If you have a Macintosh, you can enter the Unix terminal by opening Terminal.app. (There's a way to do it in Windows, too, but I don't know how.) From there, you have command-line control of your computer. If you are a Raspberry Pi aficionado, you probably know about the Linux command line. This episode of Explaining Computers has a great introduction to the Linux terminal, and shows you some of the useful things you can do in it.