to introduce the idea of scripting to newbies
I would use sh instead of bash, per tinypoodle's suggestion, however I would be unlikely to include awk and sed as they each deserve a separate intro of their own.
Having said that, I'll have to admit that I know nothing of ruby and very little of lua or python (or sed or awk, for that matter).
I'm thinking that there's not a lot one can't do with the "old-time" unix utilities if one just puts in the time to learn them - sed, awk, tcl, tk, et cetera.
What are the strengths of ruby, python, lua and such? What do they "bring to the table" that's new or easier? (I'm omitting perl because of a long standing personal bias. If I ever revisit perl, I may just come to love it.)