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More on perl like tail

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From:
Harry Putnam
Date:
October 24, 2009 13:57
Subject:
More on perl like tail
Message ID:
87k4yk5x42.fsf@newsguy.com
Sorry about being tricky with what was an older thread.  But I suspect
that thread died... and no one noticed there was an unaswered question
still there.

Shawn C originally suggested I use File::Tail.  There was a short
exchange about why and then I began trying to use File::Tail but
haven't been successful with it.  The details are below:

> Shawn H Corey <shawnhcorey@gmail.com> writes:
>
>>>   open(FILE,"<./named-pipe") or die "Can't Open ./named-pipe: $!";
>>>   while(<FILE>){
>>>     print;
>>>     if(eof){
>>>       sleep 2;
>>>       seek (FILE,0,1);
>>>     }
>>>   } 
>>> 
>>> It seems at least to survive repeated restarts of system logger.
>>> 
>>> If I write my script based on this code... what I'd be adding would be
>>> code to get 1 or 2 rgx from the cmdline, then write the hits to
>>> various files.
>
> [...]
>
>> In the general case, modules usually take care of special cases that you
>>  may not be aware of.  That makes them the preferred method of solving a
>> problem.
>>
>> In this case, the above code is a kludge.  You can tell this because the
>> program sleeps, rather than waiting on input.  When a program does
>> something to emulate what it really should be doing, it introduces code
>> that may not work in all cases.

I'm having trouble with File::Tail... or more likely the way I'm
trying to use it is wrongly setup.

But first about that sleep comment.  As I read a little of File::Tail
and its very likely I'm not really understanding what I'm reading but,
it appears to be saying that it `sleeps' at times...  the times are
a little more sophisticated... but none the less sleep.

Now the problem.

I've taken the first examples in the perldoc File::Tail output:
  (http://search.cpan.org/~mgrabnar/File-Tail-0.99.3/Tail.pm)

  use File::Tail;
  $file=File::Tail->new("/some/log/file");
  while (defined($line=$file->read)) {
      print "$line";
  }

And tied to make it work for my case (/var/adm/slpipe in the script is a
named-pipe that the system logger reads into):

cat fltr_sl

  #!/usr/local/bin/perl
  
  use strict;
  use warnings;
  use File::Tail;

  my ($file,$line);
  my $fname_in = "/var/adm/slpipe";

  $file=File::Tail->new("$fname_in");
  while (defined($line=$file->read)) {
     print "$line";
  }  

When I run it, it doesn't show any errors, and appears to be waiting
on the pipe.

But no data ever comes out.  I used the same test sequence as for the
earlier script that didn't use File::Tail (posted earlier in this
thread).   The sequence is.

 1) start the script `fltr_sl' shown above
 2) To make sure if data is flowing thru the pipe start
    start (in a different xterm) tail -f slpipe too.
 2) kill -HUP the system logger.
 3) Ensure some data is flowing thru the named-pipe by
    running ssh root@localhost from a user account

I see 7-8 lines output to the tail -f slpipe command, but nothing to
the perl script fltr_sl.

And as I write this message, quite a few more lines are appearing at
the `tail -f slpipe' cmd, but still nothing at my perl filter.

Have I made some serious mistake in my attempted usage of File::Tail? 


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