inplace_edit.rb

#original from http://stackoverflow.com/a/5452950/1305200

def inplace_edit(filename)
    File.open(filename, 'r+') do |file|
      write_pos = 0
      file.each_with_index do |line, index|
        eof      = file.eof?
        read_pos = file.pos
        file.pos = write_pos
        new_line = yield(line.chomp, index)
        eof ? file.print(new_line) : file.puts(new_line)
        write_pos = file.pos
        file.pos = read_pos
      end
      file.truncate write_pos
    end
  end