Meta question: where do you actually learn this stuff? I am a developer with an intermediate desktop level command line knowledge learnt mostly on the job but would appreciate a crash course on system-level command line, diagnostics, devops, etc.
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.
Also, talking with/demanding explanations/debating with geekier than thou friends has been really helpful with the bigger concepts.
I had the privilege of working with a badass russian unix hacker for some time, he taught me how to do black magic with find, grep, sort and uniq.
I have the feeling ops people can be somehow.. "measured".. by how many things they had exploding in their face, and then learned how to fix them.
For example, looking at TFA, the first thing I thought is "oh, `df -i` should be there too".
Because I ran out of inodes enough times that I learned to check that (though I am not a sysadmin).