Developed as the successor to Unix (How did that work out I wonder?<g>), Plan 9 is named after the 1959 film Plan 9 from Outer Space and was developed in Bell Labs in the mid 80s. The source code is free and you can visit the Plan 9 website and there is still a community who are working on it.
The source code is in C and there are a lot of utils whose source code you can view, use in the cmd folder in the sources. It's recently been ported to a Raspberry Pi. But remember it's not Unix or Linux. I've added a link to the C Operating System source code library.
- Link to C Operating Systems Code Library