In my experience, you pretty much learn this on the fly. You don't know how to do X, you google it, if you do X often enough, you'll eventually learn. After a while you end up mastering most of the unix suite (ls, grep, awk, du, etc..) so instead of googling stuff you write it.

Unlike fundamentals (think books like SICP, Introduction to Algorithms, K&R, the Dragon book, et al), this is just a collection of useful commands that do not bring barely any collateral learning aside from learning the 'UNIX' way.

If you really want to put up the effort to learn this, and you don't have any projects or anything that requires this knowledge, I've noticed lots of tech offices have a copy of UNIX in a nutshell always have a copy around. I've checked it out myself and it's pretty useful (I already 'know'), not sure how good is it for learning this stuff from the ground up.