Re: OT: tail -f | while read

["Anthony J. Breeds-Taurima" <tony@cantech.net.au>]

On Thu, 6 Sep 2001, Martin F Krafft wrote:

> hi,
> why does the following not work:
>
>   tail -f /var/log/syslog | grep something | while read i; do myprog $i; done
>
> i basically need to run myprog as soon as a new entry in syslog
> happens that matches the criteria of the grep call. it needs to happen
> immediately.


Here is a far more complicated perl script that make the "tail -f" part of the
actual application.  It is designed to run as a daemon ie
# logtail /var/log/syslog &

----
#!/usr/bin/perl -w

use strict;
use Fcntl;

sub sig_quit();
sub sig_usr1();
sub processLines($);
sub errorMSG($);
sub processLine($);

my $logName = shift;
my $maxReadSize = 4096;
my $numReads = 0;
my $data;

my %data;

my $eof;
my $currpos;

$SIG{INT}  = 'sig_quit';
$SIG{QUIT} = 'sig_quit';
$SIG{USR1} = 'sig_usr1';

sysopen(logFD,$logName,O_RDONLY);
seek(logFD, 0, 0);
$data = processLines('');
print "Done BackData\n";

$currpos=($eof=sysseek(logFD,0, 1));

while (1) {
    $eof=sysseek(logFD,0, 2);

    if ($currpos > $eof) {
        errorMSG("==> file truncated <==\n");
        $data='';
        $currpos=$eof;
    } elsif ($currpos != $eof) {
        sysseek(logFD,$currpos,0);
        $data = processLines($data);
        $currpos=($eof=sysseek(logFD,0, 1));
    } else {
        sleep 1;
    }
}

close(logFD);

sub sig_quit() {
    errorMSG("bye bye. ($numReads)\n");
    close(logFD);
    exit;
}

sub sig_usr1() {
    errorMSG("I caught a signal I _should_really do something\n");
}

sub processLines($) {
    my $data = shift;
    my $buff;
    my $bytesRead;

    while (($bytesRead = sysread(logFD,$buff,$maxReadSize))!=0) {
        $numReads++;
        $data.=$buff;
        while ($data =~ s/\A(.*?\n)//m) {
            processLine($1);
        }
    }
    return $data;
}

sub processLine($) {
    $_=shift;
	chmop;
	#Do SOMETHING HERE
}

sub errorMSG($) {
    my $buff=shift;
    syswrite(STDERR,$buff,length($buff));
}
__END__
----

I know this is way beyond the scope of your question but it may help.


Yours Tony.

/*
 * "The significant problems we face cannot be solved at the
 * same level of thinking we were at when we created them."
 * --Albert Einstein
 */