I am looking for a website that contains a reference between how to do things on the command line in different unix OSs.
I have seen such site before, I am just unable to find it.
I know such site would be beneficial for the community.
The Unix Rosetta Stone (resource for sysadmins) might be the one you had in mind.
I have a good solution: Install 2 types of Linux OS: RedHat-base and Debian-base, FreeBSD and Solaris. Forget MacOS. Read and work with them, of course start with Linux then hack so on.
Please refer to following sites:
Don't forget to consider POSIX or GNU extension, some of commands has GNU extension, some of commands are POSIX. Please read carefully.