GNU/BSD compatible cache busting with sed

cache-bust.sh

#!/usr/bin/env bash
# update cache busting strings in html file, supports GNU and BSD
function size_of () { wc -c $1 | awk '{print $1}'; }

before=`cat $1 | grep -Eo 'css\?rel=[0-9]+' | grep -Eo '([0-9]+)'`
now=`date +%s`

sed -i.bak "s/main.*css?rel=[0-9]*/main.min.css?rel=$now/" $1
sed -i.bak "s/build.*js?rel=[0-9]*/build.js?rel=$now/" $1
sed -i.bak "s/urlArgs.*$/urlArgs: \"v=$now\"/" $1

if [[ $(size_of $1) -eq $(size_of "$1.bak") ]];
then
    echo "broke cache in $1 [$before] => [$now]"
    rm "$1.bak"
    echo "removed $1.bak"
else
    echo 'error writing to $1'
fi