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

Hi All I am still trying to create a script to tail a log file and report on an instant or exception when a message is not recieved. This means I will be searching for a string in a log file which is constant meaning what ever is writing to the log file does this all the time. Now if I don't recieve a message from whatever is writing to the log within 60 seconds I would like to create a exception error e.g ( print STDERR "Have not recieved a message in the last 60 seconds"; ) I am finding creating this exception timer part the difficult bit. I have attached my code below and would like some advice on if I am on the right track. Any assistance would be much appreciated. Kind Regards James 
 use IO::Select;
 $select = IO::Select->new();
 
 # repeat next two lines for all filehandles to poll
 open(LOGFILE, "< james.log") || die;
 seek(LOGFILE, 0, 2);
 
 
 $select->add(*LOGFILE);
 
 for (;;) {
   if (@ready = $select->can_read) {
     # input waiting on the filehandles in @ready
     foreach $filehandle (@ready) {
        while (<$filehandle>) {
         chomp();
         if (m/^Hello/) {
                 print STDERR "This is a hello msg";
                 $starttime = time;
         }
        }
     }
         sleep 2;
         $endtime = time;
         print STDERR "$starttime\n";
         print STDERR "$endtime\n";
         }
   }
         $timediff = $endtime - $starttime;
         if ( $timediff = "60" ) {
                 print STDERR "Have not recieved a message in the last
+ $timediff seconds";
 }

[download]

Replies are listed 'Best First'.
Re: Log file tailing
by matija (Priest) on May 17, 2004 at 09:27 UTC
    Doing select for ordinary files won't do you any good. Disk files (as opposed to sockets and pipes) are always "ready for reading".

    I suggest you look at the code for File::Tail. While I never wrote any hooks to tell the calling program when there was no traffic in the file, I do handle it internaly and you could add it to the code fairly easily.

    Send me a patch once it's done, and if it's fairly clean, I'll include it in the next release. (Note, I suggest a callback for implementing that).

[reply]
      Hi

      Unfortunatly, Im very new to perl and just starting out, so I would not know how to write a patch for the File::Tail module. Maybe you can help me implement this ??

      Kind Regards James
[reply]
Re: Log file tailing
by rinceWind (Monsignor) on May 17, 2004 at 09:54 UTC
    I use a different approach than IO::Select when solving this type of problem. Operating systems maintain an end of file marker, which determines how much of the file is readable by another process - the size of the file in bytes.

    My approach is as follows (pseudocode): 

    while (1) {
    stat the file
    if size of file changed since last time {
        open file
        seek to previous EOF
        print file contents until EOF
        save EOF position for next time
        close the file
    }

    sleep for x seconds
}

    [download]
    Hope this helps

    --
    I'm Not Just Another Perl Hacker

[reply]
[d/l]
      Well, that is essentialy what File::Tail does. Except that File::Tail also adjusts the "sleep for x seconds" duration after every read to account for how "busy" the file is: it checks the more frequently updated files more often, and the less frequently updated files less often.

      It also checks for files that have suddenly become shorter (as when a sysadmin does a cat /dev/null >logfile in order to free space), and if the file has not been written for a long time, it closes and reopens it - thus hopefully catching cases where the file has been renamed during a log rotation.

[reply]
Re: Log file tailing
by ysth (Canon) on May 17, 2004 at 08:41 UTC
    Barely glanced at it, but surely you mean "if ( $timediff >= 60 ) {"?
[reply]
      Yes, I ment to put that back in in James
[reply]
Re: Log file tailing
by McMahon (Chaplain) on May 17, 2004 at 15:26 UTC
    James Bach has a link to a Perl program that does what you're doing here in the "TESTING THROUGH AUDIBILITY" entry. It's a neat piece of code, might give you some ideas.
[reply]
Re: Log file tailing
by tachyon (Chancellor) on May 18, 2004 at 04:38 UTC

    Why not just open a pipe to tail: open F, "|tail -f somefile" or just use File::Tail

    cheers

    tachyon

[reply]
[d/l]

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