Is OSX a POSIX OS?

Question

What is it that makes an OS a POSIX system? All versions of Linux are POSIX, right? What about OSX?

Answer 1

Yes, OS X is based on Darwin BSD, and since 10.5 (Leopard - 18-May-2007) all Intel/AMD versions have been officially certified as compliant with the Unix 03 / POSIX standard eg.

• UNIX® 03 - Mac OS X Version 10.5 Leopard
• Apple Registers Mac OS X 10.8 Mountain Lion to the UNIX® 03 Standard
• UNIX® 03 - OS X version 10.11 El Capitan