I’m unfamiliar with illumos so I went to their webpage and the very first thing it says is:
> illumos is a Unix operating system
Is illumos an actual Unix (like macOS) or a Unix-like OS (like GNU/Linux)?
> illumos is a Unix operating system
Is illumos an actual Unix (like macOS) or a Unix-like OS (like GNU/Linux)?
> It is based on OpenSolaris, which was based on System V Release 4 (SVR4) and the Berkeley Software Distribution (BSD). Illumos comprises a kernel, device drivers, system libraries, and utility software for system administration. This core is now the base for many different open-sourced Illumos distributions, in a similar way in which the Linux kernel is used in different Linux distributions.
https://www.opengroup.org/openbrand/register/
so it can't use the UNIX™ trade mark.
But it's got the AT&T Unix kernel & userland sources contained in it.
PDP-11 Unix System III: https://www.tuhs.org/cgi-bin/utree.pl?file=SysIII/usr/src/ut...
IllumOS: https://github.com/illumos/illumos-gate/blob/b8169dedfa435c0...
- Derived from Bell Labs unix source
- Legally allowed to use the UNIX trademark (AKA certified Unix)
- A unix-shaped OS (similar but not 100% the same as POSIX complacence)
and those things are basically independent. Most GNU/Linux are unix-likes but not derived from original unix code or certified, but there's been 1-2 that did get certified. The BSDs are (now quite distantly ) derived from unix source but not certified (although ex. UnixWare is IIRC). Solaris was all 3 but OpenSolaris and now illumos are obviously unix-like and still based on the original code but not certified UNIX™.
(Take all this with a grain of salt; I'm typing this all from memory and IANAL)