AWS ubuntu 18.04 setup secondary interface with routing default via interface attachment by index

set_second_interface.sh

#!/bin/bash

set_second_interface()  # $1 = interface index (aws index) to set the default route
{
  route_via="$1"

  [ "$route_via" ] || {
    echo "Must provide interface index number which to route via"
    return 1
  }

  ifaces=$(ls /sys/class/net/)

  for iface in $ifaces; do
    [ "$(cat "/sys/class/net/$iface/type")" = 1 ] || continue

    mac="$(cat "/sys/class/net/$iface/address")"

    [ "$mac" ] || continue

    # ensure we have a route
    sudo ip route add 169.254.169.254 dev "$iface"

    device_number=$(curl -fs http://169.254.169.254/latest/meta-data/network/interfaces/macs/$mac/device-number)

    sudo ip route del 169.254.169.254 dev "$iface"

    if [ "$route_via" = 0 ] && [ "$device_number" = 0 ]; then
      # We don't have to set anything only for the secondary interfaces
      sudo rm -f "/etc/netplan/51-auto-$iface.yaml"
      continue
    fi

    cat <<EOF | sudo tee "/etc/netplan/51-auto-$iface.yaml"
network:
    version: 2
    ethernets:
        $iface:
            dhcp4: true
            match:
                macaddress: $mac
            set-name: $iface
EOF
    if [ "$route_via" != "$device_number" ]; then
      cat <<EOF | sudo tee -a "/etc/netplan/51-auto-$iface.yaml"
            dhcp4-overrides:
                use-routes: false
EOF
    fi
  done


}

# If the script is sourced do nothing. If it is executed, execute the function and pass the args
(return 0 2>/dev/null) || set_second_interface "$@"

paset the above in a shell and then run
set_second_interface 0 && sudo netplan --debug apply where 0 is the default

or download the file

ROUTE_VIA_IFACE=0
curl https://gist.githubusercontent.com/ar45/4616efa6cc4093dcdf9da1daa981b381/raw/set_second_interface.sh | bash -s $ROUTE_VIA_IFACE && sudo netplan --debug apply