I'm late to the party, but I'm the author of Rash. Rash's documentation is terrible and out of date, but I've actually been planning to redo/improve all of the documentation within a week or two. I'm hoping that once I do that it will be much more approachable. Rash tries to allow both Bash style things and more normal Racket to be done with ease and mixed, and a lot of the complexity of Rash has to do with getting those two things to work together, and to allow lots of macro extensibility. But I think you'll find that it's quite powerful and useful... once I make it easier to make sense of.

Of course I'm also happy for anyone to make a new shell that they like and want to use themselves. I did it, after all...