Environment Variables in Linux/Unix

Environment variables, often referred to as ENVs, are dynamic values that wield significant influence over the behavior of programs and processes in the Linux operating system. These variables serve as a means to convey essential information to software and shape how they interact with the environment. Every Linux process is associated with a set of environment variables, which guide its behavior and interactions with other processes.

Accessing Environment Variables

In Linux, the primary conduit for interacting with environment variables is the shell. The shell acts as a command-line interpreter, executing instructions entered by the user. The most prevalent shell in the Linux world is the Bash shell (Bourne Again SHell), which comes as the default in many Linux distributions.

Scope of an environment variable

Understanding the scope of an environment variable is crucial. It dictates where the variable can be accessed or defined, making a clear distinction between global and local scopes.  

Global Environment Variables

A global environment variable, when defined in a terminal, enjoys accessibility from anywhere within that terminal’s environment. This means it can be employed by scripts, programs, or processes running within the scope of that terminal.

Local Environment Variables

On the other hand, local environment variables are confined to the specific terminal in which they are defined. They are isolated from external programs or processes, making their reach exclusive to the terminal that birthed them.

Accessing Environment Variables

Accessing environment variables is straightforward and consistent, regardless of their scope. To retrieve the value of an environment variable, the following syntax is employed:

Syntax

VARIABLE_NAME

This syntax is uniform for both local and global environment variables.

Displaying Environment Variables

To display the value of an environment variable, the echo command is employed, as demonstrated below:

Syntax:

echo $VARIABLE_NAME

To obtain a list of all global environment variables, Linux offers several commands:

1. `printenv` Command in Linux

This command provides a comprehensive list of all global environment variables.

printenv //displays all the global ENVs

2. `set` Command in Linux

Lists all environment variables, encompassing both global and local variables.

set //display all the ENVs(global as well as local)

3. `env` Command in Linux

Presents a list of global environment variables.

env //display all the global ENVs

How to Set Environment Variables in Linux?

Environment variables can be set using different approaches, depending on their intended scope.

1. Setting Global Environment Variables

Global environment variables can be defined using either the export or set command. Both methods involve specifying the variable’s name and value. For example:

$ export NAME=Value
or
$ set NAME=Value

Example:  

2. Setting Local Environment Variables

Local environment variables can be directly defined without the need for export commands. The syntax is as follows:

Syntax:  

$ NAME=Value

Example: 

To Set User-Wide Environment Variables in Linux

User-wide environment variables are configured within user-specific files like `~/.bashrc`, `~/.bash_profile`, `~/.bash_login`, or `~/.profile`. These variables are personalized and persist across system restarts. The following steps guide the process of setting user-wide environment variables:

Following steps can be followed to do so: 

Step 1: Open the terminal. 

Step 2: Edit the user-specific file (e.g., ~/.bashrc) using a text editor.

Step 3: Insert the variable in the file with the export statement:

export NAME=Value

Step 4: Save and close the file.

Step 5: Activate the changes using:

$ source ~/.bashrc

Example:

To Set System-Wide Environment Variables in Linux

System-wide environment variables are configured in files such as `/etc/environment`, `/etc/profile`, `/etc/profile.d/`, or `/etc/bash.bashrc`. These variables are accessible to all users and remain persistent across system restarts. Setting a system-wide environment variable involves the following steps:

Following steps can be followed to do so: 

Step 1: Open the terminal. 
Step 2: 

$ sudo -H vi /etc/environment

Step 3: Enter password. 
Step 4: Add variable in the file opened.  

NAME=Value

Step 5: Save and close the file. 
Step 6: To apply the changes, log out and log in again

How to unset environment variables?

To unset an environment variable, the unset command is used, or the variable can be assigned an empty value. For instance:

Syntax: 

$ unset NAME
or
$ NAME=''

Example:  

Permanent environment variables can be unset by editing the corresponding configuration files and removing the lines where they were initially defined.

Some commonly used ENVs in Linux

Example:

Conclusion

In this article we have discussed Environment Variable which are essential components of the Linux operating system., allowing processes and applications to access important configuration information. Overall, we can say it is important for a Linux User to understand the concepts of Environment Variable.