Ever try to port a POSIX utility to BeOS? I did. No mmap and no BSD sockets killed any chance of easily porting any network utility to BeOS. It had a directory called /dev, yes, but it was absolutely nothing like /dev on any Unix system; it's all home grown. No permissions; it's a single-user system. It stores file owners and modes, yes, but they don't actually mean anything; you don't need the +x bit to execute a file, you don't need the +x bit to list a directory, everything is owned by "baron", etc. It supported fork but it's incompatible with threads, and everything on BeOS is threaded. Some signals are implemented, but the C API is home grown instead of the standard POSIX calls. Etc. They built a Unix facade but the system is very un-Unix like.

If it were really Unix, they probably would not have needed to write a whole book on porting Unix applications: https://www.amazon.com/BeOS-Applications-Martin-C-Brown/dp/1...