Environment variables in Linux

Linux is an open-source operating system widely used in various applications, from mobile devices to servers. One of the essential features of Linux is environment variables. In this article, we will discuss environment variables in Linux, their usage, and how they can be helpful to developers.

What are Environment Variables in Linux?

Environment variables are a set of dynamic values that define the operating system environment. They can store information such as the user's home directory, the path to the executable files, or the default editor. Environment variables can be accessed and modified by the shell or the programs running on the system.

Environment variables in Linux have a unique name, which is usually in uppercase letters. They are set and unset by the shell or the user. The value of an environment variable can be a string or a number, depending on the variable's purpose.

How to set environment variables in Linux?

Setting environment variables in Linux can be done using the export command.

For instance, to set the PATH variable, which defines the search path for executables, type the following command:

export PATH=$PATH:/usr/local/bin

This command adds the /usr/local/bin directory to the end of the PATH variable, which means that executables in this directory will be executed if no other matching executable is found in the preceding directories.

The two files that are imperative to set environment variables in Linux are:

1. ~/.bashrc - This file is a shell script that runs every time a new terminal session is started. It is used to define environment variables, aliases, and functions that are specific to the user.

2. /etc/environment - This file is a system-wide configuration file that contains environment variables that are applied to all users on the system. It is read by the PAM (Pluggable Authentication Modules) system during login and sets environment variables for all sessions.

To set environment variables, you can edit either of these files and add the necessary variables with their values. However, changes made to ~/.bashrc only apply to the current user's terminal session, while changes made to /etc/environment apply to all users on the system.

There's one more file that every Linux user must be aware of:

The .bash_profile file is another one that is commonly used to set environment variables in Linux. This file is also a shell script, similar to the .bashrc file, but it is executed only once during login, whereas the .bashrc file is executed every time a new terminal session is started.

The .bash_profile file is typically used to set environment variables that need to be set once during login and do not need to be changed often. For example, you might set the PATH environment variable in .bash_profile to add directories containing executables to the search path.

If you are using the Bash shell, it is recommended to use the .bashrc file for defining environment variables, aliases, and functions that are specific to the user, and the bash_profile file for executing commands or scripts that need to be run only once during login.

However, not all Linux distributions use .bash_profile by default, and some use other files like profile or login instead. It is always best to check the documentation of your specific distribution to determine which file to use for setting environment variables.

How to use environment variables in Linux?

Environment variables can be used in Linux by enclosing their names in the dollar sign and enclosing braces, like $VARNAME. This notation tells the shell to replace the variable's name with its value.

For example, to display the current working directory, you can use the PWD environment variable, like this:

echo $PWD

This command prints the current working directory to the terminal.

Another example is the HOME environment variable, which contains the user's home directory. You can use this variable in a command like this:

cd $HOME

This command changes the current working directory to the user's home directory.

Scenarios of using environment variables:

Environment variables can be used in various scenarios, including shell scripts and programming languages like Python.

Here are some examples of how environment variables can be used in code blocks.

Shell Script:

Suppose you have a shell script that needs to read a configuration file. The path to the configuration file is stored in an environment variable named CONFIG_PATH. You can use the following command to read the file:

source $CONFIG_PATH

This command tells the shell to read the file specified in the CONFIG_PATH variable and execute its content.

Python:

Suppose you have a Python script that needs to access the user's home directory. You can use the following code snippet to get the home directory using the HOME environment variable:

import os

home_dir = os.environ['HOME']
print(home_dir)

This code snippet imports the os module and uses the environ dictionary to access the HOME environment variable's value.

Ending Remarks:

Environment variables are an essential feature of Linux that allows developers to store and access dynamic values. They can be used in various scenarios, including shell scripts and programming languages. Understanding how to set and use environment variables in Linux can significantly improve the development process and make scripts more flexible and portable.