Linux

You've probably got an app called "Terminal" installed. Look for it in your launcher. You'll get an interface like this:

A small black window with a bit of text that says "bash," followed by a single dollar sign.

You can type and run shell commands at this prompt:

The same image as before, but the prompt says "echo Hello!" The line of text below says "Hello!"

That's all there is here! Go to learn the basics.