Script to detect connectivity loss on a Raspberry Pi and notify with the LEDs

connectivity_alert.sh

#!/bin/bash

# This script will provide visual feedback that a Raspberry Pi 2B/3B has lost
# local network or Internet connectivity by blinking the two LEDs. Link state
# and Internet connectivity are represented as two different blink patterns,
# to help determine whether the Pi or the switch has had its cable fall out.

# The following table describes the LED flash patterns:
# Local | Internet | LED Flash Pattern
#=======|==========|=========================================================
# OK    | OK       | Unaltered (red = power, green = storage)
# OK    | Fail     | Red blinks; green is turned off.
# Fail  | OK       | This is an invalid state. Red and green set to solid on.
# Fail  | Fail     | Red and green both blink.
#
# Change INTERNET_HOST to any pingable host appropriate for your use case.
# I used my ISP's (Comcast's) DNS server.
#
# Intended to be run on a 1-minute cron, e.g.:
# [don@hermes ~]$ cat /etc/cron.d/connectivity_alert 
# * * * * * root /usr/local/bin/connectivity_alert.sh

set -euo pipefail

DEFAULT_TRIGGER_LED0=mmc0
DEFAULT_TRIGGER_LED1=input
INTERNET_HOST=75.75.75.75

function set_led() {
  echo $2 > /sys/class/leds/led$1/trigger
}

function set_leds_default() {
  set_led 0 $DEFAULT_TRIGGER_LED0
  set_led 1 $DEFAULT_TRIGGER_LED1
}

LINK_UP=0
if [ "$(cat /sys/class/net/eth0/operstate)" == "up" ]; then
  LINK_UP=1
fi

INTERNET_REACHABLE=0
if ping -c 1 $INTERNET_HOST > /dev/null 2>&1; then
  INTERNET_REACHABLE=1
fi

if [ $LINK_UP -eq 1 -a $INTERNET_REACHABLE -eq 1 ]; then # All normal.
  set_leds_default
elif [ $INTERNET_REACHABLE -eq 0 ]; then # No Internet connectivity - flash LED1.
  set_led 1 heartbeat
  if [ $LINK_UP -eq 0 ]; then # No network connectivity - flash LED0.
    set_led 0 heartbeat
  else
    set_led 0 none
  fi
else # Link up but no internet - should never get here. Complain loudly by setting both solid.
  set_led 0 default-on
  set_led 1 default-on
fi