[oracle@swnode1 ~]$ adrci

ADRCI: Release 12.2.0.1.0 - Production on Fri Jul 6 16:05:45 2018

Copyright (c) 1982, 2017, Oracle and/or its affiliates.  All rights reserved.

ADR base = "/u01/app/oracle"
adrci> show homes
ADR Homes: 
diag/clients/user_oracle/host_2908371209_107
diag/rdbms/orcl/gnnt1

[oracle@swnode1 ~]$ tail -f /u01/app/oracle/diag/rdbms/orcl/gnnt1/alert/log.xml 
 <txt>Please see the VKTM trace file for more details:
 </txt>
</msg>
<msg time='2018-07-05T09:12:34.163+08:00' org_id='oracle' comp_id='rdbms'
 type='UNKNOWN' level='16' host_id='swnode1'
 host_addr='::1' pid='23141' con_uid='1'
 con_id='1' con_name='CDB$ROOT'>
 <txt>/u01/app/oracle/diag/rdbms/orcl/gnnt1/trace/gnnt1_vktm_23141.trc
 </txt>
</msg>
^C