ARexx as implemented in Amiga software has various properties that aren't exactly duplicated anywhere. It was a system available thing, and it was an interface added to individual apps -- extensible on each end with no "privilege" barriers in the way.
Tcl/Tk was a proposed UNIX based alternative back in the day which we don't hear much about now. In actuality, just about any language interpreter is a possible alternative (basic, rebol, etc.) if you mix with a standardized message passing scheme/protocol for the commands and data sharing. AROS's regina is still broken in this way as it doesn't yet implement the "options results" data (string) buffer, along with returning the usual rexx numeric error/success code, which many ARexx scripts use and required.
