Unix / Linux - Special Variables

In this chapter, 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-pne 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 pne −

#!/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 Ap
File Name : ./test.sh
First Parameter : Zara
Second Parameter : Ap
Quoted Values: Zara Ap
Quoted Values: Zara Ap
Total Number of Parameters : 2

Special Parameters $* and $@

There are special parameters that allow accessing all the command-pne arguments at once. $* and $@ both will act the same unless they are enclosed in double quotes, "".

Both the parameters specify the command-pne arguments. However, the "$*" special parameter takes the entire pst as one argument with spaces between and the "$@" special parameter takes the entire pst and separates it into separate arguments.

We can write the shell script as shown below to process an unknown number of commandpne 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 Ap 10 Years Old
Zara
Ap
10
Years
Old

Note − Here do...done is a kind of loop that will be covered in a subsequent tutorial.

Exit Status

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.

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 Ap
File Name : ./test.sh
First Parameter : Zara
Second Parameter : Ap
Quoted Values: Zara Ap
Quoted Values: Zara Ap
Total Number of Parameters : 2
$echo $?
0
$