I talked earlier about three different definitions of UNIX – "trademark/certified UNIX", "heritage/genealogical UNIX" and "UNIX-like/UNIX-compatible". Maybe we could add a fourth, "philosophical UNIX". I don't know why we should say that is the only valid definition and ignore the validity of the other three.

The fact is that opinions differ on exactly what the "UNIX philosophy" is, and on how well various systems comply with it. The other three definitions have the advantage of being more objective/clearcut and less subject to debate or differing personal opinions.

Some would argue that UNIX itself doesn't always follow the UNIX philosophy – or at least not as well as it could – which leads to the conclusion that maybe UNIX itself isn't UNIX, and that maybe a "real UNIX" system has never actually existed.

It is claimed that one part of the UNIX philosophy is that "everything is a file". And yet, UNIX started out not treating processes as files, which leads to various problems, like how do I wait on a subprocess to terminate and a file descriptor at the same time? Even if I have an API to wait on a set of file descriptors, I can't wait on a subprocess to terminate using that API since a subprocess isn't a file descriptor.

People often point to /proc in Linux as an answer to this, but it didn't really solve the problem, since Linux's /proc was mostly read-only and the file descriptor returned by open(/proc/PID) didn't let you control or wait on the process – this is no longer true with the introduction of pidfd, but that's a rather new feature, only since 2019; Plan 9's /proc is much closer, due to the ctl file; V8 Unix's is better than the traditional Linux /proc (you can manipulate the process using ioctl) but not as good as Plan 9's (its ioctls expose more limited functionality than Plan 9's ctl file); FreeBSD's pdfork/pdkill is a good approach but they've only been around since 2012.