getMacAddr.sh

#!/bin/bash

ifconfig > /tmp/ifconfig_nodongle

while true; do
    read -p "Have you plugged in the ethernet dongle?" yn
    case $yn in
        [Yy]* ) ifconfig > /tmp/ifconfig_dongle; break;;
        [Nn]* ) exit;;
        * ) echo "Please answer yes or no.";;
    esac
done

diff /tmp/ifconfig_nodongle /tmp/ifconfig_dongle | grep ether | awk '{print $3}'
rm -rf /tmp/ifconfig_nodongle /tmp/ifconfig_dongle