Datasets:

Vikhrmodels
/

programming_books_eng

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: paul.cobbaut@gmail.com, http://www.linkedin.com/in/cobbaut

Contributors to the Linux Training project are:

• Serge van Ginderachter: serge@ginsys.eu, build scripts and infrastructure setup

• Ywein Van den Brande: ywein@crealaw.eu, license and legal sections

• Hendrik De Vloed: hendrik.devloed@ugent.be, buildheader.pl script

We'd also like to thank our reviewers:

• Wouter Verhelst: wo@uter.be, http://grep.be

• Geert

Goossens:

mail.goossens.geert@gmail.com,

http://www.linkedin.com/in/

geertgoossens

• Elie De Brauwer: elie@de-brauwer.be, http://www.de-brauwer.be

• Christophe Vandeplas: christophe@vandeplas.com, http://christophe.vandeplas.com

• Bert Desmet: bert@devnox.be, http://blog.bdesmet.be

• Rich Yonts: richyonts@gmail.com,

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. 1969 ....................................................................................................................................... 4
1.2. 1980s ...................................................................................................................................... 4
1.3. 1990s ...................................................................................................................................... 4
1.4. 2015 ....................................................................................................................................... 5
2. distributions ...................................................................................................................................... 6
2.1. Red Hat .................................................................................................................................. 7
2.2. Ubuntu ................................................................................................................................... 7
2.3. Debian .................................................................................................................................... 7
2.4. Other ...................................................................................................................................... 7
2.5. Which to choose ? ................................................................................................................. 8
3. licensing ............................................................................................................................................. 9
3.1. about software licenses ....................................................................................................... 10
3.2. public domain software and freeware ................................................................................. 10
3.3. Free Software or Open Source Software ............................................................................ 10
3.4. GNU General Public License .............................................................................................. 11
3.5. using GPLv3 software ......................................................................................................... 11
3.6. BSD license ......................................................................................................................... 12
3.7. other licenses ....................................................................................................................... 12
3.8. combination of software licenses ........................................................................................ 12
II. installing Linux ......................................................................................................................................... 13
4. installing Debian 8 ......................................................................................................................... 15
4.1. Debian .................................................................................................................................. 16
4.2. Downloading ........................................................................................................................ 16
4.3. virtualbox networking ......................................................................................................... 32
4.4. setting the hostname ............................................................................................................ 34
4.5. adding a static ip address .................................................................................................... 34
4.6. Debian package management .............................................................................................. 35
5. installing CentOS 7 ........................................................................................................................ 36
5.1. download a CentOS 7 image .............................................................................................. 37
5.2. Virtualbox ............................................................................................................................ 39
5.3. CentOS 7 installing ............................................................................................................. 44
5.4. CentOS 7 first logon ........................................................................................................... 52
5.5. Virtualbox network interface .............................................................................................. 53
5.6. configuring the network ...................................................................................................... 54
5.7. adding one static ip address ................................................................................................ 54
5.8. package management ........................................................................................................... 55
5.9. logon from Linux and MacOSX ......................................................................................... 56
5.10. logon from MS Windows ................................................................................................. 56
6. getting Linux at home ................................................................................................................... 58
6.1. download a Linux CD image .............................................................................................. 59
6.2. download Virtualbox ........................................................................................................... 59
6.3. create a virtual machine ...................................................................................................... 60
6.4. attach the CD image ............................................................................................................ 65
6.5. install Linux ......................................................................................................................... 68
III. first steps on the command line .............................................................................................................. 69
7. man pages ....................................................................................................................................... 71
7.1. man $command ................................................................................................................... 72
7.2. man $configfile .................................................................................................................... 72
7.3. man $daemon ...................................................................................................................... 72
7.4. man -k (apropos) ................................................................................................................. 72
7.5. whatis ................................................................................................................................... 72
7.6. whereis ................................................................................................................................. 72
7.7. man sections ........................................................................................................................ 73

iii

Linux Fundamentals

7.8. man $section $file ............................................................................................................... 73
7.9. man man .............................................................................................................................. 73
7.10. mandb ................................................................................................................................ 73
8. working with directories ............................................................................................................... 74
8.1. pwd ...................................................................................................................................... 75
8.2. cd .......................................................................................................................................... 75
8.3. absolute and relative paths .................................................................................................. 76
8.4. path completion ................................................................................................................... 77
8.5. ls ........................................................................................................................................... 77
8.6. mkdir .................................................................................................................................... 79
8.7. rmdir .................................................................................................................................... 79
8.8. practice: working with directories ....................................................................................... 81
8.9. solution: working with directories ...................................................................................... 82
9. working with files ........................................................................................................................... 84
9.1. all files are case sensitive ................................................................................................... 85
9.2. everything is a file .............................................................................................................. 85
9.3. file ........................................................................................................................................ 85
9.4. touch .................................................................................................................................... 86
9.5. rm ......................................................................................................................................... 87
9.6. cp .......................................................................................................................................... 88
9.7. mv ........................................................................................................................................ 89
9.8. rename .................................................................................................................................. 90
9.9. practice: working with files ................................................................................................ 91
9.10. solution: working with files .............................................................................................. 92
10. working with file contents ........................................................................................................... 94
10.1. head .................................................................................................................................... 95
10.2. tail ...................................................................................................................................... 95
10.3. cat ....................................................................................................................................... 96
10.4. tac ....................................................................................................................................... 97
10.5. more and less ..................................................................................................................... 98
10.6. strings ................................................................................................................................. 98
10.7. practice: file contents ........................................................................................................ 99
10.8. solution: file contents ...................................................................................................... 100
11. the Linux file tree ...................................................................................................................... 101
11.1. filesystem hierarchy standard .......................................................................................... 102
11.2. man hier ........................................................................................................................... 102
11.3. the root directory / ........................................................................................................... 102
11.4. binary directories ............................................................................................................. 103
11.5. configuration directories .................................................................................................. 105
11.6. data directories ................................................................................................................ 107
11.7. in memory directories ..................................................................................................... 109
11.8. /usr Unix System Resources ............................................................................................ 114
11.9. /var variable data ............................................................................................................. 116
11.10. practice: file system tree ............................................................................................... 118
11.11. solution: file system tree ............................................................................................... 120
IV. shell expansion ...................................................................................................................................... 122
12. commands and arguments ........................................................................................................ 125
12.1. arguments ......................................................................................................................... 126
12.2. white space removal ........................................................................................................ 126
12.3. single quotes .................................................................................................................... 127
12.4. double quotes ................................................................................................................... 127
12.5. echo and quotes ............................................................................................................... 127
12.6. commands ........................................................................................................................ 128
12.7. aliases ............................................................................................................................... 129
12.8. displaying shell expansion .............................................................................................. 130
12.9. practice: commands and arguments ................................................................................ 131
12.10. solution: commands and arguments .............................................................................. 133
13. control operators ........................................................................................................................ 135

Linux Fundamentals

13.1. ; semicolon ....................................................................................................................... 136
13.2. & ampersand .................................................................................................................... 136
13.3. $? dollar question mark ................................................................................................... 136
13.4. && double ampersand .................................................................................................... 137
13.5. || double vertical bar ........................................................................................................ 137
13.6. combining && and || ....................................................................................................... 137
13.7. # pound sign .................................................................................................................... 138
13.8. \ escaping special characters ........................................................................................... 138
13.9. practice: control operators ............................................................................................... 139
13.10. solution: control operators ............................................................................................. 140
14. shell variables ............................................................................................................................. 141
14.1. $ dollar sign ..................................................................................................................... 142
14.2. case sensitive ................................................................................................................... 142
14.3. creating variables ............................................................................................................. 142
14.4. quotes ............................................................................................................................... 143
14.5. set ..................................................................................................................................... 143
14.6. unset ................................................................................................................................. 143
14.7. $PS1 ................................................................................................................................. 144
14.8. $PATH ............................................................................................................................. 145
14.9. env .................................................................................................................................... 146
14.10. export ............................................................................................................................. 146
14.11. delineate variables ......................................................................................................... 147
14.12. unbound variables .......................................................................................................... 147
14.13. practice: shell variables ................................................................................................. 148
14.14. solution: shell variables ................................................................................................. 149
15. shell embedding and options ..................................................................................................... 150
15.1. shell embedding ............................................................................................................... 151
15.2. shell options ..................................................................................................................... 152
15.3. practice: shell embedding ................................................................................................ 153
15.4. solution: shell embedding ................................................................................................ 154
16. shell history ................................................................................................................................. 155
16.1. repeating the last command ............................................................................................ 156
16.2. repeating other commands .............................................................................................. 156
16.3. history .............................................................................................................................. 156
16.4. !n ...................................................................................................................................... 156
16.5. Ctrl-r ................................................................................................................................ 157
16.6. $HISTSIZE ...................................................................................................................... 157
16.7. $HISTFILE ...................................................................................................................... 157
16.8. $HISTFILESIZE .............................................................................................................. 157
16.9. prevent recording a command ......................................................................................... 158
16.10. (optional)regular expressions ........................................................................................ 158
16.11. (optional) Korn shell history ......................................................................................... 158
16.12. practice: shell history .................................................................................................... 159
16.13. solution: shell history .................................................................................................... 160
17. file globbing ................................................................................................................................ 161
17.1. * asterisk .......................................................................................................................... 162
17.2. ? question mark ............................................................................................................... 162
17.3. [] square brackets ............................................................................................................ 163
17.4. a-z and 0-9 ranges ........................................................................................................... 164
17.5. $LANG and square brackets ........................................................................................... 164
17.6. preventing file globbing .................................................................................................. 165
17.7. practice: shell globbing ................................................................................................... 166
17.8. solution: shell globbing ................................................................................................... 167
V. pipes and commands .............................................................................................................................. 169
18. I/O redirection ............................................................................................................................ 171
18.1. stdin, stdout, and stderr ................................................................................................... 172
18.2. output redirection ............................................................................................................. 173
18.3. error redirection ............................................................................................................... 175

Linux Fundamentals

18.4. output redirection and pipes ............................................................................................ 176
18.5. joining stdout and stderr ................................................................................................. 176
18.6. input redirection ............................................................................................................... 177
18.7. confusing redirection ....................................................................................................... 178
18.8. quick file clear ................................................................................................................. 178
18.9. practice: input/output redirection .................................................................................... 179
18.10. solution: input/output redirection .................................................................................. 180
19. filters ............................................................................................................................................ 181
19.1. cat ..................................................................................................................................... 182
19.2. tee ..................................................................................................................................... 182
19.3. grep .................................................................................................................................. 182
19.4. cut .................................................................................................................................... 184
19.5. tr ....................................................................................................................................... 184
19.6. wc ..................................................................................................................................... 185
19.7. sort ................................................................................................................................... 186
19.8. uniq .................................................................................................................................. 187
19.9. comm ............................................................................................................................... 188
19.10. od ................................................................................................................................... 189
19.11. sed .................................................................................................................................. 190
19.12. pipe examples ................................................................................................................ 191
19.13. practice: filters ............................................................................................................... 192
19.14. solution: filters ............................................................................................................... 193
20. basic Unix tools .......................................................................................................................... 195
20.1. find ................................................................................................................................... 196
20.2. locate ................................................................................................................................ 197
20.3. date ................................................................................................................................... 197
20.4. cal ..................................................................................................................................... 198
20.5. sleep ................................................................................................................................. 198
20.6. time .................................................................................................................................. 199
20.7. gzip - gunzip .................................................................................................................... 200
20.8. zcat - zmore ..................................................................................................................... 200
20.9. bzip2 - bunzip2 ................................................................................................................ 201
20.10. bzcat - bzmore ............................................................................................................... 201
20.11. practice: basic Unix tools .............................................................................................. 202
20.12. solution: basic Unix tools .............................................................................................. 203
21. regular expressions .................................................................................................................... 205
21.1. regex versions .................................................................................................................. 206
21.2. grep .................................................................................................................................. 207
21.3. rename .............................................................................................................................. 212
21.4. sed .................................................................................................................................... 215
21.5. bash history ...................................................................................................................... 219
VI. vi ............................................................................................................................................................ 220
22. Introduction to vi ....................................................................................................................... 222
22.1. command mode and insert mode .................................................................................... 223
22.2. start typing (a A i I o O) ................................................................................................ 223
22.3. replace and delete a character (r x X) ............................................................................. 224
22.4. undo and repeat (u .) ....................................................................................................... 224
22.5. cut, copy and paste a line (dd yy p P) ............................................................................ 224
22.6. cut, copy and paste lines (3dd 2yy) ................................................................................ 225
22.7. start and end of a line (0 or ^ and $) .............................................................................. 225
22.8. join two lines (J) and more ............................................................................................. 225
22.9. words (w b) ..................................................................................................................... 226
22.10. save (or not) and exit (:w :q :q! ) .................................................................................. 226
22.11. Searching (/ ?) ................................................................................................................ 226
22.12. replace all ( :1,$ s/foo/bar/g ) ........................................................................................ 227
22.13. reading files (:r :r !cmd) ................................................................................................ 227
22.14. text buffers ..................................................................................................................... 227
22.15. multiple files .................................................................................................................. 227

Linux Fundamentals

22.16. abbreviations .................................................................................................................. 228
22.17. key mappings ................................................................................................................. 229
22.18. setting options ................................................................................................................ 229
22.19. practice: vi(m) ............................................................................................................... 230
22.20. solution: vi(m) ............................................................................................................... 231
VII. scripting ................................................................................................................................................ 232
23. scripting introduction ................................................................................................................ 234
23.1. prerequisites ..................................................................................................................... 235
23.2. hello world ....................................................................................................................... 235
23.3. she-bang ........................................................................................................................... 235
23.4. comment ........................................................................................................................... 236
23.5. variables ........................................................................................................................... 236
23.6. sourcing a script .............................................................................................................. 236
23.7. troubleshooting a script ................................................................................................... 237
23.8. prevent setuid root spoofing ............................................................................................ 237
23.9. practice: introduction to scripting ................................................................................... 238
23.10. solution: introduction to scripting ................................................................................. 239
24. scripting loops ............................................................................................................................. 240
24.1. test [ ] ............................................................................................................................... 241
24.2. if then else ....................................................................................................................... 242
24.3. if then elif ........................................................................................................................ 242
24.4. for loop ............................................................................................................................ 242
24.5. while loop ........................................................................................................................ 243
24.6. until loop .......................................................................................................................... 243
24.7. practice: scripting tests and loops ................................................................................... 244
24.8. solution: scripting tests and loops ................................................................................... 245
25. scripting parameters .................................................................................................................. 247
25.1. script parameters .............................................................................................................. 248
25.2. shift through parameters .................................................................................................. 249
25.3. runtime input ................................................................................................................... 249
25.4. sourcing a config file ...................................................................................................... 250
25.5. get script options with getopts ........................................................................................ 251
25.6. get shell options with shopt ............................................................................................ 252
25.7. practice: parameters and options ..................................................................................... 253
25.8. solution: parameters and options ..................................................................................... 254
26. more scripting ............................................................................................................................. 255
26.1. eval ................................................................................................................................... 256
26.2. (( )) ................................................................................................................................... 256
26.3. let ..................................................................................................................................... 257
26.4. case .................................................................................................................................. 258
26.5. shell functions .................................................................................................................. 259
26.6. practice : more scripting .................................................................................................. 260
26.7. solution : more scripting .................................................................................................. 261
VIII. local user management ....................................................................................................................... 263
27. introduction to users .................................................................................................................. 266
27.1. whoami ............................................................................................................................ 267
27.2. who .................................................................................................................................. 267
27.3. who am i .......................................................................................................................... 267
27.4. w ...................................................................................................................................... 267
27.5. id ...................................................................................................................................... 267
27.6. su to another user ............................................................................................................ 268
27.7. su to root ......................................................................................................................... 268
27.8. su as root ......................................................................................................................... 268
27.9. su - $username ................................................................................................................ 268
27.10. su - ................................................................................................................................. 268
27.11. run a program as another user ...................................................................................... 269
27.12. visudo ............................................................................................................................. 269
27.13. sudo su - ........................................................................................................................ 270

vii

Linux Fundamentals

27.14. sudo logging .................................................................................................................. 270
27.15. practice: introduction to users ....................................................................................... 271
27.16. solution: introduction to users ....................................................................................... 272
28. user management ....................................................................................................................... 274
28.1. user management ............................................................................................................. 275
28.2. /etc/passwd ....................................................................................................................... 275
28.3. root ................................................................................................................................... 275
28.4. useradd ............................................................................................................................. 276
28.5. /etc/default/useradd .......................................................................................................... 276
28.6. userdel .............................................................................................................................. 276
28.7. usermod ............................................................................................................................ 276
28.8. creating home directories ................................................................................................ 277
28.9. /etc/skel/ ........................................................................................................................... 277
28.10. deleting home directories .............................................................................................. 277
28.11. login shell ...................................................................................................................... 278
28.12. chsh ................................................................................................................................ 278
28.13. practice: user management ............................................................................................ 279
28.14. solution: user management ............................................................................................ 280
29. user passwords ............................................................................................................................ 282
29.1. passwd .............................................................................................................................. 283
29.2. shadow file ...................................................................................................................... 283
29.3. encryption with passwd ................................................................................................... 284
29.4. encryption with openssl ................................................................................................... 284
29.5. encryption with crypt ...................................................................................................... 285
29.6. /etc/login.defs ................................................................................................................... 286
29.7. chage ................................................................................................................................ 286
29.8. disabling a password ....................................................................................................... 287
29.9. editing local files ............................................................................................................. 287
29.10. practice: user passwords ................................................................................................ 288
29.11. solution: user passwords ................................................................................................ 289
30. user profiles ................................................................................................................................ 291
30.1. system profile .................................................................................................................. 292
30.2. ~/.bash_profile ................................................................................................................. 292
30.3. ~/.bash_login .................................................................................................................... 293
30.4. ~/.profile .......................................................................................................................... 293
30.5. ~/.bashrc ........................................................................................................................... 293
30.6. ~/.bash_logout .................................................................................................................. 294
30.7. Debian overview .............................................................................................................. 295
30.8. RHEL5 overview ............................................................................................................. 295
30.9. practice: user profiles ...................................................................................................... 296
30.10. solution: user profiles .................................................................................................... 297
31. groups .......................................................................................................................................... 298
31.1. groupadd .......................................................................................................................... 299
31.2. group file ......................................................................................................................... 299
31.3. groups .............................................................................................................................. 299
31.4. usermod ............................................................................................................................ 300
31.5. groupmod ......................................................................................................................... 300
31.6. groupdel ........................................................................................................................... 300
31.7. gpasswd ............................................................................................................................ 301
31.8. newgrp ............................................................................................................................. 302
31.9. vigr ................................................................................................................................... 302
31.10. practice: groups ............................................................................................................. 303
31.11. solution: groups ............................................................................................................. 304
IX. file security ............................................................................................................................................ 305
32. standard file permissions .......................................................................................................... 307
32.1. file ownership .................................................................................................................. 308
32.2. list of special files ........................................................................................................... 310
32.3. permissions ...................................................................................................................... 311

viii

Linux Fundamentals

32.4. practice: standard file permissions .................................................................................. 316
32.5. solution: standard file permissions .................................................................................. 317
33. advanced file permissions .......................................................................................................... 319
33.1. sticky bit on directory ..................................................................................................... 320
33.2. setgid bit on directory ..................................................................................................... 320
33.3. setgid and setuid on regular files .................................................................................... 321
33.4. setuid on sudo .................................................................................................................. 321
33.5. practice: sticky, setuid and setgid bits ............................................................................ 322
33.6. solution: sticky, setuid and setgid bits ............................................................................ 323
34. access control lists ...................................................................................................................... 325
34.1. acl in /etc/fstab ................................................................................................................. 326
34.2. getfacl .............................................................................................................................. 326
34.3. setfacl ............................................................................................................................... 326
34.4. remove an acl entry ......................................................................................................... 327
34.5. remove the complete acl ................................................................................................. 327
34.6. the acl mask ..................................................................................................................... 327
34.7. eiciel ................................................................................................................................. 328
35. file links ....................................................................................................................................... 329
35.1. inodes ............................................................................................................................... 330
35.2. about directories .............................................................................................................. 331
35.3. hard links ......................................................................................................................... 332
35.4. symbolic links .................................................................................................................. 333
35.5. removing links ................................................................................................................. 333
35.6. practice : links ................................................................................................................. 334
35.7. solution : links ................................................................................................................. 335
X. Appendices .............................................................................................................................................. 336
A. keyboard settings ......................................................................................................................... 338
A.1. about keyboard layout ...................................................................................................... 338
A.2. X Keyboard Layout .......................................................................................................... 338
A.3. shell keyboard layout ....................................................................................................... 338
B. hardware ....................................................................................................................................... 340
B.1. buses .................................................................................................................................. 340
B.2. interrupts ........................................................................................................................... 341
B.3. io ports .............................................................................................................................. 342
B.4. dma .................................................................................................................................... 342
C. License .......................................................................................................................................... 344
Index ............................................................................................................................................................. 351

List of Tables

2.1. choosing a Linux distro ............................................................................................................................ 8
4.1. Debian releases ....................................................................................................................................... 16
22.1. getting to command mode .................................................................................................................. 223
22.2. switch to insert mode ......................................................................................................................... 223
22.3. replace and delete ............................................................................................................................... 224
22.4. undo and repeat .................................................................................................................................. 224
22.5. cut, copy and paste a line ................................................................................................................... 224
22.6. cut, copy and paste lines .................................................................................................................... 225
22.7. start and end of line ........................................................................................................................... 225
22.8. join two lines ...................................................................................................................................... 225
22.9. words ................................................................................................................................................... 226
22.10. save and exit vi ................................................................................................................................ 226
22.11. searching ........................................................................................................................................... 226
22.12. replace ............................................................................................................................................... 227
22.13. read files and input ........................................................................................................................... 227
22.14. text buffers ........................................................................................................................................ 227
22.15. multiple files ..................................................................................................................................... 228
22.16. abbreviations ..................................................................................................................................... 228
30.1. Debian User Environment .................................................................................................................. 295
30.2. Red Hat User Environment ................................................................................................................ 295
32.1. Unix special files ................................................................................................................................ 310
32.2. standard Unix file permissions ........................................................................................................... 311
32.3. Unix file permissions position ........................................................................................................... 311
32.4. Octal permissions ............................................................................................................................... 314

Part I. introduction to Linux

Table of Contents

1. Linux history .............................................................................................................................................. 3
1.1. 1969 ................................................................................................................................................. 4
1.2. 1980s ............................................................................................................................................... 4
1.3. 1990s ............................................................................................................................................... 4
1.4. 2015 ................................................................................................................................................. 5
2. distributions ................................................................................................................................................ 6
2.1. Red Hat ........................................................................................................................................... 7
2.2. Ubuntu ............................................................................................................................................. 7
2.3. Debian ............................................................................................................................................. 7
2.4. Other ................................................................................................................................................ 7
2.5. Which to choose ? ........................................................................................................................... 8
3. licensing ....................................................................................................................................................... 9
3.1. about software licenses ................................................................................................................. 10
3.2. public domain software and freeware .......................................................................................... 10
3.3. Free Software or Open Source Software ...................................................................................... 10
3.4. GNU General Public License ....................................................................................................... 11
3.5. using GPLv3 software .................................................................................................................. 11
3.6. BSD license ................................................................................................................................... 12
3.7. other licenses ................................................................................................................................. 12
3.8. combination of software licenses ................................................................................................. 12

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 8. Working with
Directories page 73.

Linux history

1.1. 1969

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: the official AT&T Unix, and the
free BSD Unix.

Development of BSD descendants like FreeBSD, OpenBSD, NetBSD, DragonFly BSD and
PC-BSD is still active today.

https://en.wikipedia.org/wiki/Dennis_Ritchie
https://en.wikipedia.org/wiki/Ken_Thompson
https://en.wikipedia.org/wiki/BSD
https://en.wikipedia.org/wiki/Comparison_of_BSD_operating_systems

1.2. 1980s

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 are GNU tools.

https://en.wikipedia.org/wiki/Richard_Stallman
https://en.wikipedia.org/wiki/IBM_AIX
https://en.wikipedia.org/wiki/HP-UX

1.3. 1990s

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.

http://en.wikipedia.org/wiki/Linus_Torvalds
https://en.wikipedia.org/wiki/History_of_Linux
https://en.wikipedia.org/wiki/Linux
https://lwn.net
http://www.levenez.com/unix/ (a huge Unix history poster)

Linux history

1.4. 2015

Today more than 97 percent of the world's supercomputers (including the complete top 10),
more than 80 percent 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, self-driving cars, space station laptops...)
run Linux. Linux is by far the most commonly used operating system in the world.

Linux kernel version 4.0 was released in April 2015. Its source code grew by several hundred
thousand lines (compared to version 3.19 from February 2015) thanks to contributions of
thousands of developers paid by hundreds of commercial companies including Red Hat,
Intel, Samsung, Broadcom, Texas Instruments, IBM, Novell, Qualcomm, Nokia, Oracle,
Google, AMD and even Microsoft (and many more).

http://kernelnewbies.org/DevelopmentStatistics
http://kernel.org
http://www.top500.org

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.

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.

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

Fedora

You want Red Hat without the support contract from Red Hat.

You want Red Hat on your laptop/desktop.

Linux Mint

You want a personal graphical desktop to play movies, music and games.

Debian

Ubuntu

Kali

others

My personal favorite for servers, laptops, and any other device.

Very popular, based on Debian, not my favorite.

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

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

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

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.

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.

Part II. installing Linux

Table of Contents

4. installing Debian 8 ................................................................................................................................... 15
4.1. Debian ........................................................................................................................................... 16
4.2. Downloading ................................................................................................................................. 16
4.3. virtualbox networking ................................................................................................................... 32
4.4. setting the hostname ..................................................................................................................... 34
4.5. adding a static ip address ............................................................................................................. 34
4.6. Debian package management ....................................................................................................... 35
5. installing CentOS 7 ................................................................................................................................. 36
5.1. download a CentOS 7 image ........................................................................................................ 37
5.2. Virtualbox ...................................................................................................................................... 39
5.3. CentOS 7 installing ....................................................................................................................... 44
5.4. CentOS 7 first logon ..................................................................................................................... 52
5.5. Virtualbox network interface ........................................................................................................ 53
5.6. configuring the network ................................................................................................................ 54
5.7. adding one static ip address .......................................................................................................... 54
5.8. package management .................................................................................................................... 55
5.9. logon from Linux and MacOSX ................................................................................................... 56
5.10. logon from MS Windows ........................................................................................................... 56
6. getting Linux at home ............................................................................................................................. 58
6.1. download a Linux CD image ....................................................................................................... 59
6.2. download Virtualbox ..................................................................................................................... 59
6.3. create a virtual machine ................................................................................................................ 60
6.4. attach the CD image ..................................................................................................................... 65
6.5. install Linux .................................................................................................................................. 68

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, ....

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

Woody

Sarge

Etch

Lenny

Squeeze

Wheezy

Jessie

number

3.0

3.1

4.0

5.0

6.0

year

2002

2005

2007

2009

2011

2013

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:

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

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.

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.

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.

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.

installing Debian 8

First select the language you want to use.

Choose your country. This information will be used to suggest a download mirror.

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).

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.

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.

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.

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.

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/.

installing Debian 8

Choose what software to install, we do not need any graphical stuff for this training.

The latest versions are being downloaded.

installing Debian 8

Say yes to install the bootloader on the virtual machine.

Booting for the first time shows the grub screen

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

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

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 paul@192.168.1.111
paul@192.168.1.111'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...

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

root@server42:~#

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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).

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.

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).

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.

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

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 ~]#

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 status
DEVICE TYPE STATE CONNECTION
enp0s3 ethernet connected enp0s3
lo loopback unmanaged --

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> <Hide>
Addresses 10.104.33.32/16__________ <Remove>

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
inet 192.168.1.110/24 brd 192.168.1.255 scope global dynamic enp0s3
inet 10.104.33.32/16 brd 10.104.255.255 scope global enp0s3
inet6 fe80::a00:27ff:fe1c:f5ab/64 scope link
[root@server33 ~]#

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.

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 root@192.168.1.110
root@192.168.1.110'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).

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).

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.

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!

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.

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).

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.

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.

getting Linux at home

Click create to create the virtual disk.

Click create to create the virtual machine.

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.

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.

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.

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!

Part III. first steps on
the command line

Table of Contents

7. man pages ................................................................................................................................................. 71
7.1. man $command ............................................................................................................................. 72
7.2. man $configfile ............................................................................................................................. 72
7.3. man $daemon ................................................................................................................................ 72
7.4. man -k (apropos) ........................................................................................................................... 72
7.5. whatis ............................................................................................................................................. 72
7.6. whereis ........................................................................................................................................... 72
7.7. man sections .................................................................................................................................. 73
7.8. man $section $file ......................................................................................................................... 73
7.9. man man ........................................................................................................................................ 73
7.10. mandb .......................................................................................................................................... 73
8. working with directories ......................................................................................................................... 74
8.1. pwd ................................................................................................................................................ 75
8.2. cd ................................................................................................................................................... 75
8.3. absolute and relative paths ........................................................................................................... 76
8.4. path completion ............................................................................................................................. 77
8.5. ls .................................................................................................................................................... 77
8.6. mkdir ............................................................................................................................................. 79
8.7. rmdir .............................................................................................................................................. 79
8.8. practice: working with directories ................................................................................................ 81
8.9. solution: working with directories ................................................................................................ 82
9. working with files .................................................................................................................................... 84
9.1. all files are case sensitive ............................................................................................................. 85
9.2. everything is a file ........................................................................................................................ 85
9.3. file .................................................................................................................................................. 85
9.4. touch .............................................................................................................................................. 86
9.5. rm .................................................................................................................................................. 87
9.6. cp ................................................................................................................................................... 88
9.7. mv .................................................................................................................................................. 89
9.8. rename ........................................................................................................................................... 90
9.9. practice: working with files .......................................................................................................... 91
9.10. solution: working with files ........................................................................................................ 92
10. working with file contents .................................................................................................................... 94
10.1. head ............................................................................................................................................. 95
10.2. tail ................................................................................................................................................ 95
10.3. cat ................................................................................................................................................ 96
10.4. tac ................................................................................................................................................ 97
10.5. more and less .............................................................................................................................. 98
10.6. strings .......................................................................................................................................... 98
10.7. practice: file contents .................................................................................................................. 99
10.8. solution: file contents ................................................................................................................ 100
11. the Linux file tree ................................................................................................................................ 101
11.1. filesystem hierarchy standard ................................................................................................... 102
11.2. man hier .................................................................................................................................... 102
11.3. the root directory / .................................................................................................................... 102
11.4. binary directories ....................................................................................................................... 103
11.5. configuration directories ........................................................................................................... 105
11.6. data directories .......................................................................................................................... 107
11.7. in memory directories ............................................................................................................... 109
11.8. /usr Unix System Resources ..................................................................................................... 114
11.9. /var variable data ....................................................................................................................... 116
11.10. practice: file system tree ......................................................................................................... 118
11.11. solution: file system tree ......................................................................................................... 120

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.

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

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 Executable programs or shell commands
2 System calls (functions provided by the kernel)
3 Library calls (functions within program libraries)
4 Special files (usually found in /dev)
5 File formats and conventions eg /etc/passwd
6 Games
7 Miscellaneous (including macro packages and conventions), e.g. man(7)
8 System administration commands (usually only for root)
9 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 # opens the first manual found
[paul@RHEL52 ~]$ man 5 passwd # 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

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.

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$ cd /etc
paul@debian8$ pwd
/etc
paul@debian8$ cd /bin
paul@debian8$ pwd
/bin
paul@debian8$ cd /home/paul/
paul@debian8$ pwd
/home/paul

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$ cd /etc
paul@debian8$ pwd
/etc
paul@debian8$ cd
paul@debian8$ pwd
/home/paul
paul@debian8$ cd ~
paul@debian8$ pwd
/home/paul

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.

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$ pwd
/home/paul
paul@debian8$ cd /etc
paul@debian8$ pwd
/etc
paul@debian8$ cd -
/home/paul
paul@debian8$ cd -
/etc

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$ pwd
/
paul@debian8$ cd home
paul@debian8$ pwd
/home
paul@debian8$ cd /
paul@debian8$ cd /home
paul@debian8$ pwd
/home

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.

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 services stuff summer.txt
paul@debian8:~$

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 stuff
.. .bash_history .bashrc services .ssh summer.txt
paul@debian8:~$

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

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 Sep 17 00:03 allfiles.txt
-rw-r--r-- 1 paul paul 95K Sep 17 00:03 dmesg.txt
-rw-r--r-- 1 paul paul 20K Sep 17 00:04 services
drwxr-xr-x 2 paul paul 4.0K Sep 17 00:04 stuff
-rw-r--r-- 1 paul paul 0 Sep 17 00:04 summer.txt
paul@debian8:~$ ls -lh
total 17M
-rw-r--r-- 1 paul paul 17M Sep 17 00:03 allfiles.txt
-rw-r--r-- 1 paul paul 95K Sep 17 00:03 dmesg.txt
-rw-r--r-- 1 paul paul 20K Sep 17 00:04 services
drwxr-xr-x 2 paul paul 4.0K Sep 17 00:04 stuff
-rw-r--r-- 1 paul paul 0 Sep 17 00:04 summer.txt
paul@debian8:~$ ls -hl
total 17M
-rw-r--r-- 1 paul paul 17M Sep 17 00:03 allfiles.txt
-rw-r--r-- 1 paul paul 95K Sep 17 00:03 dmesg.txt
-rw-r--r-- 1 paul paul 20K Sep 17 00:04 services
drwxr-xr-x 2 paul paul 4.0K Sep 17 00:04 stuff
-rw-r--r-- 1 paul paul 0 Sep 17 00:04 summer.txt
paul@debian8:~$ ls -h -l
total 17M
-rw-r--r-- 1 paul paul 17M Sep 17 00:03 allfiles.txt
-rw-r--r-- 1 paul paul 95K Sep 17 00:03 dmesg.txt
-rw-r--r-- 1 paul paul 20K Sep 17 00:04 services
drwxr-xr-x 2 paul paul 4.0K Sep 17 00:04 stuff
-rw-r--r-- 1 paul paul 0 Sep 17 00:04 summer.txt
paul@debian8:~$

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:~$

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:~$

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.

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.

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.

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 # opens the manual
/pushd # searches for pushd
n # 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

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.

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

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 200505050000 SinkoDeMayo
paul@debian7:~$ touch -t 130207111630 BigBattle.txt
paul@debian7:~$ ls -l
total 0
-rw-r--r-- 1 paul paul 0 Jul 11 1302 BigBattle.txt
-rw-r--r-- 1 paul paul 0 Oct 15 08:57 file33
-rw-r--r-- 1 paul paul 0 Oct 15 08:56 file42
-rw-r--r-- 1 paul paul 0 May 5 2005 SinkoDeMayo
paul@debian7:~$

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:~$

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:~$

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 SinkoDeMayo
paul@debian7:~$ mv file42 file33
paul@debian7:~$ ls
dir33 dir42 file33 file42.copy SinkoDeMayo
paul@debian7:~$

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 15 09:36 dir33
drwxr-xr-x 2 paul paul 4096 Oct 15 09:36 dir42
-rw-r--r-- 1 paul paul 0 Oct 15 09:38 file33
-rw-r--r-- 1 paul paul 0 Oct 15 09:16 file42.copy
-rw-r--r-- 1 paul paul 0 May 5 2005 SinkoDeMayo
paul@debian7:~$ mv dir33 backup
paul@debian7:~$ ls -l
total 8
drwxr-xr-x 2 paul paul 4096 Oct 15 09:36 backup
drwxr-xr-x 2 paul paul 4096 Oct 15 09:36 dir42
-rw-r--r-- 1 paul paul 0 Oct 15 09:38 file33
-rw-r--r-- 1 paul paul 0 Oct 15 09:16 file42.copy
-rw-r--r-- 1 paul paul 0 May 5 2005 SinkoDeMayo
paul@debian7:~$

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:~$

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 ~]$

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!)

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 ?

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

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.

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 # binkp fidonet protocol
asp 27374/tcp # Address Search Protocol
asp 27374/udp
csync2 30865/tcp # cluster synchronization tool
dircproxy 57000/tcp # Detachable IRC Proxy
tfido 60177/tcp # fidonet EMSI over telnet
fido 60179/tcp # 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.

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:~$ echo one >part1
paul@debian8:~$ echo two >part2
paul@debian8:~$ echo three >part3
paul@debian8:~$ cat part1
one
paul@debian8:~$ cat part2
two
paul@debian8:~$ cat part3
three
paul@debian8:~$ cat part1 part2 part3
one
two
three
paul@debian8:~$ cat part1 part2 part3 >all
paul@debian8:~$ cat all
one
two
three
paul@debian8:~$

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:~$ cat > winter.txt
It is very cold today!
paul@debian8:~$ cat winter.txt
It is very cold today!
paul@debian8:~$

The Ctrl d key combination will send an EOF (End of File) to the running process ending
the cat command.

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:~$ cat winter.txt
It is very cold today!
paul@debian8:~$ cat winter.txt > cold.txt
paul@debian8:~$ cat cold.txt
It is very cold today!
paul@debian8:~$

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

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
...

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.

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

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

100

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 proc sbin srv tftpboot usr
boot etc lib misc opt root selinux sys tmp var

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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 mt setupcon
autopartition false mt-gnu sh
bash fgconsole mv sh.distrib
bunzip2 fgrep nano sleep
bzcat fuser nc stralign
bzcmp fusermount nc.traditional stty
bzdiff get_mountoptions netcat su
bzegrep grep netstat sync
bzexe gunzip ntfs-3g sysfs
bzfgrep gzexe ntfs-3g.probe tailf
bzgrep gzip parted_devices tar
bzip2 hostname parted_server tempfile
bzip2recover hw-detect partman touch
bzless ip partman-commit true
bzmore kbd_mode perform_recipe ulockmgr
cat kill pidof 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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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/libcom_err.so.2.1
/lib/libcap.so.1 /lib/libcidn-2.5.so /lib/libconsole.so.0
/lib/libcap.so.1.10 /lib/libcidn.so.1 /lib/libconsole.so.0.0.0
/lib/libcfont.so.0 /lib/libcom_err.so.2 /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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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/scrollkeeper.conf
/etc/brltty.conf /etc/lftp.conf /etc/sysctl.conf
/etc/ccertificates.conf /etc/libao.conf /etc/syslog.conf
/etc/cvs-cron.conf /etc/logrotate.conf /etc/ucf.conf
/etc/ddclient.conf /etc/ltrace.conf /etc/uniconf.conf
/etc/debconf.conf /etc/mke2fs.conf /etc/updatedb.conf
/etc/deluser.conf /etc/netscsid.conf /etc/usplash.conf
/etc/fdmount.conf /etc/nsswitch.conf /etc/uswsusp.conf
/etc/hdparm.conf /etc/pam.conf /etc/vnc.conf
/etc/host.conf /etc/pnm2ppa.conf /etc/wodim.conf
/etc/inetd.conf /etc/povray.conf /etc/wvdial.conf
/etc/kernel-img.conf /etc/resolv.conf
paul@laika:~$

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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paul@RHELv4u4:~$ ls /etc/sysconfig/
apmd firstboot irda network saslauthd
apm-scripts grub irqbalance networking selinux
authconfig hidd keyboard ntpd spamassassin
autofs httpd kudzu openib.conf squid
bluetooth hwconf lm_sensors pand syslog
clock i18n mouse pcmcia sys-config-sec
console init mouse.B pgsql sys-config-users
crond installinfo named prelink sys-logviewer
desktop ipmi netdump rawdevices tux
diskdump iptables netdump_id_dsa rhn vncservers
dund iptables-cfg netdump_id_dsa.p samba xinetd
paul@RHELv4u4:~$

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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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/.ssh
/home/paul/.. /home/paul/.bashrc /home/paul/.viminfo
/home/paul/.bash_history /home/paul/.lesshst

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 CD-
ROM'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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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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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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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 6587 7201 cmdline mounts
10175 2523 4729 5421 6596 7204 cpuinfo mtrr
10211 2783 4741 5658 6599 7206 crypto net
10239 2975 4873 5661 6638 7214 devices pagetypeinfo
141 29775 4874 5665 6652 7216 diskstats partitions
15045 29792 4878 5927 6719 7218 dma sched_debug
1519 2997 4879 6 6736 7223 driver scsi
1548 3 4881 6032 6737 7224 execdomains self
1551 30228 4882 6033 6755 7227 fb slabinfo
1554 3069 5 6145 6762 7260 filesystems stat
1557 31422 5073 6298 6774 7267 fs swaps
1606 3149 5147 6414 6816 7275 ide sys
180 31507 5203 6418 6991 7282 interrupts sysrq-trigger
181 3189 5206 6419 6993 7298 iomem sysvipc
182 3193 5228 6420 6996 7319 ioports timer_list
18898 3246 5272 6421 7157 7330 irq timer_stats
19799 3248 5291 6422 7163 7345 kallsyms tty
19803 3253 5294 6423 7164 7513 kcore uptime
19804 3372 5356 6424 7171 7525 key-users version
1987 4 5370 6425 7175 7529 kmsg version_signature
1989 42 5379 6426 7188 9964 loadavg vmcore
2 45 5380 6430 7189 acpi locks vmnet
20845 4542 5412 6450 7191 asound meminfo vmstat
221 46 5414 6551 7192 buddyinfo misc zoneinfo
2338 4704 5416 6568 7199 bus modules

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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model name : AMD Athlon(tm) 64 X2 Dual Core Processor 4600+
stepping : 1
cpu MHz : 2398.628
cache size : 512 KB
fdiv_bug : no
hlt_bug : no
f00f_bug : no
coma_bug : no
fpu : yes
fpu_exception : yes
cpuid level : 1
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic mtrr pge...
bogomips : 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.

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/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 timer
1: 4533 0 IO-APIC-edge i8042
7: 0 0 IO-APIC-edge parport0
8: 6588227 0 IO-APIC-edge rtc
10: 2314 0 IO-APIC-fasteoi acpi
12: 133 0 IO-APIC-edge i8042
14: 0 0 IO-APIC-edge libata
15: 72269 0 IO-APIC-edge libata
18: 1 0 IO-APIC-fasteoi yenta
19: 115036 0 IO-APIC-fasteoi eth0
20: 126871 0 IO-APIC-fasteoi libata, ohci1394
21: 30204 0 IO-APIC-fasteoi ehci_hcd:usb1, uhci_hcd:usb2
22: 1334 0 IO-APIC-fasteoi saa7133[0], saa7133[0]
24: 234739 0 IO-APIC-fasteoi nvidia
NMI: 72 42
LOC: 860000 859994
ERR: 0

/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:~$

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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://linux-
hotplug.sourceforge.net/ ).

Basically the /sys directory contains kernel information about hardware.

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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 math.h search.h
af_vfs.h expat_external.h mcheck.h semaphore.h
aio.h expat.h memory.h setjmp.h
AL fcntl.h menu.h sgtty.h
aliases.h features.h mntent.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 sbin share src
paul@deb508:~$ du -sh /usr/local/
128K /usr/local/

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

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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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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 quagga secure.4
amanda cron.3 maillog.3 radius spooler
anaconda.log cron.4 maillog.4 rpmpkgs spooler.1
anaconda.syslog cups mailman rpmpkgs.1 spooler.2
anaconda.xlog dmesg messages rpmpkgs.2 spooler.3
audit exim messages.1 rpmpkgs.3 spooler.4
boot.log gdm messages.2 rpmpkgs.4 squid
boot.log.1 httpd messages.3 sa uucp
boot.log.2 iiim messages.4 samba vbox
boot.log.3 iptraf mysqld.log scrollkeeper.log vmware-tools-guestd
boot.log.4 lastlog news secure wtmp
canna mail pgsql secure.1 wtmp.1
cron maillog ppp secure.2 Xorg.0.log
cron.1 maillog.1 prelink.log secure.3 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 gdm man software-center
binfmts flashplugin-installer hald pm-utils
cups fontconfig jockey pppconfig
debconf fonts ldconfig samba

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).

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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.

117

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.

118

the Linux file tree

14. Read the man page of random and explain the difference between /dev/random and /
dev/urandom.

119

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.

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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 RAM-
memory, 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 systems.

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

121

Part IV. shell expansion

Table of Contents

12. commands and arguments .................................................................................................................. 125
12.1. arguments .................................................................................................................................. 126
12.2. white space removal ................................................................................................................. 126
12.3. single quotes .............................................................................................................................. 127
12.4. double quotes ............................................................................................................................ 127
12.5. echo and quotes ........................................................................................................................ 127
12.6. commands .................................................................................................................................. 128
12.7. aliases ........................................................................................................................................ 129
12.8. displaying shell expansion ........................................................................................................ 130
12.9. practice: commands and arguments .......................................................................................... 131
12.10. solution: commands and arguments ........................................................................................ 133
13. control operators .................................................................................................................................. 135
13.1. ; semicolon ................................................................................................................................ 136
13.2. & ampersand ............................................................................................................................. 136
13.3. $? dollar question mark ............................................................................................................ 136
13.4. && double ampersand .............................................................................................................. 137
13.5. || double vertical bar ................................................................................................................. 137
13.6. combining && and || ................................................................................................................ 137
13.7. # pound sign .............................................................................................................................. 138
13.8. \ escaping special characters ..................................................................................................... 138
13.9. practice: control operators ........................................................................................................ 139
13.10. solution: control operators ...................................................................................................... 140
14. shell variables ....................................................................................................................................... 141
14.1. $ dollar sign .............................................................................................................................. 142
14.2. case sensitive ............................................................................................................................. 142
14.3. creating variables ...................................................................................................................... 142
14.4. quotes ......................................................................................................................................... 143
14.5. set ............................................................................................................................................... 143
14.6. unset ........................................................................................................................................... 143
14.7. $PS1 ........................................................................................................................................... 144
14.8. $PATH ....................................................................................................................................... 145
14.9. env ............................................................................................................................................. 146
14.10. export ....................................................................................................................................... 146
14.11. delineate variables ................................................................................................................... 147
14.12. unbound variables ................................................................................................................... 147
14.13. practice: shell variables ........................................................................................................... 148
14.14. solution: shell variables .......................................................................................................... 149
15. shell embedding and options .............................................................................................................. 150
15.1. shell embedding ........................................................................................................................ 151
15.2. shell options .............................................................................................................................. 152
15.3. practice: shell embedding ......................................................................................................... 153
15.4. solution: shell embedding ......................................................................................................... 154
16. shell history .......................................................................................................................................... 155
16.1. repeating the last command ...................................................................................................... 156
16.2. repeating other commands ........................................................................................................ 156
16.3. history ........................................................................................................................................ 156
16.4. !n ................................................................................................................................................ 156
16.5. Ctrl-r .......................................................................................................................................... 157
16.6. $HISTSIZE ................................................................................................................................ 157
16.7. $HISTFILE ................................................................................................................................ 157
16.8. $HISTFILESIZE ....................................................................................................................... 157
16.9. prevent recording a command .................................................................................................. 158
16.10. (optional)regular expressions .................................................................................................. 158
16.11. (optional) Korn shell history .................................................................................................. 158
16.12. practice: shell history .............................................................................................................. 159

123

shell expansion

16.13. solution: shell history .............................................................................................................. 160
17. file globbing .......................................................................................................................................... 161
17.1. * asterisk ................................................................................................................................... 162
17.2. ? question mark ......................................................................................................................... 162
17.3. [] square brackets ...................................................................................................................... 163
17.4. a-z and 0-9 ranges .................................................................................................................... 164
17.5. $LANG and square brackets .................................................................................................... 164
17.6. preventing file globbing ............................................................................................................ 165
17.7. practice: shell globbing ............................................................................................................. 166
17.8. solution: shell globbing ............................................................................................................. 167

124

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

125

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 ~]$ echo Hello World
Hello World
[paul@RHELv4u3 ~]$ echo Hello World
Hello World
[paul@RHELv4u3 ~]$ echo Hello World
Hello World
[paul@RHELv4u3 ~]$ echo Hello 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.

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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 single quotes'
A line with single quotes
[paul@RHEL4b ~]$

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 double quotes"
A line with double quotes
[paul@RHEL4b ~]$

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 ~]$ echo -e "A line with \na newline"
A line with
a newline
[paul@RHEL4b ~]$ echo -e 'A line with \na newline'
A line with
a newline
[paul@RHEL4b ~]$ echo -e "A line with \ta tab"
A line with a tab
[paul@RHEL4b ~]$ echo -e 'A line with \ta tab'
A line with a tab
[paul@RHEL4b ~]$

The echo command can generate more than white spaces, tabs and newlines. Look in the
man page for a list of options.

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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 -i winter.txt
rm: remove regular file `winter.txt'? no
[paul@RHELv4u3 ~]$ rm winter.txt
[paul@RHELv4u3 ~]$ ls winter.txt
ls: winter.txt: No such file or directory
[paul@RHELv4u3 ~]$ touch winter.txt
[paul@RHELv4u3 ~]$ alias rm='rm -i'
[paul@RHELv4u3 ~]$ rm winter.txt
rm: remove regular empty file `winter.txt'? no
[paul@RHELv4u3 ~]$

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'

129

commands and arguments

12.7.5. unalias

You can undo an alias with the unalias command.

[paul@RHEL4b ~]$ which rm
/bin/rm
[paul@RHEL4b ~]$ alias rm='rm -i'
[paul@RHEL4b ~]$ which rm
alias rm='rm -i'
/bin/rm
[paul@RHEL4b ~]$ unalias rm
[paul@RHEL4b ~]$ which rm
/bin/rm
[paul@RHEL4b ~]$

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

130

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 =8
10+14 =24

17. Use echo to display the following exactly:

??\\

131

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.

132

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.

133

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 =8
10+14 =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 -e '??\\\\'
echo "??\\\\"
echo -e "??\\\\\\"
echo ??\\\\

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"

134

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(&).

135

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 ~]$ echo Hello
Hello
[paul@RHELv4u3 ~]$ echo World
World
[paul@RHELv4u3 ~]$ echo Hello ; echo World
Hello
World
[paul@RHELv4u3 ~]$

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 $?
1
paul@debian5:~/test$

136

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$

137

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 # we create a directory
paul@debian4:~$ cd test #### we enter the directory
paul@debian4:~/test$ ls # is it empty ?
paul@debian4:~/test$

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 ~]$ echo hello \; world
hello ; world
[paul@RHELv4u3 ~]$ echo hello\ \ \ world
hello world
[paul@RHELv4u3 ~]$ echo escaping \\\ \#\ \&\ \"\ \'
escaping \ # & " '
[paul@RHELv4u3 ~]$ echo escaping \\\?\*\"\'
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 ~]$

138

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)

139

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 \\ \* \[ \} \~ \\\\ \. \"

echo \""Hello World with strange' characters \ * [ } ~ \\ . "\"

140

Chapter 14. shell variables

In this chapter we learn to manage environment variables in the shell. These variables are
often needed by applications.

141

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 $MyVar
555
[paul@RHELv4u3 gen]$

142

shell variables

14.4. quotes

Notice that double quotes still allow the parsing of variables, whereas single quotes prevent
this.

[paul@RHELv4u3 ~]$ MyVar=555
[paul@RHELv4u3 ~]$ echo $MyVar
555
[paul@RHELv4u3 ~]$ echo "$MyVar"
555
[paul@RHELv4u3 ~]$ echo '$MyVar'
$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 ~]$ MyVar=8472
[paul@RHEL4b ~]$ echo $MyVar
8472
[paul@RHEL4b ~]$ unset MyVar
[paul@RHEL4b ~]$ echo $MyVar

[paul@RHEL4b ~]$

143

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\$ "

144

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 ~]#

145

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 ~]$ var3=three
[paul@RHEL4b ~]$ var4=four
[paul@RHEL4b ~]$ export var4
[paul@RHEL4b ~]$ echo $var3 $var4
three four
[paul@RHEL4b ~]$ bash
[paul@RHEL4b ~]$ echo $var3 $var4
four

But it will not export to the parent shell (previous screenshot continued).

[paul@RHEL4b ~]$ export var5=five
[paul@RHEL4b ~]$ echo $var3 $var4 $var5
four five
[paul@RHEL4b ~]$ exit
exit
[paul@RHEL4b ~]$ echo $var3 $var4 $var5
three four
[paul@RHEL4b ~]$

146

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.

147

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.

148

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
export
declare -x

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$ '

149

Chapter 15. shell embedding and
options

This chapter takes a brief look at child shells, embedded shells and shell options.

150

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 $var1
[paul@RHELv4u3 gen]$

151

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 ~]$ echo $-
himBH
[paul@RHEL4b ~]$ set -C ; set -u
[paul@RHEL4b ~]$ echo $-
himuBCH
[paul@RHEL4b ~]$ set +C ; set +u
[paul@RHEL4b ~]$ echo $-
himBH
[paul@RHEL4b ~]$

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.

152

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 non-
existing 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.

153

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 non-
existing 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)

154

Chapter 16. shell history

The shell makes it easy for us to repeat commands, this chapter explains how.

155

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

156

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

157

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
$

158

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 ?

159

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

when you type exit

160

Chapter 17. file globbing

The shell is also responsible for file globbing (or dynamic filename generation). This chapter
will explain file globbing.

161

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]$ ls
file1 file2 file3 File4 File55 FileA fileab Fileab FileAB fileabc
[paul@RHELv4u3 gen]$ ls File*
File4 File55 FileA Fileab FileAB
[paul@RHELv4u3 gen]$ ls file*
file1 file2 file3 fileab fileabc
[paul@RHELv4u3 gen]$ ls *ile55
File55
[paul@RHELv4u3 gen]$ ls F*ile55
File55
[paul@RHELv4u3 gen]$ ls F*55
File55
[paul@RHELv4u3 gen]$

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 FileA fileab Fileab FileAB fileabc
[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]$

162

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 fileabc
[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]$

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]$ ls
file1 file2 file3 File4 File55 FileA fileab Fileab FileAB fileabc
[paul@RHELv4u3 gen]$ ls file[a5][!Z]
fileab
[paul@RHELv4u3 gen]$ ls file[!5]*
file1 file2 file3 fileab fileabc
[paul@RHELv4u3 gen]$ ls file[!5]?
fileab
[paul@RHELv4u3 gen]$

163

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.

164

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$ echo *
file33 file42
paul@ubu1010:~/test42$ echo \*
*
paul@ubu1010:~/test42$ echo '*'
*
paul@ubu1010:~/test42$ echo "*"
*

165

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 ?

166

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 file1 file10 file11 file2 File2 File3
touch file33 fileAB filea fileA fileAAA
touch "file("
touch "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.

167

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

echo *

17. Is there another command besides cd to change directories ?

pushd popd

168

Part V. pipes and commands

Table of Contents

18. I/O redirection ...................................................................................................................................... 171
18.1. stdin, stdout, and stderr ............................................................................................................. 172
18.2. output redirection ...................................................................................................................... 173
18.3. error redirection ......................................................................................................................... 175
18.4. output redirection and pipes ..................................................................................................... 176
18.5. joining stdout and stderr ........................................................................................................... 176
18.6. input redirection ........................................................................................................................ 177
18.7. confusing redirection ................................................................................................................. 178
18.8. quick file clear .......................................................................................................................... 178
18.9. practice: input/output redirection .............................................................................................. 179
18.10. solution: input/output redirection ............................................................................................ 180
19. filters ...................................................................................................................................................... 181
19.1. cat .............................................................................................................................................. 182
19.2. tee .............................................................................................................................................. 182
19.3. grep ............................................................................................................................................ 182
19.4. cut .............................................................................................................................................. 184
19.5. tr ................................................................................................................................................. 184
19.6. wc .............................................................................................................................................. 185
19.7. sort ............................................................................................................................................. 186
19.8. uniq ............................................................................................................................................ 187
19.9. comm ......................................................................................................................................... 188
19.10. od ............................................................................................................................................. 189
19.11. sed ............................................................................................................................................ 190
19.12. pipe examples .......................................................................................................................... 191
19.13. practice: filters ......................................................................................................................... 192
19.14. solution: filters ........................................................................................................................ 193
20. basic Unix tools .................................................................................................................................... 195
20.1. find ............................................................................................................................................. 196
20.2. locate ......................................................................................................................................... 197
20.3. date ............................................................................................................................................ 197
20.4. cal .............................................................................................................................................. 198
20.5. sleep ........................................................................................................................................... 198
20.6. time ............................................................................................................................................ 199
20.7. gzip - gunzip ............................................................................................................................. 200
20.8. zcat - zmore .............................................................................................................................. 200
20.9. bzip2 - bunzip2 ......................................................................................................................... 201
20.10. bzcat - bzmore ........................................................................................................................ 201
20.11. practice: basic Unix tools ....................................................................................................... 202
20.12. solution: basic Unix tools ....................................................................................................... 203
21. regular expressions .............................................................................................................................. 205
21.1. regex versions ........................................................................................................................... 206
21.2. grep ............................................................................................................................................ 207
21.3. rename ....................................................................................................................................... 212
21.4. sed .............................................................................................................................................. 215
21.5. bash history ............................................................................................................................... 219

170

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.

171

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.

st din (0)

bash

st dout (1)

st derr (2)

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.

172

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 ~]$

173

I/O redirection

18.2.3. noclobber

Erasing a file while using > can be prevented by setting the noclobber option.

[paul@RHELv4u3 ~]$ cat winter.txt
It is cold today!
[paul@RHELv4u3 ~]$ set -o noclobber
[paul@RHELv4u3 ~]$ echo It is cold today! > winter.txt
-bash: winter.txt: cannot overwrite existing file
[paul@RHELv4u3 ~]$ set +o noclobber
[paul@RHELv4u3 ~]$

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 ~]$

174

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.

175

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:~$

176

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 ~]$ cat <<EOF > text.txt
> one
> two
> EOF
[paul@RHEL4b ~]$ cat text.txt
one
two
[paul@RHEL4b ~]$ cat <<brol > text.txt
> brel
> brol
[paul@RHEL4b ~]$ cat text.txt
brel
[paul@RHEL4b ~]$

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.

177

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

178

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.

179

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:~$

180

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.

181

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

182

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

183

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

184

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 tennis.txt
5 15 100 tennis.txt
[paul@RHEL4b pipes]$ wc -l tennis.txt
5 tennis.txt
[paul@RHEL4b pipes]$ wc -w tennis.txt
15 tennis.txt
[paul@RHEL4b pipes]$ wc -c tennis.txt
100 tennis.txt
[paul@RHEL4b pipes]$

185

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

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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

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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

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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 \n 1 2 3 4 5 6 7 \n
0000020

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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

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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

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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/ .)

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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

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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.

194

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.

195

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 {} \;

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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

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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 ~$ cal 2 1970
February 1970
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

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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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 0m0.014s
user 0m0.008s
sys 0m0.006s

The sleep 5 command takes five real seconds to execute, but consumes little cpu time.

paul@rhel55 ~$ time sleep 5

real 0m5.018s
user 0m0.005s
sys 0m0.011s

This bzip2 command compresses a file and uses a lot of cpu time.

paul@rhel55 ~$ time bzip2 text.txt

real 0m2.368s
user 0m0.847s
sys 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

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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 ?

202

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/

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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

204

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.

205

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!

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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:~$

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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:~$

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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.

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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:~$

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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 -i
grep -v
grep -w
grep -A5
grep -B5
grep -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

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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

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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 ~ $ ls
abc allfiles.TXT bllfiles.TXT Scratch tennis2.TXT
abc.conf backup cllfiles.TXT temp.TXT tennis.TXT
paul@pi ~ $ rename 's/TXT/text/' *
paul@pi ~ $ ls
abc allfiles.text bllfiles.text Scratch tennis2.text
abc.conf 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 ~ $ ls
abc allfiles.text bllfiles.text Scratch tennis2.text
abc.conf backup cllfiles.text temp.text tennis.text
paul@pi ~ $ rename 's/text/txt/' *.text
paul@pi ~ $ ls
abc allfiles.txt bllfiles.txt Scratch tennis2.txt
abc.conf backup cllfiles.txt temp.txt tennis.txt
paul@pi ~ $

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 ~ $ touch atxt.txt
paul@pi ~ $ rename 's/txt/problem/' atxt.txt
paul@pi ~ $ ls
abc allfiles.txt backup cllfiles.txt temp.txt tennis.txt
abc.conf aproblem.txt bllfiles.txt Scratch tennis2.txt
paul@pi ~ $

Only the first occurrence of the searched string is replaced.

213

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 tennis.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 ~ $

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.

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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.

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$ echo Sunday > today
paul@debian7:~/files$ cat today
Sunday
paul@debian7:~/files$ sed -i 's/Sun/Mon/' today
paul@debian7:~/files$ cat today
Monday

215

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.

216

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

217

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:~$ cat list2
ll
lol
lool
loool
paul@debian7:~$ grep -E 'o{2,3}' list2
lool
loool
paul@debian7:~$ grep 'o\{2,3\}' list2
lool
loool
paul@debian7:~$ cat list2 | sed 's/o\{2,3\}/A/'
ll
lol
lAl
lAl
paul@debian7:~$

218

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$

219

Part VI. vi

Table of Contents

22. Introduction to vi ................................................................................................................................. 222
22.1. command mode and insert mode .............................................................................................. 223
22.2. start typing (a A i I o O) .......................................................................................................... 223
22.3. replace and delete a character (r x X) ...................................................................................... 224
22.4. undo and repeat (u .) ................................................................................................................. 224
22.5. cut, copy and paste a line (dd yy p P) ..................................................................................... 224
22.6. cut, copy and paste lines (3dd 2yy) .......................................................................................... 225
22.7. start and end of a line (0 or ^ and $) ....................................................................................... 225
22.8. join two lines (J) and more ...................................................................................................... 225
22.9. words (w b) ............................................................................................................................... 226
22.10. save (or not) and exit (:w :q :q! ) ........................................................................................... 226
22.11. Searching (/ ?) ......................................................................................................................... 226
22.12. replace all ( :1,$ s/foo/bar/g ) .................................................................................................. 227
22.13. reading files (:r :r !cmd) .......................................................................................................... 227
22.14. text buffers .............................................................................................................................. 227
22.15. multiple files ........................................................................................................................... 227
22.16. abbreviations ........................................................................................................................... 228
22.17. key mappings .......................................................................................................................... 229
22.18. setting options ......................................................................................................................... 229
22.19. practice: vi(m) ......................................................................................................................... 230
22.20. solution: vi(m) ......................................................................................................................... 231

221

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).

222

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

Esc

action

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

start typing after the current character

start typing at the end of the current line

start typing before the current character

start typing at the start of the current line

start typing on a new line after the current line

start typing on a new line before the current line

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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

delete the character below the cursor

delete the character before the cursor

replace the character below the cursor

paste after the cursor (here the last deleted character)

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

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

cut the current line

(yank yank) copy the current line

paste after the current line

paste before the current line

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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

4yy

cut three lines

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

jump to start of current line

jump to end of current line

delete until start of line

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 action

yyp

ddp

join two lines

duplicate a line

switch two lines

225

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

5yb

7dw

forward one word

back one word

forward three words

delete one word

yank (copy) one word

yank five words back

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

action

save (write)

:w fname

save as fname

:wq

:q!

:w!

quit

save and quit

quit (discarding your changes)

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

action

/string

forward search for string

226

Introduction to vi

command

action

?string

backward search for string

go to next occurrence of search string

/^string

/string$

forward search string at beginning of line

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.

227

Introduction to vi

Table 22.15. multiple files

command

action

vi file1 file2 file3

start editing three files

:args

:rew

lists files and marks active file

start editing the next file

toggle with last edited file

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

action

:ab str long string

abbreviate str to be 'long string'

:una str

un-abbreviate str

228

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.

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.

231

Part VII. scripting

Table of Contents

23. scripting introduction .......................................................................................................................... 234
23.1. prerequisites ............................................................................................................................... 235
23.2. hello world ................................................................................................................................ 235
23.3. she-bang ..................................................................................................................................... 235
23.4. comment .................................................................................................................................... 236
23.5. variables ..................................................................................................................................... 236
23.6. sourcing a script ........................................................................................................................ 236
23.7. troubleshooting a script ............................................................................................................. 237
23.8. prevent setuid root spoofing ..................................................................................................... 237
23.9. practice: introduction to scripting ............................................................................................. 238
23.10. solution: introduction to scripting ........................................................................................... 239
24. scripting loops ...................................................................................................................................... 240
24.1. test [ ] ........................................................................................................................................ 241
24.2. if then else ................................................................................................................................. 242
24.3. if then elif ................................................................................................................................. 242
24.4. for loop ...................................................................................................................................... 242
24.5. while loop .................................................................................................................................. 243
24.6. until loop ................................................................................................................................... 243
24.7. practice: scripting tests and loops ............................................................................................. 244
24.8. solution: scripting tests and loops ............................................................................................ 245
25. scripting parameters ............................................................................................................................ 247
25.1. script parameters ....................................................................................................................... 248
25.2. shift through parameters ........................................................................................................... 249
25.3. runtime input ............................................................................................................................. 249
25.4. sourcing a config file ................................................................................................................ 250
25.5. get script options with getopts .................................................................................................. 251
25.6. get shell options with shopt ...................................................................................................... 252
25.7. practice: parameters and options .............................................................................................. 253
25.8. solution: parameters and options .............................................................................................. 254
26. more scripting ...................................................................................................................................... 255
26.1. eval ............................................................................................................................................ 256
26.2. (( )) ............................................................................................................................................. 256
26.3. let ............................................................................................................................................... 257
26.4. case ............................................................................................................................................ 258
26.5. shell functions ........................................................................................................................... 259
26.6. practice : more scripting ........................................................................................................... 260
26.7. solution : more scripting ........................................................................................................... 261

233

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.

234

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 ~]$ echo echo Hello World > hello_world
[paul@RHEL4a ~]$ chmod +x hello_world
[paul@RHEL4a ~]$ ./hello_world
Hello World
[paul@RHEL4a ~]$

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 --.

237

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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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

239

Chapter 24. scripting loops

240

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 ] Does the directory foo exist ?
[ -e bar ] Does the file bar exist ?
[ '/etc' = $PWD ] Is the string /etc equal to the variable $PWD ?
[ $1 != 'secret' ] Is the first parameter different from secret ?
[ 55 -lt $bar ] Is 55 less than the value of $bar ?
[ $foo -ge 1000 ] Is the value of $foo greater or equal to 1000 ?
[ "abc" < $bar ] Does abc sort before the value of $bar ?
[ -f foo ] Is foo a regular file ?
[ -r bar ] Is bar a readable file ?
[ foo -nt bar ] Is file foo newer than file bar ?
[ -o nounset ] 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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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.

242

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

243

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.

244

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 ]

245

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

246

Chapter 25. scripting parameters

247

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 \$ $$ PID of the script
echo \# $# count arguments
echo \? $? last return code
echo \* $* 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

248

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

249

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]$

250

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

251

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:~$ shopt -q cdspell ; echo $?
1
paul@laika:~$ shopt -s cdspell
paul@laika:~$ shopt -q cdspell ; echo $?
0
paul@laika:~$ cd /Etc
/etc

252

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.

253

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.

254

Chapter 26. more scripting

255

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$ (( 42 > 33 )) && echo true || echo false
true
paul@deb503:~/test42$ (( 42 > 1201 )) && echo true || echo false
false
paul@deb503:~/test42$ var42=42
paul@deb503:~/test42$ (( 42 == var42 )) && echo true || echo false
true
paul@deb503:~/test42$ (( 42 == $var42 )) && echo true || echo false
true
paul@deb503:~/test42$ var42=33
paul@deb503:~/test42$ (( 42 == var42 )) && echo true || echo false
false

256

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 ~]$ let x="3 + 4" ; echo $x
7
[paul@RHEL4b ~]$ let x="10 + 100/10" ; echo $x
20
[paul@RHEL4b ~]$ let x="10-2+100/10" ; echo $x
18
[paul@RHEL4b ~]$ let x="10*2+100/10" ; echo $x
30

The shell can also convert between different bases.

[paul@RHEL4b ~]$ let x="0xFF" ; echo $x
255
[paul@RHEL4b ~]$ let x="0xC0" ; echo $x
192
[paul@RHEL4b ~]$ let x="0xA8" ; echo $x
168
[paul@RHEL4b ~]$ let x="8#70" ; echo $x
56
[paul@RHEL4b ~]$ let x="8#77" ; echo $x
63
[paul@RHEL4b ~]$ let x="16#c0" ; echo $x
192

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$ dec=15 ; oct=017 ; hex=0x0f
kahlan@solexp11$ echo $dec $oct $hex
15 017 0x0f
kahlan@solexp11$ let dec=15 ; let oct=017 ; let hex=0x0f
kahlan@solexp11$ echo $dec $oct $hex
15 15 15

257

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 $animal in
"lion" | "tiger")
echo "You better start running fast!"
;;
"cat")
echo "Let that mouse go..."
;;
"dog")
echo "Don't worry, give it a cookie."
;;
"chicken" | "goose" | "duck" )
echo "Eggs for breakfast!"
;;
"liger")
echo "Approach and say 'Ah you big fluffy kitty...'."
;;
"babelfish")
echo "Did it fall out your ear ?"
;;
*)
echo "You discovered an unknown animal, name it!"
;;
esac
[paul@RHEL4b ~]$

258

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

259

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: 5 + 2 = 7
Product: 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.

260

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: 5 + 2 = 7
Product: 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

261

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

262

Part VIII. local user management

Table of Contents

27. introduction to users ........................................................................................................................... 266
27.1. whoami ...................................................................................................................................... 267
27.2. who ............................................................................................................................................ 267
27.3. who am i ................................................................................................................................... 267
27.4. w ................................................................................................................................................ 267
27.5. id ................................................................................................................................................ 267
27.6. su to another user ...................................................................................................................... 268
27.7. su to root ................................................................................................................................... 268
27.8. su as root ................................................................................................................................... 268
27.9. su - $username .......................................................................................................................... 268
27.10. su - ........................................................................................................................................... 268
27.11. run a program as another user ................................................................................................ 269
27.12. visudo ...................................................................................................................................... 269
27.13. sudo su - .................................................................................................................................. 270
27.14. sudo logging ............................................................................................................................ 270
27.15. practice: introduction to users ................................................................................................. 271
27.16. solution: introduction to users ................................................................................................ 272
28. user management ................................................................................................................................. 274
28.1. user management ....................................................................................................................... 275
28.2. /etc/passwd ................................................................................................................................. 275
28.3. root ............................................................................................................................................. 275
28.4. useradd ....................................................................................................................................... 276
28.5. /etc/default/useradd .................................................................................................................... 276
28.6. userdel ....................................................................................................................................... 276
28.7. usermod ..................................................................................................................................... 276
28.8. creating home directories .......................................................................................................... 277
28.9. /etc/skel/ ..................................................................................................................................... 277
28.10. deleting home directories ........................................................................................................ 277
28.11. login shell ................................................................................................................................ 278
28.12. chsh .......................................................................................................................................... 278
28.13. practice: user management ...................................................................................................... 279
28.14. solution: user management ..................................................................................................... 280
29. user passwords ..................................................................................................................................... 282
29.1. passwd ....................................................................................................................................... 283
29.2. shadow file ................................................................................................................................ 283
29.3. encryption with passwd ............................................................................................................ 284
29.4. encryption with openssl ............................................................................................................ 284
29.5. encryption with crypt ................................................................................................................ 285
29.6. /etc/login.defs ............................................................................................................................. 286
29.7. chage .......................................................................................................................................... 286
29.8. disabling a password ................................................................................................................. 287
29.9. editing local files ...................................................................................................................... 287
29.10. practice: user passwords ......................................................................................................... 288
29.11. solution: user passwords ......................................................................................................... 289
30. user profiles .......................................................................................................................................... 291
30.1. system profile ............................................................................................................................ 292
30.2. ~/.bash_profile ........................................................................................................................... 292
30.3. ~/.bash_login ............................................................................................................................. 293
30.4. ~/.profile .................................................................................................................................... 293
30.5. ~/.bashrc .................................................................................................................................... 293
30.6. ~/.bash_logout ........................................................................................................................... 294
30.7. Debian overview ....................................................................................................................... 295
30.8. RHEL5 overview ...................................................................................................................... 295
30.9. practice: user profiles ................................................................................................................ 296
30.10. solution: user profiles ............................................................................................................. 297

264

local user management

31. groups .................................................................................................................................................... 298
31.1. groupadd .................................................................................................................................... 299
31.2. group file ................................................................................................................................... 299
31.3. groups ........................................................................................................................................ 299
31.4. usermod ..................................................................................................................................... 300
31.5. groupmod ................................................................................................................................... 300
31.6. groupdel ..................................................................................................................................... 300
31.7. gpasswd ..................................................................................................................................... 301
31.8. newgrp ....................................................................................................................................... 302
31.9. vigr ............................................................................................................................................. 302
31.10. practice: groups ....................................................................................................................... 303
31.11. solution: groups ....................................................................................................................... 304

265

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.

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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

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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:~#

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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:~$

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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. This incident will be reported.
paul@rhel65:~$

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:~#

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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.

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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 2014-10-13 07:22 (10.104.33.101)
laura@debian7:~$

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, 0.00
USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT
root pts/0 10.104.33.101 07:30 6.00s 0.04s 0.00s w
root pts/1 10.104.33.101 07:46 6.00s 0.01s 0.00s sleep 42
laura@debian7:~$

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.

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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.

273

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.

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28.1. user management

User management on Linux can be done in three complementary 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 -la /etc/skel/
total 48
drwxr-xr-x 2 root root 4096 Apr 1 00:11 .
drwxr-xr-x 97 root root 12288 Jun 24 15:36 ..
-rw-r--r-- 1 root root 24 Jul 12 2006 .bash_logout
-rw-r--r-- 1 root root 176 Jul 12 2006 .bash_profile
-rw-r--r-- 1 root 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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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:~#

281

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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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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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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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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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 : Mar 27, 2014
Password expires : never
Password inactive : never
Account expires : never
Minimum number of days between password change : 0
Maximum number of days between password change : 99999
Number of days of warning before password expires : 7
root@rhel65:~#

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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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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

290

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.

291

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

293

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

294

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

~./bashrc

~/.profile

/etc/profile

/etc/bash.bashrc

yes

su -

yes

ssh

yes

gdm

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

~./bashrc

~/.bash_profile

/etc/profile

/etc/bashrc

yes

su -

yes

ssh

yes

gdm

yes

295

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, Ctrl-
alt-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 ?

296

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.

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)

297

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.

298

groups

31.1. groupadd

Groups can be created with the groupadd command. The example below shows the creation
of five (empty) groups.

root@laika:~# groupadd tennis
root@laika:~# groupadd football
root@laika:~# groupadd snooker
root@laika:~# groupadd formula1
root@laika:~# groupadd 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 ~]$

299

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:~#

300

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

301

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.

302

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.

303

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)

304

Part IX. file security

Table of Contents

32. standard file permissions .................................................................................................................... 307
32.1. file ownership ............................................................................................................................ 308
32.2. list of special files ..................................................................................................................... 310
32.3. permissions ................................................................................................................................ 311
32.4. practice: standard file permissions ........................................................................................... 316
32.5. solution: standard file permissions ........................................................................................... 317
33. advanced file permissions ................................................................................................................... 319
33.1. sticky bit on directory ............................................................................................................... 320
33.2. setgid bit on directory ............................................................................................................... 320
33.3. setgid and setuid on regular files .............................................................................................. 321
33.4. setuid on sudo ........................................................................................................................... 321
33.5. practice: sticky, setuid and setgid bits ...................................................................................... 322
33.6. solution: sticky, setuid and setgid bits ...................................................................................... 323
34. access control lists ................................................................................................................................ 325
34.1. acl in /etc/fstab .......................................................................................................................... 326
34.2. getfacl ........................................................................................................................................ 326
34.3. setfacl ......................................................................................................................................... 326
34.4. remove an acl entry .................................................................................................................. 327
34.5. remove the complete acl ........................................................................................................... 327
34.6. the acl mask .............................................................................................................................. 327
34.7. eiciel .......................................................................................................................................... 328
35. file links ................................................................................................................................................. 329
35.1. inodes ......................................................................................................................................... 330
35.2. about directories ........................................................................................................................ 331
35.3. hard links ................................................................................................................................... 332
35.4. symbolic links ........................................................................................................................... 333
35.5. removing links ........................................................................................................................... 333
35.6. practice : links ........................................................................................................................... 334
35.7. solution : links ........................................................................................................................... 335

306

Chapter 32. standard file permissions

This chapter contains details about basic file security through file ownership and file
permissions.

307

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 Apr 8 18:47 data.odt
-rw-r--r--. 1 paul paul 626K Apr 8 18:46 file1
-rw-r--r--. 1 root tennis 185 Apr 8 18:46 file2
-rw-rw-r--. 1 root root 0 Apr 8 18:47 stuff.txt
paul@rhel65:~/owners$

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 naomi
daemon mysql tom rino matthias2
bin paul wouter antonio bram
sys maarten robrecht simon fabrice
sync kevin bilal sven chimene
games yuri dimitri wouter2 messagebus
man william ahmed tarik roger
lp yves dylan jan frank
mail kris robin ian toon
news hamid matthias ivan rinus
uucp vladimir ben azeddine eddy
proxy abiy mike eric bram2
www-data david kevin2 kamel keith
backup chahid kenzo ischa jesse
list stef aaron bart frederick
irc joeri lorenzo omer hans
gnats glenn jens kurt dries
nobody yannick ruben steve steve2
libuuid christof jelle constantin tomas
Debian-exim george stefaan sam2 johan
statd joost marc bjorn tom2
sshd arno thomas ronald

308

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# ls -l FileForPaul
-rw-r--r-- 1 root paul 0 2008-08-06 14:11 FileForPaul
root@laika:/home/paul# chown paul FileForPaul
root@laika:/home/paul# ls -l FileForPaul
-rw-r--r-- 1 paul paul 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

309

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