From: Daniel Berger <djberge@...55host11.interprise.com>
Date: 2002-08-01T23:43:45+09:00
Subject: tail - latest incarnation

Hi all,

Here's the latest incarnation I've come up with, stealing bits of code from James Hranicky and 
ideas from Curt Sampson.

The inode and dev checks are untested, so if there's a logic flaw there, please let me know.

Opions & feedback welcome/desired.

Regards,

Dan

# Da code!
class Tail < File

   def initialize(file,sleeptime=10)
      @filename = file
      @fh = File.open(file,"r")
      @sleeptime = sleeptime

      @ino = @fh.stat.ino
      @dev = @fh.stat.dev

      @ino_changed = false
      @dev_changed = false
   end

   ##############################################################
   # In block form, read a line at a time in a loop.  Otherwise,
   # just return the number of lines specified as an array.
   ##############################################################
   def tail(max=10)
      if block_given?
         yield get_lines(max)
         loop do
            line = @fh.gets
            yield line unless line.nil?
            if @fh.eof?
               check_move()
               if (@ino_changed == true) || (@dev_changed == true)
                  @fh.close
                  initialize(@filename,@sleeptime)
                  next
               end
               @fh.seek(@fh.pos)
            end
            sleep @sleeptime
         end
      else
         get_lines(max)
      end
   end

   def check_move
      begin
         if(File.stat(@filename).ino != @ino)
            @ino_changed = true
         end
         if(File.stat(@filename).dev != @dev)
            @dev_changed = true
         end
      rescue Errno::ENOENT
         retry
      end
   end

   #######################################################################
   # Grab the last 'max' lines (default is 10).  This method was created
   # for two reasons.  First, even a "tail -f" reads the last 10 lines.
   # Second, it's separated from the tail() method in the event the
   # programmer just wants a "plain" tail.
   #######################################################################
   def get_lines(max)
      
      @fh.seek(0,IO::SEEK_END)
      newline_count = 0
      while newline_count < max 

         begin
            @fh.pos -= 2
         rescue Errno::EINVAL
            break
         end

         break if @fh.eof?

         if @fh.getc.chr == "\n"
            newline_count += 1
         end
      end

      @fh.readlines
   end
end