use ImageMagick to convert the given image to a favicon

favicon.sh

#!/bin/sh
# use ImageMagick to convert the given image to a favicon
# http://www.imagemagick.org/Usage/thumbnails/#favicon

usage() {
  >&2 echo "usage: $(basename "$0") input [output]"
}
set -eu
input="${1:-}"
output="${2:=favicon}"
ext=".ico"

# input filename is required
if [ ! -r "$input" ]; then
  usage
  exit 1
fi

# append file extension if necessary
[ "${2: -4}" != "$ext" ] && output=${output}${ext}

[ -f "$output" ] && echo "overwriting output file $output"

convert "$input" \
  \( -clone 0 -resize 16x16 \) \
  \( -clone 0 -resize 32x32 \) \
  \( -clone 0 -resize 48x48 \) \
  \( -clone 0 -resize 64x64 \) \
  -delete 0 -alpha remove -colors 256 "$output"

if [ "$?" ]; then
  identify "$output"
else
  >&2 echo "error converting"
fi