A variable is a character string where we stores a value. The value assigned could be a number, text, filename, device, or any other type of data.
A variable is nothing more than a pointer to the actual data stored. The shell enables you to create, assign, and delete variables.Their are some predefined convention as follows to define variables.
Variable Names
The name of a variable can contain only letters (a to z or A to Z), numbers ( 0 to 9) or the underscore character ( _).
Unix shell variables should have their names in UPPERCASE.
Below are the examples are valid variable names −
_SAMPLE RANDOM_A V_1 VAR_2 VARIABLE_3
Following are the examples of invalid variable names −
2_TEMP - Starts With numeric -VAR - Starts with &contains - VARABLE1-VAR2 - Contains - VAR_A! - Contains !
Why a variable cannot use other characters such as !, *, or – is that these characters have a special meaning for the shell?
The special character other than “_” is reversed in shell for other uses.
Defining Variables
Variables are defined with an = sing. The left part contains the name of the variable and right part contains the value of the variable.
Syntax : variable_name=variable_value
For example −
NAME="Zara Ali"
The above example defines the variable NAME and assigns the value “Zara Ali” to it. Variables of this type are called scalar variables. A scalar variable can hold only one value at a time.
Shell enables you to store any value you want in a variable. For example −
VAR1="Zara Ali" VAR2=100
Accessing Values
To access the value stored in a variable, prefix its name with the dollar sign ($) −
For example, the following script will access the value of defined variable NAME and print it on STDOUT
#!/bin/sh NAME="Zara Ali" echo $NAME
The above script will produce the following value −
Zara Ali
Read-only Variables
Shell provides a way to mark variables as read-only by using the read-only command. After a variable is marked read-only, its value cannot be changed.
For example, the following script generates an error while trying to change the value of NAME −
#!/bin/sh NAME="Zara Ali" readonly NAME NAME="Qadiri"The above script will generate the following result −
/bin/sh: NAME: This variable is read only.
Unsetting Variables
Unsetting or deleting a variable directs the shell to remove the variable from the list of variables that it tracks. Once you unset a variable, you cannot access the stored value in the variable.
Following is the syntax to unset a defined variable using the unset command −
unset variable_name
The above command unsets the value of a defined variable. Here is a simple example that demonstrates how the command works −
#!/bin/sh
NAME="Zara Ali"
unset NAME
echo $NAME
The above example does not print anything. You cannot use the unset command to unset variables that are marked readonly.