And if you're not a 50 year-old Linux admin, Arch wiki.
Edit: don't be put off by the Arch wiki if you don't use Arch. 99% of the time, Linux is Linux, and you can follow it for just about anything other than package management.
I fully own that. But I like the logical ordering of the page sections on the wiki, and if anything is unclear or info is missing there--which it is pretty rare--I'll hit up man in desperation