Unix / Linux - Special Variables
We will discuss in detail about special variable in Unix. In one of our previous chapters, we understood how to be careful when we use certain nonalphanumeric characters in variable names. This is because those characters are used in the names of special Unix variables. These variables are reserved for specific functions.
For example, the $ character represents the process ID number, or PID, of the current shell −
$echo $$
The above command writes the PID of the current shell −
29949
The following table shows a number of special variables that you can use in your shell scripts −
Command-Line Arguments
The command-line arguments $1, $2, $3, ...$9 are positional parameters, with $0 pointing to the actual command, program, shell script, or function and $1, $2, $3, ...$9 as the arguments to the command.
Following script uses various special variables related to the command line −
#!/bin/sh echo "File Name: $0" echo "First Parameter : $1" echo "Second Parameter : $2" echo "Quoted Values: $@" echo "Quoted Values: $*" echo "Total Number of Parameters : $#"
Here is a sample run for the above script −
$./test.sh Zara Ali File Name : ./test.sh First Parameter : Zara Second Parameter : Ali Quoted Values: Zara Ali Quoted Values: Zara Ali Total Number of Parameters : 2
There are special parameters that allow accessing all the command-line arguments at once. $* and $@ both will act the same unless they are enclosed in double quotes, "".
Both the parameters specify the command-line arguments. However, the "$*" special parameter takes the entire list as one argument with spaces between and the "$@" special parameter takes the entire list and separates it into separate arguments.
We can write the shell script as shown below to process an unknown number of commandline arguments with either the $* or $@ special parameters −
#!/bin/sh for TOKEN in $* do echo $TOKEN done
Here is a sample run for the above script −
$./test.sh Zara Ali 10 Years Old Zara Ali 10 Years Old
Note − Here do...done is a kind of loop that will be covered in a subsequent tutorial.
Exit StatusThe $? 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.
Some commands return additional exit statuses for particular reasons. For example, some commands differentiate between kinds of errors and will return various exit values depending on the specific type of failure.
Following is the example of successful command −
$./test.sh Zara Ali File Name : ./test.sh First Parameter : Zara Second Parameter : Ali Quoted Values: Zara Ali Quoted Values: Zara Ali Total Number of Parameters : 2 $echo $? 0 $
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