primary_iface

#!/bin/sh

min_ifindex=$((0xFFFF))

for dev in /sys/class/net/*; do
    [ $dev = "/sys/class/net/lo" ] && continue

    ifindex=$(cat $dev/ifindex)
    mac="$(cat $dev/address)"

    if [ $ifindex -lt $min_ifindex ] && [ -n "$mac" ] && [ "$mac" != "00:00:00:00" ]; then
        min_ifindex=$ifindex
        iface=$dev
    fi
done

[ -z "$iface" ] && exit 1

echo "mac=$(cat $iface/address)"