the hard way

Interesting guide here: "Learning Linux The Hard Way." 30 Unix exercises that you can try from your web browser.

From the introduction:

This is a simple guide written in “Learn X the hard way” style, author of which is Zed A. Shaw. The purpose of it is to give you some experience in working with Unix-like system. There are many good UNIX-Like systems out there, for example FreeBSD, OpenBSD, OpenSolaris and even MAC OS X, just to name a few. When deciding which system to use in this guide I chose Linux mainly because it is most popular UNIX-like server operating system, which means what you have much bigger chance encountering it in the wild in comparsion to other UNIX derivatives. And Linux-related skills are relatively easy transferable to other UNIX-likes.