Unix is a general-purpose, multi-user, interactive operating system that began at Bell Labs in 1969. In their 1974 Communications of the ACM paper “The UNIX Time-Sharing System,” Dennis Ritchie and Ken Thompson described the version then running on the DEC PDP-11/40 and PDP-11/45 computers and listed its main features: a hierarchical file system, compatible handling of files, devices, and inter-process input and output, the ability to start asynchronous processes, and a command language that each user could select.
The same paper reports that by 1974 the system already supported over 100 subsystems, including more than a dozen programming languages. Ritchie and Thompson stressed that none of this came from unusual hardware or a grand plan; the system, they wrote, was shaped by a desire for a comfortable environment in which to do their own work.
A turning point came in 1973, when the system was rewritten in the C language. Because C was portable, Unix could be moved to new hardware far more easily than systems written in assembly for one machine, and this portability let Unix spread across the industry.
From that base grew a large family of descendants. Berkeley’s BSD, the GNU and Linux projects, and Apple’s macOS all trace their design and much of their behavior back to the Bell Labs original, which is why the ideas in the 1974 paper still describe how these systems work today.