practicing_spawning.rb

module Kernel
  def system(*args)

    rd, wr = IO.pipe

    # Create a new subprocess that will just exec the requested program.
    pid = fork do
      # The sub-process closes its copy of the reading end of the pipe
      # because it only needs to write.
      rd.close

      begin
        exec(*args)
      rescue SystemCallError

        # In case of failure write a byte to the pipe to signal that an exception
        # occurred and exit with an unsuccessful code.
        wr.write('.')
        exit 1
      end
    end

    # The parent process closes its copy of the writing end of the pipe
    # because it only needs to read.
    wr.close

    # Tell the parent to wait.
    _, status = Process.waitpid2(pid)

    # If the reading end of the pipe has no data then there was no exception
    # and we fall back to the exit status code of the sub-process. Otherwise
    # we return nil to denote the error case.
    if rd.eof?
      status.success?
    else
      nil
    end
  end
  private :system

  def `(str)
    rd, wr = IO.pipe

    # Create a new subprocess that will just exec the requested program.
    pid = fork do
      # The sub-process closes its copy of the reading end of the pipe
      # because it only needs to write.
      rd.close

      # Anything that the exec'ed process would have written to $stdout will
      # be written to the pipe instead.
      $stdout.reopen(wr)

      exec(str)
    end

    # The parent process closes its copy of the writing end of the pipe
    # because it only needs to read.
    wr.close

    # The parent waits for the child to exit.
    Process.waitpid(pid)

    # The parent returns whatever it can read from the pipe.
    rd.read
  end
  private :`
end