$X=$X+$XDIS # This is what is blowing up
1 The assignment should not start with the $
2 Arithmetic is done within $(( ))
$X=$XORG # I am assuming this will not work either
$Y=$Y+YDIS # ""
I'd say those are fair assumptions.
I'm only learning myself, so if I've led you astray, my apologies.
It's a good idea to try snippets of code in a terminal.