Ask HN: Linux terminal-based learning packages?

Earlier this year someone posted a linux package that allows you to learn bash commands, or possibly something else, from in the terminal. It was in response to a post about interactive online tutorials and someone replied "why not just learn it in the terminal using {package}". I can not for the life of me find the post or similar packages.

What are some installable packages for learning bash commands and other topics from in a linux terminal?