Special Variables in Linux $0 $? $$ $# [Basic Guide]

An important feature of shell scripts is the possibility of confirming execution with success of a given command.

Special return variables

It’s called a return variable.

The return variable is filled every time a command is executed. If the value returned is 0, the command was successfully executed.

If the value is different from 0, a problem was encountered.

If used in scripts, its value must be verified immediately after the execution of the desired command.

Variable	What is for
$#	Returns the number of arguments the program received.
$_	The underscore contains the absolute name of the shell file or script that is being executed as passed in the argument list.
$?	Return variable when a program terminates execution.
$$	Provides the PID of the Shell process.
$!	Provides the PID of the last program running in the background.
$0	Returns the name of the program executed.
$n	returns the program’s n-number argument.
$* $@	Returns all arguments entered when running the program.

Take a look at the sample script where these variables are used:

#! /bin/bash 
echo “My name is: $0" 
echo “I have $# arguments passed in my execution” 
echo “This is the result of the last execution: $_” 
echo “The first argument is: $1" 
echo “The second argument is: $2" 
echo “The third argument is: $3" 
echo “My PID is: $” 
echo “My arguments are: $@” 
echo “My arguments are: $*”

When executed, it will produce the following result:

$./script one two three 
My name is:. /script 
I have 3 arguments passed in my execution 
This is the result of the last execution: I have 3 arguments passed in my execution 
The first argument is: one 
The second argument is: two 
The third argument is: three 
My PID is: 6210 
My arguments are: one two three 
My arguments are : one two three

Variable $?

The variable $? represents the exit status of the previous command.

Example:

$ echo “Uira”
Uira
$ echo $?
0

In this example the variable $? It will show the return code from the previous command echo “Will join”.

It has been agreed that when a command is executed without errors, it will return the return code 0 (zero). When there is an error, a value greater than 1 will be returned, and depending on the value, the command developer may provide a list of error codes and their meanings.

Variable $!

The variable $! will return the process ID of the last background command:

$ ls & [1] 27931

$ echo $!
27931

Don’t confuse it with the variable $$

Variable $$

The $$ variable shows the current shell process ID. For shell scripts, this is the ID of the process in which they are running:

$ echo $ 16487

$ ps -ef|grep 16487
16487 8098 0 June/21 pts/18 00:00:01 bash

Variable $#

The $# variable returns the number of arguments the program received. It can be used in scripts when you want to know if the number of arguments required has been satisfied.

Using { }

Keys can be used to highlight to the shell a variable that is used “inside” a text. See the example:

$ x="Tri”
$ y="${x}angle”
$ echo $y
Triangle

Difference between $* and $@

• $* and $@ when used outside of double quotes are identical and expand to the arguments.
• “$*” in quotes returns all arguments as if they were a single result separated by a space. Arguments “One two” “three” will return “one two three”, breaking the spaces between the arguments, as if they were three arguments and not two.
• “$@” will return the arguments exactly as they were received. “One two” “three” will return as two arguments “One two” “three” without breaking the spaces.

Conclusion

If you’re writing a shell script, it’s very important that you know the special variables.