Unix official conformance is moot for the rest of us. Most mainstream Unix, Unix like (read Gnu/Linux) distributions that do not officially comply would likely pass 99%+ of the tests.

Moreover, these standards are aimed to developers whose goal is to write portable code and scripts, not to admins or end users, not that much to the majority of software developers who usually only care of a limited set of OSes.

That's the reason why moving code, admins or users from one distribution to another is often painful.

I agree ... I care about the standards as much as I need to care about the standards, but I wouldn't make a decision about whether or not I like an operating system based on whether it's a registered product of some kind. Not taking price into consideration, I wouldn't personally buy Apple rather than use Linux. Linux is a very, very good operating system to me. If I wanted something "more UNIX" - which I think is a silly thing to worry about, really, especially when you can't make up your mind about what being "more UNIX" even is - I would use a *BSD or OpenSolaris or a commercial Unix of some kind. Going by everything I've read about this subject, there isn't "A UNIX" any more. Even all the registered UNIX operating systems like AIX and HP/UX aren't the exact same, it's just what they have in common.
Linux and UNIX have much more in common than a lot of people act like or think they do.
http://www.fas.harvard.edu/~lib215/r...inux.unix.html

From "Linux in a Nutshell", published by O'Reilly:

From IBM (who makes AIX, a registered UNIX operating system):
http://www.ibm.com/developerworks/ai...difflinux.html

Also, from kernel.org:
:p

Don't forget that the Open Group charges handsomely for testing and declaring a product "compliant." (You're talking thousands and thousands of dollars on a regular basis.)

For that reason, you're not going to see Linux distributions declared UNIX '03 compliant for a long, long, time . . . or ever, most likely.

Companies like Sun, IBM, and Apple have the extra cash to throw at obtaining a name. Since Linux is free, and, despite the nuances of its licensing, exists essentially in the public domain, no profit-motivated company has any compelling reason to pony up the hefty monies required to get it certified.

Even companies like Red Hat and Novell, who sell Linux and Linux support commercially, have almost nothing to gain from such a certification. After all, their customer base, and specifically the component of their customer base influencing the purchasing and usage decisions around their products (sysadmins, network engineers, etc) are all savvy enough that they already understand how Linux will fit into their infrastructure, regardless of what it is or can be legally called.

It's clear that we are all just traipsing around semantics only made necessary by intellectual property rights on a name that somebody owns and keeps getting sold.

That's why we always have to say, "of course, Linux isn't really UNIX (wink wink)."

I mean, come on, the very fact that you have to say that Slackware and FreeBSD are not UNIX, but MacOS X is demonstrates how laughably meaningless the label of UNIX imposed by the SUS has become.

Run Solaris, run AIX, run HP-UX, then try migrating to Slackware or FreeBSD. After you are done, migrate anew from those other real UNIXes to MacOS and tell me which migration was more seamless.

You probably don't even have to do it to tell me the answer.

It is also worth noting that UNIX has evolved FAR beyond the System VR4 days, even the directly-derived versions like Solaris. AT&T System VR4 would not meet the Open Group's SUS.

There is one great point in this thread with which I feel that I should qualify this answer:
That being said, one would have a hard time finding incidences in which code written for SUS-compliant operating systems would not port to Linux, and probably an even harder time with FreeBSD.

(FreeBSD was originally derived from 4.3BSD-Lite, which was the actual, honest-to-God, AT&T UNIX-descended version of BSD, just stripped of any AT&T code that had not been re-written yet.)

I just have to say this to satisfy my own desire, as I haven't had any opportunity to before, so I apologize: I friggin' love FreeBSD . . . I think it is just about the best free OS for a server machine. I have to say, though, that I can't imagine anything other than sheer nerdiness which would motivate someone to use it on their desktop. (Not that there's anything wrong with nerdiness . . . but a FreeBSD desktop? Now that is some hard-core nerd style.)

Summarizing, Linux may not be UNIX, but you could, speaking out loud, call certain distributions of it like Slackware, Red Hat Enterprise Linux, and Suse "you-nicks" and be correct. :)

Hence the handy contraction *nix, although its unpronounceable unless you like saying star-nix or (worse) asterisk-nix :( ;)