Stack overflow yields Where can i find Icon Composer on Mac? when I did a quick search to convert a png to icns. For future reference:
#!/bin/bash -e
set -e
set -o pipefail
if [ "$#" -ne 1 ]; then
echo "\nusage: to_icns.sh path/to/image.png\n"
exit 1
fi
readonly IMAGE_FPATH=$1
readonly BASENAME=$(basename ${IMAGE_FPATH%.*})
mkdir ${BASENAME}.iconset
sips -z 16 16 $IMAGE_FPATH --out "${BASENAME}.iconset/icon_16x16.png"
sips -z 32 32 $IMAGE_FPATH --out "${BASENAME}.iconset/icon_16x16@2x.png"
sips -z 32 32 $IMAGE_FPATH --out "${BASENAME}.iconset/icon_32x32.png"
sips -z 64 64 $IMAGE_FPATH --out "${BASENAME}.iconset/icon_32x32@2x.png"
sips -z 128 128 $IMAGE_FPATH --out "${BASENAME}.iconset/icon_128x128.png"
sips -z 256 256 $IMAGE_FPATH --out "${BASENAME}.iconset/icon_128x128@2x.png"
sips -z 256 256 $IMAGE_FPATH --out "${BASENAME}.iconset/icon_256x256.png"
sips -z 512 512 $IMAGE_FPATH --out "${BASENAME}.iconset/icon_256x256@2x.png"
sips -z 512 512 $IMAGE_FPATH --out "${BASENAME}.iconset/icon_512x512.png"
cp $IMAGE_FPATH "${BASENAME}.iconset/icon_512x512@2x.png"
iconutil -c icns ${BASENAME}.iconset
rm -R ${BASENAME}.iconset
echo Wrote ${BASENAME}.icns