Android asset resize script (run inside a folder of 3x images).

resize.sh

#!/bin/bash -e

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

mkdir ./drawable-mdpi
mkdir ./drawable-hdpi
mkdir ./drawable-xhdpi
mkdir ./drawable-xxhdpi

for f in *; do
  if [[ $f == *.png ]];
    then
    echo "...$f"
    convert $f -resize 33.3% ./drawable-mdpi/$f
    convert $f -resize 50.0% ./drawable-hdpi/$f
    convert $f -resize 66.67% ./drawable-xhdpi/$f
    convert $f -resize 100.0% ./drawable-xxhdpi/$f
  fi
done

echo "Complete"