Shell script that loads the WPA_supplicant configuration and use it to connect to available networks, if none is available it an AP is created. Include Wi-Fi watchdog service that checks that the connection is always working.

load_wifi.sh

#!/bin/bash

WPA_SUPPLICANT_CONF="/etc/wpa_supplicant/wpa_supplicant.conf"

# this funcion is called once the connection is established,
# in this case a boot sound will be played to notify the user that everything is ready.
function connected {
	aplay /root/Windows3.1.wav 2>&1 >/dev/null &
}

# function that tries to establish a connection, get a network in $temp from the WPA file
# and then goes through all available networks ($essid) to find a match.
function testConnect {
	while [ $x -lt ${#essid[@]} ]; do
		if [ "${temp[ssid]}" = "${essid[$x]}" ]
		then
			echo "Trying to connect to ${temp[ssid]}"
			ssid="${temp[ssid]}"
			ssid="${ssid%\"}"
			ssid="${ssid#\"}"
			iwconfig wlan0 essid "$ssid"
			dhclient wlan0
			connected
			exit
		fi
	(( x++ ))
	done
}

a=0
b=0
x=0

declare -a essid
declare -a address
# load in $essid and $address all available networks
while read line
do
	case $line in
		*ESSID* )
			line=${line#*ESSID:}
			essid[$a]="$line"
			a=$((a + 1))
			;;
		*Address*)
			line=${line#*Address:}
			address[$b]="$line"
			b=$((b + 1))
			;;
	esac
done < <(iwlist wlan0 scanning)

#load in $temp a network at time from the WPA supplicant file
declare -A temp
i=0
flag=0
while read line
do
	case "$line" in
		"network={")
			flag=1
			;;
		"}")
			#echo "${temp[*]}"
			testConnect temp[@]
			((i++))
			flag=0
			;;
		*)
			if [ "$flag" -eq 1 ]; then
				IFS='=' read -a array <<< "$line"
				key="${array[0]}"
				temp[$key]="${array[1]}"
			fi
	esac
done < $WPA_SUPPLICANT_CONF

# if a connection hasn't been established create an hotspot
if ifconfig wlan0 | grep -q "inet addr:" ; then
	echo "Already Connected"
	# start the watchdog service
	service wifi_watchdog start
	connected
else
	#we don't know if the watchdog service is running so stop it 'cause I haven't test it yet
	service wifi_watchdog stop
	
	echo "Network connection down! Inizializing hotspot:"
	# set a static IP
	ifconfig wlan0 192.168.0.1 netmask 255.255.255.0
  # inizialize hotspot
	hostapd -B -dd /etc/hostapd/hostapd.conf
  # start dhcp
	service udhcpd start

	connected
fi

wifi_watchdog.sh

#!/bin/bash    
# source: http://raspberrypi.stackexchange.com/a/5121
# make sure we aren't running already
what=`basename $0`
for p in `ps h -o pid -C $what`; do
	if [ $p != $$ ]; then
		exit 0
	fi
done

# source configuration
wlan=wlan0
#set router ip = gateway
router_ip=$(route -n | grep 'wlan0$' | grep '^0\.0\.0\.0' | awk '{print $2}')
log=/var/log/wifi.log
check_interval=120

exec 1> /dev/null
exec 2>> $log
echo $(date) > $log
# without check_interval set, we risk a 0 sleep = busy loop
if [ ! "$check_interval" ]; then
	echo "No check interval set!" >> $log
	exit 1
else sleep $check_interval
fi

restartWifi () {
	dhclient -v -r
	# make really sure
	killall dhclient
	ifconfig $wlan down
	ifconfig $wlan up
	#launch script to connect wifi
	/root/load_wifi.sh &
}

while [ 1 ]; do
	ping -c 1 $router_ip & wait $!
	if [ $? != 0 ]; then
		echo $(date)" attempting restart..." >> $log
		restartWifi
	fi
	sleep $check_interval
done

wifi_watchdog_service.sh

#! /bin/sh
### BEGIN INIT INFO
# Provides:          wifi_watchdog
# Required-Start:    $remote_fs $syslog
# Required-Stop:     $remote_fs $syslog
# Default-Start:     2 3 4 5
# Default-Stop:      0 1 6
# Short-Description: Watchdog service for Wi-Fi
# Description:       This script ping the router every N seconds and if it fails
#                    tries to restore the network connection.
### END INIT INFO
# this file goes in /etc/init.d/
# Author: Giulio Montagner

# Do NOT "set -e"

# PATH should only include /usr/* if it runs after the mountnfs.sh script
DESC="Description of the service"
NAME=wifi_watchdog
DAEMON=/root/$NAME.sh
DAEMON_PS=/[r]oot/$NAME.sh
DAEMON_ARGS="--options args"
PIDFILE=/var/run/$NAME.pid
SCRIPTNAME=/etc/init.d/$NAME

#Include functions
. /lib/lsb/init-functions


#
# Function that starts the daemon/service
#
do_start()
{
	# Return
	#   0 if daemon has been started
	#   1 if daemon was already running
	#   2 if daemon could not be started
	# make sure we aren't running already
	what=$(basename $DAEMON)
	for p in `ps h -o pid -C $what`; do
		if [ $p != $$ ]; then
			return 1
		fi
	done
	$DAEMON &
	return 0
}

#
# Function that stops the daemon/service
#
do_stop()
{
	# Return
	#   0 if daemon has been stopped
	#   1 if daemon was already stopped
	#   2 if daemon could not be stopped
	#   other if a failure occurred
	kill -9 $(ps aux | grep "$DAEMON_PS" | awk '{print $2}')
}

#
# Function that sends a SIGHUP to the daemon/service
#
do_reload() {
	#
	# If the daemon can reload its configuration without
	# restarting (for example, when it is sent a SIGHUP),
	# then implement that here.
	#
	return 0
}

case "$1" in
  start)
	[ "$VERBOSE" != no ] && log_daemon_msg "Starting $DESC" "$NAME"
	do_start
	case "$?" in
		0|1) [ "$VERBOSE" != no ] && log_end_msg 0 ;;
		2) [ "$VERBOSE" != no ] && log_end_msg 1 ;;
	esac
	;;
  stop)
	[ "$VERBOSE" != no ] && log_daemon_msg "Stopping $DESC" "$NAME"
	do_stop
	case "$?" in
		0|1) [ "$VERBOSE" != no ] && log_end_msg 0 ;;
		2) [ "$VERBOSE" != no ] && log_end_msg 1 ;;
	esac
	;;
  status)
	ps aux | grep "$DAEMON_PS"
	;;
  #reload|force-reload)
	#
	# If do_reload() is not implemented then leave this commented out
	# and leave 'force-reload' as an alias for 'restart'.
	#
	#log_daemon_msg "Reloading $DESC" "$NAME"
	#do_reload
	#log_end_msg $?
	#;;
  restart|force-reload)
	#
	# If the "reload" option is implemented then remove the
	# 'force-reload' alias
	#
	log_daemon_msg "Restarting $DESC" "$NAME"
	do_stop
	case "$?" in
	  0|1)
		do_start
		case "$?" in
			0) log_end_msg 0 ;;
			1) log_end_msg 1 ;; # Old process is still running
			*) log_end_msg 1 ;; # Failed to start
		esac
		;;
	  *)
		# Failed to stop
		log_end_msg 1
		;;
	esac
	;;
  *)
	#echo "Usage: $SCRIPTNAME {start|stop|restart|reload|force-reload}" >&2
	echo "Usage: $SCRIPTNAME {start|stop|status|restart|force-reload}" >&2
	exit 3
	;;
esac

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