# What are Environment Variables? 環境變數

Environment variables hold values related to the current environment, like the Operating System or user sessions.

## Path
One of the most well-known is called `PATH` on Windows, Linux and Mac OS X. It specifies the directories in which executable programs`*` are located on the machine that can be started without knowing and typing the whole path to the file on the command line. (Or in Windows, the Run dialog in the Start Menu or `Win+R`).

On Linux and Mac OS X, it usually holds all bin and sbin directories relevant for the current user. On Windows, it contains at least the C:\Windows and C:\Windows\system32 directories — that's why you can run calc.exe or notepad.exe from the command line or Run dialog, but not firefox.exe. (Firefox is located in C:\Program Files\Mozilla Firefox. For information on how to include Firefox, go here.)

For example, typing calc (the .exe can be omitted) in the command line on Windows will start up the Windows Calculator.

`*` You can add support for file extensions other than .exe by editing %PATHEXT%.

## Link
https://superuser.com/questions/284342/what-are-path-and-other-environment-variables-and-how-can-i-set-or-use-them/284351#284351