Does what it says on the tin.
swap=`free | tail -1`
total=`echo $swap | awk '{print $2}'`
used=`echo $swap | awk '{print $3}'`
used2=$(echo "scale=2;${used}/${total}"|bc -l)
used_perc=$(echo "scale=2;${used2}*100"|bc -l)echo "Swap used (%) = $used_perc"