How Linux, BSD, UNIX, and macOS Relate to Each Other
There is a family of computing standards trademarked as POSIX. The POSIX trademark is owned by IEEE.
An OS that uses the POSIX trademark must:
UNIX's requirements are a superset of POSIX's. In other words, all UNIX OSes are POSIX-compliant OSes, but not all POSIX-compliant OSes are UNIX OSes. The UNIX trademark is owned by The Open Group.
A UNIX is any operating system:
- That passes the applicable UNIX certification test suites
- Whose supplier has formally agreed to the terms of the UNIX Certification Program
MacOS is a mixed source UNIX OS.
- Each use an independently developed kernel
- Have no aim for binary compatibility among themselves
- Use the BSD License
- Each use the centrally developed Linux kernel
-
May modify the kernel for their own purposes, but there is, by virtue of the common kernel:
- Binary (not necessarily package) compatibility among Linux distros on the same CPU architecture
- Feature parity among all kernels based on the same release
- Each use the GNU General Public License (GPL) for the kernel at the very least
- The differences between the BSD License and the GPL, while outside the scope of this comparison, are significant
- Licensing and certification fees for both POSIX and UNIX are expensive relative to open source OS budgets. This appears to be the main reason BSD and Linux are neither certified nor licensed
- POSIX certification appears to involve an audited self-test, using a standardized testing suite supplied by the IEEE
- UNIX certification appears to involve approved results self-testing, using a standardized testing suite supplied by The Open Group
- There is at least 1 other standard that exists within the Linux ecosystem only: the Filesystem Hierarchy Standard
- The Open Group maintains a list of OSes that have been certified to UNIX standards