Why macOS Sequoia’s UNIX Certification Matters for Developers

Apple macOS 15 Sequoia was officially released in mid‑September and is now an officially certified UNIX™ operating system. It appears in the Open Group UNIX® certification list with two separate entries: one for the Apple Silicon version and one for the x86‑64 version.

Unix is essentially POSIX

The term “Unix” no longer refers to the original AT&T source code; after Novell’s 1993 acquisition of Bell Labs’ UNIX assets, the meaning shifted to refer to POSIX‑compatible systems. POSIX is a set of compatibility specifications and tests that, when passed, certify an OS as UNIX‑like. Major systems such as IBM’s z/OS meet these criteria.

POSIX has evolved over the years. The latest UNIX V7 (POSIX.1‑2008) was declared by IBM AIX 7, and the UNIX Version 4 specification was last revised in 2018, with a 2024 draft that sees little attention as the industry moves toward free and open‑source Unix‑like systems.

What makes an OS “Unix‑like”?

An operating system is considered Unix‑like if it looks like Unix, behaves like Unix, and allows programs written for Unix to run with little or no modification. macOS’s core meets these criteria.

The XNU Kernel

macOS uses the XNU kernel, which combines a Mach microkernel with a large BSD‑derived user space. XNU’s lineage traces back to NeXT’s acquisition by Apple, incorporating enhancements from DEC OSF/1 (later sold as Compaq Tru64) into the Mach component. The kernel source is publicly available on GitHub.

Most of the user‑space components—command‑line tools, shells, and many utilities—originate from BSD and are open source. The graphical layer, however, is proprietary to Apple and is primarily written in Objective‑C and Swift.

Darwin and Related Projects

Apple historically released the underlying system as the Darwin project, and several community‑driven forks such as OpenDarwin and PureDarwin attempted to build alternative distributions. One notable effort is NextBSD, which retains the FreeBSD kernel while integrating Apple‑specific components like launchd, effectively acting as Apple’s version of systemd.

Evolution of Apple’s Operating Systems

After acquiring NeXT in 1996, Apple introduced Rhapsody (based on NeXTstep), which later became Mac OS X Server 1.0, then Mac OS X, and eventually macOS (formerly OS X). Throughout this evolution, the core has remained rooted in BSD and Mach, while the GUI layer transitioned to modern Apple technologies.