Linux shell programming : special variables
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special shell variables
These are special shell variables which are set internally by the shell and which are available to the user:
| Variable | Description |
|---|---|
| $0 | The filename of the current script. |
| $n | These variables correspond to the arguments with which a script was invoked. Here n is a positive decimal number corresponding to the position of an argument (the first argument is $1, the second argument is $2, and so on). |
| $$ | The process ID of the current shell. For shell scripts, this is the process ID under which they are executing. |
| $# | The number of arguments supplied to a script. |
| $@ | All the arguments are individually double quoted. If a script receives two arguments, $@ is equivalent to $1 $2. |
| $* | All the arguments are double quoted. If a script receives two arguments, $* is equivalent to $1 $2. |
| $? | The exit status of the last command executed. |
| $! | The process ID of the last background command. |
| $_ | The last argument of the previous command. |
sample code
Let's see how the special shell variables are working:
#!/bin/bash # special.sh echo "Process ID of shell = $$" echo "Program name = $0" echo "Number of args = $#" echo "Argument 1 = $1" echo "Argument 2 = $2" echo "Complete list of arguments = $*" echo "Complete list of arguments = $@"
Output:
$ ./special.sh arg1 arg2 arg3 Process ID of shell = 20779 Program name = ./special.sh Number of args = 3 Argument 1 = arg1 Argument 2 = arg2 Complete list of arguments = arg1 arg2 arg3 Complete list of arguments = arg1 arg2 arg3 $ echo $? 0
The $? variable represents the exit status of the previous command.
Exit status is a numerical value returned by every command upon its completion. As a rule, most commands return an exit status of 0 if they were successful, and 1 if they were unsuccessful.
$*
#!/bin/bash for args in $* do echo $args done
Output:
$ ./unknown_args.sh 1 2 3 4 1 2 3 4
$?
A sample script that starts apache if it finds it off:
# launch.sh
#!/bin/sh
ps auxw | grep apache2 | grep -v grep > /dev/null
if [ $? != 0 ]
then
/etc/init.d/apache2 start > /dev/null
fi
Give it executable permissions:
$ chmod +x launch.sh
$!
The process ID of the last background command:
$ ls & [1] 27931 $ echo $! 27931
$$
The process ID of the current shell. For shell scripts, this is the process ID under which they are executing:
$ echo $$ 16487 $ ps -ef|grep 16487 k 16487 8098 0 Jun21 pts/18 00:00:01 bash $ ps -ef | grep `echo $$` k 16487 8098 0 Jun21 pts/18 00:00:01 bash
Note on $* AND $@
- $* and $@ when unquoted are identical and expand into the arguments.
- "$*" is a single word, comprising all the arguments to the shell, joined together with spaces. For example '1 2' 3 becomes "1 2 3".
- "$@" is identical to the arguments received by the shell, the resulting list of words completely match what was given to the shell. For example '1 2' 3 becomes "1 2" "3"
- In short, $@ when quoted ("$@") breaking up the arguments if there are spaces in them. But "$*" does not breaking the arguments.
# sp.sh
#!/bin/bash
len1=${#@}
len2=${#*}
echo "arg length len1=$len1"
echo "arg length len2=$len2"
echo
if [ $len1 -eq 0 ] || [ $len2 -eq 0 ]
then
echo "No args provided..."
return
fi
for args in $*
do
echo "$args "
done
echo
for args in $@
do
echo "$args "
done
echo
for args in "$*"
do
echo "$args "
done
echo
for args in "$@"
do
echo "$args "
done
Output 1:
$ ./sp.sh 1 2 3 4 5 arg length len1=5 arg length len2=5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5
Output 2:
$ ./sp.sh 1 2 3 '4 5' arg len=4 arg len=4 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5
{ }
Here are the usage samples of {}:
- To expand a variable:
x="tri" y=${x}angle echo $y # 'triangle' - To expand arrays:
numbers=("uno","dos","tres") echo $numbers echo ${numbers[0]} ${numbers[1]} ${numbers[2]} for item in $numbers do echo $item done for item in ${numbers[@]} do echo $item done for item in ${numbers[*]} do echo $item donewhere all five echo will produceuno,dos,tres
- To expand positional parameters beyond 9:
$8 $9 ${10} ${11} - Substring removal
${PARAMETER%PATTERN}
Sample:fullname="tiny.txt" firstname=${fullname%.txt} echo $fullname $firstname
Output:tiny.txt tiny
To see extensive usage list of "{ }" : Parameter expansion
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