UNIX is a general-purpose, multi-user, interactive operating system for the Digital Equipment Corporation PDP-1 1/40 and 11/45 computers. It offers a number of features seldom found even in larger operating systems, including 1. A hierarchical file system incorporating demountable volumes, 2. Compatible file, device, and inter-process I/O, 3. The ability to initiate asynchronous processes, 4. System command language selectable on a per-user basis, 5. Over 100 subsystems including a dozen languages. This paper discusses the usage and implementation of the file system and of the user command interface.
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Ritchie et al. (1973) studied this question.