Ruby/File Directory/seek

IO::SEEK_CUR      Seeks a certain number of bytes ahead of the current position.
IO::SEEK_END      Seeks to a position based on the end of the file. 
IO::SEEK_SET      Seeks to an absolute position in the file. Identical to pos=.
 
f = File.new("test.txt", "r+")
f.seek(-5, IO::SEEK_END)
f.putc "X"
f.close

f = File.new("test.txt", "r")
while a = f.getc
  puts a.chr
  f.seek(5, IO::SEEK_CUR)
end