Optimize image script. cf http://hugogiraudel.com/2013/07/29/optimizing-with-bash/

optimize.sh

#!/bin/bash

PROGNAME=${0##*/}
INPUT=''
QUIET='0'
NOSTATS='0'
max_input_size=0
max_output_size=0

usage()
{
  cat <<EO
Usage: $PROGNAME [options]

Script to optimize JPG and PNG images in a directory.

Options:
EO
cat <<EO | column -s\& -t
	-h, --help  	   & shows this help
	-q, --quiet 	   & disables output
	-i, --input [dir]  & specify input directory (current directory by default)
	-o, --output [dir] & specify output directory ("output" by default)
	-ns, --no-stats    & no stats at the end
EO
}

# $1: input image
# $2: output image 
optimize_image()
{
	input_file_size=$(stat -c%s "$1")
	max_input_size=$(expr $max_input_size + $input_file_size)

	if [ "${1##*.}" = "png" ]; then
		optipng -o1 -clobber -quiet $1 -out $2
		pngcrush -q -rem alla -reduce $1 $2 >/dev/null
	fi
	if [ "${1##*.}" = "jpg" -o "${1##*.}" = "jpeg" ]; then
		jpegtran -copy none -progressive $1 > $2
	fi

	output_file_size=$(stat -c%s "$2")
	max_output_size=$(expr $max_output_size + $output_file_size)
}

get_max_file_length()
{
	local maxlength=0

	IMAGES=$(find $INPUT -regextype posix-extended -regex '.*\.(jpg|jpeg|png)' | grep -v $OUTPUT)

	for CURRENT_IMAGE in $IMAGES; do
		filename=$(basename "$CURRENT_IMAGE")
		if [[ ${#filename} -gt $maxlength ]]; then
			maxlength=${#filename}
		fi
	done

	echo "$maxlength"	
}

main()
{
	# If $INPUT is empty, then we use current directory
	if [[ "$INPUT" == "" ]]; then
		INPUT=$(pwd)
	fi

	# If $OUTPUT is empty, then we use the directory "output" in the current directory
	if [[ "$OUTPUT" == "" ]]; then
		OUTPUT=$(pwd)/output
	fi

	# We create the output directory
	mkdir -p $OUTPUT

	# To avoid some troubles with filename with spaces, we store the current IFS (Internal File Separator)...
	SAVEIFS=$IFS
	# ...and we set a new one
	IFS=$(echo -en "\n\b")

	max_filelength=`get_max_file_length`
	pad=$(printf '%0.1s' "."{1..600})
	sDone=' [ DONE ]'
	linelength=$(expr $max_filelength + ${#sDone} + 5)

	# Search of all jpg/jpeg/png in $INPUT
	# We remove images from $OUTPUT if $OUTPUT is a subdirectory of $INPUT
	IMAGES=$(find $INPUT -regextype posix-extended -regex '.*\.(jpg|jpeg|png)' | grep -v $OUTPUT)

	if [ "$QUIET" == "0" ]; then
		echo --- Optimizing $INPUT ---
		echo
	fi
	for CURRENT_IMAGE in $IMAGES; do
		filename=$(basename $CURRENT_IMAGE)
		if [ "$QUIET" == "0" ]; then
		    printf '%s ' "$filename"
		    printf '%*.*s' 0 $((linelength - ${#filename} - ${#sDone} )) "$pad"
		fi

		optimize_image $CURRENT_IMAGE $OUTPUT/$filename

		if [ "$QUIET" == "0" ]; then
		    printf '%s\n' "$sDone"
		fi
	done

	# we restore the saved IFS
	IFS=$SAVEIFS

	if [ "$NOSTATS" == "0" -a "$QUIET" == "0" ]; then
		echo
		echo "Input: " $(human_readable_filesize $max_input_size)
		echo "Output: " $(human_readable_filesize $max_output_size)
		space_saved=$(expr $max_input_size - $max_output_size)
		echo "Space save: " $(human_readable_filesize $space_saved)
	fi
}

human_readable_filesize()
{
echo -n $1 | awk 'function human(x) {
     s=" b  Kb Mb Gb Tb"
     while (x>=1024 && length(s)>1) 
           {x/=1024; s=substr(s,4)}
     s=substr(s,1,4)
     xf=(s==" b ")?"%5d   ":"%.2f"
     return sprintf( xf"%s", x, s)
  }
  {gsub(/^[0-9]+/, human($1)); print}'
}

SHORTOPTS="h,i:,o:,q,s"
LONGOPTS="help,input:,output:,quiet,no-stats"
ARGS=$(getopt -s bash --options $SHORTOPTS --longoptions $LONGOPTS --name $PROGNAME -- "$@")

eval set -- "$ARGS"
while true; do
	case $1 in
		-h|--help)
			usage
			exit 0
			;;
		-i|--input)
			shift
			INPUT=$1
			;;
		-o|--output)
			shift
			OUTPUT=$1
			;;
		-q|--quiet)
			QUIET='1'
			;;
		-s|--no-stats)
			NOSTATS='1'
			;;
		--)
			shift
			break
			;;
		*)
			shift
			break
			;;
	esac
	shift
done

main

If you set INPUT and OUTPUT at the top of the file (like below), you can remove the beginning of your main function. Also no need to evaluate pwd when you can use ..

INPUT=.
OUTPUT=./output

You could have a echo_verbose function or something that would save you from checking the state of QUIET everytime:

echo_verbose ()
{
  if [ $QUIET == '0' ]; then
    echo $1
  fi
}

I have found the error ** Error: Unrecognized option: -clobber on PNG file
I am using optipng pngcrush