The following is a fragment of a bash script I use to determine the MAC of Ethernet, or if this does not exist of WiFi (for Pi Zero W).
It does not rely on ifconfig or any other method of detecting allocated IP, and just needs the system to detect the networking hardware.
This works for Jessie or Stretch
# Find MAC of eth0, or if not exist wlan0
if [ -e /sys/class/net/eth0 ]; then
MAC=$(cat /sys/class/net/eth0/address)
elif [ -e /sys/class/net/enx* ]; then
MAC=$(cat /sys/class/net/enx*/address)
else
MAC=$(cat /sys/class/net/wlan0/address)
fi
Basically you could use MAC=$(cat /sys/class/net/wlan0/address) to find the MAC of inbuilt WiFi on Pi3 or Pi Zero W.
MAC=$(cat /sys/class/net/wlx*/address) should work on WiFi dongles on Stretch, but you could easily adapt the above to work on both Jessie or Stretch for WiFi and Ethernet.