The paper introduces UNIX, a general-purpose, multi-user, interactive operating system for the Digital Equipment Corporation PDP-11/40 and 11/45 computers. It highlights several key features, including a hierarchical file system, compatible file and device I/O, asynchronous process initiation, a customizable command language, and over 100 subsystems. The authors discuss the implementation of the file system and user command interface, emphasizing the simplicity, elegance, and ease of use of UNIX. The paper also covers the hardware and software environment, the file system structure, protection mechanisms, I/O calls, and the implementation of the file system. Additionally, it details the process management, including processes, images, pipes, and synchronization. The Shell, a command-line interpreter, is described in detail, along with its capabilities and implementation. The paper concludes with a discussion on traps and the perspective behind the design of UNIX, highlighting the importance of programming convenience, size constraints, and self-maintenance.The paper introduces UNIX, a general-purpose, multi-user, interactive operating system for the Digital Equipment Corporation PDP-11/40 and 11/45 computers. It highlights several key features, including a hierarchical file system, compatible file and device I/O, asynchronous process initiation, a customizable command language, and over 100 subsystems. The authors discuss the implementation of the file system and user command interface, emphasizing the simplicity, elegance, and ease of use of UNIX. The paper also covers the hardware and software environment, the file system structure, protection mechanisms, I/O calls, and the implementation of the file system. Additionally, it details the process management, including processes, images, pipes, and synchronization. The Shell, a command-line interpreter, is described in detail, along with its capabilities and implementation. The paper concludes with a discussion on traps and the perspective behind the design of UNIX, highlighting the importance of programming convenience, size constraints, and self-maintenance.