want shell should I learn 20

Try "man sh", man "ksh". Look for ways to make the shells identify themselves, something like "sh --version". I havn't used AIX for about ten years, so I just guess they do have some variant of ksh installed somewhere. Consider looking in other directories thanbin andusr-bin, there may beusr-bin5 or similar. Look around to find out what you have available, and learn that.

I recommend that you shun csh, and probably tcsh, for a couple of reasons. The others have enough similarities that what you learn using one, transfers mostly to others. "Bourne shell" is the originalbin-sh, most others are evolutions of it.

You may be allowed to install bash, it is freely downloadable from the net, but you may have to complie it on the computer you are using, which is a bit (more than a bit) non-trivial on non-linux boxen not having the gnu tools on them. See if you can find a precompiled bash for the relevant OS-architecture. If you have ksh or pdksh, don't bother, use them. If not, and you are not allowed to install in system directories, you can most likely install in your home directory, and set up your path accordingly.

-Enrique