朝の頭の体操

shell.rb

# -*- coding: utf-8 -*-
class Shell < BasicObject
  def initialize(name, *args, &block)
    _push(name, *args)
  end

  attr_accessor :_result
  attr_writer   :_queue

  def to_s
    _queue.unshift(command: :echo, args: _result) if _result

    command = _queue.map do |c|
      [ c[:command], *c[:args] ].compact.map(&:to_s).join(" ")
    end.join(" | ")

    _queue = []

    # ここがどうしても無理
    command
    #_result = `#{command}`
  end
  alias inspect to_s

  def method_missing(name, *args, &block)
    _push(name, *args)
    self
  end

  protected
  def _push(name, *args, &block)
    _queue.push(command: name, args: args)
  end

  def _queue
    @_queue ||= []
  end
end

module Shellable
  def method_missing(name, *args, &block)
    Shell.new(name, *args)
  end
end

# usage 1
p Shell.new(:cat, "/etc/hosts").grep("localhost")

# usage 2
Class.new do
  extend Shellable
  p cat("/etc/hosts").grep("localhost")
end