Why did a side project at Bell Labs shape so many modern operating systems?

Unix shaped so many modern operating systems because a small team of Bell Labs engineers built it small, simple and portable, and then it spread. Development began in 1969 at Bell Labs under Ken Thompson and Dennis Ritchie, and it grew into a whole family of multitasking, multi-user systems.

The story begins with a failure of ambition. Bell Labs had joined an enormous project called Multics. Frustrated by its size and complexity, researchers drifted away, and the last to leave, including Thompson and Ritchie, decided to rebuild what they had learned at a smaller scale. Thompson, who had written a game called Space Travel, found an old PDP-7 machine so he could go on playing it, and on that machine the team built the pieces of a new system: a hierarchical file system, processes, device files, a command-line interpreter, pipes and small utility programs. In 1970 Brian Kernighan suggested the name Unix, a pun on Multics.

Two ideas made it spread. The first was portability. In 1973 Unix version 4 was rewritten in C, the language Ritchie created, and because almost the whole system was in C, it could run on many kinds of computers. The second was a philosophy: simple tools, each doing one limited, well-defined job, combined by pipes.

Today Unix's descendants are everywhere. Apple's macOS has a Unix system at its core, and Linux, which you will meet later in this path, follows its ideas. In 1983 Thompson and Ritchie received the Turing Award for their work.