code: def run_command(self, stream=sys.stdout, dry_run=False):
          
          command = self.formatted_command()
          if dry_run:
              stream.write("%s\n" % command)
              stream.flush()
              return 0
          proc = subprocess.Popen(command.split(),
                                  stderr=stream,
                                  stdout=stream)
          proc.communicate()
          return proc.returncode
docs: Runs the command for this link.  This method can be overridden by
          sub-classes to invoke a different command
  
          Parameters
          ----------
          stream : `file`
              Stream that this `Link` will print to,
              Must have 'write' function
  
          dry_run : bool
              Print command but do not run it
  
          Returns
          -------
          code : int
              Return code from sub-process