ping_alert.sh

#!/bin/bash

# requires 'sox' package for 'play'

# alarm parameters
ALARM_DURATION=.8
SINE_LOW=300
SINE_HIGH=3000

# target IP to watch
IP=192.168.86.119
# time to wait for ping success (seconds)
PING_TIMEOUT=1
# time to wait between attempts (seconds)
WAIT_TIME=2

# how long before we consider the ip departed
SEEN_THRESHOLD_SECS=20

# initial globals
LAST_SEEN=0
PRESENT=100

# https://stackoverflow.com/questions/51473733/convert-seconds-to-days-hour-min-sec
convertsecs2dhms () {
    # use $((...)) for arithmetic
    d=$((${1}/(60*60*24)))
    h=$(((${1}%(60*60*24))/(60*60)))
    m=$(((${1}%(60*60))/60))
    s=$((${1}%60))
    echo -n ${d}d ${h}h ${m}m ${s}s
}

sound_alarm_arrive() {
  play --no-show-progress --null --channels 1 synth $ALARM_DURATION sine ${SINE_LOW}-${SINE_HIGH}
}
sound_alarm_left() {
  play --no-show-progress --null --channels 1 synth $ALARM_DURATION sine ${SINE_HIGH}-${SINE_LOW}
}

while true; do
  # ping test
  timeout $PING_TIMEOUT ping -c 1 $IP > /dev/null
  PING_RESULT=$?

  if [ $PING_RESULT -eq 0 ]; then
    # successful ping
    LAST_SEEN=$(date +%s)
    if [ $PRESENT -eq 100 ]; then
      echo initially present
      PRESENT=1
    fi
  elif [ $PRESENT -eq 100 ]; then
    echo initially away
    PRESENT=0
  fi

  # calculate seconds since last seen
  TIME_SINCE_LAST_SEEN=$(expr $(date +%s) - $LAST_SEEN)
  PRETTY_TIME_SINCE_LAST_SEEN=$(convertsecs2dhms $TIME_SINCE_LAST_SEEN)
  #echo $PRETTY_TIME_SINCE_LAST_SEEN
  echo $(date) \($IP\): Last Seen ${PRETTY_TIME_SINCE_LAST_SEEN} ago

  if [ $TIME_SINCE_LAST_SEEN -gt $SEEN_THRESHOLD_SECS ] && [ $PRESENT -eq 1 ]; then
    # we think they're home but we haven't seen then beyond threshold
    echo Sounding alarm. they just left
    sound_alarm_left
    PRESENT=0
  fi

  # if time since last seen is under threshold and current state is not present, we sound alarm
  if [ $TIME_SINCE_LAST_SEEN -lt $SEEN_THRESHOLD_SECS ] && [ $PRESENT -eq 0 ]; then
    echo Sounding alarm, they just showed up
    sound_alarm_arrive
    PRESENT=1
  fi

  sleep $WAIT_TIME
done