Cogratulations coreplayer2. You are the first one to teach me something cool this year. I have so often wished for the -= operator but, since I typically don't use "let", I never new it was there. I guess one of the long term hazards of BASIC-as-a-first-language is the deeply ingrained notion that "VAR=VALUE" is exactly the same as "let VAR=VALUE", which clearly is not the case in shell scripting.
num=`expr ${NUM} \- 1`
let num-=1
Thanks for demonstrating such a useful tool.