#!/usr/bin/perl -w

use strict;

my (%users, @in);

while (<DATA>) {
  chomp;
  $users{$_} = 0;
}

for (;;) {
  @in = split/ /,`users`;
  foreach my $n (keys %users) {
    if ( !$users{$n} && map {/^$n$/} @in ) {
      $users{$n} = 1;
      print scalar localtime, " $n has logged in \n";
    }
    if ( $users{$n} && ! map {/^$n$/} @in ) {
      $users{$n} = 0;
      print scalar localtime, " $n has logged out \n";
    }
  }
  sleep 2;
}

__DATA__
list
of
users
to
watch