#!/usr/bin/env sh

set -u

if [ -z "$1" ]
then
	echo 'Usage: $0 [-v] <url> [<target>]' >&2
	# download url to target location if modified
	exit 64 # EX_USAGE
fi

verbose=

case "$1" in
-v)
	verbose=1
	shift
	;;
esac

source=$1
target=${2:-${source##*/}}

if [ -e "$target" ]
then
	modified=$(stat -c%y -L "$target") &&
	modified=$(date -R -d "$modified")
	[ $verbose ] && echo "download since: $modified"
fi

headers=${modified:+If-Modified-Since: $modified}
wget -nv --header="$headers" -O "$target.new" "$source" &&
mv "$target.new" "$target" ||
rm "$target.new" # clean up failed download (usually empty 304 response)