Make a mac OS icns file from a single input file using imageMagick and iconutil

makeicons.sh

#!/usr/bin/env bash
  
infile=$1
if [ ! -f $infile ]; then
  echo specify a filename as the first parameter.
  exit 1;
fi

tempdir=$(mktemp --directory --suffix=.iconset --tmpdir=`pwd`)

for size in 16x16 32x32 128x128 256x256 512x512
do
  echo convert -resize "$size" $infile "${tempdir}/icon_${size}.png"
  convert -resize "$size" $infile "${tempdir}/icon_${size}.png"
  double=$(expr 2 \* `echo $size | cut -f1 -d'x'`)
  echo convert -resize "${double}x${double}" $infile "${tempdir}/icon_${size}@2x.png"
  convert -resize "${double}x${double}" $infile "${tempdir}/icon_${size}@2x.png"
done

iconutil -c icns -o ./output.icns $tempdir