This script will generate all iOS and Android icon files (including retina) using the built-in sips command in OS X. It will also compress the PNGs if ImageOptim is installed. This is based off another awesome Gist found here: https://gist.github.com/jessedc/837916

generate-app-icons.sh

#!/bin/bash

COMPRESS=1                # Compress if ImageOptim is intalled. 0 = disabled; 1 = enabled
iOSSubdir="/ios"          # make this an empty string to save in same dir
androidSubdir="/android"  # make this an empty string to save in same dir

help() {
	echo ""
	echo "Usage: ./$(basename "$0") <your_1024x1024.png>"
	echo "       --------------------------------------------------------------------------------------"
	echo "       If you don't pass a parameter, it will look for a iTunesArtwork@2x.png in the current"
	echo "       directory. The file needs to be 1024x1024 for the rest of the files to be resized"
	echo "       correctly."
	echo ""
	echo "       If you want to automatically compress the icons, install ImageOptim."
	echo "       More info here: http://imageoptim.com/"
	exit
}

FILEPATH=$1
if [[ ! -f $FILEPATH ]]; then
	FILEPATH="./iTunesArtwork@2x.png"
fi
if [[ ! -f $FILEPATH ]]; then
	echo "ERROR: Could not find icon file"
	help
fi
FILE=$(basename "$FILEPATH")

# the root directory is where the icon passed as a parameter lives
DIR=$(cd $(dirname "$FILEPATH"); pwd)

# make subdirs
if [[ $iOSSubdir != "" && ! -d ${DIR}/$iOSSubdir ]]; then mkdir ${DIR}/${iOSSubdir}; fi
if [[ $androidSubdir != "" && ! -d ${DIR}/$androidSubdir ]]; then mkdir ${DIR}/${androidSubdir}; fi

# iOS App Store Icons
if [[ $FILE != "./iTunesArtwork@2x.png" || $iOSSubdir != "" ]]; then sips --resampleWidth 1024 "${DIR}/${FILE}" --out "${DIR}${iOSSubdir}/iTunesArtwork@2x.png" > /dev/null 2>&1; fi
sips --resampleWidth 512 "${DIR}/${FILE}" --out "${DIR}${iOSSubdir}/iTunesArtwork.png" > /dev/null 2>&1

# iOS App Icons
sips --resampleWidth 57 "${DIR}/${FILE}" --out "${DIR}${iOSSubdir}/icon.png" > /dev/null 2>&1
sips --resampleWidth 114 "${DIR}/${FILE}" --out "${DIR}${iOSSubdir}/icon@2x.png" > /dev/null 2>&1
sips --resampleWidth 60 "${DIR}/${FILE}" --out "${DIR}${iOSSubdir}/icon-60.png" > /dev/null 2>&1
sips --resampleWidth 120 "${DIR}/${FILE}" --out "${DIR}${iOSSubdir}/icon-60@2x.png" > /dev/null 2>&1
sips --resampleWidth 180 "${DIR}/${FILE}" --out "${DIR}${iOSSubdir}/icon-60@2x.png" > /dev/null 2>&1
sips --resampleWidth 72 "${DIR}/${FILE}" --out "${DIR}${iOSSubdir}/icon-72.png" > /dev/null 2>&1
sips --resampleWidth 144 "${DIR}/${FILE}" --out "${DIR}${iOSSubdir}/icon-72@2x.png" > /dev/null 2>&1
sips --resampleWidth 76 "${DIR}/${FILE}" --out "${DIR}${iOSSubdir}/icon-76.png" > /dev/null 2>&1
sips --resampleWidth 152 "${DIR}/${FILE}" --out "${DIR}${iOSSubdir}/icon-76@2x.png" > /dev/null 2>&1

# iOS Search/Settings
sips --resampleWidth 29 "${DIR}/${FILE}" --out "${DIR}${iOSSubdir}/icon-small.png" > /dev/null 2>&1
sips --resampleWidth 58 "${DIR}/${FILE}" --out "${DIR}${iOSSubdir}/icon-small@2x.png" > /dev/null 2>&1
sips --resampleWidth 40 "${DIR}/${FILE}" --out "${DIR}${iOSSubdir}/icon-40.png" > /dev/null 2>&1
sips --resampleWidth 80 "${DIR}/${FILE}" --out "${DIR}${iOSSubdir}/icon-40@2x.png" > /dev/null 2>&1
sips --resampleWidth 50 "${DIR}/${FILE}" --out "${DIR}${iOSSubdir}/icon-50.png" > /dev/null 2>&1
sips --resampleWidth 100 "${DIR}/${FILE}" --out "${DIR}${iOSSubdir}/icon-50@2x.png" > /dev/null 2>&1

# Android icons
sips --resampleWidth 144 "${DIR}/${FILE}" --out "${DIR}${androidSubdir}/xxhdpi.png" > /dev/null 2>&1
sips --resampleWidth 96 "${DIR}/${FILE}" --out "${DIR}${androidSubdir}/xhdpi.png" > /dev/null 2>&1
sips --resampleWidth 72 "${DIR}/${FILE}" --out "${DIR}${androidSubdir}/hdpi.png" > /dev/null 2>&1
sips --resampleWidth 48 "${DIR}/${FILE}" --out "${DIR}${androidSubdir}/mdpi.png" > /dev/null 2>&1
sips --resampleWidth 36 "${DIR}/${FILE}" --out "${DIR}${androidSubdir}/ldpi.png" > /dev/null 2>&1

echo "Your icons have been created!"

# optimize PNGs, if ImageOptim is installed
ImageOptim="/Applications/ImageOptim.app/Contents/MacOS/ImageOptim"
if [[ $COMPRESS -eq 1 ]]; then
	if [[ -f $ImageOptim ]]; then
		echo "Optimizing iOS icons... This is going to take a while..."
		$($ImageOptim ${DIR}${iOSSubdir}/*)
		echo "Optimizing Android icons... This is going to take a while..."
		$($ImageOptim ${DIR}${androidSubdir}/*)
		echo ""
		echo "Finished compressing the icon files!"
	else
		echo "Skipped compression. To compress the PNGs, install ImageOptim. More info here: http://imageoptim.com/"
	fi
fi

Updated paths and filenames to match Cordova app icon example code.