A command prompt is a non-graphical interface that allows you to interact with your operating system. At the command prompt, you enter commands by typing their names followed by options and arguments. Most modern computers use a Graphical User Interface (GUI), which allows users to more intuitively access programs and documents. However some programs and commands are still only available only through a command prompt. To access the command prompt, follow the instructions appropriate to your operating system (Computer).

I know how to do Windows Command Prompt but had to find out the rest (Not Copied and Pasted).

WARNING: Use At Own Caution!
Don't type codes that you dont know or havent been taught by a IT Expert .

Windows

To access the command prompt in Windows, from the start menu, select run... In the 'open' field enter cmd (Windows NT, 2000 Xp, and Vista) or command (Windows 95, 98 and Me).

Mac OS X

Mac OS X is built on a version of Unix called Darwin. To access the Unix command prompt in Mac OS X, open the terminal application (app). It is located by default inside the Utilities folde, which in turn is inside the Application folder.

Unix

When you log into a Unix computer, your shell begins to run and provides you with a command prompt. The command prompt's appearance varies depending on the shell you're using.

Thanks For Reading,
Infomation By Trist's friend.
P.S. He is trained on everything to do with IT.
No harsh comments please.

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CLI (Command-Line Interface) is a mechanism for interacting with a computers operating syystem and/or software by typing commands.

The concept of CLI originated when teletypewriter machines were connected to computers.