This guide from Librenix introduces the Linux CLI with 10 useful basic commands including pwd, ls, file, and the ever popular grep. Even as an intermediate CLI user, I found this guide very helpful. One cool tip:

…a handy command is supplied with Linux named ‘file’. For example, I have a file named ’sample.c’ in my code directory. I can learn a bit about that file by entering the following command:

[rayy@barton0 code]$ file sample.c
sample.c: C++ program text

Full Story: MiniLesson: An introduction to Linux in ten commands