Both, I would say, particularly with the GNU’s Not UNIX userland. I don’t think it matters much, though, as you can have a great non-UNIX OS, which Linux definitely is.

Also, we might forget it now, but in the past there were a lot of UNIX that were quite different in small and large ways. The differences between Linux and Darwin might be as great as the differences between SunOS and Xenix, or indeed NeXTSTEP, back in the day. Or maybe not, that was before my time, but my point is that UNIX was not a monolith and there were incompatibilities and different ways of doing things. So I guess yeah, Linux might be what one of these might have become.

I just find it ironic that so many people are keen to compare macOS to UNIX and just assume that macOS has to be the weird outsider, even though the reverse is actually true. The same people also often assume Linux-isms are the way UNIX works, like the now-dead comment about macOS’ commands not supporting GNU options.

Outside of macOS, commercial Unix is basically dead. In the old days I used to see all sorts of systems. One company had Solaris 2.x, SunOS 4.x, Digital Unix, HPUX, AIX. Now you rarely see anything other than Linux, though I still know a couple of AIX shops that went all in on IBM hardware and still swear by it.

Like it or not, right or wrong, when people say "Unix" today, they mean Linux with GNU userland.

The IBM mainframe operating system z/OS (formerly known as MVS) is a certified UNIX 95. (That's an older UNIX standard, but still valid.)

There are still a lot of IBM mainframes out there. Far less than there used to be, but IBM mainframes remain (even in 2020) a sector with many billions of dollars of annual revenue (if you add up hardware and software and services across both IBM and the ISV ecosystem). And the vast majority of those mainframes run z/OS. (They also run other operating systems – z/Linux, z/VM, z/VSE and z/TPF – but many of the sites running one of those other operating systems run z/OS as well.)

The UNIX component of z/OS is not merely a compatibility layer, a lot of operating system components and applications now rely on it. For example, the REST-based z/OS management console Zowe that IBM is now promoting runs under z/OS UNIX. My understanding is that IBM prefers (where possible) to develop new OS components and applications to run under z/OS UNIX rather than the traditional z/OS APIs. Anything written in Java (or other newer languages, such as node.js) is running on z/OS UNIX. So even if a z/OS site isn't doing anything directly on UNIX, they are almost certainly using OS components and applications which directly rely on it to function.

Obviously z/OS is absolutely dwarfed by Linux and macOS, but it is probably the healthiest of all the commercial Unixes – Oracle has put Solaris in maintenance mode, same for HPE and HP/UX, HPE killed Digital Unix years ago; AIX is healthier than Solaris or HP/UX, but I think z/OS is even healthier than AIX. z/OS licensing fees are a lot higher than AIX licensing fees, and the non-UNIX parts of z/OS (JCL, VSAM, CICS, IMS, etc) make it far harder to migrate off than AIX is.

Interesting. I had forgotten about z/OS. I don't know much about the mainframe world. Several years back, I did get it booted on the "Hercules" emulator, but making much real use of it was beyond my capabilities.

You just reminded me of this site, which I found incredibly useful back in the day when I was working closely with Solaris, Linux and OSX (as it was then known).

http://bhami.com/rosetta.html

> Linux isn't technically one, sure, but given that Linux is where the OS hot-shit is now (...) is it not fair to say it's where Unix would've been had things been different?

No, it's not fair. Linux is just the OS kernel. UNIX covers interfaces, from APIs to shells and apps distributed by default, which are way out of the scope of Linux. That's the responsibility of Linux distributionsm.

And by the way, there are linux distributions that are certified UNIX

https://unix.stackexchange.com/questions/293396/is-there-a-l...

Moreover, UNIX is standardized. The drive for standardization was derived from the fact that all vendors of UNIX-like OSes were pushing their quirky vendor-specific things. In fact, even within Linux there is no widely established standard regarding basic interfaces, let alone UNIX.

With this in mind, do note that macOS's market share is significantly larger than the aggregate market share of Linux-based UNIX-like distributions.

> And by the way, there are linux distributions that are certified UNIX

How was I not aware of this? Thanks for the link! And you make a very good point regarding kernel vs OS. I assumed Linux == GNU/Linux in this context (so ignoring Android and others), but it certainly is a gross over simplification.