Shell Script to Change Wall Paper of Mac

changewallpaper.sh

#!/bin/bash
i=0
j=1
while true
do
  command="/usr/bin/osascript -e 'tell application \"Finder\" to set desktop picture to POSIX file \"/Users/yumu/Desktop/img/img${i}.png\"'"
  if [ -e /Users/yumu/Desktop/hoge/img${j}.png ]; then
    echo ${command}
    eval ${command}
    i=`expr ${i} + 1`
  fi
  j=`expr ${i} + 1`
done