On the last day of 2025, a certified Unix died. HP-UX 11i v3, the final supported release, reached the end of its road on 31 December; what remains is a "mature support" arrangement for the Integrity servers it ran on, scheduled to lapse in 2028.¹ The cause of death had been visible for years. HP-UX ran on Intel's Itanium, Intel shipped the last Itanium in 2021, and an operating system with nowhere new to run is on a clock from that moment.²
It is worth pausing on what HP-UX was, because it was not a pretender. It held a genuine Open Group certification. It was Unix in the eyes of the body that owns the word. And the certificate, when the hardware underneath it ran out, bought it precisely nothing. This is the first thing to understand about the certified branch of the family: the paper is real, and the paper is not a promise about the future.
This Bow is four portraits of the Unix family, one branch per Monday. The map is a single tree with four boughs; this week we zoom in on the first of them, the certified.
What the Paper Actually Buys
A certified Unix is a product whose vendor paid The Open Group to measure it against the Single UNIX Specification, and passed. There are two levels still seen in the wild: UNIX 03, the common one, and UNIX V7, the stricter. The certificate says the system implements the interface, the system calls, the shell, the utilities, the behaviours an application written to the standard may rely on. It certifies that a system speaks Unix, not that it descends from Unix, and not that it has anywhere to go.
What that leaves is a register of products that share a word and almost nothing else. They run on different processors, solve different problems, and are owned by companies with very different reasons to keep them alive. The interesting thing about the certified branch is not what its members have in common. It is how differently the same certificate ages in each of their hands.
The Dead and the Frozen
HP-UX is the dead end, and it is the clearest case. No open successor exists; the codebase was proprietary, tied to proprietary hardware, and when both reached their limit there was nothing for a community to carry forward. The niche it served, big-iron HP-UX shops, is migrating to Linux and to the cloud, and the certificate did not slow that by a single quarter.
Solaris is the more instructive case, because it looks dead and is not, quite. Sun built genuinely original engineering into it: ZFS, the filesystem that treats the disk as something you can snapshot and never silently corrupt; DTrace, which let an engineer ask a running production system almost any question without stopping it; Zones, containers a decade before the word meant what it means now. Then Oracle bought Sun in 2010, and Solaris stopped being the centre of anyone's attention. Today Oracle Solaris 11.4 sits under Premier Support until November 2031 and Extended Support until 2037, a date Oracle quietly pushed out from 2034 in early 2024.³ It is not dead. It is frozen, and paid to stay frozen.
But here the certified story takes its most encouraging turn, and it has nothing to do with the certificate. When Oracle closed OpenSolaris, the open-source release Sun had made, the code did not vanish. It became illumos, carried since 2010 by the people who refused to let it go, and from illumos came a small family of living distributions: OpenIndiana, whose Hipster snapshot ships fresh in 2026; OmniOS, a minimalist server build with a stable release dated May 2026; SmartOS, tuned for virtualisation.⁴ ZFS and DTrace and Zones did not need Oracle to survive. Sun's Unix outlived its owner by being given away. The niche endured not through a support contract but through a fork.
The Still Living
AIX is the one certified Unix that is still being built rather than merely maintained. It holds the strict UNIX V7 level, the only system on the register that does, and IBM ships it forward onto new hardware: AIX 7.3 gained support for the POWER11 processors that arrived in mid-2025.⁵ Its niche is narrow and deep, the banks and insurers and back-office systems that run on IBM's POWER machines and value an operating system that behaves the same way for twenty years. It is proprietary, it is tied to one vendor's silicon, and within that niche it is genuinely alive. The certificate here is almost beside the point: customers stay for the reliability and the hardware, not the paper.
z/OS is the strangest member. It is certified Unix in the form of a POSIX environment, UNIX System Services, layered over MVS, a mainframe design from the 1960s with no Bell Labs or Berkeley anywhere in it. It runs a large share of the world's banking transactions, and it proves the point the certificate cannot escape: the mark sits at the surface, and the substance underneath it can be something else entirely. It is Unix on the outside and mainframe all the way down.
The Double Citizen: Darwin, and macOS
The largest carrier of certified Unix in the world is not on a mainframe or in a bank. It is on the desk, and it deserves to be split into the two things it actually is, because they are not the same thing, only dependent on one another.
Darwin is the open-source core. It is Apple's, released under its own licence at opensource.apple.com, and it is built from the XNU kernel, a hybrid that wraps a Mach microkernel core in a BSD layer, sitting on a userland descended from FreeBSD. Darwin holds a kind of double citizenship. On the family axis it is a genuine carrier of BSD code, a cousin of the systems in next week's piece. As an artefact it is Apple's foundation. And it is real enough on its own that a community project, PureDarwin, assembles it into a bootable system independent of macOS.⁶ Darwin can be held in the hand without Apple.
macOS is the other half, and the half that holds the certificate. It is the proprietary, complete operating system built on top of Darwin: the window server, the frameworks, the applications, the polish. The Open Group certifies macOS, the full distribution, at UNIX 03, not Darwin alone. Every Mac sold is a certified Unix, which makes Apple the largest certified-Unix vendor by a margin no enterprise vendor approaches.
The relationship is worth stating precisely, because it is the hinge of this whole Bow. macOS needs Darwin; Darwin does not need macOS. The certified citizen rests on an open-source one that carries BSD blood in a hybrid kernel. The paper belongs to the top layer; the lineage belongs to the layer beneath it; and they are held together in one product that most people never think of as Unix at all. That split, certified surface over inherited open core, is the thread the next two Mondays pull on.
The certificate is a pass to a niche, not a bond of family, and the niches end in every possible way: one dead, one forked free, one still built, one a Unix surface over a mainframe heart, and the largest of all really two systems in one coat. What keeps each alive is something the paper never certified.
The Point
The thing the certified members share is a word and a measurement. What keeps each of them alive, or fails to, is something the paper never certified: a forward hardware path, a community willing to fork, a vendor still building, an open core beneath the shell. The register tells you who paid to be measured. It does not tell you who will be here in five years.
Next Monday: the same lineage, minus the paper. Four siblings who forked and stayed whole.