Enable Unified Audit in Oracle 12c

First, stop all oracle processes (instances, listeners)
$ cd $ORACLE_HOME/rdbms/lib
$ make -f ins_rdbms.mk uniaud_on ioracle ORACLE_HOME=$ORACLE_HOME

To monitor recent RMAN backup and recovery

SQL> select dbusername,rman_operation
  2  from unified_audit_trail
  3  where rman_operation is not null;

DBUSERNAME               RMAN_OPERATION
------------------------------ --------------------
SYS                   Backup
SYS                   Restore
SYS                   Recover
SYS                   Backup
SYS                   Restore
SYS                   Recover

6 rows selected.