Linux Environment Variables List, Set, Create & Remove

Environment variables enhance and standardize shell environment on Linux systems. Standard environment variables are variables that the system sets up. Custom environment variables are variables that we set up for our specific requirements.

Viewing the value of environment variables

The echo command displays the specified text on the prompt. If you provide an environment variable, it shows the value of the specified environment variable. A dollar sign with the variable name instructs the shell to interpret the given text as the variable. Without it, it interprets the given text as regular and displays it as it is. Let us take an example. The SHLVL environment variable saves the levels of the shell. If we use it without the echo command with the $ sign, the echo command prints it as it is.

$echo SHLVL
SHLVL

To view the value of the SHLVL environment variable, use $ with it.

$echo $SHLVL
1

This variable keeps the number of subshells we open. The default is 1. For example, if we open three subshells, it shows the value 4 (1+3). The bash command opens a subshell. Enter this command three times and view the value of this variable again.

$bash
$bash
$bash
$echo $SHLVL
4

The echo command displays the value of a single variable. It requires the variable name. If you do not know the variable name or want to display a list of all environmental variables, use the env or printenv command. Both commands list all environment variables of the user who runs it. They show environment variables on the left and their values on the right. All default environment variables are in caps letters.

$env

$printenv

Creating/updating environmental variables (temporarily)

To create a new variable for the current session, specify its name and value as a command on the shell prompt. For example, the following commands define and verify an environmental variable XYZ with the value test.

$XYZ=test
$echo $XYZ
test

Use the same method to update or append an existing variable. For example, the PATH variable contains the search path for executing commands and scripts. It uses a colon (:) as the separator. The following command prints the current value of it.

$echo $PATH

The following command appends the current path and adds the /opt/bin directory to the search list.

$PATH=$PATH:/opt/bin

This change is temporary. It applies only to the current session. Shell removes it when the user logs out.

Creating/updating environmental variables (permanently)

A user's home directory contains the ~/.bash_profile file. It is a script file. It holds all custom environmental variables. Shell executes it every time the user logs in. Add your custom environmental variable at the end of this file. You also need to export the added environmental variable using the export command. Let us take an example. Open the ~/.bash_profile file. Add the following lines at the end of the file.

HISTSIZE=50
PATH=$PATH:/opt/bin
export HISTSIZE PATH

Save the file and log out to terminate the current session. Log in again and check the value of the HISTSIZE and PATH variables.

Creating/updating environmental variables for all users

Linux has a default profile file for all users. It runs this file every time a user logs in to provide a default user profile. This file is the /etc/profile. It is part of the default installation package. If an updated version of this file is available, the update process overwrites the existing file with the updated version. Because of this, administrators do not use this file to add custom environmental variables. Instead of editing this file, they create a custom script in the /etc/profile.d/ directory and use it to deploy to custom environmental variables. Shell gives priority to the variables set in the /etc/profile.d/ directory over the defaults set in the /etc/profile file. This override feature allows administrators to set custom environmental variables without worrying about updates.

Let us take an example. Login from the root user and create a file custom.sh in the /etc/profile.d/ directory. Add a new custom environmental variable. Save the file and log out from the root user account.

Login with a regular user account and verify the custom environmental variable.

Changing default or creating new environment variables only for new accounts

The /etc/skel directory contains default user profile files. When we add a new user account, the shell copies all files from this directory to the user's home directory. If we change profile files in this directory or add a new profile file, new user accounts get the updated files with the newly added custom profile file. Let us take an example. Login from the root user account. Create a backup of the default .bash_profile file and open it for editing. Add a custom environmental variable. Save the file. Add a new user account and logout from the root account.

Login from the new user account and verify the custom environmental variable. It does not affect existing user accounts. To verify it, log in with an account created before this change and check it.

Default environmental variables

Conclusion

Environmental variables define the shell environment. You can create new or update existing ones to customize the shell environment. The /etc/profile file defines the default environmental variables for all users. The /etc/profile.d/* files override the default environmental variables for all users. The ~/.bash_profile file overrides the default environmental variables for the particular user.