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Advanced Bash-Scripting Guide
An in-depth exploration of the art of shell scripting
Mendel Cooper
<thegrendel.abs@gmail.com>
10
10 Mar 2014
Revision History
Revision 6.5
05 Apr 2012
Revised by: mc
'TUNGSTENBERRY' release
Revision 6.6
27 Nov 2012
Revised by: mc
'YTTERBIUMBERRY' release
Revision 10
10 Mar 2014
Revised by: mc
'PUBLICDOMAIN' release
This tutorial assumes no previous knowledge of scripting or programming, yet progresses rapidly toward an
intermediate/advanced level of instruction . . . all the while sneaking in little nuggets of UNIX® wisdom and
lore. It serves as a textbook, a manual for self-study, and as a reference and source of knowledge on shell
scripting techniques. The exercises and heavily-commented examples invite active reader participation, under
the premise that the only way to really learn scripting is to write scripts.
This book is suitable for classroom use as a general introduction to programming concepts.
This document is herewith granted to the Public Domain. No copyright!
Dedication
For Anita, the source of all the magic
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Table of Contents
Chapter 1. Shell Programming!.........................................................................................................................1
Chapter 2. Starting Off With a Sha-Bang........................................................................................................3
2.1. Invoking the script............................................................................................................................6
2.2. Preliminary Exercises.......................................................................................................................6
Part 2. Basics.......................................................................................................................................................7
Chapter 3. Special Characters...........................................................................................................................8
Chapter 4. Introduction to Variables and Parameters..................................................................................30
4.1. Variable Substitution......................................................................................................................30
4.2. Variable Assignment.......................................................................................................................33
4.3. Bash Variables Are Untyped..........................................................................................................34
4.4. Special Variable Types...................................................................................................................35
Chapter 5. Quoting...........................................................................................................................................41
5.1. Quoting Variables...........................................................................................................................41
5.2. Escaping..........................................................................................................................................43
Chapter 6. Exit and Exit Status.......................................................................................................................51
Chapter 7. Tests................................................................................................................................................54
7.1. Test Constructs...............................................................................................................................54
7.2. File test operators............................................................................................................................62
7.3. Other Comparison Operators..........................................................................................................65
7.4. Nestedif/then Condition Tests.......................................................................................................70
7.5. Testing Your Knowledge of Tests..................................................................................................71
Chapter 8. Operations and Related Topics....................................................................................................72
8.1. Operators.........................................................................................................................................72
8.2. Numerical Constants.......................................................................................................................78
8.3. The Double-Parentheses Construct.................................................................................................80
8.4. Operator Precedence.......................................................................................................................81
Part 3. Beyond the Basics.................................................................................................................................84
Chapter 9. Another Look at Variables...........................................................................................................85
9.1. Internal Variables............................................................................................................................85
9.2. Typing variables: declare or typeset.............................................................................................104
9.2.1. Another use for declare.......................................................................................................107
9.3. $RANDOM: generate random integer..........................................................................................107
Chapter 10. Manipulating Variables.............................................................................................................119
10.1. Manipulating Strings...................................................................................................................119
10.1.1. Manipulating strings using awk........................................................................................127
10.1.2. Further Reference..............................................................................................................127
10.2. Parameter Substitution................................................................................................................128
Advanced Bash-Scripting Guide
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Table of Contents
Chapter 11. Loops and Branches..................................................................................................................138
11.1. Loops..........................................................................................................................................138
11.2. Nested Loops..............................................................................................................................152
11.3. Loop Control...............................................................................................................................153
11.4. Testing and Branching................................................................................................................156
Chapter 12. Command Substitution.............................................................................................................165
Chapter 13. Arithmetic Expansion................................................................................................................171
Chapter 14. Recess Time................................................................................................................................172
Part 4. Commands..........................................................................................................................................173
Chapter 15. Internal Commands and Builtins.............................................................................................181
15.1. Job Control Commands..............................................................................................................210
Chapter 16. External Filters, Programs and Commands...........................................................................215
16.1. Basic Commands........................................................................................................................215
16.2. Complex Commands...................................................................................................................221
16.3. Time / Date Commands..............................................................................................................231
16.4. Text Processing Commands........................................................................................................235
16.5. File and Archiving Commands...................................................................................................258
16.6. Communications Commands......................................................................................................276
16.7. Terminal Control Commands.....................................................................................................291
16.8. Math Commands.........................................................................................................................292
16.9. Miscellaneous Commands..........................................................................................................303
Chapter 17. System and Administrative Commands..................................................................................318
17.1. Analyzing a System Script..........................................................................................................349
Part 5. Advanced Topics.................................................................................................................................351
Chapter 18. Regular Expressions..................................................................................................................353
18.1. A Brief Introduction to Regular Expressions..............................................................................353
18.2. Globbing.....................................................................................................................................357
Chapter 19. Here Documents.........................................................................................................................359
19.1. Here Strings................................................................................................................................369
Chapter 20. I/O Redirection...........................................................................................................................373
20.1. Using exec...................................................................................................................................376
20.2. Redirecting Code Blocks............................................................................................................379
20.3. Applications................................................................................................................................384
Chapter 21. Subshells.....................................................................................................................................386
Advanced Bash-Scripting Guide
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Table of Contents
Chapter 22. Restricted Shells.........................................................................................................................391
Chapter 23. Process Substitution...................................................................................................................393
Chapter 24. Functions....................................................................................................................................398
24.1. Complex Functions and Function Complexities.........................................................................402
24.2. Local Variables...........................................................................................................................413
24.2.1. Local variables and recursion............................................................................................414
24.3. Recursion Without Local Variables............................................................................................417
Chapter 25. Aliases.........................................................................................................................................420
Chapter 26. List Constructs...........................................................................................................................423
Chapter 27. Arrays.........................................................................................................................................427
Chapter 28. Indirect References....................................................................................................................456
Chapter 29./dev and/proc.............................................................................................................................460
29.1./dev..............................................................................................................................................460
29.2./proc............................................................................................................................................463
Chapter 30. Network Programming.............................................................................................................469
Chapter 31. Of Zeros and Nulls.....................................................................................................................472
Chapter 32. Debugging...................................................................................................................................476
Chapter 33. Options........................................................................................................................................487
Chapter 34. Gotchas.......................................................................................................................................490
Chapter 35. Scripting With Style..................................................................................................................499
35.1. Unofficial Shell Scripting Stylesheet..........................................................................................499
Chapter 36. Miscellany...................................................................................................................................502
36.1. Interactive and non-interactive shells and scripts.......................................................................502
36.2. Shell Wrappers............................................................................................................................503
36.3. Tests and Comparisons: Alternatives..........................................................................................509
36.4. Recursion: a script calling itself..................................................................................................509
36.5. "Colorizing" Scripts....................................................................................................................512
36.6. Optimizations..............................................................................................................................525
36.7. Assorted Tips..............................................................................................................................528
36.7.1. Ideas for more powerful scripts.........................................................................................528
36.7.2. Widgets..............................................................................................................................539
36.8. Security Issues............................................................................................................................541
36.8.1. Infected Shell Scripts.........................................................................................................541
36.8.2. Hiding Shell Script Source................................................................................................541
Advanced Bash-Scripting Guide
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Table of Contents
Chapter 36. Miscellany
36.8.3. Writing Secure Shell Scripts.............................................................................................541
36.9. Portability Issues.........................................................................................................................541
36.9.1. A Test Suite.......................................................................................................................542
36.10. Shell Scripting Under Windows...............................................................................................543
Chapter 37. Bash, versions 2, 3, and 4..........................................................................................................544
37.1. Bash, version 2............................................................................................................................544
37.2. Bash, version 3............................................................................................................................548
37.2.1. Bash, version 3.1...............................................................................................................551
37.2.2. Bash, version 3.2...............................................................................................................552
37.3. Bash, version 4............................................................................................................................552
37.3.1. Bash, version 4.1...............................................................................................................559
37.3.2. Bash, version 4.2...............................................................................................................560
Chapter 38. Endnotes.....................................................................................................................................564
38.1. Author's Note..............................................................................................................................564
38.2. About the Author........................................................................................................................564
38.3. Where to Go For Help.................................................................................................................565
38.4. Tools Used to Produce This Book..............................................................................................565
38.4.1. Hardware...........................................................................................................................565
38.4.2. Software and Printware.....................................................................................................565
38.5. Credits.........................................................................................................................................566
38.6. Disclaimer...................................................................................................................................567
Bibliography....................................................................................................................................................569
Appendix A. Contributed Scripts..................................................................................................................577
Appendix B. Reference Cards........................................................................................................................787
Appendix C. A Sed and Awk Micro-Primer................................................................................................792
C.1. Sed................................................................................................................................................792
C.2. Awk..............................................................................................................................................795
Appendix D. Parsing and Managing Pathnames.........................................................................................798
Appendix E. Exit Codes With Special Meanings.........................................................................................802
Appendix F. A Detailed Introduction to I/O and I/O Redirection.............................................................803
Appendix G. Command-Line Options..........................................................................................................805
G.1. Standard Command-Line Options...............................................................................................805
G.2. Bash Command-Line Options......................................................................................................806
Appendix H. Important Files.........................................................................................................................808
Advanced Bash-Scripting Guide
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Table of Contents
Appendix I. Important System Directories..................................................................................................809
Appendix J. An Introduction to Programmable Completion.....................................................................811
Appendix K. Localization...............................................................................................................................814
Appendix L. History Commands...................................................................................................................818
Appendix M. Sample.bashrc and.bash_profile Files.................................................................................820
Appendix N. Converting DOS Batch Files to Shell Scripts.........................................................................837
Appendix O. Exercises....................................................................................................................................841
O.1. Analyzing Scripts.........................................................................................................................841
O.2. Writing Scripts.............................................................................................................................843
Appendix P. Revision History........................................................................................................................853
Appendix Q. Download and Mirror Sites.....................................................................................................856
Appendix R. To Do List..................................................................................................................................857
Appendix S. Copyright...................................................................................................................................858
Appendix T. ASCII Table..............................................................................................................................860
Index....................................................................................................................................................862
Notes..............................................................................................................................................899
Advanced Bash-Scripting Guide
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Chapter 1. Shell Programming!
No programming language is perfect. There is
not even a single best language; there are only
languages well suited or perhaps poorly suited
for particular purposes.
--Herbert Mayer
A working knowledge of shell scripting is essential to anyone wishing to become reasonably proficient at
system administration, even if they do not anticipate ever having to actually write a script. Consider that as a
Linux machine boots up, it executes the shell scripts in /etc/rc.d to restore the system configuration and
set up services. A detailed understanding of these startup scripts is important for analyzing the behavior of a
system, and possibly modifying it.
The craft of scripting is not hard to master, since scripts can be built in bite-sized sections and there is only a
fairly small set of shell-specific operators and options [1] to learn. The syntax is simple -- even austere --
similar to that of invoking and chaining together utilities at the command line, and there are only a few "rules"
governing their use. Most short scripts work right the first time, and debugging even the longer ones is
straightforward.
In the early days of personal computing, the BASIC language enabled
anyone reasonably computer proficient to write programs on an early
generation of microcomputers. Decades later, the Bash scripting
language enables anyone with a rudimentary knowledge of Linux or
UNIX to do the same on modern machines.
We now have miniaturized single-board computers with amazing
capabilities, such as the Raspberry Pi.
Bash scripting provides a way to explore the capabilities of these
fascinating devices.
A shell script is a quick-and-dirty method of prototyping a complex application. Getting even a limited subset
of the functionality to work in a script is often a useful first stage in project development. In this way, the
structure of the application can be tested and tinkered with, and the major pitfalls found before proceeding to
the final coding in C, C++, Java, Perl, or Python.
Shell scripting hearkens back to the classic UNIX philosophy of breaking complex projects into simpler
subtasks, of chaining together components and utilities. Many consider this a better, or at least more
esthetically pleasing approach to problem solving than using one of the new generation of high-powered
all-in-one languages, such as Perl, which attempt to be all things to all people, but at the cost of forcing you to
alter your thinking processes to fit the tool.
According to Herbert Mayer, "a useful language needs arrays, pointers, and a generic mechanism for building
data structures." By these criteria, shell scripting falls somewhat short of being "useful." Or, perhaps not. . . .
When not to use shell scripts
Chapter 1. Shell Programming!
1
Resource-intensive tasks, especially where speed is a factor (sorting, hashing, recursion [2] ...)
• 
Procedures involving heavy-duty math operations, especially floating point arithmetic, arbitrary
precision calculations, or complex numbers (use C++ or FORTRAN instead)
• 
Cross-platform portability required (use C or Java instead)
• 
Complex applications, where structured programming is a necessity (type-checking of variables,
function prototypes, etc.)
• 
Mission-critical applications upon which you are betting the future of the company
• 
Situations where security is important, where you need to guarantee the integrity of your system and
protect against intrusion, cracking, and vandalism
• 
Project consists of subcomponents with interlocking dependencies
• 
Extensive file operations required (Bash is limited to serial file access, and that only in a
particularly clumsy and inefficient line-by-line fashion.)
• 
Need native support for multi-dimensional arrays
• 
Need data structures, such as linked lists or trees
• 
Need to generate / manipulate graphics or GUIs
• 
Need direct access to system hardware or external peripherals
• 
Need port or socket I/O
• 
Need to use libraries or interface with legacy code
• 
Proprietary, closed-source applications (Shell scripts put the source code right out in the open for all
the world to see.)
• 
If any of the above applies, consider a more powerful scripting language -- perhaps Perl, Tcl, Python, Ruby
-- or possibly a compiled language such as C, C++, or Java. Even then, prototyping the application as a
shell script might still be a useful development step.
We will be using Bash, an acronym [3] for "Bourne-Again shell" and a pun on Stephen Bourne's now classic
Bourne shell. Bash has become a de facto standard for shell scripting on most flavors of UNIX. Most of the
principles this book covers apply equally well to scripting with other shells, such as the Korn Shell, from
which Bash derives some of its features, [4] and the C Shell and its variants. (Note that C Shell programming
is not recommended due to certain inherent problems, as pointed out in an October, 1993 Usenet post by Tom
Christiansen.)
What follows is a tutorial on shell scripting. It relies heavily on examples to illustrate various features of the
shell. The example scripts work -- they've been tested, insofar as possible -- and some of them are even useful
in real life. The reader can play with the actual working code of the examples in the source archive
(scriptname.sh or scriptname.bash), [5] give them execute permission (chmod u+rx
scriptname), then run them to see what happens. Should the source archive not be available, then
cut-and-paste from the HTML or pdf rendered versions. Be aware that some of the scripts presented here
introduce features before they are explained, and this may require the reader to temporarily skip ahead for
enlightenment.
Unless otherwise noted, the author of this book wrote the example scripts that follow.
His countenance was bold and bashed not.
--Edmund Spenser
Advanced Bash-Scripting Guide
Chapter 1. Shell Programming!
2
Chapter 2. Starting Off With a Sha-Bang
Shell programming is a 1950s juke box . . .
--Larry Wall
In the simplest case, a script is nothing more than a list of system commands stored in a file. At the very least,
this saves the effort of retyping that particular sequence of commands each time it is invoked.
Example 2-1. cleanup: A script to clean up log files in /var/log
# Cleanup
# Run as root, of course.
cd /var/log
cat /dev/null > messages
cat /dev/null > wtmp
echo "Log files cleaned up."
There is nothing unusual here, only a set of commands that could just as easily have been invoked one by one
from the command-line on the console or in a terminal window. The advantages of placing the commands in a
script go far beyond not having to retype them time and again. The script becomes a program -- a tool -- and it
can easily be modified or customized for a particular application.
Example 2-2. cleanup: An improved clean-up script
#!/bin/bash
# Proper header for a Bash script.
# Cleanup, version 2
# Run as root, of course.
# Insert code here to print error message and exit if not root.
LOG_DIR=/var/log
# Variables are better than hard-coded values.
cd $LOG_DIR
cat /dev/null > messages
cat /dev/null > wtmp
echo "Logs cleaned up."
exit #  The right and proper method of "exiting" from a script.
 A bare "exit" (no parameter) returns the exit status
#+ of the preceding command. 
Now that's beginning to look like a real script. But we can go even farther . . .
Example 2-3. cleanup: An enhanced and generalized version of above scripts.
#!/bin/bash
# Cleanup, version 3
Chapter 2. Starting Off With a Sha-Bang
3
 Warning:
 -------
 This script uses quite a number of features that will be explained
#+ later on.
 By the time you've finished the first half of the book,
#+ there should be nothing mysterious about it.
LOG_DIR=/var/log
ROOT_UID=0     # Only users with $UID 0 have root privileges.
LINES=50       # Default number of lines saved.
E_XCD=86       # Can't change directory?
E_NOTROOT=87   # Non-root exit error.
# Run as root, of course.
if [ "$UID" -ne "$ROOT_UID" ]
then
echo "Must be root to run this script."
exit $E_NOTROOT
fi  
if [ -n "$1" ]
# Test whether command-line argument is present (non-empty).
then
lines=$1
else  
lines=$LINES # Default, if not specified on command-line.
fi  
 Stephane Chazelas suggests the following,
#+ as a better way of checking command-line arguments,
#+ but this is still a bit advanced for this stage of the tutorial.
#
   E_WRONGARGS=85  # Non-numerical argument (bad argument format).
#
   case "$1" in
   ""      ) lines=50;;
   *[!0-9]*) echo "Usage: `basename $0` lines-to-cleanup";
    exit $E_WRONGARGS;;
   *       ) lines=$1;;
   esac
#
#* Skip ahead to "Loops" chapter to decipher all this.
cd $LOG_DIR
if [ `pwd` != "$LOG_DIR" ]  # or   if [ "$PWD" != "$LOG_DIR" ]
# Not in /var/log?
then
echo "Can't change to $LOG_DIR."
exit $E_XCD
fi  # Doublecheck if in right directory before messing with log file.
# Far more efficient is:
#
# cd /var/log || {
  echo "Cannot change to necessary directory." >&2
  exit $E_XCD;
Advanced Bash-Scripting Guide
Chapter 2. Starting Off With a Sha-Bang
4
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