[mdasen]

It's not simply that certification costs money. It's that a lot of modern UNIX-like operating systems don't adhere to the UNIX spec. For example, the OpenBSD man pages specify the ways in which they diverge from POSIX and UNIX in the Standards section: https://man.openbsd.org/sh.1#STANDARDS, https://man.openbsd.org/awk.1#STANDARDS. Often times these are small deviations that might not matter to most people, but it means that they aren't UNIX.

Terry Lambert has written about the work he did to make Mac OS X UNIX compliant: https://www.quora.com/What-goes-into-making-an-OS-to-be-Unix...

I don't know how much it costs for the Open Group to certify an OS as UNIX compliant (if anything).

A lot of the time, things are compliant in most of the common stuff, but they might not implement all of UNIX or POSIX. Close isn't compliant, but it's enough that most people don't care. For example, do you care if your Linux disto comes with the `pax` command? Or do you just use `tar`?

Apple had started calling OS X UNIX and was being sued for calling it UNIX when it wasn't actually UNIX. They decided it'd be cheaper to do the work to make OS X UNIX compliant than to deal with the lawsuit and that's what they did. BSDs never cared about calling themselves UNIX so they never went into the nitty gritty of making themselves UNIX compliant.

[linguae]

I agree, though I feel compelled to post that the BSDs learned their lesson about the use of the UNIX trademark the hard way. Part of what caused the classic lawsuit between the Regents of the University of California (the "Berkeley" in Berkeley Software Distribution) and AT&T was the fact that a spinoff company selling BSD had the phone number 1-800-ITS-UNIX:

https://en.wikipedia.org/wiki/UNIX_System_Laboratories,_Inc.....

This lawsuit happened (1992-1994) when Linux was in its infancy (Linus Torvalds started work on Linux in 1991). Had the future of BSD not been so uncertain during this lawsuit, it is quite possible that some derivative of BSD would be the dominant FOSS operating system today instead of Linux.

[deafpolygon]

Linux's dominance might still have emerged due to the individuals behind it. Beyond Torvalds' work on the kernel, Richard Stallman and others had a significant influence in shaping its early trajectory. Their philosophies and actions were key in driving the open-source movement forward.

The BSD license, while appealing to corporations due to its permissive nature, allows proprietary entities to incorporate the code into their products without giving back to the community. This has happened multiple times, with examples like Apple's Darwin, NetApp, QNAP, and Sony's PlayStation systems.

The GPL, on the other hand, attracted early contributors because it ensured that their work would remain open and shared with the world, rather than being exploited for profit.