Bash wifi network switcher script for Raspberry Pi

I had a situation where I had a Raspberry Pi 3B+ in a location where there were two usable wifi networks in range, but neither were particularly reliable in terms of Internet access. The wifi networks themselves would stay up, but the ADSL connections used by the  routers would sometimes drop and stay down for a while (in the case of one, until someone rebooted it).

So I wanted the Pi to switch from one network to the other in the event of losing Internet connectivity.

So I wrote a bash script that can be run periodically by cron.

There are a number of ways to achieve this,  including using additional packages like nmcli, but I wanted something that should just work on a plain version of Raspbian.

I also decided to test it on older Pis without built-in wifi and using a doingle instead. I found that a few tweaks were required to get the script to work reliably. 

I also encountered one situation where the Pi under test ended up not connected to any wifi network at all. Annoyingly I have not been able to reproduce that outcome to identify what went wrong.

The script is set to switch between two networks, SSID1 and SSID1 - obviously you'll need to change these names to reflect your own environment. 

To use the script you'll need two different copies of your wpa_supplicant file - which contains the network name and pre-shared key (password) required to authenticate - , one for each SSID.  Give them sensible names like  wpa_supplicant_1.conf and  wpa_supplicant_2.conf.

The script logs to a file wifi.log - this can be useful for debugging. It also echoes to the terminal too, to make it easier to see what's going on when testing. If you don't want any of this, just remove or comment out the 'echo' lines. 

#!/bin/bash
# Checks for Internet connectivity and changes to an alternative wifi network if
# it detects a problem.
# Needs to have a wpa_supplicant file for each network:
#  wpa_supplicant_1 for SSID! and wpa_supplicant_2 for SSID2
# Can easily be extended to more than 2 possible networks

# function to bring down wireless interface, stop wpa_supplicant service and
# then bring up wlan0 again
reset_wifi(){
    /sbin/ifconfig wlan0 down
    sleep 1
    killall wpa_supplicant
    sleep 1
    /sbin/ifconfig wlan0 up
    sleep 1
}
# Logging information saved to a file
# useful for testing and debugging
echo "START" >> /home/pi/wifi.log
echo "Date:" `date`  >> /home/pi/wifi.log
echo "WLAN:" `/sbin/iwgetid` >> /home/pi/wifi.log
if /sbin/iwgetid | grep SSID1 > /dev/null; then
  echo "SSID1" >> /home/pi/wifi.log
  SSID="SSID1"
elif /sbin/iwgetid | grep SSID2 > /dev/null; then
  echo "SSID2"
  echo "SSID2" >> /home/pi/wifi.log
  SSID="SSID2"
else
  echo "unknown SSID"
  echo "unknown SSID" >> /home/pi/wifi.log
  SSID="UN"
fi
echo "$SSID" >> /home/pi/wifi.log
if ping -q -c 1 -W 1 8.8.8.8 >/dev/null; then
  echo "Net is up"
  echo "Net is up" >> /home/pi/wifi.log
else
  if ping -q -c 1 -W 1 8.8.8.8 >/dev/null; then
    echo "Net is up (after retry)"
    echo "Net is up (after retry)" >> /home/pi/wifi.log
  else
    echo "Net is down (after retry)"
    echo "Net is down (after retry)" >> /home/pi/wifi.log
    if [ "$SSID" = "SSID1" ]; then
      echo "trying SSID2"
      echo "trying SSID2"  >> /home/pi/wifi.log
      reset_wifi
      cp /etc/wpa_supplicant/wpa_supplicant_2.conf /etc/wpa_supplicant/wpa_supplicant.conf
      /sbin/wpa_supplicant -B -c/etc/wpa_supplicant/wpa_supplicant.conf -iwlan0

  elif [ "$SSID" = "SSID2" ]; then
      echo "trying SSID1"
      echo "trying SSID1"  >> /home/pi/wifi.log
      reset_wifi
      cp /etc/wpa_supplicant/wpa_supplicant_1.conf /etc/wpa_supplicant/wpa_supplicant.conf
      /sbin/wpa_supplicant -B -c/etc/wpa_supplicant/wpa_supplicant.conf -iwlan0
    else
      echo "dunno"  >> /home/pi/wifi.log
      echo "dunno"
      if [ `date +%M` = 10 ] || [ `date +%M` = 30 ] || [ `date +%M` = 50 ] ; then
         echo "yep"
         echo "trying SSID1 anyway"
         echo "trying SSID1 anayway"  >> /home/pi/wifi.log
         reset_wifi
         cp /etc/wpa_supplicant/wpa_supplicant_1.conf /etc/wpa_supplicant/wpa_supplicant.conf
         /sbin/wpa_supplicant -B -c/etc/wpa_supplicant/wpa_supplicant.conf -iwlan0
      else
         echo "trying SSID2 anyway"
         echo "trying SSID2 anyway"  >> /home/pi/wifi.log
         reset_wifi
         cp /etc/wpa_supplicant/wpa_supplicant_2.conf /etc/wpa_supplicant/wpa_supplicant.conf
         /sbin/wpa_supplicant -B -c/etc/wpa_supplicant/wpa_supplicant.conf -iwlan0
      fi
    fi
  fi
fi
echo "END" >> /home/pi/wifi.log

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There's no reason that this shouldn't work on other Debian distros or more widely on Linux, but I have not tested it.

The script needs to be run on a Pi with sudo. If you want to schedule using cron, then type 

sudo crontab -e

and then add a line to run as frequently as you like. For example:

0 */2 * * * /home/pi/wifitest.sh

Don't forget to make the file executable

chmod +x /home/pi/wifitest.sh