SQL> conn system/manager@localhost/orcl
Connected.
SQL> sho con_name

CON_NAME
------------------------------
CDB$ROOT
SQL> select sys_context('userenv', 'con_name') from dual;

SYS_CONTEXT('USERENV','CON_NAME')
--------------------------------------------------------------------------------
CDB$ROOT

SQL> select con_id from v$session where sid=sys_context('userenv', 'sid');

    CON_ID
----------
         1

SQL> conn system/manager@localhost/pdb
Connected.
SQL> sho con_name

CON_NAME
------------------------------
PDB
SQL> select sys_context('userenv', 'con_name') from dual;

SYS_CONTEXT('USERENV','CON_NAME')
--------------------------------------------------------------------------------
PDB

SQL> select con_id from v$session where sid=sys_context('userenv', 'sid');

    CON_ID
----------
         3