Unixes (and Linuxes) have most features, commands, concepts, etc. in common. If you learn one Unix - any Unix - and sit down in front of another one you will be able to do anything a user wants to do without even thinking. Alas, the finer points of administration - management of users, filesystems, network adapters, tuning, etc. - can differ widely. So, while any Unix system is almost the same from a user POV it can differ widely for the administrator.

The position of software developers is somewhere in between: Unix is more like a standard than a certain system and in principle, if you use all the standard Unix system calls you are guaranteed to be able to compile the same source and have it run on any other Unix-system unchanged. Well, that is the theory and while the practice is of course different it comes remarkably close to being so.

Always bear in mind that Unix is not only an OS at all. Unix comes out of academic research and it is as much a culture as it is an OS. Most times it isn't about doing something, but doing it right. In this regard right usually means "being in tune with a long-term strategic plan and according to some bigger concept of how things can and should be done". This makes Unix systems, diverse as they may be, quite predictable on a certain level. There are only so many ways to do something in the "right" way.

This "right way" is often something hard to point at. Like, say, the concept of "fairness" or "justice" it is hard to explain what exactly it encompasses. Still, anyone immersed in the same culture will immediately recognize a thing to be "just" or the opposite in a very consistent way. The judgement of Unix people about attempts at a solution being right or wrong will converge over time to a similar degree.

I hope this helps.

bakunin