[clue] arithmetic in bash

[Aaron D. Johnson]

David L. Willson writes:
> Thanks for the well-thought-out answer. I'll return to using
> '$(( EXPRESSION ))', even though I can depend on bash for all
> my work.

You're very welcome.  I've done some more googling since this morning
and came across https://wiki.ubuntu.com/DashAsBinSh which is a pretty
nice document covering differences between Bash and other, more
strictly POSIX, Bourne family shells.  Use of $RANDOM is specifically
addressed there.

- Aaron