UNIX Scripting

A summary of useful tips and tricks as can be found on different (web)-pages such as:

• Russel Quong , A guide for writing scripts.
• Mendel Cooper , Advanced Bash Scripting Guide. An in-depth exploration of the art of shell scripting.

Debugging Scripts

1. Use echo statements.
2. Run bash -n script to check for syntax errors.
3. Use the command set -v to get a verbose dump of each line the shell reads.
   Use set +v to turn off verbose mode.
4. Use the command set -x to see what each command expands to.
   Again, set +x turns this mode off.

Command line arguments

The command line parameters to a script are stored in the nearly identical variables $* and $.
The following table summarizes the variables you would use for command line processing.
For the example values, assume you wrote a bash script /usr/bin/args.sh and ran it as shown below.

Variable	Meaning	Example: echoArgs -t two "let's go"
$*	Command line args	-t two let's go
$@	Command line args	-t two "let's go"
$#	Number of args	3
$0	Name of script	/usr/bin/args.sh
$1	First arg in $*	-t
$2	Second arg in $*	two
$3	Third arg in $*	let's go
$4	Fourth arg in $*	(empty)

echoArgs () { 
    echo $#
    for i in "$@"; do
        echo "($i)";
    done;
    for i in $*; do
        echo "(($i))";
    done
}

$ echoArgs -t two "let's go"
  3
  (-t)
  (two)
  (let's go)
  ((-t))
  ((two))
  ((let's))
  ((go))

Command Line Options

I prefer the usage of case statements in stead of the getops.

Flag	Description
-o OUT	Sends output to file OUT
-n	Shows what you would do but do not do it
-v	Gives more output, each -v increases verboseness
-l	Same as -verbose
-version	Shows the version and quit

nflag=0
vlevel=0
OUT=
while [ $# -gt 0 ] 
do
  case "$1" in 
    -o ) OUT=$2 ; shift 2 ;;
    -n ) nflag=1 ; shift ;;
    -l | -v ) vlevel=$(( vlevel+1 )) ; shift ;;
    -ver* ) echo "Version $version"  ; exit 1 ;;
    * ) echo "Saw non flag $arg" ; break ;;
  esac
done

Tests

UNIX tests have the format:

if [ 0 ]      # zero
then
  echo "0 is true."
else          # Or else ...
  echo "0 is false."
fi    

Suppose you want to check if the first parameter (a filename) exists then:

if [ -e $1 ] 
then
   echo "File $1 exists"
else
   echo "File $1 does not exist"
fi

Tests overview

There are several testing categories:

• Test for files
• Test for Strings
• Internal Test
• Test for Integers
• Combined Forms

Test for Files

Test	Description
-a	File exists. Deprecated, its use is discouraged.
-b	File is a block device .
-c	File is a character device .
-d	File is a directory.
-e	File exists. The file maybe a file or a directory. Use other operators to find out.
-f	File is a regular file (not a directory or device file).
-g	File exists and its set-group-id bit is set. The file maybe a file or a directory .
-h	File exists and is a symbolic link.
-i	Not used
-j	Not used
-k	File exists and its sticky bit is set .
-L	File exists and is a symbolic link.
-p	File exists and is a named pipe (FIFO).
-r	File exists and is readable.
-s	File exists and has a size larger than zero.
-S	File exists and is a socket.
-t[n]	The open file is associated with a terminal device. Default value for n is 1.
-u	File exists and its set-user-bit is set.
-x	File exists and is executable.

Test for Strings

Test	Description
string	String is not null.
-n s1	String s1 has nonzero length.
-z s1	String s1 has zero length.
s1 = s2	String s1 and s2 are identical. String s2 can be a wildcard pattern. Quote s2 to treat it literally. Although correct do not use!. Better use ==, see below .
s1 == s2	String s1 and s2 are identical. String s2 can be a wildcard pattern. Quote s2 to treat it literally. Preferred above =.
s1 != s2	String s1 and s2 are not identical. String s2 can be a wildcard pattern. Quote s2 to treat it literally.
s1 =~ s2	String s1 matches extended regular expression s2. Quote s2 to keep the shell from expanding embedded shell meta-characters. Strings matched by parentheszied sunexpressions are placed into elements of the BASH_REMATCH array .
s1 < s2	ASCII value of s1 precedes that of s2. Only valid when used inside [[ ]] construct.
s1 > s2	ASCII value of s1 follows that of s2. Only valid when used inside [[ ]] construct.

Internal Test

Test	Description
-o opt	Option opt' for set -o is on. .

Test for Integers

Test	Description
n1 -eq n2	n1 equals n2.
n1 -ge n2	n1 greater than or equal to n2.
n1 -gt n2	n1 greater than n2.
n1 -le n2	n1 less than or equal to n2.
n1 -lt n2	n1 less than n2.
n1 -ne n2	n1 not equal n2.

Combined Forms

Test	Description
(condition)	True if condition is true (used for grouping). For test and [], the ()s should be quoted by a \. The form [[ ]] does not require quoting the parentheses.
! condition	True if the condition is false.
c1 -a c2	True if both conditions are true.
c1 && c2	True if both conditions are true. Only valid within a [[ ]] construct.
c1 -o c2	True if either condition is true.
c1 || c2	True if either condition is true. Only valid within a [[ ]] construct.

Set options

Option	Name	Alternative	ksh88	ksh93	Bash	Description
-a	allexport	-o allexport	x	x	x	Exports all defined variables.
+A name			x	x		Assigns remaining arguments as elements of array name.
-A name			x	x		Same as +A name, but unset name before making assignments.
-b	notify	-o notify		x	x	Prints job completion message as soon as the jobs terminate; don't wait until the next prompt.
-B	brace expansion	-o braceexpand			x	Enables brace expansion. On by default.
-C	noclobber	-o noclobber			x	Prevents overwriting via > redirection; use >| to overwrite files.
-e	errexit	-o errexit	x	x	x	Exits if a command yields a nonzero exit status. The ERR trap executes before the shell exits.
-E					x	Causes shell functions, command substitutions and subshells to inherit the ERR trap.
-f	noglob	-o noglob	x	x	x	Ignores filename metacharacters (Globbing disabled).
-G	globstar	-o globstar		x		Causes ** to also match subdirectories during filename expansion.
-h	hashall	-o hashall			x	Locates commands as they are defined. Bash hashes command names. On by default.
-h	trackall	-o trackhall	x	x		Locates commands as they are defined. Korn shell creates tracked aliases. On by default.
-k	keyword	-o keyword	x	x	x	Assignment of environment variables (var=value) takes effect regardless of where they appear on the command line. Normally assignments must precede the command line.
-m	monitor	-o monitor	x	x	x	Enables job control. Background jobs executes in a separate process group. Usually -m is set automatically.
-n	noexec	-o noexec	x	x	x	Reads commands in script, but do not execute them (syntax check). Both shells ignore this option if it interactive.
-p	privileged	-o privileged	x	x	x	Start up as privileged user. Bash: Does not read $ENV or $BASH_ENV, does not import functions from the environment, ignores the value of $SHELLOPTS. Korn: Does not process $HOME/.profile, reads /etc/suid_profile instead.
-s			x	x		Sorts the positional parameters.
-t	onecmd	-o onecmd	x	x	x	Exits after the execution of one command.
-T	functrace	-o functrace			x	Causes shell functions, command substitutions and subshells to inherit the DEBUG trap.
-u	nounset	-o nounset	x	x	x	Attempt to use undefined variable outputs error message, and forces an exit.
-v	verbose	-o verbose	x	x	x	Prints each command to stdout before executing it.
-x	xtrace	-o xtrace	x	x	x	Similar to -v, but expands commands (preceded by the value of PS4. Provides step-by-step tracing of shell scripts.
-			x	x	x	End of options flag. All other arguments are positional parameters.
--			x	x	x	Unset positional parameters. If arguments given (-- arg1 arg2), positional parameters set to arguments.

The +o [mode] disables the given shell option, while the -o [option] enables the given shell option.

See also

• Shell-Differences Short overview of different shells
• Overview, Short overview of different shells
• Csh Whynot, List of csh problems