A shell script to convert a single image of arbitrary size to an ICNS file in OS X.

icns.sh

#!/bin/sh

# exit on error
set -e

if [ "$#" -ne 1 ]; then
  echo "Usage: icns myicon.png"
  exit 0
fi

if [ ! -f "$1" ]; then
  echo "File not found: $1"
  exit 0
fi

filename=$(basename "$1")
filename="${filename%.*}"

if [ -d "$filename.iconset" ]; then
  rm -r $filename.iconset
fi

mkdir $filename.iconset

echo "Creating $filename.icns..."
convert $1 -resize 16x16 -background transparent -gravity center -extent 16x16 $filename.iconset/icon_16x16.png
convert $1 -resize 32x32 -background transparent -gravity center -extent 32x32 $filename.iconset/icon_16x16@2x.png
convert $1 -resize 32x32 -background transparent -gravity center -extent 32x32 $filename.iconset/icon_32x32.png
convert $1 -resize 64x64 -background transparent -gravity center -extent 64x64 $filename.iconset/icon_32x32@2x.png
convert $1 -resize 128x128 -background transparent -gravity center -extent 128x128 $filename.iconset/icon_128x128.png
convert $1 -resize 256x256 -background transparent -gravity center -extent 256x256 $filename.iconset/icon_128x128@2x.png
convert $1 -resize 256x256 -background transparent -gravity center -extent 256x256 $filename.iconset/icon_256x256.png
convert $1 -resize 512x512 -background transparent -gravity center -extent 512x512 $filename.iconset/icon_256x256@2x.png
convert $1 -resize 512x512 -background transparent -gravity center -extent 512x512 $filename.iconset/icon_512x512.png
convert $1 -resize 1024x1024 -background transparent -gravity center -extent 1024x1024 $filename.iconset/icon_512x512@2x.png

iconutil -c icns $filename.iconset
rm -R $filename.iconset
echo "Done!"