blackcode6 has asked for the wisdom of the Perl Monks concerning the following question:

Fellow monks, I wrote a small inefficient perl script that checks to see if the time has changed on the /var/log/httpd/access_log file. If the file time is the same it prints a period. Once the file is "touch"ed by anything and the time changes, the script prints an "x". Eventually, I want to move to a specific response instead of an "x". The file uses 12% of cpu process because of its while loop nature. Is there a better way to do this? 
#!/usr/bin/perl

print "Testing to see if file changes.\n";

$listing = `ls -al access_log`;
@fields = split /" "/,$listing;

foreach (@fields) {
    if (/[0-9][0-9]:[0-9][0-9]/) {
       $status = $&;
       print "Status is :",$status,"\n"; }
    else {
     print "Didn't find anything.\n";
    }
}
@fields = split /:/, $status;
$hours = @fields[0];
$minutes = @fields[1];
print "hours :",$hours,"\n";
print "minutes :",$minutes,"\n";

$hours2 = $hours; 
$minutes2 = $minutes;
@fields = ();

while (1) {
while ($hours == $hours2 && $minutes == $minutes2)  {
    $listing = `ls -al access_log`;
    @fields = split /" "/,$listing;

    foreach (@fields) {
    if (/[0-9][0-9]:[0-9][0-9]/) {
       $status = $&;
 }
    else {
#      print "Didn't find anything suckker.\n";
    }
  }
    @fields = split/:/,$status;
    $hours = @fields[0];
    $minutes=@fields[1];
    @fields= ();
    print "."; # meaning the time is the still the same
}
    $hours2 = $hours;
    $minutes2 = $minutes;    
    print "x";  # meaning the time has changed; file touched
}

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Replies are listed 'Best First'.
Re: Perl process to constantly detect the time change of a file?
by Rich36 (Chaplain) on May 03, 2002 at 20:33 UTC

    You could cut out a lot of code by using stat - specifically "mtime", which is the "last modify time in seconds since the epoch". I would suggest putting a small sleep time in the loop as well, just so you're not constantly hitting your machine - unless it's absolutely crucial that you know the second when this file has changed. 

    my $file = "somefile.txt";

# Establish a base "last accessed time"
my $lastaccessed = (stat($file))[9]; 

while(1) {
  if ((stat($file))[9] == $lastaccessed) {
    # The file is the same
  } else {
    # File is different
  }
  sleep 5;
}

    [download]

    Check the docs for stat - it provides a lot of useful info about files.


    Rich36
    There's more than one way to screw it up...
[reply]
[d/l]
      Right -- of course one would usually want to make sure it goes like this (Rich36 assumes this goes without saying): 
      while(1) {
    my $modtime = (stat($file))[9];
    if ($modtime == $lastaccessed) {
      # The file is the same
    } else {
      # File is different; do whatever, and also:
      $lastaccessed = $modtime;
    }
    sleep 5;
}

      [download]
[reply]
[d/l]
Re: Perl process to constantly detect the time change of a file?
by stefp (Vicar) on May 04, 2002 at 02:31 UTC
    May be you want first to check out packages that do similar stuff like File::Tail, File::MultiTail and even POE::Wheel::FollowTail if you want to insert you stuff in an event framework. Even when I don't use a package or don't borrow code from it, I sometimes notice they deal with issues I had overlooked.

    -- stefp -- check out TeXmacs wiki

[reply]
      I think this is an excellent advice as it looks at the problem from a different perspective. Instead of polling the time the file was last changed, you wait for the file to change. (this of course assumes you can read from the file)

      ++stefp++

      Update: Actually, coming to read the original post again, this will not work if the file is "touched" by another process, only if appended to. Nonetheless, --stefp++ :-)

[reply]
Re: Perl process to constantly detect the time change on a file?
by bluto (Curate) on May 03, 2002 at 20:30 UTC
    To answer your question, use the sleep function in between each iteration. If you only sleep for a second or two each time through, you will save a boat load of cpu time.

    If you can get away with just checking once per minute, and you are on some unix variant, you can run a script via cron.

    On a broader note: This is a very long winded way of doing this. Instead of forking off a process and parsing the output, you can save a lot more CPU time (not to mention coding headaches) by using the stat function and just checking the mtime field.

    bluto

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