Unix / Linux for Beginners
Unix / Linux Shell Programming
Advanced Unix / Linux
Unix / Linux Useful Resources
Selected Reading
- Unix / Linux - The vi Editor
- Unix / Linux - Communication
- Unix / Linux - Processes
- Unix / Linux - Pipes & Filters
- Unix / Linux - Basic Utilities
- Unix / Linux - Environment
- Unix / Linux - File Permission
- Unix / Linux - Directories
- Unix / Linux - File Management
- Unix / Linux - Getting Started
- Unix / Linux - Home
Unix / Linux Shell Programming
- Unix / Linux - Manpage Help
- Unix / Linux - Shell Functions
- Unix / Linux - IO Redirections
- Unix / Linux - Quoting Mechanisms
- Unix / Linux - Shell Substitutions
- Unix / Linux - Loop Control
- Unix / Linux - Shell Loops
- Unix / Linux - Decision Making
- Unix / Linux - Basic Operators
- Unix / Linux - Using Arrays
- Unix / Linux - Special Variables
- Unix / Linux - Using Variables
- Unix / Linux - What is Shell?
- Unix / Linux - Shell Scripting
Advanced Unix / Linux
- Unix / Linux - Signals and Traps
- Unix / Linux - System Logging
- Unix / Linux - System Performance
- Unix / Linux - User Administration
- Unix / Linux - File System Basics
- Unix / Linux - Regular Expressions
Unix / Linux Useful Resources
- Unix / Linux - Discussion
- Unix / Linux - Useful Resources
- Unix / Linux - Commands List
- Unix / Linux - System Calls
- Unix / Linux - Builtin Functions
- Unix / Linux - Quick Guide
- Unix / Linux - Useful Commands
- Unix / Linux - Questions & Answers
Selected Reading
- Who is Who
- Computer Glossary
- HR Interview Questions
- Effective Resume Writing
- Questions and Answers
- UPSC IAS Exams Notes
Unix / Linux - Using Variables
Unix / Linux - Using Shell Variables
In this chapter, we will learn how to use Shell variables in Unix. 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 will have their names in UPPERCASE.
The following examples are vapd variable names −
_ALI TOKEN_A VAR_1 VAR_2
Following are the examples of invapd 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 Ap"
The above example defines the variable NAME and assigns the value "Zara Ap" 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 Ap" 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 Ap" echo $NAME
The above script will produce the following value −
Zara Ap
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 Ap" 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 pst 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 Ap" unset NAME echo $NAME
The above example does 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 the command prompt.
Environment Variables − An environment variable 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.