Oracle Database 12c does not have mechanism to automatically open pluggable databases i.e. container database starts, but all pluggable database stay in the mount state, not open. To run automatically, we can use a trigger.
CREATE or REPLACE trigger OPEN_PDS After Startup on Database BEGIN execute Immediate 'alter pluggable All Open'; END;
sql> shutdown immediate sql> startup sql> select con_id, name, guid, open_mode from v$pdbs order by 1;