Trigger a systemd service when a host becomes unreachable via ping. Machine can be hostname or ip address.

start-service-if-host-unreachable.sh

#!/bin/sh
# add to your crontab?
# 45 * * * * /path/to/start-service-if-host-unreachable.sh

machine="moto"
service="reverse-tunnel"

if [ "$USER" != "root" ] ; then
	echo "You are not root. bye."
	exit 1;
fi

is_started () {
	systemctl status $service
	[ $? -eq 0 ] && return 0		# 0 indicates that service isrunning
	return 1
}

ping -c 1 "$machine"

if [ $? -eq 0 ] ; then			# if true, indicates machine is online
	if is_started ; then	# 0 indicates service is running
		echo "Service is running.. stopping now."
		systemctl stop $service
	fi

	exit 0
fi

echo $machine did not respond to ping.. starting service..
# start tunnel

is_started || systemctl start "$service"