Android app script for Android Studio / Android SDK. This script resizes android xhdpi-sized png image assets into the ldpi, mdpi, hdpi, xhdpi folders. Now just maintain one folder of images rather than 4 or 5. Updated to create directories if needed and also to separately convert images in the mipmap folder (used for app icons)

resizeImagesFromXHDPI.sh

#!/bin/bash -e

# Ensure we're running in location of script.
cd "`dirname $0`"

function fail {
	echo "FAILED with $1"
	exit 1
}

function doConversion() {
	local folderName=$1
	echo "Using $folderName..."

	cd "res/$folderName-xhdpi" || fail "No $folderName-xhdpi folder"

	for f in *; do
	  if [[ ${f} == *.png ]];
	    then
	    echo "...$f"
	    if [[ ${f} != *.9.png ]];
	    then

	    	# ensure folders exist
	    	mkdir -p "../$folderName-xhdpi" || fail "can't make folder"
		mkdir -p "../$folderName-hdpi" || fail "can't make folder"
		mkdir -p "../$folderName-mdpi" || fail "can't make folder"
		mkdir -p "../$folderName-ldpi" || fail "can't make folder"

		convert ${f} -resize 75% "../$folderName-hdpi/${f}" || fail "Failed with ../$folderName-hdpi/${f}"
		convert ${f} -resize 50% "../$folderName-mdpi/${f}" || fail "Failed with ../$folderName-mdpi/${f}"
		convert ${f} -resize 37.5% "../$folderName-ldpi/${f}" || fail "Failed with ../$folderName-ldpi/${f}"	        
	    else
	        echo "not resizing ${f} as it's a 9-patch"
	    fi
	  fi
	done

	cd -
}

# Convert graphic images
doConversion "drawable"

# Convert app icon (now goes in the mipmap folder)
doConversion "mipmap"

echo Complete

You should place this script at the same level as the 'res' folder. Put your master images in the xhdpi folder and then run the script.

I tried to run it on mac and it says: convert: command not found. Am I missing something?

This script depends on ImageMagick - try this http://cactuslab.com/imagemagick/