Last Monday's piece set out the map: where Unix actually runs in 2026. Apple's certified macOS on hundreds of millions of desks. Netflix's FreeBSD-based Open Connect under a substantial share of evening internet. Sony PlayStation, Juniper Networks, IBM AIX, Oracle Solaris, HP-UX, z/OS. Linux, we noted, was not on that list: not because it does not matter, but because it is Unix-like, not Unix.
Last week's comments made that list look modest, and rightly. The FreeBSD security-appliance tier belongs on it: pfSense, OPNsense, Stormshield, Zscaler's service edge, millions of firewalls and gateways quietly running BSD in the gap between the public internet and everyone's office. So does Nintendo, with a twist worth keeping. The Nintendo Switch is not a FreeBSD machine the way Sony's consoles are; its Horizon system carries FreeBSD's networking code without being a FreeBSD derivative outright. Counted alongside Sony, it tilts the games-console world heavily towards BSD and away from the Linux-based Steam Deck. And the Switch's twist, code from one family running inside another, is a quiet preview of this whole piece. Hold the thought; we will want it before the end.
We promised, last week, to give the Linux distinction its own room. This is the room.
The question is not academic. "Unix" is a word with a trademark, an owner, a register and a procedure for joining. When a system claims the word casually, the casualness is itself the tell: nobody who had read the register would step across the line without asking. So we begin with the line.
Who Owns the Word
The trademark UNIX belongs to The Open Group, an industry consortium based in the United States and the United Kingdom. The Open Group has kept the mark since 1996, when it was formed from the merger of X/Open and the Open Software Foundation. X/Open had received the trademark from Novell in 1993, and Novell had it from AT&T's Unix System Laboratories. The chain is longer than most people expect, and the present keeper is not a company selling an operating system. It is a standards body that keeps a list.
To use the word commercially, as a brand on a shipping product, a system must be certified against a written specification. That specification is the Single UNIX Specification (SUS), revised over the decades; the current edition, Issue 8, aligns with POSIX.1-2024. Products are certified against one of its brand levels: most against UNIX 03, a few against the newer and stricter UNIX V7. A vendor submits a system, passes the conformance tests, pays the licence fee, and is added to the register. The register is public, and anyone with an afternoon and a mild curiosity may consult it.
This is what being Unix means in 2026. Not a vibe. A registration.
Three Ways to the Word, and a Trademark That Keeps Its Distance
Sort systems by how they come to the word "Unix", and they fall into three relationships. There is a fourth thing in the picture, the trademark, but it does not preside over the other three the way one might expect. It keeps, by legal necessity, firmly to one side.
First, inheritance. You are Unix because you descend from it. The BSD family (FreeBSD, OpenBSD, NetBSD, DragonFly), together with a scattering of older and now mostly moribund branches, carries the actual AT&T-and-Berkeley line in its source, traceable commit by commit, back to Bell Labs in the late 1960s. These systems are Unix the way a child is family: by blood, having done nothing to earn it and able to do nothing to lose it. No certificate, no fee, no renewal date. It is the oldest claim and the least revocable, and it is the one this column keeps closest to, because the BSD line is where the Unix tradition is still maintained as a single coherent tree rather than reassembled each release. Most of the BSDs have never been certified, and they are none the poorer for it. They do not need the paperwork; they have the pedigree.
Second, certification. You build a system and have it certified. This is the route a new product takes to use the word commercially: submit to The Open Group, pass the conformance tests, pay the fee, join the register. Apple's macOS is the cleanest case (UNIX 03, certified since 10.5 Leopard in 2007), alongside IBM's AIX (UNIX V7, the only product at that stricter level), IBM's z/OS and HPE's HP-UX. Most of these are descended as well (macOS through its FreeBSD-derived Darwin userland, AIX and HP-UX down from AT&T's System V), so certification is usually a commercial layer over an inheritance, not a replacement for one. But the certificate is its own kind of object, and it behaves like a subscription: it lapses when unpaid. Oracle's Solaris, Huawei's EulerOS and Inspur's K-UX were each certified once, and not one is on the register today; EulerOS lapsed in 2022. The systems did not get worse the day their entry expired. They simply stopped renewing a word.
Third, resemblance. You build something new with Unix as the model. These are the Unix-like systems, the Unixoids: independent creations that took the Unix design as inspiration and rewrote the substance from scratch. BeOS did it in its own idiom; Linux did it most famously, the kernel Linus Torvalds began in 1991, not out of any quarrel with Unix but out of admiration for it (steeped in Unix and Minix, he wanted a free Unix of his own to run on a 386), with the GNU userland that Richard Stallman's project had been building since 1983; QNX did it on a microkernel of its own design. None inherited a line of Bell Labs code. They are in the family by resemblance, not by blood and not by paperwork, and in the good cases the resemblance is deliberate homage rather than accident.
And off to one side, keeping its distance, the trademark. UNIX is not a roof over the family. It is a certification mark, and a certification mark survives in law only by staying narrow: the sign may sit on tested, approved, fee-paying products and nowhere else, because the moment it is let to mean "any Unix-ish system" it turns generic and dies, the way aspirin and escalator died. So The Open Group guards it in the opposite direction from a roof. It disapproves of the very phrase Unix-like as a misuse of the mark; it has gone to law to keep the mark narrow, seeing off a 2007 challenge to the trademark's validity and pressing a German university to drop the short form "UNIK". The mark is not the word's parent and not its umbrella. It is a tightly fenced commercial sign with every legal reason to disown most of the systems this piece is content to call Unix. A BSD is Unix to the marrow and may not wear it; the mark would, strictly, rather you did not say "Unix-like" out loud either. The word and the thing are not the same object, and the holder of the word would prefer to keep it that way.
The three relationships share a surface. Most commands behave alike, most system calls carry the same names, and a POSIX-conformant program written on macOS will, as a rule, compile and run on Linux. The standard exists precisely so that it can. Underneath, they tell three different stories, in descending order of how hard each is to come by: descent, which you inherit; certification, which you buy; resemblance, which you build.
The Most Interesting Unixoid: Code Travelling Under Another Name
Among the Unixoids, one repays a closer look, because it spoils the neat picture of resemblance: it resembles Unix and also carries a piece of the real thing inside it. It came up in last week's comments, and it is the cleanest illustration of how porous the register's boundary really is.
QNX, the real-time operating system that Quantum Software Systems began building near Ottawa in 1982, is neither certified UNIX nor part of the BSD line. Its kernel is a microkernel of its own design: a small message-passing core that keeps the drivers, the file systems and the network stack out of kernel space, so that a failed driver restarts in milliseconds instead of taking the machine down with it. This is the architecture the rest of the industry has spent decades rediscovering, and QNX has shipped it in safety-critical production, in medical devices, industrial control and cars, for over forty years. It is not a curiosity. It is one of the most widely deployed operating systems that almost nobody can name.
What matters here is what QNX carries. Its io-pkt networking stack is built directly on NetBSD's: Berkeley's code is in the source, compiled in, running in QNX's own address space. And QNX holds a formal POSIX certification of its own, the IEEE and Open Group realtime profile, PSE52, a certificate issued by the very bodies that keep the UNIX register, only against a different standard. It runs in more than 175 million vehicles, behind the infotainment and instrument clusters of Audi, BMW, Ford, General Motors and a long list of others. Berkeley's code travels in every one of them.
That puts QNX in a position worth naming precisely, and the cleanest way to name it is to set it beside Linux. Both were built outside the lineage, on kernels of their own design; neither is a descendant of Bell Labs or Berkeley, and by the strict test of this piece neither is Unix. But the two are not quite level. QNX carries actual BSD source where Linux carries none, and QNX holds a genuine POSIX certificate where Linux only conforms by attempt. If Linux is Unix-like, QNX is Unix-like with a graft of the real thing running inside it. On the family tree the two sit close together, out on their own distant branches, with QNX a half-step nearer the trunk for the Berkeley code compiled into it.
Whether that half-step is enough to call QNX Unix depends entirely on how far past the register one is willing to draw the family, and honest people draw it in different places. The register says no: QNX is not on it. The source tree says: in part, yes, and in the part that does most of the work whenever a system talks to a network. Both answers are true at once, which is precisely what makes QNX the interesting case.
One note in passing, because it belongs to a later argument. Family membership and discipline (fidelity to the living practice of building things the Unix way) are two separate axes, and on the second QNX, with its small core and its four decades of disciplined production, arguably sits closer to the tradition than one or two certified members do. That deserves its own discussion rather than a parenthesis here.
What the UNIX Certificate Actually Certifies
Return to the second relationship and ask the sharp question: when The Open Group certifies a system as UNIX, what has it actually measured? Not the kernel, not the lineage, not the architecture. The interface, and only the interface: the C library and system calls, the shell, and the roughly 174 standard utilities the Single UNIX Specification lays down, all behaving as the text requires. The certificate attests that a system offers the Unix interface. It is silent on whether the thing underneath is Unix at all.
The register's own membership holds the cleanest proof. IBM's z/OS is certified UNIX, and z/OS is not a Unix. Its kernel is the mainframe MVS line, a design from the 1960s with no Bell Labs or Berkeley in it; the certified Unix is UNIX System Services, a POSIX environment layered on top of MVS. One account calls that environment the first truly POSIX-compliant system and then adds, with admirable candour, that it was not even a Unix operating system as such. It is Unix at the surface and mainframe all the way down.
So here is the answer to the question worth asking aloud: what does a certified system share with a Unix in the older, blood sense? The interface, the portable contract a program sees, and that alone. A BSD shares the interface and the substance, because it is the substance. z/OS shares the interface and none of the substance. The certificate cannot tell the two apart, because measuring the substance was never its job.
It certifies that you speak Unix, not that you are one.
What POSIX Does
That shared interface has a name and a history. POSIX (the Portable Operating System Interface, IEEE 1003) is the technical text the Single UNIX Specification incorporates, first published as IEEE 1003.1 in 1988; Richard Stallman is credited with the name, having offered "POSIX" in place of the committee's leaden "IEEE-IX". It is what makes the surface shareable at all. It specifies what a program sees, what open() does, what fork() does, how a shell environment behaves, and is deliberately silent on what implements it. Two systems that pass the same tests may have nothing whatever in common below the call.
So POSIX and certified UNIX are not one claim but a short ladder of them. A system can be POSIX-conformant by attempt (Linux), POSIX-certified in its own right (QNX, against the realtime profile), or certified UNIX on the full Single UNIX Specification (macOS, which sits on both registers). Each rung is a firmer statement about the interface. Not one of them is a statement about descent.
Why the Line Matters
In an age when "Unix" has become a loose and friendly word, it is tempting to wave the distinction away as paperwork. Linux is everywhere; surely this is a registry quibble?
It is not. The line matters for the same reason any line naming a responsibility matters. When a vendor says "certified UNIX", a customer can open the register, find the conformance document, read the test conditions, and know what was measured and when. When a vendor says "Unix-like", the resemblance is real but the contract is not; the customer is being told the truth, only a vaguer one.
The BSDs make the cleanest case of the lot: no Open Group contract, but a lineage anyone can read in the source. Unix by descent, certified by nobody, answerable to no renewal date. For most engineering decisions that is the stronger assurance and not the weaker one: a fact about the code outlasts a subscription to a brand.
All three can carry production. The certificate proves a vendor paid to be measured; the lineage proves the code is what it claims to be. Different promises, and the older of the two is descent.
The Point
"Unix" turns out to be four things wearing one word. It is a trademark, a certification mark held by The Open Group and kept deliberately narrow against genericide, so narrow that the Group frowns on the very phrase Unix-like; a system earns it by certification and forfeits it by not renewing. It is, older and deeper than any register, a lineage: the BSD family and a few surviving relatives, Unix by descent, having done nothing to earn the word and unable to lose it. It is a resemblance, the idiom that BeOS and Linux and QNX rebuilt out of admiration without inheriting it. And it is the gap between those, the reason a system can be Unix to the marrow with no right to the mark, and another can hold the mark while sharing little of the marrow.
The register is short, specific and, as Solaris reminds us, revocable. Descent is none of those things. The systems off the register are not lesser; several are larger and busier than the ones on it. They are standing on a different part of the claim, and the part that does not expire is the part you inherit.
Where Unix runs, last week's piece showed. What Unix is, this week's has tried to draw. Where Unix came from, the question sitting beneath both, is still open.
Four things in one word: a trademark earned by certification, a lineage held by descent and never lost, a resemblance rebuilt from admiration, and the gap between them. The part that does not expire is the part you inherit.
With thanks to the readers who widened last week's map: Greg Wallace (NetActuate; FreeBSD Foundation Enterprise Working Group) for the QNX case, and Devin Teske (FreeBSD src/ports committer) for the Nintendo addition, by whose count the consoles run some fifteen-to-one for BSD over the Linux-based Steam Deck.