check remote image before downloading

update_img.sh

 img_tmp="${img}.tmp"
 img_url="http://srv.ordiclic.eu/prololive.img"
 
+if [ ! -f "${img}" ]
+then
+  local_size=0
+  remote_size=1
+else
+  remote_size=$(curl -s --head "${img_url}" | grep Content-Length | grep -oE [0-9]+)
+  local_size=$(stat --printf="%s" "${img}")
+fi
+
+if [ "${local_size}" -eq "${remote_size}" ]
+then
+  echo "Remote image is probably the same"
+  exit
+fi
+
+rm -f "${img}"
+
 wget "${img_url}" -O "${img_tmp}" || {
   rm "${img_tmp}"
   echo "Failed to download image" >&2