shift.rb

# "Shift" the first line off of the file

class File
  def self.shift(file)
    File.open(file) do |input|
      first_line = input.gets
      
      File.open(file, 'w') do |output|
        input.each_line {|line| output.puts line }
      end
      
      first_line
    end
  end
end

puts File.shift("shift_file.txt")
puts
puts File.read("shift_file.txt")