What is Plan 9?

FQA INDEX | FQA 1 - Introduction To 9front

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0.1 - What is Plan 9?

Plan 9 is a research operating system from the same group who created UNIX at Bell Labs Computing Sciences Research Center (CSRC). It emerged in the late 1980s, and its early development coincided with continuing development of the later versions of Research UNIX. Plan 9 can be seen as an attempt to push some of the same ideas that informed UNIX even further into the era of networking and graphics. Rob Pike has described Plan 9 as "an argument" for simplicity and clarity, while others have described it as "UNIX, only moreso."

From The Use of Name Spaces in Plan 9:

Plan 9 argues that given a few carefully implemented abstractions it is possible to produce a small operating system that provides support for the largest systems on a variety of architectures and networks.

From the intro(1) man page:

Plan 9 is a distributed computing environment assembled from separate machines acting as terminals, CPU servers, and file servers. A user works at a terminal, running a window system on a raster display. Some windows are connected to CPU servers; the intent is that heavy computing should be done in those windows but it is also possible to compute on the terminal. A separate file server provides file storage for terminals and CPU servers alike.

The two most important ideas in Plan 9 are:

• private namespaces (each process constructs a unique view of the hierarchical file system)

• file interfaces (familiar from UNIX, but taken to the extreme: all resources in Plan 9 look like file systems)

Most everything else in the system falls out of these two basic ideas.

Plan 9 really pushes hard on some ideas that Unix has that haven't really been fully developed, in particular, the notion that just about everything in the system is accessible through a file. In other words, things look like an ordinary disk file. So all the devices are controlled this way by means of ASCII strings, not complicated data structures. For example, you make network calls by writing an ASCII string, not the files. This notion is something that's actually leaking quite fast.

The second thing is sort of more subtle and sort of hard to appreciate until you've actually played with it. That is that the set of files an individual program can see depends on that program itself. In a standard kind of system, either with Unix remote file systems or Windows attached file systems, all the programs running in the machine see the same thing. In Plan 9, that's adjustable per program. You can set up specialized name stations that are unique to a particular program. I mean, it's not associated with the program itself but with the process, with the execution of the process.

— Dennis Ritchie

Read: intro(1); Plan 9 from Bell Labs; Designing Plan 9, originally delivered at the UKUUG Conference in London, July 1990; and FQA 7 - System Management; for a more detailed overview of Plan 9's design.