You can use the man command on Unix, and the help command in Windows to find information about commands. I've been playing a very dirty trick on you this whole time. I could have just told you to do this and read about each command rather than memorize what they do. But, if I did this, you'd be lost because you wouldn't know the basic commands you have to know, or how directories work, or what a wildcard is, etc etc.
Hopefully you forgive me for not telling you about this awesome tool until now, but now when you forget what a command does, just use the help.
For the price of most other course's PDFs only, you can get the full PDF for this class and 2 videos demonstrating the whole book for both Unix/OSX Terminal and Windows PowerShell. The course is self-paced so you can go through it any time you want, as many times as you want.