Program in UNIX which checks after every one-minute whether user has logged in or not

A friend of yours has promised to log in at a particular time. However, he has not kept the promise. You want to contact him as soon as he logs in. Write a shell script which checks after every one-minute whether your friend has logged in or not. The logname should be supplied to the shell script at command prompt.
echo "Enter User number :"
read un

x=`who | grep $un |wc -l`

if [ $x -ne 0 ]
then
echo "User $un logged in "
else
while [ $x -eq 0 ]
do
sleep 2
x=`who|grep $un|wc -l`
if [ $x -ge 1 ]
then
echo "now user logged in "
write $un >hi
exit
fi
done
fi