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=${CHECK_DELAY:-4}
CHECK_IP=${CHECK_IP:-8.8.8.8}
PRIMARY_IF=${PRIMARY_IF:-eth0} PRIMARY_GW=${PRIMARY_GW:-192.168.1.1}
BACKUP_IF=${BACKUP_IF:-wlan0}
BACKUP_GW=${BACKUP_GW:-192.168.43.1}

# Compare arg with current default gateway interface for route to healthcheck IP
gateway_if() {
	[[ "$1" = "$(ip r g "$CHECK_IP" | sed -rn 's/^.*dev ([^ ]*).*$/\1/p')" ]]
}

# Cycle healthcheck continuously with specified delay
while sleep "$CHECK_DELAY"
do
	# If healthcheck succeedscd 
	if ! ping -c1 "$CHECK_IP" &>/dev/null
	then
		# Are we using the backup?
		if gateway_if "$BACKUP_IF"
		then # Switch to primary
			ip r d default via "$BACKUP_GW" dev "$BACKUP_IF"
			ip r a default via "$PRIMARY_GW" dev "$PRIMARY_IF"
		else
			ip r d default via "$PRIMARY_GW" dev "$PRIMARY_IF"
			ip r a default via "$BACKUP_GW" dev "$BACKUP_IF"
		fi
	fi
done