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# Unix - Using Shell Variables

A variable is a character string to which we assign 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. The shell enables you to create, assign, and delete 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 ( _).

By convention, Unix Shell variables would have their names in UPPERCASE.

The following examples are valid variable names:

    _ALI
    TOKEN_A
    VAR_1
    VAR_2

Following are the examples of invalid variable names:

    2_VAR
    -VARIABLE
    VAR1-VAR2
    VAR_A!

The reason you cannot use other characters such as !,*, or - is that these characters have a special meaning for the shell.

# Defining Variables:

Variables are defined as follows::

    variable_name=variable_value

For example:

    NAME="Zara Ali"

Above example defines the variable NAME and assigns it the value "Zara Ali". Variables of this type are called scalar variables. A scalar variable can hold only one value at a time.

The 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, following script would access the value of defined variable NAME and would print it on STDOUT:

    #!/bin/sh

    NAME="Zara Ali"
    echo $NAME

This would produce following value:

    Zara Ali

# Read-only Variables:

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

For example, following script would give error while trying to change the value of NAME:

    #!/bin/sh

    NAME="Zara Ali"
    readonly NAME
    NAME="Qadiri"

This would produce following result:

    /bin/sh: NAME: This variable is read only.

# Unsetting Variables:

Unsetting or deleting a variable tells the shell to remove the variable from the list of variables that it tracks. Once you unset a variable, you would not be able to access stored value in the variable.

Following is the syntax to unset a defined variable using the unset command:

    unset variable_name

Above command would unset the value of a defined variable. Here is a simple example:

    #!/bin/sh

    NAME="Zara Ali"
    unset NAME
    echo $NAME

Above example would not print anything. You cannot use the unset command to unset variables that are marked readonly.

# Variable Types:

When a shell is running, three main types of variables are present:

- Local Variables: A local variable is a variable that is present within the current instance of the shell. It is not available to programs that are started by the shell. They are set at command prompt.

- Environment Variables: An environment variable is a variable that is available to any child process of the shell. Some programs need environment variables in order to function correctly. Usually a shell script defines only those environment variables that are needed by the programs that it runs.

- Shell Variables: A shell variable is a special variable that is set by the shell and is required by the shell in order to function correctly. Some of these variables are environment variables whereas others are local variables.

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