[dillab2]

Which folks haven't mentioned yet, but z/OS is also a proprietary UNIX, albeit in a very strange way given that it is sorta like a subsystem/different way of looking at the same things, but is also in implemented in the base control program (kernel), so it isn't really a subsystem.

And it isn't the same as AIX. IBM has two alive commercial UNIXs, which is kinda wild.

[racingmars]

Yes, and the z/OS USS is what I call aggressively POSIX-compliant. Its mission in life is to hurt you with strict adherence to the spec, not ever help you!

If there's any optional part of POSIX, it's not in there. You only get the 'MUST' parts of the base spec. No optional extensions, etc. When you're porting over C code to z/OS USS, you are often surprised when things you assumed were just part of every POSIX implementation aren't there. Fun times for all!