Unix
termprogrammingops culture
The operating system built at Bell Labs from 1969, whose design - files, small composable tools, processes forked from processes - became the shape almost everything since has copied.
What spread was not the code but the model, and the reason is that the model was small enough to reimplement. Linux, the BSDs and macOS descend from the design rather than from one source tree, and POSIX exists to write down what they have to agree on. For anyone operating systems today, the inheritance is practical: a shell that pipes one program into another, a filesystem that represents devices as files, and a permission model that predates the internet and still decides who can read what.
Also known as: UNIX