3.6. apt-get remove
You can remove one or more applications by appending their name behind apt-get remove.
The screenshot shows how to remove the rsync package.
root@ubu1204~# apt-get remove rsync
Reading package lists... Done
Building dependency tree
Reading state information... Done
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package management
The following packages will be REMOVED:
rsync ubuntu-standard
0 upgraded, 0 newly installed, 2 to remove and 8 not upgraded.
After this operation, 692 kB disk space will be freed.
Do you want to continue [Y/n]?
(Reading database ... 323681 files and directories currently installed.)
Removing ubuntu-standard ...
Removing rsync ...
* Stopping rsync daemon rsync
Processing triggers for ureadahead ...
Processing triggers for man-db ...
root@ubu1204~#
Note however that some configuration information is not removed.
root@ubu1204~# dpkg -l rsync | tail -1 | tr -s ' '
rc rsync 3.0.9-1ubuntu1 fast, versatile, remote (and local) file-copying tool
55.3.7. apt-get purge
You can purge one or more applications by appending their name behind apt-get purge.
Purging will also remove all existing configuration files related to that application. The
screenshot shows how to purge the rsync package.
root@ubu1204~# apt-get purge rsync
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages will be REMOVED:
rsync*
0 upgraded, 0 newly installed, 1 to remove and 8 not upgraded.
After this operation, 0 B of additional disk space will be used.
Do you want to continue [Y/n]?
(Reading database ... 323651 files and directories currently installed.)
Removing rsync ...
Purging configuration files for rsync ...
Processing triggers for ureadahead ...
root@ubu1204~#
Note that dpkg has no information about a purged package, except that it is uninstalled and
no configuration is left on the system.
root@ubu1204~# dpkg -l rsync | tail -1 | tr -s ' '
un rsync <none> (no description available)
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package management
55.4. aptitude
Most people use aptitude for package management on Debian, Mint and Ubuntu systems.
To synchronize with the repositories.
aptitude update
To patch and upgrade all software to the latest version on Debian.
aptitude upgrade
To patch and upgrade all software to the latest version on Ubuntu and Mint.
aptitude safe-upgrade
To install an application with all dependencies.
aptitude install $package
To search the repositories for applications that contain a certain string in their name or
description.
aptitude search $string
To remove an application.
aptitude remove $package
To remove an application and all configuration files.
aptitude purge $package
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package management
55.5. apt
Both apt-get and aptitude use the same configuration information in /etc/apt/. Thus adding
a repository for one of them, will automatically add it for both.
55.5.1. /etc/apt/sources.list
The resource list used by apt-get and aptitude is located in /etc/apt/sources.list. This file
contains a list of http or ftp sources where packages for the distribution can be downloaded.
This is what that list looks like on my Debian server.
root@debian6:~# cat /etc/apt/sources.list
deb http://ftp.be.debian.org/debian/ squeeze main
deb-src http://ftp.be.debian.org/debian/ squeeze main
deb http://security.debian.org/ squeeze/updates main
deb-src http://security.debian.org/ squeeze/updates main
# squeeze-updates, previously known as 'volatile'
deb http://ftp.be.debian.org/debian/ squeeze-updates main
deb-src http://ftp.be.debian.org/debian/ squeeze-updates main
On my Ubuntu there are four times as many online repositories in use.
root@ubu1204~# wc -l /etc/apt/sources.list
63 /etc/apt/sources.list
There is much more to learn about apt, explore commands like add-apt-repository, aptkey and apropos apt.
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package management
55.6. rpm
55.6.1. about rpm
The Red Hat package manager can be used on the command line with rpm or in a graphical
way going to Applications--System Settings--Add/Remove Applications. Type rpm --help
to see some of the options.
Software distributed in the rpm format will be named foo-version.platform.rpm .
55.6.2. rpm -qa
To obtain a list of all installed software, use the rpm -qa command.
[root@RHEL52 ~]# rpm -qa | grep samba
system-config-samba-1.2.39-1.el5
samba-3.0.28-1.el5_2.1
samba-client-3.0.28-1.el5_2.1
samba-common-3.0.28-1.el5_2.1
55.6.3. rpm -q
To verify whether one package is installed, use rpm -q.
root@RHELv4u4:~# rpm -q gcc
gcc-3.4.6-3
root@RHELv4u4:~# rpm -q laika
package laika is not installed
55.6.4. rpm -Uvh
To install or upgrade a package, use the -Uvh switches. The -U switch is the same as -i
for install, except that older versions of the software are removed. The -vh switches are for
nicer output.
root@RHELv4u4:~# rpm -Uvh gcc-3.4.6-3
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package management
55.6.5. rpm -e
To remove a package, use the -e switch.
root@RHELv4u4:~# rpm -e gcc-3.4.6-3
rpm -e verifies dependencies, and thus will prevent you from accidentailly erasing packages
that are needed by other packages.
[root@RHEL52 ~]# rpm -e gcc-4.1.2-42.el5
error: Failed dependencies:
gcc = 4.1.2-42.el5 is needed by (installed) gcc-c++-4.1.2-42.el5.i386
gcc = 4.1.2-42.el5 is needed by (installed) gcc-gfortran-4.1.2-42.el5.i386
gcc is needed by (installed) systemtap-0.6.2-1.el5_2.2.i386
55.6.6. /var/lib/rpm
The rpm database is located at /var/lib/rpm. This database contains all meta information
about packages that are installed (via rpm). It keeps track of all files, which enables complete
removes of software.
55.6.7. rpm2cpio
We can use rpm2cpio to convert an rpm to a cpio archive.
[root@RHEL53 ~]# file kernel.src.rpm
kernel.src.rpm: RPM v3 src PowerPC kernel-2.6.18-92.1.13.el5
[root@RHEL53 ~]# rpm2cpio kernel.src.rpm > kernel.cpio
[root@RHEL53 ~]# file kernel.cpio
kernel.cpio: ASCII cpio archive (SVR4 with no CRC)
But why would you want to do this ?
Perhaps just to see of list of files in the rpm file.
[root@RHEL53 ~]# rpm2cpio kernel.src.rpm | cpio -t | head -5
COPYING.modules
Config.mk
Module.kabi_i686
Module.kabi_i686PAE
Module.kabi_i686xen
Or to extract one file from an rpm package.
[root@RHEL53 ~]# rpm2cpio kernel.src.rpm | cpio -iv Config.mk
Config.mk
246098 blocks
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package management
55.7. yum
55.7.1. about yum
The Yellowdog Updater, Modified (yum) is an easier command to work with rpm
packages. It is installed by default on Fedora and Red Hat Enterprise Linux since version 5.2.
55.7.2. yum list
Issue yum list available to see a list of available packages. The available parameter is
optional.
root@rhel65:/etc# yum list | wc -l
This system is receiving updates from Red Hat Subscription Management.
3935
root@rhel65:/etc#
Issue yum list $package to get all versions (in different repositories) of one package.
[root@rhel55 ~]# yum list samba
Loaded plugins: rhnplugin, security
Installed Packages
samba.i386
3.0.33-3.28.el5
Available Packages
samba.i386
3.0.33-3.29.el5_5
installed
rhel-i386-server-5
55.7.3. yum search
To search for a package containing a certain string in the description or name use yum
search $string.
[root@rhel55 ~]# yum search gcc44
Loaded plugins: rhnplugin, security
========================== Matched: gcc44 ===========================
gcc44.i386 : Preview of GCC version 4.4
gcc44-c++.i386 : C++ support for GCC version 4.4
gcc44-gfortran.i386 : Fortran support for GCC 4.4 previe
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package management
55.7.4. yum provides
To search for a package containing a certain file (you might need for compiling things) use
yum provides $filename.
root@rhel65:/etc# yum provides /usr/share/man/man5/passwd.5.gz
Loaded plugins: product-id, subscription-manager
This system is receiving updates from Red Hat Subscription Management.
rhel-6-server-cf-tools-1-rpms
| 2.8 kB
00:00
rhel-6-server-rpms
| 3.7 kB
00:00
man-pages-3.22-12.el6.noarch : Man (manual) pages from the Linux Documenta...
Repo
: rhel-6-server-rpms
Matched from:
Filename
: /usr/share/man/man5/passwd.5.gz
man-pages-3.22-20.el6.noarch : Man (manual) pages from the Linux Documenta...
Repo
: rhel-6-server-rpms
Matched from:
Filename
: /usr/share/man/man5/passwd.5.gz
man-pages-3.22-17.el6.noarch : Man (manual) pages from the Linux Documenta...
Repo
: rhel-6-server-rpms
Matched from:
Filename
: /usr/share/man/man5/passwd.5.gz
man-pages-3.22-20.el6.noarch : Man (manual) pages from the Linux Documenta...
Repo
: installed
Matched from:
Other
: Provides-match: /usr/share/man/man5/passwd.5.gz
root@rhel65:/etc#
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package management
55.7.5. yum install
To install an application, use yum install $package. Naturally yum will install all the
necessary dependencies.
[root@rhel55 ~]# yum install sudo
Loaded plugins: rhnplugin, security
Setting up Install Process
Resolving Dependencies
--> Running transaction check
---> Package sudo.i386 0:1.7.2p1-7.el5_5 set to be updated
--> Finished Dependency Resolution
Dependencies Resolved
=======================================================================
Package
Arch
Version
Repository
Size
=======================================================================
Installing:
sudo
i386
1.7.2p1-7.el5_5
rhel-i386-server-5
230 k
Transaction Summary
=======================================================================
Install
1 Package(s)
Upgrade
0 Package(s)
Total download size: 230 k
Is this ok [y/N]: y
Downloading Packages:
sudo-1.7.2p1-7.el5_5.i386.rpm
Running rpm_check_debug
Running Transaction Test
Finished Transaction Test
Transaction Test Succeeded
Running Transaction
Installing
: sudo
| 230 kB
00:00
1/1
Installed:
sudo.i386 0:1.7.2p1-7.el5_5
Complete!
You can add more than one parameter here.
yum install $package1 $package2 $package3
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package management
55.7.6. yum update
To bring all applications up to date, by downloading and installing them, issue yum update.
All software that was installed via yum will be updated to the latest version that is available
in the repository.
yum update
If you only want to update one package, use yum update $package.
[root@rhel55 ~]# yum update sudo
Loaded plugins: rhnplugin, security
Skipping security plugin, no data
Setting up Update Process
Resolving Dependencies
Skipping security plugin, no data
--> Running transaction check
---> Package sudo.i386 0:1.7.2p1-7.el5_5 set to be updated
--> Finished Dependency Resolution
Dependencies Resolved
=====================================================================
Package
Arch
Version
Repository
Size
=====================================================================
Updating:
sudo
i386
1.7.2p1-7.el5_5
rhel-i386-server-5
230 k
Transaction Summary
=====================================================================
Install
0 Package(s)
Upgrade
1 Package(s)
Total download size: 230 k
Is this ok [y/N]: y
Downloading Packages:
sudo-1.7.2p1-7.el5_5.i386.rpm
Running rpm_check_debug
Running Transaction Test
Finished Transaction Test
Transaction Test Succeeded
Running Transaction
Updating
: sudo
Cleanup
: sudo
| 230 kB
00:00
1/2
2/2
Updated:
sudo.i386 0:1.7.2p1-7.el5_5
Complete!
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package management
55.7.7. yum software groups
Issue yum grouplist to see a list of all available software groups.
[root@rhel55 ~]# yum grouplist
Loaded plugins: rhnplugin, security
Setting up Group Process
Installed Groups:
Administration Tools
Authoring and Publishing
DNS Name Server
Development Libraries
Development Tools
Editors
GNOME Desktop Environment
GNOME Software Development
Graphical Internet
Graphics
Legacy Network Server
Legacy Software Development
Legacy Software Support
Mail Server
Network Servers
Office/Productivity
Printing Support
Server Configuration Tools
System Tools
Text-based Internet
Web Server
Windows File Server
X Software Development
X Window System
Available Groups:
Engineering and Scientific
FTP Server
Games and Entertainment
Java Development
KDE (K Desktop Environment)
KDE Software Development
MySQL Database
News Server
OpenFabrics Enterprise Distribution
PostgreSQL Database
Sound and Video
Done
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package management
To install a set of applications, brought together via a group, use yum groupinstall
$groupname.
[root@rhel55 ~]# yum groupinstall 'Sound and video'
Loaded plugins: rhnplugin, security
Setting up Group Process
Package alsa-utils-1.0.17-1.el5.i386 already installed and latest version
Package sox-12.18.1-1.i386 already installed and latest version
Package 9:mkisofs-2.01-10.7.el5.i386 already installed and latest version
Package 9:cdrecord-2.01-10.7.el5.i386 already installed and latest version
Package cdrdao-1.2.1-2.i386 already installed and latest version
Resolving Dependencies
--> Running transaction check
---> Package cdda2wav.i386 9:2.01-10.7.el5 set to be updated
---> Package cdparanoia.i386 0:alpha9.8-27.2 set to be updated
---> Package sound-juicer.i386 0:2.16.0-3.el5 set to be updated
--> Processing Dependency: libmusicbrainz >= 2.1.0 for package: sound-juicer
--> Processing Dependency: libmusicbrainz.so.4 for package: sound-juicer
---> Package vorbis-tools.i386 1:1.1.1-3.el5 set to be updated
--> Processing Dependency: libao >= 0.8.4 for package: vorbis-tools
--> Processing Dependency: libao.so.2 for package: vorbis-tools
--> Running transaction check
---> Package libao.i386 0:0.8.6-7 set to be updated
---> Package libmusicbrainz.i386 0:2.1.1-4.1 set to be updated
--> Finished Dependency Resolution
...
Read the manual page of yum for more information about managing groups in yum.
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package management
55.7.8. /etc/yum.conf and repositories
The configuration of yum repositories is done in /etc/yum/yum.conf and /etc/yum/repos.d/
.
Configurating yum itself is done in /etc/yum.conf. This file will contain the location of a
log file and a cache directory for yum and can also contain a list of repositories.
Recently yum started accepting several repo files with each file containing a list of
repositories. These repo files are located in the /etc/yum.repos.d/ directory.
One important flag for yum is enablerepo. Use this command if you want to use a repository
that is not enabled by default.
yum $command $foo --enablerepo=$repo
An example of the contents of the repo file: MyRepo.repo
[$repo]
name=My Repository
baseurl=http://path/to/MyRepo
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-MyRep
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package management
55.8. alien
alien is experimental software that converts between rpm and deb package formats (and
others).
Below an example of how to use alien to convert an rpm package to a deb package.
paul@barry:~$ ls -l netcat*
-rw-r--r-- 1 paul paul 123912 2009-06-04 14:58 netcat-0.7.1-1.i386.rpm
paul@barry:~$ alien --to-deb netcat-0.7.1-1.i386.rpm
netcat_0.7.1-2_i386.deb generated
paul@barry:~$ ls -l netcat*
-rw-r--r-- 1 paul paul 123912 2009-06-04 14:58 netcat-0.7.1-1.i386.rpm
-rw-r--r-- 1 root root 125236 2009-06-04 14:59 netcat_0.7.1-2_i386.deb
In real life, use the netcat tool provided by your distribution, or use the .deb file from their
website.
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package management
55.9. downloading software outside the
repository
First and most important, whenever you download software, start by reading the README
file!
Normally the readme will explain what to do after download. You will probably receive
a .tar.gz or a .tgz file. Read the documentation, then put the compressed file in a directory.
You can use the following to find out where the package wants to install.
tar tvzpf $downloadedFile.tgz
You unpack them like with tar xzf, it will create a directory called applicationName-1.2.3
tar xzf $applicationName.tgz
Replace the z with a j when the file ends in .tar.bz2. The tar, gzip and bzip2 commands are
explained in detail in the Linux Fundamentals course.
If you download a .deb file, then you'll have to use dpkg to install it, .rpm's can be installed
with the rpm command.
55.10. compiling software
First and most important, whenever you download source code for installation, start by
reading the README file!
Usually the steps are always the same three : running ./configure followed by make (which
is the actual compiling) and then by make install to copy the files to their proper location.
./configure
make
make install
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55.11. practice: package management
1. Verify whether gcc, sudo and wesnoth are installed.
2. Use yum or aptitude to search for and install the scp, tmux, and man-pages packages. Did
you find them all ?
3. Search the internet for 'webmin' and figure out how to install it.
4. If time permits, search for and install samba including the samba docs pdf files (thousands
of pages in two pdf's).
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package management
55.12. solution: package management
1. Verify whether gcc, sudo and wesnoth are installed.
On Red Hat/CentOS:
rpm -qa | grep gcc
rpm -qa | grep sudo
rpm -qa | grep wesnoth
On Debian/Ubuntu:
dpkg -l | grep gcc
dpkg -l | grep sudo
dpkg -l | grep wesnoth
2. Use yum or aptitude to search for and install the scp, tmux, and man-pages packages. Did
you find them all ?
On Red Hat/CentOS:
yum search scp
yum search tmux
yum search man-pages
On Debian/Ubuntu:
aptitude search scp
aptitude search tmux
aptitude search man-pages
3. Search the internet for 'webmin' and figure out how to install it.
Google should point you to webmin.com.
There are several formats available there choose .rpm, .deb or .tgz .
4. If time permits, search for and install samba including the samba docs pdf files (thousands
of pages in two pdf's).
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Part XIV. network management
Table of Contents
56. general networking ........................................................................................................ 578
56.1. network layers ...................................................................................................... 579
56.2. unicast, multicast, broadcast, anycast ........................................................................ 582
56.3. lan-wan-man ........................................................................................................ 584
56.4. internet - intranet - extranet .................................................................................... 586
56.5. tcp/ip .................................................................................................................. 587
57. interface configuration ................................................................................................... 588
57.1. to gui or not to gui ............................................................................................... 589
57.2. Debian/Ubuntu nic configuration ............................................................................. 590
57.3. Red Hat/Fedora nic configuration ............................................................................ 592
57.4. ifconfig ............................................................................................................... 594
57.5. hostname ............................................................................................................. 596
57.6. arp ..................................................................................................................... 597
57.7. route ................................................................................................................... 598
57.8. ping .................................................................................................................... 598
57.9. optional: ethtool .................................................................................................... 599
57.10. practice: interface configuration ............................................................................. 600
57.11. solution: interface configuration ............................................................................. 601
58. network sniffing ............................................................................................................. 603
58.1. wireshark ............................................................................................................. 604
58.2. tcpdump .............................................................................................................. 608
58.3. practice: network sniffing ....................................................................................... 609
58.4. solution: network sniffing ....................................................................................... 610
59. binding and bonding ...................................................................................................... 611
59.1. binding on Redhat/Fedora ....................................................................................... 612
59.2. binding on Debian/Ubuntu ...................................................................................... 613
59.3. bonding on Redhat/Fedora ...................................................................................... 614
59.4. bonding on Debian/Ubuntu ..................................................................................... 616
59.5. practice: binding and bonding ................................................................................. 618
59.6. solution: binding and bonding ................................................................................. 619
60. ssh client and server ...................................................................................................... 620
60.1. about ssh ............................................................................................................. 621
60.2. log on to a remote server ....................................................................................... 623
60.3. executing a command in remote .............................................................................. 623
60.4. scp ..................................................................................................................... 624
60.5. setting up passwordless ssh ..................................................................................... 625
60.6. X forwarding via ssh ............................................................................................. 626
60.7. troubleshooting ssh ................................................................................................ 627
60.8. sshd .................................................................................................................... 628
60.9. sshd keys ............................................................................................................ 628
60.10. ssh-agent ........................................................................................................... 628
60.11. practice: ssh ....................................................................................................... 629
60.12. solution: ssh ....................................................................................................... 630
61. introduction to nfs ......................................................................................................... 632
61.1. nfs protocol versions ............................................................................................. 633
61.2. rpcinfo ................................................................................................................ 633
61.3. server configuration ............................................................................................... 634
61.4. /etc/exports ........................................................................................................... 634
61.5. exportfs ............................................................................................................... 634
61.6. client configuration ............................................................................................... 635
61.7. practice: introduction to nfs .................................................................................... 636
62. introduction to networking ............................................................................................. 637
62.1. introduction to iptables .......................................................................................... 638
62.2. practice : iptables .................................................................................................. 639
62.3. solution : iptables .................................................................................................. 640
576
network management
62.4.
62.5.
62.6.
62.7.
xinetd and inetd ....................................................................................................
practice : inetd and xinetd ......................................................................................
network file system ...............................................................................................
practice : network file system ..................................................................................
577
641
643
644
646
Chapter 56. general networking
While this chapter is not directly about Linux, it does contain general networking concepts
that will help you in troubleshooting networks on Linux.
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general networking
56.1. network layers
56.1.1. seven OSI layers
When talking about protocol layers, people usually mention the seven layers of the osi
protocol (Application, Presentation, Session, Transport, Network, Data Link and Physical).
We will discuss layers 2 and 3 in depth, and focus less on the other layers. The reason is
that these layers are important for understanding networks. You will hear administrators use
words like "this is a layer 2 device" or "this is a layer 3 broadcast", and you should be able
to understand what they are talking about.
56.1.2. four DoD layers
The DoD (or tcp/ip) model has only four layers, roughly mapping its network access layer
to OSI layers 1 and 2 (Physical and Datalink), its internet (IP) layer to the OSI network
layer, its host-to-host (tcp, udp) layer to OSI layer 4 (transport) and its application layer
to OSI layers 5, 6 and 7.
Below an attempt to put OSI and DoD layers next to some protocols and devices.
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general networking
56.1.3. short introduction to the physical layer
The physical layer, or layer 1, is all about voltage, electrical signals and mechanical
connections. Some networks might still use coax cables, but most will have migrated to utp
(cat 5 or better) with rj45 connectors.
Devices like repeaters and hubs are part of this layer. You cannot use software to 'see'
a repeater or hub on the network. The only thing these devices are doing is amplifying
electrical signals on cables. Passive hubs are multiport amplifiers that amplify an incoming
electrical signal on all other connections. Active hubs do this by reading and retransmitting
bits, without interpreting any meaning in those bits.
Network technologies like csma/cd and token ring are defined on this layer.
This is all we have to say about layer 1 in this book.
56.1.4. short introduction to the data link layer
The data link layer, or layer 2 is about frames. A frame has a crc (cyclic redundancy check).
In the case of ethernet (802.3), each network card is identifiable by a unique 48-bit mac
address (media access control address).
On this layer we find devices like bridges and switches. A bridge is more intelligent than a
hub because a bridge can make decisions based on the mac address of computers. A switch
also understands mac addresses.
In this book we will discuss commands like arp and ifconfig to explore this layer.
56.1.5. short introduction to the network layer
Layer 3 is about ip packets. This layer gives every host a unique 32-bit ip address. But ip
is not the only protocol on this layer, there is also icmp, igmp, ipv6 and more. A complete
list can be found in the /etc/protocols file.
On this layer we find devices like routers and layer 3 switches, devices that know (and
have) an ip address.
In tcp/ip this layer is commonly referred to as the internet layer.
56.1.6. short introduction to the transport layer
We will discuss the tcp and udp protocols in the context of layer 4. The DoD model calls
this the host-to-host layer.
56.1.7. layers 5, 6 and 7
The tcp/ip application layer includes layers 5, 6 and 7. Details on the difference between
these layers are out of scope of this course.
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general networking
56.1.8. network layers in this book
Stacking of layers in this book is based on the Protocols in Frame explanation in the
wireshark sniffer. When sniffing a dhcp packet, we notice the following in the sniffer.
[Protocols in Frame: eth:ip:udp:bootp]
Sniffing for ntp (Network Time Protocol) packets gives us this line, which makes us
conclude to put ntp next to bootp in the protocol chart below.
[Protocols in Frame: eth:ip:udp:ntp]
Sniffing an arp broadcast makes us put arp next to ip. All these protocols are explained
later in this chapter.
[Protocols in Frame: eth:arp]
Below is a protocol chart based on wireshark's knowledge. It contains some very common
protocols that are discussed in this book. The chart does not contain all protocols.
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general networking
56.2. unicast, multicast, broadcast, anycast
56.2.1. unicast
A unicast communication originates from one computer and is destined for exactly one other
computer (or host). It is common for computers to have many unicast communications.
56.2.2. multicast
A multicast is destined for a group (of computers).
Some examples of multicast are Realplayer (.sdp files) and ripv2 (a routing protocol).
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general networking
56.2.3. broadcast
A broadcast is meant for everyone.
Typical example here is the BBC (British Broadcasting Corporation) broadcasting to
everyone. In datacommunications a broadcast is most common confined to the lan.
Careful, a layer 2 broadcast is very different from a layer 3 broadcast. A layer two
broadcast is received by all network cards on the same segment (it does not pass any router),
whereas a layer 3 broadcast is received by all hosts in the same ip subnet.
56.2.4. anycast
The root name servers of the internet use anycast. An anycast signal goes the the
(geographically) nearest of a well defined group.
With thanks to the nice anonymous wikipedia contributor to put these pictures in the public
domain.
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general networking
56.3. lan-wan-man
The term lan is used for local area networks, as opposed to a wan for wide area networks.
The difference between the two is determined by the distance between the computers, and
not by the number of computers in a network. Some protocols like atm are designed for use
in a wan, others like ethernet are designed for use in a lan.
56.3.1. lan
A lan (Local Area Network) is a local network. This can be one room, or one floor, or even
one big building. We say lan as long as computers are close to each other. You can also
define a lan when all computers are ethernet connected.
A lan can contain multiple smaller lan's. The picture below shows three lan's that together
make up one lan.
56.3.2. man
A man (Metropolitan Area Network) is something inbetween a lan and a wan, often
comprising several buildings on the same campus or in the same city. A man can use fddi
or ethernet or other protocols for connectivity.
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general networking
56.3.3. wan
A wan (Wide Area Network) is a network with a lot of distance between the computers (or
hosts). These hosts are often connected by leased lines. A wan does not use ethernet, but
protocols like fddi, frame relay, ATM or X.25 to connect computers (and networks).
The picture below shows a branch office that is connected through Frame Relay with
headquarters.
The acronym wan is also used for large surface area networks like the internet.
Cisco is known for their wan technology. They make routers that connect many lan
networks using wan protocols.
56.3.4. pan-wpan
Your home network is called a pan (Personal Area Network). A wireless pan is a wpan.
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general networking
56.4. internet - intranet - extranet
The internet is a global network. It connects many networks using the tcp/ip protocol stack.
The origin of the internet is the arpanet. The arpanet was created in 1969, that year only
four computers were connected in the network. In 1971 the first e-mail was sent over the
arpanet. E-mail took 75 percent of all arpanet traffic in 1973. 1973 was also the year ftp
was introduced, and saw the connection of the first European countries (Norway and UK). In
2009 the internet was available to 25 percent of the world population. In 2011 it is estimated
that only a quarter of internet webpages are in English.
An intranet is a private tcp/ip network. An intranet uses the same protocols as the internet,
but is only accessible to people from within one organization.
An extranet is similar to an intranet, but some trusted organizations (partners/clients/
suppliers/...) also get access.
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general networking
56.5. tcp/ip
56.5.1. history of tcp/ip
In the Sixties development of the tcp/ip protocol stack was started by the US Department of
Defense. In the Eighties a lot of commercial enterprises developed their own protocol stack:
IBM created sna, Novell had ipx/spx, Microsoft completed netbeui and Apple worked with
appletalk. All the efforts from the Eighties failed to survive the Nineties. By the end of the
Nineties, almost all computers in the world were able to speak tcp/ip.
In my humble opinion, the main reason for the survival of tcp/ip over all the other protocols
is its openness. Everyone is free to develop and use the tcp/ip protocol suite.
56.5.2. rfc (request for comment)
The protocols that are used on the internet are defined in rfc's. An rfc or request
for comment describes the inner working of all internet protocols. The IETF (Internet
Engineering Task Force) is the sole publisher of these protocols since 1986.
The official website for the rfc's is http://www.rfc-editor.org. This website contains all
rfc's in plain text, for example rfc2132 (which defines dhcp and bootp) is accessible at http://
www.rfc-editor.org/rfc/rfc2132.txt.
56.5.3. many protocols
For reliable connections, you use tcp, whereas udp is connectionless but faster. The icmp
error messages are used by ping, multicast groups are managed by igmp.
These protocols are visible in the protocol field of the ip header, and are listed in the /etc/
protocols file.
paul@debian5:~$ grep tcp /etc/protocols
tcp
6
TCP
# transmission control protocol
56.5.4. many services
Network cards are uniquely identified by their mac address, hosts by their ip address and
applications by their port number.
Common application level protocols like smtp, http, ssh, telnet and ftp have fixed port
numbers. There is a list of port numbers in /etc/services.
paul@ubu1010:~$ grep ssh /etc/services
ssh
22/tcp
# SSH Remote Login Protocol
ssh
22/udp
587
Chapter 57. interface configuration
This chapter explains how to configure network interface cards to work with tcp/ip.
588
interface configuration
57.1. to gui or not to gui
Recent Linux distributions often include a graphical application to configure the network.
Some people complain that these applications mess networking configurations up when
used simultaneously with command line configurations. Notably Network Manager (often
replaced by wicd) and yast are known to not care about configuration changes via the
command line.
Since the goal of this course is server administration, we will assume our Linux servers are
always administered through the command line.
This chapter only focuses on using the command line for network interface configuration!
Unfortunately there is no single combination of Linux commands and /etc files that works on
all Linux distributions. We discuss networking on two (large but distinct) Linux distribution
families.
We start with Debian/Ubuntu, then continue with Fedora/RHEL.
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interface configuration
57.2. Debian/Ubuntu nic configuration
57.2.1. /etc/network/interfaces
The /etc/network/interfaces file is a core network interface card configuration file on
Ubuntu and Debian.
dhcp client
The screenshot below shows that our current Ubuntu 11.04 is configured for dhcp on eth0
(the first network interface card or nic).
root@ubu1104srv:~# cat /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# The loopback network interface
auto lo
iface lo inet loopback
# The primary network interface
auto eth0
iface eth0 inet dhcp
Configuring network cards for dhcp is good practice for clients, but servers usually require
a fixed ip address.
fixed ip
The screenshot below shows /etc/network/interfaces configured with a fixed ip address.
root@ubu1104srv:~# cat /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# The loopback network interface
auto lo
iface lo inet loopback
# The primary network interface
auto eth0
iface eth0 inet static
address 192.168.33.100
network 192.168.33.0
netmask 255.255.255.0
gateway 192.168.33.1
The screenshot above also shows that you can provide more configuration than just the
ip address. See interfaces(5) for help on setting a gateway, netmask or any of the other
options.
590
interface configuration
57.2.2. /sbin/ifdown
It is adviced (but not mandatory) to down an interface before changing its configuration.
This can be done with the ifdown command.
The command will not give any output when downing an interface with a fixed ip address.
However ifconfig will no longer show the interface.
root@ubu1104srv:~# ifdown eth0
root@ubu1104srv:~# ifconfig
lo
Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:106 errors:0 dropped:0 overruns:0 frame:0
TX packets:106 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:11162 (11.1 KB) TX bytes:11162 (11.1 KB)
An interface that is down cannot be used to connect to the network.
57.2.3. /sbin/ifup
Below a screenshot of ifup bringing the eth0 ethernet interface up using dhcp. (Note that
this is a Ubuntu 10.10 screenshot, Ubuntu 11.04 omits ifup output by default.)
root@ubu1010srv:/etc/network# ifup eth0
Internet Systems Consortium DHCP Client V3.1.3
Copyright 2004-2009 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/
Listening on LPF/eth0/08:00:27:cd:7f:fc
Sending on
LPF/eth0/08:00:27:cd:7f:fc
Sending on
Socket/fallback
DHCPREQUEST of 192.168.1.34 on eth0 to 255.255.255.255 port 67
DHCPNAK from 192.168.33.100
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 3
DHCPOFFER of 192.168.33.77 from 192.168.33.100
DHCPREQUEST of 192.168.33.77 on eth0 to 255.255.255.255 port 67
DHCPACK of 192.168.33.77 from 192.168.33.100
bound to 192.168.33.77 -- renewal in 95 seconds.
ssh stop/waiting
ssh start/running, process 1301
root@ubu1010srv:/etc/network#
The details of dhcp are covered in a separate chapter in the Linux Servers course.
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interface configuration
57.3. Red Hat/Fedora nic configuration
57.3.1. /etc/sysconfig/network
The /etc/sysconfig/network file is a global (across all network cards) configuration file.
It allows us to define whether we want networking (NETWORKING=yes|no), what the
hostname should be (HOSTNAME=) and which gateway to use (GATEWAY=).
[root@rhel6 ~]# cat /etc/sysconfig/network
NETWORKING=yes
HOSTNAME=rhel6
GATEWAY=192.168.1.1
There are a dozen more option settable in this file, details can be found in /usr/share/doc/
initscripts-*/sysconfig.txt.
57.3.2. /etc/sysconfig/network-scripts/ifcfgEach network card can be configured individually using the /etc/sysconfig/network-scripts/
ifcfg-* files. When you have only one network card, then this will probably be /etc/
sysconfig/network-scripts/ifcfg-eth0.
dhcp client
Below a screenshot of /etc/sysconfig/network-scripts/ifcfg-eth0 configured for dhcp
(BOOTPROTO="dhcp"). Note also the NM_CONTROLLED paramater to disable control
of this nic by Network Manager. This parameter is not explained (not even mentioned) in
/usr/share/doc/initscripts-*/sysconfig.txt, but many others are.
[root@rhel6 ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE="eth0"
HWADDR="08:00:27:DD:0D:5C"
NM_CONTROLLED="no"
BOOTPROTO="dhcp"
ONBOOT="yes"
The BOOTPROTO variable can be set to either dhcp or bootp, anything else will be
considered static meaning there should be no protocol used at boot time to set the interface
values.
fixed ip
Below a screenshot of a fixed ip configuration in /etc/sysconfig/network-scripts/ifcfgeth0.
[root@rhel6 ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0
DEVICE="eth0"
HWADDR="08:00:27:DD:0D:5C"
NM_CONTROLLED="no"
BOOTPROTO="none"
IPADDR="192.168.1.99"
NETMASK="255.255.255.0"
GATEWAY="192.168.1.1"
ONBOOT="yes"
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interface configuration
The HWADDR can be used to make sure that each network card gets the correct name
when multiple network cards are present in the computer. It can not be used to assign a mac
address to a network card. For this, you need to specify the MACADDR variable. Do not
use HWADDR and MACADDR in the same ifcfg-ethx file.
The BROADCAST= and NETWORK= parameters from previous RHEL/Fedora versions
are obsoleted.
57.3.3. /sbin/ifup and /sbin/ifdown
The ifup and ifdown commands will set an interface up or down, using the configuration
discussed above. This is identical to their behaviour in Debian and Ubuntu.
[root@rhel6 ~]# ifdown eth0 && ifup eth0
[root@rhel6 ~]# ifconfig eth0
eth0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::a00:27ff:fedd:d5c/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:2452 errors:0 dropped:0 overruns:0 frame:0
TX packets:1881 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:257036 (251.0 KiB) TX bytes:184767 (180.4 KiB)
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interface configuration
57.4. ifconfig
The use of /sbin/ifconfig without any arguments will present you with a list of all active
network interface cards, including wireless and the loopback interface. In the screenshot
below eth0 has no ip address.
root@ubu1010:~# ifconfig
eth0 Link encap:Ethernet HWaddr 00:26:bb:5d:2e:52
UP BROADCAST MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)
Interrupt:43 Base address:0xe000
eth1 Link encap:Ethernet HWaddr 00:26:bb:12:7a:5e
inet addr:192.168.1.30 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::226:bbff:fe12:7a5e/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:11141791 errors:202 dropped:0 overruns:0 frame:11580126
TX packets:6473056 errors:3860 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:3476531617 (3.4 GB) TX bytes:2114919475 (2.1 GB)
Interrupt:23
lo
Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:2879 errors:0 dropped:0 overruns:0 frame:0
TX packets:2879 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:486510 (486.5 KB) TX bytes:486510 (486.5 KB)
You can also use ifconfig to obtain information about just one network card.
[root@rhel6 ~]# ifconfig eth0
eth0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::a00:27ff:fedd:d5c/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:2969 errors:0 dropped:0 overruns:0 frame:0
TX packets:1918 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:335942 (328.0 KiB) TX bytes:190157 (185.7 KiB)
When /sbin is not in the $PATH of a normal user you will have to type the full path, as
seen here on Debian.
paul@debian5:~$ /sbin/ifconfig eth3
eth3 Link encap:Ethernet HWaddr 08:00:27:ab:67:30
inet addr:192.168.1.29 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::a00:27ff:feab:6730/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:27155 errors:0 dropped:0 overruns:0 frame:0
TX packets:30527 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:13095386 (12.4 MiB) TX bytes:25767221 (24.5 MiB)
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interface configuration
57.4.1. up and down
You can also use ifconfig to bring an interface up or down. The difference with ifup is that
ifconfig eth0 up will re-activate the nic keeping its existing (current) configuration, whereas
ifup will read the correct file that contains a (possibly new) configuration and use this config
file to bring the interface up.
[root@rhel6 ~]# ifconfig eth0 down
[root@rhel6 ~]# ifconfig eth0 up
[root@rhel6 ~]# ifconfig eth0
eth0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::a00:27ff:fedd:d5c/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:2995 errors:0 dropped:0 overruns:0 frame:0
TX packets:1927 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:339030 (331.0 KiB) TX bytes:191583 (187.0 KiB)
57.4.2. setting ip address
You can temporary set an ip address with ifconfig. This ip address is only valid until the
next ifup/ifdown cycle or until the next reboot.
[root@rhel6 ~]# ifconfig eth0 | grep 192
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
[root@rhel6 ~]# ifconfig eth0 192.168.33.42 netmask 255.255.0.0
[root@rhel6 ~]# ifconfig eth0 | grep 192
inet addr:192.168.33.42 Bcast:192.168.255.255 Mask:255.255.0.0
[root@rhel6 ~]# ifdown eth0 && ifup eth0
[root@rhel6 ~]# ifconfig eth0 | grep 192
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
57.4.3. setting mac address
You can also use ifconfig to set another mac address than the one hard coded in the network
card. This screenshot shows you how.
[root@rhel6 ~]# ifconfig eth0 | grep HWaddr
eth0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C
[root@rhel6 ~]# ifconfig eth0 hw ether 00:42:42:42:42:42
[root@rhel6 ~]# ifconfig eth0 | grep HWaddr
eth0 Link encap:Ethernet HWaddr 00:42:42:42:42:42
57.4.4. dhclient
Home and client Linux desktops often have /sbin/dhclient running. This is a daemon that
enables a network interface to lease an ip configuration from a dhcp server. When your
adapter is configured for dhcp or bootp, then /sbin/ifup will start the dhclient daemon.
When a lease is renewed, dhclient will override your ifconfig set ip address!
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interface configuration
57.5. hostname
Every host receives a hostname, often placed in a DNS name space forming the fqdn or
Fully Qualified Domain Name.
This screenshot shows the hostname command and the configuration of the hostname on
Red Hat/Fedora.
[root@rhel6 ~]# grep rhel /etc/sysconfig/network
HOSTNAME=rhel6
[root@rhel6 ~]# hostname
rhel6
Ubuntu/Debian uses the /etc/hostname file to configure the hostname.
paul@ubu1010:~$ cat /etc/hostname
ubu1010
paul@ubu1010:~$ hostname
ubu1010
On all Linux distributions you can change the hostname using the hostname $newname
command. This is not a permanent change.
[root@rhel6 ~]# hostname server42
[root@rhel6 ~]# hostname
server42
On any Linux you can use sysctl to display and set the hostname.
[root@rhel6 ~]#
kernel.hostname
[root@rhel6 ~]#
kernel.hostname
[root@rhel6 ~]#
kernel.hostname
[root@rhel6 ~]#
rhel6
sysctl kernel.hostname
= server42
sysctl kernel.hostname=rhel6
= rhel6
sysctl kernel.hostname
= rhel6
hostname
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interface configuration
57.6. arp
The ip to mac resolution is handled by the layer two broadcast protocol arp. The arp table
can be displayed with the arp tool. The screenshot below shows the list of computers that
this computer recently communicated with.
root@barry:~# arp -a
? (192.168.1.191) at 00:0C:29:3B:15:80 [ether] on eth1
agapi (192.168.1.73) at 00:03:BA:09:7F:D2 [ether] on eth1
anya (192.168.1.1) at 00:12:01:E2:87:FB [ether] on eth1
faith (192.168.1.41) at 00:0E:7F:41:0D:EB [ether] on eth1
kiss (192.168.1.49) at 00:D0:E0:91:79:95 [ether] on eth1
laika (192.168.1.40) at 00:90:F5:4E:AE:17 [ether] on eth1
pasha (192.168.1.71) at 00:03:BA:02:C3:82 [ether] on eth1
shaka (192.168.1.72) at 00:03:BA:09:7C:F9 [ether] on eth1
root@barry:~#
Anya is a Cisco Firewall, faith is a laser printer, kiss is a Kiss DP600, laika is a laptop and
Agapi, Shaka and Pasha are SPARC servers. The question mark is a Red Hat Enterprise
Linux server running on a virtual machine.
You can use arp -d to remove an entry from the arp table.
[root@rhel6 ~]# arp
Address
HWtype HWaddress
Flags Mask
Iface
ubu1010
ether
00:26:bb:12:7a:5e
C
eth0
anya
ether
00:02:cf:aa:68:f0
C
eth0
[root@rhel6 ~]# arp -d anya
[root@rhel6 ~]# arp
Address
HWtype HWaddress
Flags Mask
Iface
ubu1010
ether
00:26:bb:12:7a:5e
C
eth0
anya
(incomplete)
eth0
[root@rhel6 ~]# ping anya
PING anya (192.168.1.1) 56(84) bytes of data.
64 bytes from anya (192.168.1.1): icmp_seq=1 ttl=254 time=10.2 ms
...
[root@rhel6 ~]# arp
Address
HWtype HWaddress
Flags Mask
Iface
ubu1010
ether
00:26:bb:12:7a:5e
C
eth0
anya
ether
00:02:cf:aa:68:f0
C
eth0
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57.7. route
You can see the computer's local routing table with the /sbin/route command (and also with
netstat -r ).
root@RHEL4b ~]# netstat -r
Kernel IP routing table
Destination
Gateway
Genmask
192.168.1.0
*
255.255.255.0
[root@RHEL4b ~]# route
Kernel IP routing table
Destination
Gateway
Genmask
192.168.1.0
*
255.255.255.0
[root@RHEL4b ~]#
Flags
U
MSS Window
0 0
Flags Metric Ref
U
0
0
irtt Iface
0 eth0
Use Iface
0 eth0
It appears this computer does not have a gateway configured, so we use route add default
gw to add a default gateway on the fly.
[root@RHEL4b ~]# route add default gw 192.168.1.1
[root@RHEL4b ~]# route
Kernel IP routing table
Destination
Gateway
Genmask
Flags Metric Ref
192.168.1.0
*
255.255.255.0 U
0
0
default
192.168.1.1 0.0.0.0
UG
0
0
[root@RHEL4b ~]#
Use Iface
0 eth0
0 eth0
Unless you configure the gateway in one of the /etc/ file from the start of this chapter, your
computer will forget this gateway after a reboot.
57.8. ping
If you can ping to another host, then tcp/ip is configured.
[root@RHEL4b ~]# ping 192.168.1.5
PING 192.168.1.5 (192.168.1.5) 56(84)
64 bytes from 192.168.1.5: icmp_seq=0
64 bytes from 192.168.1.5: icmp_seq=1
64 bytes from 192.168.1.5: icmp_seq=2
64 bytes from 192.168.1.5: icmp_seq=3
bytes of data.
ttl=64 time=1004 ms
ttl=64 time=1.19 ms
ttl=64 time=0.494 ms
ttl=64 time=0.419 ms
--- 192.168.1.5 ping statistics --4 packets transmitted, 4 received, 0% packet loss, time 3009ms
rtt min/avg/max/mdev = 0.419/251.574/1004.186/434.520 ms, pipe 2
[root@RHEL4b ~]#
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interface configuration
57.9. optional: ethtool
To display or change network card settings, use ethtool. The results depend on the
capabilities of your network card. The example shows a network that auto-negotiates it's
bandwidth.
root@laika:~# ethtool eth0
Settings for eth0:
Supported ports: [ TP ]
Supported link modes:
10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Supports auto-negotiation: Yes
Advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Advertised auto-negotiation: Yes
Speed: 1000Mb/s
Duplex: Full
Port: Twisted Pair
PHYAD: 0
Transceiver: internal
Auto-negotiation: on
Supports Wake-on: pumbg
Wake-on: g
Current message level: 0x00000033 (51)
Link detected: yes
This example shows how to use ethtool to switch the bandwidth from 1000Mbit to 100Mbit
and back. Note that some time passes before the nic is back to 1000Mbit.
root@laika:~# ethtool
Speed: 1000Mb/s
root@laika:~# ethtool
root@laika:~# ethtool
Speed: 100Mb/s
root@laika:~# ethtool
root@laika:~# ethtool
Speed: 1000Mb/s
eth0 | grep Speed
-s eth0 speed 100
eth0 | grep Speed
-s eth0 speed 1000
eth0 | grep Speed
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interface configuration
57.10. practice: interface configuration
1. Verify whether dhclient is running.
2. Display your current ip address(es).
3. Display the configuration file where this ip address is defined.
4. Follow the nic configuration in the book to change your ip address from dhcp client to
fixed. Keep the same ip address to avoid conflicts!
5. Did you also configure the correct gateway in the previous question ? If not, then do
this now.
6. Verify that you have a gateway.
7. Verify that you can connect to the gateway, that it is alive.
8. Change the last two digits of your mac address.
9. Which ports are used by http, pop3, ssh, telnet, nntp and ftp ?
Note that sctp was ommitted from the screenshot.
10. Explain why e-mail and websites are sent over tcp and not udp.
11. Display the hostname of your computer.
12. Which ip-addresses did your computer recently have contact with ?
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interface configuration
57.11. solution: interface configuration
1. Verify whether dhclient is running.
paul@debian5:~$ ps fax | grep dhclient
2. Display your current ip address(es).
paul@debian5:~$ /sbin/ifconfig | grep 'inet '
inet addr:192.168.1.31 Bcast:192.168.1.255
inet addr:127.0.0.1 Mask:255.0.0.0
Mask:255.255.255.0
3. Display the configuration file where this ip address is defined.
Ubuntu/Debian: cat /etc/network/interfaces
Redhat/Fedora: cat /etc/sysconfig/network-scripts/ifcfg-eth*
4. Follow the nic configuration in the book to change your ip address from dhcp client to
fixed. Keep the same ip address to avoid conflicts!
Ubuntu/Debian:
ifdown eth0
vi /etc/network/interfaces
ifup eth0
Redhat/Fedora:
ifdown eth0
vi /etc/sysconfig/network-scripts/ifcfg-eth0
ifup eth0
5. Did you also configure the correct gateway in the previous question ? If not, then do
this now.
6. Verify that you have a gateway.
paul@debian5:~$ /sbin/route
Kernel IP routing table
Destination
Gateway
Genmask
192.168.1.0
*
255.255.255.0
default
192.168.1.1
0.0.0.0
Flags Metric Ref
U
0
0
UG
0
0
Use Iface
0 eth0
0 eth0
7. Verify that you can connect to the gateway, that it is alive.
paul@debian5:~$ ping -c3 192.168.1.1
PING 192.168.1.1 (192.168.1.1) 56(84)
64 bytes from 192.168.1.1: icmp_seq=1
64 bytes from 192.168.1.1: icmp_seq=2
64 bytes from 192.168.1.1: icmp_seq=3
bytes of data.
ttl=254 time=2.28 ms
ttl=254 time=2.94 ms
ttl=254 time=2.34 ms
--- 192.168.1.1 ping statistics --3 packets transmitted, 3 received, 0% packet loss, time 2008ms
rtt min/avg/max/mdev = 2.283/2.524/2.941/0.296 ms
8. Change the last two digits of your mac address.
[root@rhel6 ~]# ifconfig eth0 hw ether 08:00:27:ab:67:XX
9. Which ports are used by http, pop3, ssh, telnet, nntp and ftp ?
root@rhel6 ~# grep ^'http ' /etc/services
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interface configuration
http
http
root@rhel6
smtp
smtp
root@rhel6
ssh
ssh
root@rhel6
telnet
telnet
root@rhel6
nntp
nntp
root@rhel6
ftp
ftp
80/tcp
80/udp
~# grep
25/tcp
25/udp
~# grep
22/tcp
22/udp
~# grep
23/tcp
23/udp
~# grep
119/tcp
119/udp
~# grep
21/tcp
21/udp
www www-http
# WorldWideWeb HTTP
www www-http
# HyperText Transfer Protocol
^'smtp ' /etc/services
mail
mail
^'ssh ' /etc/services
# The Secure Shell (SSH) Protocol
# The Secure Shell (SSH) Protocol
^'telnet ' /etc/services
^'nntp ' /etc/services
readnews untp
readnews untp
^'ftp ' /etc/services
# USENET News Transfer Protocol
# USENET News Transfer Protocol
fsp fspd
Note that sctp was ommitted from the screenshot.
10. Explain why e-mail and websites are sent over tcp and not udp.
Because tcp is reliable and udp is not.
11. Display the hostname of your computer.
paul@debian5:~$ hostname
debian5
12. Which ip-addresses did your computer recently have contact with ?
root@rhel6 ~# arp -a
? (192.168.1.1) at 00:02:cf:aa:68:f0 [ether] on eth2
? (192.168.1.30) at 00:26:bb:12:7a:5e [ether] on eth2
? (192.168.1.31) at 08:00:27:8e:8a:a8 [ether] on eth2
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Chapter 58. network sniffing
A network administrator should be able to use a sniffer like wireshark or tcpdump to
troubleshoot network problems.
A student should often use a sniffer to learn about networking. This chapter introduces you
to network sniffing.
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network sniffing
58.1. wireshark
58.1.1. installing wireshark
This example shows how to install wireshark on .deb based distributions (including Debian,
Mint, Xubuntu, and others).
root@debian8:~# apt-get install wireshark
Reading package lists... Done
Building dependency tree
Reading state information... Done
... (output truncated)
On .rpm based distributions like CentOS, RHEL and Fedora you can use yum to install
wireshark.
[root@centos7 ~]# yum install wireshark
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
... (output truncated)
58.1.2. selecting interface
When you start wireshark for the first time, you will need to select an interface. You will
see a dialog box that looks similar to this one.
It is possible that there are no interfaces available because some distributions only allow
root to sniff the network. You may need to use sudo wireshark.
Or you can follow the general advice to sniff using tcpdump or any other tool, and save the
capture to a file. Any saved capture can be analyzed using wireshark at a later time.
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network sniffing
58.1.3. minimize traffic
Sniffing a network can generate many thousands of packets in a very short time. This can
be overwhelming. Try to mitigate by isolating your sniffer on the network. Preferably sniff
an isolated virtual network interface over which you control all traffic.
If you are at home to learn sniffing, then it could help to close all network programs on
your computer, and disconnect other computers and devices like smartphones and tablets
to minimize the traffic.
Even more important than this is the use of filters which will be discussed in this chapter.
58.1.4. sniffing ping
I started the sniffer and captured all packets while doing these three ping commands (there
is no need for root to do this):
root@debian7:~# ping -c2 ns1.paul.local
PING ns1.paul.local (10.104.33.30) 56(84) bytes of data.
64 bytes from 10.104.33.30: icmp_req=1 ttl=64 time=0.010 ms
64 bytes from 10.104.33.30: icmp_req=2 ttl=64 time=0.023 ms
--- ns1.paul.local ping statistics --2 packets transmitted, 2 received, 0% packet loss, time 1001ms
rtt min/avg/max/mdev = 0.010/0.016/0.023/0.007 ms
root@debian7:~# ping -c3 linux-training.be
PING linux-training.be (188.93.155.87) 56(84) bytes of data.
64 bytes from antares.ginsys.net (188.93.155.87): icmp_req=1 ttl=56 time=15.6 ms
64 bytes from antares.ginsys.net (188.93.155.87): icmp_req=2 ttl=56 time=17.8 ms
64 bytes from antares.ginsys.net (188.93.155.87): icmp_req=3 ttl=56 time=14.7 ms
--- linux-training.be ping statistics --3 packets transmitted, 3 received, 0% packet loss, time 2003ms
rtt min/avg/max/mdev = 14.756/16.110/17.881/1.309 ms
root@debian7:~# ping -c1 centos7.paul.local
PING centos7.paul.local (10.104.33.31) 56(84) bytes of data.
64 bytes from 10.104.33.31: icmp_req=1 ttl=64 time=0.590 ms
--- centos7.paul.local ping statistics --1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.590/0.590/0.590/0.000 ms
In total more than 200 packets were sniffed from the network. Things become clearer when
you enter icmp in the filter field and press the apply button.
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network sniffing
58.1.5. sniffing ping and dns
Using the same capture as before, but now with a different filter. We want to see both dns
and icmp traffic, so we enter both in the filter field.
We put dns or icmp in the filter to achieve this. Putting dns and icmp would render nothing
because there is no packet that matches both protocols.
In the screenshot above you can see that packets 25 and 26 both have 10.104.33.30 as source
and destination ip address. That is because the dns client is the same computer as the dns
server.
The same is true for packets 31 and 32, since the machine is actually pinging itself.
58.1.6. specific ip address
This is a screenshot that filters for dns packets that contain a certain ip address. The filter
in use is ip.addr==10.104.33.30 and dns. The and directive forces each displayed packet
to match both conditions.
Packet 93 is the dns query for the A record of linux-training.be. Packet 98 is the response
from the dns server. What do you think happened in the packets between 93 and 98 ? Try
to answer this before reading on (it always helps to try to predict what you will see, and
then checking your prediction).
58.1.7. filtering by frame
The correct technical term for a packet as sniffed is a frame (because we sniff on layer two).
So to display packets with certain numbers, we use frame.number in the filter.
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network sniffing
58.1.8. looking inside packets
The middle pane can be expanded. When selecting a line in this pane, you can see the
corresponding bytes in the frame in the bottom panel.
This screenshot shows the middle pane with the source address of my laptop selected.
Note that the above works fine when sniffing one interface. When sniffing with for example
tcpdump -i any you will end up with Linux cooked at this level.
58.1.9. other filter examples
You can combine two protocols with a logical or between them. The example below shows
how to filter only arp and bootp (or dhcp) packets.
This example shows how to filter for dns traffic containing a certain ip address.
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network sniffing
58.2. tcpdump
Sniffing on the command line can be done with tcpdump. Here are some examples.
Using the tcpdump host $ip command displays all traffic with one host (192.168.1.38 in
this example).
root@ubuntu910:~# tcpdump host 192.168.1.38
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes
Capturing only ssh (tcp port 22) traffic can be done with tcpdump tcp port $port. This
screenshot is cropped to 76 characters for readability in the pdf.
root@deb503:~# tcpdump tcp port 22
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 96 bytes
14:22:20.716313 IP deb503.local.37973 > rhel53.local.ssh: P 666050963:66605
14:22:20.719936 IP rhel53.local.ssh > deb503.local.37973: P 1:49(48) ack 48
14:22:20.720922 IP rhel53.local.ssh > deb503.local.37973: P 49:113(64) ack
14:22:20.721321 IP rhel53.local.ssh > deb503.local.37973: P 113:161(48) ack
14:22:20.721820 IP deb503.local.37973 > rhel53.local.ssh: . ack 161 win 200
14:22:20.722492 IP rhel53.local.ssh > deb503.local.37973: P 161:225(64) ack
14:22:20.760602 IP deb503.local.37973 > rhel53.local.ssh: . ack 225 win 200
14:22:23.108106 IP deb503.local.54424 > ubuntu910.local.ssh: P 467252637:46
14:22:23.116804 IP ubuntu910.local.ssh > deb503.local.54424: P 1:81(80) ack
14:22:23.116844 IP deb503.local.54424 > ubuntu910.local.ssh: . ack 81 win 2
^C
10 packets captured
10 packets received by filter
0 packets dropped by kernel
Same as above, but write the output to a file with the tcpdump -w $filename command.
root@ubuntu910:~# tcpdump -w sshdump.tcpdump tcp port 22
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes
^C
17 packets captured
17 packets received by filter
0 packets dropped by kernel
With tcpdump -r $filename the file created above can be displayed.
root@ubuntu910:~# tcpdump -r sshdump.tcpdump
Many more examples can be found in the manual page of tcpdump.
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58.3. practice: network sniffing
1. Install wireshark on your computer (not inside a virtual machine).
2. Start a ping between your computer and another computer.
3. Start sniffing the network.
4. Display only the ping echo's in the top pane using a filter.
5. Now ping to a name (like www.linux-training.be) and try to sniff the DNS query and
response. Which DNS server was used ? Was it a tcp or udp query and response ?
6. Find an amateur/hobby/club website that features a login prompt. Attempt to login with
user 'paul' and password 'hunter2' while your sniffer is running. Now find this information
in the sniffer.
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network sniffing
58.4. solution: network sniffing
1. Install wireshark on your computer (not inside a virtual machine).
Debian/Ubuntu: aptitude install wireshark
Red Hat/Mandriva/Fedora: yum install wireshark
2. Start a ping between your computer and another computer.
ping $ip_address
3. Start sniffing the network.
(sudo) wireshark
select an interface (probably eth0)
4. Display only the ping echo's in the top pane using a filter.
type 'icmp' (without quotes) in the filter box, and then click 'apply'
5. Now ping to a name (like www.linux-training.be) and try to sniff the DNS query and
response. Which DNS server was used ? Was it a tcp or udp query and response ?
First start the sniffer.
Enter 'dns' in the filter box and click apply.
root@ubuntu910:~# ping www.linux-training.be
PING www.linux-training.be (88.151.243.8) 56(84) bytes of data.
64 bytes from fosfor.openminds.be (88.151.243.8): icmp_seq=1 ttl=58 time=14.9 ms
64 bytes from fosfor.openminds.be (88.151.243.8): icmp_seq=2 ttl=58 time=16.0 ms
^C
--- www.linux-training.be ping statistics --2 packets transmitted, 2 received, 0% packet loss, time 1002ms
rtt min/avg/max/mdev = 14.984/15.539/16.095/0.569 ms
The wireshark screen should look something like this.
The details in wireshark will say the DNS query was inside a udp packet.
6. Find an amateur/hobby/club website that features a login prompt. Attempt to login with
user 'paul' and password 'hunter2' while your sniffer is running. Now find this information
in the sniffer.
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Chapter 59. binding and bonding
Sometimes a server needs more than one ip address on the same network card, we call this
binding ip addresses.
Linux can also activate multiple network cards behind the same ip address, this is called
bonding.
This chapter will teach you how to configure binding and bonding on the most common
Linux distributions.
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binding and bonding
59.1. binding on Redhat/Fedora
59.1.1. binding extra ip addresses
To bind more than one ip address to the same interface, use ifcfg-eth0:0, where the last
zero can be anything else. Only two directives are required in the files.
[root@rhel6 ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0:0
DEVICE="eth0:0"
IPADDR="192.168.1.133"
[root@rhel6 ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0:1
DEVICE="eth0:0"
IPADDR="192.168.1.142"
59.1.2. enabling extra ip-addresses
To activate a virtual network interface, use ifup, to deactivate it, use ifdown.
[root@rhel6 ~]# ifup eth0:0
[root@rhel6 ~]# ifconfig | grep 'inet '
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
inet addr:192.168.1.133 Bcast:192.168.1.255 Mask:255.255.255.0
inet addr:127.0.0.1 Mask:255.0.0.0
[root@rhel6 ~]# ifup eth0:1
[root@rhel6 ~]# ifconfig | grep 'inet '
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
inet addr:192.168.1.133 Bcast:192.168.1.255 Mask:255.255.255.0
inet addr:192.168.1.142 Bcast:192.168.1.255 Mask:255.255.255.0
inet addr:127.0.0.1 Mask:255.0.0.0
59.1.3. verifying extra ip-addresses
Use ping from another computer to check the activation, or use ifconfig like in this
screenshot.
[root@rhel6 ~]# ifconfig
eth0
Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C
inet addr:192.168.1.99 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::a00:27ff:fedd:d5c/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:1259 errors:0 dropped:0 overruns:0 frame:0
TX packets:545 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:115260 (112.5 KiB) TX bytes:84293 (82.3 KiB)
eth0:0 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C
inet addr:192.168.1.133 Bcast:192.168.1.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
eth0:1 Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C
inet addr:192.168.1.142 Bcast:192.168.1.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
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59.2. binding on Debian/Ubuntu
59.2.1. binding extra ip addresses
The configuration of multiple ip addresses on the same network card is done in /etc/network/
interfaces by adding eth0:x devices. Adding the netmask is mandatory.
debian5:~# cat /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).
# The loopback network interface
auto lo
iface lo inet loopback
# The primary network interface
iface eth0 inet static
address 192.168.1.34
network 192.168.1.0
netmask 255.255.255.0
gateway 192.168.1.1
auto eth0
auto eth0:0
iface eth0:0 inet static
address 192.168.1.233
netmask 255.255.255.0
auto eth0:1
iface eth0:1 inet static
address 192.168.1.242
netmask 255.255.255.0
59.2.2. enabling extra ip-addresses
Use ifup to enable the extra addresses.
debian5:~# ifup eth0:0
debian5:~# ifup eth0:1
59.2.3. verifying extra ip-addresses
Use ping from another computer to check the activation, or use ifconfig like in this
screenshot.
debian5:~#
inet
inet
inet
inet
ifconfig | grep 'inet '
addr:192.168.1.34 Bcast:192.168.1.255 Mask:255.255.255.0
addr:192.168.1.233 Bcast:192.168.1.255 Mask:255.255.255.0
addr:192.168.1.242 Bcast:192.168.1.255 Mask:255.255.255.0
addr:127.0.0.1 Mask:255.0.0.0
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59.3. bonding on Redhat/Fedora
We start with ifconfig -a to get a list of all the network cards on our system.
[root@rhel6 network-scripts]# ifconfig -a | grep Ethernet
eth0
Link encap:Ethernet HWaddr 08:00:27:DD:0D:5C
eth1
Link encap:Ethernet HWaddr 08:00:27:DA:C1:49
eth2
Link encap:Ethernet HWaddr 08:00:27:40:03:3B
In this demo we decide to bond eth1 and eth2.
We will name our bond bond0 and add this entry to modprobe so the kernel can load the
bonding module when we bring the interface up.
[root@rhel6 network-scripts]# cat /etc/modprobe.d/bonding.conf
alias bond0 bonding
Then we create /etc/sysconfig/network-scripts/ifcfg-bond0 to configure our bond0
interface.
[root@rhel6 network-scripts]# pwd
/etc/sysconfig/network-scripts
[root@rhel6 network-scripts]# cat ifcfg-bond0
DEVICE=bond0
IPADDR=192.168.1.199
NETMASK=255.255.255.0
ONBOOT=yes
BOOTPROTO=none
USERCTL=no
Next we create two files, one for each network card that we will use as slave in bond0.
[root@rhel6 network-scripts]# cat ifcfg-eth1
DEVICE=eth1
BOOTPROTO=none
ONBOOT=yes
MASTER=bond0
SLAVE=yes
USERCTL=no
[root@rhel6 network-scripts]# cat ifcfg-eth2
DEVICE=eth2
BOOTPROTO=none
ONBOOT=yes
MASTER=bond0
SLAVE=yes
USERCTL=no
Finally we bring the interface up with ifup bond0.
[root@rhel6 network-scripts]# ifup bond0
[root@rhel6 network-scripts]# ifconfig bond0
bond0
Link encap:Ethernet HWaddr 08:00:27:DA:C1:49
inet addr:192.168.1.199 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::a00:27ff:feda:c149/64 Scope:Link
UP BROADCAST RUNNING MASTER MULTICAST MTU:1500 Metric:1
RX packets:251 errors:0 dropped:0 overruns:0 frame:0
TX packets:21 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:39852 (38.9 KiB) TX bytes:1070 (1.0 KiB)
The bond should also be visible in /proc/net/bonding.
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binding and bonding
[root@rhel6 network-scripts]# cat /proc/net/bonding/bond0
Ethernet Channel Bonding Driver: v3.5.0 (November 4, 2008)
Bonding Mode: load balancing (round-robin)
MII Status: up
MII Polling Interval (ms): 0
Up Delay (ms): 0
Down Delay (ms): 0
Slave Interface: eth1
MII Status: up
Link Failure Count: 0
Permanent HW addr: 08:00:27:da:c1:49
Slave Interface: eth2
MII Status: up
Link Failure Count: 0
Permanent HW addr: 08:00:27:40:03:3b
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binding and bonding
59.4. bonding on Debian/Ubuntu
We start with ifconfig -a to get a list of all the network cards on our system.
debian5:~# ifconfig -a | grep Ethernet
eth0
Link encap:Ethernet HWaddr 08:00:27:bb:18:a4
eth1
Link encap:Ethernet HWaddr 08:00:27:63:9a:95
eth2
Link encap:Ethernet HWaddr 08:00:27:27:a4:92
In this demo we decide to bond eth1 and eth2.
We also need to install the ifenslave package.
debian5:~# aptitude search ifenslave
p ifenslave
- Attach and detach slave interfaces to a bonding device
p ifenslave-2.6 - Attach and detach slave interfaces to a bonding device
debian5:~# aptitude install ifenslave
Reading package lists... Done
...
Next we update the /etc/network/interfaces file with information about the bond0 interface.
debian5:~# tail -7 /etc/network/interfaces
iface bond0 inet static
address 192.168.1.42
netmask 255.255.255.0
gateway 192.168.1.1
slaves eth1 eth2
bond-mode active-backup
bond_primary eth1
On older version of Debian/Ubuntu you needed to modprobe bonding, but this is no longer
required. Use ifup to bring the interface up, then test that it works.
debian5:~# ifup bond0
debian5:~# ifconfig bond0
bond0
Link encap:Ethernet HWaddr 08:00:27:63:9a:95
inet addr:192.168.1.42 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::a00:27ff:fe63:9a95/64 Scope:Link
UP BROADCAST RUNNING MASTER MULTICAST MTU:1500 Metric:1
RX packets:212 errors:0 dropped:0 overruns:0 frame:0
TX packets:39 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:31978 (31.2 KiB) TX bytes:6709 (6.5 KiB)
The bond should also be visible in /proc/net/bonding.
debian5:~# cat /proc/net/bonding/bond0
Ethernet Channel Bonding Driver: v3.2.5 (March 21, 2008)
Bonding Mode: fault-tolerance (active-backup)
Primary Slave: eth1
Currently Active Slave: eth1
MII Status: up
MII Polling Interval (ms): 0
Up Delay (ms): 0
Down Delay (ms): 0
Slave Interface: eth1
MII Status: up
Link Failure Count: 0
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binding and bonding
Permanent HW addr: 08:00:27:63:9a:95
Slave Interface: eth2
MII Status: up
Link Failure Count: 0
Permanent HW addr: 08:00:27:27:a4:92
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binding and bonding
59.5. practice: binding and bonding
1. Add an extra ip address to one of your network cards. Test that it works (have your
neighbour ssh to it)!
2. Use ifdown to disable this extra ip address.
3. Make sure your neighbour also succeeded in binding an extra ip address before you
continue.
4. Add an extra network card (or two) to your virtual machine and use the theory to bond
two network cards.
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binding and bonding
59.6. solution: binding and bonding
1. Add an extra ip address to one of your network cards. Test that it works (have your
neighbour ssh to it)!
Redhat/Fedora:
add an /etc/sysconfig/network-scripts/ifcfg-ethX:X file
as shown in the theory
Debian/Ubuntu:
expand the /etc/network/interfaces file
as shown in the theory
2. Use ifdown to disable this extra ip address.
ifdown eth0:0
3. Make sure your neighbour also succeeded in binding an extra ip address before you
continue.
ping $extra_ip_neighbour
or
ssh $extra_ip_neighbour
4. Add an extra network card (or two) to your virtual machine and use the theory to bond
two network cards.
Redhat/Fedora:
add ifcfg-ethX and ifcfg-bondX files in /etc/sysconfig/network-scripts
as shown in the theory
and don't forget the modprobe.conf
Debian/Ubuntu:
expand the /etc/network/interfaces file
as shown in the theory
and don't forget to install the ifenslave package
619
Chapter 60. ssh client and server
The secure shell or ssh is a collection of tools using a secure protocol for communications
with remote Linux computers.
This chapter gives an overview of the most common commands related to the use of the
sshd server and the ssh client.
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ssh client and server
60.1. about ssh
60.1.1. secure shell
Avoid using telnet, rlogin and rsh to remotely connect to your servers. These older protocols
do not encrypt the login session, which means your user id and password can be sniffed by
tools like wireshark or tcpdump. To securely connect to your servers, use ssh.
The ssh protocol is secure in two ways. Firstly the connection is encrypted and secondly
the connection is authenticated both ways.
An ssh connection always starts with a cryptographic handshake, followed by encryption of
the transport layer using a symmetric cypher. In other words, the tunnel is encrypted before
you start typing anything.
Then authentication takes place (using user id/password or public/private keys) and
communication can begin over the encrypted connection.
The ssh protocol will remember the servers it connected to (and warn you in case something
suspicious happened).
The openssh package is maintained by the OpenBSD people and is distributed with a lot of
operating systems (it may even be the most popular package in the world).
60.1.2. /etc/ssh/
Configuration of ssh client and server is done in the /etc/ssh directory. In the next sections
we will discuss most of the files found in /etc/ssh/.
60.1.3. ssh protocol versions
The ssh protocol has two versions (1 and 2). Avoid using version 1 anywhere, since it
contains some known vulnerabilities. You can control the protocol version via /etc/ssh/
ssh_config for the client side and /etc/ssh/sshd_config for the openssh-server daemon.
paul@ubu1204:/etc/ssh$ grep Protocol ssh_config
#
Protocol 2,1
paul@ubu1204:/etc/ssh$ grep Protocol sshd_config
Protocol 2
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ssh client and server
60.1.4. public and private keys
The ssh protocol uses the well known system of public and private keys. The below
explanation is succinct, more information can be found on wikipedia.
http://en.wikipedia.org/wiki/Public-key_cryptography
Imagine Alice and Bob, two people that like to communicate with each other. Using public
and private keys they can communicate with encryption and with authentication.
When Alice wants to send an encrypted message to Bob, she uses the public key of Bob.
Bob shares his public key with Alice, but keeps his private key private! Since Bob is the
only one to have Bob's private key, Alice is sure that Bob is the only one that can read the
encrypted message.
When Bob wants to verify that the message came from Alice, Bob uses the public key of
Alice to verify that Alice signed the message with her private key. Since Alice is the only
one to have Alice's private key, Bob is sure the message came from Alice.
60.1.5. rsa and dsa algorithms
This chapter does not explain the technical implementation of cryptographic algorithms,
it only explains how to use the ssh tools with rsa and dsa. More information about these
algorithms can be found here:
http://en.wikipedia.org/wiki/RSA_(algorithm)
http://en.wikipedia.org/wiki/Digital_Signature_Algorithm
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ssh client and server
60.2. log on to a remote server
The following screenshot shows how to use ssh to log on to a remote computer running
Linux. The local user is named paul and he is logging on as user admin42 on the remote
system.
paul@ubu1204:~$ ssh [email protected]
The authenticity of host '192.168.1.30 (192.168.1.30)' can't be established.
RSA key fingerprint is b5:fb:3c:53:50:b4:ab:81:f3:cd:2e:bb:ba:44:d3:75.
Are you sure you want to continue connecting (yes/no)?
As you can see, the user paul is presented with an rsa authentication fingerprint from the
remote system. The user can accepts this bu typing yes. We will see later that an entry will
be added to the ~/.ssh/known_hosts file.
paul@ubu1204:~$ ssh [email protected]
The authenticity of host '192.168.1.30 (192.168.1.30)' can't be established.
RSA key fingerprint is b5:fb:3c:53:50:b4:ab:81:f3:cd:2e:bb:ba:44:d3:75.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '192.168.1.30' (RSA) to the list of known hosts.
[email protected]'s password:
Welcome to Ubuntu 12.04 LTS (GNU/Linux 3.2.0-26-generic-pae i686)
* Documentation:
https://help.ubuntu.com/
1 package can be updated.
0 updates are security updates.
Last login: Wed Jun 6 19:25:57 2012 from 172.28.0.131
admin42@ubuserver:~$
The user can get log out of the remote server by typing exit or by using Ctrl-d.
admin42@ubuserver:~$ exit
logout
Connection to 192.168.1.30 closed.
paul@ubu1204:~$
60.3. executing a command in remote
This screenshot shows how to execute the pwd command on the remote server. There is no
need to exit the server manually.
paul@ubu1204:~$ ssh [email protected] pwd
[email protected]'s password:
/home/admin42
paul@ubu1204:~$
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ssh client and server
60.4. scp
The scp command works just like cp, but allows the source and destination of the copy to
be behind ssh. Here is an example where we copy the /etc/hosts file from the remote server
to the home directory of user paul.
paul@ubu1204:~$ scp [email protected]:/etc/hosts /home/paul/serverhosts
[email protected]'s password:
hosts
100% 809
0.8KB/s
00:00
Here is an example of the reverse, copying a local file to a remote server.
paul@ubu1204:~$ scp ~/serverhosts [email protected]:/etc/hosts.new
[email protected]'s password:
serverhosts
100% 809
0.8KB/s
00:00
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ssh client and server
60.5. setting up passwordless ssh
To set up passwordless ssh authentication through public/private keys, use ssh-keygen to
generate a key pair without a passphrase, and then copy your public key to the destination
server. Let's do this step by step.
In the example that follows, we will set up ssh without password between Alice and Bob.
Alice has an account on a Red Hat Enterprise Linux server, Bob is using Ubuntu on his
laptop. Bob wants to give Alice access using ssh and the public and private key system. This
means that even if Bob changes his password on his laptop, Alice will still have access.
60.5.1. ssh-keygen
The example below shows how Alice uses ssh-keygen to generate a key pair. Alice does
not enter a passphrase.
[alice@RHEL5 ~]$ ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/alice/.ssh/id_rsa):
Created directory '/home/alice/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/alice/.ssh/id_rsa.
Your public key has been saved in /home/alice/.ssh/id_rsa.pub.
The key fingerprint is:
9b:ac:ac:56:c2:98:e5:d9:18:c4:2a:51:72:bb:45:eb alice@RHEL5
[alice@RHEL5 ~]$
You can use ssh-keygen -t dsa in the same way.
60.5.2. ~/.ssh
While ssh-keygen generates a public and a private key, it will also create a hidden .ssh
directory with proper permissions. If you create the .ssh directory manually, then you need
to chmod 700 it! Otherwise ssh will refuse to use the keys (world readable private keys are
not secure!).
As you can see, the .ssh directory is secure in Alice's home directory.
[alice@RHEL5 ~]$ ls -ld .ssh
drwx------ 2 alice alice 4096 May
[alice@RHEL5 ~]$
1 07:38 .ssh
Bob is using Ubuntu at home. He decides to manually create the .ssh directory, so he needs
to manually secure it.
bob@laika:~$
bob@laika:~$
drwxr-xr-x 2
bob@laika:~$
bob@laika:~$
mkdir .ssh
ls -ld .ssh
bob bob 4096 2008-05-14 16:53 .ssh
chmod 700 .ssh/
60.5.3. id_rsa and id_rsa.pub
The ssh-keygen command generate two keys in .ssh. The public key is named ~/.ssh/
id_rsa.pub. The private key is named ~/.ssh/id_rsa.
625
ssh client and server
[alice@RHEL5 ~]$ ls -l .ssh/
total 16
-rw------- 1 alice alice 1671 May
-rw-r--r-- 1 alice alice 393 May
1 07:38 id_rsa
1 07:38 id_rsa.pub
The files will be named id_dsa and id_dsa.pub when using dsa instead of rsa.
60.5.4. copy the public key to the other computer
To copy the public key from Alice's server tot Bob's laptop, Alice decides to use scp.
[alice@RHEL5 .ssh]$ scp id_rsa.pub [email protected]:~/.ssh/authorized_keys
[email protected]'s password:
id_rsa.pub
100% 393
0.4KB/s
00:00
Be careful when copying a second key! Do not overwrite the first key, instead append the
key to the same ~/.ssh/authorized_keys file!
cat id_rsa.pub >> ~/.ssh/authorized_keys
Alice could also have used ssh-copy-id like in this example.
ssh-copy-id -i .ssh/id_rsa.pub [email protected]
60.5.5. authorized_keys
In your ~/.ssh directory, you can create a file called authorized_keys. This file can contain
one or more public keys from people you trust. Those trusted people can use their private
keys to prove their identity and gain access to your account via ssh (without password). The
example shows Bob's authorized_keys file containing the public key of Alice.
bob@laika:~$ cat .ssh/authorized_keys
ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEApCQ9xzyLzJes1sR+hPyqW2vyzt1D4zTLqk\
MDWBR4mMFuUZD/O583I3Lg/Q+JIq0RSksNzaL/BNLDou1jMpBe2Dmf/u22u4KmqlJBfDhe\
yTmGSBzeNYCYRSMq78CT9l9a+y6x/shucwhaILsy8A2XfJ9VCggkVtu7XlWFDL2cum08/0\
mRFwVrfc/uPsAn5XkkTscl4g21mQbnp9wJC40pGSJXXMuFOk8MgCb5ieSnpKFniAKM+tEo\
/vjDGSi3F/bxu691jscrU0VUdIoOSo98HUfEf7jKBRikxGAC7I4HLa+/zX73OIvRFAb2hv\
tUhn6RHrBtUJUjbSGiYeFTLDfcTQ== alice@RHEL5
60.5.6. passwordless ssh
Alice can now use ssh to connect passwordless to Bob's laptop. In combination with ssh's
capability to execute commands on the remote host, this can be useful in pipes across
different machines.
[alice@RHEL5 ~]$ ssh [email protected] "ls -l .ssh"
total 4
-rw-r--r-- 1 bob bob 393 2008-05-14 17:03 authorized_keys
[alice@RHEL5 ~]$
60.6. X forwarding via ssh
Another popular feature of ssh is called X11 forwarding and is implemented with ssh -X.
Below an example of X forwarding: user paul logs in as user greet on her computer to start the
graphical application mozilla-thunderbird. Although the application will run on the remote
computer from greet, it will be displayed on the screen attached locally to paul's computer.
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ssh client and server
paul@debian5:~/PDF$ ssh -X [email protected] -p 55555
Warning: Permanently added the RSA host key for IP address \
'81.240.174.161' to the list of known hosts.
Password:
Linux raika 2.6.8-2-686 #1 Tue Aug 16 13:22:48 UTC 2005 i686 GNU/Linux
Last login: Thu Jan 18 12:35:56 2007
greet@raika:~$ ps fax | grep thun
greet@raika:~$ mozilla-thunderbird &
[1] 30336
60.7. troubleshooting ssh
Use ssh -v to get debug information about the ssh connection attempt.
paul@debian5:~$ ssh -v [email protected]
OpenSSH_4.3p2 Debian-8ubuntu1, OpenSSL 0.9.8c 05 Sep 2006
debug1: Reading configuration data /home/paul/.ssh/config
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Applying options for *
debug1: Connecting to 192.168.1.192 [192.168.1.192] port 22.
debug1: Connection established.
debug1: identity file /home/paul/.ssh/identity type -1
debug1: identity file /home/paul/.ssh/id_rsa type 1
debug1: identity file /home/paul/.ssh/id_dsa type -1
debug1: Remote protocol version 1.99, remote software version OpenSSH_3
debug1: match: OpenSSH_3.9p1 pat OpenSSH_3.*
debug1: Enabling compatibility mode for protocol 2.0
...
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ssh client and server
60.8. sshd
The ssh server is called sshd and is provided by the openssh-server package.
root@ubu1204~# dpkg -l openssh-server | tail -1
ii openssh-server
1:5.9p1-5ubuntu1
secure shell (SSH) server,...
60.9. sshd keys
The public keys used by the sshd server are located in /etc/ssh and are world readable. The
private keys are only readable by root.
root@ubu1204~# ls
-rw------- 1 root
-rw-r--r-- 1 root
-rw------- 1 root
-rw-r--r-- 1 root
-l /etc/ssh/ssh_host_*
root 668 Jun 7 2011
root 598 Jun 7 2011
root 1679 Jun 7 2011
root 390 Jun 7 2011
/etc/ssh/ssh_host_dsa_key
/etc/ssh/ssh_host_dsa_key.pub
/etc/ssh/ssh_host_rsa_key
/etc/ssh/ssh_host_rsa_key.pub
60.10. ssh-agent
When generating keys with ssh-keygen, you have the option to enter a passphrase to protect
access to the keys. To avoid having to type this passphrase every time, you can add the key
to ssh-agent using ssh-add.
Most Linux distributions will start the ssh-agent automatically when you log on.
root@ubu1204~# ps -ef | grep ssh-agent
paul
2405 2365 0 08:13 ?
00:00:00 /usr/bin/ssh-agent...
This clipped screenshot shows how to use ssh-add to list the keys that are currently added
to the ssh-agent
paul@debian5:~$ ssh-add -L
ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAvgI+Vx5UrIsusZPl8da8URHGsxG7yivv3/\
...
wMGqa48Kelwom8TGb4Sgcwpp/VO/ldA5m+BGCw== paul@deb503
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ssh client and server
60.11. practice: ssh
0. Make sure that you have access to two Linux computers, or work together with a partner
for this exercise. For this practice, we will name one of the machines the server.
1. Install sshd on the server
2. Verify in the ssh configuration files that only protocol version 2 is allowed.
3. Use ssh to log on to the server, show your current directory and then exit the server.
4. Use scp to copy a file from your computer to the server.
5. Use scp to copy a file from the server to your computer.
6. (optional, only works when you have a graphical install of Linux) Install the xeyes package
on the server and use ssh to run xeyes on the server, but display it on your client.
7. (optional, same as previous) Create a bookmark in firefox, then quit firefox on client and
server. Use ssh -X to run firefox on your display, but on your neighbour's computer. Do you
see your neighbour's bookmark ?
8. Use ssh-keygen to create a key pair without passphrase. Setup passwordless ssh between
you and your neighbour. (or between your client and your server)
9.Verify that the permissions on the server key files are correct; world readable for the public
keys and only root access for the private keys.
10. Verify that the ssh-agent is running.
11. (optional) Protect your keypair with a passphrase, then add this key to the ssh-agent
and test your passwordless ssh to the server.
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ssh client and server
60.12. solution: ssh
0. Make sure that you have access to two Linux computers, or work together with a partner
for this exercise. For this practice, we will name one of the machines the server.
1. Install sshd on the server
apt-get install openssh-server (on Ubuntu/Debian)
yum -y install openssh-server (on Centos/Fedora/Red Hat)
2. Verify in the ssh configuration files that only protocol version 2 is allowed.
grep Protocol /etc/ssh/ssh*_config
3. Use ssh to log on to the server, show your current directory and then exit the server.
user@client$ ssh user@server-ip-address
user@server$ pwd
/home/user
user@server$ exit
4. Use scp to copy a file from your computer to the server.
scp localfile user@server:~
5. Use scp to copy a file from the server to your computer.
scp user@server:~/serverfile .
6. (optional, only works when you have a graphical install of Linux) Install the xeyes package
on the server and use ssh to run xeyes on the server, but display it on your client.
on the server:
apt-get install xeyes
on the client:
ssh -X user@server-ip
xeyes
7. (optional, same as previous) Create a bookmark in firefox, then quit firefox on client and
server. Use ssh -X to run firefox on your display, but on your neighbour's computer. Do you
see your neighbour's bookmark ?
8. Use ssh-keygen to create a key pair without passphrase. Setup passwordless ssh between
you and your neighbour. (or between your client and your server)
See solution in book "setting up passwordless ssh"
9. Verify that the permissions on the server key files are correct; world readable for the
public keys and only root access for the private keys.
ls -l /etc/ssh/ssh_host_*
10. Verify that the ssh-agent is running.
ps fax | grep ssh-agent
11. (optional) Protect your keypair with a passphrase, then add this key to the ssh-agent
and test your passwordless ssh to the server.
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ssh client and server
man ssh-keygen
man ssh-agent
man ssh-add
631
Chapter 61. introduction to nfs
The network file system (or simply nfs) enables us since the Eighties to share a directory
with other computers on the network.
In this chapter we see how to setup an nfs server and an nfs client computer.
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introduction to nfs
61.1. nfs protocol versions
The older nfs versions 2 and 3 are stateless (udp) by default (but they can use tcp). The more
recent nfs version 4 brings a stateful protocol with better performance and stronger security.
NFS version 4 was defined in rfc 3010 in 2000 and rfc 3530 in 2003 and requires tcp (port
2049). It also supports Kerberos user authentication as an option when mounting a share.
NFS versions 2 and 3 authenticate only the host.
61.2. rpcinfo
Clients connect to the server using rpc (on Linux this can be managed by the portmap
daemon). Look at rpcinfo to verify that nfs and its related services are running.
root@RHELv4u2:~# /etc/init.d/portmap status
portmap (pid 1920) is running...
root@RHELv4u2:~# rpcinfo -p
program vers proto
port
100000
2
tcp
111 portmapper
100000
2
udp
111 portmapper
100024
1
udp 32768 status
100024
1
tcp 32769 status
root@RHELv4u2:~# service nfs start
Starting NFS services:
Starting NFS quotas:
Starting NFS daemon:
Starting NFS mountd:
The same rpcinfo command when nfs is started.
root@RHELv4u2:~# rpcinfo -p
program vers proto
port
100000
2
tcp
111 portmapper
100000
2
udp
111 portmapper
100024
1
udp 32768 status
100024
1
tcp 32769 status
100011
1
udp
985 rquotad
100011
2
udp
985 rquotad
100011
1
tcp
988 rquotad
100011
2
tcp
988 rquotad
100003
2
udp
2049 nfs
100003
3
udp
2049 nfs
100003
4
udp
2049 nfs
100003
2
tcp
2049 nfs
100003
3
tcp
2049 nfs
100003
4
tcp
2049 nfs
100021
1
udp 32770 nlockmgr
100021
3
udp 32770 nlockmgr
100021
4
udp 32770 nlockmgr
100021
1
tcp 32789 nlockmgr
100021
3
tcp 32789 nlockmgr
100021
4
tcp 32789 nlockmgr
100005
1
udp
1004 mountd
100005
1
tcp
1007 mountd
100005
2
udp
1004 mountd
100005
2
tcp
1007 mountd
100005
3
udp
1004 mountd
100005
3
tcp
1007 mountd
633
[
[
[
[
OK
OK
OK
OK
]
]
]
]
introduction to nfs
61.3. server configuration
nfs is configured in /etc/exports. You might want some way (ldap?) to synchronize userid's
across computers when using nfs a lot.
The rootsquash option will change UID 0 to the UID of a nobody (or similar) user account.
The sync option will write writes to disk before completing the client request.
61.4. /etc/exports
Here is a sample /etc/exports to explain the syntax:
paul@laika:~$ cat /etc/exports
# Everyone can read this share
/mnt/data/iso *(ro)
# Only the computers named pasha and barry can readwrite this one
/var/www pasha(rw) barry(rw)
# same, but without root squashing for barry
/var/ftp pasha(rw) barry(rw,no_root_squash)
# everyone from the netsec.local domain gets access
/var/backup
*.netsec.local(rw)
# ro for one network, rw for the other
/var/upload
192.168.1.0/24(ro) 192.168.5.0/24(rw)
More recent incarnations of nfs require the subtree_check option to be explicitly set (or
unset with no_subtree_check). The /etc/exports file then looks like this:
root@debian6 ~# cat /etc/exports
# Everyone can read this share
/srv/iso *(ro,no_subtree_check)
# Only the computers named pasha and barry can readwrite this one
/var/www pasha(rw,no_subtree_check) barry(rw,no_subtree_check)
# same, but without root squashing for barry
/var/ftp pasha(rw,no_subtree_check) barry(rw,no_root_squash,no_subtree_check)
61.5. exportfs
You don't need to restart the nfs server to start exporting your newly created exports. You
can use the exportfs -va command to do this. It will write the exported directories to /var/
lib/nfs/etab, where they are immediately applied.
root@debian6 ~# exportfs -va
exporting pasha:/var/ftp
exporting barry:/var/ftp
exporting pasha:/var/www
exporting barry:/var/www
exporting *:/srv/iso
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introduction to nfs
61.6. client configuration
We have seen the mount command and the /etc/fstab file before.
root@RHELv4u2:~# mount -t nfs barry:/mnt/data/iso /home/project55/
root@RHELv4u2:~# cat /etc/fstab | grep nfs
barry:/mnt/data/iso
/home/iso
nfs
defaults
0 0
root@RHELv4u2:~#
Here is another simple example. Suppose the project55 people tell you they only need a
couple of CD-ROM images, and you already have them available on an nfs server. You
could issue the following command to mount this storage on their /home/project55 mount
point.
root@RHELv4u2:~# mount -t nfs 192.168.1.40:/mnt/data/iso /home/project55/
root@RHELv4u2:~# ls -lh /home/project55/
total 3.6G
drwxr-xr-x 2 1000 1000 4.0K Jan 16 17:55 RHELv4u1
drwxr-xr-x 2 1000 1000 4.0K Jan 16 14:14 RHELv4u2
drwxr-xr-x 2 1000 1000 4.0K Jan 16 14:54 RHELv4u3
drwxr-xr-x 2 1000 1000 4.0K Jan 16 11:09 RHELv4u4
-rw-r--r-- 1 root root 1.6G Oct 13 15:22 sled10-vmwarews5-vm.zip
root@RHELv4u2:~#
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introduction to nfs
61.7. practice: introduction to nfs
1. Create two directories with some files. Use nfs to share one of them as read only, the other
must be writable. Have your neighbour connect to them to test.
2. Investigate the user owner of the files created by your neighbour.
3. Protect a share by ip-address or hostname, so only your neighbour can connect.
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Chapter 62. introduction to
networking
637
introduction to networking
62.1. introduction to iptables
62.1.1. iptables firewall
The Linux kernel has a built-in stateful firewall named iptables. To stop the iptables firewall
on Red Hat, use the service command.
root@RHELv4u4:~# service iptables stop
Flushing firewall rules:
Setting chains to policy ACCEPT: filter
Unloading iptables modules:
root@RHELv4u4:~#
[
[
[
OK
OK
OK
]
]
]
The easy way to configure iptables, is to use a graphical tool like KDE's kmyfirewall
or Security Level Configuration Tool. You can find the latter in the graphical menu,
somewhere in System Tools - Security, or you can start it by typing system-configsecuritylevel in bash. These tools allow for some basic firewall configuration. You can
decide whether to enable or disable the firewall, and what typical standard ports are allowed
when the firewall is active. You can even add some custom ports. When you are done, the
configuration is written to /etc/sysconfig/iptables on Red Hat.
root@RHELv4u4:~# cat /etc/sysconfig/iptables
# Firewall configuration written by system-config-securitylevel
# Manual customization of this file is not recommended.
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:RH-Firewall-1-INPUT - [0:0]
-A INPUT -j RH-Firewall-1-INPUT
-A FORWARD -j RH-Firewall-1-INPUT
-A RH-Firewall-1-INPUT -i lo -j ACCEPT
-A RH-Firewall-1-INPUT -p icmp --icmp-type any -j ACCEPT
-A RH-Firewall-1-INPUT -p 50 -j ACCEPT
-A RH-Firewall-1-INPUT -p 51 -j ACCEPT
-A RH-Firewall-1-INPUT -p udp --dport 5353 -d 224.0.0.251 -j ACCEPT
-A RH-Firewall-1-INPUT -p udp -m udp --dport 631 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
-A RH-F...NPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT
-A RH-F...NPUT -m state --state NEW -m tcp -p tcp --dport 80 -j ACCEPT
-A RH-F...NPUT -m state --state NEW -m tcp -p tcp --dport 21 -j ACCEPT
-A RH-F...NPUT -m state --state NEW -m tcp -p tcp --dport 25 -j ACCEPT
-A RH-Firewall-1-INPUT -j REJECT --reject-with icmp-host-prohibited
COMMIT
root@RHELv4u4:~#
To start the service, issue the service iptables start command. You can configure iptables
to start at boot time with chkconfig.
root@RHELv4u4:~# service iptables start
Applying iptables firewall rules:
root@RHELv4u4:~# chkconfig iptables on
root@RHELv4u4:~#
638
[
OK
]
introduction to networking
One of the nice features of iptables is that it displays extensive status information when
queried with the service iptables status command.
root@RHELv4u4:~# service iptables status
Table: filter
Chain INPUT (policy ACCEPT)
target
prot opt source
destination
RH-Firewall-1-INPUT all -- 0.0.0.0/0
0.0.0.0/0
Chain FORWARD (policy ACCEPT)
target
prot opt source
RH-Firewall-1-INPUT all -- 0.0.0.0/0
destination
0.0.0.0/0
Chain OUTPUT (policy ACCEPT)
target
prot opt source
destination
Chain RH-Firewall-1-INPUT (2
target prot opt source
ACCEPT all -- 0.0.0.0/0
ACCEPT icmp -- 0.0.0.0/0
ACCEPT esp -- 0.0.0.0/0
ACCEPT ah
-- 0.0.0.0/0
ACCEPT udp -- 0.0.0.0/0
ACCEPT udp -- 0.0.0.0/0
ACCEPT all -- 0.0.0.0/0
ACCEPT tcp -- 0.0.0.0/0
ACCEPT tcp -- 0.0.0.0/0
ACCEPT tcp -- 0.0.0.0/0
ACCEPT tcp -- 0.0.0.0/0
REJECT all -- 0.0.0.0/0
references)
destination
0.0.0.0/0
0.0.0.0/0
0.0.0.0/0
0.0.0.0/0
224.0.0.251
0.0.0.0/0
0.0.0.0/0
0.0.0.0/0
0.0.0.0/0
0.0.0.0/0
0.0.0.0/0
0.0.0.0/0
icmp type 255
udp dpt:5353
udp dpt:631
state RELATED,ESTABLISHED
state NEW tcp dpt:22
state NEW tcp dpt:80
state NEW tcp dpt:21
state NEW tcp dpt:25
reject-with icmp-host-prohibited
root@RHELv4u4:~#
Mastering firewall configuration requires a decent knowledge of tcp/ip. Good iptables
tutorials can be found online here http://iptables-tutorial.frozentux.net/iptables-tutorial.html
and here http://tldp.org/HOWTO/IP-Masquerade-HOWTO/.
62.2. practice : iptables
1. Verify whether the firewall is running.
2. Stop the running firewall.
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introduction to networking
62.3. solution : iptables
1. Verify whether the firewall is running.
root@rhel55 ~# service iptables status | head
Table: filter
Chain INPUT (policy ACCEPT)
num target
prot opt source
destination
1
RH-Firewall-1-INPUT all -- 0.0.0.0/0
0.0.0.0/0
Chain FORWARD (policy ACCEPT)
num target
prot opt source
1
RH-Firewall-1-INPUT all --
0.0.0.0/0
destination
0.0.0.0/0
Chain OUTPUT (policy ACCEPT)
2. Stop the running firewall.
root@rhel55 ~# service iptables stop
Flushing firewall rules:
Setting chains to policy ACCEPT: filter
Unloading iptables modules:
root@rhel55 ~# service iptables status
Firewall is stopped.
640
[
[
[
OK
OK
OK
]
]
]
introduction to networking
62.4. xinetd and inetd
62.4.1. the superdaemon
Back when resources like RAM memory were limited, a super-server was devised to listen
to all sockets and start the appropriate daemon only when needed. Services like swat, telnet
and ftp are typically served by such a super-server. The xinetd superdaemon is more recent
than inetd. We will discuss the configuration both daemons.
Recent Linux distributions like RHEL5 and Ubuntu10.04 do not activate inetd or xinetd by
default, unless an application requires it.
62.4.2. inetd or xinetd
First verify whether your computer is running inetd or xinetd. This Debian 4.0 Etch is
running inetd.
root@barry:~# ps fax | grep inet
3870 ?
Ss
0:00 /usr/sbin/inetd
This Red Hat Enterprise Linux 4 update 4 is running xinetd.
[root@RHEL4b ~]# ps fax | grep inet
3003 ?
Ss
0:00 xinetd -stayalive -pidfile /var/run/xinetd.pid
Both daemons have the same functionality (listening to many ports, starting other daemons
when they are needed), but they have different configuration files.
62.4.3. xinetd superdaemon
The xinetd daemon is often called a superdaemon because it listens to a lot of incoming
connections, and starts other daemons when they are needed. When a connection request
is received, xinetd will first check TCP wrappers (/etc/hosts.allow and /etc/hosts.deny) and
then give control of the connection to the other daemon. This superdaemon is configured
through /etc/xinetd.conf and the files in the directory /etc/xinetd.d. Let's first take a look
at /etc/xinetd.conf.
paul@RHELv4u2:~$ cat /etc/xinetd.conf
#
# Simple configuration file for xinetd
#
# Some defaults, and include /etc/xinetd.d/
defaults
{
instances
log_type
log_on_success
log_on_failure
cps
=
=
=
=
=
60
SYSLOG authpriv
HOST PID
HOST
25 30
641
introduction to networking
}
includedir /etc/xinetd.d
paul@RHELv4u2:~$
According to the settings in this file, xinetd can handle 60 client requests at once. It uses the
authpriv facility to log the host ip-address and pid of successful daemon spawns. When a
service (aka protocol linked to daemon) gets more than 25 cps (connections per second), it
holds subsequent requests for 30 seconds.
The directory /etc/xinetd.d contains more specific configuration files. Let's also take a look
at one of them.
paul@RHELv4u2:~$ ls /etc/xinetd.d
amanda
chargen-udp echo
klogin
rexec
talk
amandaidx cups-lpd
echo-udp krb5-telnet rlogin telnet
amidxtape daytime
eklogin
kshell
rsh
tftp
auth
daytime-udp finger
ktalk
rsync
time
chargen
dbskkd-cdb
gssftp
ntalk
swat
time-udp
paul@RHELv4u2:~$ cat /etc/xinetd.d/swat
# default: off
# description: SWAT is the Samba Web Admin Tool. Use swat \
#
to configure your Samba server. To use SWAT, \
#
connect to port 901 with your favorite web browser.
service swat
{
port
= 901
socket_type
= stream
wait
= no
only_from
= 127.0.0.1
user
= root
server
= /usr/sbin/swat
log_on_failure += USERID
disable
= yes
}
paul@RHELv4u2:~$
The services should be listed in the /etc/services file. Port determines the service port, and
must be the same as the port specified in /etc/services. The socket_type should be set to
stream for tcp services (and to dgram for udp). The log_on_failure += concats the userid
to the log message formatted in /etc/xinetd.conf. The last setting disable can be set to yes
or no. Setting this to no means the service is enabled!
Check the xinetd and xinetd.conf manual pages for many more configuration options.
62.4.4. inetd superdaemon
This superdaemon has only one configuration file /etc/inetd.conf. Every protocol or daemon
that it is listening for, gets one line in this file.
root@barry:~# grep ftp /etc/inetd.conf
tftp dgram udp wait nobody /usr/sbin/tcpd /usr/sbin/in.tftpd /boot/tftp
root@barry:~#
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introduction to networking
You can disable a service in inetd.conf above by putting a # at the start of that line. Here an
example of the disabled vmware web interface (listening on tcp port 902).
paul@laika:~$ grep vmware /etc/inetd.conf
#902 stream tcp nowait root /usr/sbin/vmware-authd vmware-authd
62.5. practice : inetd and xinetd
1. Verify on all systems whether they are using xinetd or inetd.
2. Look at the configuration files.
3. (If telnet is installable, then replace swat in these questions with telnet) Is swat installed ?
If not, then install swat and look at the changes in the (x)inetd configuration. Is swat enabled
or disabled ?
4. Disable swat, test it. Enable swat, test it.
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introduction to networking
62.6. network file system
62.6.1. protocol versions
The older nfs versions 2 and 3 are stateless (udp) by default, but they can use tcp. Clients
connect to the server using rpc (on Linux this is controlled by the portmap daemon. Look
at rpcinfo to verify that nfs and its related services are running.
root@RHELv4u2:~# /etc/init.d/portmap status
portmap (pid 1920) is running...
root@RHELv4u2:~# rpcinfo -p
program vers proto
port
100000
2
tcp
111 portmapper
100000
2
udp
111 portmapper
100024
1
udp 32768 status
100024
1
tcp 32769 status
root@RHELv4u2:~# service nfs start
Starting NFS services:
Starting NFS quotas:
Starting NFS daemon:
Starting NFS mountd:
[
[
[
[
OK
OK
OK
OK
]
]
]
]
The same rpcinfo command when nfs is started.
root@RHELv4u2:~# rpcinfo -p
program vers proto
port
100000
2
tcp
111 portmapper
100000
2
udp
111 portmapper
100024
1
udp 32768 status
100024
1
tcp 32769 status
100011
1
udp
985 rquotad
100011
2
udp
985 rquotad
100011
1
tcp
988 rquotad
100011
2
tcp
988 rquotad
100003
2
udp
2049 nfs
100003
3
udp
2049 nfs
100003
4
udp
2049 nfs
100003
2
tcp
2049 nfs
100003
3
tcp
2049 nfs
100003
4
tcp
2049 nfs
100021
1
udp 32770 nlockmgr
100021
3
udp 32770 nlockmgr
100021
4
udp 32770 nlockmgr
100021
1
tcp 32789 nlockmgr
100021
3
tcp 32789 nlockmgr
100021
4
tcp 32789 nlockmgr
100005
1
udp
1004 mountd
100005
1
tcp
1007 mountd
100005
2
udp
1004 mountd
100005
2
tcp
1007 mountd
100005
3
udp
1004 mountd
100005
3
tcp
1007 mountd
root@RHELv4u2:~#
nfs version 4 requires tcp (port 2049) and supports Kerberos user authentication as an
option. nfs authentication only takes place when mounting the share. nfs versions 2 and 3
authenticate only the host.
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introduction to networking
62.6.2. server configuration
nfs is configured in /etc/exports. Here is a sample /etc/exports to explain the syntax. You
need some way (NIS domain or LDAP) to synchronize userid's across computers when
using nfs a lot. The rootsquash option will change UID 0 to the UID of the nfsnobody user
account. The sync option will write writes to disk before completing the client request.
paul@laika:~$ cat /etc/exports
# Everyone can read this share
/mnt/data/iso *(ro)
# Only the computers barry and pasha can readwrite this one
/var/www pasha(rw) barry(rw)
# same, but without root squashing for barry
/var/ftp pasha(rw) barry(rw,no_root_squash)
# everyone from the netsec.lan domain gets access
/var/backup
*.netsec.lan(rw)
# ro for one network, rw for the other
/var/upload
192.168.1.0/24(ro) 192.168.5.0/24(rw)
You don't need to restart the nfs server to start exporting your newly created exports. You
can use the exportfs -va command to do this. It will write the exported directories to /var/
lib/nfs/etab, where they are immediately applied.
62.6.3. client configuration
We have seen the mount command and the /etc/fstab file before.
root@RHELv4u2:~# mount -t nfs barry:/mnt/data/iso /home/project55/
root@RHELv4u2:~# cat /etc/fstab | grep nfs
barry:/mnt/data/iso
/home/iso
nfs
defaults
0 0
root@RHELv4u2:~#
Here is another simple example. Suppose the project55 people tell you they only need a
couple of CD-ROM images, and you already have them available on an nfs server. You
could issue the following command to mount this storage on their /home/project55 mount
point.
root@RHELv4u2:~# mount -t nfs 192.168.1.40:/mnt/data/iso /home/project55/
root@RHELv4u2:~# ls -lh /home/project55/
total 3.6G
drwxr-xr-x 2 1000 1000 4.0K Jan 16 17:55 RHELv4u1
drwxr-xr-x 2 1000 1000 4.0K Jan 16 14:14 RHELv4u2
drwxr-xr-x 2 1000 1000 4.0K Jan 16 14:54 RHELv4u3
drwxr-xr-x 2 1000 1000 4.0K Jan 16 11:09 RHELv4u4
-rw-r--r-- 1 root root 1.6G Oct 13 15:22 sled10-vmwarews5-vm.zip
root@RHELv4u2:~#
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introduction to networking
62.7. practice : network file system
1. Create two directories with some files. Use nfs to share one of them as read only, the other
must be writable. Have your neighbour connect to them to test.
2. Investigate the user owner of the files created by your neighbour.
3. Protect a share by ip-address or hostname, so only your neighbour can connect.
646
Part XV. kernel management
Table of Contents
63. the Linux kernel ............................................................................................................
63.1. about the Linux kernel ...........................................................................................
63.2. Linux kernel source ...............................................................................................
63.3. kernel boot files ....................................................................................................
63.4. Linux kernel modules ............................................................................................
63.5. compiling a kernel ................................................................................................
63.6. compiling one module ...........................................................................................
64. library management .......................................................................................................
64.1. introduction .........................................................................................................
64.2. /lib and /usr/lib .....................................................................................................
64.3. ldd .....................................................................................................................
64.4. ltrace ..................................................................................................................
64.5. dpkg -S and debsums ............................................................................................
64.6. rpm -qf and rpm -V ..............................................................................................
64.7. tracing with strace .................................................................................................
648
649
650
653
657
659
664
667
669
670
670
670
671
671
672
673
Chapter 63. the Linux kernel
649
the Linux kernel
63.1. about the Linux kernel
63.1.1. kernel versions
In 1991 Linux Torvalds wrote (the first version of) the Linux kernel. He put it online, and
other people started contributing code. Over 4000 individuals contributed source code to the
latest kernel release (version 2.6.27 in November 2008).
Major Linux kernel versions used to come in even and odd numbers. Versions 2.0, 2.2, 2.4
and 2.6 are considered stable kernel versions. Whereas 2.1, 2.3 and 2.5 were unstable (read
development) versions. Since the release of 2.6.0 in January 2004, all development has been
done in the 2.6 tree. There is currently no v2.7.x and according to Linus the even/stable vs
odd/development scheme is abandoned forever.
63.1.2. uname -r
To see your current Linux kernel version, issue the uname -r command as shown below.
This first example shows Linux major version 2.6 and minor version 24. The rest -22-generic
is specific to the distribution (Ubuntu in this case).
paul@laika:~$ uname -r
2.6.24-22-generic
The same command on Red Hat Enterprise Linux shows an older kernel (2.6.18) with
-92.1.17.el5 being specific to the distribution.
[paul@RHEL52 ~]$ uname -r
2.6.18-92.1.17.el5
63.1.3. /proc/cmdline
The parameters that were passed to the kernel at boot time are in /proc/cmdline.
paul@RHELv4u4:~$ cat /proc/cmdline
ro root=/dev/VolGroup00/LogVol00 rhgb quiet
650
the Linux kernel
63.1.4. single user mode
When booting the kernel with the single parameter, it starts in single user mode. Linux can
start in a bash shell with the root user logged on (without password).
Some distributions prevent the use of this feature (at kernel compile time).
63.1.5. init=/bin/bash
Normally the kernel invokes init as the first daemon process. Adding init=/bin/bash to the
kernel parameters will instead invoke bash (again with root logged on without providing a
password).
63.1.6. /var/log/messages
The kernel reports during boot to syslog which writes a lot of kernel actions in /var/log/
messages. Looking at this file reveals when the kernel was started, including all the devices
that were detected at boot time.
[root@RHEL53 ~]# grep -A16 "syslogd 1.4.1:" /var/log/messages|cut -b24syslogd 1.4.1: restart.
kernel: klogd 1.4.1, log source = /proc/kmsg started.
kernel: Linux version 2.6.18-128.el5 ([email protected]...
kernel: BIOS-provided physical RAM map:
kernel: BIOS-e820: 0000000000000000 - 000000000009f800 (usable)
kernel: BIOS-e820: 000000000009f800 - 00000000000a0000 (reserved)
kernel: BIOS-e820: 00000000000ca000 - 00000000000cc000 (reserved)
kernel: BIOS-e820: 00000000000dc000 - 0000000000100000 (reserved)
kernel: BIOS-e820: 0000000000100000 - 000000001fef0000 (usable)
kernel: BIOS-e820: 000000001fef0000 - 000000001feff000 (ACPI data)
kernel: BIOS-e820: 000000001feff000 - 000000001ff00000 (ACPI NVS)
kernel: BIOS-e820: 000000001ff00000 - 0000000020000000 (usable)
kernel: BIOS-e820: 00000000fec00000 - 00000000fec10000 (reserved)
kernel: BIOS-e820: 00000000fee00000 - 00000000fee01000 (reserved)
kernel: BIOS-e820: 00000000fffe0000 - 0000000100000000 (reserved)
kernel: 0MB HIGHMEM available.
kernel: 512MB LOWMEM available.
This example shows how to use /var/log/messages to see kernel information about /dev/sda.
[root@RHEL53 ~]# grep sda /var/log/messages | cut -b24kernel: SCSI device sda: 41943040 512-byte hdwr sectors (21475 MB)
kernel: sda: Write Protect is off
kernel: sda: cache data unavailable
kernel: sda: assuming drive cache: write through
kernel: SCSI device sda: 41943040 512-byte hdwr sectors (21475 MB)
kernel: sda: Write Protect is off
kernel: sda: cache data unavailable
kernel: sda: assuming drive cache: write through
kernel: sda: sda1 sda2
kernel: sd 0:0:0:0: Attached scsi disk sda
kernel: EXT3 FS on sda1, internal journal
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the Linux kernel
63.1.7. dmesg
The dmesg command prints out all the kernel bootup messages (from the last boot).
[root@RHEL53 ~]# dmesg | head
Linux version 2.6.18-128.el5 ([email protected])
BIOS-provided physical RAM map:
BIOS-e820: 0000000000000000 - 000000000009f800 (usable)
BIOS-e820: 000000000009f800 - 00000000000a0000 (reserved)
BIOS-e820: 00000000000ca000 - 00000000000cc000 (reserved)
BIOS-e820: 00000000000dc000 - 0000000000100000 (reserved)
BIOS-e820: 0000000000100000 - 000000001fef0000 (usable)
BIOS-e820: 000000001fef0000 - 000000001feff000 (ACPI data)
BIOS-e820: 000000001feff000 - 000000001ff00000 (ACPI NVS)
BIOS-e820: 000000001ff00000 - 0000000020000000 (usable)
Thus to find information about /dev/sda, using dmesg will yield only kernel messages from
the last boot.
[root@RHEL53 ~]# dmesg | grep sda
SCSI device sda: 41943040 512-byte hdwr sectors (21475 MB)
sda: Write Protect is off
sda: Mode Sense: 5d 00 00 00
sda: cache data unavailable
sda: assuming drive cache: write through
SCSI device sda: 41943040 512-byte hdwr sectors (21475 MB)
sda: Write Protect is off
sda: Mode Sense: 5d 00 00 00
sda: cache data unavailable
sda: assuming drive cache: write through
sda: sda1 sda2
sd 0:0:0:0: Attached scsi disk sda
EXT3 FS on sda1, internal journal
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the Linux kernel
63.2. Linux kernel source
63.2.1. ftp.kernel.org
The home of the Linux kernel source is ftp.kernel.org. It contains all official releases of
the Linux kernel source code from 1991. It provides free downloads over http, ftp and rsync
of all these releases, as well as changelogs and patches. More information can be otained
on the website www.kernel.org.
Anyone can anonymously use an ftp client to access ftp.kernel.org
paul@laika:~$ ftp ftp.kernel.org
Connected to pub3.kernel.org.
220 Welcome to ftp.kernel.org.
Name (ftp.kernel.org:paul): anonymous
331 Please specify the password.
Password:
230Welcome to the
230230LINUX KERNEL ARCHIVES
230ftp.kernel.org
All the Linux kernel versions are located in the pub/linux/kernel/ directory.
ftp> ls pub/linux/kernel/v*
200 PORT command successful. Consider using PASV.
150 Here comes the directory listing.
drwxrwsr-x
2 536
536
4096 Mar 20 2003 v1.0
drwxrwsr-x
2 536
536
20480 Mar 20 2003 v1.1
drwxrwsr-x
2 536
536
8192 Mar 20 2003 v1.2
drwxrwsr-x
2 536
536
40960 Mar 20 2003 v1.3
drwxrwsr-x
3 536
536
16384 Feb 08 2004 v2.0
drwxrwsr-x
2 536
536
53248 Mar 20 2003 v2.1
drwxrwsr-x
3 536
536
12288 Mar 24 2004 v2.2
drwxrwsr-x
2 536
536
24576 Mar 20 2003 v2.3
drwxrwsr-x
5 536
536
28672 Dec 02 08:14 v2.4
drwxrwsr-x
4 536
536
32768 Jul 14 2003 v2.5
drwxrwsr-x
7 536
536
110592 Dec 05 22:36 v2.6
226 Directory send OK.
ftp>
653
the Linux kernel
63.2.2. /usr/src
On your local computer, the kernel source is located in /usr/src. Note though that the
structure inside /usr/src might be different depending on the distribution that you are using.
First let's take a look at /usr/src on Debian. There appear to be two versions of the complete
Linux source code there. Looking for a specific file (e1000_main.c) with find reveals it's
exact location.
paul@barry:~$ ls -l /usr/src/
drwxr-xr-x 20 root root
4096 2006-04-04 22:12 linux-source-2.6.15
drwxr-xr-x 19 root root
4096 2006-07-15 17:32 linux-source-2.6.16
paul@barry:~$ find /usr/src -name e1000_main.c
/usr/src/linux-source-2.6.15/drivers/net/e1000/e1000_main.c
/usr/src/linux-source-2.6.16/drivers/net/e1000/e1000_main.c
This is very similar to /usr/src on Ubuntu, except there is only one kernel here (and it is
newer).
paul@laika:~$ ls -l /usr/src/
drwxr-xr-x 23 root root
4096 2008-11-24 23:28 linux-source-2.6.24
paul@laika:~$ find /usr/src -name "e1000_main.c"
/usr/src/linux-source-2.6.24/drivers/net/e1000/e1000_main.c
Now take a look at /usr/src on Red Hat Enterprise Linux.
[paul@RHEL52 ~]$ ls -l /usr/src/
drwxr-xr-x 5 root root 4096 Dec 5 19:23 kernels
drwxr-xr-x 7 root root 4096 Oct 11 13:22 redhat
We will have to dig a little deeper to find the kernel source on Red Hat!
[paul@RHEL52 ~]$ cd /usr/src/redhat/BUILD/
[paul@RHEL52 BUILD]$ find . -name "e1000_main.c"
./kernel-2.6.18/linux-2.6.18.i686/drivers/net/e1000/e1000_main.c
654
the Linux kernel
63.2.3. downloading the kernel source
Debian
Installing the kernel source on Debian is really simple with aptitude install linux-source.
You can do a search for all linux-source packeges first, like in this screenshot.
root@barry:~# aptitude search linux-source
v
linux-source
v
linux-source-2.6
id linux-source-2.6.15
- Linux kernel source
i
linux-source-2.6.16
- Linux kernel source
p
linux-source-2.6.18
- Linux kernel source
p
linux-source-2.6.24
- Linux kernel source
for
for
for
for
version
version
version
version
2.6.15
2.6.16
2.6.18
2.6.24
And then use aptitude install to download and install the Debian Linux kernel source code.
root@barry:~# aptitude install linux-source-2.6.24
When the aptitude is finished, you will see a new file named /usr/src/linux-source<version>.tar.bz2
root@barry:/usr/src# ls -lh
drwxr-xr-x 20 root root 4.0K 2006-04-04 22:12 linux-source-2.6.15
drwxr-xr-x 19 root root 4.0K 2006-07-15 17:32 linux-source-2.6.16
-rw-r--r-- 1 root root 45M 2008-12-02 10:56 linux-source-2.6.24.tar.bz2
Ubuntu
Ubuntu is based on Debian and also uses aptitude, so the task is very similar.
root@laika:~# aptitude search linux-source
i
linux-source
- Linux kernel source with Ubuntu patches
v
linux-source-2.6
i A linux-source-2.6.24
- Linux kernel source for version 2.6.24
root@laika:~# aptitude install linux-source
And when aptitude finishes, we end up with a /usr/src/linux-source-<version>.tar.bz file.
oot@laika:~# ll /usr/src
total 45M
-rw-r--r-- 1 root root 45M 2008-11-24 23:30 linux-source-2.6.24.tar.bz2
655
the Linux kernel
Red Hat Enterprise Linux
The Red Hat kernel source is located on the fourth source cdrom. The file is called
kernel-2.6.9-42.EL.src.rpm (example for RHELv4u4). It is also available online at ftp://
ftp.redhat.com/pub/redhat/linux/enterprise/5Server/en/os/SRPMS/ (example for RHEL5).
To download the kernel source on RHEL, use this long wget command (on one line, without
the trailing \).
wget ftp://ftp.redhat.com/pub/redhat/linux/enterprise/5Server/en/os/\
SRPMS/kernel-`uname -r`.src.rpm
When the wget download is finished, you end up with a 60M .rpm file.
[root@RHEL52
total 60M
-rw-r--r-- 1
drwxr-xr-x 5
drwxr-xr-x 7
src]# ll
root root 60M Dec 5 20:54 kernel-2.6.18-92.1.17.el5.src.rpm
root root 4.0K Dec 5 19:23 kernels
root root 4.0K Oct 11 13:22 redhat
We will need to perform some more steps before this can be used as kernel source code.
First, we issue the rpm -i kernel-2.6.9-42.EL.src.rpm command to install this Red Hat
package.
[root@RHEL52
total 60M
-rw-r--r-- 1
drwxr-xr-x 5
drwxr-xr-x 7
[root@RHEL52
src]# ll
root root 60M Dec 5 20:54 kernel-2.6.18-92.1.17.el5.src.rpm
root root 4.0K Dec 5 19:23 kernels
root root 4.0K Oct 11 13:22 redhat
src]# rpm -i kernel-2.6.18-92.1.17.el5.src.rpm
Then we move to the SPECS directory and perform an rpmbuild.
[root@RHEL52 ~]# cd /usr/src/redhat/SPECS
[root@RHEL52 SPECS]# rpmbuild -bp -vv --target=i686 kernel-2.6.spec
The rpmbuild command put the RHEL Linux kernel source code in /usr/src/redhat/BUILD/
kernel-<version>/.
[root@RHEL52 kernel-2.6.18]# pwd
/usr/src/redhat/BUILD/kernel-2.6.18
[root@RHEL52 kernel-2.6.18]# ll
total 20K
drwxr-xr-x 2 root root 4.0K Dec 6 2007 config
-rw-r--r-- 1 root root 3.1K Dec 5 20:58 Config.mk
drwxr-xr-x 20 root root 4.0K Dec 5 20:58 linux-2.6.18.i686
drwxr-xr-x 19 root root 4.0K Sep 20 2006 vanilla
drwxr-xr-x 8 root root 4.0K Dec 6 2007 xen
656
the Linux kernel
63.3. kernel boot files
63.3.1. vmlinuz
The vmlinuz file in /boot is the compressed kernel.
paul@barry:~$ ls -lh /boot | grep vmlinuz
-rw-r--r-- 1 root root 1.2M 2006-03-06 16:22 vmlinuz-2.6.15-1-486
-rw-r--r-- 1 root root 1.1M 2006-03-06 16:30 vmlinuz-2.6.15-1-686
-rw-r--r-- 1 root root 1.3M 2008-02-11 00:00 vmlinuz-2.6.18-6-686
paul@barry:~$
63.3.2. initrd
The kernel uses initrd (an initial RAM disk) at boot time. The initrd is mounted before
the kernel loads, and can contain additional drivers and modules. It is a compressed cpio
archive, so you can look at the contents in this way.
root@RHELv4u4:/boot# mkdir /mnt/initrd
root@RHELv4u4:/boot# cp initrd-2.6.9-42.0.3.EL.img TMPinitrd.gz
root@RHELv4u4:/boot# gunzip TMPinitrd.gz
root@RHELv4u4:/boot# file TMPinitrd
TMPinitrd: ASCII cpio archive (SVR4 with no CRC)
root@RHELv4u4:/boot# cd /mnt/initrd/
root@RHELv4u4:/mnt/initrd# cpio -i | /boot/TMPinitrd
4985 blocks
root@RHELv4u4:/mnt/initrd# ls -l
total 76
drwxr-xr-x 2 root root 4096 Feb 5 08:36 bin
drwxr-xr-x 2 root root 4096 Feb 5 08:36 dev
drwxr-xr-x 4 root root 4096 Feb 5 08:36 etc
-rwxr-xr-x 1 root root 1607 Feb 5 08:36 init
drwxr-xr-x 2 root root 4096 Feb 5 08:36 lib
drwxr-xr-x 2 root root 4096 Feb 5 08:36 loopfs
drwxr-xr-x 2 root root 4096 Feb 5 08:36 proc
lrwxrwxrwx 1 root root
3 Feb 5 08:36 sbin -> bin
drwxr-xr-x 2 root root 4096 Feb 5 08:36 sys
drwxr-xr-x 2 root root 4096 Feb 5 08:36 sysroot
root@RHELv4u4:/mnt/initrd#
657
the Linux kernel
63.3.3. System.map
The System.map contains the symbol table and changes with every kernel compile. The
symbol table is also present in /proc/kallsyms (pre 2.6 kernels name this file /proc/ksyms).
root@RHELv4u4:/boot# head System.map-`uname -r`
00000400 A __kernel_vsyscall
0000041a A SYSENTER_RETURN_OFFSET
00000420 A __kernel_sigreturn
00000440 A __kernel_rt_sigreturn
c0100000 A _text
c0100000 T startup_32
c01000c6 t checkCPUtype
c0100147 t is486
c010014e t is386
c010019f t L6
root@RHELv4u4:/boot# head /proc/kallsyms
c0100228 t _stext
c0100228 t calibrate_delay_direct
c0100228 t stext
c0100337 t calibrate_delay
c01004db t rest_init
c0100580 t do_pre_smp_initcalls
c0100585 t run_init_process
c01005ac t init
c0100789 t early_param_test
c01007ad t early_setup_test
root@RHELv4u4:/boot#
63.3.4. .config
The last file copied to the /boot directory is the kernel configuration used for compilation.
This file is not necessary in the /boot directory, but it is common practice to put a copy
there. It allows you to recompile a kernel, starting from the same configuration as an existing
working one.
658
the Linux kernel
63.4. Linux kernel modules
63.4.1. about kernel modules
The Linux kernel is a monolithic kernel with loadable modules. These modules contain parts
of the kernel used typically for device drivers, file systems and network protocols. Most of
the time the necessary kernel modules are loaded automatically and dynamically without
administrator interaction.
63.4.2. /lib/modules
The modules are stored in the /lib/modules/<kernel-version> directory. There is a separate
directory for each kernel that was compiled for your system.
paul@laika:~$ ll /lib/modules/
total 12K
drwxr-xr-x 7 root root 4.0K 2008-11-10 14:32 2.6.24-16-generic
drwxr-xr-x 8 root root 4.0K 2008-12-06 15:39 2.6.24-21-generic
drwxr-xr-x 8 root root 4.0K 2008-12-05 12:58 2.6.24-22-generic
63.4.3. <module>.ko
The file containing the modules usually ends in .ko. This screenshot shows the location of
the isdn module files.
paul@laika:~$ find /lib/modules -name isdn.ko
/lib/modules/2.6.24-21-generic/kernel/drivers/isdn/i4l/isdn.ko
/lib/modules/2.6.24-22-generic/kernel/drivers/isdn/i4l/isdn.ko
/lib/modules/2.6.24-16-generic/kernel/drivers/isdn/i4l/isdn.ko
63.4.4. lsmod
To see a list of currently loaded modules, use lsmod. You see the name of each loaded
module, the size, the use count, and the names of other modules using this one.
[root@RHEL52 ~]# lsmod | head
Module
Size
autofs4
24517
hidp
23105
rfcomm
42457
l2cap
29505
-5
Used by
2
2
0
10 hidp,rfcomm
659
the Linux kernel
63.4.5. /proc/modules
/proc/modules lists all modules loaded by the kernel. The output would be too long to
display here, so lets grep for the vm module.
We see that vmmon and vmnet are both loaded. You can display the same information with
lsmod. Actually lsmod only reads and reformats the output of /proc/modules.
paul@laika:~$ cat /proc/modules | grep vm
vmnet 36896 13 - Live 0xffffffff88b21000 (P)
vmmon 194540 0 - Live 0xffffffff88af0000 (P)
paul@laika:~$ lsmod | grep vm
vmnet
36896 13
vmmon
194540 0
paul@laika:~$
63.4.6. module dependencies
Some modules depend on others. In the following example, you can see that the nfsd module
is used by exportfs, lockd and sunrpc.
paul@laika:~$ cat /proc/modules | grep nfsd
nfsd 267432 17 - Live 0xffffffff88a40000
exportfs 7808 1 nfsd, Live 0xffffffff88a3d000
lockd 73520 3 nfs,nfsd, Live 0xffffffff88a2a000
sunrpc 185032 12 nfs,nfsd,lockd, Live 0xffffffff889fb000
paul@laika:~$ lsmod | grep nfsd
nfsd
267432 17
exportfs
7808 1 nfsd
lockd
73520 3 nfs,nfsd
sunrpc
185032 12 nfs,nfsd,lockd
paul@laika:~$
660
the Linux kernel
63.4.7. insmod
Kernel modules can be manually loaded with the insmod command. This is a very simple
(and obsolete) way of loading modules. The screenshot shows insmod loading the fat
module (for fat file system support).
root@barry:/lib/modules/2.6.17-2-686# lsmod | grep fat
root@barry:/lib/modules/2.6.17-2-686# insmod kernel/fs/fat/fat.ko
root@barry:/lib/modules/2.6.17-2-686# lsmod | grep fat
fat
46588 0
insmod is not detecting dependencies, so it fails to load the isdn module (because the isdn
module depends on the slhc module).
[root@RHEL52 drivers]# pwd
/lib/modules/2.6.18-92.1.18.el5/kernel/drivers
[root@RHEL52 kernel]# insmod isdn/i4l/isdn.ko
insmod: error inserting 'isdn/i4l/isdn.ko': -1 Unknown symbol in module
63.4.8. modinfo
As you can see in the screenshot of modinfo below, the isdn module depends in the slhc
module.
[root@RHEL52 drivers]# modinfo isdn/i4l/isdn.ko | head -6
filename:
isdn/i4l/isdn.ko
license:
GPL
author:
Fritz Elfert
description:
ISDN4Linux: link layer
srcversion:
99650346E708173496F6739
depends:
slhc
63.4.9. modprobe
The big advantage of modprobe over insmod is that modprobe will load all necessary
modules, whereas insmod requires manual loading of dependencies. Another advantage is
that you don't need to point to the filename with full path.
This screenshot shows how modprobe loads the isdn module, automatically loading slhc in
background.
[root@RHEL52
[root@RHEL52
[root@RHEL52
isdn
slhc
[root@RHEL52
kernel]# lsmod | grep isdn
kernel]# modprobe isdn
kernel]# lsmod | grep isdn
122433 0
10561 1 isdn
kernel]#
661
the Linux kernel
63.4.10. /lib/modules/<kernel>/modules.dep
Module dependencies are stored in modules.dep.
[root@RHEL52 2.6.18-92.1.18.el5]# pwd
/lib/modules/2.6.18-92.1.18.el5
[root@RHEL52 2.6.18-92.1.18.el5]# head -3 modules.dep
/lib/modules/2.6.18-92.1.18.el5/kernel/drivers/net/tokenring/3c359.ko:
/lib/modules/2.6.18-92.1.18.el5/kernel/drivers/net/pcmcia/3c574_cs.ko:
/lib/modules/2.6.18-92.1.18.el5/kernel/drivers/net/pcmcia/3c589_cs.ko:
63.4.11. depmod
The modules.dep file can be updated (recreated) with the depmod command. In this
screenshot no modules were added, so depmod generates the same file.
root@barry:/lib/modules/2.6.17-2-686# ls -l modules.dep
-rw-r--r-- 1 root root 310676 2008-03-01 16:32 modules.dep
root@barry:/lib/modules/2.6.17-2-686# depmod
root@barry:/lib/modules/2.6.17-2-686# ls -l modules.dep
-rw-r--r-- 1 root root 310676 2008-12-07 13:54 modules.dep
63.4.12. rmmod
Similar to insmod, the rmmod command is rarely used anymore.
[root@RHELv4u3 ~]#
[root@RHELv4u3 ~]#
ERROR: Module slhc
[root@RHELv4u3 ~]#
[root@RHELv4u3 ~]#
[root@RHELv4u3 ~]#
[root@RHELv4u3 ~]#
modprobe isdn
rmmod slhc
is in use by isdn
rmmod isdn
rmmod slhc
lsmod | grep isdn
63.4.13. modprobe -r
Contrary to rmmod, modprobe will automatically remove unneeded modules.
[root@RHELv4u3
[root@RHELv4u3
isdn
slhc
[root@RHELv4u3
[root@RHELv4u3
[root@RHELv4u3
[root@RHELv4u3
~]# modprobe isdn
~]# lsmod | grep isdn
133537 0
7233 1 isdn
~]# modprobe -r isdn
~]# lsmod | grep isdn
~]# lsmod | grep slhc
~]#
63.4.14. /etc/modprobe.conf
The /etc/modprobe.conf file and the /etc/modprobe.d directory can contain aliases (used
by humans) and options (for dependent modules) for modprobe.
[root@RHEL52 ~]# cat /etc/modprobe.conf
alias scsi_hostadapter mptbase
alias scsi_hostadapter1 mptspi
alias scsi_hostadapter2 ata_piix
alias eth0 pcnet32
alias eth2 pcnet32
662
the Linux kernel
alias eth1 pcnet32
663
the Linux kernel
63.5. compiling a kernel
63.5.1. extraversion
Enter into /usr/src/redhat/BUILD/kernel-2.6.9/linux-2.6.9/ and change the extraversion
in the Makefile.
[root@RHEL52 linux-2.6.18.i686]# pwd
/usr/src/redhat/BUILD/kernel-2.6.18/linux-2.6.18.i686
[root@RHEL52 linux-2.6.18.i686]# vi Makefile
[root@RHEL52 linux-2.6.18.i686]# head -4 Makefile
VERSION = 2
PATCHLEVEL = 6
SUBLEVEL = 18
EXTRAVERSION = -paul2008
63.5.2. make mrproper
Now clean up the source from any previous installs with make mrproper. If this is your
first after downloading the source code, then this is not needed.
[root@RHEL52 linux-2.6.18.i686]# make mrproper
CLEAN
scripts/basic
CLEAN
scripts/kconfig
CLEAN
include/config
CLEAN
.config .config.old
63.5.3. .config
Now copy a working .config from /boot to our kernel directory. This file contains the
configuration that was used for your current working kernel. It determines whether modules
are included in compilation or not.
[root@RHEL52 linux-2.6.18.i686]# cp /boot/config-2.6.18-92.1.18.el5 .config
63.5.4. make menuconfig
Now run make menuconfig (or the graphical make xconfig). This tool allows you to select
whether to compile stuff as a module (m), as part of the kernel (*), or not at all (smaller
kernel size). If you remove too much, your kernel will not work. The configuration will be
stored in the hidden .config file.
[root@RHEL52 linux-2.6.18.i686]# make menuconfig
63.5.5. make clean
Issue a make clean to prepare the kernel for compile. make clean will remove most
generated files, but keeps your kernel configuration. Running a make mrproper at this point
would destroy the .config file that you built with make menuconfig.
[root@RHEL52 linux-2.6.18.i686]# make clean
664
the Linux kernel
63.5.6. make bzImage
And then run make bzImage, sit back and relax while the kernel compiles. You can use
time make bzImage to know how long it takes to compile, so next time you can go for a
short walk.
[root@RHEL52 linux-2.6.18.i686]# time make bzImage
HOSTCC scripts/basic/fixdep
HOSTCC scripts/basic/docproc
HOSTCC scripts/kconfig/conf.o
HOSTCC scripts/kconfig/kxgettext.o
...
This command will end with telling you the location of the bzImage file (and with time info
if you also specified the time command.
Kernel: arch/i386/boot/bzImage is ready
(#1)
real 13m59.573s
user 1m22.631s
sys 11m51.034s
[root@RHEL52 linux-2.6.18.i686]#
You can already copy this image to /boot with cp arch/i386/boot/bzImage /boot/vmlinuz<kernel-version>.
63.5.7. make modules
Now run make modules. It can take 20 to 50 minutes to compile all the modules.
[root@RHEL52 linux-2.6.18.i686]# time make modules
CHK
include/linux/version.h
CHK
include/linux/utsrelease.h
CC [M] arch/i386/kernel/msr.o
CC [M] arch/i386/kernel/cpuid.o
CC [M] arch/i386/kernel/microcode.o
63.5.8. make modules_install
To copy all the compiled modules to /lib/modules just run make modules_install (takes
about 20 seconds). Here's a screenshot from before the command.
[root@RHEL52
total 20
drwxr-xr-x 6
drwxr-xr-x 6
drwxr-xr-x 6
[root@RHEL52
linux-2.6.18.i686]# ls -l /lib/modules/
root root 4096 Oct 15 13:09 2.6.18-92.1.13.el5
root root 4096 Nov 11 08:51 2.6.18-92.1.17.el5
root root 4096 Dec 6 07:11 2.6.18-92.1.18.el5
linux-2.6.18.i686]# make modules_install
And here is the same directory after. Notice that make modules_install created a new
directory for the new kernel.
[root@RHEL52
total 24
drwxr-xr-x 6
drwxr-xr-x 6
drwxr-xr-x 6
drwxr-xr-x 3
linux-2.6.18.i686]# ls -l /lib/modules/
root
root
root
root
root
root
root
root
4096
4096
4096
4096
Oct 15 13:09 2.6.18-92.1.13.el5
Nov 11 08:51 2.6.18-92.1.17.el5
Dec 6 07:11 2.6.18-92.1.18.el5
Dec 6 08:50 2.6.18-paul2008
665
the Linux kernel
63.5.9. /boot
We still need to copy the kernel, the System.map and our configuration file to /boot. Strictly
speaking the .config file is not obligatory, but it might help you in future compilations of
the kernel.
[root@RHEL52 ]# pwd
/usr/src/redhat/BUILD/kernel-2.6.18/linux-2.6.18.i686
[root@RHEL52 ]# cp System.map /boot/System.map-2.6.18-paul2008
[root@RHEL52 ]# cp .config /boot/config-2.6.18-paul2008
[root@RHEL52 ]# cp arch/i386/boot/bzImage /boot/vmlinuz-2.6.18-paul2008
63.5.10. mkinitrd
The kernel often uses an initrd file at bootup. We can use mkinitrd to generate this file.
Make sure you use the correct kernel name!
[root@RHEL52 ]# pwd
/usr/src/redhat/BUILD/kernel-2.6.18/linux-2.6.18.i686
[root@RHEL52 ]# mkinitrd /boot/initrd-2.6.18-paul2008 2.6.18-paul2008
63.5.11. bootloader
Compilation is now finished, don't forget to create an additional stanza in grub or lilo.
666
the Linux kernel
63.6. compiling one module
63.6.1. hello.c
A little C program that will be our module.
[root@rhel4a kernel_module]# cat hello.c
#include <linux/module.h>
#include <section>
int init_module(void)
{
printk(KERN_INFO "Start Hello World...\n");
return 0;
}
void cleanup_module(void)
{
printk(KERN_INFO "End Hello World... \n");
}
63.6.2. Makefile
The make file for this module.
[root@rhel4a kernel_module]# cat Makefile
obj-m += hello.o
all:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean
These are the only two files needed.
[root@rhel4a kernel_module]# ll
total 16
-rw-rw-r-- 1 paul paul 250 Feb 15 19:14 hello.c
-rw-rw-r-- 1 paul paul 153 Feb 15 19:15 Makefile
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the Linux kernel
63.6.3. make
The running of the make command.
[root@rhel4a kernel_module]# make
make -C /lib/modules/2.6.9-paul-2/build M=~/kernel_module modules
make[1]: Entering dir... `/usr/src/redhat/BUILD/kernel-2.6.9/linux-2.6.9'
CC [M] /home/paul/kernel_module/hello.o
Building modules, stage 2.
MODPOST
CC
/home/paul/kernel_module/hello.mod.o
LD [M] /home/paul/kernel_module/hello.ko
make[1]: Leaving dir... `/usr/src/redhat/BUILD/kernel-2.6.9/linux-2.6.9'
[root@rhel4a kernel_module]#
Now we have more files.
[root@rhel4a kernel_module]# ll
total 172
-rw-rw-r-- 1 paul paul
250 Feb
-rw-r--r-- 1 root root 64475 Feb
-rw-r--r-- 1 root root
632 Feb
-rw-r--r-- 1 root root 37036 Feb
-rw-r--r-- 1 root root 28396 Feb
-rw-rw-r-- 1 paul paul
153 Feb
[root@rhel4a kernel_module]#
15
15
15
15
15
15
19:14
19:15
19:15
19:15
19:15
19:15
hello.c
hello.ko
hello.mod.c
hello.mod.o
hello.o
Makefile
63.6.4. hello.ko
Use modinfo to verify that it is really a module.
[root@rhel4a kernel_module]# modinfo hello.ko
filename:
hello.ko
vermagic:
2.6.9-paul-2 SMP 686 REGPARM 4KSTACKS gcc-3.4
depends:
[root@rhel4a kernel_module]#
Good, so now we can load our hello module.
[root@rhel4a kernel_module]# lsmod | grep hello
[root@rhel4a kernel_module]# insmod ./hello.ko
[root@rhel4a kernel_module]# lsmod | grep hello
hello
5504 0
[root@rhel4a kernel_module]# tail -1 /var/log/messages
Feb 15 19:16:07 rhel4a kernel: Start Hello World...
[root@rhel4a kernel_module]# rmmod hello
[root@rhel4a kernel_module]#
Finally /var/log/messages has a little surprise.
[root@rhel4a kernel_module]# tail -2 /var/log/messages
Feb 15 19:16:07 rhel4a kernel: Start Hello World...
Feb 15 19:16:35 rhel4a kernel: End Hello World...
[root@rhel4a kernel_module]#
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Chapter 64. library management
669
library management
64.1. introduction
With libraries we are talking about dynamically linked libraries (aka shared objects). These
are binaries that contain functions and are not started themselves as programs, but are called
by other binaries.
Several programs can use the same library. The name of the library file usually starts with
lib, followed by the actual name of the library, then the chracters .so and finally a version
number.
64.2. /lib and /usr/lib
When you look at the /lib or the /usr/lib directory, you will see a lot of symbolic links. Most
libraries have a detailed version number in their name, but receive a symbolic link from a
filename which only contains the major version number.
root@rhel53 ~# ls -l /lib/libext*
lrwxrwxrwx 1 root root
16 Feb 18 16:36 /lib/libext2fs.so.2 -> libext2fs.so.2.4
-rwxr-xr-x 1 root root 113K Jun 30 2009 /lib/libext2fs.so.2.4
64.3. ldd
Many programs have dependencies on the installation of certain libraries. You can display
these dependencies with ldd.
This example shows the dependencies of the su command.
paul@RHEL5 ~$ ldd /bin/su
linux-gate.so.1 => (0x003f7000)
libpam.so.0 => /lib/libpam.so.0 (0x00d5c000)
libpam_misc.so.0 => /lib/libpam_misc.so.0 (0x0073c000)
libcrypt.so.1 => /lib/libcrypt.so.1 (0x00aa4000)
libdl.so.2 => /lib/libdl.so.2 (0x00800000)
libc.so.6 => /lib/libc.so.6 (0x00ec1000)
libaudit.so.0 => /lib/libaudit.so.0 (0x0049f000)
/lib/ld-linux.so.2 (0x4769c000)
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library management
64.4. ltrace
The ltrace program allows to see all the calls made to library functions by a program. The
example below uses the -c option to get only a summary count (there can be many calls),
and the -l option to only show calls in one library file. All this to see what calls are made
when executing su - serena as root.
root@deb503:~# ltrace -c -l /lib/libpam.so.0 su - serena
serena@deb503:~$ exit
logout
% time
seconds usecs/call
calls
function
------ ----------- ----------- --------- -------------------70.31
0.014117
14117
1 pam_start
12.36
0.002482
2482
1 pam_open_session
5.17
0.001039
1039
1 pam_acct_mgmt
4.36
0.000876
876
1 pam_end
3.36
0.000675
675
1 pam_close_session
3.22
0.000646
646
1 pam_authenticate
0.48
0.000096
48
2 pam_set_item
0.27
0.000054
54
1 pam_setcred
0.25
0.000050
50
1 pam_getenvlist
0.22
0.000044
44
1 pam_get_item
------ ----------- ----------- --------- -------------------100.00
0.020079
11 total
64.5. dpkg -S and debsums
Find out on Debian/Ubuntu to which package a library belongs.
paul@deb503:/lib$ dpkg -S libext2fs.so.2.4
e2fslibs: /lib/libext2fs.so.2.4
You can then verify the integrity of all files in this package using debsums.
paul@deb503:~$ debsums e2fslibs
/usr/share/doc/e2fslibs/changelog.Debian.gz
/usr/share/doc/e2fslibs/copyright
/lib/libe2p.so.2.3
/lib/libext2fs.so.2.4
Should a library be broken, then reinstall it with aptitude reinstall $package.
root@deb503:~# aptitude reinstall e2fslibs
Reading package lists... Done
Building dependency tree
Reading state information... Done
Reading extended state information
Initializing package states... Done
Reading task descriptions... Done
The following packages will be REINSTALLED:
e2fslibs
...
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OK
OK
OK
library management
64.6. rpm -qf and rpm -V
Find out on Red Hat/Fedora to which package a library belongs.
paul@RHEL5 ~$ rpm -qf /lib/libext2fs.so.2.4
e2fsprogs-libs-1.39-8.el5
You can then use rpm -V to verify all files in this package. In the example below the output
shows that the Size and the Time stamp of the file have changed since installation.
root@rhel53 ~# rpm -V e2fsprogs-libs
prelink: /lib/libext2fs.so.2.4: prelinked file size differs
S.?....T
/lib/libext2fs.so.2.4
You can then use yum reinstall $package to overwrite the existing library with an original
version.
root@rhel53 lib# yum reinstall e2fsprogs-libs
Loaded plugins: rhnplugin, security
Setting up Reinstall Process
Resolving Dependencies
--> Running transaction check
---> Package e2fsprogs-libs.i386 0:1.39-23.el5 set to be erased
---> Package e2fsprogs-libs.i386 0:1.39-23.el5 set to be updated
--> Finished Dependency Resolution
...
The package verification now reports no problems with the library.
root@rhel53 lib# rpm -V e2fsprogs-libs
root@rhel53 lib#
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library management
64.7. tracing with strace
More detailed tracing of all function calls can be done with strace. We start by creating a
read only file.
root@deb503:~# echo hello > 42.txt
root@deb503:~# chmod 400 42.txt
root@deb503:~# ls -l 42.txt
-r-------- 1 root root 6 2011-09-26 12:03 42.txt
We open the file with vi, but include the strace command with an output file for the trace
before vi. This will create a file with all the function calls done by vi.
root@deb503:~# strace -o strace.txt vi 42.txt
The file is read only, but we still change the contents, and use the :w! directive to write to
this file. Then we close vi and take a look at the trace log.
root@deb503:~# grep chmod strace.txt
chmod("42.txt", 0100600)
= -1 ENOENT (No such file or directory)
chmod("42.txt", 0100400)
= 0
root@deb503:~# ls -l 42.txt
-r-------- 1 root root 12 2011-09-26 12:04 42.txt
Notice that vi changed the permissions on the file twice. The trace log is too long to show
a complete screenshot in this book.
root@deb503:~# wc -l strace.txt
941 strace.txt
673
Part XVI. backup management
Table of Contents
65. backup ..........................................................................................................................
65.1. About tape devices ................................................................................................
65.2. Compression ........................................................................................................
65.3. tar ......................................................................................................................
65.4. Backup Types ......................................................................................................
65.5. dump and restore ..................................................................................................
65.6. cpio ....................................................................................................................
65.7. dd ......................................................................................................................
65.8. split ....................................................................................................................
65.9. practice: backup ....................................................................................................
675
676
676
677
677
679
680
680
681
682
682
Chapter 65. backup
65.1. About tape devices
Don't forget that the name of a device strictly speaking has no meaning since the kernel will
use the major and minor number to find the hardware! See the man page of mknod and the
devices.txt file in the Linux kernel source for more info.
65.1.1. SCSI tapes
On the official Linux device list (http://www.lanana.org/docs/device-list/) we find the names
for SCSI tapes (major 9 char). SCSI tape devices are located underneath /dev/st and are
numbered starting with 0 for the first tape device.
/dev/st0
/dev/st1
/dev/st2
First tape device
Second tape device
Third tape device
To prevent automatic rewinding of tapes, prefix them with the letter n.
/dev/nst0
/dev/nst1
/dev/nst2
First no rewind tape device
Second no rewind tape device
Third no rewind tape device
By default, SCSI tapes on Linux will use the highest hardware compression that is supported
by the tape device. To lower the compression level, append one of the letters l (low), m
(medium) or a (auto) to the tape name.
/dev/st0l
/dev/st0m
/dev/nst2m
First low compression tape device
First medium compression tape device
Third no rewind medium compression tape device
65.1.2. IDE tapes
On the official Linux device list (http://www.lanana.org/docs/device-list/) we find the names
for IDE tapes (major 37 char). IDE tape devices are located underneath /dev/ht and are
numbered starting with 0 for the first tape device. No rewind and compression is similar
to SCSI tapes.
/dev/ht0
/dev/nht0
/dev/ht0m
First IDE tape device
Second no rewind IDE tape device
First medium compression IDE tape device
65.1.3. mt
To manage your tapes, use mt (Magnetic Tape). Some examples.
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backup
To receive information about the status of the tape.
mt -f /dev/st0 status
To rewind a tape...
mt -f /dev/st0 rewind
To rewind and eject a tape...
mt -f /dev/st0 eject
To erase a tape...
mt -f /dev/st0 erase
65.2. Compression
It can be beneficial to compress files before backup. The two most popular tools for
compression of regular files on Linux are gzip/gunzip and bzip2/bunzip2. Below you can
see gzip in action, notice that it adds the .gz extension to the file.
paul@RHELv4u4:~/test$ ls -l allfiles.tx*
-rw-rw-r-- 1 paul paul 8813553 Feb 27 05:38 allfiles.txt
paul@RHELv4u4:~/test$ gzip allfiles.txt
paul@RHELv4u4:~/test$ ls -l allfiles.tx*
-rw-rw-r-- 1 paul paul 931863 Feb 27 05:38 allfiles.txt.gz
paul@RHELv4u4:~/test$ gunzip allfiles.txt.gz
paul@RHELv4u4:~/test$ ls -l allfiles.tx*
-rw-rw-r-- 1 paul paul 8813553 Feb 27 05:38 allfiles.txt
paul@RHELv4u4:~/test$
In general, gzip is much faster than bzip2, but the latter one compresses a lot better. Let us
compare the two.
paul@RHELv4u4:~/test$ cp allfiles.txt bllfiles.txt
paul@RHELv4u4:~/test$ time gzip allfiles.txt
real
0m0.050s
user
0m0.041s
sys
0m0.009s
paul@RHELv4u4:~/test$ time bzip2 bllfiles.txt
real
0m5.968s
user
0m5.794s
sys
0m0.076s
paul@RHELv4u4:~/test$ ls -l ?llfiles.tx*
-rw-rw-r-- 1 paul paul 931863 Feb 27 05:38 allfiles.txt.gz
-rw-rw-r-- 1 paul paul 708871 May 12 10:52 bllfiles.txt.bz2
paul@RHELv4u4:~/test$
65.3. tar
The tar utility gets its name from Tape ARchive. This tool will receive and send files to
a destination (typically a tape or a regular file). The c option is used to create a tar archive
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backup
(or tarfile), the f option to name/create the tarfile. The example below takes a backup of /
etc into the file /backup/etc.tar .
root@RHELv4u4:~# tar cf /backup/etc.tar /etc
root@RHELv4u4:~# ls -l /backup/etc.tar
-rw-r--r-- 1 root root 47800320 May 12 11:47 /backup/etc.tar
root@RHELv4u4:~#
Compression can be achieved without pipes since tar uses the z flag to compress with gzip,
and the j flag to compress with bzip2.
root@RHELv4u4:~# tar czf /backup/etc.tar.gz /etc
root@RHELv4u4:~# tar cjf /backup/etc.tar.bz2 /etc
root@RHELv4u4:~# ls -l /backup/etc.ta*
-rw-r--r-- 1 root root 47800320 May 12 11:47 /backup/etc.tar
-rw-r--r-- 1 root root 6077340 May 12 11:48 /backup/etc.tar.bz2
-rw-r--r-- 1 root root 8496607 May 12 11:47 /backup/etc.tar.gz
root@RHELv4u4:~#
The t option is used to list the contents of a tar file. Verbose mode is enabled with v (also
useful when you want to see the files being archived during archiving).
root@RHELv4u4:~# tar tvf /backup/etc.tar
drwxr-xr-x root/root
0 2007-05-12
-rw-r--r-- root/root
2657 2004-09-27
-rw-r--r-- root/root
13136 2006-11-03
drwxr-xr-x root/root
0 2004-11-03
...
09:38:21
10:15:03
17:34:50
13:35:50
etc/
etc/warnquota.conf
etc/mime.types
etc/sound/
To list a specific file in a tar archive, use the t option, added with the filename (without
leading /).
root@RHELv4u4:~# tar tvf /backup/etc.tar etc/resolv.conf
-rw-r--r-- root/root
77 2007-05-12 08:31:32 etc/resolv.conf
root@RHELv4u4:~#
Use the x flag to restore a tar archive, or a single file from the archive. Remember that by
default tar will restore the file in the current directory.
root@RHELv4u4:~# tar xvf /backup/etc.tar etc/resolv.conf
etc/resolv.conf
root@RHELv4u4:~# ls -l /etc/resolv.conf
-rw-r--r-- 2 root root 40 May 12 12:05 /etc/resolv.conf
root@RHELv4u4:~# ls -l etc/resolv.conf
-rw-r--r-- 1 root root 77 May 12 08:31 etc/resolv.conf
root@RHELv4u4:~#
You can preserve file permissions with the p flag. And you can exclude directories or file
with --exclude.
root ~# tar cpzf /backup/etc_with_perms.tgz /etc
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backup
root ~# tar cpzf /backup/etc_no_sysconf.tgz /etc --exclude /etc/sysconfig
root ~# ls -l /backup/etc_*
-rw-r--r-- 1 root root 8434293 May 12 12:48 /backup/etc_no_sysconf.tgz
-rw-r--r-- 1 root root 8496591 May 12 12:48 /backup/etc_with_perms.tgz
root ~#
You can also create a text file with names of files and directories to archive, and then supply
this file to tar with the -T flag.
root@RHELv4u4:~# find /etc -name *.conf > files_to_archive.txt
root@RHELv4u4:~# find /home -name *.pdf >> files_to_archive.txt
root@RHELv4u4:~# tar cpzf /backup/backup.tgz -T files_to_archive.txt
The tar utility can receive filenames from the find command, with the help of xargs.
find /etc -type f -name "*.conf" | xargs tar czf /backup/confs.tar.gz
You can also use tar to copy a directory, this is more efficient than using cp -r.
(cd /etc; tar -cf - . ) | (cd /backup/copy_of_etc/; tar -xpf - )
Another example of tar, this copies a directory securely over the network.
(cd /etc;tar -cf - . )|(ssh user@srv 'cd /backup/cp_of_etc/; tar -xf - ')
tar can be used together with gzip and copy a file to a remote server through ssh
cat backup.tar | gzip | ssh [email protected] "cat - > backup.tgz"
Compress the tar backup when it is on the network, but leave it uncompressed at the
destination.
cat backup.tar | gzip | ssh [email protected] "gunzip|cat - > backup.tar"
Same as the previous, but let ssh handle the compression
cat backup.tar | ssh -C [email protected] "cat - > backup.tar"
65.4. Backup Types
Linux uses multilevel incremental backups using distinct levels. A full backup is a backup
at level 0. A higher level x backup will include all changes since the last level x-1 backup.
Suppose you take a full backup on Monday (level 0) and a level 1 backup on Tuesday, then
the Tuesday backup will contain all changes since Monday. Taking a level 2 on Wednesday
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backup
will contain all changes since Tuesday (the last level 2-1). A level 3 backup on Thursday
will contain all changes since Wednesday (the last level 3-1). Another level 3 on Friday
will also contain all changes since Wednesday. A level 2 backup on Saturday would take
all changes since the last level 1 from Tuesday.
65.5. dump and restore
While dump is similar to tar, it is also very different because it looks at the file system.
Where tar receives a lists of files to backup, dump will find files to backup by itself by
examining ext2. Files found by dump will be copied to a tape or regular file. In case the
target is not big enough to hold the dump (end-of-media), it is broken into multiple volumes.
Restoring files that were backed up with dump is done with the restore command. In the
example below we take a full level 0 backup of two partitions to a SCSI tape. The no rewind
is mandatory to put the volumes behind each other on the tape.
dump 0f /dev/nst0 /boot
dump 0f /dev/nst0 /
Listing files in a dump archive is done with dump -t, and you can compare files with dump
-C.
You can omit files from a dump by changing the dump attribute with the chattr command.
The d attribute on ext will tell dump to skip the file, even during a full backup. In the
following example, /etc/hosts is excluded from dump archives.
chattr +d /etc/hosts
To restore the complete file system with restore, use the -r option. This can be useful to
change the size or block size of a file system. You should have a clean file system mounted
and cd'd into it. Like this example shows.
mke2fs /dev/hda3
mount /dev/hda3 /mnt/data
cd /mnt/data
restore rf /dev/nst0
To extract only one file or directory from a dump, use the -x option.
restore -xf /dev/st0 /etc
65.6. cpio
Different from tar and dump is cpio (Copy Input and Output). It can be used to receive
filenames, but copies the actual files. This makes it an easy companion with find! Some
examples below.
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backup
find sends filenames to cpio, which puts the files in an archive.
find /etc -depth -print | cpio -oaV -O archive.cpio
The same, but compressed with gzip
find /etc -depth -print | cpio -oaV | gzip -c > archive.cpio.gz
Now pipe it through ssh (backup files to a compressed file on another machine)
find /etc -depth -print|cpio -oaV|gzip -c|ssh server "cat - > etc.cpio.gz"
find sends filenames to cpio | cpio sends files to ssh | ssh sends files to cpio 'cpio extracts files'
find /etc -depth -print | cpio -oaV | ssh user@host 'cpio -imVd'
the same but reversed: copy a dir from the remote host to the local machine
ssh user@host "find path -depth -print | cpio -oaV" | cpio -imVd
65.7. dd
65.7.1. About dd
Some people use dd to create backups. This can be very powerful, but dd backups can only
be restored to very similar partitions or devices. There are however a lot of useful things
possible with dd. Some examples.
65.7.2. Create a CDROM image
The easiest way to create a .ISO file from any CD. The if switch means Input File, of is the
Output File. Any good tool can burn a copy of the CD with this .ISO file.
dd if=/dev/cdrom of=/path/to/cdrom.ISO
65.7.3. Create a floppy image
A little outdated maybe, but just in case : make an image file from a 1.44MB floppy.
Blocksize is defined by bs, and count contains the number of blocks to copy.
dd if=/dev/floppy of=/path/to/floppy.img bs=1024 count=1440
65.7.4. Copy the master boot record
Use dd to copy the MBR (Master Boot Record) of hard disk /dev/hda to a file.
dd if=/dev/hda of=/MBR.img bs=512 count=1
65.7.5. Copy files
This example shows how dd can copy files. Copy the file summer.txt to
copy_of_summer.txt .
dd if=~/summer.txt of=~/copy_of_summer.txt
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65.7.6. Image disks or partitions
And who needs ghost when dd can create a (compressed) image of a partition.
dd if=/dev/hdb2 of=/image_of_hdb2.IMG
dd if=/dev/hdb2 | gzip > /image_of_hdb2.IMG.gz
65.7.7. Create files of a certain size
dd can be used to create a file of any size. The first example creates a one MEBIbyte file,
the second a one MEGAbyte file.
dd if=/dev/zero of=file1MB count=1024 bs=1024
dd if=/dev/zero of=file1MB count=1000 bs=1024
65.7.8. CDROM server example
And there are of course endless combinations with ssh and bzip2. This example puts a bzip2
backup of a cdrom on a remote server.
dd if=/dev/cdrom |bzip2|ssh user@host "cat - > /backups/cd/cdrom.iso.bz2"
65.8. split
The split command is useful to split files into smaller files. This can be useful to fit the file
onto multiple instances of a medium too small to contain the complete file. In the example
below, a file of size 5000 bytes is split into three smaller files, with maximum 2000 bytes
each.
paul@laika:~/test$ ls -l
total 8
-rw-r--r-- 1 paul paul 5000
paul@laika:~/test$ split -b
paul@laika:~/test$ ls -l
total 20
-rw-r--r-- 1 paul paul 5000
-rw-r--r-- 1 paul paul 2000
-rw-r--r-- 1 paul paul 2000
-rw-r--r-- 1 paul paul 1000
2007-09-09 20:46 bigfile1
2000 bigfile1 splitfile.
2007-09-09
2007-09-09
2007-09-09
2007-09-09
20:46
20:47
20:47
20:47
bigfile1
splitfile.aa
splitfile.ab
splitfile.ac
65.9. practice: backup
!! Careful with tar options and the position of the backup file, mistakes can destroy your
system!!
1. Create a directory (or partition if you like) for backups. Link (or mount) it under /mnt/
backup.
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2a. Use tar to backup /etc in /mnt/backup/etc_date.tgz, the backup must be gzipped. (Replace
date with the current date)
2b. Use tar to backup /bin to /mnt/backup/bin_date.tar.bz2, the backup must be bzip2'd.
2c. Choose a file in /etc and /bin and verify with tar that the file is indeed backed up.
2d. Extract those two files to your home directory.
3a. Create a backup directory for your neighbour, make it accessible under /mnt/
neighbourName
3b. Combine ssh and tar to put a backup of your /boot on your neighbours computer in /
mnt/YourName
4a. Combine find and cpio to create a cpio archive of /etc.
4b. Choose a file in /etc and restore it from the cpio archive into your home directory.
5. Use dd and ssh to put a backup of the master boot record on your neighbours computer.
6. (On the real computer) Create and mount an ISO image of the ubuntu cdrom.
7. Combine dd and gzip to create a 'ghost' image of one of your partitions on another partition.
8. Use dd to create a five megabyte file in ~/testsplit and name it biggest. Then split this file
in smaller two megabyte parts.
mkdir testsplit
dd if=/dev/zero of=~/testsplit/biggest count=5000 bs=1024
split -b 2000000 biggest parts
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Part XVII. apache and squid
Table of Contents
66. apache web server .........................................................................................................
66.1. introduction to apache ...........................................................................................
66.2. port virtual hosts on Debian ....................................................................................
66.3. named virtual hosts on Debian ................................................................................
66.4. password protected website on Debian ......................................................................
66.5. port virtual hosts on CentOS ...................................................................................
66.6. named virtual hosts on CentOS ...............................................................................
66.7. password protected website on CentOS .....................................................................
66.8. troubleshooting apache ...........................................................................................
66.9. virtual hosts example .............................................................................................
66.10. aliases and redirects .............................................................................................
66.11. more on .htaccess ................................................................................................
66.12. traffic ................................................................................................................
66.13. self signed cert on Debian ....................................................................................
66.14. self signed cert on RHEL/CentOS ..........................................................................
66.15. practice: apache ..................................................................................................
67. introduction to squid ......................................................................................................
67.1. about proxy servers ...............................................................................................
67.2. installing squid .....................................................................................................
67.3. port 3128 ............................................................................................................
67.4. starting and stopping .............................................................................................
67.5. client proxy settings ..............................................................................................
67.6. upside down images ..............................................................................................
67.7. /var/log/squid ........................................................................................................
67.8. access control .......................................................................................................
67.9. testing squid ........................................................................................................
67.10. name resolution ..................................................................................................
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Chapter 66. apache web server
In this chapter we learn how to setup a web server with the apache software.
According to NetCraft (http://news.netcraft.com/archives/web_server_survey.html) about
seventy percent of all web servers are running on Apache. The name is derived from a
patchy web server, because of all the patches people wrote for the NCSA httpd server.
Later chapters will expand this web server into a LAMP stack (Linux, Apache, Mysql, Perl/
PHP/Python).
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apache web server
66.1. introduction to apache
66.1.1. installing on Debian
This screenshot shows that there is no apache server installed, nor does the /var/www
directory exist.
root@debian7:~# ls -l /var/www
ls: cannot access /var/www: No such file or directory
root@debian7:~# dpkg -l | grep apache
To install apache on Debian:
root@debian7:~# aptitude install apache2
The following NEW packages will be installed:
apache2 apache2-mpm-worker{a} apache2-utils{a} apache2.2-bin{a} apache2.2-com\
mon{a} libapr1{a} libaprutil1{a} libaprutil1-dbd-sqlite3{a} libaprutil1-ldap{a}\
ssl-cert{a}
0 packages upgraded, 10 newly installed, 0 to remove and 0 not upgraded.
Need to get 1,487 kB of archives. After unpacking 5,673 kB will be used.
Do you want to continue? [Y/n/?]
After installation, the same two commands as above will yield a different result:
root@debian7:~# ls -l /var/www
total 4
-rw-r--r-- 1 root root 177 Apr 29 11:55 index.html
root@debian7:~# dpkg -l | grep apache | tr -s ' '
ii apache2 2.2.22-13+deb7u1 amd64 Apache HTTP Server metapackage
ii apache2-mpm-worker 2.2.22-13+deb7u1 amd64 Apache HTTP Server - high speed th\
readed model
ii apache2-utils 2.2.22-13+deb7u1 amd64 utility programs for webservers
ii apache2.2-bin 2.2.22-13+deb7u1 amd64 Apache HTTP Server common binary files
ii apache2.2-common 2.2.22-13+deb7u1 amd64 Apache HTTP Server common files
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apache web server
66.1.2. installing on RHEL/CentOS
Note that Red Hat derived distributions use httpd as package and process name instead of
apache.
To verify whether apache is installed in CentOS/RHEL:
[root@centos65 ~]# rpm -q httpd
package httpd is not installed
[root@centos65 ~]# ls -l /var/www
ls: cannot access /var/www: No such file or directory
To install apache on CentOS:
[root@centos65 ~]# yum install httpd
After running the yum install httpd command, the Centos 6.5 server has apache installed
and the /var/www directory exists.
[root@centos65 ~]# rpm -q httpd
httpd-2.2.15-30.el6.centos.x86_64
[root@centos65 ~]# ls -l /var/www
total 16
drwxr-xr-x. 2 root root 4096 Apr
drwxr-xr-x. 3 root root 4096 May
drwxr-xr-x. 2 root root 4096 Apr
drwxr-xr-x. 3 root root 4096 May
[root@centos65 ~]#
3
6
3
6
23:57
13:08
23:57
13:08
688
cgi-bin
error
html
icons
apache web server
66.1.3. running apache on Debian
This is how you start apache2 on Debian.
root@debian7:~# service apache2 status
Apache2 is NOT running.
root@debian7:~# service apache2 start
Starting web server: apache2apache2: Could not reliably determine the server's \
fully qualified domain name, using 127.0.1.1 for ServerName
.
To verify, run the service apache2 status command again or use ps.
root@debian7:~# service apache2 status
Apache2 is running (pid 3680).
root@debian7:~# ps -C apache2
PID TTY
TIME CMD
3680 ?
00:00:00 apache2
3683 ?
00:00:00 apache2
3684 ?
00:00:00 apache2
3685 ?
00:00:00 apache2
root@debian7:~#
Or use wget and file to verify that your web server serves an html document.
root@debian7:~# wget 127.0.0.1
--2014-05-06 13:27:02-- http://127.0.0.1/
Connecting to 127.0.0.1:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 177 [text/html]
Saving to: `index.html'
100%[==================================================>] 177
--.-K/s
in 0s
2014-05-06 13:27:02 (15.8 MB/s) - `index.html' saved [177/177]
root@debian7:~# file index.html
index.html: HTML document, ASCII text
root@debian7:~#
Or verify that apache is running by opening a web browser, and browse to the ip-address of
your server. An Apache test page should be shown.
You can do the following to quickly avoid the 'could not reliably determine the fqdn' message
when restarting apache.
root@debian7:~# echo ServerName Debian7 >> /etc/apache2/apache2.conf
root@debian7:~# service apache2 restart
Restarting web server: apache2 ... waiting .
root@debian7:~#
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apache web server
66.1.4. running apache on CentOS
Starting the httpd on RHEL/CentOS is done with the service command.
[root@centos65 ~]# service httpd status
httpd is stopped
[root@centos65 ~]# service httpd start
Starting httpd: httpd: Could not reliably determine the server's fully qualifie\
d domain name, using 127.0.0.1 for ServerName
[ OK ]
[root@centos65 ~]#
To verify that apache is running, use ps or issue the service httpd status command again.
[root@centos65 ~]# service httpd status
httpd (pid 2410) is running...
[root@centos65 ~]# ps -C httpd
PID TTY
TIME CMD
2410 ?
00:00:00 httpd
2412 ?
00:00:00 httpd
2413 ?
00:00:00 httpd
2414 ?
00:00:00 httpd
2415 ?
00:00:00 httpd
2416 ?
00:00:00 httpd
2417 ?
00:00:00 httpd
2418 ?
00:00:00 httpd
2419 ?
00:00:00 httpd
[root@centos65 ~]#
To prevent the 'Could not reliably determine the fqdn' message, issue the following
command.
[root@centos65 ~]# echo ServerName Centos65 >> /etc/httpd/conf/httpd.conf
[root@centos65 ~]# service httpd restart
Stopping httpd:
[ OK ]
Starting httpd:
[ OK ]
[root@centos65 ~]#
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apache web server
66.1.5. index file on CentOS
CentOS does not provide a standard index.html or index.php file. A simple wget gives an
error.
[root@centos65 ~]# wget 127.0.0.1
--2014-05-06 15:10:22-- http://127.0.0.1/
Connecting to 127.0.0.1:80... connected.
HTTP request sent, awaiting response... 403 Forbidden
2014-05-06 15:10:22 ERROR 403: Forbidden.
Instead when visiting the ip-address of your server in a web browser you get a noindex.html
page. You can verify this using wget.
[root@centos65 ~]# wget http://127.0.0.1/error/noindex.html
--2014-05-06 15:16:05-- http://127.0.0.1/error/noindex.html
Connecting to 127.0.0.1:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 5039 (4.9K) [text/html]
Saving to: “noindex.html”
100%[=============================================>] 5,039
--.-K/s
in 0s
2014-05-06 15:16:05 (289 MB/s) - “noindex.html” saved [5039/5039]
[root@centos65 ~]# file noindex.html
noindex.html: HTML document text
[root@centos65 ~]#
Any custom index.html file in /var/www/html will immediately serve as an index for this
web server.
[root@centos65 ~]# echo 'Welcome to my website' > /var/www/html/index.html
[root@centos65 ~]# wget http://127.0.0.1
--2014-05-06 15:19:16-- http://127.0.0.1/
Connecting to 127.0.0.1:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 22 [text/html]
Saving to: “index.html”
100%[=============================================>] 22
2014-05-06 15:19:16 (1.95 MB/s) - “index.html” saved [22/22]
[root@centos65 ~]# cat index.html
Welcome to my website
691
--.-K/s
in 0s
apache web server
66.1.6. default website
Changing the default website of a freshly installed apache web server is easy. All you need
to do is create (or change) an index.html file in the DocumentRoot directory.
To locate the DocumentRoot directory on Debian:
root@debian7:~# grep DocumentRoot /etc/apache2/sites-available/default
DocumentRoot /var/www
This means that /var/www/index.html is the default web site.
root@debian7:~# cat /var/www/index.html
<html><body><h1>It works!</h1>
<p>This is the default web page for this server.</p>
<p>The web server software is running but no content has been added, yet.</p>
</body></html>
root@debian7:~#
This screenshot shows how to locate the DocumentRoot directory on RHEL/CentOS.
[root@centos65 ~]# grep ^DocumentRoot /etc/httpd/conf/httpd.conf
DocumentRoot "/var/www/html"
RHEL/CentOS have no default web page (only the noindex.html error page mentioned
before). But an index.html file created in /var/www/html/ will automatically be used as
default page.
[root@centos65 ~]# echo '<html><head><title>Default website</title></head><body\
><p>A new web page</p></body></html>' > /var/www/html/index.html
[root@centos65 ~]# cat /var/www/html/index.html
<html><head><title>Default website</title></head><body><p>A new web page</p></b\
ody></html>
[root@centos65 ~]#
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apache web server
66.1.7. apache configuration
There are many similarities, but also a couple of differences when configuring apache on
Debian or on CentOS. Both Linux families will get their own chapters with examples.
All configuration on RHEL/CentOS is done in /etc/httpd.
[root@centos65 ~]#
total 8
drwxr-xr-x. 2 root
drwxr-xr-x. 2 root
lrwxrwxrwx. 1 root
lrwxrwxrwx. 1 root
les
lrwxrwxrwx. 1 root
[root@centos65 ~]#
ls -l /etc/httpd/
root 4096 May
root 4096 May
root
19 May
root
29 May
6
6
6
6
root
6 13:08 run -> ../../var/run/httpd
19 May
13:08
13:08
13:08
13:08
conf
conf.d
logs -> ../../var/log/httpd
modules -> ../../usr/lib64/httpd/modu\
Debian (and ubuntu/mint/...) use /etc/apache2.
root@debian7:~# ls -l /etc/apache2/
total 72
-rw-r--r-- 1 root root 9659 May 6
drwxr-xr-x 2 root root 4096 May 6
-rw-r--r-- 1 root root 1465 Jan 31
-rw-r--r-- 1 root root 31063 Jul 20
drwxr-xr-x 2 root root 4096 May 6
drwxr-xr-x 2 root root 4096 May 6
-rw-r--r-- 1 root root
750 Jan 26
drwxr-xr-x 2 root root 4096 May 6
drwxr-xr-x 2 root root 4096 May 6
root@debian7:~#
14:23
13:19
18:35
2013
13:19
13:19
12:13
13:19
13:19
693
apache2.conf
conf.d
envvars
magic
mods-available
mods-enabled
ports.conf
sites-available
sites-enabled
apache web server
66.2. port virtual hosts on Debian
66.2.1. default virtual host
Debian has a virtualhost configuration file for its default website in /etc/apache2/sitesavailable/default.
root@debian7:~# head -2 /etc/apache2/sites-available/default
<VirtualHost *:80>
ServerAdmin webmaster@localhost
66.2.2. three extra virtual hosts
In this scenario we create three additional websites for three customers that share a clubhouse
and want to jointly hire you. They are a model train club named Choo Choo, a chess club
named Chess Club 42 and a hackerspace named hunter2.
One way to put three websites on one web server, is to put each website on a different port.
This screenshot shows three newly created virtual hosts, one for each customer.
root@debian7:~# vi /etc/apache2/sites-available/choochoo
root@debian7:~# cat /etc/apache2/sites-available/choochoo
<VirtualHost *:7000>
ServerAdmin webmaster@localhost
DocumentRoot /var/www/choochoo
</VirtualHost>
root@debian7:~# vi /etc/apache2/sites-available/chessclub42
root@debian7:~# cat /etc/apache2/sites-available/chessclub42
<VirtualHost *:8000>
ServerAdmin webmaster@localhost
DocumentRoot /var/www/chessclub42
</VirtualHost>
root@debian7:~# vi /etc/apache2/sites-available/hunter2
root@debian7:~# cat /etc/apache2/sites-available/hunter2
<VirtualHost *:9000>
ServerAdmin webmaster@localhost
DocumentRoot /var/www/hunter2
</VirtualHost>
Notice the different port numbers 7000, 8000 and 9000. Notice also that we specified a
unique DocumentRoot for each website.
Are you using Ubuntu or Mint, then these configfiles need to end in .conf.
694
apache web server
66.2.3. three extra ports
We need to enable these three ports on apache in the ports.conf file. Open this file with vi
and add three lines to listen on three extra ports.
root@debian7:~# vi /etc/apache2/ports.conf
Verify with grep that the Listen directives are added correctly.
root@debian7:~# grep ^Listen /etc/apache2/ports.conf
Listen 80
Listen 7000
Listen 8000
Listen 9000
66.2.4. three extra websites
Next we need to create three DocumentRoot directories.
root@debian7:~# mkdir /var/www/choochoo
root@debian7:~# mkdir /var/www/chessclub42
root@debian7:~# mkdir /var/www/hunter2
And we have to put some really simple website in those directories.
root@debian7:~# echo 'Choo Choo model train Choo Choo' > /var/www/choochoo/inde\
x.html
root@debian7:~# echo 'Welcome to chess club 42' > /var/www/chessclub42/index.ht\
ml
root@debian7:~# echo 'HaCkInG iS fUn At HuNtEr2' > /var/www/hunter2/index.html
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apache web server
66.2.5. enabling extra websites
The last step is to enable the websites with the a2ensite command. This command will create
links in sites-enabled.
The links are not there yet...
root@debian7:~# cd /etc/apache2/
root@debian7:/etc/apache2# ls sites-available/
chessclub42 choochoo default default-ssl hunter2
root@debian7:/etc/apache2# ls sites-enabled/
000-default
So we run the a2ensite command for all websites.
root@debian7:/etc/apache2# a2ensite choochoo
Enabling site choochoo.
To activate the new configuration, you need to run:
service apache2 reload
root@debian7:/etc/apache2# a2ensite chessclub42
Enabling site chessclub42.
To activate the new configuration, you need to run:
service apache2 reload
root@debian7:/etc/apache2# a2ensite hunter2
Enabling site hunter2.
To activate the new configuration, you need to run:
service apache2 reload
The links are created, so we can tell apache.
root@debian7:/etc/apache2# ls sites-enabled/
000-default chessclub42 choochoo hunter2
root@debian7:/etc/apache2# service apache2 reload
Reloading web server config: apache2.
root@debian7:/etc/apache2#
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apache web server
66.2.6. testing the three websites
Testing the model train club named Choo Choo on port 7000.
root@debian7:/etc/apache2# wget 127.0.0.1:7000
--2014-05-06 21:16:03-- http://127.0.0.1:7000/
Connecting to 127.0.0.1:7000... connected.
HTTP request sent, awaiting response... 200 OK
Length: 32 [text/html]
Saving to: `index.html'
100%[============================================>] 32
--.-K/s
in 0s
2014-05-06 21:16:03 (2.92 MB/s) - `index.html' saved [32/32]
root@debian7:/etc/apache2# cat index.html
Choo Choo model train Choo Choo
Testing the chess club named Chess Club 42 on port 8000.
root@debian7:/etc/apache2# wget 127.0.0.1:8000
--2014-05-06 21:16:20-- http://127.0.0.1:8000/
Connecting to 127.0.0.1:8000... connected.
HTTP request sent, awaiting response... 200 OK
Length: 25 [text/html]
Saving to: `index.html.1'
100%[===========================================>] 25
--.-K/s
in 0s
--.-K/s
in 0s
2014-05-06 21:16:20 (2.16 MB/s) - `index.html.1' saved [25/25]
root@debian7:/etc/apache2# cat index.html.1
Welcome to chess club 42
Testing the hacker club named hunter2 on port 9000.
root@debian7:/etc/apache2# wget 127.0.0.1:9000
--2014-05-06 21:16:30-- http://127.0.0.1:9000/
Connecting to 127.0.0.1:9000... connected.
HTTP request sent, awaiting response... 200 OK
Length: 26 [text/html]
Saving to: `index.html.2'
100%[===========================================>] 26
2014-05-06 21:16:30 (2.01 MB/s) - `index.html.2' saved [26/26]
root@debian7:/etc/apache2# cat index.html.2
HaCkInG iS fUn At HuNtEr2
Cleaning up the temporary files.
root@debian7:/etc/apache2# rm index.html index.html.1 index.html.2
Try testing from another computer using the ip-address of your server.
697
apache web server
66.3. named virtual hosts on Debian
66.3.1. named virtual hosts
The chess club and the model train club find the port numbers too hard to remember. They
would prefere to have their website accessible by name.
We continue work on the same server that has three websites on three ports. We need to
make sure those websites are accesible using the names choochoo.local, chessclub42.local
and hunter2.local.
We start by creating three new virtualhosts.
root@debian7:/etc/apache2/sites-available#
root@debian7:/etc/apache2/sites-available#
root@debian7:/etc/apache2/sites-available#
root@debian7:/etc/apache2/sites-available#
<VirtualHost *:80>
ServerAdmin webmaster@localhost
ServerName choochoo.local
DocumentRoot /var/www/choochoo
</VirtualHost>
root@debian7:/etc/apache2/sites-available#
<VirtualHost *:80>
ServerAdmin webmaster@localhost
ServerName chessclub42.local
DocumentRoot /var/www/chessclub42
</VirtualHost>
root@debian7:/etc/apache2/sites-available#
<VirtualHost *:80>
ServerAdmin webmaster@localhost
ServerName hunter2.local
DocumentRoot /var/www/hunter2
</VirtualHost>
root@debian7:/etc/apache2/sites-available#
vi choochoo.local
vi chessclub42.local
vi hunter2.local
cat choochoo.local
cat chessclub42.local
cat hunter2.local
Notice that they all listen on port 80 and have an extra ServerName directive.
66.3.2. name resolution
We need some way to resolve names. This can be done with DNS, which is discussed in
another chapter. For this demo it is also possible to quickly add the three names to the /etc/
hosts file.
root@debian7:/etc/apache2/sites-available# grep ^192 /etc/hosts
192.168.42.50 choochoo.local
192.168.42.50 chessclub42.local
192.168.42.50 hunter2.local
Note that you may have another ip address...
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apache web server
66.3.3. enabling virtual hosts
Next we enable them with a2ensite.
root@debian7:/etc/apache2/sites-available# a2ensite choochoo.local
Enabling site choochoo.local.
To activate the new configuration, you need to run:
service apache2 reload
root@debian7:/etc/apache2/sites-available# a2ensite chessclub42.local
Enabling site chessclub42.local.
To activate the new configuration, you need to run:
service apache2 reload
root@debian7:/etc/apache2/sites-available# a2ensite hunter2.local
Enabling site hunter2.local.
To activate the new configuration, you need to run:
service apache2 reload
66.3.4. reload and verify
After a service apache2 reload the websites should be available by name.
root@debian7:/etc/apache2/sites-available# service apache2 reload
Reloading web server config: apache2.
root@debian7:/etc/apache2/sites-available# wget chessclub42.local
--2014-05-06 21:37:13-- http://chessclub42.local/
Resolving chessclub42.local (chessclub42.local)... 192.168.42.50
Connecting to chessclub42.local (chessclub42.local)|192.168.42.50|:80... conne\
cted.
HTTP request sent, awaiting response... 200 OK
Length: 25 [text/html]
Saving to: `index.html'
100%[=============================================>] 25
2014-05-06 21:37:13 (2.06 MB/s) - `index.html' saved [25/25]
root@debian7:/etc/apache2/sites-available# cat index.html
Welcome to chess club 42
699
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in 0s
apache web server
66.4. password protected website on Debian
You can secure files and directories in your website with a .htaccess file that refers to a
.htpasswd file. The htpasswd command can create a .htpasswd file that contains a userid
and an (encrypted) password.
This screenshot creates a user and password for the hacker named cliff and uses the -c flag
to create the .htpasswd file.
root@debian7:~# htpasswd -c /var/www/.htpasswd cliff
New password:
Re-type new password:
Adding password for user cliff
root@debian7:~# cat /var/www/.htpasswd
cliff:$apr1$vujll0KL$./SZ4w9q0swhX93pQ0PVp.
Hacker rob also wants access, this screenshot shows how to add a second user and password
to .htpasswd.
root@debian7:~# htpasswd /var/www/.htpasswd rob
New password:
Re-type new password:
Adding password for user rob
root@debian7:~# cat /var/www/.htpasswd
cliff:$apr1$vujll0KL$./SZ4w9q0swhX93pQ0PVp.
rob:$apr1$HNln1FFt$nRlpF0H.IW11/1DRq4lQo0
Both Cliff and Rob chose the same password (hunter2), but that is not visible in the
.htpasswd file because of the different salts.
Next we need to create a .htaccess file in the DocumentRoot of the website we want to
protect. This screenshot shows an example.
root@debian7:~# cd /var/www/hunter2/
root@debian7:/var/www/hunter2# cat .htaccess
AuthUserFile /var/www/.htpasswd
AuthName "Members only!"
AuthType Basic
require valid-user
Note that we are protecting the website on port 9000 that we created earlier.
And because we put the website for the Hackerspace named hunter2 in a subdirectory of the
default website, we will need to adjust the AllowOvveride parameter in /etc/apache2/sitesavailable/default as this screenshot shows (with line numbers on Debian7, your may vary).
9
10
11
12
13
14
<Directory /var/www/>
Options Indexes FollowSymLinks MultiViews
AllowOverride Authconfig
Order allow,deny
allow from all
</Directory
Now restart the apache2 server and test that it works!
700
apache web server
66.5. port virtual hosts on CentOS
66.5.1. default virtual host
Unlike Debian, CentOS has no virtualHost configuration file for its default website. Instead
the default configuration will throw a standard error page when no index file can be found
in the default location (/var/www/html).
66.5.2. three extra virtual hosts
In this scenario we create three additional websites for three customers that share a clubhouse
and want to jointly hire you. They are a model train club named Choo Choo, a chess club
named Chess Club 42 and a hackerspace named hunter2.
One way to put three websites on one web server, is to put each website on a different port.
This screenshot shows three newly created virtual hosts, one for each customer.
[root@CentOS65 ~]# vi /etc/httpd/conf.d/choochoo.conf
[root@CentOS65 ~]# cat /etc/httpd/conf.d/choochoo.conf
<VirtualHost *:7000>
ServerAdmin webmaster@localhost
DocumentRoot /var/www/html/choochoo
</VirtualHost>
[root@CentOS65 ~]# vi /etc/httpd/conf.d/chessclub42.conf
[root@CentOS65 ~]# cat /etc/httpd/conf.d/chessclub42.conf
<VirtualHost *:8000>
ServerAdmin webmaster@localhost
DocumentRoot /var/www/html/chessclub42
</VirtualHost>
[root@CentOS65 ~]# vi /etc/httpd/conf.d/hunter2.conf
[root@CentOS65 ~]# cat /etc/httpd/conf.d/hunter2.conf
<VirtualHost *:9000>
ServerAdmin webmaster@localhost
DocumentRoot /var/www/html/hunter2
</VirtualHost>
Notice the different port numbers 7000, 8000 and 9000. Notice also that we specified a
unique DocumentRoot for each website.
66.5.3. three extra ports
We need to enable these three ports on apache in the httpd.conf file.
[root@CentOS65 ~]# vi /etc/httpd/conf/httpd.conf
root@debian7:~# grep ^Listen /etc/httpd/conf/httpd.conf
Listen 80
Listen 7000
Listen 8000
Listen 9000
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apache web server
66.5.4. SELinux guards our ports
If we try to restart our server, we will notice the following error:
[root@CentOS65 ~]# service httpd restart
Stopping httpd:
[ OK ]
Starting httpd:
(13)Permission denied: make_sock: could not bind to address 0.0.0.0:7000
no listening sockets available, shutting down
[FAILED]
This is due to SELinux reserving ports 7000 and 8000 for other uses. We need to tell SELinux
we want to use these ports for http traffic
[root@CentOS65 ~]# semanage port -m -t http_port_t -p tcp 7000
[root@CentOS65 ~]# semanage port -m -t http_port_t -p tcp 8000
[root@CentOS65 ~]# service httpd restart
Stopping httpd:
[ OK
Starting httpd:
[ OK
]
]
66.5.5. three extra websites
Next we need to create three DocumentRoot directories.
[root@CentOS65 ~]# mkdir /var/www/html/choochoo
[root@CentOS65 ~]# mkdir /var/www/html/chessclub42
[root@CentOS65 ~]# mkdir /var/www/html/hunter2
And we have to put some really simple website in those directories.
[root@CentOS65 ~]# echo 'Choo Choo model train Choo Choo' > /var/www/html/chooc\
hoo/index.html
[root@CentOS65 ~]# echo 'Welcome to chess club 42' > /var/www/html/chessclub42/\
index.html
[root@CentOS65 ~]# echo 'HaCkInG iS fUn At HuNtEr2' > /var/www/html/hunter2/ind\
ex.html
66.5.6. enabling extra websites
The only way to enable or disable configurations in RHEL/CentOS is by renaming or
moving the configuration files. Any file in /etc/httpd/conf.d ending on .conf will be loaded
by Apache. To disable a site we can either rename the file or move it to another directory.
The files are created, so we can tell apache.
[root@CentOS65 ~]# ls /etc/httpd/conf.d/
chessclub42.conf choochoo.conf hunter2.conf
[root@CentOS65 ~]# service httpd reload
Reloading httpd:
702
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welcome.conf
apache web server
66.5.7. testing the three websites
Testing the model train club named Choo Choo on port 7000.
[root@CentOS65 ~]# wget 127.0.0.1:7000
--2014-05-11 11:59:36-- http://127.0.0.1:7000/
Connecting to 127.0.0.1:7000... connected.
HTTP request sent, awaiting response... 200 OK
Length: 32 [text/html]
Saving to: `index.html'
100%[===========================================>] 32
--.-K/s
in 0s
--.-K/s
in 0s
--.-K/s
in 0s
2014-05-11 11:59:36 (4.47 MB/s) - `index.html' saved [32/32]
[root@CentOS65 ~]# cat index.html
Choo Choo model train Choo Choo
Testing the chess club named Chess Club 42 on port 8000.
[root@CentOS65 ~]# wget 127.0.0.1:8000
--2014-05-11 12:01:30-- http://127.0.0.1:8000/
Connecting to 127.0.0.1:8000... connected.
HTTP request sent, awaiting response... 200 OK
Length: 25 [text/html]
Saving to: `index.html.1'
100%[===========================================>] 25
2014-05-11 12:01:30 (4.25 MB/s) - `index.html.1' saved [25/25]
root@debian7:/etc/apache2# cat index.html.1
Welcome to chess club 42
Testing the hacker club named hunter2 on port 9000.
[root@CentOS65 ~]# wget 127.0.0.1:9000
--2014-05-11 12:02:37-- http://127.0.0.1:9000/
Connecting to 127.0.0.1:9000... connected.
HTTP request sent, awaiting response... 200 OK
Length: 26 [text/html]
Saving to: `index.html.2'
100%[===========================================>] 26
2014-05-11 12:02:37 (4.49 MB/s) - `index.html.2' saved [26/26]
root@debian7:/etc/apache2# cat index.html.2
HaCkInG iS fUn At HuNtEr2
Cleaning up the temporary files.
[root@CentOS65 ~]# rm index.html index.html.1 index.html.2
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66.5.8. firewall rules
If we attempt to access the site from another machine however, we will not be able to view the
website yet. The firewall is blocking incoming connections. We need to open these incoming
ports first
[root@CentOS65
[root@CentOS65
[root@CentOS65
[root@CentOS65
~]#
~]#
~]#
~]#
iptables
iptables
iptables
iptables
-I
-I
-I
-I
INPUT
INPUT
INPUT
INPUT
-p
-p
-p
-p
tcp
tcp
tcp
tcp
--dport
--dport
--dport
--dport
80 -j ACCEPT
7000 -j ACCEPT
8000 -j ACCEPT
9000 -j ACCEPT
And if we want these rules to remain active after a reboot, we need to save them
[root@CentOS65 ~]# service iptables save
iptables: Saving firewall rules to /etc/sysconfig/iptables:[
704
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66.6. named virtual hosts on CentOS
66.6.1. named virtual hosts
The chess club and the model train club find the port numbers too hard to remember. They
would prefere to have their website accessible by name.
We continue work on the same server that has three websites on three ports. We need to
make sure those websites are accesible using the names choochoo.local, chessclub42.local
and hunter2.local.
First, we need to enable named virtual hosts in the configuration
[root@CentOS65 ~]# vi /etc/httpd/conf/httpd.conf
[root@CentOS65 ~]# grep ^NameVirtualHost /etc/httpd/conf/httpd.conf
NameVirtualHost *:80
[root@CentOS65 ~]#
Next we need to create three new virtualhosts.
[root@CentOS65 ~]# vi /etc/httpd/conf.d/choochoo.local.conf
[root@CentOS65 ~]# vi /etc/httpd/conf.d/chessclub42.local.conf
[root@CentOS65 ~]# vi /etc/httpd/conf.d/hunter2.local.conf
[root@CentOS65 ~]# cat /etc/httpd/conf.d/choochoo.local.conf
<VirtualHost *:80>
ServerAdmin webmaster@localhost
ServerName choochoo.local
DocumentRoot /var/www/html/choochoo
</VirtualHost>
[root@CentOS65 ~]# cat /etc/httpd/conf.d/chessclub42.local.conf
<VirtualHost *:80>
ServerAdmin webmaster@localhost
ServerName chessclub42.local
DocumentRoot /var/www/html/chessclub42
</VirtualHost>
[root@CentOS65 ~]# cat /etc/httpd/conf.d/hunter2.local.conf
<VirtualHost *:80>
ServerAdmin webmaster@localhost
ServerName hunter2.local
DocumentRoot /var/www/html/hunter2
</VirtualHost>
[root@CentOS65 ~]#
Notice that they all listen on port 80 and have an extra ServerName directive.
66.6.2. name resolution
We need some way to resolve names. This can be done with DNS, which is discussed in
another chapter. For this demo it is also possible to quickly add the three names to the /etc/
hosts file.
[root@CentOS65 ~]# grep ^192 /etc/hosts
192.168.1.225 choochoo.local
192.168.1.225 chessclub42.local
192.168.1.225 hunter2.local
Note that you may have another ip address...
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apache web server
66.6.3. reload and verify
After a service httpd reload the websites should be available by name.
[root@CentOS65 ~]# service httpd reload
Reloading httpd:
[root@CentOS65 ~]# wget chessclub42.local
--2014-05-25 16:59:14-- http://chessclub42.local/
Resolving chessclub42.local... 192.168.1.225
Connecting to chessclub42.local|192.168.1.225|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 25 [text/html]
Saving to: âindex.htmlâ
100%[=============================================>] 25
2014-05-25 16:59:15 (1014 KB/s) - `index.html' saved [25/25]
[root@CentOS65 ~]# cat index.html
Welcome to chess club 42
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--.-K/s
in 0s
apache web server
66.7. password protected website on CentOS
You can secure files and directories in your website with a .htaccess file that refers to a
.htpasswd file. The htpasswd command can create a .htpasswd file that contains a userid
and an (encrypted) password.
This screenshot creates a user and password for the hacker named cliff and uses the -c flag
to create the .htpasswd file.
[root@CentOS65 ~]# htpasswd -c /var/www/.htpasswd cliff
New password:
Re-type new password:
Adding password for user cliff
[root@CentOS65 ~]# cat /var/www/.htpasswd
cliff:QNwTrymMLBctU
Hacker rob also wants access, this screenshot shows how to add a second user and password
to .htpasswd.
[root@CentOS65 ~]# htpasswd /var/www/.htpasswd rob
New password:
Re-type new password:
Adding password for user rob
[root@CentOS65 ~]# cat /var/www/.htpasswd
cliff:QNwTrymMLBctU
rob:EC2vOCcrMXDoM
[root@CentOS65 ~]#
Both Cliff and Rob chose the same password (hunter2), but that is not visible in the
.htpasswd file because of the different salts.
Next we need to create a .htaccess file in the DocumentRoot of the website we want to
protect. This screenshot shows an example.
[root@CentOS65 ~]# cat /var/www/html/hunter2/.htaccess
AuthUserFile /var/www/.htpasswd
AuthName "Members only!"
AuthType Basic
require valid-user
Note that we are protecting the website on port 9000 that we created earlier.
And because we put the website for the Hackerspace named hunter2 in a subdirectory of the
default website, we will need to adjust the AllowOvveride parameter in /etc/httpd/conf/
httpd.conf under the <Directory "/var/www/html"> directive as this screenshot shows.
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apache web server
[root@CentOS65 ~]# vi /etc/httpd/conf/httpd.conf
<Directory "/var/www/html">
#
#
#
#
#
#
#
#
#
#
#
#
Possible values for the Options directive are "None", "All",
or any combination of:
Indexes Includes FollowSymLinks SymLinksifOwnerMatch ExecCGI MultiViews
Note that "MultiViews" must be named *explicitly* --- "Options All"
doesn't give it to you.
The Options directive is both complicated and important.
http://httpd.apache.org/docs/2.2/mod/core.html#options
for more information.
Please see
Options Indexes FollowSymLinks
#
# AllowOverride controls what directives may be placed in .htaccess files.
# It can be "All", "None", or any combination of the keywords:
#
Options FileInfo AuthConfig Limit
#
AllowOverride Authconfig
#
# Controls who can get stuff from this server.
#
Order allow,deny
Allow from all
</Directory>
Now restart the apache2 server and test that it works!
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apache web server
66.8. troubleshooting apache
When apache restarts, it will verify the syntax of files in the configuration folder /etc/
apache2 on debian or /etc/httpd on CentOS and it will tell you the name of the faulty file,
the line number and an explanation of the error.
root@debian7:~# service apache2 restart
apache2: Syntax error on line 268 of /etc/apache2/apache2.conf: Syntax error o\
n line 1 of /etc/apache2/sites-enabled/chessclub42: /etc/apache2/sites-enabled\
/chessclub42:4: <VirtualHost> was not closed.\n/etc/apache2/sites-enabled/ches\
sclub42:1: <VirtualHost> was not closed.
Action 'configtest' failed.
The Apache error log may have more information.
failed!
Below you see the problem... a missing / before on line 4.
root@debian7:~# cat /etc/apache2/sites-available/chessclub42
<VirtualHost *:8000>
ServerAdmin webmaster@localhost
DocumentRoot /var/www/chessclub42
<VirtualHost>
Let us force another error by renaming the directory of one of our websites:
root@debian7:~# mv /var/www/choochoo/ /var/www/chooshoo
root@debian7:~# !ser
service apache2 restart
Restarting web server: apache2Warning: DocumentRoot [/var/www/choochoo] does n\
ot exist
Warning: DocumentRoot [/var/www/choochoo] does not exist
... waiting Warning: DocumentRoot [/var/www/choochoo] does not exist
Warning: DocumentRoot [/var/www/choochoo] does not exist
.
As you can see, apache will tell you exactly what is wrong.
You can also troubleshoot by connecting to the website via a browser and then checking the
apache log files in /var/log/apache.
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apache web server
66.9. virtual hosts example
Below is a sample virtual host configuration. This virtual hosts overrules the default Apache
ErrorDocument directive.
<VirtualHost 83.217.76.245:80>
ServerName cobbaut.be
ServerAlias www.cobbaut.be
DocumentRoot /home/paul/public_html
ErrorLog /home/paul/logs/error_log
CustomLog /home/paul/logs/access_log common
ScriptAlias /cgi-bin/ /home/paul/cgi-bin/
<Directory /home/paul/public_html>
Options Indexes IncludesNOEXEC FollowSymLinks
allow from all
</Directory>
ErrorDocument 404 http://www.cobbaut.be/cobbaut.php
</VirtualHost>
66.10. aliases and redirects
Apache supports aliases for directories, like this example shows.
Alias /paul/ "/home/paul/public_html/"
Similarly, content can be redirected to another website or web server.
Redirect permanent /foo http://www.foo.com/bar
66.11. more on .htaccess
You can do much more with .htaccess. One example is to use .htaccess to prevent people
from certain domains to access your website. Like in this case, where a number of referer
spammers are blocked from the website.
paul@lounge:~/cobbaut.be$ cat .htaccess
# Options +FollowSymlinks
RewriteEngine On
RewriteCond %{HTTP_REFERER} ^http://(www\.)?buy-adipex.fw.nu.*$ [OR]
RewriteCond %{HTTP_REFERER} ^http://(www\.)?buy-levitra.asso.ws.*$ [NC,OR]
RewriteCond %{HTTP_REFERER} ^http://(www\.)?buy-tramadol.fw.nu.*$ [NC,OR]
RewriteCond %{HTTP_REFERER} ^http://(www\.)?buy-viagra.lookin.at.*$ [NC,OR]
...
RewriteCond %{HTTP_REFERER} ^http://(www\.)?www.healthinsurancehelp.net.*$ [NC]
RewriteRule .* - [F,L]
paul@lounge:~/cobbaut.be$
66.12. traffic
Apache keeps a log of all visitors. The webalizer is often used to parse this log into nice
html statistics.
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apache web server
66.13. self signed cert on Debian
Below is a very quick guide on setting up Apache2 on Debian 7 with a self-signed certificate.
Chances are these packages are already installed.
root@debian7:~# aptitude install apache2 openssl
No packages will be installed, upgraded, or removed.
0 packages upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 0 B of archives. After unpacking 0 B will be used.
Create a directory to store the certs, and use openssl to create a self signed cert that is valid
for 999 days.
root@debian7:~# mkdir /etc/ssl/localcerts
root@debian7:~# openssl req -new -x509 -days 999 -nodes -out /etc/ssl/local\
certs/apache.pem -keyout /etc/ssl/localcerts/apache.key
Generating a 2048 bit RSA private key
...
...
writing new private key to '/etc/ssl/localcerts/apache.key'
----You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
----Country Name (2 letter code) [AU]:BE
State or Province Name (full name) [Some-State]:Antwerp
Locality Name (eg, city) []:Antwerp
Organization Name (eg, company) [Internet Widgits Pty Ltd]:linux-training.be
Organizational Unit Name (eg, section) []:
Common Name (e.g. server FQDN or YOUR name) []:Paul
Email Address []:
A little security never hurt anyone.
root@debian7:~# ls -l /etc/ssl/localcerts/
total 8
-rw-r--r-- 1 root root 1704 Sep 16 18:24 apache.key
-rw-r--r-- 1 root root 1302 Sep 16 18:24 apache.pem
root@debian7:~# chmod 600 /etc/ssl/localcerts/*
root@debian7:~# ls -l /etc/ssl/localcerts/
total 8
-rw------- 1 root root 1704 Sep 16 18:24 apache.key
-rw------- 1 root root 1302 Sep 16 18:24 apache.pem
Enable the apache ssl mod.
root@debian7:~# a2enmod ssl
Enabling module ssl.
See /usr/share/doc/apache2.2-common/README.Debian.gz on how to configure SSL\
and create self-signed certificates.
To activate the new configuration, you need to run:
service apache2 restart
Create the website configuration.
root@debian7:~# vi /etc/apache2/sites-available/choochoos
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apache web server
root@debian7:~# cat /etc/apache2/sites-available/choochoos
<VirtualHost *:7000>
ServerAdmin webmaster@localhost
DocumentRoot /var/www/choochoos
SSLEngine On
SSLCertificateFile /etc/ssl/localcerts/apache.pem
SSLCertificateKeyFile /etc/ssl/localcerts/apache.key
</VirtualHost>
root@debian7:~#
And create the website itself.
root@debian7:/var/www/choochoos# vi index.html
root@debian7:/var/www/choochoos# cat index.html
Choo Choo HTTPS secured model train Choo Choo
Enable the website and restart (or reload) apache2.
root@debian7:/var/www/choochoos# a2ensite choochoos
Enabling site choochoos.
To activate the new configuration, you need to run:
service apache2 reload
root@debian7:/var/www/choochoos# service apache2 restart
Restarting web server: apache2 ... waiting .
Chances are your browser will warn you about the self signed certificate.
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apache web server
66.14. self signed cert on RHEL/CentOS
Below is a quick way to create a self signed cert for https on RHEL/CentOS. You may need
these packages:
[root@paulserver ~]# yum install httpd openssl mod_ssl
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
* base: ftp.belnet.be
* extras: ftp.belnet.be
* updates: mirrors.vooservers.com
base
| 3.7 kB
00:00
Setting up Install Process
Package httpd-2.2.15-31.el6.centos.x86_64 already installed and latest version
Package openssl-1.0.1e-16.el6_5.15.x86_64 already installed and latest version
Package 1:mod_ssl-2.2.15-31.el6.centos.x86_64 already ins... and latest version
Nothing to do
We use openssl to create the certificate.
[root@paulserver ~]# mkdir certs
[root@paulserver ~]# cd certs
[root@paulserver certs]# openssl genrsa -out ca.key 2048
Generating RSA private key, 2048 bit long modulus
.........+++
.........................................................+++
e is 65537 (0x10001)
[root@paulserver certs]# openssl req -new -key ca.key -out ca.csr
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
----Country Name (2 letter code) [XX]:BE
State or Province Name (full name) []:antwerp
Locality Name (eg, city) [Default City]:antwerp
Organization Name (eg, company) [Default Company Ltd]:antwerp
Organizational Unit Name (eg, section) []:
Common Name (eg, your name or your server's hostname) []:paulserver
Email Address []:
Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:
An optional company name []:
[root@paulserver certs]# openssl x509 -req -days 365 -in ca.csr -signkey ca.ke\
y -out ca.crt
Signature ok
subject=/C=BE/ST=antwerp/L=antwerp/O=antwerp/CN=paulserver
Getting Private key
We copy the keys to the right location (You may be missing SELinux info here).
[root@paulserver certs]# cp ca.crt /etc/pki/tls/certs/
[root@paulserver certs]# cp ca.key ca.csr /etc/pki/tls/private/
We add the location of our keys to this file, and also add the NameVirtualHost *:443
directive.
[root@paulserver certs]# vi /etc/httpd/conf.d/ssl.conf
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apache web server
[root@paulserver certs]# grep ^SSLCerti /etc/httpd/conf.d/ssl.conf
SSLCertificateFile /etc/pki/tls/certs/ca.crt
SSLCertificateKeyFile /etc/pki/tls/private/ca.key
Create a website configuration.
[root@paulserver certs]# vi /etc/httpd/conf.d/choochoos.conf
[root@paulserver certs]# cat /etc/httpd/conf.d/choochoos.conf
<VirtualHost *:443>
SSLEngine on
SSLCertificateFile /etc/pki/tls/certs/ca.crt
SSLCertificateKeyFile /etc/pki/tls/private/ca.key
DocumentRoot /var/www/choochoos
ServerName paulserver
</VirtualHost>
[root@paulserver certs]#
Create a simple website and restart apache.
[root@paulserver certs]# mkdir /var/www/choochoos
[root@paulserver certs]# echo HTTPS model train choochoos > /var/www/choochoos/\
index.html
[root@paulserver httpd]# service httpd restart
Stopping httpd:
[ OK ]
Starting httpd:
[ OK ]
And your browser will probably warn you that this certificate is self signed.
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apache web server
66.15. practice: apache
1. Verify that Apache is installed and running.
2. Browse to the Apache HTML manual.
3. Create three virtual hosts that listen on ports 8472, 31337 and 1201. Test that it all works.
4. Create three named virtual hosts startrek.local, starwars.local and stargate.local. Test that
it all works.
5. Create a virtual hosts that listens on another ip-address.
6. Protect one of your websites with a user/password combo.
715
Chapter 67. introduction to squid
67.1. about proxy servers
67.1.1. usage
A proxy server is a server that caches the internet. Clients connect to the proxy server with
a request for an internet server. The proxy server will connect to the internet server on behalf
of the client. The proxy server will also cache the pages retrieved from the internet server.
A proxy server may provide pages from his cache to a client, instead of connecting to the
internet server to retrieve the (same) pages.
A proxy server has two main advantages. It improves web surfing speed when returning
cached data to clients, and it reduces the required bandwidth (cost) to the internet.
Smaller organizations sometimes put the proxy server on the same physical computer that
serves as a NAT to the internet. In larger organizations, the proxy server is one of many
servers in the DMZ.
When web traffic passes via a proxy server, it is common practice to configure the proxy
with extra settings for access control. Access control in a proxy server can mean user account
access, but also website(url), ip-address or dns restrictions.
67.1.2. open proxy servers
You can find lists of open proxy servers on the internet that enable you to surf anonymously.
This works when the proxy server connects on your behalf to a website, without logging
your ip-address. But be careful, these (listed) open proxy servers could be created in order
to eavesdrop upon their users.
67.1.3. squid
This module is an introduction to the squid proxy server (http://www.squid-cache.org). We
will first configure squid as a normal proxy server.
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introduction to squid
67.2. installing squid
This screenshot shows how to install squid on Debian with aptitude. Use yum if you are
on Red Hat/CentOS.
root@debian7:~# aptitude install squid
The following NEW packages will be installed:
squid squid-common{a} squid-langpack{a}
0 packages upgraded, 3 newly installed, 0 to remove and 0 not upgraded.
Need to get 1,513 kB of archives. After unpacking 4,540 kB will be used.
Do you want to continue? [Y/n/?]
...output truncated...
Setting up squid-langpack (20120616-1) ...
Setting up squid-common (2.7.STABLE9-4.1) ...
Setting up squid (2.7.STABLE9-4.1) ...
Creating squid spool directory structure
2014/08/01 15:19:31| Creating Swap Directories
Restarting Squid HTTP proxy: squid.
squid's main configuration file is /etc/squid/squid.conf. The file explains every parameter
in great detail.
root@debian7:~# wc -l /etc/squid/squid.conf
4948 /etc/squid/squid.conf
67.3. port 3128
By default the squid proxy server will lsiten to port 3128.
root@debian7:~# grep ^http_port /etc/squid/squid.conf
http_port 3128
root@debian7:~#
67.4. starting and stopping
You can manage squid with the standard service command as shown in this screenshot.
root@debian7:~# service squid start
Starting Squid HTTP proxy: squid.
root@debian7:~# service squid restart
Restarting Squid HTTP proxy: squid.
root@debian7:~# service squid status
squid is running.
root@debian7:~# service squid stop
Stopping Squid HTTP proxy: squid.
root@debian7:~#
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introduction to squid
67.5. client proxy settings
To enable a proxy server in Firefox or Iceweasel go to Edit Preferences and configure as
shown in this screenshot (replace 192.168.1.60 with the ip address of your proxy server).
Test that your internet works with the proxy enabled. Also test that after a service squid
stop command on your proxy server that you get a message similar to this schreenshot.
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introduction to squid
To enable a proxy server with Google Chrome (or Debian Chromium) start the program
from the command line like this:
paul@debian7:~$ chromium --proxy-server='192.168.1.60:3128'
Disabling the proxy with service squid stop should result in an error message similar to
this screenshot.
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introduction to squid
67.6. upside down images
A proxy server sits inbetween your browser and the internet. So besides caching of internet
data (the original function of a proxy server) and besides firewall like restrictions based on
www content, a proxy server is in the perfect position to alter the webpages that you visit.
You could for instance change the advertising on a webpage (or remove certain advertisers),
or like we do in this example; change all images so they are upside down.
The server needs command line tools to manipulate images and a perl script that uses these
tools (and wget to download the images locally and serve them with apache2). In this
example we use imagemagick (which provides tools like convert and mogrify).
root@debian7:~# aptitude install imagemagick wget perl apache2
...output truncated...
root@debian7:~# dpkg -S $(readlink -f $(which mogrify))
imagemagick: /usr/bin/mogrify.im6
root@debian7:~#
The perl script that is shown in the screenshot below can be found on several websites, yet
I have not found the original author. It is however a very simple script that uses wget and
mogrify to download images (.jpg .gif and .png), flip them and store them in /var/www/
images.
root@debian7:~# cat /usr/local/bin/flip.pl
#!/usr/bin/perl
$|=1;
$count = 0;
$pid = $$;
while (<>) {
chomp $_;
if ($_ =~ /(.*\.jpg)/i) {
$url = $1;
system("/usr/bin/wget", "-q", "-O","/var/www/images/$pid-$count.jpg", "$url");
system("/usr/bin/mogrify", "-flip","/var/www/images/$pid-$count.jpg");
print "http://127.0.0.1/images/$pid-$count.jpg\n";
}
elsif ($_ =~ /(.*\.gif)/i) {
$url = $1;
system("/usr/bin/wget", "-q", "-O","/var/www/images/$pid-$count.gif", "$url");
system("/usr/bin/mogrify", "-flip","/var/www/images/$pid-$count.gif");
print "http://127.0.0.1/images/$pid-$count.gif\n";
}
elsif ($_ =~ /(.*\.png)/i) {
$url = $1;
system("/usr/bin/wget", "-q", "-O","/var/www/images/$pid-$count.png", "$url");
system("/usr/bin/mogrify", "-flip","/var/www/images/$pid-$count.png");
print "http://127.0.0.1/images/$pid-$count.png\n";
}
else {
print "$_\n";;
}
$count++;
}
Change (or enable) also the following line in /etc/squid/suiqd.conf.
http_access allow localnet
http_port 3128 transparent
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introduction to squid
url_rwwrite_program /usr/local/bin/flip.pl
The directory this script uses is /var/www/images and should be accessible by both the
squid server (which uses the user named proxy and by the apache2 webserver (which
uses the user www-data. The screenshot below shows how to create this directory, set the
permissions and make the users a member of the other groups.
root@debian7:~#
root@debian7:~#
root@debian7:~#
root@debian7:~#
root@debian7:~#
mkdir /var/www/images
chown www-data:www-data /var/www/images
chmod 755 /var/www/images
usermod -aG www-data proxy
usermod -aG proxy www-data
Test that it works after restarting squid and apache2.
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introduction to squid
67.7. /var/log/squid
The standard log file location for squid is /var/log/squid.
[root@RHEL4 ~]# grep "/var/log" /etc/squid/squid.conf
# cache_access_log /var/log/squid/access.log
# cache_log /var/log/squid/cache.log
# cache_store_log /var/log/squid/store.log
67.8. access control
The default squid setup only allows localhost access. To enable access for a private network
range, look for the "INSERT YOUR OWN RULE(S) HERE..." sentence in squid.conf and
add two lines similar to the screenshot below.
# INSERT YOUR OWN RULE(S) HERE TO ALLOW ACCESS FROM YOUR CLIENTS
acl company_network src 192.168.1.0/24
http_access allow company_network
67.9. testing squid
First, make sure that the server running squid has access to the internet.
[root@RHEL4
[root@RHEL4
-rw-r--r-[root@RHEL4
~]# wget -q http://linux-training.be/index.html
~]# ls -l index.html
1 root root 2269 Sep 18 13:18 index.html
~]#
Then configure a browser on a client to use the proxy server, or you could set the
HTTP_PROXY (sometimes http_proxy) variable to point command line programs to the
proxy.
[root@fedora ~]# export HTTP_PROXY=http://192.168.1.39:8080
[root@ubuntu ~]# export http_proxy=http://192.168.1.39:8080
Testing a client machine can then be done with wget (wget -q is used to simplify the
screenshot).
[root@RHEL5 ~]# > /etc/resolv.conf
[root@RHEL5 ~]# wget -q http://www.linux-training.be/index.html
[root@RHEL5 ~]# ls -l index.html
-rw-r--r-- 1 root root 2269 Sep 18 2008 index.html
[root@RHEL5 ~]#
67.10. name resolution
You need name resolution working on the squid server, but you don't need name resolution
on the clients.
[paul@RHEL5 ~]$ wget http://grep.be
--14:35:44-- http://grep.be
Resolving grep.be... failed: Temporary failure in name resolution.
[paul@RHEL5 ~]$ export http_proxy=http://192.168.1.39:8080
[paul@RHEL5 ~]$ wget http://grep.be
--14:35:49-- http://grep.be/
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introduction to squid
Connecting to 192.168.1.39:8080... connected.
Proxy request sent, awaiting response... 200 OK
Length: 5390 (5.3K) [text/html]
Saving to: `index.html.1'
100%[================================>] 5,390
--.-K/s
14:38:29 (54.8 KB/s) - `index.html' saved [5390/5390]
[paul@RHEL5 ~]$
723
in 0.1s
Part XVIII. mysql database
Table of Contents
68. introduction to sql using mysql .......................................................................................
68.1. installing mysql ....................................................................................................
68.2. accessing mysql ....................................................................................................
68.3. mysql databases ....................................................................................................
68.4. mysql tables .........................................................................................................
68.5. mysql records .......................................................................................................
68.6. joining two tables .................................................................................................
68.7. mysql triggers ......................................................................................................
725
726
727
728
730
732
734
737
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Chapter 68. introduction to sql using
mysql
mysql is a database server that understands Structured Query Language (SQL). MySQL
was developed by the Swedish Company MySQL AB. The first release was in 1995. In
2008 MySQL AB was bought by Sun Microsystems (which is now owned by Oracle).
mysql is very popular for websites in combination with php and apache (the m in lamp
servers), but mysql is also used in organizations with huge databases like Facebook, Flickr,
Google, Nokia, Wikipedia and Youtube.
This chapter will teach you sql by creating and using small databases, tables, queries and a
simple trigger in a local mysql server.
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introduction to sql using mysql
68.1. installing mysql
On Debian/Ubuntu you can use aptitude install mysql-server to install the mysql server
and client.
root@ubu1204~# aptitude install mysql-server
The following NEW packages will be installed:
libdbd-mysql-perl{a} libdbi-perl{a} libhtml-template-perl{a}
libnet-daemon-perl{a} libplrpc-perl{a} mysql-client-5.5{a}
mysql-client-core-5.5{a} mysql-server mysql-server-5.5{a}
mysql-server-core-5.5{a}
0 packages upgraded, 10 newly installed, 0 to remove and 1 not upgraded.
Need to get 25.5 MB of archives. After unpacking 88.4 MB will be used.
Do you want to continue? [Y/n/?]
During the installation you will be asked to provide a password for the root mysql user,
remember this password (or use hunter2 like i do.
To verify the installed version, use dpkg -l on Debian/Ubuntu. This screenshot shows
version 5.0 installed.
root@ubu1204~# dpkg -l mysql-server | tail -1 | tr -s ' ' | cut -c-72
ii mysql-server 5.5.24-0ubuntu0.12.04.1 MySQL database server (metapacka
Issue rpm -q to get version information about MySQL on Red Hat/Fedora/CentOS.
[paul@RHEL52 ~]$ rpm -q mysql-server
mysql-server-5.0.45-7.el5
You will need at least version 5.0 to work with triggers.
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introduction to sql using mysql
68.2. accessing mysql
68.2.1. Linux users
The installation of mysql creates a user account in /etc/passwd and a group account in /
etc/group.
kevin@ubu1204:~$ tail -1 /etc/passwd
mysql:x:120:131:MySQL Server,,,:/nonexistent:/bin/false
kevin@ubu1204:~$ tail -1 /etc/group
mysql:x:131:
The mysql daemon mysqld will run with the credentials of this user and group.
root@ubu1204~# ps -eo uid,user,gid,group,comm | grep mysqld
120 mysql
131 mysql
mysqld
68.2.2. mysql client application
You can now use mysql from the commandline by just typing mysql -u root -p and you
'll be asked for the password (of the mysql root account). In the screenshot below the user
typed exit to exit the mysql console.
root@ubu1204~# mysql -u root -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 43
Server version: 5.5.24-0ubuntu0.12.04.1 (Ubuntu)
Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql> exit
Bye
You could also put the password in clear text on the command line, but that would not be
very secure. Anyone with access to your bash history would be able to read your mysql root
password.
root@ubu1204~# mysql -u root -phunter2
Welcome to the MySQL monitor. Commands end with ; or \g.
...
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introduction to sql using mysql
68.2.3. ~/.my.cnf
You can save configuration in your home directory in the hidden file .my.cnf. In the
screenshot below we put the root user and password in .my.cnf.
kevin@ubu1204:~$ pwd
/home/kevin
kevin@ubu1204:~$ cat .my.cnf
[client]
user=root
password=hunter2
kevin@ubu1204:~$
This enables us to log on as the root mysql user just by typing mysql.
kevin@ubu1204:~$ mysql
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 56
Server version: 5.5.24-0ubuntu0.12.04.1 (Ubuntu)
68.2.4. the mysql command line client
You can use the mysql command to take a look at the databases, and to execute SQL queries
on them. The screenshots below show you how.
Here we execute the command show databases. Every command must be terminated by a
delimiter. The default delimiter is ; (the semicolon).
mysql> show databases;
+--------------------+
| Database
|
+--------------------+
| information_schema |
| mysql
|
| performance_schema |
| test
|
+--------------------+
4 rows in set (0.00 sec)
We will use this prompt in the next sections.
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introduction to sql using mysql
68.3. mysql databases
68.3.1. listing all databases
You can use the mysql command to take a look at the databases, and to execute SQL queries
on them. The screenshots below show you how. First, we log on to our MySQL server and
execute the command show databases to see which databases exist on our mysql server.
kevin@ubu1204:~$ mysql
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 57
Server version: 5.5.24-0ubuntu0.12.04.1 (Ubuntu)
Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql> show databases;
+--------------------+
| Database
|
+--------------------+
| information_schema |
| mysql
|
| performance_schema |
| test
|
+--------------------+
4 rows in set (0.00 sec)
68.3.2. creating a database
You can create a new database with the create database command.
mysql> create database famouspeople;
Query OK, 1 row affected (0.00 sec)
mysql> show databases;
+--------------------+
| Database
|
+--------------------+
| information_schema |
| famouspeople
|
| mysql
|
| performance_schema |
| test
|
+--------------------+
5 rows in set (0.00 sec)
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introduction to sql using mysql
68.3.3. using a database
Next we tell mysql to use one particular database with the use $database command. This
screenshot shows how to make wikidb the current database (in use).
mysql> use famouspeople;
Database changed
mysql>
68.3.4. access to a database
To give someone access to a mysql database, use the grant command.
mysql> grant all on famouspeople.* to kevin@localhost IDENTIFIED BY "hunter2";
Query OK, 0 rows affected (0.00 sec)
68.3.5. deleting a database
When a database is no longer needed, you can permanently remove it with the drop database
command.
mysql> drop database demodb;
Query OK, 1 row affected (0.09 sec)
68.3.6. backup and restore a database
You can take a backup of a database, or move it to another computer using the mysql and
mysqldump commands. In the screenshot below, we take a backup of the wikidb database
on the computer named laika.
mysqldump -u root famouspeople > famouspeople.backup.20120708.sql
Here is a screenshot of a database restore operation from this backup.
mysql -u root famouspeople < famouspeople.backup.20120708.sql
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introduction to sql using mysql
68.4. mysql tables
68.4.1. listing tables
You can see a list of tables in the current database with the show tables; command. Our
famouspeople database has no tables yet.
mysql> use famouspeople;
Database changed
mysql> show tables;
Empty set (0.00 sec)
68.4.2. creating a table
The create table command will create a new table.
This screenshot shows the creation of a country table. We use the countrycode as a primary
key (all country codes are uniquely defined). Most country codes are two or three letters, so
a char of three uses less space than a varchar of three. The country name and the name of
the capital are both defined as varchar. The population can be seen as an integer.
mysql> create table country (
-> countrycode char(3) NOT NULL,
-> countryname varchar(70) NOT NULL,
-> population int,
-> countrycapital varchar(50),
-> primary key (countrycode)
-> );
Query OK, 0 rows affected (0.19 sec)
mysql> show tables;
+------------------------+
| Tables_in_famouspeople |
+------------------------+
| country
|
+------------------------+
1 row in set (0.00 sec)
mysql>
You are allowed to type the create table command on one long line, but administrators often
use multiple lines to improve readability.
mysql> create table country ( countrycode char(3) NOT NULL, countryname\
varchar(70) NOT NULL, population int, countrycapital varchar(50), prim\
ary key (countrycode) );
Query OK, 0 rows affected (0.18 sec)
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introduction to sql using mysql
68.4.3. describing a table
To see a description of the structure of a table, issue the describe $tablename command
as shown below.
mysql> describe country;
+----------------+-------------+------+-----+---------+-------+
| Field
| Type
| Null | Key | Default | Extra |
+----------------+-------------+------+-----+---------+-------+
| countrycode
| char(3)
| NO
| PRI | NULL
|
|
| countryname
| varchar(70) | NO
|
| NULL
|
|
| population
| int(11)
| YES |
| NULL
|
|
| countrycapital | varchar(50) | YES |
| NULL
|
|
+----------------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)
68.4.4. removing a table
To remove a table from a database, issue the drop table $tablename command as shown
below.
mysql> drop table country;
Query OK, 0 rows affected (0.00 sec)
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introduction to sql using mysql
68.5. mysql records
68.5.1. creating records
Use insert to enter data into the table. The screenshot shows several insert statements that
insert values depending on the position of the data in the statement.
mysql> insert into country values ('BE','Belgium','11000000','Brussels');
Query OK, 1 row affected (0.05 sec)
mysql> insert into country values ('DE','Germany','82000000','Berlin');
Query OK, 1 row affected (0.05 sec)
mysql> insert into country values ('JP','Japan','128000000','Tokyo');
Query OK, 1 row affected (0.05 sec)
Some administrators prefer to use uppercase for sql keywords. The mysql client accepts
both.
mysql> INSERT INTO country VALUES ('FR','France','64000000','Paris');
Query OK, 1 row affected (0.00 sec)
Note that you get an error when using a duplicate primary key.
mysql> insert into country values ('DE','Germany','82000000','Berlin');
ERROR 1062 (23000): Duplicate entry 'DE' for key 'PRIMARY'
68.5.2. viewing all records
Below an example of a simple select query to look at the contents of a table.
mysql> select * from country;
+-------------+---------------+------------+----------------+
| countrycode | countryname
| population | countrycapital |
+-------------+---------------+------------+----------------+
| BE
| Belgium
|
11000000 | Brussels
|
| CN
| China
| 1400000000 | Beijing
|
| DE
| Germany
|
82000000 | Berlin
|
| FR
| France
|
64000000 | Paris
|
| IN
| India
| 1300000000 | New Delhi
|
| JP
| Japan
| 128000000 | Tokyo
|
| MX
| Mexico
| 113000000 | Mexico City
|
| US
| United States | 313000000 | Washington
|
+-------------+---------------+------------+----------------+
8 rows in set (0.00 sec)
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introduction to sql using mysql
68.5.3. updating records
Consider the following insert statement. The capital of Spain is not Barcelona, it is Madrid.
mysql> insert into country values ('ES','Spain','48000000','Barcelona');
Query OK, 1 row affected (0.08 sec)
Using an update statement, the record can be updated.
mysql> update country set countrycapital='Madrid' where countrycode='ES';
Query OK, 1 row affected (0.07 sec)
Rows matched: 1 Changed: 1 Warnings: 0
We can use a select statement to verify this change.
mysql> select * from country;
+-------------+---------------+------------+----------------+
| countrycode | countryname
| population | countrycapital |
+-------------+---------------+------------+----------------+
| BE
| Belgium
|
11000000 | Brussels
|
| CN
| China
| 1400000000 | Beijing
|
| DE
| Germany
|
82000000 | Berlin
|
| ES
| Spain
|
48000000 | Madrid
|
| FR
| France
|
64000000 | Paris
|
| IN
| India
| 1300000000 | New Delhi
|
| JP
| Japan
| 128000000 | Tokyo
|
| MX
| Mexico
| 113000000 | Mexico City
|
| US
| United States | 313000000 | Washington
|
+-------------+---------------+------------+----------------+
9 rows in set (0.00 sec)
68.5.4. viewing selected records
Using a where clause in a select statement, you can specify which record(s) you want to see.
mysql> SELECT * FROM country WHERE countrycode='ES';
+-------------+-------------+------------+----------------+
| countrycode | countryname | population | countrycapital |
+-------------+-------------+------------+----------------+
| ES
| Spain
|
48000000 | Madrid
|
+-------------+-------------+------------+----------------+
1 row in set (0.00 sec)
Another example of the where clause.
mysql> select * from country where countryname='Spain';
+-------------+-------------+------------+----------------+
| countrycode | countryname | population | countrycapital |
+-------------+-------------+------------+----------------+
| ES
| Spain
|
48000000 | Madrid
|
+-------------+-------------+------------+----------------+
1 row in set (0.00 sec)
68.5.5. primary key in where clause ?
The primary key of a table is a field that uniquely identifies every record (every row) in
the table. when using another field in the where clause, it is possible to get multiple rows
returned.
mysql> insert into country values ('EG','Egypt','82000000','Cairo');
735
introduction to sql using mysql
Query OK, 1 row affected (0.33 sec)
mysql> select * from country where population='82000000';
+-------------+-------------+------------+----------------+
| countrycode | countryname | population | countrycapital |
+-------------+-------------+------------+----------------+
| DE
| Germany
|
82000000 | Berlin
|
| EG
| Egypt
|
82000000 | Cairo
|
+-------------+-------------+------------+----------------+
2 rows in set (0.00 sec)
68.5.6. ordering records
We know that select allows us to see all records in a table. Consider this table.
mysql> select countryname,population from country;
+---------------+------------+
| countryname
| population |
+---------------+------------+
| Belgium
|
11000000 |
| China
| 1400000000 |
| Germany
|
82000000 |
| Egypt
|
82000000 |
| Spain
|
48000000 |
| France
|
64000000 |
| India
| 1300000000 |
| Japan
| 128000000 |
| Mexico
| 113000000 |
| United States | 313000000 |
+---------------+------------+
10 rows in set (0.00 sec)
Using the order by clause, we can change the order in which the records are presented.
mysql> select countryname,population from country order by countryname;
+---------------+------------+
| countryname
| population |
+---------------+------------+
| Belgium
|
11000000 |
| China
| 1400000000 |
| Egypt
|
82000000 |
| France
|
64000000 |
| Germany
|
82000000 |
| India
| 1300000000 |
| Japan
| 128000000 |
| Mexico
| 113000000 |
| Spain
|
48000000 |
| United States | 313000000 |
+---------------+------------+
10 rows in set (0.00 sec)
68.5.7. grouping records
Consider this table of people. The screenshot shows how to use the avg function to calculate
an average.
mysql> select * from people;
+-----------------+-----------+-----------+-------------+
| Name
| Field
| birthyear | countrycode |
+-----------------+-----------+-----------+-------------+
| Barack Obama
| politics | 1961
| US
|
| Deng Xiaoping
| politics | 1904
| CN
|
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introduction to sql using mysql
| Guy Verhofstadt | politics | 1953
| BE
|
| Justine Henin
| tennis
| 1982
| BE
|
| Kim Clijsters
| tennis
| 1983
| BE
|
| Li Na
| tennis
| 1982
| CN
|
| Liu Yang
| astronaut | 1978
| CN
|
| Serena Williams | tennis
| 1981
| US
|
| Venus Williams | tennis
| 1980
| US
|
+-----------------+-----------+-----------+-------------+
9 rows in set (0.00 sec)
mysql> select Field,AVG(birthyear) from people;
+----------+-------------------+
| Field
| AVG(birthyear)
|
+----------+-------------------+
| politics | 1967.111111111111 |
+----------+-------------------+
1 row in set (0.00 sec)
Using the group by clause, we can have an average per field.
mysql> select Field,AVG(birthyear) from people group by Field;
+-----------+--------------------+
| Field
| AVG(birthyear)
|
+-----------+--------------------+
| astronaut |
1978 |
| politics | 1939.3333333333333 |
| tennis
|
1981.6 |
+-----------+--------------------+
3 rows in set (0.00 sec)
68.5.8. deleting records
You can use the delete to permanently remove a record from a table.
mysql> delete from country where countryname='Spain';
Query OK, 1 row affected (0.06 sec)
mysql> select * from country where countryname='Spain';
Empty set (0.00 sec)
68.6. joining two tables
68.6.1. inner join
With an inner join you can take values from two tables and combine them in one result.
Consider the country and the people tables from the previous section when looking at this
screenshot of an inner join.
mysql> select Name,Field,countryname
-> from country
-> inner join people on people.countrycode=country.countrycode;
+-----------------+-----------+---------------+
| Name
| Field
| countryname
|
+-----------------+-----------+---------------+
| Barack Obama
| politics | United States |
| Deng Xiaoping
| politics | China
|
| Guy Verhofstadt | politics | Belgium
|
| Justine Henin
| tennis
| Belgium
|
| Kim Clijsters
| tennis
| Belgium
|
| Li Na
| tennis
| China
|
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introduction to sql using mysql
| Liu Yang
| astronaut | China
|
| Serena Williams | tennis
| United States |
| Venus Williams | tennis
| United States |
+-----------------+-----------+---------------+
9 rows in set (0.00 sec)
This inner join will show only records with a match on countrycode in both tables.
68.6.2. left join
A left join is different from an inner join in that it will take all rows from the left table,
regardless of a match in the right table.
mysql> select Name,Field,countryname from country left join people on people.countrycode=countr
+-----------------+-----------+---------------+
| Name
| Field
| countryname
|
+-----------------+-----------+---------------+
| Guy Verhofstadt | politics | Belgium
|
| Justine Henin
| tennis
| Belgium
|
| Kim Clijsters
| tennis
| Belgium
|
| Deng Xiaoping
| politics | China
|
| Li Na
| tennis
| China
|
| Liu Yang
| astronaut | China
|
| NULL
| NULL
| Germany
|
| NULL
| NULL
| Egypt
|
| NULL
| NULL
| Spain
|
| NULL
| NULL
| France
|
| NULL
| NULL
| India
|
| NULL
| NULL
| Japan
|
| NULL
| NULL
| Mexico
|
| Barack Obama
| politics | United States |
| Serena Williams | tennis
| United States |
| Venus Williams | tennis
| United States |
+-----------------+-----------+---------------+
16 rows in set (0.00 sec)
You can see that some countries are present, even when they have no matching records in
the people table.
68.7. mysql triggers
68.7.1. using a before trigger
Consider the following create table command. The last field (amount) is the multiplication
of the two fields named unitprice and unitcount.
mysql> create table invoices (
-> id char(8) NOT NULL,
-> customerid char(3) NOT NULL,
-> unitprice int,
-> unitcount smallint,
-> amount int );
Query OK, 0 rows affected (0.00 sec)
We can let mysql do the calculation for that by using a before trigger. The screenshot below
shows the creation of a trigger that calculates the amount by multiplying two fields that are
about to be inserted.
mysql> create trigger total_amount before INSERT on invoices
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introduction to sql using mysql
-> for each row set new.amount = new.unitprice * new.unitcount ;
Query OK, 0 rows affected (0.02 sec)
Here we verify that the trigger works by inserting a new record, without providing the total
amount.
mysql> insert into invoices values ('20090526','ABC','199','10','');
Query OK, 1 row affected (0.02 sec)
Looking at the record proves that the trigger works.
mysql> select * from invoices;
+----------+------------+-----------+-----------+--------+
| id
| customerid | unitprice | unitcount | amount |
+----------+------------+-----------+-----------+--------+
| 20090526 | ABC
|
199 |
10 |
1990 |
+----------+------------+-----------+-----------+--------+
1 row in set (0.00 sec)
68.7.2. removing a trigger
When a trigger is no longer needed, you can delete it with the drop trigger command.
mysql> drop trigger total_amount;
Query OK, 0 rows affected (0.00 sec)
739
Part XIX. dns server
Table of Contents
69. introduction to DNS .......................................................................................................
69.1. about dns ............................................................................................................
69.2. dns namespace .....................................................................................................
69.3. caching only servers ..............................................................................................
69.4. authoritative dns servers .........................................................................................
69.5. primary and secondary ...........................................................................................
69.6. zone transfers .......................................................................................................
69.7. master and slave ...................................................................................................
69.8. SOA record .........................................................................................................
69.9. full or incremental zone transfers .............................................................................
69.10. DNS cache .........................................................................................................
69.11. forward lookup zone example ................................................................................
69.12. example: caching only DNS server .........................................................................
69.13. example: caching only with forwarder .....................................................................
69.14. example: primary authoritative server ......................................................................
69.15. example: a DNS slave server .................................................................................
69.16. practice: dns .......................................................................................................
69.17. solution: dns .......................................................................................................
70. advanced DNS ...............................................................................................................
70.1. example: DNS round robin .....................................................................................
70.2. DNS delegation ....................................................................................................
70.3. example: DNS delegation .......................................................................................
70.4. example: split-horizon dns ......................................................................................
70.5. old dns topics .......................................................................................................
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Chapter 69. introduction to DNS
dns is a fundamental part of every large computer network. dns is used by many network
services to translate names into network addresses and to locate services on the network
(by name).
Whenever you visit a web site, send an e-mail, log on to Active Directory, play Minecraft,
chat, or use VoIP, there will be one or (many) more queries to dns services.
Should dns fail at your organization, then the whole network will grind to a halt (unless you
hardcoded the network addresses).
You will notice that even the largest of organizations benefit greatly from having one dns
infrastructure. Thus dns requires all business units to work together.
Even at home, most home modems and routers have builtin dns functionality.
This module will explain what dns actually is and how to set it up using Linux and bind9.
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introduction to DNS
69.1. about dns
69.1.1. name to ip address resolution
The domain name system or dns is a service on a tcp/ip network that enables clients to
translate names into ip addresses. Actually dns is much more than that, but let's keep it
simple for now.
When you use a browser to go to a website, then you type the name of that website in the
url bar. But for your computer to actually communicate with the web server hosting said
website, your computer needs the ip address of that web server. That is where dns comes in.
In wireshark you can use the dns filter to see this traffic.
69.1.2. history
In the Seventies, only a few hundred computers were connected to the internet. To resolve
names, computers had a flat file that contained a table to resolve hostnames to ip addresses.
This local file was downloaded from hosts.txt on an ftp server in Stanford.
In 1984 Paul Mockapetris created dns, a distributed treelike hierarchical database that will
be explained in detail in these chapters.
Today, dns or domain name system is a worldwide distributed hierarchical database
controlled by ICANN. Its primary function is to resolve names to ip addresses, and to point
to internet servers providing smtp or ldap services.
The old hosts.txt file is still active today on most computer systems under the name /etc/
hosts (or C:/Windows/System32/Drivers/etc/hosts). We will discuss this file later, as it can
influence name resolution.
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introduction to DNS
69.1.3. forward and reverse lookup queries
The question a client asks a dns server is called a query. When a client queries for an ip
address, this is called a forward lookup query (as seen in the previous drawing).
The reverse, a query for the name of a host, is called a reverse lookup query.
Below a picture of a reverse lookup query.
Here is a screenshot of a reverse lookup query in nslookup.
root@debian7:~# nslookup
> set type=PTR
> 188.93.155.87
Server:
192.168.1.42
Address:
192.168.1.42#53
Non-authoritative answer:
87.155.93.188.in-addr.arpa
name = antares.ginsys.net.
This is what a reverse lookup looks like when sniffing with tcpdump.
root@debian7:~# tcpdump udp port 53
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
11:01:29.357685 IP 192.168.1.103.42041 > 192.168.1.42.domain: 14763+ PT\
R? 87.155.93.188.in-addr.arpa. (44)
11:01:29.640093 IP 192.168.1.42.domain > 192.168.1.103.42041: 14763 1/0\
/0 PTR antares.ginsys.net. (76)
And here is what it looks like in wireshark (note this is an older screenshot).
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introduction to DNS
69.1.4. /etc/resolv.conf
A client computer needs to know the ip address of the dns server to be able to send queries
to it. This is either provided by a dhcp server or manually entered.
Linux clients keep this information in the /etc/resolv.conf file.
root@debian7:~# cat /etc/resolv.conf
domain linux-training.be
search linux-training.be
nameserver 192.168.1.42
root@debian7:~#
You can manually change the ip address in this file to use another dns server. For example
Google provides a public name server at 8.8.8.8 and 8.8.4.4.
root@debian7:~# cat /etc/resolv.conf
nameserver 8.8.8.8
root@debian7:~#
Please note that on dhcp clients this value can be overwritten when the dhcp lease is
renewed.
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69.2. dns namespace
69.2.1. hierarchy
The dns namespace is hierarchical tree structure, with the root servers (aka dot-servers) at
the top. The root servers are usually represented by a dot.
Below the root-servers are the Top Level Domains or tld's.
There are more tld's than shown in the picture. Currently about 200 countries have a tld. And
there are several general tld's like .com, .edu, .org, .gov, .net, .mil, .int and more recently
also .aero, .info, .museum, ...
69.2.2. root servers
There are thirteen root servers on the internet, they are named A to M. Journalists often
refer to these servers as the master servers of the internet, because if these servers go
down, then nobody can (use names to) connect to websites.
The root servers are not thirteen physical machines, they are many more. For example the
F root server consists of 46 physical machines that all behave as one (using anycast).
http://root-servers.org
http://f.root-servers.org
http://en.wikipedia.org/wiki/Root_nameserver.
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69.2.3. root hints
Every dns server software will come with a list of root hints to locate the root servers.
This screenshot shows a small portion of the root hints file that comes with bind 9.8.4.
root@debian7:~# grep -w 'A ' /etc/bind/db.root
A.ROOT-SERVERS.NET.
3600000
A
198.41.0.4
B.ROOT-SERVERS.NET.
3600000
A
192.228.79.201
C.ROOT-SERVERS.NET.
3600000
A
192.33.4.12
D.ROOT-SERVERS.NET.
3600000
A
199.7.91.13
E.ROOT-SERVERS.NET.
3600000
A
192.203.230.10
F.ROOT-SERVERS.NET.
3600000
A
192.5.5.241
G.ROOT-SERVERS.NET.
3600000
A
192.112.36.4
H.ROOT-SERVERS.NET.
3600000
A
128.63.2.53
I.ROOT-SERVERS.NET.
3600000
A
192.36.148.17
J.ROOT-SERVERS.NET.
3600000
A
192.58.128.30
K.ROOT-SERVERS.NET.
3600000
A
193.0.14.129
L.ROOT-SERVERS.NET.
3600000
A
199.7.83.42
M.ROOT-SERVERS.NET.
3600000
A
202.12.27.33
root@debian7:~#
69.2.4. domains
One level below the top level domains are the domains. Domains can have subdomains
(also called child domains).
This picture shows dns domains like google.com, chess.com, linux-training.be (there are
millions more).
DNS domains are registered at the tld servers, the tld servers are registered at the dot
servers.
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69.2.5. top level domains
Below the root level are the top level domains or tld's. Originally there were only seven
defined:
Table 69.1. the first top level domains
year
TLD
purpose
1985
.arpa
Reverse lookup via in-addr.arpa
1985
.com
Commercial Organizations
1985
.edu
US Educational Institutions
1985
.gov
US Government Institutions
1985
.mil
US Military
1985
.net
Internet Service Providers, Internet Infrastructure
1985
.org
Non profit Organizations
1988
.int
International Treaties like nato.int
Country tld's were defined for individual countries, like .uk in 1985 for Great Britain (yes
really), .be for Belgium in 1988 and .fr for France in 1986. See RFC 1591 for more info.
In 1998 seven new general purpose tld's where chosen, they became active in the 21st
century.
Table 69.2. new general purpose tld's
year
TLD
purpose
2002
.aero
aviation related
2001
.biz
businesses
2001
.coop
for co-operatives
2001
.info
informative internet resources
2001
.museum
2001
.name
2004
.pro
for museums
for all kinds of names, pseudonyms and labels...
for professionals
Many people were surprised by the choices, claiming not much use for them and wanting
a separate .xxx domain (introduced in 2011) for adult content, and .kidz a save haven for
children. In the meantime more useless tld's were create like .travel (for travel agents) and
.tel (for internet communications) and .jobs (for jobs sites).
In 2012 ICANN released a list of 2000 new tld's that would gradually become available.
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69.2.6. fully qualified domain name
The fully qualified domain name or fqdn is the combination of the hostname of a machine
appended with its domain name.
If for example a system is called gwen and it is in the domain linux-training.be, then the
fqdn of this system is gwen.linux-training.be.
On Linux systems you can use the hostname and dnsdomainname commands to verify
this information.
root@gwen:~# hostname
gwen
root@gwen:~# dnsdomainname
linux-training.be
root@gwen:~# hostname --fqdn
gwen.linux-training.be
root@gwen:~# cat /etc/debian_version
6.0.10
69.2.7. dns zones
A zone (aka a zone of authority) is a portion of the DNS tree that covers one domain name
or child domain name. The picture below represents zones as blue ovals. Some zones will
contain delegate authority over a child domain to another zone.
A dns server can be authoritative over 0, 1 or more dns zones. We will see more details
later on the relation between a dns server and a dns zone.
A dns zone consists of records, also called resource records. We will list some of those
resource records on the next page.
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69.2.8. dns records
A record
The A record, which is also called a host record contains the ipv4-address of a computer.
When a DNS client queries a DNS server for an A record, then the DNS server will resolve
the hostname in the query to an ip address. An AAAA record is similar but contains an ipv6
address instead of ipv4.
PTR record
A PTR record is the reverse of an A record. It contains the name of a computer and can be
used to resolve an ip address to a hostname.
NS record
A NS record or nameserver record is a record that points to a DNS name server (in this
zone). You can list all your name servers for your DNS zone in distinct NS records.
glue A record
An A record that maps the name of an NS record to an ip address is said to be a glue record.
SOA record
The SOA record of a zone contains meta information about the zone itself. The contents of
the SOA record is explained in detail in the section about zone transfers. There is exactly
one SOA record for each zone.
CNAME record
A CNAME record maps a hostname to a hostname, creating effectively an alias for an
existing hostname. The name of the mail server is often aliased to mail or smtp, and the
name of a web server to www.
MX record
The MX record points to an smtp server. When you send an email to another domain, then
your mail server will need the MX record of the target domain's mail server.
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69.3. caching only servers
A dns server that is set up without authority over a zone, but that is connected to other
name servers and caches the queries is called a caching only name server. Caching only
name servers do not have a zone database with resource records. Instead they connect to
other name servers and cache that information.
There are two kinds of caching only name servers. Those with a forwarder, and those that
use the root servers.
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69.3.1. caching only server without forwarder
A caching only server without forwarder will have to get information elsewhere. When it
receives a query from a client, then it will consult one of the root servers. The root server
will refer it to a tld server, which will refer it to another dns server. That last server might
know the answer to the query, or may refer to yet another server. In the end, our hard working
dns server will find an answer and report this back to the client.
In the picture below, the clients asks for the ip address of linux-training.be. Our caching only
server will contact the root server, and be refered to the .be server. It will then contact the .be
server and be refered to one of the name servers of Openminds. One of these name servers
(in this cas ns1.openminds.be) will answer the query with the ip address of linux-training.be.
When our caching only server reports this to the client, then the client can connect to this
website.
Sniffing with tcpdump will give you this (the first 20 characters of each line are cut).
192.168.1.103.41251 > M.ROOT-SERVERS.NET.domain: 37279% [1au] A? linux-tr\
aining.be. (46)
M.ROOT-SERVERS.NET.domain > 192.168.1.103.41251: 37279- 0/11/13 (740)
192.168.1.103.65268 > d.ns.dns.be.domain: 38555% [1au] A? linux-training.\
be. (46)
d.ns.dns.be.domain > 192.168.1.103.65268: 38555- 0/7/5 (737)
192.168.1.103.7514 > ns2.openminds.be.domain: 60888% [1au] A? linux-train\
ing.be. (46)
ns2.openminds.be.domain > 192.168.1.103.7514: 60888*- 1/0/1 A 188.93.155.\
87 (62)
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introduction to DNS
69.3.2. caching only server with forwarder
A caching only server with a forwarder is a DNS server that will get all its information
from the forwarder. The forwarder must be a dns server for example the dns server of
an internet service provider.
This picture shows a dns server on the company LAN that has set the dns server from their
isp as a forwarder. If the ip address of the isp dns server is 212.71.8.10, then the following
lines would occur in the named.conf file of the company dns server:
forwarders {
212.71.8.10;
};
You can also configure your dns server to work with conditional forwarder(s). The
definition of a conditional forwarder looks like this.
zone "someotherdomain.local" {
type forward;
forward only;
forwarders { 10.104.42.1; };
};
69.3.3. iterative or recursive query
A recursive query is a DNS query where the client that is submitting the query expects a
complete answer (Like the fat red arrow above going from the Macbook to the DNS server).
An iterative query is a DNS query where the client does not expect a complete answer (the
three black arrows originating from the DNS server in the picture above). Iterative queries
usually take place between name servers. The root name servers do not respond to recursive
queries.
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introduction to DNS
69.4. authoritative dns servers
A DNS server that is controlling a zone, is said to be the authoritative DNS server for that
zone. Remember that a zone is a collection of resource records.
69.5. primary and secondary
When you set up the first authoritative dns server for a zone, then this is called the primary
dns server. This server will have a readable and writable copy of the zone database. For
reasons of fault tolerance, performance or load balancing you may decide to set up another
dns server with authority over that zone. This is called a secondary dns server.
69.6. zone transfers
The slave server receives a copy of the zone database from the master server using a
zone transfer. Zone transfers are requested by the slave servers at regular intervals. Those
intervals are defined in the soa record.
You can force a refresh from a zone with rndc. The example below force a transfer of the
fred.local zone, and shows the log from /var/log/syslog.
root@debian7:/etc/bind# rndc refresh fred.local
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introduction to DNS
root@debian7:/etc/bind# grep fred /var/log/syslog | tail -7 | cut -c38zone fred.local/IN: sending notifies (serial 1)
received control channel command 'refresh fred.local'
zone fred.local/IN: Transfer started.
transfer of 'fred.local/IN' from 10.104.109.1#53: connected using 10.104.33.30#57367
zone fred.local/IN: transferred serial 2
transfer of 'fred.local/IN' from 10.104.109.1#53: Transfer completed: 1 messages, 10 records, 2
zone fred.local/IN: sending notifies (serial 2)
root@debian7:/etc/bind#
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introduction to DNS
69.7. master and slave
When adding a secondary dns server to a zone, then you will configure this server as a
slave server to the primary server. The primary server then becomes the master server
of the slave server.
Often the primary dns server is the master server of all slaves. Sometimes a slave server
is master server for a second line slave server. In the picture below ns1 is the primary dns
server and ns2, ns3 and ns4 are secondaries. The master for slaves ns2 and ns3 is ns1, but
the master for ns4 is ns2.
69.8. SOA record
The soa record contains a refresh value. If this is set to 30 minutes, then the slave server
will request a copy of the zone file every 30 minutes. There is also a retry value. The retry
value is used when the master server did not reply to the last zone transfer request. The value
for expiry time says how long the slave server will answer to queries, without receiving
a zone update.
Below an example of how to use nslookup to query the soa record of a zone (linuxtraining.be).
root@debian6:~# nslookup
> set type=SOA
> server ns1.openminds.be
> linux-training.be
Server:
ns1.openminds.be
Address:
195.47.215.14#53
linux-training.be
origin = ns1.openminds.be
mail addr = hostmaster.openminds.be
serial = 2321001133
refresh = 14400
retry = 3600
expire = 604800
minimum = 3600
Zone transfers only occur when the zone database was updated (meaning when one or more
resource records were added, removed or changed on the master server). The slave server
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introduction to DNS
will compare the serial number of its own copy of the SOA record with the serial number
of its master's SOA record. When both serial numbers are the same, then no update is needed
(because no records were added, removed or deleted). When the slave has a lower serial
number than its master, then a zone transfer is requested.
Below a zone transfer captured in wireshark.
69.9. full or incremental zone transfers
When a zone tranfer occurs, this can be either a full zone transfer or an incremental zone
transfer. The decision depends on the size of the transfer that is needed to completely update
the zone on the slave server. An incremental zone transfer is prefered when the total size
of changes is smaller than the size of the zone database. Full zone transfers use the axfr
protocol, incremental zone transfer use the ixfr protocol.
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introduction to DNS
69.10. DNS cache
DNS is a caching protocol.
When a client queries its local DNS server, and the local DNS server is not authoritative
for the query, then this server will go looking for an authoritative name server in the DNS
tree. The local name server will first query a root server, then a tld server and then a domain
server. When the local name server resolves the query, then it will relay this information to
the client that submitted the query, and it will also keep a copy of these queries in its cache.
So when a(nother) client submits the same query to this name server, then it will retrieve
this information form its cache.
For example, a client queries for the A record on www.linux-training.be to its local server.
This is the first query ever received by this local server. The local server checks that it is
not authoritative for the linux-training.be domain, nor for the .be tld, and it is also not a root
server. So the local server will use the root hints to send an iterative query to a root server.
The root server will reply with a reference to the server that is authoritative for the .be
domain (root DNS servers do not resolve fqdn's, and root servers do not respond to recursive
queries).
The local server will then sent an iterative query to the authoritative server for the .be tld.
This server will respond with a reference to the name server that is authoritative for the
linux-training.be domain.
The local server will then sent the query for www.linux-training.be to the authoritative server
(or one of its slave servers) for the linux-training.be domain. When the local server receives
the ip address for www.linux-training.be, then it will provide this information to the client
that submitted this query.
Besides caching the A record for www.linux-training.be, the local server will also cache the
NS and A record for the linux-training.be name server and the .be name server.
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introduction to DNS
69.11. forward lookup zone example
The way to set up zones in /etc/bind/named.conf.local is to create a zone entry with a
reference to another file (this other file contains the zone database).
Here is an example of such an entry in /etc/bind/named.conf.local:
root@debian7:~# cat /etc/bind/named.conf.local
//
// Do any local configuration here
//
// Consider adding the 1918 zones here, if they are not used in your
// organization
//include "/etc/bind/zones.rfc1918";
zone "paul.local" IN {
type master;
file "/etc/bind/db.paul.local";
allow-update { none; };
};
root@debian7:~#
To create the zone file, the easy method is to copy an existing zone file (this is easier than
writing from scratch).
root@debian7:/etc/bind# cp db.empty db.paul.local
root@debian7:/etc/bind# vi db.paul.local
Here is an example of a zone file.
root@debian7:/etc/bind# cat db.paul.local
; zone for classroom teaching
$TTL
86400
@
IN
SOA
debianpaul.paul.local. root.paul.local (
2014100100
; Serial
1h
; Refresh
1h
; Retry
2h
; Expire
86400 )
; Negative Cache TTL
;
; name servers
;
IN
NS
ns1
IN
NS
debianpaul
IN
NS
debian7
;
; servers
;
debianpaul
IN
A
10.104.33.30
debian7
IN
A
10.104.33.30
ns1
IN
A
10.104.33.30
;www
IN
A
10.104.33.30
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introduction to DNS
69.12. example: caching only DNS server
1. installing DNS software on Debian
root@debian7:~# aptitude update && aptitude upgrade
...
root@debian7:~# aptitude install bind9
...
root@debian7:~# dpkg -l | grep bind9 | tr -s ' '
ii bind9 1:9.8.4.dfsg.P1-6+nmu2+deb7u2 amd64 Internet Domain Name Server
ii bind9-host 1:9.8.4.dfsg.P1-6+nmu2+deb7u2 amd64 Version of 'host' bundled...
ii bind9utils 1:9.8.4.dfsg.P1-6+nmu2+deb7u2 amd64 Utilities for BIND
ii libbind9-80 1:9.8.4.dfsg.P1-6+nmu2+deb7u2 amd64 BIND9 Shared Library use...
root@debian7:~#
2. Discover the default configuration files. Can you define the purpose of each file ?
root@debian7:~# ls -l /etc/bind
total 52
-rw-r--r-- 1 root root 2389 Sep
-rw-r--r-- 1 root root 237 Sep
-rw-r--r-- 1 root root 271 Sep
-rw-r--r-- 1 root root 237 Sep
-rw-r--r-- 1 root root 353 Sep
-rw-r--r-- 1 root root 270 Sep
-rw-r--r-- 1 root root 3048 Sep
-rw-r--r-- 1 root bind 463 Sep
-rw-r--r-- 1 root bind 490 Sep
-rw-r--r-- 1 root bind 374 Oct
-rw-r--r-- 1 root bind 913 Oct
-rw-r----- 1 bind bind
77 Oct
-rw-r--r-- 1 root root 1317 Sep
5
5
5
5
5
5
5
5
5
1
1
1
5
20:25
20:25
20:25
20:25
20:25
20:25
20:25
20:25
20:25
20:01
13:24
11:14
20:25
bind.keys
db.0
db.127
db.255
db.empty
db.local
db.root
named.conf
named.conf.default-zones
named.conf.local
named.conf.options
rndc.key
zones.rfc191
3. Setup caching only dns server. This is normally the default setup. A caching-only name
server will look up names for you and cache them. Many tutorials will tell you to add a
forwarder, but we first try without this!
Hey this seems to work without a forwarder. Using a sniffer you can find out what really
happens. Your freshly install dns server is not using a cache, and it is not using your local
dns server (from /etc/resolv.conf). So where is this information coming from ? And what
can you learn from sniffing this dns traffic ?
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4. Explain in detail what happens when you enable a caching only dns server without
forwarder. This wireshark screenshot can help, but you learn more by sniffing the traffic
yourself.
You should see traffic to a root name server whenever you try a new tld for the first time.
Remember that dns is a caching protocol, which means that repeating a query will generate
a lot less traffic since your dns server will still have the answer in its memory.
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69.13. example: caching only with forwarder
5. Add the public Google dns server as a forwarder. The ip address of this server is 8.8.8.8 .
Before the change:
root@debian7:~# grep -A2 'forwarders {' /etc/bind/named.conf.options
// forwarders {
//
0.0.0.0;
// };
changing:
root@debian7:~# vi /etc/bind/named.conf.options
After the change:
root@debian7:~# grep -A2 'forwarders {' /etc/bind/named.conf.options
forwarders {
8.8.8.8;
};
Restart the server:
root@debian7:~# service bind9 restart
Stopping domain name service...: bind9.
Starting domain name service...: bind9.
6. Explain the purpose of adding the forwarder. What is our dns server doing when it
receives a query ?
root@debian7:~# nslookup
> server
Default server: 10.104.33.30
Address: 10.104.33.30#53
> linux-training.be
Server:
10.104.33.30
Address:
10.104.33.30#53
Non-authoritative answer:
Name:
linux-training.be
Address: 188.93.155.87
>
This is the output of tcpdump udp port 53 while executing the above query for linuxtraining.be in nslookup.
root@debian7:~# tcpdump udp port 53
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
You should find the following two lines in the output of tcpdump:
10.104.33.30.19381 > google-public-dns-a.google.com.domain: 18237+% [1au] A? \
linux-training.be. (46)
google-public-dns-a.google.com.domain > 10.104.33.30.19381: 18237 1/0/1 A 188\
.93.155.87 (62)
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introduction to DNS
Below is an (old) wireshark screenshot that can help, you should see something similar (but
with different ip addresses).
7. What happens when you query for the same domain name more than once ?
8. Why does it say "non-authoritative answer" ? When is a dns server authoritative ?
9. You can also use dig instead of nslookup.
root@debian7:~# dig @10.104.33.30 linux-training.be +short
188.93.155.87
root@debian7:~#
10. How can we avoid having to set the server in dig or nslookup ?
Change this:
root@debian7:~# cat /etc/resolv.conf
nameserver 10.46.101.1
root@debian7:~#
into this:
root@debian7:~# cat /etc/resolv.conf
nameserver 10.104.33.30
root@debian7:~#
11. When you use dig for the first time for a domain, where is the answer coming from ?
And the second time ? How can you tell ?
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introduction to DNS
69.14. example: primary authoritative server
1. Instead of only cachng the information from other servers, we will now make our server
authoritative for our own domain.
2. I choose the top level domain .local and the domain paul.local and put the information
in /etc/bind/named.conf.local.
root@debian7:~# cat /etc/bind/named.conf.local
//
// Do any local configuration here
//
// Consider adding the 1918 zones here, if they are not used in your
// organization
//include "/etc/bind/zones.rfc1918";
zone "paul.local" IN {
type master;
file "/etc/bind/db.paul.local";
allow-update { none; };
};
3. Also add a zone database file, similar to this one (add some A records for testing). Set
the Refresh and Retry values not too high so you can sniff this traffic (this example makes
the slave server contact the master every hour).
root@debian7:~# cat /etc/bind/db.paul.local
; zone for classroom teaching
$TTL
86400
@
IN
SOA
debianpaul.paul.local. root.paul.local (
2014100101
; Serial
1h
; Refresh
1h
; Retry
2h
; Expire
900 )
; Negative Cache TTL
;
; name servers
;
IN
NS
ns1
IN
NS
debianpaul
IN
NS
debian7
;
; servers
;
debianpaul
IN
A
10.104.33.30
debian7
IN
A
10.104.33.30
ns1
IN
A
10.104.33.30
;www
IN
A
10.104.33.30
root@debian7:~#
Note that the www record is commented out, so it will not resolve.
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introduction to DNS
69.14.1. using your own DNS server
If you are confident that your dns server works, then set it as default and only dns server
in /etc/resolv.conf.
root@debian7:~# cat /etc/resolv.conf
nameserver 10.104.33.30
root@debian7:~#
In case you also use dhclient, you will need to add your dns server to /etc/dhcp/
dhclient.conf.
root@debian7:~# diff /etc/dhcp/dhclient.conf /etc/dhcp/dhclient.conf.original
21c21
< prepend domain-name-servers 10.104.33.30;
--> #prepend domain-name-servers 127.0.0.1;
23,24c23
< #
domain-name, domain-name-servers, domain-search, host-name,
<
domain-name, domain-search, host-name,
-->
domain-name, domain-name-servers, domain-search, host-name,
root@debian7:~#
The above screenshot shows that 10.104.33.30 is now a default option that the dhcp client
should no longer request from the dhcp server.
Adjust /etc/hosts to reflect your domain name and verify with hostname and
dnsdomainname.
root@debian7:~# grep debian7 /etc/hosts
127.0.1.1 debian7.paul.local debian7
root@debian7:~# hostname
debian7
root@debian7:~# hostname --fqdn
debian7.paul.local
root@debian7:~# dnsdomainname
paul.local
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introduction to DNS
69.14.2. using your own domain
Consider the following screenshot:
root@debian7b:~# cat /etc/resolv.conf
nameserver 10.104.33.30
root@debian7b:~# ping -c1 www
ping: unknown host www
root@debian7b:~# vi /etc/resolv.conf
root@debian7b:~# cat /etc/resolv.conf
nameserver 10.104.33.30
domain paul.local
root@debian7b:~# ping -c1 www
PING www.paul.local (10.104.33.31) 56(84) bytes of data.
64 bytes from 10.104.33.31: icmp_req=1 ttl=64 time=0.021 ms
--- www.paul.local ping statistics --1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.021/0.021/0.021/0.000 ms
root@debian7b:~#
Adding the domain paul.local directive to /etc/resolv.conf allows omitting the domain
when using hostnames.
You can accomplish this feature automatically by adjusting dhclient.conf.
root@debian7:~# grep paul.local /etc/dhcp/dhclient.conf
prepend domain-name "paul.local";
prepend domain-search "paul.local";
root@debian7:~#
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introduction to DNS
4. Restart the DNS server and check your zone in the error log.
root@debian7:~# service bind9 restart
Stopping domain name service...: bind9.
Starting domain name service...: bind9.
root@debian7:~# grep paul.local /var/log/syslog
Oct 6 09:22:18 debian7 named[2707]: zone paul.local/IN: loaded seria\
l 2014100101
Oct 6 09:22:18 debian7 named[2707]: zone paul.local/IN: sending noti\
fies (serial 2014100101)
5. Use dig or nslookup (or even ping) to test your A records.
root@debian7:~# ping -c1 ns1.paul.local
PING ns1.paul.local (10.104.33.30) 56(84) bytes of data.
64 bytes from 10.104.33.30: icmp_req=1 ttl=64 time=0.006 ms
--- ns1.paul.local ping statistics --1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.006/0.006/0.006/0.000 ms
root@debian7:~# ping -c1 www.paul.local
ping: unknown host www.paul.local
Note that the www record was commented out, so it should fail.
root@debian7:~# dig debian7.paul.local
; <<>> DiG 9.8.4-rpz2+rl005.12-P1 <<>> debian7.paul.local
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 50491
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 3, ADDITIONAL: 2
;; QUESTION SECTION:
;debian7.paul.local.
IN
A
;; ANSWER SECTION:
debian7.paul.local.
86400
IN
A
10.104.33.30
;; AUTHORITY SECTION:
paul.local.
paul.local.
paul.local.
86400
86400
86400
IN
IN
IN
NS
NS
NS
ns1.paul.local.
debian7.paul.local.
debianpaul.paul.local.
;; ADDITIONAL SECTION:
ns1.paul.local.
debianpaul.paul.local.
86400
86400
IN
IN
A
A
10.104.33.30
10.104.33.30
;;
;;
;;
;;
Query time: 4 msec
SERVER: 10.104.33.30#53(10.104.33.30)
WHEN: Mon Oct 6 09:35:25 2014
MSG SIZE rcvd: 141
root@debian7:~#
6. Our primary server appears to be up and running. Note the information here:
server os :
ip address :
domain name:
server name:
Debian 7
10.104.33.30
paul.local
ns1.paul.local
767
introduction to DNS
69.15. example: a DNS slave server
1. A slave server transfers zone information over the network from a master server (a slave
can also be a master). A primary server maintains zone records in its local file system. As
an exercise, and to verify the work of all students, set up a slave server of all the master
servers in the classroom.
2. Before configuring the slave server, we may have to allow transfers from our zone to this
server. Remember that this is not very secure since transfers are in clear text and limited to
an ip address. This example follows our demo from above.
Imagine a student named Jesse having completed the setup as shown before, with the domain
name jesse.local and the ip address 10.104.15.20. The goal is to have a slave server of
paul.local on Jesse's computer and a slave zone of jesse.local on my computer.
Below is an example of an allow-transfer statement. Careful, maybe the default allows
transfer to any.
root@debian7:/etc/bind# cat named.conf.local
//
// Do any local configuration here
//
// Consider adding the 1918 zones here, if they are not used in your
// organization
//include "/etc/bind/zones.rfc1918";
zone "paul.local" IN {
type master;
file "/etc/bind/db.paul.local";
allow-update { none; };
allow-transfer { 10.104.15.20; };
};
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introduction to DNS
3. With the configuration below I can make my server a slave for the jesse.local zone.
root@debian7:/etc/bind# tail -6 named.conf.local
zone "jesse.local" IN {
type slave;
file "/var/cache/named/db.jesse.local";
masters { 10.104.15.20; };
};
root@debian7:/etc/bind# mkdir /var/cache/named/
root@debian7:/etc/bind# chown bind:bind /var/cache/named/
root@debian7:/etc/bind# ls -ld /var/cache/named/
drwxr-xr-x 2 bind bind 4096 Oct 1 20:01 /var/cache/named/
Note that we put the slave zones in /var/cache/named and not in /etc/bind.
4. Restarting bind on the slave server should transfer the zone database file. Verify this in /
var/log/syslog. (time and date are truncated from the screenshot, and Jesse did not use the
current date in the serial number...)
root@debian7:/etc/bind# grep jesse /var/log/syslog
named[2731]: zone jesse.local/IN: Transfer started.
named[2731]: transfer of 'jesse.local/IN' from 10.104.15.20#53: connected u\
sing 10.104.33.30#44719
named[2731]: zone jesse.local/IN: transferred serial 20110516
named[2731]: transfer of 'jesse.local/IN' from 10.104.15.20#53: Transfer co\
mpleted: 1 messages, 8 records, 239 bytes, 0.001 secs (239000 bytes/sec)
And the contents of the slave zone:
root@debian7:/etc/bind# cat /var/cache/named/db.jesse.local
$ORIGIN .
$TTL 604800
; 1 week
jesse.local
IN SOA ns.jesse.local. root.jesse.local.jesse.local. (
20110516
; serial
300
; refresh (5 minutes)
200
; retry (3 minutes 20 seconds)
2419200
; expire (4 weeks)
604800
; minimum (1 week)
)
NS
ns.jesse.local.
$ORIGIN jesse.local.
anya
A
10.104.15.1
mac
A
10.104.15.30
ns
A
10.104.15.20
ubu1010srv
A
10.104.15.20
www
A
10.104.15.25
root@debian7:/etc/bind#
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introduction to DNS
69.16. practice: dns
1. Install bind9 and verify with a sniffer how it works.
2. Add a forwarder and verify that it works.
3. Create a primary forward lookup zone named yourname.local with at least two NS
records and four A records.
4. Use dig and nslookup to verify your NS and A records.
5. Create a slave of your primary zone (on another server) and verify the zone transfer.
6. Set up two primary zones on two servers and implement a conditional forwarder (you
can use the two servers from before).
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introduction to DNS
69.17. solution: dns
1. Install bind9 and verify with a sniffer how it works.
You should see queries to the root name servers with tcpdump or wireshark.
2. Add a forwarder and verify that it works.
The forwarder van be added in named.conf.options as seen in the theory.
3. Create a primary forward lookup zone named yourname.local with at least two NS
records and four A records.
This is literally explained in the theory.
4. Use dig and nslookup to verify your NS and A records.
This is literally explained in the theory.
5. Create a slave of your primary zone (on another server) and verify the zone transfer.
This is literally explained in the theory.
6. Set up two primary zones on two servers and implement a conditional forwarder (you
can use the two servers from before).
A conditional forwarder is set in named.conf.local as a zone.
(see the theory on forwarder)
771
Chapter 70. advanced DNS
This chapter expands your DNS server with topics like round robin dns for load balancing
servers, dns delegation to delegate child domains to another team and split horizon dns so
you can provide local service locations to clients.
There is more to dns, content will be added rsn.
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advanced DNS
70.1. example: DNS round robin
When you create multiple A records for the same name, then bind will do a round robin of
the order in which the records are returned. This allows the use of DNS as a load balancer
between hosts, since clients will usually take the first ip-address offered.
Consider this example from the /etc/bind/db.paul.local zone configuration file. There are
two A records for www pointing to two distinct ip addresses.
root@debian7:~# grep www /etc/bind/db.paul.local
www
IN
A
10.104.33.30
www
IN
A
10.104.33.31
Below a screenshot of nslookup querying a load balanced A record. Notice the order of ip
addresses returned.
root@debian7:~# nslookup www.paul.local 10.104.33.30
Server:
10.104.33.30
Address:
10.104.33.30#53
Name:
www.paul.local
Address: 10.104.33.31
Name:
www.paul.local
Address: 10.104.33.30
root@debian7:~# nslookup www.paul.local 10.104.33.30
Server:
10.104.33.30
Address:
10.104.33.30#53
Name:
www.paul.local
Address: 10.104.33.30
Name:
www.paul.local
Address: 10.104.33.31
Try to set up a website on two web servers (with a small difference so you can distinguish
the websites) and test the round robin.
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advanced DNS
70.2. DNS delegation
You can delegate a child domain to another DNS server. The child domain then becomes
a new zone, with authority at the new dns server.
When delegation is properly set up, then clients that query your parent zone will also be
able to resolve the delegated child zones.
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advanced DNS
70.3. example: DNS delegation
We have another Linux server named debian7b and we want to make it responsible for the
child domain test42.paul.local.
Note the name of the servers in the screenshots are either debian7 (hosting the parent
domain) or debian7b (hosting the child domain).
We start by adjusting the /etc/bind/named.comf.local file (on the server hosting the parent
domain) to make sure that no forwarder will be used when resolving authoritative names.
root@debian7:~# grep -A4 paul.local /etc/bind/named.conf.local
zone "paul.local" IN {
type master;
file "/etc/bind/db.paul.local";
allow-update { none; };
allow-transfer { 10.104.15.20; };
forwarders { };
};
root@debian7:~#
Technically, you could also set allow-transfer to { any; }; while troubleshooting and then
refine it later, but this is not needed for delegation.
Then we add the delegation to our zone database:
root@debian7:~# tail -3 /etc/bind/db.paul.local
$ORIGIN test42.paul.local.
@
IN
NS
ns2.test42.paul.local.
ns2
IN
A
10.104.33.31
; the glue record
root@debian7:~#
Don't forget to restart bind and verify /var/log/syslog.
root@debian7:~# service bind9 restart
Stopping domain name service...: bind9.
Starting domain name service...: bind9.
root@debian7:~# grep paul.local /var/log/syslog | cut -c28- | tail -2
named[3202]: zone paul.local/IN: loaded serial 2014100801
named[3202]: zone paul.local/IN: sending notifies (serial 2014100801)
root@debian7:~#
Note that on your terminal you can type tail -40 /var/log/syslog because the only reason I
use grep, cut and tail -2 is to limit the size of the screenshots in this book.
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advanced DNS
Next we create a zone database file on the second server, as seen in this screenshot:
root@debian7b:~# cat /etc/bind/db.test42.paul.local
; child zone for classroom teaching
$TTL
86400
$ORIGIN test42.paul.local.
@
IN
SOA
ns2.test42.paul.local. root.test42.paul.local. (
2014100802
; Serial
1h
; Refresh
1h
; Retry
2h
; Expire
900 )
; Negative Cache TTL
;
; name servers
;
IN
NS
ns2.test42.paul.local.
IN
NS
debian7b.test42.paul.local.
;
; servers
;
ns2
IN
A
10.104.33.31
debian7b
IN
A
10.104.33.31
testsrv
IN
A
10.104.33.31
root@debian7b:~#
The second server also needs a zone definition in named.conf.local, followed by a restart
of bind.
root@debian7b:~# cat /etc/bind/named.conf.local
//
// Do any local configuration here
//
// Consider adding the 1918 zones here, if they are not used in your
// organization
//include "/etc/bind/zones.rfc1918";
zone "test42.paul.local" IN {
type master;
file "/etc/bind/db.test42.paul.local";
allow-update { none; };
allow-transfer { any; };
};
root@debian7b:~#
Testing on the parent server:
root@debian7:~# dig ns1.paul.local +short
10.104.33.30
root@debian7:~# dig ns2.test42.paul.local +short
10.104.33.31
root@debian7:~# dig debian7b.test42.paul.local +short
10.104.33.31
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advanced DNS
70.4. example: split-horizon dns
Suppose you want to answer dns queries depending on who is asking. For example
when someone from the 10.104.15.0/24 network (managed by Jesse) asks for the A
record www.paul.local, then dns answers with 10.104.33.30. But when someone from the
10.104.42.0/24 network (managed by Keith) asks for the same A record of www.paul.local,
he will get 10.104.33.31 as an answer.
A split-horizon setup can be used to redirect people to local copies of certain services.
In this example we want to decide on specific answers for two networks (Jesse's and Keith's)
and prevent them from using our dns server for recursion, while maintaining the capability
to resolve the internet and our paul.local zone from our own network.
We start by creating three view clauses in named.conf.local.
root@debian7:/etc/bind# cat named.conf.local
view "paul" {
match-clients { 10.104.33.0; localhost; };
include "/etc/bind/named.conf.default-zones";
zone "paul.local" IN {
type master;
file "/etc/bind/db.paul.local";
allow-update { none; };
};
};
// end view internal
view "jesse" {
match-clients { 10.104.15/24; };
zone "paul.local" IN {
type master;
file "/etc/bind/db.paul.local.jesse";
allow-update { none; };
};
};
// end view jesse
view "keith" {
match-clients { 10.104.42/24; };
zone "paul.local" IN {
type master;
file "/etc/bind/db.paul.local.keith";
allow-update { none; };
};
};
// end view keith
Note that we included the default-zones in the internal zone. It is mandatory to put all zones
inside views when using a view.
The zone files are identical copies, except for the www record. You can see that the
round robin is still active for internal users, computers from 10.104.15.0/24 (Jesse) will
always receive 10.104.33.30 while computers from 10.104.42.0/24 (Keith) will receive
10.104.33.31.
root@debian7:/etc/bind# grep www db.paul.local db.paul.local.[jk]*
db.paul.local:www
IN
A
10.104.33.30
db.paul.local:www
IN
A
10.104.33.31
db.paul.local.jesse:www
IN
A
10.104.33.30
db.paul.local.keith:www
IN
A
10.104.33.31
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advanced DNS
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advanced DNS
70.5. old dns topics
All the dns things below this paragraph are old and in urgent need of review.
70.5.1. old example: reverse DNS
1. We can add ip to name resolution to our dns-server using a reverse dns zone.
2. Start by adding a .arpa zone to /etc/bind/named.conf.local like this (we set notify to no to
avoid sending of notify messages to other name servers):
root@ubu1010srv:/etc/bind# grep -A4 arpa named.conf.local
zone "1.168.192.in-addr.arpa" {
type master;
notify no;
file "/etc/bind/db.192";
};
3. Also create a zone database file for this reverse lookup zone.
root@ubu1010srv:/etc/bind# cat db.192
;
; BIND reverse data file for 192.168.1.0/24 network
;
$TTL 604800
@ IN SOA ns.cobbaut.paul root.cobbaut.paul. (
20110516 ; Serial
604800 ; Refresh
86400 ; Retry
2419200 ; Expire
604800 ) ; Negative Cache TTL
;
@ IN NS ns.
37 IN PTR ns.cobbaut.paul.
1 IN PTR anya.cobbaut.paul.
30 IN PTR mac.cobbaut.paul.
root@ubu1010srv:/etc/bind#
4. Test with nslookup or dig:
root@ubu1010srv:/etc/bind# dig 1.168.192.in-addr.arpa AXFR
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advanced DNS
70.5.2. old DNS load balancing
Not as above. When you have more than one DNS server authoritative for a zone, you can
spread queries amongst all server. One way to do this is by creating NS records for all servers
that participate in the load balancing of external queries.
You could also configure different name servers on internal clients.
70.5.3. old DNS notify
The original design of DNS in rfc 1034 and rfc 1035 implemented a refresh time in the
SOA record to configure a time loop for slaves to query their master server. This can result
in a lot of useless pull requests, or in a significant lag between updates.
For this reason dns notify (rfc 1996) was designed. The server will now notify slaves
whenever there is an update. By default this feature is activated in bind.
Notify can be disabled as in this screenshot.
zone "1.168.192.in-addr.arpa" {
type master;
notify no;
file "/etc/bind/db.192";
};
70.5.4. old testing IXFR and AXFR
Full zone transfers (AXFR) are initiated when you restart the bind server, or when you
manually update the zone database file directly. With nsupdate you can update a zone
database and initiate an incremental zone transfer.
You need DDNS allowed for nsupdate to work.
root@ubu1010srv:/etc/bind# nsupdate
> server 127.0.0.1
> update add mac14.linux-training.be 86400 A 192.168.1.23
> send
update failed: REFUSED
70.5.5. old DDNS integration with DHCP
Some organizations like to have all their client computers in DNS. This can be cumbersome
to maintain. Luckily rfc 2136 describes integration of DHCP servers with a DNS server.
Whenever DHCP acknowledges a client ip configuration, it can notify DNS with this clients
ip-address and name. This is called dynamic updates or DDNS.
70.5.6. old reverse is forward in-addr.arpa
Reverse lookup is actually iomplemented as a forward lookup in the in-addr.arpa domain.
This domain has 256 child domains (from 0.in-addr.arpa to 255.in-addr.arpa), with each
child domain having again 256 child domains. And this twice more to a structure of over
four billion (2 to the power 32) domains.
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advanced DNS
70.5.7. old ipv6
With rfc 3596 came ipv6 extensions for DNS. There is the AAAA record for ipv6 hosts on
the network, and there is the ip6.int domain for reverse lookup (having 16 child domains
from 0.ip6.int to f.ip6.int, each of those having again 16 child domains...and this 16 times.
70.5.8. old DNS security: file corruption
To mitigate file corruption on the zone files and the bind configuration files protect them
with Unix permissions and take regular backups.
70.5.9. old DNS security: zone transfers
Limit zone transfers to certain ip addresses instead of to any. Nevermind that ip-addresses
can be spoofed, still use this.
70.5.10. old DNS security: zone transfers, ip spoofing
You could setup DNSSEC (which is not the easiest to maintain) and with rfc 2845(tsig?) and
with rfc 2930(tkey, but this is open to brute force), or you could disable all zone transfers
and use a script with ssh to copy them manually.
70.5.11. old DNS security: queries
Allow recursion only from the local network, and iterative queries from outside only when
necessary. This can be configured on master and slave servers.
view "internal" {
match-clients { 192.168.42/24; };
recursion yes;
...
};
view "external" {
match-clients { any; };
recursion no;
...
};
Or allow only queries from the local network.
options {
allow-query { 192.168.42.0/24; localhost; };
};
zone "cobbaut.paul" {
allow-query { any; };
};
Or only allow recursive queries from internal clients.
options {
allow-recursion { 192.168.42.0/24; localhost; };
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advanced DNS
};
70.5.12. old DNS security: chrooted bind
Most Linux distributions allow an easy setup of bind in a chrooted environment.
70.5.13. old DNS security: DNSSEC
DNSSEC uses public/private keys to secure communications, this is described in rfc's 4033,
4034 and 4035.
70.5.14. old DNS security: root
Do not run bind as root. Do not run any application daemon as root.
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Part XX. dhcp server
Table of Contents
71. introduction to dhcp ......................................................................................................
71.1. four broadcasts .....................................................................................................
71.2. picturing dhcp ......................................................................................................
71.3. installing a dhcp server ..........................................................................................
71.4. dhcp server on Red Hat .........................................................................................
71.5. dhcp options ........................................................................................................
71.6. client reservations .................................................................................................
71.7. example config files ..............................................................................................
71.8. older example config files ......................................................................................
71.9. advanced dhcp ......................................................................................................
71.10. Practice: dhcp .....................................................................................................
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Chapter 71. introduction to dhcp
Dynamic Host Configuration Protocol (or short dhcp) is a standard tcp/ip protocol that
distributes ip configurations to clients. dhcp is defined in rfc 2131 (before that it was defined
as an update to bootp in rfc 1531/1541.
The alternative to dhcp is manually entering the ip configuration on each client computer.
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introduction to dhcp
71.1. four broadcasts
dhcp works with layer 2 broadcasts. A dhcp client that starts, will send a dhcp discover
on the network. All dhcp servers (that have a lease available) will respond with a dhcp
offer. The client will choose one of those offers and will send a dhcp request containing
the chosen offer. The dhcp server usually responds with a dhcp ack(knowledge).
In wireshark it looks like this.
When this procedure is finished, then the client is allowed to use that ip-configuration until
the end of its lease time.
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71.2. picturing dhcp
Here we have a small network with two dhcp servers named DHCP-SRV1 and DHCPSRV2 and two clients (SunWS1 and Mac42). All computers are connected by a hub or switch
(pictured in the middle). All four computers have a cable to the hub (cables not pictured).
1. The client SunWS1 sends a dhcp discover on the network. All computers receive this
broadcast.
2. Both dhcp servers answer with a dhcp offer. DHCP-SRV1 is a dedicated dhcp server
and is faster in sending a dhcp offer than DHCP-SRV2 (who happens to also be a file server).
3. The client chooses the offer from DHCP-SRV1 and sends a dhcp request on the network.
4. DHCP-SRV1 answers with a dhcp ack (short for acknowledge).
All four broadcasts (or five when you count both offers) can be layer 2 ethernet broadcast
to mac address ff:ff:ff:ff:ff:ff and a layer 3 ip broadcast to 255.255.255.255.
The same story can be read in rfc 2131.
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71.3. installing a dhcp server
On Debian/Ubuntu
debian5:~# aptitude install dhcp3-server
Reading package lists... Done
Building dependency tree
Reading state information... Done
Reading extended state information
Initializing package states... Done
Reading task descriptions... Done
The following NEW packages will be installed:
dhcp3-server
You get a configuration file with many examples.
debian5:~# ls -l /etc/dhcp3/dhcpd.conf
-rw-r--r-- 1 root root 3551 2011-04-10 21:23 /etc/dhcp3/dhcpd.conf
71.4. dhcp server on Red Hat
After installing we get a /etc/dhcpd.conf that points us to an example file named
dhcpd.conf.sample.
[root@localhost ~]# cat /etc/dhcpd.conf
#
# DHCP Server Configuration file.
#
see /usr/share/doc/dhcp*/dhcpd.conf.sample
So we copy the sample and adjust it for our real situation. We name the copy /etc/
dhcpd.conf.
subnet 192.168.1.0 netmask 255.255.255.0 {
range 192.168.1.140 192.168.1.159
option routers
192.168.1.1;
option subnet-mask
255.255.255.0;
option domain-name
"classdemo.local";
option domain-name-servers
192.168.1.1;
default-lease-time
21600;
}
71.5. dhcp options
Options can be set on the global, scope, client-reservation level.
option
option
option
option
subnet-mask 255.255.255.0;
domain-name "linux-training.be";
domain-name-servers "ns1.openminds.be";
routers 192.168.42.1;
71.6. client reservations
You can reserve an ip configuration for a client using the mac address.
host pc42 {
hardware ethernet 11:22:33:44:55:66;
fixed-address 192.168.42.42;
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}
You can add individual options to this reservation.
host pc42 {
hardware ethernet 11:22:33:44:55:66;
fixed-address 192.168.42.42;
option domain-name "linux-training.be";
option routers 192.168.42.1;
}
71.7. example config files
Below you see several sections of /etc/dhcp/dhcpd.conf on a Debian 6 server.
# NetSec Antwerp Network
subnet 192.168.1.0 netmask 255.255.255.0 {
range 192.168.1.20 192.168.1.199;
option domain-name-servers ns1.netsec.local;
option domain-name "netsec.local";
option routers 192.168.1.1;
option broadcast-address 192.168.1.255;
default-lease-time 7200;
max-lease-time 7200;
}
Above the general configuration for the network, with a pool of 180 addresses.
Below two client reservations:
#
# laptops
#
host mac {
hardware ethernet 00:26:bb:xx:xx:xx;
fixed-address mac.netsec.local;
}
host vmac {
hardware ethernet 8c:7b:9d:xx:xx:xx;
fixed-address vmac.netsec.local;
}
71.8. older example config files
For dhcpd.conf on Fedora with dynamic updates for a DNS domain.
[root@fedora14 ~]# cat /etc/dhcp/dhcpd.conf
authoritative;
include "/etc/rndc.key";
log-facility local6;
server-identifier
fedora14;
ddns-domainname "office.linux-training.be";
ddns-update-style interim;
ddns-updates on;
update-static-leases on;
option domain-name "office.linux-training.be";
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option domain-name-servers 192.168.42.100;
option ip-forwarding off;
default-lease-time 1800;
max-lease-time 3600;
zone office.linux-training.be {
primary 192.168.42.100;
}
subnet 192.168.4.0 netmask 255.255.255.0 {
range 192.168.4.24 192.168.4.40;
}
Allowing any updates in the zone database (part of the named.conf configuration)
zone "office.linux-training.be" {
type master;
file "/var/named/db.office.linux-training.be";
allow-transfer { any; };
allow-update { any; };
};
Allowing secure key updates in the zone database (part of the named.conf configuration)
zone "office.linux-training.be" {
type master;
file "/var/named/db.office.linux-training.be";
allow-transfer { any; };
allow-update { key mykey; };
};
Sample key file contents:
[root@fedora14 ~]# cat /etc/rndc.key
key "rndc-key" {
algorithm hmac-md5;
secret "4Ykd58uIeUr3Ve6ad1qTfQ==";
};
Generate your own keys with dnssec-keygen.
How to include a key in a config file:
include "/etc/bind/rndc.key";
Also make sure that bind can write to your db.zone file (using chmod/chown). For Ubuntu
this can be in /etc/bind, for Fedora in /var/named.
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71.9. advanced dhcp
71.9.1. 80/20 rule
DHCP servers should not be a single point of failure. Let us discuss redundant dhcp server
setups.
71.9.2. relay agent
To avoid having to place a dhcp server on every segment, we can use dhcp relay agents.
71.9.3. rogue dhcp servers
Rogue dhcp servers are a problem without a solution. For example accidental connection of
a (believed to be simple) hub/switch to a network with an internal dhcp server.
71.9.4. dhcp and ddns
DHCP can dynamically update DNS when it configures a client computer. DDNS can be
used with or without secure keys.
When set up properly records can be added automaticall to the zone file:
root@fedora14~# tail -2 /var/named/db.office.linux-training.be
ubu1010srv
A
192.168.42.151
TXT
"00dfbb15e144a273c3cf2d6ae933885782"
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introduction to dhcp
71.10. Practice: dhcp
1. Make sure you have a unique fixed ip address for your DNS and DHCP server (easier
on the same machine).
2. Install DHCP and browse the explanation in the default configuration file /etc/dhcp/
dhcpd.conf or /etc/dhcp3/dhcpd.conf.
3. Decide on a valid scope and activate it.
4. Test with a client that your DHCP server works.
5. Use wireshark to capture the four broadcasts when a client receives an ip (for the first
time).
6. Use wireshark to capture a DHCPNAK and a DHCPrelease.
7. Reserve a configuration for a particular client (using mac address).
8. Configure your DHCP/DNS server(s) with a proper hostname and domainname (/etc/
hosts, /etc/hostname, /etc/sysconfig/network on Fedora/RHEL, /etc/resolv.conf ...). You
may need to disable NetworkManager on *buntu-desktops.
9. Make sure your DNS server still works, and is master over (at least) one domain.
There are several ways to do steps 10-11-12. Google is your friend in exploring DDNS with
keys, with key-files or without keys.
10. Configure your DNS server to allow dynamic updates from your DHCP server.
11. Configure your DHCP server to send dynamic updates to your DNS server.
12. Test the working of Dynamic DNS.
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Part XXI. iptables firewall
Table of Contents
72. introduction to routers ...................................................................................................
72.1. router or firewall ..................................................................................................
72.2. packet forwarding .................................................................................................
72.3. packet filtering .....................................................................................................
72.4. stateful ................................................................................................................
72.5. nat (network address translation) .............................................................................
72.6. pat (port address translation) ...................