#!/bin/bash

...

fix_route () {
  good_wifi=$(ip route | perl -ne '/default\s+via\s+(\S+).+?metric\s+600/; if ($1) {print $1; exit}')
  good_eth0=$(ip route | perl -ne '/default\s+via\s+(\S+).+?metric\s+100/; if ($1) {print $1; exit}')
  bad_eth0=$(ip route | perl -ne '/default\s+via\s+(\S+).+?metric\s+20100/; if ($1) {print $1; exit}')
  # if there are no good routes with metrics 100 or 600, and there is a "bad"
  # route with metric 20100, assume the bad route is actually good and use it
  if [ -z "$good_wifi" -a -z "$good_eth0" -a -n "$bad_eth0" ]; then
    echo ip route add 0.0.0.0/0 via "$bad_eth0" metric 100
  fi
}

while true; do
  fix_route
  provisioning
  echo "*** provisioning exited; waiting 5s before trying again"
  sleep 5
done