SQL> select name,open_mode from v$pdbs;
NAME OPEN_MODE
------------------------------ ----------
PDB$SEED READ ONLY
PDB1 MOUNTED
PDB2 MOUNTED
SQL> alter pluggable database all open;
Pluggable database altered.
SQL> select name,open_mode from v$pdbs;
NAME OPEN_MODE
------------------------------ ----------
PDB$SEED READ ONLY
PDB1 READ WRITE
PDB2 READ WRITE
SQL> alter pluggable database all close;
Pluggable database altered.
SQL> select name,open_mode from v$pdbs;
NAME OPEN_MODE
------------------------------ ----------
PDB$SEED READ ONLY
PDB1 MOUNTED
PDB2 MOUNTED
SQL>