Lets get the obvious out of the way first:
The best bet is just to install UNIX to a PC.
Hint: Solaris = UNIX
The next best is to install a unix-like OS like linux.
If you do not want to install to a PC, then you could try one of the live distributions. Knoppix is a favorite for learning linux and is often used in a school environment where installing a new OS is frowned upon. Alternatively, consider installing something like vmware and install a linux VM to that.
If none of the above works for you (perhaps the computer owner/sysadmin won't let you) then you want an online unix simulator.
Presumably you want to learn the shell commands? In which case, L-Nix
may be right for you. It is available for windows under a demo license.
Eduflex has a unix course
- their simulator uses JAVA.
While you are at it, take a look at:
However, you wanted one you could use online: and I simply do not know of any. I cannot think why anyone would maintain one for free either. (Note: there will probably be some that will be available to university students etc by people who think that web-based solutions are just the bees knees.)
Hmmm... of course it used to be possible to use Richard Stallman's account at MIT and so practise on a real unix machine... (famously without a password, don't know if that is still the case.)