SQL> select count(*) from dba_audit_trail;

  COUNT(*)
----------
         0

SQL> audit session by scott;

Audit succeeded.
#on another terminal connect with scott user.
SQL>  select username, action_name from dba_audit_trail;

USERNAME                       ACTION_NAME
------------------------------ ----------------------------
SCOTT                          LOGON
#on another terminal connect with scott user.
SQL> /

USERNAME                       ACTION_NAME
------------------------------ ----------------------------
SCOTT                          LOGON
SCOTT                          LOGON
#on another terminal connect with scott user.
SQL> /

USERNAME                       ACTION_NAME
------------------------------ ----------------------------
SCOTT                          LOGON
SCOTT                          LOGON
SCOTT                          LOGON
#on another terminal connect with scott user.plus disconnect
SQL> /

USERNAME                       ACTION_NAME
------------------------------ ----------------------------
SCOTT                          LOGON
SCOTT                          LOGOFF
SCOTT                          LOGON
SCOTT                          LOGON

SQL> Delete from SYS.AUD$;

4 rows deleted.

SQL> audit connect by scott;

Audit succeeded.

SQL>  select username, action_name from dba_audit_trail;

USERNAME                       ACTION_NAME
------------------------------ ----------------------------
SCOTT                          LOGON

SQL> /

USERNAME                       ACTION_NAME
------------------------------ ----------------------------
SCOTT                          LOGON
SCOTT                          LOGON

SQL> /

USERNAME                       ACTION_NAME
------------------------------ ----------------------------
SCOTT                          LOGON
SCOTT                          LOGOFF
SCOTT                          LOGON

SQL>