ls.rb

middle = ->(array) { array[array.size / 2] }

def plural(string, plural_ending)
  return string if string.end_with?(plural_ending)

  "#{string}#{plural_ending}"
end

files = `ls -a`.split("\n")

files
  .reject { |file| file.start_with?('.') }
  .sort
  .yield_self(&middle)
  .yield_self { |file| plural(file, 's') }
  .yield_self(&:upcase)
  .tap { |file| puts file }