Environment Variables

In Linux, Environment variables are named key-value pairs that store information about your operating environment and configuration. They are crucial because they let the operating system, applications, and scripts know about elements such as :

• Where programs are location; _via _PATH.
• Your user/currently logged-in directory; HOME.
• Language or locale settings.
• System-wide or app-specific configurations.

In simple terms, environment variables are a foundational way that Linux customizes, configues, and manages the actions of programs and users through simple “key=value” pairs.

1. Structure

Each environment variable has a name (usually all-uppercase by convention, like PATH, HOME, EDITOR) and a value (a string or set of strings).

Syntax:

NAME=value

Example:

PATH=/usr/local/bin:/usr/bin:/bin
HOME=/home/iamyaash

You can see that we are separating directory location using : colon.

2. Purpose

• Customization: Set your preferred editor, default shell, time zone, etc.
• Application Configuration: Programs often check environment variables for things like database passwords or settings.
• System Management: Admins set global variables to manage environment settings for all users.
• Dynamic Context: Environments variables adapt applications to different users, folders or hardware settings. Eg: Try running this command echo $HOME with different users, and you will see different output each time.

3. Types

• System Environment Variables: Affect all users and entire system. Set in files like /etc/environment or /etc/profile.
• User Environment Variables: Affect only user session. Typically set in ~/.bashrc, ~/.profile, etc.
• Session Variables: Exist only for the life of current terminal/session.
• Persistent Variables: Remain across reboots/sessions because they are defined in configuration files.
• Read-Only Variables: Cannot be changed after being set in a session.

4. Usage

Programs and the shell read these variables to determine settings like which directories to search for command ($PATH), where your home folder is (HOME), or what language to use (LANG).

Note: You can see your environment variables with commands like printenv, env, or set.

5. Features

• Values are case-sensitive and, by convention, variable names are ALL CAPS.
• Some variables, like PATH, use colons (:) to separate multiple values.
• Environment variables are inherited by child processes, making them essential for scripts and subprocesses.

Common Environment Variables