From: Robert Klemme <shortcutter@...>
Date: 2008-12-21T23:46:43+09:00
Subject: Re: get first and last line from txt file - how?

On 20.12.2008 17:46, Thomas Preymesser wrote:
> 2008/12/20 Yaser Sulaiman <yaserbuntu@gmail.com>:
>> I'm just wondering..
>> Let's say that we only need to read the last line. Can we do that without
>> reading the other lines?
> 
> Yes. Position your file pointer to the last byte in a file, read and
> collect backwards each byte until you find a newline character (or the
> first byte of the file). This is the last line.

You have to admit that this approach is rather inefficient.  Here's a 
more efficient variant - especially for large files:

$ cat r.rb
#!/bin/env ruby

OFFSET = 512 # > 2 * assumed avg line length

file = ARGV.shift or abort "ERROR: need a file name"

File.open file do |io|
   first = io.gets
   break unless first
   puts first

   limit = io.stat.size
   offset = OFFSET
   lines = []

   while lines.size < 2 && offset <= limit
     io.seek -offset, IO::SEEK_END
     lines = io.readlines
     offset += OFFSET
   end # while lines.size < 2

   puts lines.last unless lines.empty?
end

Cheers

	robert