For the command line commands:

http://linuxcommand.org/learning_the_shell.php

Read this. Learn. That was one of the first things I did when getting my N900, and it keeps you from floundering around completely when something has to get done with the command line.

As for corrupted file system fixes, just scroll up to the power search, type in "corrupted filesystem" or something, and read through the threads. Or, alternatively, just go to Google, and type "corrupted filesystem". Or wait until someone on here has the time to search the relevant stuff for you and post it.