Is Mac OS X a UNIX-based OS? [duplicate]

Question

Is OS X Mountain Lion a pure UNIX-based OS?

Some tutorials say that it is a Unix-like OS, but after version 10.5, all Mac operating systems are based on pure UNIX.

Is this true?

Answer 1

There's a lot of discussion on this topic, but as far as Apple's concerned, yes it's UNIX:

Open source UNIX foundation

• POSIX-compliant, Open Brand UNIX 03 Registered Product
• Open source kernel based on FreeBSD and Mach 3.0
• 64-bit OS using LP64 data model
• Support for multiple CPU and GPU cores via Grand Central Dispatch and OpenCL
• Hand-tuned, standards-compliant scalar and vector math libraries

Standards-based networking

• Complete IP-based architecture supporting IPv4, IPv6, and L2TP/IPSec VPN
• Rich zero-configuration discovery and naming via Bonjour and Dynamic DNS
• Interoperable file serving via NFS, AFP, SMB/CIFS, and FTP
• Powerful Apache web services
• Open Directory services built on LDAP and Kerberos for single sign-on

Comprehensive UNIX user environment

• Standards-based graphics built on PDF (Quartz), OpenGL, and H.264 (QuickTime)
• Full-screen terminal with xterm-256color support
• Familiar UNIX/Linux utilities (such as emacs, vim, and bash)
• Free Xcode developer tools based on Clang/LLVM

Answer 2

The UNIX trademark is owned by The Open Group, which basically uses it as a certification mark for SUS-compliance. OS X has been registered as a UNIX 03-compliant product since 10.5, so it qualifies as UNIX (not just UNX or Unix-like) in that sense. Most GNU/Linux distributions are not registered as SUS-compliant, so they should be called something like UNIX-like or UNX in that sense, even though neither of those terms is used by The Open Group.

OS X could also be considered to be genetic Unix in the sense that Darwin was based on FreeBSD (or it has used FreeBSD as a reference platform), and BSDs are usually considered to be genetic Unix, since the original BSD was based on AT&T code. Both GNU and Linux were rewritten from scratch though, so GNU/Linux platforms are not generally considered to be genetic Unix.

In a functional or spiritual sense, OS X is maybe less Unix (whatever that means) than platforms that are merely "Unix-like" in the first or second senses, but OS X could still considered to "be Unix" in all three senses.