Except that this doesn't work. Try reading the last N lines from a file with N-5 lines, or a file with < $bufsiz bytes. :)

I had a version that used buffers and was a virtual clone of the algorithm in tail.c, except that I got lost and frustrated in the boundary conditions and really didn't care anymore. Laziness and impatience.

If you want to take a stab at doing this right, be my guest. I just don't want to do the requisite testing, because the test conditions are yucky:

• File of L lines reading:
  • L lines
  • L+l lines
  • L-l lines
  • 0 lines
• Where bufsiz:
  • < size of the file
  • > size of the file
  • Some even multiple of the size of the file
  • Some even multiple of the size of the file less some portion of bufsz
  • == size of the file

Basically all of the combinations of these. I got all but the last two coded with nice buffering action.

After consideration, I figured I'd let the OS worry about buffering and JFDI. As a matter of fact, if you use getc()instead of sysread() (and seek instead of sysseek, etc..) the STDIO package would take care of most of this buffering nonsense anyway.

sub lastn {
        my($file, $lines)=@_;
        my $fh;
        $lines++;
        if (! open($fh, $file) ) {
                print "Can't open $file: $!";
                return;
        }
        binmode($fh);
        seek($fh, 0, 2);  # Seek to end
        my $nlcount=0;
        while($nlcount<$lines) {
                last unless seek($fh, -1, 1);
                $_=getc($fh);
                die unless defined $_;
                $nlcount++ if ( $_ eq "\n");
                last if $nlcount==$lines;
                last unless (seek($fh, -1, 1));
        }
        $fh;
}
[download]

There is such as thing as too much optimizing. :)

Update: with example.

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