Source: http://sirlagz.net/2013/01/22/script-starting-hostapd-when-wifi-goes-down/

Source: https://www.raspberrypi.org/forums/viewtopic.php?f=26&t=38323

I’ve had issues with my WiFi in the past, and with my Pi running headless it’s sometimes a pain to get it connected back up to WiFi, so I’ve created this little script to start hostapd and dnsmasq whenever the WiFi connection went down, which allowed me to SSH into the Pi even though it wasn’t connected to the network, because it was broadcasting it’s own network !

This can be customised to do different things if either WiFi or ethernet go down.


#!/bin/bash
#
# Interface checker
# Checks to see whether interface has an IP address, if it doesn't assume it's down and start hostapd
# Author : SirLagz
#
Interface='wlan0'
HostAPDIP='10.0.0.1'
echo "-----------------------------------"
echo "Checking connectivity of $Interface"
NetworkUp=`/sbin/ifconfig $Interface`
IP=`echo "$NetworkUp" | grep inet | wc -l`
if [[ $IP -eq 0 ]]; then
echo "Connection is down"


hostapd=`pidof hostapd`
if [[ -z $hostapd ]]; then
# If there are any more actions required when the interface goes down, add them here
echo "Attempting to start hostapd"
/etc/init.d/hostapd start
echo "Attempting to start dnsmasq"
/etc/init.d/dnsmasq start
echo "Setting IP Address for wlan0"
/sbin/ifconfig wlan0 $HostAPDIP netmask 255.255.255.0 up
fi
elif [[ $IP -eq 1 && $NetworkUp =~ $HostAPDIP ]]; then
echo "IP is $HostAPDIP - hostapd is running"
else
echo "Connection is up"
hostapd=`pidof hostapd`
if [[ ! -z $hostapd ]]; then
echo "Attempting to stop hostapd"
/etc/init.d/hostapd stop
echo "Attempting to stop dnsmasq"
/etc/init.d/dnsmasq stop
echo "Renewing IP Address for $Interface"
/sbin/dhclient wlan0
fi
fi
echo "-----------------------------------"