OpenJDK + Groovy

If you're looking to move it onto another language/VM platform for the final version and are
considering the JDK, you might want to start with Groovy. It's interactive interpreter
features and generally more dynamic structure would be a better fit for building a shell than
compiled Java would be.

In particular, being able to load and run Groovy scripts (as well as compiled .class / .jar
format commands shipped by libraries etc) would be really handy. Libraries could ship fast,
precompiled jars that your command classloader could quickly pick up, but users could write
groovy snippets and scripts without needing the `javac' step. Handily, groovy can be compiled
too, so if you do then want to package your groovy scripts as fast precompiled commands you
can.

Sounds good to me.

I would personally much prefer to see it take the Java path both because the JVM is now GPL
and because Java is subject to the JCP. It also has excellent backward compat. Mono, while the
codebase is free, is still very tied to Microsoft's language direction, and I'd be a bit
nervous about building new core OS infrastructure on it myself.