See a lengthier reply elsewhere in this thread, re: motivations etc. but it's almost all from scratch. Frankly, a simple terminal is not a huge amount of work, and you can get something working "well enough" to start with with very little code.

The huge amount of work comes from nailing all the quirks you will have to deal with if you let a bunch of other users loose on it, with their expectations of running all kinds of applications I've never tested that does weird stuff.