[t-3]

http://man.openbsd.org/sh

That's all you need to read to learn the shell. Learning about the commands requires actual hands-on work with a system, because the commands vary between systems and which ones you use depends on what you're doing. A general "how to be good at *nix command line" class will always be boring rote memorization, because there aren't really any concepts or skills to learn, just details.