Well, so is z/OS (the old IBM mainframe operating system formerly known as "MVS"). I just looked it up: It has actually passed the full OpenGroup testing suite for Unix certification, as long as you use the correct version of the C++ compiler on it. In reality, z/OS is so far away from the "Unix philosophy", it isn't funny. To begin with, even on recent versions, you actually had to write JCL decks (on virtual punched cards) to perform certain system administration tasks. No shell here! It also contains no single line from the old Bell Labs or BSD kernels. And it usually runs in EBCDIC, not ASCII.Well...MacOS is certified UNIX.
MacOS 10.14 is UNIX certified. FreeBSD can be certified. It just costs a lot of money and every version would need to be re-certified. Same for POSIX. FreeBSD is POSIX compliant but not certified. Again because this costs a lot of money. As far as I know no Linux distribution is UNIX or POSIX certified. Windows was POSIX certified but only for NT 3.5, 3.51 and 4.0.Regarding Mac OS, since Apple switched his OS to the NextStep code (before I don't know), this is a BSD and POSIX compliant OS.
Is this a "POSIX" certfied ? Probably yes, but in any case Mac OS can't be UNIX certified because basically BSD IS NOT UNIX, and as FreeBSD can't be a UNIX certified OS.
The UNIX 03 Product Standard is the mark for systems conforming to Version 3 of the Single UNIX Specification. It is a significantly enhanced version of the UNIX 98 Product Standard. The mandatory enhancements include alignment with ISO/IEC 9989:1999 C Programming Language, IEEE Std 1003.1-2001 and ISO/IEC 9945:2002. This Product Standard includes the following mandatory Product Standards: Internationalized System Calls and Libraries Extended V3,Commands and Utilities V4, C Language V2, and Internationalized Terminal Interfaces.
[…] To begin with, even on recent versions, you actually had to write JCL decks (on virtual punched cards) to perform certain system administration tasks. No shell here! […]