Bash-Beginners-Guide.pdf

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Bash Guide for Beginners
Bash Guide for Beginners
Machtelt Garrels
Xalasys.com
Version 1.6 Last updated 20050301 Edition
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Bash Guide for Beginners
Table of Contents
Introduction. ........................................................................................................................................................1
1. Why this guide?. ..................................................................................................................................1
2. Who should read this book?. ................................................................................................................1
3. New versions and availability. .............................................................................................................2
4. Revision History. .................................................................................................................................2
5. Contributions. ......................................................................................................................................3
6. Feedback. .............................................................................................................................................3
7. Copyright information .........................................................................................................................3
8. What do you need?. ..............................................................................................................................4
9. Conventions used in this document. ....................................................................................................4
10. Organization of this documen. ..........................................................................................................4
Chapter 1. Bash and Bash scripts. .....................................................................................................................6
1.1. Common shell programs. ..................................................................................................................6
1.1.1. General shell functions. ...........................................................................................................6
1.1.2. Shell types. ..............................................................................................................................6
1.2. Advantages of the Bourne Again SHel. ...........................................................................................7
1.2.1. Bash is the GNU shell. ............................................................................................................7
1.2.2. Features only found in bash.. ...................................................................................................7
1.3. Executing commands. .....................................................................................................................12
1.3.1. General. .................................................................................................................................12
1.3.2. Shell built−in commands. ......................................................................................................12
1.3.3. Executing programs from a script. ........................................................................................13
1.4. Building blocks. ..............................................................................................................................13
1.4.1. Shell building blocks. ............................................................................................................13
1.5. Developing good scripts. ................................................................................................................15
1.5.1. Properties of good scripts. .....................................................................................................15
1.5.2. Structure. ...............................................................................................................................16
1.5.3. Terminology. .........................................................................................................................16
1.5.4. A word on order and logic. ....................................................................................................16
1.5.5. An example Bash script: mysystem.sh. .................................................................................17
1.5.6. Example init scrip. ................................................................................................................18
1.6. Summary. ........................................................................................................................................19
1.7. Exercises. ........................................................................................................................................19
Chapter 2. Writing and debugging scripts. ....................................................................................................21
2.1. Creating and running a script. .........................................................................................................21
2.1.1. Writing and naming. ..............................................................................................................21
2.1.2. script1.sh. ...............................................................................................................................21
2.1.3. Executing the script. ..............................................................................................................22
2.2. Script basics. ...................................................................................................................................24
2.2.1. Which shell will run the script?. ............................................................................................24
2.2.2. Adding comments.. ................................................................................................................24
2.3. Debugging Bash scripts. .................................................................................................................25
2.3.1. Debugging on the entire script. .............................................................................................25
2.3.2. Debugging on part(s) of the scrip. ........................................................................................26
2.4. Summary. ........................................................................................................................................28
2.5. Exercises. ........................................................................................................................................28
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Bash Guide for Beginners
Table of Contents
Chapter 3. The Bash environment. .................................................................................................................29
3.1. Shell initialization files. ..................................................................................................................29
3.1.1. System−wide configuration files. ..........................................................................................29
3.1.2. Individual user configuration files. .......................................................................................31
3.1.3. Changing shell configuration files. .......................................................................................33
3.2. Variables. ........................................................................................................................................34
3.2.1. Types of variables. ................................................................................................................34
3.2.2. Creating variables. .................................................................................................................37
3.2.3. Exporting variables. ...............................................................................................................38
3.2.4. Reserved variables. ................................................................................................................39
3.2.5. Special parameters. ................................................................................................................41
3.2.6. Script recycling with variables. .............................................................................................43
3.3. Quoting characters. .........................................................................................................................44
3.3.1. Why?. ....................................................................................................................................44
3.3.2. Escape characters. .................................................................................................................45
3.3.3. Single quotes. ........................................................................................................................45
3.3.4. Double quotes. .......................................................................................................................45
3.3.5. ANSI−C quoting ...................................................................................................................46
3.3.6. Locales.. .................................................................................................................................46
3.4. Shell expansion. ..............................................................................................................................46
3.4.1. General. .................................................................................................................................46
3.4.2. Brace expansion. ...................................................................................................................46
3.4.3. Tilde expansion. ....................................................................................................................47
3.4.4. Shell parameter and variable expansion. ...............................................................................47
3.4.5. Command substitution. ..........................................................................................................48
3.4.6. Arithmetic expansion. ...........................................................................................................49
3.4.7. Process substitution. ..............................................................................................................50
3.4.8. Word splitting. .......................................................................................................................50
3.4.9. File name expansion. .............................................................................................................51
3.5. Aliases. ............................................................................................................................................51
3.5.1. What are aliases?. ..................................................................................................................51
3.5.2. Creating and removing aliases. .............................................................................................52
3.6. More Bash options. .........................................................................................................................53
3.6.1. Displaying options. ................................................................................................................53
3.6.2. Changing options. ..................................................................................................................54
3.7. Summary. ........................................................................................................................................55
3.8. Exercises. ........................................................................................................................................55
Chapter 4. Regular expressions. ......................................................................................................................56
4.1. Regular expressions. .......................................................................................................................56
4.1.1. What are regular expressions?. ..............................................................................................56
4.1.2. Regular expression metacharacters. ......................................................................................56
4.1.3. Basic versus extended regular expressions ...........................................................................57
4.2. Examples using grep. ......................................................................................................................57
4.2.1. What is grep?. ........................................................................................................................57
4.2.2. Grep and regular expressions. ...............................................................................................58
4.3. Pattern matching using Bash features. ............................................................................................60
4.3.1. Character ranges. ...................................................................................................................60
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Table of Contents
4.3.2. Character classes. ..................................................................................................................60
4.4. Summary. ........................................................................................................................................61
4.5. Exercises. ........................................................................................................................................61
Chapter 5. The GNU sed stream editor. .........................................................................................................62
5.1. Introduction. ....................................................................................................................................62
5.1.1. What is sed?.. .........................................................................................................................62
5.1.2. sed commands. ......................................................................................................................62
5.2. Interactive editing. ..........................................................................................................................63
5.2.1. Printing lines containing a pattern. ........................................................................................63
5.2.2. Deleting lines of input containing a pattern. .........................................................................64
5.2.3. Ranges of lines. .....................................................................................................................64
5.2.4. Find and replace with sed. .....................................................................................................65
5.3. Non−interactive editing. .................................................................................................................66
5.3.1. Reading sed commands from a file. ......................................................................................66
5.3.2. Writing output files ...............................................................................................................66
5.4. Summary. ........................................................................................................................................67
5.5. Exercises. ........................................................................................................................................68
Chapter 6. The GNU awk programming language. .......................................................................................69
6.1. Getting started with gawk. ..............................................................................................................69
6.1.1. What is gawk?. ......................................................................................................................69
6.1.2. Gawk commands. ..................................................................................................................69
6.2. The print program. ..........................................................................................................................70
6.2.1. Printing selected fields. .........................................................................................................70
6.2.2. Formatting fields. ..................................................................................................................71
6.2.3. The print command and regular expressions. ........................................................................72
6.2.4. Special patterns.. ....................................................................................................................72
6.2.5. Gawk scripts. .........................................................................................................................73
6.3. Gawk variables. ...............................................................................................................................73
6.3.1. The input field separato. .......................................................................................................73
6.3.2. The output separators. ...........................................................................................................74
6.3.3. The number of records. .........................................................................................................75
6.3.4. User defined variables. ..........................................................................................................75
6.3.5. More examples. .....................................................................................................................76
6.3.6. The printf program. ...............................................................................................................76
6.4. Summary. ........................................................................................................................................77
6.5. Exercises. ........................................................................................................................................77
Chapter 7. Conditional statements. .................................................................................................................79
7.1. Introduction to if. ............................................................................................................................79
7.1.1. General. .................................................................................................................................79
7.1.2. Simple applications of i.. ......................................................................................................82
7.2. More advanced if usage. .................................................................................................................84
7.2.1. if/then/else constructs. ...........................................................................................................84
7.2.2. if/then/elif/else constructs.. ....................................................................................................87
7.2.3. Nested if statements.. .............................................................................................................87
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7.2.4. Boolean operations. ...............................................................................................................88
7.2.5. Using the exit statement and if. .............................................................................................89
7.3. Using case statements. ....................................................................................................................90
7.3.1. Simplified conditions. ...........................................................................................................90
7.3.2. Initscript example. .................................................................................................................91
7.4. Summary. ........................................................................................................................................92
7.5. Exercises. ........................................................................................................................................92
Chapter 8. Writing interactive scripts. ...........................................................................................................94
8.1. Displaying user messages. ..............................................................................................................94
8.1.1. Interactive or not?. .................................................................................................................94
8.1.2. Using the echo built−in command ........................................................................................94
8.2. Catching user input. ........................................................................................................................97
8.2.1. Using the read built−in command. ........................................................................................97
8.2.2. Prompting for user inpu. .......................................................................................................98
8.2.3. Redirection and file descriptors. ............................................................................................99
8.2.4. File input and outpu.. ..........................................................................................................101
8.3. Summary. ......................................................................................................................................105
8.4. Exercises. ......................................................................................................................................106
Chapter 9. Repetitive tasks. ...........................................................................................................................107
9.1. The for loop. ..................................................................................................................................107
9.1.1. How does it work?. ..............................................................................................................107
9.1.2. Examples. ............................................................................................................................107
9.2. The while loop. .............................................................................................................................108
9.2.1. What is it?. ...........................................................................................................................108
9.2.2. Examples. ............................................................................................................................108
9.3. The until loop. ...............................................................................................................................111
9.3.1. What is it?. ...........................................................................................................................111
9.3.2. Example. ..............................................................................................................................111
9.4. I/0 redirection and loops. ..............................................................................................................112
9.4.1. Input redirection. .................................................................................................................112
9.4.2. Output redirection.. ..............................................................................................................112
9.5. Break and continue. ......................................................................................................................113
9.5.1. The break built−in. ..............................................................................................................113
9.5.2. The continue built−in. .........................................................................................................114
9.5.3. Examples. ............................................................................................................................114
9.6. Making menus with the select built−in. ........................................................................................115
9.6.1. General. ...............................................................................................................................115
9.6.2. Submenus. ...........................................................................................................................117
9.7. The shift built−in ..........................................................................................................................117
9.7.1. What does it do?. .................................................................................................................117
9.7.2. Examples. ............................................................................................................................117
9.8. Summary. ......................................................................................................................................118
9.9. Exercises. ......................................................................................................................................119
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