A not (yet) very posixy shell implementation in Python.
For no good reason, except that I was reading the posix shell documentation.
That seems, on the whole, to have been written to describe a particular implementation - especially the way that backtick parsing works. It's all slightly reminiscent of Tcl's evaluation model.
It's also led me to nose around looking at corner-cases of the implementation of various shells.
Consider this a playground, not anything useful.