grouping many image files into sub directories

mv2dirs.sh

#!/bin/bash
dirno=1
fileno=0
for i in `ls *.JPG`
    do
        mod=$(($fileno%40))
        if  [ "$mod" -eq "0" ]
        then
            dir="g$dirno"
            mkdir $dir
            ((dirno+=1))
        fi
        mv $i $dir/
        ((fileno+=1))
    done