DOKK Library

Linux Fundamentals

Authors Paul Cobbaut

License GFDL-1.3-no-invariants-or-later

Plaintext
Linux Fundamentals
    Paul Cobbaut
Linux Fundamentals
Paul Cobbaut

Publication date 2015-05-24 CEST

                                            Abstract

This book is meant to be used in an instructor-led training. For self-study, the intent is to read
this book next to a working Linux computer so you can immediately do every subject, practicing
each command.

This book is aimed at novice Linux system administrators (and might be interesting and useful
for home users that want to know a bit more about their Linux system). However, this book
is not meant as an introduction to Linux desktop applications like text editors, browsers, mail
clients, multimedia or office applications.

More information and free .pdf available at http://linux-training.be .


Feel free to contact the author:

• Paul Cobbaut: 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,

Copyright 2007-2015 Netsec BVBA, Paul Cobbaut

Permission is granted to copy, distribute and/or modify this document under the terms of the
GNU Free Documentation License, Version 1.3 or any later version published by the Free
Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts. A copy of the license is included in the section entitled 'GNU Free Documentation
License'.
Table of Contents
 I. introduction to Linux ................................................................................................................................... 1
         1. Linux history .................................................................................................................................... 3
                1.1. 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


                                                                            iv
                                                             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


                                                                           v
                                                                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


                                                                              vi
                                                              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




                                                                              ix
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




                                                                              x
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




                                                                               2
Chapter 1. Linux history
 This chapter briefly tells the history of Unix and where Linux fits in.

 If you are eager to start working with Linux without this blah, blah, blah over history,
 distributions, and licensing then jump straight to Part II - Chapter 8. Working with
 Directories page 73.




                                             3
                                       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)




                                            4
                                       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




                                            5
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.




                                             6
                                        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.




                                             7
                                            distributions



2.5. Which to choose ?
 Below are some very personal opinions on some of the most popular Linux Distributions.
 Keep in mind that any of the below Linux distributions can be a stable server and a nice
 graphical desktop client.

 Table 2.1. choosing a Linux distro
      distribution name      reason(s) for using
  Red Hat Enterprise (RHEL) You are a manager and you want a good support contract.
           CentOS            You want Red Hat without the support contract from Red Hat.
           Fedora            You want Red Hat on your laptop/desktop.
         Linux Mint          You want a personal graphical desktop to play movies, music and games.
           Debian            My personal favorite for servers, laptops, and any other device.
           Ubuntu            Very popular, based on Debian, not my favorite.
            Kali             You want a pointy-clicky hacking interface.
            others           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




                                                   8
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




                                            9
                                           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


                                              10
                                           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.


                                              11
                                          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.




                                             12
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




                                                                             14
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, ....




                                             15
                                    installing Debian 8



4.1. Debian
 Debian is one of the oldest Linux distributions. I use Debian myself on almost every
 computer that I own (including raspbian on the Raspberry Pi).

 Debian comes in releases named after characters in the movie Toy Story. The Jessie release
 contains about 36000 packages.

 Table 4.1. Debian releases
      name          number          year
     Woody            3.0           2002
      Sarge           3.1           2005
      Etch            4.0           2007
     Lenny            5.0           2009
     Squeeze          6.0           2011
     Wheezy           7             2013
      Jessie          8             2015

 There is never a fixed date for the next Debian release. The next version is released when
 it is ready.

4.2. Downloading
 All these screenshots were made in November 2014, which means Debian 8 was still in
 'testing' (but in 'freeze', so there will be no major changes when it is released).

 Download Debian here:




                                            16
                                    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




                                            17
                                   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.




                                           18
                                    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.




                                             19
                                    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.




                                            20
                                    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.




                                            21
                                    installing Debian 8


First select the language you want to use.




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




                                             22
                                  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).




                                          23
                                     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.




                                             24
                                    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.




                                            25
                                   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.




                                           26
                                   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.




                                           27
                                   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/.




                                           28
                                    installing Debian 8


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




The latest versions are being downloaded.




                                            29
                                    installing Debian 8


Say yes to install the bootloader on the virtual machine.




Booting for the first time shows the grub screen




                                            30
                                   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




                                           31
                                     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


                                             32
                                    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...




                                            33
                                      installing Debian 8



4.4. setting the hostname
 The hostname of the server is asked during installation, so there is no need to configure this
 manually.
 root@server42:~# hostname
 server42
 root@server42:~# cat /etc/hostname
 server42
 root@server42:~# dnsdomainname
 paul.local
 root@server42:~# grep server42 /etc/hosts
 127.0.1.1        server42.paul.local    server42
 root@server42:~#


4.5. adding a static ip address
 This example shows how to add a static ip address to your server.

 You can use ifconfig to set a static address that is active until the next reboot (or until the
 next ifdown).
 a

 root@server42:~# ifconfig eth0:0 10.104.33.39

 Adding a couple of lines to the /etc/network/interfaces file to enable an extra ip address
 forever.
 root@server42:~# vi /etc/network/interfaces
 root@server42:~# tail -4 /etc/network/interfaces
 auto eth0:0
 iface eth0:0 inet static
 address 10.104.33.39
 netmask 255.255.0.0
 root@server42:~# ifconfig
 eth0      Link encap:Ethernet HWaddr 08:00:27:f5:74:cf
           inet addr:192.168.1.111 Bcast:192.168.1.255 Mask:255.255.255.0
           inet6 addr: fe80::a00:27ff:fef5:74cf/64 Scope:Link
           UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
           RX packets:528 errors:0 dropped:0 overruns:0 frame:0
           TX packets:333 errors:0 dropped:0 overruns:0 carrier:0
           collisions:0 txqueuelen:1000
           RX bytes:45429 (44.3 KiB) TX bytes:48763 (47.6 KiB)

 eth0:0     Link encap:Ethernet HWaddr 08:00:27:f5:74:cf
            inet addr:10.104.33.39 Bcast:10.255.255.255 Mask:255.0.0.0
            UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1

 lo         Link encap:Local Loopback
            inet addr:127.0.0.1 Mask:255.0.0.0
            inet6 addr: ::1/128 Scope:Host
            UP LOOPBACK RUNNING MTU:65536 Metric:1
            RX packets:0 errors:0 dropped:0 overruns:0 frame:0
            TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
            collisions:0 txqueuelen:0
            RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)

 root@server42:~#




                                              34
                                    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.




                                            35
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.




                                           36
                                    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.




                                            37
                                     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.




                                             38
                                    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.




                                            39
                                  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.




                                          40
                                    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.




                                            41
                                    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.




                                            42
                                     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.




                                             43
                                    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.




                                            44
                                   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).




                                           45
                                   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.




                                           46
                                     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.




                                             47
                                     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).




                                             48
                                     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.




                                             49
                                     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).




                                             50
                                      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.




                                              51
                                      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




                                              52
                                     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 ~]#




                                             53
                                     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 ~]#




                                               54
                                    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.




                                            55
                                  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).




                                          56
                                    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).




                                            57
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.




                                             58
                                    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!




                                             59
                                   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.




                                            60
                                 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).




                                          61
                                   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.




                                             62
                                   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.




                                            63
                                    getting Linux at home


Click create to create the virtual disk.




Click create to create the virtual machine.




                                              64
                                    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.




                                             65
                                  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.




                                           66
                                   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.




                                            67
                                   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!




                                            68
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




                                                                              70
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.




                                         71
                                          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



                                             72
                                        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




                                           73
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.




                                         74
                                     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.




                                               75
                                      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.


                                                76
                                     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




                                               77
                                    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:~$




                                                78
                                    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:~$


                                              79
                              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:~$




                                          80
                                      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.




                                                 81
                                      working with directories



8.9. solution: working with directories
 1. Display your current directory.
 pwd

 2. Change to the /etc directory.
 cd /etc

 3. Now change to your home directory using only three key presses.
 cd (and the enter key)

 4. Change to the /boot/grub directory using only eleven key presses.
 cd /boot/grub (use the tab key)

 5. Go to the parent directory of the current directory.
 cd .. (with space between cd and ..)

 6. Go to the root directory.
 cd /

 7. List the contents of the root directory.
 ls

 8. List a long listing of the root directory.
 ls -l

 9. Stay where you are, and list the contents of /etc.
 ls /etc

 10. Stay where you are, and list the contents of /bin and /sbin.
 ls /bin /sbin

 11. Stay where you are, and list the contents of ~.
 ls ~

 12. List all the files (including hidden files) in your home directory.
 ls -al ~

 13. List the files in /boot in a human readable format.
 ls -lh /boot

 14. Create a directory testdir in your home directory.
 mkdir ~/testdir

 15. Change to the /etc directory, stay here and create a directory newdir in your home
 directory.


                                                 82
                                    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




                                              83
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.




                                        84
                                        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




                                               85
                                     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:~$




                                            86
                                      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:~$




                                             87
                                         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:~$




                                                88
                                       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:~$




                                              89
                                      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 ~]$




                                             90
                                           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!)




                                                  91
                                           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 ?


                                                  92
                                   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




                                          93
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.




                                              94
                                   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.


                                              95
                                      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.




                                                  96
                                   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




                                              97
                                   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
 ...




                                              98
                                     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.




                                                99
                                     working with file contents



10.8. solution: file contents
 1. Display the first 12 lines of /etc/services.
 head -12 /etc/services

 2. Display the last line of /etc/passwd.
 tail -1 /etc/passwd

 3. Use cat to create a file named count.txt that looks like this:
 cat > count.txt
 One
 Two
 Three
 Four
 Five (followed by Ctrl-d)

 4. Use cp to make a backup of this file to cnt.txt.
 cp count.txt cnt.txt

 5. Use cat to make a backup of this file to catcnt.txt.
 cat count.txt > catcnt.txt

 6. Display catcnt.txt, but with all lines in reverse order (the last line first).
 tac catcnt.txt

 7. Use more to display /etc/services.
 more /etc/services

 8. Display the readable character strings from the /usr/bin/passwd command.
 strings /usr/bin/passwd

 9. Use ls to find the biggest file in /etc.
 ls -lrS /etc

 10. Open two terminal windows (or tabs) and make sure you are in the same directory in
 both. Type echo this is the first line > tailing.txt in the first terminal, then issue tail -f
 tailing.txt in the second terminal. Now go back to the first terminal and type echo This is
 another line >> tailing.txt (note the double >>), verify that the tail -f in the second terminal
 shows both lines. Stop the tail -f with Ctrl-C.

 11. Use cat to create a file named tailing.txt that contains the contents of tailing.txt followed
 by the contents of /etc/passwd.
 cat /etc/passwd >> tailing.txt

 12. Use cat to create a file named tailing.txt that contains the contents of tailing.txt preceded
 by the contents of /etc/passwd.
 mv tailing.txt tmp.txt ; cat /etc/passwd tmp.txt > tailing.txt


                                                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.




                                            101
                                       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




                                              102
                                      the Linux file tree



11.4. binary directories
  Binaries are files that contain compiled source code (or machine code). Binaries can be
  executed on the computer. Sometimes binaries are called executables.

11.4.1. /bin
  The /bin directory contains binaries for use by all users. According to the FHS the /bin
  directory should contain /bin/cat and /bin/date (among others).

  In the screenshot below you see common Unix/Linux commands like cat, cp, cpio, date, dd,
  echo, grep, and so on. Many of these will be covered in this book.
  paul@laika:~$ ls /bin
  archdetect       egrep                 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




                                             103
                                       the Linux file tree



11.4.4. /lib
  Binaries found in /bin and /sbin often use shared libraries located in /lib. Below is a
  screenshot of the partial contents of /lib.
  paul@laika:~$ ls /lib/libc*
  /lib/libc-2.5.so     /lib/libcfont.so.0.0.0            /lib/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.




                                              104
                                       the Linux file tree



11.5. configuration directories
11.5.1. /boot
  The /boot directory contains all files needed to boot the computer. These files don't change
  very often. On Linux systems you typically find the /boot/grub directory here. /boot/grub
  contains /boot/grub/grub.cfg (older systems may still have /boot/grub/grub.conf) which
  defines the boot menu that is displayed before the kernel starts.

11.5.2. /etc
  All of the machine-specific configuration files should be located in /etc. Historically /etc
  stood for etcetera, today people often use the Editable Text Configuration backronym.

  Many times the name of a configuration files is the same as the application, daemon, or
  protocol with .conf added as the extension.
  paul@laika:~$ ls /etc/*.conf
  /etc/adduser.conf        /etc/ld.so.conf                   /etc/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.


                                              105
                                    the Linux file tree


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.




                                           106
                                        the Linux file tree



11.6. data directories
11.6.1. /home
  Users can store personal or project data under /home. It is common (but not mandatory by
  the fhs) practice to name the users home directory after the user name in the format /home/
  $USERNAME. For example:
  paul@ubu606:~$ ls /home
  geert annik sandra paul         tom

  Besides giving every user (or every project or group) a location to store personal files, the
  home directory of a user also serves as a location to store the user profile. A typical Unix
  user profile contains many hidden files (files whose file name starts with a dot). The hidden
  files of the Unix user profiles contain settings specific for that user.

  paul@ubu606:~$ ls -d /home/paul/.*
  /home/paul/.              /home/paul/.bash_profile          /home/paul/.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).


                                               107
                                    the Linux file tree


 Unix and Linux administrators used to create many directories here, like /mnt/something/.
 You likely will encounter many systems with more than one directory created and/or
 mounted inside /mnt to be used for various local and remote filesystems.

11.6.6. /tmp
 Applications and users should use /tmp to store temporary data when needed. Data stored
 in /tmp may use either disk space or RAM. Both of which are managed by the operating
 system. Never use /tmp to store data that is important or which you wish to archive.




                                           108
                                        the Linux file tree



11.7. in memory directories
11.7.1. /dev
  Device files in /dev appear to be ordinary files, but are not actually located on the hard disk.
  The /dev directory is populated with files as the kernel is recognising hardware.

common physical devices
  Common hardware such as hard disk devices are represented by device files in /dev. Below
  a screenshot of SATA device files on a laptop and then IDE attached drives on a desktop.
  (The detailed meaning of these devices will be discussed later.)

  #
  # SATA or SCSI or USB
  #
  paul@laika:~$ ls /dev/sd*
  /dev/sda /dev/sda1 /dev/sda2          /dev/sda3      /dev/sdb   /dev/sdb1   /dev/sdb2

  #
  # IDE or ATAPI
  #
  paul@barry:~$ ls /dev/hd*
  /dev/hda /dev/hda1 /dev/hda2          /dev/hdb     /dev/hdb1    /dev/hdb2   /dev/hdc



  Besides representing physical hardware, some device files are special. These special devices
  can be very useful.

/dev/tty and /dev/pts
  For example, /dev/tty1 represents a terminal or console attached to the system. (Don't
  break your head on the exact terminology of 'terminal' or 'console', what we mean here is
  a command line interface.) When typing commands in a terminal that is part of a graphical
  interface like Gnome or KDE, then your terminal will be represented as /dev/pts/1 (1 can
  be another number).

/dev/null
  On Linux you will find other special devices such as /dev/null which can be considered
  a black hole; it has unlimited storage, but nothing can be retrieved from it. Technically
  speaking, anything written to /dev/null will be discarded. /dev/null can be useful to discard
  unwanted output from commands. /dev/null is not a good location to store your backups ;-).

11.7.2. /proc conversation with the kernel
  /proc is another special directory, appearing to be ordinary files, but not taking up disk
  space. It is actually a view of the kernel, or better, what the kernel manages, and is a means
  to interact with it directly. /proc is a proc filesystem.

  paul@RHELv4u4:~$ mount -t proc


                                               109
                                     the Linux file tree


none on /proc type proc (rw)


When listing the /proc directory you will see many numbers (on any Unix) and some
interesting files (on Linux)

mul@laika:~$ ls /proc
1      2339   4724 5418      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


                                            110
                                      the Linux file tree


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.




                                             111
                                       the Linux file tree


/proc/interrupts
  On the x86 architecture, /proc/interrupts displays the interrupts.

  paul@RHELv4u4:~$ cat /proc/interrupts
             CPU0
    0:   13876877    IO-APIC-edge timer
    1:         15    IO-APIC-edge i8042
    8:          1    IO-APIC-edge rtc
    9:          0   IO-APIC-level acpi
   12:         67    IO-APIC-edge i8042
   14:        128    IO-APIC-edge ide0
   15:     124320    IO-APIC-edge ide1
  169:     111993   IO-APIC-level ioc0
  177:       2428   IO-APIC-level eth0
  NMI:          0
  LOC:   13878037
  ERR:          0
  MIS:          0

  On a machine with two CPU's, the file looks like this.
  paul@laika:~$ cat /proc/interrupts
            CPU0      CPU1
    0:    860013        0 IO-APIC-edge              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:~$




                                              112
                                     the Linux file tree



11.7.3. /sys Linux 2.6 hot plugging
 The /sys directory was created for the Linux 2.6 kernel. Since 2.6, Linux uses sysfs
 to support usb and IEEE 1394 (FireWire) hot plug devices. See the manual pages
 of udev(8) (the successor of devfs) and hotplug(8) for more info (or visit http://linux-
 hotplug.sourceforge.net/ ).

 Basically the /sys directory contains kernel information about hardware.




                                            113
                                        the Linux file tree



11.8. /usr Unix System Resources
  Although /usr is pronounced like user, remember that it stands for Unix System Resources.
  The /usr hierarchy should contain shareable, read only data. Some people choose to mount
  /usr as read only. This can be done from its own partition or from a read only NFS share
  (NFS is discussed later).

11.8.1. /usr/bin
  The /usr/bin directory contains a lot of commands.
  paul@deb508:~$ ls /usr/bin | wc -l
  1395

  (On Solaris the /bin directory is a symbolic link to /usr/bin.)

11.8.2. /usr/include
  The /usr/include directory contains general use include files for C.
  paul@ubu1010:~$ ls /usr/include/
  aalib.h        expat_config.h            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


                                               114
                                     the Linux file tree


 263
 paul@deb508:~$ du -sh /usr/share/
 1.3G /usr/share/

 This directory typically contains /usr/share/man for manual pages.
 paul@deb508:~$ ls /usr/share/man
 cs fr     hu it.UTF-8 man2 man6 pl.ISO8859-2 sv
 de fr.ISO8859-1 id ja       man3 man7 pl.UTF-8     tr
 es fr.UTF-8    it ko    man4 man8 pt_BR      zh_CN
 fi gl     it.ISO8859-1 man1    man5 pl   ru     zh_TW

 And it contains /usr/share/games for all static game data (so no high-scores or play logs).
 paul@ubu1010:~$ ls /usr/share/games/
 openttd wesnoth


11.8.6. /usr/src
 The /usr/src directory is the recommended location for kernel source files.
 paul@deb508:~$ ls -l /usr/src/
 total 12
 drwxr-xr-x 4 root root 4096 2011-02-01 14:43 linux-headers-2.6.26-2-686
 drwxr-xr-x 18 root root 4096 2011-02-01 14:43 linux-headers-2.6.26-2-common
 drwxr-xr-x 3 root root 4096 2009-10-28 16:01 linux-kbuild-2.6.26




                                            115
                                        the Linux file tree



11.9. /var variable data
  Files that are unpredictable in size, such as log, cache and spool files, should be located in
  /var.

11.9.1. /var/log
  The /var/log directory serves as a central point to contain all log files.
  [paul@RHEL4b ~]$ ls /var/log
  acpid           cron.2    maillog.2           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).


                                               116
                                       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.


                                             120
                                      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 sys

11. Are ifconfig, fdisk, parted, shutdown and grub-install present in /sbin ? Why are these
binaries in /sbin and not in /bin ?
Because those files are only meant for system administrators.

12. Is /var/log a file or a directory ? What about /var/spool ?
Both are directories.

13. Open two command prompts (Ctrl-Shift-T in gnome-terminal) or terminals (Ctrl-Alt-F1,
Ctrl-Alt-F2, ...) and issue the who am i in both. Then try to echo a word from one terminal
to the other.
tty-terminal: echo Hello > /dev/tty1

pts-terminal: echo Hello > /dev/pts/1

14. Read the man page of random and explain the difference between /dev/random and /
dev/urandom.
man 4 random




                                             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.




                                              126
                                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.




                                         127
                                   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




                                             128
                                   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 \\ \* \[ \} \~ \\\\ \. \"

 or

 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

 9. Open more than one console (by press Ctrl-shift-t in gnome-terminal, or by opening an
 extra putty.exe in MS Windows) with the same user account. When is command history
 written to the history file ?
 when you type exit




                                            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

16. You receive information that one of your servers was cracked, the cracker probably
replaced the ls command. You know that the echo command is safe to use. Can echo replace
ls ? How can you list the files in the current directory with echo ?
echo *

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




                                               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 dout (1)
             st din (0)                    bash
                                                                   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




                                              186
                                          filters



19.8. uniq
 With uniq you can remove duplicates from a sorted list.
 paul@debian5:~/pipes$ cat music.txt
 Queen
 Brel
 Queen
 Abba
 paul@debian5:~/pipes$ sort music.txt
 Abba
 Brel
 Queen
 Queen
 paul@debian5:~/pipes$ sort music.txt |uniq
 Abba
 Brel
 Queen

 uniq can also count occurrences with the -c option.
 paul@debian5:~/pipes$ sort music.txt |uniq -c
       1 Abba
       1 Brel
       2 Queen




                                           187
                                         filters



19.9. comm
 Comparing streams (or files) can be done with the comm. By default comm will output
 three columns. In this example, Abba, Cure and Queen are in both lists, Bowie and Sweet
 are only in the first file, Turner is only in the second.
 paul@debian5:~/pipes$ cat > list1.txt
 Abba
 Bowie
 Cure
 Queen
 Sweet
 paul@debian5:~/pipes$ cat > list2.txt
 Abba
 Cure
 Queen
 Turner
 paul@debian5:~/pipes$ comm list1.txt list2.txt
                 Abba
 Bowie
                 Cure
                 Queen
 Sweet
         Turner

 The output of comm can be easier to read when outputting only a single column. The digits
 point out which output columns should not be displayed.

 paul@debian5:~/pipes$ comm -12 list1.txt list2.txt
 Abba
 Cure
 Queen
 paul@debian5:~/pipes$ comm -13 list1.txt list2.txt
 Turner
 paul@debian5:~/pipes$ comm -23 list1.txt list2.txt
 Bowie
 Sweet




                                          188
                                            filters



19.10. od
 European humans like to work with ascii characters, but computers store files in bytes. The
 example below creates a simple file, and then uses od to show the contents of the file in
 hexadecimal bytes

 paul@laika:~/test$ cat > text.txt
 abcdefg
 1234567
 paul@laika:~/test$ od -t x1 text.txt
 0000000 61 62 63 64 65 66 67 0a 31 32 33 34 35 36 37 0a
 0000020

 The same file can also be displayed in octal bytes.

 paul@laika:~/test$ od -b text.txt
 0000000 141 142 143 144 145 146 147 012 061 062 063 064 065 066 067 012
 0000020

 And here is the file in ascii (or backslashed) characters.

 paul@laika:~/test$ od -c text.txt
 0000000   a   b   c   d   e   f   g        \n        1   2   3   4   5   6   7   \n
 0000020




                                             189
                                           filters



19.11. sed
 The stream editor sed can perform editing functions in the stream, using regular
 expressions.

 paul@debian5:~/pipes$ echo level5 | sed 's/5/42/'
 level42
 paul@debian5:~/pipes$ echo level5 | sed 's/level/jump/'
 jump5



 Add g for global replacements (all occurrences of the string per line).

 paul@debian5:~/pipes$ echo level5 level7 | sed 's/level/jump/'
 jump5 level7
 paul@debian5:~/pipes$ echo level5 level7 | sed 's/level/jump/g'
 jump5 jump7



 With d you can remove lines from a stream containing a character.

 paul@debian5:~/test42$ cat tennis.txt
 Venus Williams, USA
 Martina Hingis, SUI
 Justine Henin, BE
 Serena williams, USA
 Kim Clijsters, BE
 Yanina Wickmayer, BE
 paul@debian5:~/test42$ cat tennis.txt | sed '/BE/d'
 Venus Williams, USA
 Martina Hingis, SUI
 Serena williams, USA




                                            190
                                             filters



19.12. pipe examples
19.12.1. who | wc
 How many users are logged on to this system ?
 [paul@RHEL4b pipes]$ who
 root     tty1         Jul 25 10:50
 paul     pts/0        Jul 25 09:29 (laika)
 Harry    pts/1        Jul 25 12:26 (barry)
 paul     pts/2        Jul 25 12:26 (pasha)
 [paul@RHEL4b pipes]$ who | wc -l
 4


19.12.2. who | cut | sort
 Display a sorted list of logged on users.
 [paul@RHEL4b pipes]$ who | cut -d' ' -f1 | sort
 Harry
 paul
 paul
 root

 Display a sorted list of logged on users, but every user only once .
 [paul@RHEL4b pipes]$ who | cut -d' ' -f1 | sort | uniq
 Harry
 paul
 root


19.12.3. grep | cut
 Display a list of all bash user accounts on this computer. Users accounts are explained in
 detail later.
 paul@debian5:~$ grep bash /etc/passwd
 root:x:0:0:root:/root:/bin/bash
 paul:x:1000:1000:paul,,,:/home/paul:/bin/bash
 serena:x:1001:1001::/home/serena:/bin/bash
 paul@debian5:~$ grep bash /etc/passwd | cut -d: -f1
 root
 paul
 serena




                                              191
                                             filters



19.13. practice: filters
 1. Put a sorted list of all bash users in bashusers.txt.

 2. Put a sorted list of all logged on users in onlineusers.txt.

 3. Make a list of all filenames in /etc that contain the string conf in their filename.

 4. Make a sorted list of all files in /etc that contain the case insensitive string conf in their
 filename.

 5. Look at the output of /sbin/ifconfig. Write a line that displays only ip address and the
 subnet mask.

 6. Write a line that removes all non-letters from a stream.

 7. Write a line that receives a text file, and outputs all words on a separate line.

 8. Write a spell checker on the command line. (There may be a dictionary in /usr/share/
 dict/ .)




                                              192
                                             filters



19.14. solution: filters
 1. Put a sorted list of all bash users in bashusers.txt.
 grep bash /etc/passwd | cut -d: -f1 | sort > bashusers.txt

 2. Put a sorted list of all logged on users in onlineusers.txt.
 who | cut -d' ' -f1 | sort > onlineusers.txt

 3. Make a list of all filenames in /etc that contain the string conf in their filename.
 ls /etc | grep conf

 4. Make a sorted list of all files in /etc that contain the case insensitive string conf in their
 filename.
 ls /etc | grep -i conf | sort

 5. Look at the output of /sbin/ifconfig. Write a line that displays only ip address and the
 subnet mask.
 /sbin/ifconfig | head -2 | grep 'inet ' | tr -s ' ' | cut -d' ' -f3,5

 6. Write a line that removes all non-letters from a stream.
 paul@deb503:~$ cat text
 This is, yes really! , a text with ?&* too many str$ange# characters ;-)
 paul@deb503:~$ cat text | tr -d ',!$?.*&^%#@;()-'
 This is yes really a text with too many strange characters



 7. Write a line that receives a text file, and outputs all words on a separate line.

 paul@deb503:~$ cat text2
 it is very cold today without the sun

 paul@deb503:~$ cat text2 | tr ' ' '\n'
 it
 is
 very
 cold
 today
 without
 the
 sun



 8. Write a spell checker on the command line. (There may be a dictionary in /usr/share/
 dict/ .)

 paul@rhel ~$ echo "The zun is shining today" > text

 paul@rhel ~$ cat > DICT
 is
 shining
 sun
 the


                                              193
                                        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 {} \;




                                                196
                                        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




                                              197
                                      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 ~$




                                            198
                                     basic Unix tools



20.6. time
 The time command can display how long it takes to execute a command. The date command
 takes only a little time.
 paul@rhel55 ~$ time date
 Sat Apr 17 13:08:27 CEST 2010

 real    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




                                           199
                                     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




                                           200
                                   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




                                         201
                                        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/


                                              203
                                      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!




                                             206
                                        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:~$




                                               207
                                     regular expressions



21.2.3. one or the other
 PRCE and ERE both use the pipe symbol to signify OR. In this example we grep for lines
 containing the letter i or the letter a.
 paul@debian7:~$ cat list
 Tania
 Laura
 paul@debian7:~$ grep -E 'i|a' list
 Tania
 Laura

 Note that we use the -E switch of grep to force interpretion of our string as an ERE.

 We need to escape the pipe symbol in a BRE to get the same logical OR.
 paul@debian7:~$ grep -G 'i|a' list
 paul@debian7:~$ grep -G 'i\|a' list
 Tania
 Laura


21.2.4. one or more
 The * signifies zero, one or more occurences of the previous and the + signifies one or more
 of the previous.
 paul@debian7:~$ cat list2
 ll
 lol
 lool
 loool
 paul@debian7:~$ grep -E 'o*' list2
 ll
 lol
 lool
 loool
 paul@debian7:~$ grep -E 'o+' list2
 lol
 lool
 loool
 paul@debian7:~$




                                            208
                                      regular expressions



21.2.5. match the end of a string
 For the following examples, we will use this file.
 paul@debian7:~$ cat names
 Tania
 Laura
 Valentina
 Fleur
 Floor

 The two examples below show how to use the dollar character to match the end of a string.
 paul@debian7:~$ grep a$ names
 Tania
 Laura
 Valentina
 paul@debian7:~$ grep r$ names
 Fleur
 Floor


21.2.6. match the start of a string
 The caret character (^) will match a string at the start (or the beginning) of a line.

 Given the same file as above, here are two examples.
 paul@debian7:~$ grep ^Val names
 Valentina
 paul@debian7:~$ grep ^F names
 Fleur
 Floor

 Both the dollar sign and the little hat are called anchors in a regex.




                                             209
                                    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:~$




                                           210
                                      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




                                             211
                                    regular expressions



21.3. rename
21.3.1. the rename command
  On Debian Linux the /usr/bin/rename command is a link to /usr/bin/prename installed by
  the perl package.
  paul@pi ~ $ dpkg -S $(readlink -f $(which rename))
  perl: /usr/bin/prename

  Red Hat derived systems do not install the same rename command, so this section does not
  describe rename on Red Hat (unless you copy the perl script manually).

  There is often confusion on the internet about the rename command because solutions
  that work fine in Debian (and Ubuntu, xubuntu, Mint, ...) cannot be used in Red Hat
  (and CentOS, Fedora, ...).

21.3.2. perl
  The rename command is actually a perl script that uses perl regular expressions. The
  complete manual for these can be found by typing perldoc perlrequick (after installing
  perldoc).
  root@pi:~# aptitude install perl-doc
  The following NEW packages will be installed:
    perl-doc
  0 packages upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
  Need to get 8,170 kB of archives. After unpacking 13.2 MB will be used.
  Get: 1 http://mirrordirector.raspbian.org/raspbian/ wheezy/main perl-do...
  Fetched 8,170 kB in 19s (412 kB/s)
  Selecting previously unselected package perl-doc.
  (Reading database ... 67121 files and directories currently installed.)
  Unpacking perl-doc (from .../perl-doc_5.14.2-21+rpi2_all.deb) ...
  Adding 'diversion of /usr/bin/perldoc to /usr/bin/perldoc.stub by perl-doc'
  Processing triggers for man-db ...
  Setting up perl-doc (5.14.2-21+rpi2) ...

  root@pi:~# perldoc perlrequick




                                           212
                                      regular expressions



21.3.3. well known syntax
 The most common use of the rename is to search for filenames matching a certain string
 and replacing this string with an other string.

 This is often presented as s/string/other string/ as seen in this example:
 paul@pi ~   $ 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.




                                             214
                                      regular expressions



21.4. sed
21.4.1. stream editor
 The stream editor or short sed uses regex for stream editing.

 In this example sed is used to replace a string.
 echo Sunday | sed 's/Sun/Mon/'
 Monday

 The slashes can be replaced by a couple of other characters, which can be handy in some
 cases to improve readability.
 echo Sunday | sed 's:Sun:Mon:'
 Monday
 echo Sunday | sed 's_Sun_Mon_'
 Monday
 echo Sunday | sed 's|Sun|Mon|'
 Monday


21.4.2. interactive editor
 While sed is meant to be used in a stream, it can also be used interactively on a file.
 paul@debian7:~/files$    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      action
     Esc      set vi(m) in command mode.


22.2. start typing (a A i I o O)
 The difference between a A i I o and O is the location where you can start typing. a will
 append after the current character and A will append at the end of the line. i will insert before
 the current character and I will insert at the beginning of the line. o will put you in a new
 line after the current line and O will put you in a new line before the current line.

 Table 22.2. switch to insert mode
  command     action
      a       start typing after the current character
      A       start typing at the end of the current line
       i      start typing before the current character
       I      start typing at the start of the current line
      o       start typing on a new line after the current line
      O       start typing on a new line before the current line




                                                     223
                                                Introduction to vi



22.3. replace and delete a character (r x X)
 When in command mode (it doesn't hurt to hit the escape key more than once) you can use
 the x key to delete the current character. The big X key (or shift x) will delete the character
 left of the cursor. Also when in command mode, you can use the r key to replace one single
 character. The r key will bring you in insert mode for just one key press, and will return you
 immediately to command mode.

 Table 22.3. replace and delete
    command          action
           x         delete the character below the cursor
           X         delete the character before the cursor
           r         replace the character below the cursor
           p         paste after the cursor (here the last deleted character)
           xp        switch two characters


22.4. undo and repeat (u .)
 When in command mode, you can undo your mistakes with u. You can do your mistakes
 twice with . (in other words, the . will repeat your last command).

 Table 22.4. undo and repeat
  command       action
      u         undo the last action
       .        repeat the last action


22.5. cut, copy and paste a line (dd yy p P)
 When in command mode, dd will cut the current line. yy will copy the current line. You can
 paste the last copied or cut line after (p) or before (P) the current line.

 Table 22.5. cut, copy and paste a line
  command       action
      dd        cut the current line
      yy        (yank yank) copy the current line
      p         paste after the current line
      P         paste before the current line




                                                       224
                                              Introduction to vi



22.6. cut, copy and paste lines (3dd 2yy)
 When in command mode, before typing dd or yy, you can type a number to repeat the
 command a number of times. Thus, 5dd will cut 5 lines and 4yy will copy (yank) 4 lines.
 That last one will be noted by vi in the bottom left corner as "4 line yanked".

 Table 22.6. cut, copy and paste lines
  command     action
     3dd      cut three lines
     4yy      copy four lines


22.7. start and end of a line (0 or ^ and $)
 When in command mode, the 0 and the caret ^ will bring you to the start of the current line,
 whereas the $ will put the cursor at the end of the current line. You can add 0 and $ to the d
 command, d0 will delete every character between the current character and the start of the
 line. Likewise d$ will delete everything from the current character till the end of the line.
 Similarly y0 and y$ will yank till start and end of the current line.

 Table 22.7. start and end of line
  command     action
      0       jump to start of current line
      ^       jump to start of current line
      $       jump to end of current line
      d0      delete until start of line
      d$      delete until end of line


22.8. join two lines (J) and more
 When in command mode, pressing J will append the next line to the current line. With yyp
 you duplicate a line and with ddp you switch two lines.

 Table 22.8. join two lines
  command     action
      J       join two lines
     yyp      duplicate a line
     ddp      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
      w         forward one word
      b         back one word
     3w         forward three words
     dw         delete one word
     yw         yank (copy) one word
     5yb        yank five words back
     7dw        delete seven words


22.10. save (or not) and exit (:w :q :q! )
 Pressing the colon : will allow you to give instructions to vi (technically speaking, typing
 the colon will open the ex editor). :w will write (save) the file, :q will quit an unchanged
 file without saving, and :q! will quit vi discarding any changes. :wq will save and quit and
 is the same as typing ZZ in command mode.

 Table 22.10. save and exit vi
   command        action
       :w         save (write)
    :w fname      save as fname
       :q         quit
      :wq         save and quit
       ZZ         save and quit
       :q!        quit (discarding your changes)
       :w!        save (and write to non-writable file!)

 The last one is a bit special. With :w! vi will try to chmod the file to get write permission
 (this works when you are the owner) and will chmod it back when the write succeeds. This
 should always work when you are root (and the file system is writable).

22.11. Searching (/ ?)
 When in command mode typing / will allow you to search in vi for strings (can be a regular
 expression). Typing /foo will do a forward search for the string foo and typing ?bar will do
 a backward search for bar.

 Table 22.11. searching
    command         action
      /string       forward search for string


                                                   226
                                                Introduction to vi


    command            action
      ?string          backward search for string
           n           go to next occurrence of search string
      /^string         forward search string at beginning of line
      /string$         forward search string at end of line
     /br[aeio]l        search for bral brel bril and brol
      /\<he\>          search for the word he (and not for here or the)


22.12. replace all ( :1,$ s/foo/bar/g )
 To replace all occurrences of the string foo with bar, first switch to ex mode with : . Then
 tell vi which lines to use, for example 1,$ will do the replace all from the first to the last
 line. You can write 1,5 to only process the first five lines. The s/foo/bar/g will replace all
 occurrences of foo with bar.

 Table 22.12. replace
        command                 action
      :4,8 s/foo/bar/g          replace foo with bar on lines 4 to 8
      :1,$ s/foo/bar/g          replace foo with bar on all lines


22.13. reading files (:r :r !cmd)
 When in command mode, :r foo will read the file named foo, :r !foo will execute the
 command foo. The result will be put at the current location. Thus :r !ls will put a listing of
 the current directory in your text file.

 Table 22.13. read files and input
  command         action
   :r fname       (read) file fname and paste contents
    :r !cmd       execute cmd and paste its output


22.14. text buffers
 There are 36 buffers in vi to store text. You can use them with the " character.

 Table 22.14. text buffers
  command         action
     "add         delete current line and put text in buffer a
    "g7yy         copy seven lines into buffer g
     "ap          paste from buffer a


22.15. multiple files
 You can edit multiple files with vi. Here are some tips.


                                                       227
                                           Introduction to vi


 Table 22.15. multiple files
        command             action
     vi file1 file2 file3   start editing three files
            :args           lists files and marks active file
             :n             start editing the next file
             :e             toggle with last edited file
            :rew            rewind file pointer to first file


22.16. abbreviations
 With :ab you can put abbreviations in vi. Use :una to undo the abbreviation.

 Table 22.16. abbreviations
          command             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:~$




                                             229
                                      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 ?




                                             230
                                      Introduction to vi



22.20. solution: vi(m)
 1. Start the vimtutor and do some or all of the exercises. You might need to run aptitude
 install vim on xubuntu.
 vimtutor

 2. What 3 key sequence in command mode will duplicate the current line.
 yyp

 3. What 3 key sequence in command mode will switch two lines' place (line five becomes
 line six and line six becomes line five).
 ddp

 4. What 2 key sequence in command mode will switch a character's place with the next one.
 xp

 5. vi can understand macro's. A macro can be recorded with q followed by the name of
 the macro. So qa will record the macro named a. Pressing q again will end the recording.
 You can recall the macro with @ followed by the name of the macro. Try this example: i 1
 'Escape Key' qa yyp 'Ctrl a' q 5@a (Ctrl a will increase the number with one).

 6. Copy /etc/passwd to your ~/passwd. Open the last one in vi and press Ctrl v. Use the arrow
 keys to select a Visual Block, you can copy this with y or delete it with d. Try pasting it.
 cp /etc/passwd ~
 vi passwd
 (press Ctrl-V)

 7. What does dwwP do when you are at the beginning of a word in a sentence ?

 dwwP can switch the current word with the next word.




                                             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`




                                               235
                                     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 $var1
 4
 [paul@RHEL4a ~]$




                                              236
                                     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.




                                              238
                                     scripting introduction



23.10. solution: introduction to scripting
 0. Give each script a different name, keep them for later!

 1. Write a script that outputs the name of a city.
 $ echo 'echo Antwerp' > first.bash
 $ chmod +x first.bash
 $ ./first.bash
 Antwerp

 2. Make sure the script runs in the bash shell.
 $ cat first.bash
 #!/bin/bash
 echo Antwerp

 3. Make sure the script runs in the Korn shell.
 $ cat first.bash
 #!/bin/ksh
 echo Antwerp

 Note that while first.bash will technically work as a Korn shell script, the name ending
 in .bash is confusing.

 4. Create a script that defines two variables, and outputs their value.
 $ cat second.bash
 #!/bin/bash

 var33=300
 var42=400

 echo $var33 $var42

 5. The previous script does not influence your current shell (the variables do not exist outside
 of the script). Now run the script so that it influences your current shell.
 source second.bash

 6. Is there a shorter way to source the script ?
 . ./second.bash

 7. Comment your scripts so that you know what they are doing.
 $ cat second.bash
 #!/bin/bash
 # script to test variables and sourcing

 # define two variables
 var33=300
 var42=400

 # output the value of these variables
 echo $var33 $var42




                                              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




                                               241
                                        scripting loops



24.2. if then else
 The if then else construction is about choice. If a certain condition is met, then execute
 something, else execute something else. The example below tests whether a file exists, and
 if the file exists then a proper message is echoed.
 #!/bin/bash

 if [ -f isit.txt ]
 then echo isit.txt exists!
 else echo isit.txt not found!
 fi

 If we name the above script 'choice', then it executes like this.
 [paul@RHEL4a scripts]$ ./choice
 isit.txt not found!
 [paul@RHEL4a scripts]$ touch isit.txt
 [paul@RHEL4a scripts]$ ./choice
 isit.txt exists!
 [paul@RHEL4a scripts]$


24.3. if then elif
 You can nest a new if inside an else with elif. This is a simple example.
 #!/bin/bash
 count=42
 if [ $count -eq 42 ]
 then
   echo "42 is correct."
 elif [ $count -gt 42 ]
 then
   echo "Too much."
 else
   echo "Not enough."
 fi


24.4. for loop
 The example below shows the syntax of a classical for loop in bash.
 for i in 1 2 4
 do
    echo $i
 done

 An example of a for loop combined with an embedded shell.
 #!/bin/ksh
 for counter in `seq 1 20`
 do
    echo counting from 1 to 20, now at $counter
    sleep 1
 done

 The same example as above can be written without the embedded shell using the bash
 {from..to} shorthand.


                                             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

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.




                                             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.




                                            266
                                   introduction to users



27.1. whoami
 The whoami command tells you your username.
 [paul@centos7 ~]$ whoami
 paul
 [paul@centos7 ~]$


27.2. who
 The who command will give you information about who is logged on the system.
 [paul@centos7 ~]$ who
 root     pts/0        2014-10-10 23:07 (10.104.33.101)
 paul     pts/1        2014-10-10 23:30 (10.104.33.101)
 laura    pts/2        2014-10-10 23:34 (10.104.33.96)
 tania    pts/3        2014-10-10 23:39 (10.104.33.91)
 [paul@centos7 ~]$


27.3. who am i
 With who am i the who command will display only the line pointing to your current session.
 [paul@centos7 ~]$ who am i
 paul     pts/1        2014-10-10 23:30 (10.104.33.101)
 [paul@centos7 ~]$


27.4. w
 The w command shows you who is logged on and what they are doing.
 [paul@centos7 ~]$ w
  23:34:07 up 31 min, 2 users, load average: 0.00, 0.01, 0.02
 USER     TTY         LOGIN@   IDLE  JCPU  PCPU WHAT
 root     pts/0      23:07   15.00s 0.01s 0.01s top
 paul     pts/1      23:30    7.00s 0.00s 0.00s w
 [paul@centos7 ~]$


27.5. id
 The id command will give you your user id, primary group id, and a list of the groups that
 you belong to.
 paul@debian7:~$ id
 uid=1000(paul) gid=1000(paul) groups=1000(paul)

 On RHEL/CentOS you will also get SELinux context information with this command.
 [root@centos7 ~]# id
 uid=0(root) gid=0(root) groups=0(root) context=unconfined_u:unconfined_r\
 :unconfined_t:s0-s0:c0.c1023




                                           267
                                    introduction to users



27.6. su to another user
 The su command allows a user to run a shell as another user.
 laura@debian7:~$ su tania
 Password:
 tania@debian7:/home/laura$


27.7. su to root
 Yes you can also su to become root, when you know the root password.
 laura@debian7:~$ su root
 Password:
 root@debian7:/home/laura#


27.8. su as root
 You need to know the password of the user you want to substitute to, unless your are logged
 in as root. The root user can become any existing user without knowing that user's password.
 root@debian7:~# id
 uid=0(root) gid=0(root) groups=0(root)
 root@debian7:~# su - valentina
 valentina@debian7:~$


27.9. su - $username
 By default, the su command maintains the same shell environment. To become another user
 and also get the target user's environment, issue the su - command followed by the target
 username.
 root@debian7:~# su laura
 laura@debian7:/root$ exit
 exit
 root@debian7:~# su - laura
 laura@debian7:~$ pwd
 /home/laura


27.10. su -
 When no username is provided to su or su -, the command will assume root is the target.
 tania@debian7:~$ su -
 Password:
 root@debian7:~#




                                            268
                                     introduction to users



27.11. run a program as another user
 The sudo program allows a user to start a program with the credentials of another user.
 Before this works, the system administrator has to set up the /etc/sudoers file. This can be
 useful to delegate administrative tasks to another user (without giving the root password).

 The screenshot below shows the usage of sudo. User paul received the right to run useradd
 with the credentials of root. This allows paul to create new users on the system without
 becoming root and without knowing the root password.

 First the command fails for paul.
 paul@debian7:~$ /usr/sbin/useradd -m valentina
 useradd: Permission denied.
 useradd: cannot lock /etc/passwd; try again later.

 But with sudo it works.
 paul@debian7:~$ sudo /usr/sbin/useradd -m valentina
 [sudo] password for paul:
 paul@debian7:~$


27.12. visudo
 Check the man page of visudo before playing with the /etc/sudoers file. Editing the sudoers
 is out of scope for this fundamentals book.
 paul@rhel65:~$ apropos visudo
 visudo               (8) - edit the sudoers file
 paul@rhel65:~$




                                             269
                                     introduction to users



27.13. sudo su -
 On some Linux systems like Ubuntu and Xubuntu, the root user does not have a password
 set. This means that it is not possible to login as root (extra security). To perform tasks as
 root, the first user is given all sudo rights via the /etc/sudoers. In fact all users that are
 members of the admin group can use sudo to run all commands as root.
 root@laika:~# grep admin /etc/sudoers
 # Members of the admin group may gain root privileges
 %admin ALL=(ALL) ALL

 The end result of this is that the user can type sudo su - and become root without having to
 enter the root password. The sudo command does require you to enter your own password.
 Thus the password prompt in the screenshot below is for sudo, not for su.
 paul@laika:~$ sudo su -
 Password:
 root@laika:~#


27.14. sudo logging
 Using sudo without authorization will result in a severe warning:
 paul@rhel65:~$ sudo su -

 We trust you have received the usual lecture from the local System
 Administrator. It usually boils down to these three things:

     #1) Respect the privacy of others.
     #2) Think before you type.
     #3) With great power comes great responsibility.

 [sudo] password for paul:
 paul is not in the sudoers file.       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:~#




                                             270
                                      introduction to users



27.15. practice: introduction to users
 1. Run a command that displays only your currently logged on user name.

 2. Display a list of all logged on users.

 3. Display a list of all logged on users including the command they are running at this very
 moment.

 4. Display your user name and your unique user identification (userid).

 5. Use su to switch to another user account (unless you are root, you will need the password
 of the other account). And get back to the previous account.

 6. Now use su - to switch to another user and notice the difference.

 Note that su - gets you into the home directory of Tania.

 7. Try to create a new user account (when using your normal user account). this should fail.
 (Details on adding user accounts are explained in the next chapter.)

 8. Now try the same, but with sudo before your command.




                                              271
                                      introduction to users



27.16. solution: introduction to users
 1. Run a command that displays only your currently logged on user name.
 laura@debian7:~$ whoami
 laura
 laura@debian7:~$ echo $USER
 laura

 2. Display a list of all logged on users.
 laura@debian7:~$ who
 laura     pts/0            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.




                                              272
                                   introduction to users


7. Try to create a new user account (when using your normal user account). this should fail.
(Details on adding user accounts are explained in the next chapter.)
laura@debian7:~$ useradd valentina
-su: useradd: command not found
laura@debian7:~$ /usr/sbin/useradd valentina
useradd: Permission denied.
useradd: cannot lock /etc/passwd; try again later.

It is possible that useradd is located in /sbin/useradd on your computer.

8. Now try the same, but with sudo before your command.
laura@debian7:~$ sudo /usr/sbin/useradd valentina
[sudo] password for laura:
laura is not in the sudoers file. This incident will be reported.
laura@debian7:~$

Notice that laura has no permission to use the sudo on this system.




                                           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.




                                          274
                                       user management



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




                                             275
                                      user management



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




                                            276
                                       user management



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




                                             277
                                      user management



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.




                                            278
                                       user management



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.




                                             279
                                      user management



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.




                                           280
                                      user management


8. Create a file named welcome.txt and make sure every new user will see this file in their
home directory.
root@debian7:~# echo Hello > /etc/skel/welcome.txt

9. Verify this setup by creating (and deleting) a test user account.
root@debian7:~# useradd -m test
root@debian7:~# ls -l /home/test
total 4
-rw-r--r-- 1 test test 6 Oct 15 11:16 welcome.txt
root@debian7:~# userdel -r test
root@debian7:~#

10. Change the default login shell for the serena user to /bin/bash. Verify before and after
you make this change.
root@debian7:~# grep serena /etc/passwd
serena:x:1008:1010:Serena Williams:/home/serena:/bin/sh
root@debian7:~# usermod -s /bin/bash serena
root@debian7:~# grep serena /etc/passwd
serena:x:1008:1010:Serena Williams:/home/serena:/bin/bash
root@debian7:~#




                                            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.




                                           282
                                       user passwords



29.1. passwd
 Passwords of users can be set with the passwd command. Users will have to provide their
 old password before twice entering the new one.
 [tania@centos7 ~]$ passwd
 Changing password for user tania.
 Changing password for tania.
 (current) UNIX password:
 New password:
 BAD PASSWORD: The password is shorter than 8 characters
 New password:
 BAD PASSWORD: The password is a palindrome
 New password:
 BAD PASSWORD: The password is too similar to the old one
 passwd: Have exhausted maximum number of retries for service

 As you can see, the passwd tool will do some basic verification to prevent users from using
 too simple passwords. The root user does not have to follow these rules (there will be
 a warning though). The root user also does not have to provide the old password before
 entering the new password twice.
 root@debian7:~# passwd tania
 Enter new UNIX password:
 Retype new UNIX password:
 passwd: password updated successfully


29.2. shadow file
 User passwords are encrypted and kept in /etc/shadow. The /etc/shadow file is read only
 and can only be read by root. We will see in the file permissions section how it is possible
 for users to change their password. For now, you will have to know that users can change
 their password with the /usr/bin/passwd command.
 [root@centos7 ~]# tail -4 /etc/shadow
 paul:$6$ikp2Xta5BT.Tml.p$2TZjNnOYNNQKpwLJqoGJbVsZG5/Fti8ovBRd.VzRbiDSl7TEq\
 IaSMH.TeBKnTS/SjlMruW8qffC0JNORW.BTW1:16338:0:99999:7:::
 tania:$6$8Z/zovxj$9qvoqT8i9KIrmN.k4EQwAF5ryz5yzNwEvYjAa9L5XVXQu.z4DlpvMREH\
 eQpQzvRnqFdKkVj17H5ST.c79HDZw0:16356:0:99999:7:::
 laura:$6$glDuTY5e$/NYYWLxfHgZFWeoujaXSMcR.Mz.lGOxtcxFocFVJNb98nbTPhWFXfKWG\
 SyYh1WCv6763Wq54.w24Yr3uAZBOm/:16356:0:99999:7:::
 valentina:$6$jrZa6PVI$1uQgqR6En9mZB6mKJ3LXRB4CnFko6LRhbh.v4iqUk9MVreui1lv7\
 GxHOUDSKA0N55ZRNhGHa6T2ouFnVno/0o1:16356:0:99999:7:::
 [root@centos7 ~]#

 The /etc/shadow file contains nine colon separated columns. The nine fields contain (from
 left to right) the user name, the encrypted password (note that only inge and laura have an
 encrypted password), the day the password was last changed (day 1 is January 1, 1970),
 number of days the password must be left unchanged, password expiry day, warning number
 of days before password expiry, number of days after expiry before disabling the account,
 and the day the account was disabled (again, since 1970). The last field has no meaning yet.

 All the passwords in the screenshot above are hashes of hunter2.




                                            283
                                        user passwords



29.3. encryption with passwd
 Passwords are stored in an encrypted format. This encryption is done by the crypt function.
 The easiest (and recommended) way to add a user with a password to the system is to add
 the user with the useradd -m user command, and then set the user's password with passwd.
 [root@RHEL4 ~]# useradd -m xavier
 [root@RHEL4 ~]# passwd xavier
 Changing password for user xavier.
 New UNIX password:
 Retype new UNIX password:
 passwd: all authentication tokens updated successfully.
 [root@RHEL4 ~]#


29.4. encryption with openssl
 Another way to create users with a password is to use the -p option of useradd, but that
 option requires an encrypted password. You can generate this encrypted password with the
 openssl passwd command.

 The openssl passwd command will generate several distinct hashes for the same password,
 for this it uses a salt.
 paul@rhel65:~$ openssl passwd hunter2
 86jcUNlnGDFpY
 paul@rhel65:~$ openssl passwd hunter2
 Yj7mDO9OAnvq6
 paul@rhel65:~$ openssl passwd hunter2
 YqDcJeGoDbzKA
 paul@rhel65:~$

 This salt can be chosen and is visible as the first two characters of the hash.
 paul@rhel65:~$ openssl passwd -salt 42 hunter2
 42ZrbtP1Ze8G.
 paul@rhel65:~$ openssl passwd -salt 42 hunter2
 42ZrbtP1Ze8G.
 paul@rhel65:~$ openssl passwd -salt 42 hunter2
 42ZrbtP1Ze8G.
 paul@rhel65:~$

 This example shows how to create a user with password.
 root@rhel65:~# useradd -m -p $(openssl passwd hunter2) mohamed

 Note that this command puts the password in your command history!




                                             284
                                       user passwords



29.5. encryption with crypt
 A third option is to create your own C program using the crypt function, and compile this
 into a command.
 paul@rhel65:~$ cat MyCrypt.c
 #include <stdio.h>
 #define __USE_XOPEN
 #include <unistd.h>

 int main(int argc, char** argv)
 {
   if(argc==3)
     {
         printf("%s\n", crypt(argv[1],argv[2]));
     }
     else
     {
         printf("Usage: MyCrypt $password $salt\n" );
     }
    return 0;
 }

 This little program can be compiled with gcc like this.
 paul@rhel65:~$ gcc MyCrypt.c -o MyCrypt -lcrypt

 To use it, we need to give two parameters to MyCrypt. The first is the unencrypted password,
 the second is the salt. The salt is used to perturb the encryption algorithm in one of 4096
 different ways. This variation prevents two users with the same password from having the
 same entry in /etc/shadow.
 paul@rhel65:~$ ./MyCrypt hunter2 42
 42ZrbtP1Ze8G.
 paul@rhel65:~$ ./MyCrypt hunter2 33
 33d6taYSiEUXI

 Did you notice that the first two characters of the password are the salt?

 The standard output of the crypt function is using the DES algorithm which is old and can
 be cracked in minutes. A better method is to use md5 passwords which can be recognized
 by a salt starting with $1$.
 paul@rhel65:~$ ./MyCrypt hunter2 '$1$42'
 $1$42$7l6Y3xT5282XmZrtDOF9f0
 paul@rhel65:~$ ./MyCrypt hunter2 '$6$42'
 $6$42$OqFFAVnI3gTSYG0yI9TZWX9cpyQzwIop7HwpG1LLEsNBiMr4w6OvLX1KDa./UpwXfrFk1i...

 The md5 salt can be up to eight characters long. The salt is displayed in /etc/shadow between
 the second and third $, so never use the password as the salt!
 paul@rhel65:~$ ./MyCrypt hunter2 '$1$hunter2'
 $1$hunter2$YVxrxDmidq7Xf8Gdt6qM2.




                                            285
                                       user passwords



29.6. /etc/login.defs
 The /etc/login.defs file contains some default settings for user passwords like password
 aging and length settings. (You will also find the numerical limits of user ids and group ids
 and whether or not a home directory should be created by default).
 root@rhel65:~# grep ^PASS /etc/login.defs
 PASS_MAX_DAYS   99999
 PASS_MIN_DAYS   0
 PASS_MIN_LEN    5
 PASS_WARN_AGE   7

 Debian also has this file.
 root@debian7:~# grep PASS /etc/login.defs
 # PASS_MAX_DAYS    Maximum number of days a password may be used.
 # PASS_MIN_DAYS    Minimum number of days allowed between password changes.
 # PASS_WARN_AGE    Number of days warning given before a password expires.
 PASS_MAX_DAYS   99999
 PASS_MIN_DAYS   0
 PASS_WARN_AGE   7
 #PASS_CHANGE_TRIES
 #PASS_ALWAYS_WARN
 #PASS_MIN_LEN
 #PASS_MAX_LEN
 # NO_PASSWORD_CONSOLE
 root@debian7:~#


29.7. chage
 The chage command can be used to set an expiration date for a user account (-E), set a
 minimum (-m) and maximum (-M) password age, a password expiration date, and set the
 number of warning days before the password expiration date. Much of this functionality is
 also available from the passwd command. The -l option of chage will list these settings for
 a user.
 root@rhel65:~# chage -l paul
 Last password change                                            :   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:~#




                                            286
                                        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)




                                             287
                                      user passwords



29.10. practice: user passwords
 1. Set the password for serena to hunter2.

 2. Also set a password for venus and then lock the venus user account with usermod. Verify
 the locking in /etc/shadow before and after you lock it.

 3. Use passwd -d to disable the serena password. Verify the serena line in /etc/shadow
 before and after disabling.

 4. What is the difference between locking a user account and disabling a user account's
 password like we just did with usermod -L and passwd -d?

 5. Try changing the password of serena to serena as serena.

 6. Make sure serena has to change her password in 10 days.

 7. Make sure every new user needs to change their password every 10 days.

 8. Take a backup as root of /etc/shadow. Use vi to copy an encrypted hunter2 hash from
 venus to serena. Can serena now log on with hunter2 as a password ?

 9. Why use vipw instead of vi ? What could be the problem when using vi or vim ?

 10. Use chsh to list all shells (only works on RHEL/CentOS/Fedora), and compare to cat /
 etc/shells.

 11. Which useradd option allows you to name a home directory ?

 12. How can you see whether the password of user serena is locked or unlocked ? Give a
 solution with grep and a solution with passwd.




                                           288
                                      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)




                                           289
                                    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 ~]#




                                            292
                                         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                    su     su -      ssh      gdm
 ~./bashrc                 no     yes       yes          yes
 ~/.profile                no     yes       yes          yes
 /etc/profile              no     yes       yes          yes
 /etc/bash.bashrc         yes     no        no           yes


30.8. RHEL5 overview
 Below is a table overview of when Red Hat Enterprise Linux 5 is running any of these bash
 startup scripts.

 Table 30.2. Red Hat User Environment
 script                    su     su -      ssh      gdm
 ~./bashrc                yes     yes       yes          yes
 ~/.bash_profile           no     yes       yes          yes
 /etc/profile              no     yes       yes          yes
 /etc/bashrc              yes     yes       yes          yes




                                             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.

 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?
 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
        d                    directory
         l                 symbolic link
        p                   named pipe
        b                   block device
        c                 character device
         s                     socket

 Below a screenshot of a character device (the console) and a block device (the hard disk).
 paul@debian6lt~$ ls -ld /dev/console /dev/sda
 crw-------   1 root root 5, 1 Mar 15 12:45 /dev/console
 brw-rw----   1 root disk 8, 0 Mar 15 12:45 /dev/sda

 And here you can see a directory, a regular file and a symbolic link.
 paul@debian6lt~$ ls     -ld /etc /etc/hosts /etc/motd
 drwxr-xr-x 128 root     root 12288 Mar 15 18:34 /etc
 -rw-r--r--   1 root     root   372 Dec 10 17:36 /etc/hosts
 lrwxrwxrwx   1 root     root    13 Dec 5 10:36 /etc/motd -> /var/run/motd




                                                  310
                                      standard file permissions



32.3. permissions
32.3.1. rwx
 The nine characters following the file type denote the permissions in three triplets. A
 permission can be r for read access, w for write access, and x for execute. You need the r
 permission to list (ls) the contents of a directory. You need the x permission to enter (cd) a
 directory. You need the w permission to create files in or remove files from a directory.

 Table 32.2. standard Unix file permissions
    permission                   on a file                                 on a directory
      r (read)           read file contents (cat)                    read directory contents (ls)
     w (write)          change file contents (vi)                       create files in (touch)
    x (execute)               execute the file                         enter the directory (cd)


32.3.2. three sets of rwx
 We already know that the output of ls -l starts with ten characters for each file. This
 screenshot shows a regular file (because the first character is a - ).
 paul@RHELv4u4:~/test$ ls -l proc42.bash
 -rwxr-xr-- 1 paul proj 984 Feb 6 12:01 proc42.bash

 Below is a table describing the function of all ten characters.

 Table 32.3. Unix file permissions position
     position       characters                                 function
        1               -                                 this is a regular file
       2-4             rwx                       permissions for the user owner
       5-7             r-x                       permissions for the group owner
       8-10             r--                           permissions for others

 When you are the user owner of a file, then the user owner permissions apply to you. The
 rest of the permissions have no influence on your access to the file.

 When you belong to the group that is the group owner of a file, then the group owner
 permissions apply to you. The rest of the permissions have no influence on your access to
 the file.

 When you are not the user owner of a file and you do not belong to the group owner, then
 the others permissions apply to you. The rest of the permissions have no influence on your
 access to the file.




                                                    311
                                   standard file permissions



32.3.3. permission examples
 Some example combinations on files and directories are seen in this screenshot. The name
 of the file explains the permissions.
 paul@laika:~/perms$ ls    -lh
 total 12K
 drwxr-xr-x 2 paul paul    4.0K   2007-02-07   22:26   AllEnter_UserCreateDelete
 -rwxrwxrwx 1 paul paul       0   2007-02-07   22:21   EveryoneFullControl.txt
 -r--r----- 1 paul paul       0   2007-02-07   22:21   OnlyOwnersRead.txt
 -rwxrwx--- 1 paul paul       0   2007-02-07   22:21   OwnersAll_RestNothing.txt
 dr-xr-x--- 2 paul paul    4.0K   2007-02-07   22:25   UserAndGroupEnter
 dr-x------ 2 paul paul    4.0K   2007-02-07   22:25   OnlyUserEnter
 paul@laika:~/perms$

 To summarise, the first rwx triplet represents the permissions for the user owner. The
 second triplet corresponds to the group owner; it specifies permissions for all members
 of that group. The third triplet defines permissions for all other users that are not the user
 owner and are not a member of the group owner.




                                             312
                                  standard file permissions



32.3.4. setting permissions (chmod)
 Permissions can be changed with chmod. The first example gives the user owner execute
 permissions.
 paul@laika:~/perms$ ls -l permissions.txt
 -rw-r--r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
 paul@laika:~/perms$ chmod u+x permissions.txt
 paul@laika:~/perms$ ls -l permissions.txt
 -rwxr--r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt

 This example removes the group owners read permission.
 paul@laika:~/perms$ chmod g-r permissions.txt
 paul@laika:~/perms$ ls -l permissions.txt
 -rwx---r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt

 This example removes the others read permission.
 paul@laika:~/perms$ chmod o-r permissions.txt
 paul@laika:~/perms$ ls -l permissions.txt
 -rwx------ 1 paul paul 0 2007-02-07 22:34 permissions.txt

 This example gives all of them the write permission.
 paul@laika:~/perms$ chmod a+w permissions.txt
 paul@laika:~/perms$ ls -l permissions.txt
 -rwx-w--w- 1 paul paul 0 2007-02-07 22:34 permissions.txt

 You don't even have to type the a.
 paul@laika:~/perms$ chmod +x permissions.txt
 paul@laika:~/perms$ ls -l permissions.txt
 -rwx-wx-wx 1 paul paul 0 2007-02-07 22:34 permissions.txt

 You can also set explicit permissions.
 paul@laika:~/perms$ chmod u=rw permissions.txt
 paul@laika:~/perms$ ls -l permissions.txt
 -rw--wx-wx 1 paul paul 0 2007-02-07 22:34 permissions.txt

 Feel free to make any kind of combination.
 paul@laika:~/perms$ chmod u=rw,g=rw,o=r permissions.txt
 paul@laika:~/perms$ ls -l permissions.txt
 -rw-rw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt

 Even fishy combinations are accepted by chmod.
 paul@laika:~/perms$ chmod u=rwx,ug+rw,o=r permissions.txt
 paul@laika:~/perms$ ls -l permissions.txt
 -rwxrw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt




                                            313
                                 standard file permissions



32.3.5. setting octal permissions
 Most Unix administrators will use the old school octal system to talk about and set
 permissions. Look at the triplet bitwise, equating r to 4, w to 2, and x to 1.

 Table 32.4. Octal permissions
     binary         octal        permission
      000             0              ---
      001             1             --x
      010             2             -w-
      011             3             -wx
      100             4              r--
      101             5             r-x
      110             6             rw-
      111             7             rwx

 This makes 777 equal to rwxrwxrwx and by the same logic, 654 mean rw-r-xr-- . The chmod
 command will accept these numbers.
 paul@laika:~/perms$ chmod 777 permissions.txt
 paul@laika:~/perms$ ls -l permissions.txt
 -rwxrwxrwx 1 paul paul 0 2007-02-07 22:34 permissions.txt
 paul@laika:~/perms$ chmod 664 permissions.txt
 paul@laika:~/perms$ ls -l permissions.txt
 -rw-rw-r-- 1 paul paul 0 2007-02-07 22:34 permissions.txt
 paul@laika:~/perms$ chmod 750 permissions.txt
 paul@laika:~/perms$ ls -l permissions.txt
 -rwxr-x--- 1 paul paul 0 2007-02-07 22:34 permissions.txt




                                           314
                                  standard file permissions



32.3.6. umask
 When creating a file or directory, a set of default permissions are applied. These default
 permissions are determined by the umask. The umask specifies permissions that you do
 not want set on by default. You can display the umask with the umask command.
 [Harry@RHEL4b   ~]$ umask
 0002
 [Harry@RHEL4b   ~]$ touch test
 [Harry@RHEL4b   ~]$ ls -l test
 -rw-rw-r-- 1    Harry Harry 0 Jul 24 06:03 test
 [Harry@RHEL4b   ~]$

 As you can also see, the file is also not executable by default. This is a general security
 feature among Unixes; newly created files are never executable by default. You have to
 explicitly do a chmod +x to make a file executable. This also means that the 1 bit in the
 umask has no meaning--a umask of 0022 is the same as 0033.

32.3.7. mkdir -m
 When creating directories with mkdir you can use the -m option to set the mode. This
 screenshot explains.
 paul@debian5~$ mkdir -m 700 MyDir
 paul@debian5~$ mkdir -m 777 Public
 paul@debian5~$ ls -dl MyDir/ Public/
 drwx------ 2 paul paul 4096 2011-10-16 19:16 MyDir/
 drwxrwxrwx 2 paul paul 4096 2011-10-16 19:16 Public/


32.3.8. cp -p
 To preserve permissions and time stamps from source files, use cp -p.
 paul@laika:~/perms$ cp file* cp
 paul@laika:~/perms$ cp -p file* cpp
 paul@laika:~/perms$ ll *
 -rwx------ 1 paul paul    0 2008-08-25 13:26 file33
 -rwxr-x--- 1 paul paul    0 2008-08-25 13:26 file42

 cp:
 total 0
 -rwx------ 1 paul paul 0 2008-08-25 13:34 file33
 -rwxr-x--- 1 paul paul 0 2008-08-25 13:34 file42

 cpp:
 total 0
 -rwx------ 1 paul paul 0 2008-08-25 13:26 file33
 -rwxr-x--- 1 paul paul 0 2008-08-25 13:26 file42




                                            315
                                    standard file permissions



32.4. practice: standard file permissions
 1. As normal user, create a directory ~/permissions. Create a file owned by yourself in there.

 2. Copy a file owned by root from /etc/ to your permissions dir, who owns this file now ?

 3. As root, create a file in the users ~/permissions directory.

 4. As normal user, look at who owns this file created by root.

 5. Change the ownership of all files in ~/permissions to yourself.

 6. Make sure you have all rights to these files, and others can only read.

 7. With chmod, is 770 the same as rwxrwx--- ?

 8. With chmod, is 664 the same as r-xr-xr-- ?

 9. With chmod, is 400 the same as r-------- ?

 10. With chmod, is 734 the same as rwxr-xr-- ?

 11a. Display the umask in octal and in symbolic form.

 11b. Set the umask to 077, but use the symbolic format to set it. Verify that this works.

 12. Create a file as root, give only read to others. Can a normal user read this file ? Test
 writing to this file with vi.

 13a. Create a file as normal user, give only read to others. Can another normal user read this
 file ? Test writing to this file with vi.

 13b. Can root read this file ? Can root write to this file with vi ?

 14. Create a directory that belongs to a group, where every member of that group can read
 and write to files, and create files. Make sure that people can only delete their own files.




                                              316
                                    standard file permissions



32.5. solution: standard file permissions
 1. As normal user, create a directory ~/permissions. Create a file owned by yourself in there.
 mkdir ~/permissions ; touch ~/permissions/myfile.txt

 2. Copy a file owned by root from /etc/ to your permissions dir, who owns this file now ?
 cp /etc/hosts ~/permissions/

 The copy is owned by you.

 3. As root, create a file in the users ~/permissions directory.
 (become root)# touch /home/username/permissions/rootfile

 4. As normal user, look at who owns this file created by root.
 ls -l ~/permissions

 The file created by root is owned by root.

 5. Change the ownership of all files in ~/permissions to yourself.
 chown user ~/permissions/*

 You cannot become owner of the file that belongs to root.

 6. Make sure you have all rights to these files, and others can only read.
 chmod 644 (on files)

 chmod 755 (on directories)

 7. With chmod, is 770 the same as rwxrwx--- ?

 yes

 8. With chmod, is 664 the same as r-xr-xr-- ?

 No

 9. With chmod, is 400 the same as r-------- ?

 yes

 10. With chmod, is 734 the same as rwxr-xr-- ?

 no

 11a. Display the umask in octal and in symbolic form.
 umask ; umask -S

 11b. Set the umask to 077, but use the symbolic format to set it. Verify that this works.
 umask -S u=rwx,go=


                                              317
                                   standard file permissions


12. Create a file as root, give only read to others. Can a normal user read this file ? Test
writing to this file with vi.
(become root)

# echo hello > /home/username/root.txt

# chmod 744 /home/username/root.txt

(become user)

vi ~/root.txt

13a. Create a file as normal user, give only read to others. Can another normal user read this
file ? Test writing to this file with vi.
echo hello > file ; chmod 744 file

Yes, others can read this file

13b. Can root read this file ? Can root write to this file with vi ?

Yes, root can read and write to this file. Permissions do not apply to root.

14. Create a directory that belongs to a group, where every member of that group can read
and write to files, and create files. Make sure that people can only delete their own files.
mkdir /home/project42 ; groupadd project42

chgrp project42 /home/project42 ; chmod 775 /home/project42

You can not yet do the last part of this exercise...




                                             318
Chapter 33. advanced file
permissions




                  319
                                     advanced file permissions



33.1. sticky bit on directory
 You can set the sticky bit on a directory to prevent users from removing files that they do
 not own as a user owner. The sticky bit is displayed at the same location as the x permission
 for others. The sticky bit is represented by a t (meaning x is also there) or a T (when there
 is no x for others).
 root@RHELv4u4:~# mkdir /project55
 root@RHELv4u4:~# ls -ld /project55
 drwxr-xr-x 2 root root 4096 Feb 7 17:38 /project55
 root@RHELv4u4:~# chmod +t /project55/
 root@RHELv4u4:~# ls -ld /project55
 drwxr-xr-t 2 root root 4096 Feb 7 17:38 /project55
 root@RHELv4u4:~#

 The sticky bit can also be set with octal permissions, it is binary 1 in the first of four triplets.
 root@RHELv4u4:~# chmod 1775 /project55/
 root@RHELv4u4:~# ls -ld /project55
 drwxrwxr-t 2 root root 4096 Feb 7 17:38 /project55
 root@RHELv4u4:~#

 You will typically find the sticky bit on the /tmp directory.
 root@barry:~# ls -ld /tmp
 drwxrwxrwt 6 root root 4096 2009-06-04 19:02 /tmp


33.2. setgid bit on directory
 setgid can be used on directories to make sure that all files inside the directory are owned
 by the group owner of the directory. The setgid bit is displayed at the same location as the x
 permission for group owner. The setgid bit is represented by an s (meaning x is also there)
 or a S (when there is no x for the group owner). As this example shows, even though root
 does not belong to the group proj55, the files created by root in /project55 will belong to
 proj55 since the setgid is set.
 root@RHELv4u4:~# groupadd proj55
 root@RHELv4u4:~# chown root:proj55 /project55/
 root@RHELv4u4:~# chmod 2775 /project55/
 root@RHELv4u4:~# touch /project55/fromroot.txt
 root@RHELv4u4:~# ls -ld /project55/
 drwxrwsr-x 2 root proj55 4096 Feb 7 17:45 /project55/
 root@RHELv4u4:~# ls -l /project55/
 total 4
 -rw-r--r-- 1 root proj55 0 Feb 7 17:45 fromroot.txt
 root@RHELv4u4:~#

 You can use the find command to find all setgid directories.
 paul@laika:~$ find / -type d -perm -2000 2> /dev/null
 /var/log/mysql
 /var/log/news
 /var/local
 ...




                                               320
                                    advanced file permissions



33.3. setgid and setuid on regular files
 These two permissions cause an executable file to be executed with the permissions of the
 file owner instead of the executing owner. This means that if any user executes a program
 that belongs to the root user, and the setuid bit is set on that program, then the program
 runs as root. This can be dangerous, but sometimes this is good for security.

 Take the example of passwords; they are stored in /etc/shadow which is only readable by
 root. (The root user never needs permissions anyway.)

 root@RHELv4u4:~# ls -l /etc/shadow
 -r-------- 1 root root 1260 Jan 21 07:49 /etc/shadow



 Changing your password requires an update of this file, so how can normal non-root users
 do this? Let's take a look at the permissions on the /usr/bin/passwd.

 root@RHELv4u4:~# ls -l /usr/bin/passwd
 -r-s--x--x 1 root root 21200 Jun 17 2005 /usr/bin/passwd



 When running the passwd program, you are executing it with root credentials.

 You can use the find command to find all setuid programs.

 paul@laika:~$ find /usr/bin -type f -perm -04000
 /usr/bin/arping
 /usr/bin/kgrantpty
 /usr/bin/newgrp
 /usr/bin/chfn
 /usr/bin/sudo
 /usr/bin/fping6
 /usr/bin/passwd
 /usr/bin/gpasswd
 ...



 In most cases, setting the setuid bit on executables is sufficient. Setting the setgid bit will
 result in these programs to run with the credentials of their group owner.

33.4. setuid on sudo
 The sudo binary has the setuid bit set, so any user can run it with the effective userid of root.
 paul@rhel65:~$ ls -l $(which sudo)
 ---s--x--x. 1 root root 123832 Oct         7    2013 /usr/bin/sudo
 paul@rhel65:~$




                                                321
                                    advanced file permissions



33.5. practice: sticky, setuid and setgid bits
 1a. Set up a directory, owned by the group sports.

 1b. Members of the sports group should be able to create files in this directory.

 1c. All files created in this directory should be group-owned by the sports group.

 1d. Users should be able to delete only their own user-owned files.

 1e. Test that this works!

 2. Verify the permissions on /usr/bin/passwd. Remove the setuid, then try changing your
 password as a normal user. Reset the permissions back and try again.

 3. If time permits (or if you are waiting for other students to finish this practice), read about
 file attributes in the man page of chattr and lsattr. Try setting the i attribute on a file and
 test that it works.




                                              322
                                    advanced file permissions



33.6. solution: sticky, setuid and setgid bits
 1a. Set up a directory, owned by the group sports.
 groupadd sports

 mkdir /home/sports

 chown root:sports /home/sports

 1b. Members of the sports group should be able to create files in this directory.
 chmod 770 /home/sports

 1c. All files created in this directory should be group-owned by the sports group.
 chmod 2770 /home/sports

 1d. Users should be able to delete only their own user-owned files.
 chmod +t /home/sports

 1e. Test that this works!

 Log in with different users (group members and others and root), create files and watch the
 permissions. Try changing and deleting files...

 2. Verify the permissions on /usr/bin/passwd. Remove the setuid, then try changing your
 password as a normal user. Reset the permissions back and try again.

 root@deb503:~# ls -l /usr/bin/passwd
 -rwsr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd
 root@deb503:~# chmod 755 /usr/bin/passwd
 root@deb503:~# ls -l /usr/bin/passwd
 -rwxr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd



 A normal user cannot change password now.

 root@deb503:~# chmod 4755 /usr/bin/passwd
 root@deb503:~# ls -l /usr/bin/passwd
 -rwsr-xr-x 1 root root 31704 2009-11-14 15:41 /usr/bin/passwd



 3. If time permits (or if you are waiting for other students to finish this practice), read about
 file attributes in the man page of chattr and lsattr. Try setting the i attribute on a file and
 test that it works.

 paul@laika:~$ sudo su -
 [sudo] password for paul:
 root@laika:~# mkdir attr
 root@laika:~# cd attr/
 root@laika:~/attr# touch file42
 root@laika:~/attr# lsattr
 ------------------ ./file42
 root@laika:~/attr# chattr +i file42


                                              323
                             advanced file permissions


root@laika:~/attr# lsattr
----i------------- ./file42
root@laika:~/attr# rm -rf file42
rm: cannot remove `file42': Operation not permitted
root@laika:~/attr# chattr -i file42
root@laika:~/attr# rm -rf file42
root@laika:~/attr#




                                       324
Chapter 34. access control lists
 Standard Unix permissions might not be enough for some organisations. This chapter
 introduces access control lists or acl's to further protect files and directories.




                                       325
                                       access control lists



34.1. acl in /etc/fstab
 File systems that support access control lists, or acls, have to be mounted with the acl
 option listed in /etc/fstab. In the example below, you can see that the root file system has
 acl support, whereas /home/data does not.
 root@laika:~# tail -4 /etc/fstab
 /dev/sda1        /               ext3              acl,relatime     0   1
 /dev/sdb2        /home/data      auto              noacl,defaults   0   0
 pasha:/home/r    /home/pasha     nfs               defaults         0   0
 wolf:/srv/data   /home/wolf      nfs               defaults         0   0


34.2. getfacl
 Reading acls can be done with /usr/bin/getfacl. This screenshot shows how to read the acl
 of file33 with getfacl.
 paul@laika:~/test$ getfacl file33
 # file: file33
 # owner: paul
 # group: paul
 user::rw-
 group::r--
 mask::rwx
 other::r--


34.3. setfacl
 Writing or changing acls can be done with /usr/bin/setfacl. These screenshots show how
 to change the acl of file33 with setfacl.

 First we add user sandra with octal permission 7 to the acl.
 paul@laika:~/test$ setfacl -m u:sandra:7 file33

 Then we add the group tennis with octal permission 6 to the acl of the same file.
 paul@laika:~/test$ setfacl -m g:tennis:6 file33

 The result is visible with getfacl.
 paul@laika:~/test$ getfacl file33
 # file: file33
 # owner: paul
 # group: paul
 user::rw-
 user:sandra:rwx
 group::r--
 group:tennis:rw-
 mask::rwx
 other::r--




                                              326
                                     access control lists



34.4. remove an acl entry
 The -x option of the setfacl command will remove an acl entry from the targeted file.
 paul@laika:~/test$   setfacl -m u:sandra:7 file33
 paul@laika:~/test$   getfacl file33 | grep sandra
 user:sandra:rwx
 paul@laika:~/test$   setfacl -x sandra file33
 paul@laika:~/test$   getfacl file33 | grep sandra

 Note that omitting the u or g when defining the acl for an account will default it to a user
 account.

34.5. remove the complete acl
 The -b option of the setfacl command will remove the acl from the targeted file.
 paul@laika:~/test$ setfacl -b file33
 paul@laika:~/test$ getfacl file33
 # file: file33
 # owner: paul
 # group: paul
 user::rw-
 group::r--
 other::r--


34.6. the acl mask
 The acl mask defines the maximum effective permissions for any entry in the acl. This
 mask is calculated every time you execute the setfacl or chmod commands.

 You can prevent the calculation by using the --no-mask switch.

 paul@laika:~/test$ setfacl --no-mask -m u:sandra:7 file33
 paul@laika:~/test$ getfacl file33
 # file: file33
 # owner: paul
 # group: paul
 user::rw-
 user:sandra:rwx   #effective:rw-
 group::r--
 mask::rw-
 other::r--




                                            327
                                     access control lists



34.7. eiciel
 Desktop users might want to use eiciel to manage acls with a graphical tool.




 You will need to install eiciel and nautilus-actions to have an extra tab in nautilus to
 manage acls.

 paul@laika:~$ sudo aptitude install eiciel nautilus-actions




                                            328
Chapter 35. file links
 An average computer using Linux has a file system with many hard links and symbolic
 links.

 To understand links in a file system, you first have to understand what an inode is.




                                           329
                                           file links



35.1. inodes
35.1.1. inode contents
 An inode is a data structure that contains metadata about a file. When the file system stores
 a new file on the hard disk, it stores not only the contents (data) of the file, but also extra
 properties like the name of the file, the creation date, its permissions, the owner of the file,
 and more. All this information (except the name of the file and the contents of the file) is
 stored in the inode of the file.

 The ls -l command will display some of the inode contents, as seen in this screenshot.
 root@rhel53 ~# ls -ld /home/project42/
 drwxr-xr-x 4 root pro42 4.0K Mar 27 14:29 /home/project42/


35.1.2. inode table
 The inode table contains all of the inodes and is created when you create the file system
 (with mkfs). You can use the df -i command to see how many inodes are used and free on
 mounted file systems.
 root@rhel53 ~# df -i
 Filesystem            Inodes    IUsed  IFree IUse% Mounted on
 /dev/mapper/VolGroup00-LogVol00
                      4947968 115326 4832642     3% /
 /dev/hda1              26104       45  26059    1% /boot
 tmpfs                  64417        1  64416    1% /dev/shm
 /dev/sda1             262144     2207 259937    1% /home/project42
 /dev/sdb1              74400     5519  68881    8% /home/project33
 /dev/sdb5                  0        0      0    - /home/sales
 /dev/sdb6             100744       11 100733    1% /home/research


 In the df -i screenshot above you can see the inode usage for several mounted file systems.
 You don't see numbers for /dev/sdb5 because it is a fat file system.

35.1.3. inode number
 Each inode has a unique number (the inode number). You can see the inode numbers with
 the ls -li command.
 paul@RHELv4u4:~/test$ touch file1
 paul@RHELv4u4:~/test$ touch file2
 paul@RHELv4u4:~/test$ touch file3
 paul@RHELv4u4:~/test$ ls -li
 total 12
 817266 -rw-rw-r-- 1 paul paul 0 Feb          5 15:38 file1
 817267 -rw-rw-r-- 1 paul paul 0 Feb          5 15:38 file2
 817268 -rw-rw-r-- 1 paul paul 0 Feb          5 15:38 file3
 paul@RHELv4u4:~/test$


 These three files were created one after the other and got three different inodes (the first
 column). All the information you see with this ls command resides in the inode, except for
 the filename (which is contained in the directory).


                                             330
                                             file links



35.1.4. inode and file contents
  Let's put some data in one of the files.
  paul@RHELv4u4:~/test$ ls -li
  total 16
  817266 -rw-rw-r-- 1 paul paul 0 Feb            5 15:38 file1
  817270 -rw-rw-r-- 1 paul paul 92 Feb           5 15:42 file2
  817268 -rw-rw-r-- 1 paul paul 0 Feb            5 15:38 file3
  paul@RHELv4u4:~/test$ cat file2
  It is winter now and it is very cold.
  We do not like the cold, we prefer hot         summer nights.
  paul@RHELv4u4:~/test$

  The data that is displayed by the cat command is not in the inode, but somewhere else on
  the disk. The inode contains a pointer to that data.

35.2. about directories
35.2.1. a directory is a table
  A directory is a special kind of file that contains a table which maps filenames to inodes.
  Listing our current directory with ls -ali will display the contents of the directory file.
  paul@RHELv4u4:~/test$     ls -ali
  total 32
  817262 drwxrwxr-x   2     paul   paul 4096 Feb      5   15:42   .
  800768 drwx------ 16      paul   paul 4096 Feb      5   15:42   ..
  817266 -rw-rw-r--   1     paul   paul    0 Feb      5   15:38   file1
  817270 -rw-rw-r--   1     paul   paul   92 Feb      5   15:42   file2
  817268 -rw-rw-r--   1     paul   paul    0 Feb      5   15:38   file3
  paul@RHELv4u4:~/test$


35.2.2. . and ..
  You can see five names, and the mapping to their five inodes. The dot . is a mapping to itself,
  and the dotdot .. is a mapping to the parent directory. The three other names are mappings
  to different inodes.




                                               331
                                          file links



35.3. hard links
35.3.1. creating hard links
 When we create a hard link to a file with ln, an extra entry is added in the directory. A new
 file name is mapped to an existing inode.
 paul@RHELv4u4:~/test$ ln file2 hardlink_to_file2
 paul@RHELv4u4:~/test$ ls -li
 total 24
 817266 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file1
 817270 -rw-rw-r-- 2 paul paul 92 Feb 5 15:42 file2
 817268 -rw-rw-r-- 1 paul paul 0 Feb 5 15:38 file3
 817270 -rw-rw-r-- 2 paul paul 92 Feb 5 15:42 hardlink_to_file2
 paul@RHELv4u4:~/test$

 Both files have the same inode, so they will always have the same permissions and the same
 owner. Both files will have the same content. Actually, both files are equal now, meaning
 you can safely remove the original file, the hardlinked file will remain. The inode contains
 a counter, counting the number of hard links to itself. When the counter drops to zero, then
 the inode is emptied.

35.3.2. finding hard links
 You can use the find command to look for files with a certain inode. The screenshot below
 shows how to search for all filenames that point to inode 817270. Remember that an inode
 number is unique to its partition.
 paul@RHELv4u4:~/test$ find / -inum 817270 2> /dev/null
 /home/paul/test/file2
 /home/paul/test/hardlink_to_file2




                                            332
                                          file links



35.4. symbolic links
 Symbolic links (sometimes called soft links) do not link to inodes, but create a name to
 name mapping. Symbolic links are created with ln -s. As you can see below, the symbolic
 link gets an inode of its own.
 paul@RHELv4u4:~/test$ ln -s file2 symlink_to_file2
 paul@RHELv4u4:~/test$ ls -li
 total 32
 817273 -rw-rw-r-- 1 paul paul 13 Feb 5 17:06 file1
 817270 -rw-rw-r-- 2 paul paul 106 Feb 5 17:04 file2
 817268 -rw-rw-r-- 1 paul paul    0 Feb 5 15:38 file3
 817270 -rw-rw-r-- 2 paul paul 106 Feb 5 17:04 hardlink_to_file2
 817267 lrwxrwxrwx 1 paul paul    5 Feb 5 16:55 symlink_to_file2 -> file2
 paul@RHELv4u4:~/test$

 Permissions on a symbolic link have no meaning, since the permissions of the target apply.
 Hard links are limited to their own partition (because they point to an inode), symbolic links
 can link anywhere (other file systems, even networked).

35.5. removing links
 Links can be removed with rm.
 paul@laika:~$   touch data.txt
 paul@laika:~$   ln -s data.txt sl_data.txt
 paul@laika:~$   ln data.txt hl_data.txt
 paul@laika:~$   rm sl_data.txt
 paul@laika:~$   rm hl_data.txt




                                            333
                                             file links



35.6. practice : links
 1. Create two files named winter.txt and summer.txt, put some text in them.

 2. Create a hard link to winter.txt named hlwinter.txt.

 3. Display the inode numbers of these three files, the hard links should have the same inode.

 4. Use the find command to list the two hardlinked files

 5. Everything about a file is in the inode, except two things : name them!

 6. Create a symbolic link to summer.txt called slsummer.txt.

 7. Find all files with inode number 2. What does this information tell you ?

 8. Look at the directories /etc/init.d/ /etc/rc2.d/ /etc/rc3.d/ ... do you see the links ?

 9. Look in /lib with ls -l...

 10. Use find to look in your home directory for regular files that do not(!) have one hard link.




                                               334
                                             file links



35.7. solution : links
 1. Create two files named winter.txt and summer.txt, put some text in them.
 echo cold > winter.txt ; echo hot > summer.txt

 2. Create a hard link to winter.txt named hlwinter.txt.
 ln winter.txt hlwinter.txt

 3. Display the inode numbers of these three files, the hard links should have the same inode.
 ls -li winter.txt summer.txt hlwinter.txt

 4. Use the find command to list the two hardlinked files
 find . -inum xyz #replace xyz with the inode number

 5. Everything about a file is in the inode, except two things : name them!

 The name of the file is in a directory, and the contents is somewhere on the disk.

 6. Create a symbolic link to summer.txt called slsummer.txt.
 ln -s summer.txt slsummer.txt

 7. Find all files with inode number 2. What does this information tell you ?

 It tells you there is more than one inode table (one for every formatted partition + virtual
 file systems)

 8. Look at the directories /etc/init.d/ /etc/rc.d/ /etc/rc3.d/ ... do you see the links ?
 ls -l /etc/init.d

 ls -l /etc/rc2.d

 ls -l /etc/rc3.d

 9. Look in /lib with ls -l...
 ls -l /lib

 10. Use find to look in your home directory for regular files that do not(!) have one hard link.
 find ~ ! -links 1 -type f




                                               335
Part X. Appendices
Table of Contents
 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




                                                                              337
Appendix A. keyboard settings
A.1. about keyboard layout
 Many people (like US-Americans) prefer the default US-qwerty keyboard layout. So when
 you are not from the USA and want a local keyboard layout on your system, then the best
 practice is to select this keyboard at installation time. Then the keyboard layout will always
 be correct. Also, whenever you use ssh to remotely manage a Linux system, your local
 keyboard layout will be used, independent of the server keyboard configuration. So you will
 not find much information on changing keyboard layout on the fly on linux, because not
 many people need it. Below are some tips to help you.

A.2. X Keyboard Layout
 This is the relevant portion in /etc/X11/xorg.conf, first for Belgian azerty, then for US-
 qwerty.

 [paul@RHEL5 ~]$ grep -i xkb /etc/X11/xorg.conf
         Option      "XkbModel" "pc105"
         Option      "XkbLayout" "be"


 [paul@RHEL5 ~]$ grep -i xkb /etc/X11/xorg.conf
         Option      "XkbModel" "pc105"
         Option      "XkbLayout" "us"

 When in Gnome or KDE or any other graphical environment, look in the graphical menu in
 preferences, there will be a keyboard section to choose your layout. Use the graphical menu
 instead of editing xorg.conf.

A.3. shell keyboard layout
 When in bash, take a look in the /etc/sysconfig/keyboard file. Below a sample US-qwerty
 configuration, followed by a Belgian azerty configuration.

 [paul@RHEL5 ~]$ cat /etc/sysconfig/keyboard
 KEYBOARDTYPE="pc"
 KEYTABLE="us"



 [paul@RHEL5 ~]$ cat /etc/sysconfig/keyboard
 KEYBOARDTYPE="pc"
 KEYTABLE="be-latin1"



 The keymaps themselves can be found in /usr/share/keymaps or /lib/kbd/keymaps.

 [paul@RHEL5 ~]$ ls -l /lib/kbd/keymaps/
 total 52
 drwxr-xr-x 2 root root 4096 Apr 1 00:14 amiga


                                            338
                                            keyboard settings


drwxr-xr-x   2   root   root   4096   Apr   1   00:14   atari
drwxr-xr-x   8   root   root   4096   Apr   1   00:14   i386
drwxr-xr-x   2   root   root   4096   Apr   1   00:14   include
drwxr-xr-x   4   root   root   4096   Apr   1   00:14   mac
lrwxrwxrwx   1   root   root      3   Apr   1   00:14   ppc -> mac
drwxr-xr-x   2   root   root   4096   Apr   1   00:14   sun




                                                  339
Appendix B. hardware
B.1. buses
B.1.1. about buses
 Hardware components communicate with the Central Processing Unit or cpu over a bus.
 The most common buses today are usb, pci, agp, pci-express and pcmcia aka pc-card.
 These are all Plag and Play buses.

 Older x86 computers often had isa buses, which can be configured using jumpers or dip
 switches.

B.1.2. /proc/bus
 To list the buses recognised by the Linux kernel on your computer, look at the contents of
 the /proc/bus/ directory (screenshot from Ubuntu 7.04 and RHEL4u4 below).

 root@laika:~# ls /proc/bus/
 input pccard pci usb



 [root@RHEL4b ~]# ls /proc/bus/
 input pci usb


 Can you guess which of these two screenshots was taken on a laptop ?

B.1.3. /usr/sbin/lsusb
 To list all the usb devices connected to your system, you could read the contents of /proc/
 bus/usb/devices (if it exists) or you could use the more readable output of lsusb, which is
 executed here on a SPARC system with Ubuntu.

 root@shaka:~# lsusb
 Bus 001 Device 002: ID 0430:0100 Sun Microsystems, Inc. 3-button Mouse
 Bus 001 Device 003: ID 0430:0005 Sun Microsystems, Inc. Type 6 Keyboard
 Bus 001 Device 001: ID 04b0:0136 Nikon Corp. Coolpix 7900 (storage)
 root@shaka:~#



B.1.4. /var/lib/usbutils/usb.ids
 The /var/lib/usbutils/usb.ids file contains a gzipped list of all known usb devices.

 paul@barry:~$ zmore /var/lib/usbutils/usb.ids | head
 ------> /var/lib/usbutils/usb.ids <------
 #
 # List of USB ID's
 #
 # Maintained by Vojtech Pavlik <vojtech@suse.cz>


                                            340
                                          hardware


 # If you have any new entries, send them to the maintainer.
 # The latest version can be obtained from
 # http://www.linux-usb.org/usb.ids
 #
 # $Id: usb.ids,v 1.225 2006/07/13 04:18:02 dbrownell Exp $



B.1.5. /usr/sbin/lspci
 To get a list of all pci devices connected, you could take a look at /proc/bus/pci or run lspci
 (partial output below).

 paul@laika:~$ lspci
 ...
 00:06.0 FireWire (IEEE 1394): Texas Instruments TSB43AB22/A IEEE-139...
 00:08.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL-816...
 00:09.0 Multimedia controller: Philips Semiconductors SAA7133/SAA713...
 00:0a.0 Network controller: RaLink RT2500 802.11g Cardbus/mini-PCI
 00:0f.0 RAID bus controller: VIA Technologies, Inc. VIA VT6420 SATA ...
 00:0f.1 IDE interface: VIA Technologies, Inc. VT82C586A/B/VT82C686/A...
 00:10.0 USB Controller: VIA Technologies, Inc. VT82xxxxx UHCI USB 1....
 00:10.1 USB Controller: VIA Technologies, Inc. VT82xxxxx UHCI USB 1....
 ...



B.2. interrupts
B.2.1. about interrupts
 An interrupt request or IRQ is a request from a device to the CPU. A device raises an
 interrupt when it requires the attention of the CPU (could be because the device has data
 ready to be read by the CPU).

 Since the introduction of pci, irq's can be shared among devices.

 Interrupt 0 is always reserved for the timer, interrupt 1 for the keyboard. IRQ 2 is used as a
 channel for IRQ's 8 to 15, and thus is the same as IRQ 9.

B.2.2. /proc/interrupts
 You can see a listing of interrupts on your system in /proc/interrupts.

 paul@laika:~$ cat /proc/interrupts
       CPU0     CPU1
 0: 1320048      555 IO-APIC-edge             timer
 1:    10224       7 IO-APIC-edge             i8042
 7:         0      0 IO-APIC-edge             parport0
 8:         2      1 IO-APIC-edge             rtc
 10:     3062     21 IO-APIC-fasteoi          acpi
 12:      131      2 IO-APIC-edge             i8042
 15:    47073      0 IO-APIC-edge             ide1
 18:         0     1 IO-APIC-fasteoi          yenta
 19:    31056      1 IO-APIC-fasteoi          libata, ohci1394
 20:    19042      1 IO-APIC-fasteoi          eth0
 21:    44052      1 IO-APIC-fasteoi          uhci_hcd:usb1, uhci_hcd:usb2,...
 22:   188352      1 IO-APIC-fasteoi          ra0


                                             341
                                          hardware


 23:   632444        1   IO-APIC-fasteoi      nvidia
 24:     1585        1   IO-APIC-fasteoi      VIA82XX-MODEM, VIA8237



B.2.3. dmesg
 You can also use dmesg to find irq's allocated at boot time.
 paul@laika:~$ dmesg | grep "irq 1[45]"
 [ 28.930069] ata3: PATA max UDMA/133 cmd 0x1f0 ctl 0x3f6 bmdma 0x2090 irq 14
 [ 28.930071] ata4: PATA max UDMA/133 cmd 0x170 ctl 0x376 bmdma 0x2098 irq 15



B.3. io ports
B.3.1. about io ports
 Communication in the other direction, from CPU to device, happens through IO ports. The
 CPU writes data or control codes to the IO port of the device. But this is not only a one way
 communication, the CPU can also use a device's IO port to read status information about the
 device. Unlike interrupts, ports cannot be shared!

B.3.2. /proc/ioports
 You can see a listing of your system's IO ports via /proc/ioports.

 [root@RHEL4b ~]# cat /proc/ioports
 0000-001f : dma1
 0020-0021 : pic1
 0040-0043 : timer0
 0050-0053 : timer1
 0060-006f : keyboard
 0070-0077 : rtc
 0080-008f : dma page reg
 00a0-00a1 : pic2
 00c0-00df : dma2
 00f0-00ff : fpu
 0170-0177 : ide1
 02f8-02ff : serial
 ...



B.4. dma
B.4.1. about dma
 A device that needs a lot of data, interrupts and ports can pose a heavy load on the cpu. With
 dma or Direct Memory Access a device can gain (temporary) access to a specific range
 of the ram memory.

B.4.2. /proc/dma
 Looking at /proc/dma might not give you the information that you want, since it only
 contains currently assigned dma channels for isa devices.


                                            342
                                        hardware



root@laika:~# cat /proc/dma
1: parport0
4: cascade



pci devices that are using dma are not listed in /proc/dma, in this case dmesg can be useful.
The screenshot below shows that during boot the parallel port received dma channel 1, and
the Infrared port received dma channel 3.

root@laika:~# dmesg | egrep -C 1 'dma 1|dma 3'
[   20.576000] parport: PnPBIOS parport detected.
[   20.580000] parport0: PC-style at 0x378 (0x778), irq 7, dma 1...
[   20.764000] irda_init()
--
[   21.204000] pnp: Device 00:0b activated.
[   21.204000] nsc_ircc_pnp_probe() : From PnP, found firbase 0x2F8...
[   21.204000] nsc-ircc, chip->init




                                           343
Appendix C. License
 GNU Free Documentation License

 Version 1.3, 3 November 2008

 Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc.

 Everyone is permitted to copy and distribute verbatim copies of this
 license document, but changing it is not allowed.

 0. PREAMBLE

 The purpose of this License is to make a manual, textbook, or other
 functional and useful document "free" in the sense of freedom: to
 assure everyone the effective freedom to copy and redistribute it,
 with or without modifying it, either commercially or noncommercially.
 Secondarily, this License preserves for the author and publisher a way
 to get credit for their work, while not being considered responsible
 for modifications made by others.

 This License is a kind of "copyleft", which means that derivative
 works of the document must themselves be free in the same sense. It
 complements the GNU General Public License, which is a copyleft
 license designed for free software.

 We have designed this License in order to use it for manuals for free
 software, because free software needs free documentation: a free
 program should come with manuals providing the same freedoms that the
 software does. But this License is not limited to software manuals; it
 can be used for any textual work, regardless of subject matter or
 whether it is published as a printed book. We recommend this License
 principally for works whose purpose is instruction or reference.

 1. APPLICABILITY AND DEFINITIONS

 This License applies to any manual or other work, in any medium, that
 contains a notice placed by the copyright holder saying it can be
 distributed under the terms of this License. Such a notice grants a
 world-wide, royalty-free license, unlimited in duration, to use that
 work under the conditions stated herein. The "Document", below, refers
 to any such manual or work. Any member of the public is a licensee,
 and is addressed as "you". You accept the license if you copy, modify
 or distribute the work in a way requiring permission under copyright
 law.

 A "Modified Version" of the Document means any work containing the
 Document or a portion of it, either copied verbatim, or with
 modifications and/or translated into another language.

 A "Secondary Section" is a named appendix or a front-matter section of
 the Document that deals exclusively with the relationship of the
 publishers or authors of the Document to the Document's overall
 subject (or to related matters) and contains nothing that could fall
 directly within that overall subject. (Thus, if the Document is in
 part a textbook of mathematics, a Secondary Section may not explain
 any mathematics.) The relationship could be a matter of historical
 connection with the subject or with related matters, or of legal,
 commercial, philosophical, ethical or political position regarding
 them.

 The "Invariant Sections" are certain Secondary Sections whose titles


                                      344
                                    License


are designated, as being those of Invariant Sections, in the notice
that says that the Document is released under this License. If a
section does not fit the above definition of Secondary then it is not
allowed to be designated as Invariant. The Document may contain zero
Invariant Sections. If the Document does not identify any Invariant
Sections then there are none.

The "Cover Texts" are certain short passages of text that are listed,
as Front-Cover Texts or Back-Cover Texts, in the notice that says that
the Document is released under this License. A Front-Cover Text may be
at most 5 words, and a Back-Cover Text may be at most 25 words.

A "Transparent" copy of the Document means a machine-readable copy,
represented in a format whose specification is available to the
general public, that is suitable for revising the document
straightforwardly with generic text editors or (for images composed of
pixels) generic paint programs or (for drawings) some widely available
drawing editor, and that is suitable for input to text formatters or
for automatic translation to a variety of formats suitable for input
to text formatters. A copy made in an otherwise Transparent file
format whose markup, or absence of markup, has been arranged to thwart
or discourage subsequent modification by readers is not Transparent.
An image format is not Transparent if used for any substantial amount
of text. A copy that is not "Transparent" is called "Opaque".

Examples of suitable formats for Transparent copies include plain
ASCII without markup, Texinfo input format, LaTeX input format, SGML
or XML using a publicly available DTD, and standard-conforming simple
HTML, PostScript or PDF designed for human modification. Examples of
transparent image formats include PNG, XCF and JPG. Opaque formats
include proprietary formats that can be read and edited only by
proprietary word processors, SGML or XML for which the DTD and/or
processing tools are not generally available, and the
machine-generated HTML, PostScript or PDF produced by some word
processors for output purposes only.

The "Title Page" means, for a printed book, the title page itself,
plus such following pages as are needed to hold, legibly, the material
this License requires to appear in the title page. For works in
formats which do not have any title page as such, "Title Page" means
the text near the most prominent appearance of the work's title,
preceding the beginning of the body of the text.

The "publisher" means any person or entity that distributes copies of
the Document to the public.

A section "Entitled XYZ" means a named subunit of the Document whose
title either is precisely XYZ or contains XYZ in parentheses following
text that translates XYZ in another language. (Here XYZ stands for a
specific section name mentioned below, such as "Acknowledgements",
"Dedications", "Endorsements", or "History".) To "Preserve the Title"
of such a section when you modify the Document means that it remains a
section "Entitled XYZ" according to this definition.

The Document may include Warranty Disclaimers next to the notice which
states that this License applies to the Document. These Warranty
Disclaimers are considered to be included by reference in this
License, but only as regards disclaiming warranties: any other
implication that these Warranty Disclaimers may have is void and has
no effect on the meaning of this License.

2. VERBATIM COPYING

You may copy and distribute the Document in any medium, either



                                     345
                                    License


commercially or noncommercially, provided that this License, the
copyright notices, and the license notice saying this License applies
to the Document are reproduced in all copies, and that you add no
other conditions whatsoever to those of this License. You may not use
technical measures to obstruct or control the reading or further
copying of the copies you make or distribute. However, you may accept
compensation in exchange for copies. If you distribute a large enough
number of copies you must also follow the conditions in section 3.

You may also lend copies, under the same conditions stated above, and
you may publicly display copies.

3. COPYING IN QUANTITY

If you publish printed copies (or copies in media that commonly have
printed covers) of the Document, numbering more than 100, and the
Document's license notice requires Cover Texts, you must enclose the
copies in covers that carry, clearly and legibly, all these Cover
Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on
the back cover. Both covers must also clearly and legibly identify you
as the publisher of these copies. The front cover must present the
full title with all words of the title equally prominent and visible.
You may add other material on the covers in addition. Copying with
changes limited to the covers, as long as they preserve the title of
the Document and satisfy these conditions, can be treated as verbatim
copying in other respects.

If the required texts for either cover are too voluminous to fit
legibly, you should put the first ones listed (as many as fit
reasonably) on the actual cover, and continue the rest onto adjacent
pages.

If you publish or distribute Opaque copies of the Document numbering
more than 100, you must either include a machine-readable Transparent
copy along with each Opaque copy, or state in or with each Opaque copy
a computer-network location from which the general network-using
public has access to download using public-standard network protocols
a complete Transparent copy of the Document, free of added material.
If you use the latter option, you must take reasonably prudent steps,
when you begin distribution of Opaque copies in quantity, to ensure
that this Transparent copy will remain thus accessible at the stated
location until at least one year after the last time you distribute an
Opaque copy (directly or through your agents or retailers) of that
edition to the public.

It is requested, but not required, that you contact the authors of the
Document well before redistributing any large number of copies, to
give them a chance to provide you with an updated version of the
Document.

4. MODIFICATIONS

You may copy and distribute a Modified Version of the Document under
the conditions of sections 2 and 3 above, provided that you release
the Modified Version under precisely this License, with the Modified
Version filling the role of the Document, thus licensing distribution
and modification of the Modified Version to whoever possesses a copy
of it. In addition, you must do these things in the Modified Version:

   * A. Use in the Title Page (and on the covers, if any) a title
distinct from that of the Document, and from those of previous
versions (which should, if there were any, be listed in the History
section of the Document). You may use the same title as a previous
version if the original publisher of that version gives permission.



                                     346
                                    License


   * B. List on the Title Page, as authors, one or more persons or
entities responsible for authorship of the modifications in the
Modified Version, together with at least five of the principal authors
of the Document (all of its principal authors, if it has fewer than
five), unless they release you from this requirement.
   * C. State on the Title page the name of the publisher of the
Modified Version, as the publisher.
   * D. Preserve all the copyright notices of the Document.
   * E. Add an appropriate copyright notice for your modifications
adjacent to the other copyright notices.
   * F. Include, immediately after the copyright notices, a license
notice giving the public permission to use the Modified Version under
the terms of this License, in the form shown in the Addendum below.
   * G. Preserve in that license notice the full lists of Invariant
Sections and required Cover Texts given in the Document's license
notice.
   * H. Include an unaltered copy of this License.
   * I. Preserve the section Entitled "History", Preserve its Title,
and add to it an item stating at least the title, year, new authors,
and publisher of the Modified Version as given on the Title Page. If
there is no section Entitled "History" in the Document, create one
stating the title, year, authors, and publisher of the Document as
given on its Title Page, then add an item describing the Modified
Version as stated in the previous sentence.
   * J. Preserve the network location, if any, given in the Document
for public access to a Transparent copy of the Document, and likewise
the network locations given in the Document for previous versions it
was based on. These may be placed in the "History" section. You may
omit a network location for a work that was published at least four
years before the Document itself, or if the original publisher of the
version it refers to gives permission.
   * K. For any section Entitled "Acknowledgements" or "Dedications",
Preserve the Title of the section, and preserve in the section all the
substance and tone of each of the contributor acknowledgements and/or
dedications given therein.
   * L. Preserve all the Invariant Sections of the Document,
unaltered in their text and in their titles. Section numbers or the
equivalent are not considered part of the section titles.
   * M. Delete any section Entitled "Endorsements". Such a section
may not be included in the Modified Version.
   * N. Do not retitle any existing section to be Entitled
"Endorsements" or to conflict in title with any Invariant Section.
   * O. Preserve any Warranty Disclaimers.

If the Modified Version includes new front-matter sections or
appendices that qualify as Secondary Sections and contain no material
copied from the Document, you may at your option designate some or all
of these sections as invariant. To do this, add their titles to the
list of Invariant Sections in the Modified Version's license notice.
These titles must be distinct from any other section titles.

You may add a section Entitled "Endorsements", provided it contains
nothing but endorsements of your Modified Version by various
parties—for example, statements of peer review or that the text has
been approved by an organization as the authoritative definition of a
standard.

You may add a passage of up to five words as a Front-Cover Text, and a
passage of up to 25 words as a Back-Cover Text, to the end of the list
of Cover Texts in the Modified Version. Only one passage of
Front-Cover Text and one of Back-Cover Text may be added by (or
through arrangements made by) any one entity. If the Document already
includes a cover text for the same cover, previously added by you or
by arrangement made by the same entity you are acting on behalf of,



                                     347
                                    License


you may not add another; but you may replace the old one, on explicit
permission from the previous publisher that added the old one.

The author(s) and publisher(s) of the Document do not by this License
give permission to use their names for publicity for or to assert or
imply endorsement of any Modified Version.

5. COMBINING DOCUMENTS

You may combine the Document with other documents released under this
License, under the terms defined in section 4 above for modified
versions, provided that you include in the combination all of the
Invariant Sections of all of the original documents, unmodified, and
list them all as Invariant Sections of your combined work in its
license notice, and that you preserve all their Warranty Disclaimers.

The combined work need only contain one copy of this License, and
multiple identical Invariant Sections may be replaced with a single
copy. If there are multiple Invariant Sections with the same name but
different contents, make the title of each such section unique by
adding at the end of it, in parentheses, the name of the original
author or publisher of that section if known, or else a unique number.
Make the same adjustment to the section titles in the list of
Invariant Sections in the license notice of the combined work.

In the combination, you must combine any sections Entitled "History"
in the various original documents, forming one section Entitled
"History"; likewise combine any sections Entitled "Acknowledgements",
and any sections Entitled "Dedications". You must delete all sections
Entitled "Endorsements".

6. COLLECTIONS OF DOCUMENTS

You may make a collection consisting of the Document and other
documents released under this License, and replace the individual
copies of this License in the various documents with a single copy
that is included in the collection, provided that you follow the rules
of this License for verbatim copying of each of the documents in all
other respects.

You may extract a single document from such a collection, and
distribute it individually under this License, provided you insert a
copy of this License into the extracted document, and follow this
License in all other respects regarding verbatim copying of that
document.

7. AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other separate
and independent documents or works, in or on a volume of a storage or
distribution medium, is called an "aggregate" if the copyright
resulting from the compilation is not used to limit the legal rights
of the compilation's users beyond what the individual works permit.
When the Document is included in an aggregate, this License does not
apply to the other works in the aggregate which are not themselves
derivative works of the Document.

If the Cover Text requirement of section 3 is applicable to these
copies of the Document, then if the Document is less than one half of
the entire aggregate, the Document's Cover Texts may be placed on
covers that bracket the Document within the aggregate, or the
electronic equivalent of covers if the Document is in electronic form.
Otherwise they must appear on printed covers that bracket the whole
aggregate.



                                        348
                                       License



8. TRANSLATION

Translation is considered a kind of modification, so you may
distribute translations of the Document under the terms of section 4.
Replacing Invariant Sections with translations requires special
permission from their copyright holders, but you may include
translations of some or all Invariant Sections in addition to the
original versions of these Invariant Sections. You may include a
translation of this License, and all the license notices in the
Document, and any Warranty Disclaimers, provided that you also include
the original English version of this License and the original versions
of those notices and disclaimers. In case of a disagreement between
the translation and the original version of this License or a notice
or disclaimer, the original version will prevail.

If a section in the Document is Entitled "Acknowledgements",
"Dedications", or "History", the requirement (section 4) to Preserve
its Title (section 1) will typically require changing the actual
title.

9. TERMINATION

You may not copy, modify, sublicense, or distribute the Document
except as expressly provided under this License. Any attempt otherwise
to copy, modify, sublicense, or distribute it is void, and will
automatically terminate your rights under this License.

However, if you cease all violation of this License, then your license
from a particular copyright holder is reinstated (a) provisionally,
unless and until the copyright holder explicitly and finally
terminates your license, and (b) permanently, if the copyright holder
fails to notify you of the violation by some reasonable means prior to
60 days after the cessation.

Moreover, your license from a particular copyright holder is
reinstated permanently if the copyright holder notifies you of the
violation by some reasonable means, this is the first time you have
received notice of violation of this License (for any work) from that
copyright holder, and you cure the violation prior to 30 days after
your receipt of the notice.

Termination of your rights under this section does not terminate the
licenses of parties who have received copies or rights from you under
this License. If your rights have been terminated and not permanently
reinstated, receipt of a copy of some or all of the same material does
not give you any rights to use it.

10. FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions of the
GNU Free Documentation License from time to time. Such new versions
will be similar in spirit to the present version, but may differ in
detail to address new problems or concerns. See
http://www.gnu.org/copyleft/.

Each version of the License is given a distinguishing version number.
If the Document specifies that a particular numbered version of this
License "or any later version" applies to it, you have the option of
following the terms and conditions either of that specified version or
of any later version that has been published (not as a draft) by the
Free Software Foundation. If the Document does not specify a version
number of this License, you may choose any version ever published (not
as a draft) by the Free Software Foundation. If the Document specifies



                                        349
                                    License


that a proxy can decide which future versions of this License can be
used, that proxy's public statement of acceptance of a version
permanently authorizes you to choose that version for the Document.

11. RELICENSING

"Massive Multiauthor Collaboration Site" (or "MMC Site") means any
World Wide Web server that publishes copyrightable works and also
provides prominent facilities for anybody to edit those works. A
public wiki that anybody can edit is an example of such a server. A
"Massive Multiauthor Collaboration" (or "MMC") contained in the site
means any set of copyrightable works thus published on the MMC site.

"CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0
license published by Creative Commons Corporation, a not-for-profit
corporation with a principal place of business in San Francisco,
California, as well as future copyleft versions of that license
published by that same organization.

"Incorporate" means to publish or republish a Document, in whole or in
part, as part of another Document.

An MMC is "eligible for relicensing" if it is licensed under this
License, and if all works that were first published under this License
somewhere other than this MMC, and subsequently incorporated in whole
or in part into the MMC, (1) had no cover texts or invariant sections,
and (2) were thus incorporated prior to November 1, 2008.

The operator of an MMC Site may republish an MMC contained in the site
under CC-BY-SA on the same site at any time before August 1, 2009,
provided the MMC is eligible for relicensing.




                                     350
                                              /lib32, 104
Index                                         /lib64, 104
                                              /media, 107
                                              /opt, 104
Symbols                                       /proc, 85, 109
; (shell), 136                                /proc/bus, 340
!! (shell), 156                               /proc/bus/pci, 341
! (bash history), 156                         /proc/bus/usb/devices, 340
! (file globbing), 163                        /proc/cpuinfo, 110
? (file globbing), 162                        /proc/dma, 342
/, 76, 102                                    /proc/interrupts, 112, 341
/bin, 103, 128                                /proc/ioports, 342
/bin/bash, 125, 292                           /proc/kcore, 112
/bin/cat, 103                                 /proc/sys, 111
/bin/csh, 125                                 /root, 107
/bin/date, 103                                /run, 117
/bin/ksh, 125, 292                            /sbin, 103, 128
/bin/rm, 129                                  /srv, 107
/bin/sh, 125                                  /sys, 113
/boot, 105                                    /tmp, 108, 320
/boot/grub, 105                               /usr, 114
/boot/grub/grub.cfg, 105                      /usr/bin, 114
/boot/grub/grub.conf, 105                     /usr/bin/getfacl, 326
/dev, 85, 109                                 /usr/bin/passwd, 321
/dev/null, 109, 175                           /usr/bin/setfacl, 326
/dev/pts/1, 109                               /usr/include, 114
/dev/random, 120                              /usr/lib, 114
/dev/tty1, 109                                /usr/local, 114
/dev/urandom, 119, 121                        /usr/share, 114
/dev/zero, 120                                /usr/share/games, 115
/etc, 105                                     /usr/share/man, 115
/etc/bashrc, 293                              /usr/src, 115
/etc/default/useradd, 276                     /var, 116
/etc/fstab, 326                               /var/cache, 116
/etc/group, 299, 308                          /var/lib, 117
/etc/gshadow, 301                             /var/lib/rpm, 117
/etc/hosts, 120                               /var/lib/usbutils/usb.ids, 340
/etc/init.d/, 105                             /var/lock, 117
/etc/inputrc, 292                             /var/log, 116
/etc/login.defs, 286                          /var/log/messages, 116
/etc/passwd, 191, 275, 278, 287, 287, 308     /var/log/syslog, 116
/etc/profile, 292                             /var/run, 117
/etc/resolv.conf, 120                         /var/spool, 116
/etc/shadow, 283, 285, 321                    /var/tmp, 117
/etc/shells, 235, 278                         ., 75
/etc/skel, 105, 277                           .., 75
/etc/sudoers, 269, 270                        .. (directory), 331
/etc/sysconfig, 105                           . (directory), 331
/etc/sysconfig/firstboot, 106                 . (shell), 236
/etc/sysconfig/harddisks, 106                 .bash_history, 157
/etc/sysconfig/hwconf, 106                    .bash_login, 293
/etc/sysconfig/keyboard, 106                  .bash_logout, 294
/etc/X11/xorg.conf, 105                       .bash_profile, 292
/export, 107                                  .bashrc, 292, 293
/home, 107                                    .exrc, 229
/lib, 104                                     .vimrc, 229
/lib/kbd/keymaps/, 106                        `(backtick), 151
/lib/modules, 104                             ~, 75


                                            351
                            Index


'(single quote), 151           C
" (double quotes), 127
                               cal, 198
(( (shell), 256
                               case, 258
-- (shell), 237
                               case sensitive, 85
[ (file globbing), 163
                               cat, 96, 182
[ (shell), 241
                               cd, 75
$? (shell variables), 136
                               cd -, 76
$() embedded shell, 151
                               CentOS, 7
$ (shell variables), 142
                               chage, 286
$HISTFILE, 157
                               chgrp(1), 309
$HISTFILESIZE, 157
                               chkconfig, 106
$HISTSIZE, 157
                               chmod, 277, 314
$LANG, 164
                               chmod(1), 226, 313
$PATH, 128, 145
                               chmod +x, 235, 315
$PS1, 76
                               chown, 277
* (file globbing), 162
                               chown(1), 309
\ (backslash), 138
                               chsh(1), 278
&, 136
                               comm(1), 188
&&, 137
                               command line scan, 126
#!/bin/bash, 235
                               command mode(vi), 223
#! (shell), 235
                               copyleft, 11
# (pound sign), 138
                               copyright, 10, 10
>, 173
                               cp, 88
>>, 174
                               cp(1), 88
>|, 174
                               cpu, 340
||, 137
                               crypt, 284
1>, 175
                               csh, 235
2>, 175
                               Ctrl d, 96
2>&1, 175
                               ctrl-r, 157
777, 314
                               current directory, 75
                               cut, 191
A                              cut(1), 184
access control list, 326
acl, 328                       D
acls, 326                      daemon, 72
agp, 340                       date, 197
AIX, 4                         Debian, 7
alias(bash), 129               Dennis Ritchie, 4
alias(shell), 129              devfs, 113
apropos, 72                    df -i, 330
arguments(shell), 126          directory, 331
                               distribution, 6
B                              distributions, 102
backticks, 151                 dma, 342
base64, 177                    dmesg(1), 342, 343
bash, 219, 248                 dumpkeys(1), 106
bash history, 156
bash -x, 237                   E
binaries, 103                  echo, 126
Bourne again shell, 125        echo(1), 125, 127
BSD, 4                         echo $-, 152
bunzip2, 201                   echo *, 165
bus, 340                       Edubuntu, 7
bzcat, 201                     eiciel, 328
bzip2, 199, 201, 201           ELF, 104
bzmore, 201                    elif, 242
                               embedding(shell), 151


                            352
                                     Index


env(1), 146, 146                        hidden files, 77
environment variable, 142               HP, 4
EOF, 96, 177                            HP-UX, 4
escaping (shell), 165                   http://www.pathname.com/fhs/, 102
eval, 256
executables, 103                        I
exit (bash), 157                        IBM, 4
export, 146                             id, 267
                                        IEEE 1394, 113
F                                       if then else (bash), 242
Fedora, 7                               inode, 329, 332
FHS, 102                                inode table, 330
file, 85                                insert mode(vi), 223
file(1), 104                            interrupt, 341
file globbing, 161                      IO Ports, 342
file ownership, 308                     IRQ, 341
Filesystem Hierarchy Standard, 102      isa, 340
filters, 181
find(1), 196, 320, 321, 332             K
FireWire, 113                           Ken Thompson, 4
for (bash), 242                         kernel, 104
FOSS, 10                                keymaps(5), 106
four freedoms, 11                       Korn shell, 158
Free Software, 10                       Korn Shell, 278
free software, 10                       ksh, 158, 235
freeware, 10                            kudzu, 106
function (shell), 259
                                        L
G                                       less(1), 98
gcc(1), 285                             let, 257
getfacl, 326                            Linus Torvalds, 4
getopts, 251                            Linux Mint, 7
GID, 299                                ln, 333
glob(7), 162                            ln(1), 332
GNU, 4                                  loadkeys(1), 106
gpasswd, 301                            locate(1), 197
GPL, 11                                 logical AND, 137
GPLv3, 11                               logical OR, 137
grep, 206, 207, 210                     Logiciel Libre, 10
grep(1), 182                            ls, 77, 311, 330
grep -i, 182                            ls(1), 77, 330, 331
grep -v, 183                            ls -l, 310
groupadd(1), 299                        lspci, 341
groupdel(1), 300                        lsusb, 340
groupmod(1), 300
groups, 298                             M
groups(1), 299                          magic, 85
gunzip(1), 200                          makewhatis, 73
gzip, 200                               man(1), 72, 72, 73
gzip(1), 200                            mandb(1), 73
                                        man hier, 102
H                                       man -k, 72
hard link, 332                          md5, 285
head(1), 95                             mkdir, 277
here directive, 97                      mkdir(1), 79, 315
here document, 177                      mkdir -p, 79
here string, 177                        mkfs, 330


                                     353
                                Index


more(1), 98                        S
mv, 89
                                   salt (encryption), 285
                                   Scientific, 7
N                                  sed, 190, 215, 216
noclobber, 174                     set, 152
nounset(shell), 147                set(shell), 143
                                   set +x, 130
O                                  setfacl, 326
                                   setgid, 320, 320
octal permissions, 314
                                   setuid, 237, 321, 321, 321
od(1), 189
                                   set -x, 130
OEL, 7
                                   she-bang (shell), 235
open source, 10
                                   shell, 291
open source definition, 11
                                   shell comment, 138
open source software, 10
                                   shell embedding, 151
openssl, 284
                                   shell escaping, 138
Oracle Enterprise Linux, 7
                                   shell expansion, 126, 126
owner, 311
                                   shell functions, 259
                                   shift, 249
P                                  shopt, 252
parent directory, 75               skeleton, 105
passwd, 283, 283, 284, 286         sleep, 198
passwd(1), 73, 321                 soft link, 333
passwd(5), 73                      Solaris, 4
path, 76, 77                       sort, 191
pc-card, 340                       sort(1), 186
pci, 340                           source, 236, 250
pci-express, 340                   standard input, 96
pcmcia, 340                        standard output, 96
perl, 212                          stderr, 172
perldoc, 212                       stdin, 172, 182
popd, 83                           stdout, 172, 182
prename, 212                       sticky bit, 320
primary group, 276                 strings(1), 98
proprietary, 10                    su, 268, 268, 287, 301
public domain, 10                  su -, 145
pushd, 83                          sudo, 269, 270, 287
pwd, 75                            sudo su -, 270
pwd(1), 76                         Sun, 4
                                   SunOS, 4
R                                  superuser, 275
random number generator, 120       symbolic link, 333
read, 249                          sysfs, 113
reboot, 157                        System V, 104
Red Hat, 7
regular expressions, 158           T
rename, 90, 212, 213, 214          tab key(bash), 77
repository, 6                      tac, 97
Richard Stallman, 4                tail(1), 95
rm, 87                             tee(1), 182
rm(1), 333                         test, 241
rmdir(1), 79                       time, 199
rmdir -p, 80                       touch(1), 86
rm -rf, 87                         tr, 185
root, 103, 268, 269, 270, 275      tr(1), 184
root directory, 102                type(shell), 128
rpm, 117



                                354
                          Index


U
Ubuntu, 7
umask(1), 315
unalias(bash), 130
uniq, 191
uniq(1), 187
Unix, 4
unset, 152
unset(shell), 143
until (bash), 243
updatedb(1), 197
usb, 113, 340
useradd, 276, 277, 284
useradd(1), 277
useradd -D, 276
userdel(1), 276
usermod, 287, 287, 300
usermod(1), 276

V
vi, 302
vi(1), 222
vigr(1), 302
vim(1), 222
vimtutor(1), 222
vipw, 287
visudo, 269
vrije software, 10

W
w, 267
wc(1), 185
whatis(1), 72
whereis(1), 72
which(1), 128
while (bash), 243
white space(shell), 126
who, 191, 267
whoami, 267
who am i, 267
wild cards, 163

X
X, 105
X Window System, 105

Z
zcat, 200
zmore, 200




                          355