Suppose I have three friends Rupesh Parag and Rizwan having their laptops then
the following script will help me to check whether they are online or offline and notify me if any change in the status

save this script as notification 
and set cron to run this script every minute

#!/bin/bash
rupesh=192.168.100.102
parag=192.168.100.101
rizwan=192.168.100.110
 
rupeshstatus=/tmp/rupeshstatus
paragstatus=/tmp/paragstatus
rizwanstatus=/tmp/rizwanstatus

start_time=$(date +%s)

######### Checking for Rupesh ##########################3

if [[  ! -f $rupeshstatus ]];
then
echo offline > $rupeshstatus
fi

ping -c1 $rupesh
rustatus=`echo $?`
#echo sstatus is $rustatus

if [ "$rustatus" -eq 0 ];
then
rup=online
else
rup=offline
fi

#echo $rup

ru=`cat $rupeshstatus`

#echo $ru
#echo "$rup" -ne "$s"
if [[ "$rup" != "$ru" ]];
then
#xmessage  -center "Rupesh $rup" -display :0
sshpass -p a ssh -o StrictHostKeyChecking=no akash.l@192.168.100.199 "xmessage -timeout 2 -c "Rupesh is $rup" -display :0 & "
echo $rup > $rupeshstatus
fi

if [[  ! -f $rupeshstatus ]];
then
echo offline > $rupeshstatus
fi
############ checking for Parag #############################################


if [[  ! -f $paragstatus ]];
then
echo offline > $paragstatus
fi

ping -c1 $parag
pstatus=`echo $?`
#echo sstatus is $pstatus

if [ "$pstatus" -eq 0 ];
then
prg=online
else
prg=offline
fi

#echo $prg

p=`cat $paragstatus`

#echo $v
#echo "$prg" -ne "$v"
if [[ "$prg" != "$p" ]];
then
#xmessage  -center "Parag is $prg" -display :0
sshpass -p a ssh -o StrictHostKeyChecking=no akash.l@192.168.100.199 "xmessage -timeout 2 -c Parag is $prg -display :0 &"
echo $prg > $paragstatus
fi

######## checking for Rizwan ##########################

if [[  ! -f $rizwanstatus ]];
then
echo offline > $rizwanstatus
fi

ping -c1 $rizwan
ristatus=`echo $?`
#echo sstatus is $sstatus

if [ "$ristatus" -eq 0 ];
then
riz=online
else
riz=offline
fi

#echo $rup

ri=`cat $rizwanstatus`

#echo $s
#echo "$rup" -ne "$s"
if [[ "$riz" != "$ri" ]];
then
#xmessage  -center "Rizwan is $riz" -display :0
sshpass -p a ssh -o StrictHostKeyChecking=no akash.l@192.168.100.199 "xmessage -timeout 2 -c Rizwan is $riz -display :0 &"

echo $riz > $rizwanstatus
fi


finish_time=$(date +%s)
echo "Time duration: $((finish_time - start_time)) secs."