$ sqlplus '/as sysdba'

SQL*Plus: Release 12.1.0.2. Production on Sun Nov  :: 

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

Connected to an idle instance.

SQL> startup nomount;

ORACLE instance started.

Total System Global Area  bytes

Fixed Size                   bytes

Variable Size              bytes

Database Buffers           bytes

Redo Buffers                bytes

SQL> select name,open_mode from v$pdbs;

no rows selected

SQL> alter database db12 mount;                

Database altered.

SQL> select name,open_mode from v$pdbs;

NAME                           OPEN_MODE

------------------------------ ----------

PDB$SEED                       MOUNTED

PDB1                           MOUNTED

PDB2                           MOUNTED

SQL> alter database db12 open;

Database altered.

SQL> select name,open_mode from v$pdbs;

NAME                           OPEN_MODE

------------------------------ ----------

PDB$SEED                       READ ONLY

PDB1                           MOUNTED

PDB2                           MOUNTED

SQL> connect sys@pdb1 as sysdba

Enter password:

Connected.

SQL> alter pluggable database pdb1 open;

Pluggable database altered.

SQL> select name,open_mode from v$pdbs;  

NAME                           OPEN_MODE

------------------------------ ----------

PDB1                           READ WRITE

SQL>