I came across this site the other day, as someone pointed it out to me. Looks to be very handy for many Unix commands.
It has many different sections, all with explanations of what the commands do it does pretty much cover everything on there, I recommend bookmarking this site if you do work in the Unix/Linux world. It covers everything from looking at users to simple C programming, Shell scripts, Sed & Awk example and references, I am sure there is something to learn whether you are new to Linux or you are a veteran in the OS.
The latest page can be found here, you can also download a PDF version or a booklet version.
I am always interested in feedback so please feel free to add any comments, or you can mail me here. If you would like to submit a quick tip with full credit and links back to your site then also feel free to contact me.
