We maintain a handout of linux commands for MGCF users and a link can be found at:
Linux - Basic Commands

If you want more extensive background then here is a nice tutorial:
http://www.ee.surrey.ac.uk/Teaching/Unix/
If you can understand the Intro and Tutorials 1 and 2, then you have most of the skills you will need for everyday work in the MGCF.

The above page was down as of Dec 7, 2017:
This is a good alternative. If you can understand the Intro and Tutorials 1-4, then you have most of the skills you will need for everyday work in the MGCF. Do the rest if you have more advanced needs but skip sections 6 and 7.