Nobody really likes to type boring filenames, and that's what got completion started back in the day—type a few letters of a filename, press Tab, and the shell will do the rest for you. Zsh goes the extra mile though and actually allows you to complete almost anything. By default, the Tab key is bound to a completion command in zsh.

Like Bash, zsh defaults to filename completion. Unlike anything else, however, zsh can enable the completion for practically everything that dares to rear its head in the command line—paths, external and built-in commands, aliases, functions, and options; you name it. And even if you can't name it, you can program it, as we will learn shortly.

Originally, zsh used a built-in module with a special syntax in order to provide completion. Luckily for us, this was eventually replaced by an even simpler mechanism. We'll focus on the new completion system that is entirely based on shell functions.

Go ahead and pop open that .zshrc file of...