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

Test	Description
-a	File exists. Deprecated, its use is discouraged.
-b	File is a block device .
-c	File is a character device

See also

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