def init(self):
        """
        Sub-command init.

        Branches out to sharded, replicaset or single node methods.
        """
        # check for existing environment. Only allow subsequent
        # 'mlaunch init' if they are identical.
        if self._load_parameters():
            if self.loaded_args != self.args:
                raise SystemExit('A different environment already exists '
                                 'at %s.' % self.dir)
            first_init = False
        else:
            first_init = True

        self.ssl_pymongo_options = self._get_ssl_pymongo_options(self.args)
        self.tls_pymongo_options = self._get_tls_pymongo_options(self.args)

        if (self._get_ssl_server_args() and not
                self.args['sslAllowConnectionsWithoutCertificates'] and not
                self.args['sslClientCertificate'] and not
                self.args['sslClientPEMKeyFile']):
            sys.stderr.write('warning: server requires certificates but no'
                             ' --sslClientCertificate provided\n')

        if (self._get_tls_server_args() and not
                self.args['tlsAllowConnectionsWithoutCertificates'] and not
                self.args['tlsClientCertificate'] and not
                self.args['tlsClientCertificateKeyFile']):
            sys.stderr.write('warning: server requires certificates but no'
                             ' --tlsClientCertificate provided\n')
        # number of default config servers
        if self.args['config'] == -1:
            self.args['config'] = 1

        # Exit with error if --csrs is set and MongoDB < 3.1.0
        if (self.args['csrs'] and
                LooseVersion(self.current_version) < LooseVersion("3.1.0") and
                LooseVersion(self.current_version) != LooseVersion("0.0.0")):
            errmsg = (" \n * The '--csrs' option requires MongoDB version "
                      "3.2.0 or greater, the current version is %s.\n"
                      % self.current_version)
            raise SystemExit(errmsg)

        # add the 'csrs' parameter as default for MongoDB >= 3.3.0
        if (LooseVersion(self.current_version) >= LooseVersion("3.3.0") or
                LooseVersion(self.current_version) == LooseVersion("0.0.0")):
            self.args['csrs'] = True

        # construct startup strings
        self._construct_cmdlines()

        # write out parameters
        if self.args['verbose']:
            print("writing .mlaunch_startup file.")
        self._store_parameters()

        # exit if running in testing mode
        if self.test:
            return

        # check if authentication is enabled, make key file
        if self.args['auth'] and first_init:
            if not os.path.exists(self.dir):
                os.makedirs(self.dir)

            if '--keyFile' in self.unknown_args:
                # Check if keyfile is readable
                keyfile = None
                try:
                    keyfile_idx = self.unknown_args.index('--keyFile') + 1
                    keyfile_path = self.unknown_args[keyfile_idx]
                    keyfile = self._read_key_file(keyfile_path)
                except:
                    print(f'\n WARNING: Specified keyFile does not appear readable: {keyfile_path}\n')
            else:
                keyfile = os.path.join(self.dir, "keyfile")
                print(f'Generating keyfile: {keyfile}')
                os.system('openssl rand -base64 753 > "%s"' % keyfile)
                if os.name != 'nt':
                    os.system(f'chmod 600 "{keyfile}"')

        # if not all ports are free, complain and suggest alternatives.
        all_ports = self.get_tagged(['all'])
        ports_avail = self.wait_for(all_ports, 1, 1, to_start=False)

        if not all(map(itemgetter(1), ports_avail)):
            dir_addon = (' --dir %s' % self.relative_dir
                         if self.relative_dir != './data' else '')
            errmsg = ('\nThe following ports are not available: %s\n\n'
                      % ', '.join([str(p[0])
                                   for p in ports_avail if not p[1]]))
            errmsg += (" * If you want to restart nodes from this "
                       "environment, use 'mlaunch start%s' instead.\n"
                       % dir_addon)
            errmsg += (" * If the ports are used by a different mlaunch "
                       "environment, stop those first with 'mlaunch stop "
                       "--dir <env>'.\n")
            errmsg += (" * You can also specify a different port range with "
                       "an additional '--port <startport>'\n")
            raise SystemExit(errmsg)

        if self.args['sharded']:

            shard_names = self._get_shard_names(self.args)

            # start mongod (shard and config) nodes and wait
            nodes = self.get_tagged(['mongod', 'down'])
            self._start_on_ports(nodes, wait=True, override_auth=True)

            # initiate replica sets if init is called for the first time
            if first_init:
                if self.args['csrs']:
                    # Initiate config servers in a replicaset
                    if self.args['verbose']:
                        print('Initiating config server replica set.')
                    members = sorted(self.get_tagged(["config"]))
                    self._initiate_replset(members[0], "configRepl")
                for shard in shard_names:
                    # initiate replica set on first member
                    if self.args['verbose']:
                        print('Initiating shard replica set %s.' % shard)
                    members = sorted(self.get_tagged([shard]))
                    self._initiate_replset(members[0], shard)

            # add mongos
            mongos = sorted(self.get_tagged(['mongos', 'down']))
            self._start_on_ports(mongos, wait=True, override_auth=True)

            if first_init:
                # add shards
                mongos = sorted(self.get_tagged(['mongos']))
                con = self.client('localhost:%i' % mongos[0])

                shards_to_add = len(self.shard_connection_str)
                nshards = con['config']['shards'].count_documents({})
                if nshards < shards_to_add:
                    if self.args['replicaset']:
                        print("adding shards. can take up to 30 seconds...")
                    else:
                        print("adding shards.")

                shard_conns_and_names = list(zip(self.shard_connection_str,
                                                 shard_names))
                while True:
                    try:
                        nshards = con['config']['shards'].count_documents({})
                    except Exception:
                        nshards = 0
                    if nshards >= shards_to_add:
                        break

                    for conn_str, name in shard_conns_and_names:
                        try:
                            res = con['admin'].command(SON([('addShard',
                                                             conn_str),
                                                       ('name', name)]))
                        except Exception as e:
                            if self.args['verbose']:
                                print('%s will retry in a moment.' % e)
                            continue

                        if res['ok']:
                            if self.args['verbose']:
                                print("shard %s added successfully" % conn_str)
                                shard_conns_and_names.remove((conn_str, name))
                                break
                        else:
                            if self.args['verbose']:
                                print(res + ' - will retry')

                    time.sleep(1)

        elif self.args['single']:
            # just start node
            nodes = self.get_tagged(['single', 'down'])
            self._start_on_ports(nodes, wait=False)

        elif self.args['replicaset']:
            # start nodes and wait
            nodes = sorted(self.get_tagged(['mongod', 'down']))
            self._start_on_ports(nodes, wait=True)

            # initiate replica set
            if first_init:
                self._initiate_replset(nodes[0], self.args['name'])

        # wait for all nodes to be running
        nodes = self.get_tagged(['all'])
        self.wait_for(nodes)

        # now that nodes are running, add admin user if authentication enabled
        if self.args['auth'] and self.args['initial-user'] and first_init:
            self.discover()
            nodes = []

            if self.args['sharded']:
                nodes = self.get_tagged(['mongos', 'running'])
            elif self.args['single']:
                nodes = self.get_tagged(['single', 'running'])
            elif self.args['replicaset']:
                print("waiting for primary to add a user.")
                if self._wait_for_primary():
                    nodes = self.get_tagged(['primary', 'running'])
                else:
                    raise RuntimeError("failed to find a primary, so adding "
                                       "admin user isn't possible")

            if not nodes:
                raise RuntimeError("can't connect to server, so adding admin "
                                   "user isn't possible")

            roles = []
            found_cluster_admin = False
            if self.args['auth_role_docs']:
                for role_str in self.args['auth_roles']:
                    role_doc = json.loads(role_str)
                    roles.append(role_doc)
                    if role_doc['role'] == "clusterAdmin":
                        found_cluster_admin = True
            else:
                roles = self.args['auth_roles']
                found_cluster_admin = "clusterAdmin" in roles

            if not found_cluster_admin:
                warnings.warn("the stop command will not work with auth "
                              "because the user does not have the "
                              "clusterAdmin role")

            self._add_user(sorted(nodes)[0], name=self.args['username'],
                           password=self.args['password'],
                           database=self.args['auth_db'],
                           roles=roles)

            if self.args['sharded']:
                for shard in shard_names:
                    members = sorted(self.get_tagged([shard]))
                    if self.args['verbose']:
                        print("adding users to %s" % shard)
                    self._add_user(members[0],
                                   name=self.args['username'],
                                   password=self.args['password'],
                                   database=self.args['auth_db'],
                                   roles=roles)

            if self.args['verbose']:
                print("added user %s on %s database" % (self.args['username'],
                                                        self.args['auth_db']))

        # in sharded env, if --mongos 0, kill the dummy mongos
        if self.args['sharded'] and self.args['mongos'] == 0:
            port = self.args['port']
            print("shutting down temporary mongos on localhost:%s" % port)
            username = self.args['username'] if self.args['auth'] else None
            password = self.args['password'] if self.args['auth'] else None
            authdb = self.args['auth_db'] if self.args['auth'] else None
            shutdown_host(port, username, password, authdb)

        # discover again, to get up-to-date info
        self.discover()

        # for sharded authenticated clusters, restart after first_init
        # to enable auth
        if self.args['sharded'] and self.args['auth'] and first_init:
            if self.args['verbose']:
                print("restarting cluster to enable auth...")
            self.restart()

        if self.args['auth'] and self.args['initial-user']:
            print('Username "%s", password "%s"'
                  % (self.args['username'], self.args['password']))

        if self.args['verbose']:
            print("done.")