You could look at the introductory exercises I wrote up here
. The first thread "The Command Line and Why Microsoft is Different" introduces how files work and gets into details about why. It's useful as background but not a quick reference. The second thread "More Command-line Goodness" then has a table of the very common Linux commands.
I've still got an article to write for MagPi about the command line... must get around to writing it.