You can chain string with shell command output. -> StdinString.new(`ls -l`).run('grep *.pid')

stdin_string.rb

require 'tempfile'

class StdinString < String
  def run(*commands)
    r = `cat #{tf.path} | #{commands.join(' ')}`
    remove_tf
    StdinString.new r
  end

  private
  def tf
    @tf ||= build_tf
  end
  def remove_tf
    if @tf
      @tf.unlink
      @tf = nil
    end
  end

  def build_tf
    f = Tempfile.new('stdin-string')
    f.write(self)
    f.close(false)
    f
  end
end