Re: Unix shells
If you can afford it, by a couple of really really cheap boxes and put a few different Unix flavors on them.
recommend: FreeBSD, OpenBSD, NetBSD, & Solaris 7.
That will not only give you the most common Unix flavors on today's servers but you will be able to see their intricate details like the difference between how `ps' acts on each.
For shells, you will get: ksh, csh, sh, tcsh, zsh, pksh, bash and a couple more obscure ones.
I would recommend sticking to these at first.
ksh: unix standard shell ( default shell on Solaris & SunOS)
csh: standard and default shell on BSD Unix's
bash: standard and default shell on Linux
As for commands go and get O'reilly's
Unix in a Nutshell
Linux in a Nutshell
Learning the Korn Shell
Learning the C Shell
Learning the Bash Shell
all of which can be found at http://www.oreilly.com/
hope that helped