In the beginning…was the REPL

What is a command-line interface (CLI)? It’s a text-based environment for interacting with your computer that:

1. Reads some input from you,
2. Evaluates (or processes) that input,
3. Prints some output to the screen in response, and then
4. Loops back to the beginning to repeat that process.

Let’s look at what happens at each step, on a practical level, with the ls (list) command, which you’ll see in a few pages. For now, it’s enough to know that the ls command lists the contents of a directory.

Step	What it means
1. Read input	You type the ls command and press Enter.
2.Evaluate command	The shell looks up the ls binary, finds it, and tells the machine...