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Advanced Bash-Scripting Guide

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Original source (www.iitk.ac.in)
Tags: bash shell-scripting
Clipped on: 2014-09-06

Advanced Bash-Scripting Guide

An in-depth exploration of the art of shell scripting

Mendel Cooper

5.3

11 May 2008

Revision History
Revision 5.110 Nov 2007Revised by: mc
'LINGONBERRY' release: Minor Update.
Revision 5.216 Mar 2008Revised by: mc
'SILVERBERRY' release: Important Update.

This tutorial assumes no previous knowledge of scripting or programming, but progresses rapidly toward an intermediate/advanced level of instruction . . . all the while sneaking in little snippets of UNIX® wisdom and lore. It serves as a textbook, a manual for self-study, and 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.

The latest update of this document, as an archived, bzip2-ed "tarball" including both the SGML source and rendered HTML, may be downloaded from the author's home site. A pdf version is also available ( pdf mirror site). See the change log for a revision history.



Dedication

For Anita, the source of all the magic

Table of Contents
Part 1. Introduction
1. Why Shell Programming?
2. Starting Off With a Sha-Bang
Part 2. Basics
3. Special Characters
4. Introduction to Variables and Parameters
5. Quoting
6. Exit and Exit Status
7. Tests
8. Operations and Related Topics
Part 3. Beyond the Basics
9. Variables Revisited
10. Loops and Branches
11. Command Substitution
12. Arithmetic Expansion
13. Recess Time
Part 4. Commands
14. Internal Commands and Builtins
15. External Filters, Programs and Commands
16. System and Administrative Commands
Part 5. Advanced Topics
17. Regular Expressions
18. Here Documents
19. I/O Redirection
20. Subshells
21. Restricted Shells
22. Process Substitution
23. Functions
24. Aliases
25. List Constructs
26. Arrays
27. /dev and /proc
28. Of Zeros and Nulls
29. Debugging
30. Options
31. Gotchas
32. Scripting With Style
33. Miscellany
34. Bash, versions 2 and 3
35. Endnotes
35.1. Author's Note
35.2. About the Author
35.3. Where to Go For Help
35.4. Tools Used to Produce This Book
35.5. Credits
35.6. Disclaimer
Bibliography
A. Contributed Scripts
B. Reference Cards
C. A Sed and Awk Micro-Primer
C.1. Sed
C.2. Awk
D. Exit Codes With Special Meanings
E. A Detailed Introduction to I/O and I/O Redirection
F. Command-Line Options
F.1. Standard Command-Line Options
F.2. Bash Command-Line Options
G. Important Files
H. Important System Directories
I. Localization
J. History Commands
K. A Sample .bashrc File
L. Converting DOS Batch Files to Shell Scripts
M. Exercises
M.1. Analyzing Scripts
M.2. Writing Scripts
N. Revision History
O. Mirror Sites
P. To Do List
Q. Copyright
R. ASCII Table
Index
List of Examples
2-1. cleanup: A script to clean up the log files in /var/log
2-2. cleanup: An improved clean-up script
2-3. cleanup: An enhanced and generalized version of above scripts.
3-1. Code blocks and I/O redirection
3-2. Saving the output of a code block to a file
3-3. Running a loop in the background
3-4. Backup of all files changed in last day
4-1. Variable assignment and substitution
4-2. Plain Variable Assignment
4-3. Variable Assignment, plain and fancy
4-4. Integer or string?
4-5. Positional Parameters
4-6. wh, whois domain name lookup
4-7. Using shift
5-1. Echoing Weird Variables
5-2. Escaped Characters
6-1. exit / exit status
6-2. Negating a condition using !
7-1. What is truth?
7-2. Equivalence of test, /usr/bin/test, [ ], and /usr/bin/[
7-3. Arithmetic Tests using (( ))
7-4. Testing for broken links
7-5. Arithmetic and string comparisons
7-6. Testing whether a string is null
7-7. zmore
8-1. Greatest common divisor
8-2. Using Arithmetic Operations
8-3. Compound Condition Tests Using && and ||
8-4. Representation of numerical constants
9-1. $IFS and whitespace
9-2. Timed Input
9-3. Once more, timed input
9-4. Timed read
9-5. Am I root?
9-6. arglist: Listing arguments with $* and $@
9-7. Inconsistent $* and $@ behavior
9-8. $* and $@ when $IFS is empty
9-9. Underscore variable
9-10. Inserting a blank line between paragraphs in a text file
9-11. Generating an 8-character "random" string
9-12. Converting graphic file formats, with filename change
9-13. Converting streaming audio files to ogg
9-14. Emulating getopt
9-15. Alternate ways of extracting substrings
9-16. Using parameter substitution and error messages
9-17. Parameter substitution and "usage" messages
9-18. Length of a variable
9-19. Pattern matching in parameter substitution
9-20. Renaming file extensions:
9-21. Using pattern matching to parse arbitrary strings
9-22. Matching patterns at prefix or suffix of string
9-23. Using declare to type variables
9-24. Indirect Variable References
9-25. Passing an indirect reference to awk
9-26. Generating random numbers
9-27. Picking a random card from a deck
9-28. Brownian Motion Simulation
9-29. Random between values
9-30. Rolling a single die with RANDOM
9-31. Reseeding RANDOM
9-32. Pseudorandom numbers, using awk
9-33. C-style manipulation of variables
10-1. Simple for loops
10-2. for loop with two parameters in each [list] element
10-3. Fileinfo: operating on a file list contained in a variable
10-4. Operating on files with a for loop
10-5. Missing in [list] in a for loop
10-6. Generating the [list] in a for loop with command substitution
10-7. A grep replacement for binary files
10-8. Listing all users on the system
10-9. Checking all the binaries in a directory for authorship
10-10. Listing the symbolic links in a directory
10-11. Symbolic links in a directory, saved to a file
10-12. A C-style for loop
10-13. Using efax in batch mode
10-14. Simple while loop
10-15. Another while loop
10-16. while loop with multiple conditions
10-17. C-style syntax in a while loop
10-18. until loop
10-19. Nested Loop
10-20. Effects of break and continue in a loop
10-21. Breaking out of multiple loop levels
10-22. Continuing at a higher loop level
10-23. Using continue N in an actual task
10-24. Using case
10-25. Creating menus using case
10-26. Using command substitution to generate the case variable
10-27. Simple string matching
10-28. Checking for alphabetic input
10-29. Creating menus using select
10-30. Creating menus using select in a function
11-1. Stupid script tricks
11-2. Generating a variable from a loop
11-3. Finding anagrams
14-1. A script that forks off multiple instances of itself
14-2. printf in action
14-3. Variable assignment, using read
14-4. What happens when read has no variable
14-5. Multi-line input to read
14-6. Detecting the arrow keys
14-7. Using read with file redirection
14-8. Problems reading from a pipe
14-9. Changing the current working directory
14-10. Letting let do arithmetic.
14-11. Showing the effect of eval
14-12. Echoing the command-line parameters
14-13. Forcing a log-off
14-14. A version of rot13
14-15. Using eval to force variable substitution in a Perl script
14-16. Using set with positional parameters
14-17. Reversing the positional parameters
14-18. Reassigning the positional parameters
14-19. "Unsetting" a variable
14-20. Using export to pass a variable to an embedded awk script
14-21. Using getopts to read the options/arguments passed to a script
14-22. "Including" a data file
14-23. A (useless) script that sources itself
14-24. Effects of exec
14-25. A script that exec's itself
14-26. Waiting for a process to finish before proceeding
14-27. A script that kills itself
15-1. Using ls to create a table of contents for burning a CDR disk
15-2. Hello or Good-bye
15-3. Badname, eliminate file names in current directory containing bad characters and whitespace.
15-4. Deleting a file by its inode number
15-5. Logfile: Using xargs to monitor system log
15-6. Copying files in current directory to another
15-7. Killing processes by name
15-8. Word frequency analysis using xargs
15-9. Using expr
15-10. Using date
15-11. Date calculations
15-12. Word Frequency Analysis
15-13. Which files are scripts?
15-14. Generating 10-digit random numbers
15-15. Using tail to monitor the system log
15-16. Printing out the From lines in stored e-mail messages
15-17. Emulating grep in a script
15-18. Crossword puzzle solver
15-19. Looking up definitions in Webster's 1913 Dictionary
15-20. Checking words in a list for validity
15-21. toupper: Transforms a file to all uppercase.
15-22. lowercase: Changes all filenames in working directory to lowercase.
15-23. du: DOS to UNIX text file conversion.
15-24. rot13: ultra-weak encryption.
15-25. Generating "Crypto-Quote" Puzzles
15-26. Formatted file listing.
15-27. Using column to format a directory listing
15-28. nl: A self-numbering script.
15-29. manview: Viewing formatted manpages
15-30. Using cpio to move a directory tree
15-31. Unpacking an rpm archive
15-32. Stripping comments from C program files
15-33. Exploring /usr/X11R6/bin
15-34. An "improved" strings command
15-35. Using cmp to compare two files within a script.
15-36. basename and dirname
15-37. A script that copies itself in sections
15-38. Checking file integrity
15-39. Uudecoding encoded files
15-40. Finding out where to report a spammer
15-41. Analyzing a spam domain
15-42. Getting a stock quote
15-43. Updating FC4
15-44. Using ssh
15-45. A script that mails itself
15-46. Monthly Payment on a Mortgage
15-47. Base Conversion
15-48. Invoking bc using a here document
15-49. Calculating PI
15-50. Converting a decimal number to hexadecimal
15-51. Factoring
15-52. Calculating the hypotenuse of a triangle
15-53. Using seq to generate loop arguments
15-54. Letter Count"
15-55. Using getopt to parse command-line options
15-56. A script that copies itself
15-57. Exercising dd
15-58. Capturing Keystrokes
15-59. Securely deleting a file
15-60. Filename generator
15-61. Converting meters to miles
15-62. Using m4
16-1. Setting a new password
16-2. Setting an erase character
16-3. secret password: Turning off terminal echoing
16-4. Keypress detection
16-5. Checking a remote server for identd
16-6. pidof helps kill a process
16-7. Checking a CD image
16-8. Creating a filesystem in a file
16-9. Adding a new hard drive
16-10. Using umask to hide an output file from prying eyes
16-11. killall, from /etc/rc.d/init.d
18-1. broadcast: Sends message to everyone logged in
18-2. dummyfile: Creates a 2-line dummy file
18-3. Multi-line message using cat
18-4. Multi-line message, with tabs suppressed
18-5. Here document with parameter substitution
18-6. Upload a file pair to Sunsite incoming directory
18-7. Parameter substitution turned off
18-8. A script that generates another script
18-9. Here documents and functions
18-10. "Anonymous" Here Document
18-11. Commenting out a block of code
18-12. A self-documenting script
18-13. Prepending a line to a file
18-14. Parsing a mailbox
19-1. Redirecting stdin using exec
19-2. Redirecting stdout using exec
19-3. Redirecting both stdin and stdout in the same script with exec
19-4. Avoiding a subshell
19-5. Redirected while loop
19-6. Alternate form of redirected while loop
19-7. Redirected until loop
19-8. Redirected for loop
19-9. Redirected for loop (both stdin and stdout redirected)
19-10. Redirected if/then test
19-11. Data file names.data for above examples
19-12. Logging events
20-1. Variable scope in a subshell
20-2. List User Profiles
20-3. Running parallel processes in subshells
21-1. Running a script in restricted mode
23-1. Simple functions
23-2. Function Taking Parameters
23-3. Functions and command-line args passed to the script
23-4. Passing an indirect reference to a function
23-5. Dereferencing a parameter passed to a function
23-6. Again, dereferencing a parameter passed to a function
23-7. Maximum of two numbers
23-8. Converting numbers to Roman numerals
23-9. Testing large return values in a function
23-10. Comparing two large integers
23-11. Real name from username
23-12. Local variable visibility
23-13. Demonstration of a simple recursive function
23-14. Recursion, using a local variable
23-15. The Fibonacci Sequence
23-16. The Towers of Hanoi
24-1. Aliases within a script
24-2. unalias: Setting and unsetting an alias
25-1. Using an and list to test for command-line arguments
25-2. Another command-line arg test using an and list
25-3. Using or lists in combination with an and list
26-1. Simple array usage
26-2. Formatting a poem
26-3. Various array operations
26-4. String operations on arrays
26-5. Loading the contents of a script into an array
26-6. Some special properties of arrays
26-7. Of empty arrays and empty elements
26-8. Initializing arrays
26-9. Copying and concatenating arrays
26-10. More on concatenating arrays
26-11. The Bubble Sort
26-12. Embedded arrays and indirect references
26-13. The Sieve of Eratosthenes
26-14. The Sieve of Eratosthenes, Optimized
26-15. Emulating a push-down stack
26-16. Complex array application: Exploring a weird mathematical series
26-17. Simulating a two-dimensional array, then tilting it
27-1. Using /dev/tcp for troubleshooting
27-2. Finding the process associated with a PID
27-3. On-line connect status
28-1. Hiding the cookie jar
28-2. Setting up a swapfile using /dev/zero
28-3. Creating a ramdisk
29-1. A buggy script
29-2. Missing keyword
29-3. test24: another buggy script
29-4. Testing a condition with an assert
29-5. Trapping at exit
29-6. Cleaning up after Control-C
29-7. Tracing a variable
29-8. Running multiple processes (on an SMP box)
31-1. Numerical and string comparison are not equivalent
31-2. Subshell Pitfalls
31-3. Piping the output of echo to a read
33-1. shell wrapper
33-2. A slightly more complex shell wrapper
33-3. A generic shell wrapper that writes to a logfile
33-4. A shell wrapper around an awk script
33-5. A shell wrapper around another awk script
33-6. Perl embedded in a Bash script
33-7. Bash and Perl scripts combined
33-8. A (useless) script that recursively calls itself
33-9. A (useful) script that recursively calls itself
33-10. Another (useful) script that recursively calls itself
33-11. A "colorized" address database
33-12. Drawing a box
33-13. Echoing colored text
33-14. A "horserace" game
33-15. Return value trickery
33-16. Even more return value trickery
33-17. Passing and returning arrays
33-18. Fun with anagrams
33-19. Widgets invoked from a shell script
34-1. String expansion
34-2. Indirect variable references - the new way
34-3. Simple database application, using indirect variable referencing
34-4. Using arrays and other miscellaneous trickery to deal four random hands from a deck of cards
A-1. mailformat: Formatting an e-mail message
A-2. rn: A simple-minded file renaming utility
A-3. blank-rename: Renames filenames containing blanks
A-4. encryptedpw: Uploading to an ftp site, using a locally encrypted password
A-5. copy-cd: Copying a data CD
A-6. Collatz series
A-7. days-between: Days between two dates
A-8. Making a dictionary
A-9. Soundex conversion
A-10. Game of Life
A-11. Data file for Game of Life
A-12. behead: Removing mail and news message headers
A-13. ftpget: Downloading files via ftp
A-14. password: Generating random 8-character passwords
A-15. fifo: Making daily backups, using named pipes
A-16. Generating prime numbers using the modulo operator
A-17. tree: Displaying a directory tree
A-18. tree2: Alternate directory tree script
A-19. string functions: C-style string functions
A-20. Directory information
A-21. Object-oriented database
A-22. Library of hash functions
A-23. Colorizing text using hash functions
A-24. More on hash functions
A-25. Mounting USB keychain storage devices
A-26. Converting to HTML
A-27. Preserving weblogs
A-28. Protecting literal strings
A-29. Unprotecting literal strings
A-30. Spammer Identification
A-31. Spammer Hunt
A-32. Making wget easier to use
A-33. A podcasting script
A-34. Nightly backup to a firewire HD
A-35. An expanded cd command
A-36. A soundcard setup script
A-37. Locating split paragraphs in a text file
A-38. Insertion sort
A-39. A pad file generator for shareware authors
A-40. Petals Around the Rose
A-41. Quacky: a Perquackey-type word game
A-42. An all-purpose shell scripting homework assignment solution
A-43. Basics Reviewed
C-1. Counting Letter Occurrences
K-1. Sample .bashrc file
L-1. VIEWDATA.BAT: DOS Batch File
L-2. viewdata.sh: Shell Script Conversion of VIEWDATA.BAT
P-1. Print the server environment
R-1. A script that generates an ASCII table

Part 1. Introduction

The shell is a command interpreter. More than just the insulating layer between the operating system kernel and the user, it's also a fairly powerful programming language. A shell program, called a script, is an easy-to-use tool for building applications by "gluing together" system calls, tools, utilities, and compiled binaries. Virtually the entire repertoire of UNIX commands, utilities, and tools is available for invocation by a shell script. If that were not enough, internal shell commands, such as testing and loop constructs, lend additional power and flexibility to scripts. Shell scripts are especially well suited for administrative system tasks and other routine repetitive tasks not requiring the bells and whistles of a full-blown tightly structured programming language.


Chapter 1. Why 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 the 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 and straightforward, 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.

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. This way, the structure of the application can be tested and played 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

  • 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

  • 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 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 all 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, [3] 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 was 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), [4] 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.


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 the log files in /var/log

# Cleanup
# Run as root, of course.

cd /var/log
cat /dev/null > messages
cat /dev/null > wtmp
echo "Logs cleaned up."

There is nothing unusual here, only a set of commands that could just as easily be invoked one by one from the command line on the console or in an xterm. The advantages of placing the commands in a script go beyond not having to retype them time and again. The script becomes a tool, and 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.

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

#  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=66       # Can't change directory?
E_NOTROOT=67   # 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 if command line argument 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=65  # Non-numerical argument (bad arg format)
#
#    case "$1" in
#    ""      ) lines=50;;
#    *[!0-9]*) echo "Usage: `basename $0` file-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;
# }




tail -n $lines messages > mesg.temp # Saves last section of message log file.
mv mesg.temp messages               # Becomes new log directory.


# cat /dev/null > messages
#* No longer needed, as the above method is safer.

cat /dev/null > wtmp  #  ': > wtmp' and '> wtmp'  have the same effect.
echo "Logs cleaned up."

exit 0
#  A zero return value from the script upon exit
#+ indicates success to the shell.

Since you may not wish to wipe out the entire system log, this version of the script keeps the last section of the message log intact. You will constantly discover ways of fine-tuning previously written scripts for increased effectiveness.

The sha-bang ( #!) [5] at the head of a script tells your system that this file is a set of commands to be fed to the command interpreter indicated. The #! is actually a two-byte [6] magic number, a special marker that designates a file type, or in this case an executable shell script (type man magic for more details on this fascinating topic). Immediately following the sha-bang is a path name. This is the path to the program that interprets the commands in the script, whether it be a shell, a programming language, or a utility. This command interpreter then executes the commands in the script, starting at the top (line following the sha-bang line), ignoring comments. [7]

#!/bin/sh
#!/bin/bash
#!/usr/bin/perl
#!/usr/bin/tcl
#!/bin/sed -f
#!/usr/awk -f

Each of the above script header lines calls a different command interpreter, be it /bin/sh, the default shell (bash in a Linux system) or otherwise. [8] Using #!/bin/sh, the default Bourne shell in most commercial variants of UNIX, makes the script portable to non-Linux machines, though you sacrifice Bash-specific features. The script will, however, conform to the POSIX [9] sh standard.

Note that the path given at the "sha-bang" must be correct, otherwise an error message -- usually "Command not found." -- will be the only result of running the script. [10]

#! can be omitted if the script consists only of a set of generic system commands, using no internal shell directives. The second example, above, requires the initial #!, since the variable assignment line, lines=50, uses a shell-specific construct. [11] Note again that #!/bin/sh invokes the default shell interpreter, which defaults to /bin/bash on a Linux machine.

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