#!/bin/sh
#
# notice that different version of ls may return different file
# information differently. Some verision of ls return the file size
# in the fourth field and thus $5 should be replaced with $4 below.
#
# save current status of the mail box.
#
set -- `ls -l $MAIL`
previous_size=$5
set -- `wc -l $MAIL`
previous_length=$1
while true
do
# Do a sleep so less computation power is used.
  sleep 5
  #
  # get new status of mail box
  #
  set -- `ls -l $MAIL`
  current_size=$5
  set -- `wc -l $MAIL`
  current_length=$1 
  #
  # if new mails arrived (note that this script doesn't consider
  #                       mail deletion. You may want to handle it
  #                       as an exercise)
  #
  if [ $current_size -gt $previous_size ]
  then
     echo New mail received!
     extra_line=`expr $current_length - $previous_length`
     echo The mail file has $extra_line more new lines 
     echo Here is the contents:
     echo
     tail -$extra $MAIL 
     #
     # remember current status of mail box
     #
     previous_size=$current_size
     previous_length=$current_length
  fi
done