To start a CDB instance, the current user must be a common user whose current container is the root.
##启动CDB,需要使用root用户

When you open a CDB, its PDBs are mounted. Use the ALTER PLUGGABLE DATABASE statement to modify the open mode of one or more PDBs.
##打开一个CDB,PDB确实mount，需要使用alter pluggable database 修改open模式

$sqlplus / as sysdba

SQL*Plus: Release 12.1.0.2.0 Production on Mon Mar 5 20:13:46 2018

Copyright (c) 1982, 2014, Oracle.  All rights reserved.

Connected to an idle instance.

SQL> startup
ORACLE instance started.

Total System Global Area 1174405120 bytes
Fixed Size                  2923680 bytes
Variable Size             436208480 bytes
Database Buffers          721420288 bytes
Redo Buffers               13852672 bytes
Database mounted.
Database opened.
SQL> show con_name

CON_NAME
------------------------------
CDB$ROOT
SQL> show pdbs

    CON_ID CON_NAME                       OPEN MODE  RESTRICTED
---------- ------------------------------ ---------- ----------
         2 PDB$SEED                       READ ONLY  NO
         3 PDB11                          MOUNTED

SQL> alter pluggable database all open;

Pluggable database altered.

SQL> show pdbs;

    CON_ID CON_NAME                       OPEN MODE  RESTRICTED
---------- ------------------------------ ---------- ----------
         2 PDB$SEED                       READ ONLY  NO
         3 PDB11                          READ WRITE NO  ##默认是读写的
SQL>