UNIX Shell variable

The shell variable

A variable is a string that is assigned. T he value assigned to a variable can be a number, text, file name, device, or other type of data.

In essence, a variable is a pointer to the actual data. S hells can create, assign, and delete variables.

The name of the variable

Variable names can contain only letters, numbers, or underscores.

By convention, the variable names of UNIX shells are capitaled.

Here are some examples of valid variable names:

    _ALI
    TOKEN_A
    VAR_1
    VAR_2

Here are some examples of invalid variable names:

    2_VAR
    -VARIABLE
    VAR1-VAR2
    VAR_A!

Cannot be ！ The * the - , - and so on is that they have special uses in the shell.

Define variables

Variables can be defined as follows:

    variable_name=variable_value

Like what:

    NAME="Zara Ali"

The above example defines the variable NAME and then assigns "Zara Ali". This type of variable is a regular variable that can only be assigned one at a time.

Shells can be assigned as they please. Like what:

    VAR1="Zara Ali"
    VAR2=100

Access variables

In order to get the value stored in the variable, you need to add $before the variable name.

For example, the following script can access the value in the variable NAME and then print it to STDOUT:

    #!/bin/sh

    NAME="Zara Ali"
    echo $NAME

The following values appear:

    Zara Ali

Read-only variables

Shell uses read-only commands to provide read-only functionality for variables. Such variables cannot be changed.

For example, in the following script, the value of the variable NAME is modified and the system reports an error:

    #!/bin/sh

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

The following results occur:

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

Delete the variable

The deletion of a variable tells the shell to remove the variable from the list of variables so that it cannot be tracked. Once the user deletes a variable, it cannot be accessed and stored in the variable.

Here's an example of using the unset instruction:

    unset variable_name

The above instruction cancels the defined variable. Here's a simple example:

    #!/bin/sh

    NAME="Zara Ali"
    unset NAME
    echo $NAME

The above example does not display any information and cannot use the unset instruction to cancel variables that are marked as read-only.

The type of variable

When a shell script is executed, there are three main types of variables:

• Local variable: This type variable is valid only within the current shell instance. T hey cannot be applied to programs started by the shell. They are set only at the command prompt.
• Environment variables: Environment variables are valid for any child process of the shell. S ome programs require the correct call function to require environment variables. Typically, shell scripts define only the environment variables that a program needs to run.
• Shell variable: This type of variable is a specialized variable set by the shell and is used to call functions correctly. Sometimes these variables are environment variables, sometimes they are local variables.