Since its on topic, just thought I would mention that I have ported my documentation and spec building tools to Linux and OSX. I do not yet have a Linux installer but that will probably happen in a couple of months time.
The code is all in C# which has just acquired a fully MIT Licensed open source implementation. The tools can be used as standalone command line tools or as integrated Visual Studio components. I use Visual Studio for my build environment on every platform, Linux and OSX included. There is a tool that will convert Visual Studio projects to Gnu Makefiles. The only serious limitation being that I can't run unit tests on platforms other than Windows at the moment.
OK so what tools?
First, I have my own RFC documentation tool that can take source from MarkDown, Word, XML2RFC or HTML. It does not yet support the new format but will in the near future.
Second, I have a protocol compiler that allows a protocol to be defined in a very high level schema from which the reference sections of the specification, examples, reference and production code are generated.
My approach is at a higher level of abstraction to YANG. I believe that the process of designing a standard is to remove choices that don't matter. There are hundreds of ways that an integer value can be encoded and DNS uses at least 6 different forms. If you are designing a protocol you should choose one and stick to it. I don't currently deal in YANG but if I had to, YANG would be an output from my tools.
The same libraries also include an ASN.1 encoder (and partial decoder), JSON, RFC822 and DNS serialization/deserialization tools.
The tool building tool is also included.