Plan 9 is an operating system developed by Bell Laboratories. It is a distributed computing environment assembled from separate machines--terminals that sit on users' desks, file servers that store permanent data, and other servers that provide faster CPUs, user authentication, and network gateways, all using a network protocol called 9P. The protocol is used to refer to and communicate with processes, programs, and data, including both the user interface and the network.
It is based on the best features of UNIX but was developed to demonstrate the concept of making communication the central function of the computing system--all system resources are named and accessed as if they were files and allowing multiple views of the distributed system to be dynamically defined dynamically for each program running on a particular machine. This approach improves generality and modularity of application design by encouraging servers that hold any information to appear to users and to applications just like collections of ordinary files
Plan 9 runs on Intel, MIPS, DEC Alpha, and PowerPC architectures. It is written in a strict dialect of ISO/ANSI C. It can import POSIX applications and can emulate the Berkeley socket interface. It has complete UTF-8 support, and a windowing system called rio.
The first edition of Plan 9 was released in 1993, a second shrink-wrapped version in 1995 and the third under a open-source agreement in 2000.