If you plan to work only on the GUI, then you only need to know a handful of commands to start virtuoso.
If you plan to do some command-line simulations or do some direct netlist editing or things like that, then you probably need to know only a little bit more.
You DON'T need to be an EXPERT.
Just being able to navigate through linux terminal will mostly be enough. And also perhaps some of the shortcuts in the text editors like gvim or emacs. These will help speed up some of the stuff, but you will pick them up as you go along.