Environment variables are settings within a computer's operating system or shell that control how programs and processes behave.
According to the reference provided, an environment variable is a user-definable value that can affect the way running processes will behave on a computer. Environment variables are part of the environment in which a process runs.
Understanding Environment Variables
Think of environment variables as named values that store information relevant to the operational environment of a program or command. When a process starts, it inherits a set of environment variables from its parent process or the system. These variables can influence various aspects, such as where the system looks for executable files, the default text editor to use, or specific configuration settings for an application.
Why Are They Important?
Environment variables provide a flexible way to configure applications and scripts without hardcoding specific paths or settings directly into the code. This makes software more portable and easier to manage across different systems or environments (like development, testing, and production).
- Configuration: Store database connection strings, API keys, or other settings that change between environments.
- Path Management: Tell the system where to find executable programs.
- Localization: Define language and regional settings.
- User Preferences: Specify default editors, home directories, etc.
Common Examples
Many environment variables are standard across operating systems (though their specific names might vary slightly between Windows, macOS, and Linux). Here are a few well-known examples:
| Variable Name | Purpose | Example Value (Linux/macOS) |
|---|---|---|
PATH |
List of directories to search for commands. | /usr/local/bin:/usr/bin |
HOME |
Path to the current user's home directory. | /home/username |
USER |
The name of the current user. | username |
PWD |
The current working directory. | /home/username/documents |
LANG |
Default locale and character encoding. | en_US.UTF-8 |
Note: On Windows, similar variables exist like Path, UserProfile, Username, etc.
How They Affect Processes
When you launch a program, it gets a copy of the environment variables from the process that launched it (e.g., the command shell). The program can then read these variables to adjust its behavior.
For instance, when you type a command like python, the operating system looks through each directory listed in the PATH environment variable until it finds the python executable. Without PATH, you would have to type the full path to the executable every time.
Similarly, a web application might read an environment variable called DATABASE_URL to know how to connect to its database, allowing you to easily switch databases by simply changing the variable's value in the environment where the application runs.
Setting Environment Variables
Environment variables can be set temporarily for a single command, for a user's entire session, or permanently at the system level.
- Temporary (Shell Session):
- Linux/macOS:
export MY_VARIABLE="some_value" - Windows (Command Prompt):
set MY_VARIABLE=some_value - Windows (PowerShell):
$env:MY_VARIABLE = "some_value"
- Linux/macOS:
- Persistent (User/System Level): This involves modifying configuration files (like
.bashrc,.zshrc,/etc/environmenton Linux/macOS) or using system settings UIs (on Windows). Changes typically require logging out and back in or rebooting.
By leveraging environment variables, system administrators and developers can manage configurations efficiently and maintain flexibility in how applications run.
