File::ReadBackwards has similar, but not quite the same, functionality. Rather than seeking and reading one character at a time, though, it'd be more efficient in general to read larger chunks, like File::ReadBackwards does. Perhaps like so...and autovivifing filehandles is only compatible with 5.6+, so I changed that also to be backward compatible:
sub lastn {
        my($file, $lines, $bufsiz)=@_;
        $bufsiz ||= 1024;
        # Changed FH to STDOUT to avoid warning
        my $fh = \do { local *STDOUT };
        $lines++;
        if (! open($fh, $file) ) {
                print "Can't open $file: $!<P/>";
                return;
        }
        binmode($fh);
        my $pos = sysseek($fh, 0, 2);  # Seek to end
        my $nlcount=0;
        while($nlcount<=$lines) {
                $bufsiz = $pos if $bufsiz > $pos;
                $pos = sysseek($fh, -$bufsiz, 1);
                die "Bad seek: $!" unless defined $pos;
                my $bytes = sysread($fh, $_, $bufsiz, 0);
                die "Bad read: $!" unless defined $bytes;
                $nlcount += tr/\n//;
                $pos = sysseek($fh, -$bufsiz, 1);
                die "Bad seek: $!" unless defined $pos;
                last if $pos == 0;
        }
        seek($fh, sysseek($fh, 0, 1), 0) || warn;
        <$fh> for $lines..$nlcount;
        $fh;
}

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Update: It does work when requesting more lines than the file contains, though I fixed it to work with various buffer sizes. I don't think a 20000% speed improvement (for tailing 400 10 byte lines) is 'too much' optimization :-) It does miscount if the last line does not have a line feed, though do you actually count that as a line or not? Besides, your's 'miscounts' in that situation also.

In reply to Re: Last N lines from file (tail) by SpongeBob
in thread Last N lines from file (tail) by clintp

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