Bash script to optimize png files

optimize_png.sh

#!/bin/bash
if [ $# -lt 1 ] 
then
	echo "Missing argument"
	echo "Usage : $0 <APPLICATION_PROJECT_DIRECTORY>"
	exit 1;
fi
APPLICATION_PROJECT_DIRECTORY=$1

echo "Optimizing PNGs with pngquant"
let "optimizedCount = 0"
for png in $(find $APPLICATION_PROJECT_DIRECTORY -iname "*[^9].png")
do
	is_optimized=$(exiftool $png | grep -e "Color Type.*:.*Palette")
	if [ -z "$is_optimized" ] 
	then
		already_optimized=$(pngquant $png -ext .png -force 2> last_error)
		if [ $? -eq 0 ] 
		then
			echo "Optimized - $png" 
			let "optimizedCount++"
		else
			echo "Optimization failed for : $png - Reason : $(cat last_error)" >&2
		fi
	fi
done
rm last_error 2> /dev/null
echo "Optimized $optimizedCount pngs"