Sub-command start.

def start(self):
        """Sub-command start."""
        self.discover()

        # startup_info only gets loaded from protocol version 2 on,
        # check if it's loaded
        if not self.startup_info:
            # hack to make environment startable with older protocol
            # versions < 2: try to start nodes via init if all nodes are down
            if len(self.get_tagged(['down'])) == len(self.get_tagged(['all'])):
                self.args = self.loaded_args
                print("upgrading mlaunch environment meta-data.")
                return self.init()
            else:
                raise SystemExit("These nodes were created with an older "
                                 "version of mlaunch (v1.1.1 or below). To "
                                 "upgrade this environment and make use of "
                                 "the start/stop/list commands, stop all "
                                 "nodes manually, then run 'mlaunch start' "
                                 "again. You only have to do this once.")

        # if new unknown_args are present, compare them with loaded ones
        # (here we can be certain of protocol v2+)
        if (self.args['binarypath'] is not None or
                (self.unknown_args and
                 set(self.unknown_args) != set(self.loaded_unknown_args))):

            # store current args, use self.args from file (self.loaded_args)
            start_args = self.args
            self.args = self.loaded_args

            self.args['binarypath'] = start_args['binarypath']
            # construct new startup strings with updated unknown args.
            # They are for this start only and will not be persisted in
            # the .mlaunch_startup file
            self._construct_cmdlines()

            # reset to original args for this start command
            self.args = start_args

        matches = self._get_ports_from_args(self.args, 'down')
        if len(matches) == 0:
            raise SystemExit('no nodes started.')

        # start config servers first
        config_matches = self.get_tagged(['config']).intersection(matches)
        self._start_on_ports(config_matches, wait=True)

        # start shards next
        mongod_matches = (self.get_tagged(['mongod']) -
                          self.get_tagged(['config']))
        mongod_matches = mongod_matches.intersection(matches)
        self._start_on_ports(mongod_matches, wait=True)

        # now start mongos
        mongos_matches = self.get_tagged(['mongos']).intersection(matches)
        self._start_on_ports(mongos_matches)

        # wait for all matched nodes to be running
        self.wait_for(matches)

        # refresh discover
        self.discover()