An example failover script for dual WAN, using a ping healthcheck and managing default routes appropriately

failover.sh

#!/bin/bash

# Set defaults if not provided by environment
CHECK_DELAY=${CHECK_DELAY:-5}
CHECK_IP=${CHECK_IP:-1.1.1.1}
PRIMARY_IF=${PRIMARY_IF:-eno8303}
PRIMARY_GW=${PRIMARY_GW:-192.168.15.1}

# Cycle healthcheck continuously with specified delay
while sleep "$CHECK_DELAY"
do
  # If healthcheck succeeds from primary interface
  if ping -I "$PRIMARY_IF" -c1 "$CHECK_IP" &>/dev/null
  then
    # Set Primary with more priority (ie. lower metric)
    ip route add default via "$PRIMARY_GW" dev "$PRIMARY_IF" metric 100
 
  else
    # Set Primary with less priority (ie. higher metric)
    ip route add default via "$PRIMARY_GW" dev "$PRIMARY_IF" metric 200
  fi
done