So, I've been using linux for a while. If you want it to be, it's a constant learning process - always finding new tricks and shortcuts and so on.
Here is something that I wish I had known when I started using linux:
If you install a package on a .deb-based system, and want to know what files it installed, you can check with dpkg --contents /path/to/package. On Ubuntu, /path/to/package will normally be /var/cache/apt/archives/packagename. When this is useful, it is extremely useful.
Also, I wish that I would have done a man bash much earlier in my linux history. There's a lot of time-saving stuff in there.