Each version of the UNIX Time-Sharing System evolved from the version before, with version one evolving from the prototypal Unix. Not all variants and descendants are displayed.
The versions leading to v7 are also sometimes called Ancient UNIX. After the release of Version 10, the Unix research team at Bell Labs turned its focus to Plan 9 from Bell Labs, a distinct operating system that was first released to the public in 1993. All versions of BSD from its inception up to 4.3BSD-Reno are based on Research Unix, with versions starting with 4.4 BSD and Net/2 instead becoming Unix-like. Furthermore, 8th Edition Research Unix and on-wards had a close relationship to BSD. This began by using 4.1cBSD as the basis for the 8th Edition. In a Usenet post from 2000, Dennis Ritchie described these later versions of Research Unix as being closer to BSD than they were to UNIX System V,[1] which also included some BSD code:[2]
Research Unix 8th Edition started from (I think) BSD 4.1c, but with enormous amounts scooped out and replaced by our own stuff. This continued with 9th and 10th. The ordinary user command-set was, I guess, a bit more BSD-flavored than SysVish, but it was pretty eclectic.
Each of the systems in this list is evolved from the version before, with Unix System III evolving from both the UNIX Time-Sharing System v7 and the descendants of the UNIX Time-Sharing System v6.
Below are other certified Unix operating systems:[4]