> For the complete documentation index, see [llms.txt](https://kryptocoder.gitbook.io/hacking-knowledge/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://kryptocoder.gitbook.io/hacking-knowledge/linux/functional-structure/environment-variables.md).

# Environment Variables

An **environment variable** is a user-defined value that can affect the way running processes behave on a computer. In Linux, we can check them with the command `env` and identify them because they start with the `$` symbol.

Some of the most common preset environment variables are:

* **`$PATH`:** List of directories for searching executable files or other directories. By default, contains the directories of system applications and installed apps
* **`$USER`:** The user who is currently logged in
* **`$PWD`:** Current working directory
* **`$HOME`**: Home Directory path
* **`$$`:** Current shell PID
* **`$SHELL`:** The path to the current shell that determines which command-line interface is being used
* **`$TERM`:** Indicates the type of terminal to emulate when running the shell
* **`$LANG`:** Specifies the language and locale settings for the user environment
* **`$EDITOR`:** The default text editor set for the user
* **`$MAIL`:** The path to the user's mailbox
* **`$DISPLAY`:** Used in graphical environments to specify the display server to connect to
* **`$HISTSIZE`:** Defines the number of commands to remember in the command history for the current user session