UNIX, BSD, Linux
UNIX began with Ken Thompson who first thought of the idea of UNIX.
Dennis Ritchie was the first collaborator.
Ritchie created C, then used C to implement UNIX.
Both previously worked on Multics, reused ideas from it.
Douglas McIlroy introduced the idea of pipes, a major element of UNIX.
BSD is a descendant of the original Bell Labs source code,
developed further by University of California, Berkeley.
The Internet programming interface (TCP/IP sockets) originated on BSD.
Linux is a clone of the UNIX kernel, written from scratch.
Most of the surrounding user-space software was developed by GNU.
Both BSD and GNU/Linux are powerful stable operating-systems.
Arguably, BSD is more stable and faster than GNU/Linux (stable also means less changes to the OS).
BSD's C library is simpler and faster than GNU glibc.
Linux kernel is well-organized, modular, easy to modify, which led to the popularity of Linux.
BSD kernel needs better organization, much code is arcane.
GNU/Linux is actually sssllloooooooowww.
That GNU/Linux is slow is hidden by today's fast hardware.
GNU/Linux was installed on a 68040 Amiga which ran Amiga OS 2.x fast,
but the Amiga crawled trying to run GNU/Linux.
Ironically, the lead architect of another operating-system,
after failing to correct its defects, ultimately converted to GNU/Linux.