Linux is not UNIX®.
FreeBSD is not UNIX®.
OSX is not UNIX®.
AIX, HP-UX, IRIX, SCO OpenServer, Solaris and UNIXware are all UNIX® systems.
The Open Group owns the UNIX® brand, and has the sole right to say what is UNIX®. POSIX is a set of standards that don't themselves define UNIX®. BSD, the Berkeley System Distribution, is defunct. There hasn't been a BSD for nearly a decade. FreeBSD, OpenBSD etc. are all descendants of 4.4BSD-Lite, which is a legacy of the BSD project that is not itself UNIX®. Corruptions of "UNIX", like "Unix" or "unix" may have specific meanings to individuals, but don't have any official meaning.
Q. Is Linux really like Unix?
A. Yes. Linux works like
UNIX® does in many ways. It just isn't UNIX®.
Q. Is Linux a "Unix OS"?
A. Linux is not a recognized UNIX® system. "Unix OS" is an imprecise term that has no official meaning.
Q. Is BSD pure UNIX?
A. BSD UNIX is no longer available. But when it was, it did include genuine AT&T UNIX. To use BSD, you had to purchase a UNIX license from AT&T first. Since then, the UINX codebase and rights have changed hands several times. Currently the Open Group owns the rights, while Caldera owns the reference code. In that respect, Caldera's UNIXware product is the most pure UNIX® system, being directly descended from the last AT&T UNIX product. Getting back to the question, BSD wasn't "pure" UNIX because it was pure UNIX plus some proprietary extensions.
You should note that since AT&T divested, UNIX® systems don't come from the same codebase; conformance comes through testing instead. In other words, today anybody can develop a UNIX® system, whereas before it all came from AT&T. FreeBSD and other 4.4BSD-Lite derived projects include no UNIX® code or license. They came from the University of California, not AT&T. The U of C was never a producer of UNIX.
Q. Is "Unix" a generic term, like the famous Xerox case?
A. No, UNIX® is still a protected copyright. Using the term improperly ranges from being a personal faux pas
to being outright fraud when the trademark is used without consent for monetary gain. In practical terms, it's a good way to distinguish between dilettante and professional. Because the UNIX paradigm is highly precise, you know that you don't want to give root access to a person who might accidentally issue a command like `rm -Rf /*'
Q. So whats the difference between BSD Unix and Linux like SuSe?
A. BSD UNIX was a custom UNIX distribution from the University of California at Berkeley. BSD UNIX started with real AT&T UNIX, and added certain proprietary features and utilities to it. S.u.S.E. is a Linux distribution that uses the Linux OS kernel, similar to how BSD was a distribution of the UNIX OS. But S.u.S.E. is not a UNIX® distribution, it's a Linux distribution. The difference is the software -- it's not the same. Although some aspect resemble
each other, they're actually quite different.
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