Shell script arguments or parameters held in '$@' and '$*'

[J65nko]

When passing arguments to a shell script it good to know the differences between the behaviour of the '$@' and '$*' special variables. Both of them allow you to retrieve the arguments or parameters passed to your script, but there are subtle differences when you surround them with quotes or not.

A shell script that illustrates these quirks:

#!/bin/sh
# J65nko - daemonforums.org

echo; echo 'Parameters in "$@" -------'

nr=0
for THIS in "$@" ; do
  nr=$((nr+1))
  echo "Parameter $nr :\t$THIS"
done

echo; echo 'Parameters in $@ -------'

nr=0
for THIS in $@ ; do
  nr=$((nr+1))
  echo "Parameter $nr :\t$THIS"
done

echo; echo 'Parameters in "$*" -------'

nr=0
for THIS in "$*" ; do
  nr=$((nr+1))
  echo "Parameter $nr :\t$THIS"
done

echo; echo 'Parameters in $* -------'

nr=0
for THIS in $* ; do
  nr=$((nr+1))
  echo "Parameter $nr :\t$THIS"
done

Running it with some arguments shows the differences:

$ ./paramtst.sh "King Alfred the Great" 1 2

Parameters in "$@" -------
Parameter 1 :   King Alfred the Great
Parameter 2 :   1
Parameter 3 :   2

Parameters in $@ -------
Parameter 1 :   King
Parameter 2 :   Alfred
Parameter 3 :   the
Parameter 4 :   Great
Parameter 5 :   1
Parameter 6 :   2

Parameters in "$*" -------
Parameter 1 :   King Alfred the Great 1 2

Parameters in $* -------
Parameter 1 :   King
Parameter 2 :   Alfred
Parameter 3 :   the
Parameter 4 :   Great
Parameter 5 :   1
Parameter 6 :   2

[J65nko]

Another approach to process the passed arguments is to use "shift". Using the above example:

$1 will contain "King Alfred the Great"
$2 holds the number "1"
$3 holds the number "2"

The special variable "$#" contains the number of arguments.
The situation after a shift:

$1 holds the number "1"
$2 holds the number "2"
and $# will be decreased by one to reflect the number of the remaining parameters.

A script to demonstrate that we can keep printing the first parameter in "$1" when we use shift:

#!/bin/sh
# J65nko - daemonforums.org

while [ $# -ne 0 ]; do
    nr=$((nr+1))
    echo  "Parameter $nr :\t$1"
    shift
done

Running with same arguments as in the previous example:

./paramshift.sh "King Alfred the Great" 1 2   
Parameter 1 :   King Alfred the Great
Parameter 2 :   1
Parameter 3 :   2

[ctac]

Hi
I think it can be interesting to add this 2 lines at the start of the script and try it with and without double quotes around the string "King Alfred the Great"

echo; echo "Number of Parameters :\t$#"
echo "Original Parameter 1 :\t$1"