seek

Seeks to a given offset anInteger in the stream according to the value of whence:

IO::SEEK_CUR  | Seeks to _amount_ plus current position
--------------+----------------------------------------------------
IO::SEEK_END  | Seeks to _amount_ plus end of stream (you probably
              | want a negative value for _amount_)
--------------+----------------------------------------------------
IO::SEEK_SET  | Seeks to the absolute location given by _amount_

Example:

f = File.new("testfile")
f.seek(-13, IO::SEEK_END)   #=> 0
f.readline                  #=> "And so on...\n"