In Perl, how can I watch a directory for changes?

Question

use Text::Diff;
for($count = 0; $count <= 1000; $count++){
   my $data_dir="archive/oswiostat/oracleapps.*dat";
   my $data_file= `ls -t $data_dir | head -1`;
   while($data_file){
      print $data_file;
      open (DAT,$data_file) || die("Could not open file! $!");
      $stats1 = (stat $data_file)[9];
      print "Stats: \n";
      @raw_data=<DAT>;
      close(DAT);
      print "Stats1 is :$stats1\n";
      sleep(5);
      if($stats1 != $stats2){
         @diff = diff \@raw_data, $data_file, { STYLE => "Context" };
         $stats2 = $stats1;
      }
      print @diff || die ("Didn't see any updates $!");
   }
}

Output:

$ perl client_socket.pl
archive/oswiostat/oracleapps.localdomain_iostat_12.06.28.1500.dat
Stats:
Stats1 is :
Didn't see any updates  at client_socket.pl line 18.

Can you tell me why the stats are missing and how to fix it?

Answer 1

Note that I'm answering your original question, why the stat() seemed to fail, rather than the newly edited question title, which asks something different.

This is the fix:

my $data_file= `ls -t $data_dir | head -1`;
chomp($data_file);

The reason this is the fix is a little murky. Without that chomp(), $data_file contains a trailing newline: "some_filename\n". The two argument form of open() ignores trailing newlines in filenames and I don't know why because two-arg open mimics shell behavior. Your call to stat(), however, does not ignore the newline in the filename, so it is stat()ing a non-existent file and thus $stats1 is undef.

Answer 2

The real fix is File::ChangeNotify or File::Monitor or something similar (e.g., on Windows, Win32::ChangeNotify).

use File::ChangeNotify;

my $watcher = File::ChangeNotify->instantiate_watcher(
    directories => [ 'archive/oswiostat' ],
    filter => qr/\Aoracleapps[.].*dat\z/,
);

while (my @events = $watcher->wait_for_events) {
    # ...
}