Variable Names in Unix // Linux: Use Variables in Shell

A variable is a string of characters from which we assign a value. The assigned value can be a number, text, file name, device or any other type of data.

A variable is nothing but a pointer to real data. Shell allows you to create, assign, and delete variables.

Variable names in Unix / Linux

The name of a variable can contain only characters (a to z or from A to Z), numbers (from 0 to 9), or underscores (_).

By convention, Unix variables will have their names in capital letters.

the example below is about a valid variable name.

 _ALI TOKEN_A VAR_1 VAR_2 

The following is an example of invalid variable names:

 2 _VAR - VARIABLE VAR1 - VAR2 VAR_A ! 

The reason is that you cannot use other characters like!, *, Or -, because these characters have special meaning for the shell.

Definition of variables in Unix / Linux

The variables are defined as follows:

 variable_name = variable_value 

For instance,:

 NAME = "Zara Ali" 

For instance,, define the Name variable and assign Zara Ali value to it. Variables of this type are called scala variables. A scalar variable can only hold one value at a time.

Shell allows you to keep any value you want in a variable. For instance,:

 VAR1 = "Zara Ali" VAR2 = 100 

Access values in Unix / Linux

To access the values held in a variable, precede the variable name with the symbol ($):

For instance,, the following script will access the value of the defined NAME variable and print it on STDOUT:

 #! / bin / sh NAME = "Zara Ali" echo $ NAME 

It will produce the following result:

 Zara Ali 

The variables are read-only (read-only) in Unix / Linux

The shell provides a way to mark variables as read-only using the readonly command. After a variable is read-only, its value cannot be changed.

For instance,, the following script will fail while trying to change the value of NAME variable.

 #! / bin / sh NAME = "Zara Ali" readonly NAME NAME = "Qadiri" 

It will produce the following result:

 / bin / sh : NAME : bi?n này ???c ??c ch? . 

Delete a variable in Unix / Linux

Delete a variable for the shell to remove that variable from the list of variables it tracks. Once you delete a variable, you will not be able to access the value stored in that variable.

Here is the syntax to delete a defined variable using the unset command:

 unset variable_name 

The above command will delete the value of a defined variable. Here is a simple example:

 #! / bin / sh NAME = "Zara Ali" unset NAME echo $ NAME 

The above command will not print anything. You cannot use the unset command to delete variables that are read-only.

Variable types in Unix / Linux

When a Shell is running, there are three types of variables that exist:

Local Variables : An internal variable is a variable that exists in the shell's execution process. It is not available in programs that were started by Shell. They are set at the command prompt.

Environment Variables : An environment variable is a variable available in any shell subprocess. Some programs need environment variables to execute functions correctly. typically a Shell script only defines the environment variables it needs by programs when it runs.