Well, in addition to being huge (850K lines of C# by my count), I would say it's only marginally relevant to someone who wants to write a Unix shell. The parts of the kernel that Unix uses -- fork/exec/wait/pipe/dup -- are exactly the things that Windows does in a completely different manner (whereas the file system is probably more similar).

The shell is almost what distinguishes Unix from Windows, and that is apparent at the lowest levels.