rescale_images.sh

#!/bin/bash
#Recently switched an in-house level app with 20+ targets to universal app so we were in need to update 200+ icons, 
#this is a small quick and dirty script used to resize all icons to what ios expects, checking Contents.json (solves warnings). 
#WARNING USE IT AT YOUR OWN RISK!!
#Please note that this may downscale or upscale your images based on the original ones.
#Must be run at Images.xcassets level
#Requirements
# jq for JSON parsing
# imagemagick for image rescaling
for d in *.appiconset; do
  cd "$d" #push folder
  if [ $? -ne 0 ]; then
    echo "Could not change folder"
    exit 1
  fi

  FILELIST=`cat Contents.json |jq -r '.images | map(.filename) | join("\n")'`
  for f in $FILELIST; do
    FILESIZE=`cat Contents.json | jq  --arg f "$f" '.images[] | select(.filename==$f) | .size'|replace \" ""|cut -dx -f1`
    FILESCALE=`cat Contents.json | jq  --arg f "$f" '.images[] | select(.filename==$f) | .scale'|replace \" ""|cut -dx -f1`
    FILESIDE=`bc -l <<< "$FILESIZE * $FILESCALE"` #Calculate size based on scale and size property

    convert $f -resize ${FILESIDE}x${FILESIDE}  $f
  done
  cd .. #back to root
done