Howto loop based on some iterator

Say you have some files named a100 to a200 that you want to rename to b200 to b100 (it really does happen).

y=200
for x in `seq 100 200`
do
   mv a$x b$((y--))
done

seq is a fairly useful tool, it can also take a reversed list.

for x in `seq 200 -1 100`
do
printf "$x "
done
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