Question about alerts in Oracle 11.2
An Oracle database can generate "Server-Generated Alerts".

You can view the alert thresholds in DBA_THRESHOLDS view, and query the alerts from the DBA_OUTSTANDING_ALERTS and DBA_ALERT_HISTORY views.

But I do not think this is what you are looking for. I have yet to see someone who uses this feature specifically and deliberately for alerting and notifications.

Usually there is Enterprise Manager (provided by Oracle) or some kind of monitoring software like Nagios, Zabbix, etc. that periodically logs in to the database, runs its checks, and sends notifications if needed. Still, some companies just simply use custom scripts scheduled in crontab for monitoring the database. Others use custom Scheduler jobs for monitoring. So no, there is no universal way to find what you are looking for.

The command you tried to execute is not a sqlplus command, it can be executed in adrci, and all it does it shows the default log of the selected location. Example:

[oracle@o71 ~]$ adrci

ADRCI: Release 12.2.0.1.0 - Production on Tue Sep 18 18:59:29 2018

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

ADR base = "/u01/app/oracle"
adrci&gt; show alert

Choose the home from which to view the alert log:

1: diag/rdbms/etalon/ETALON
2: diag/clients/user_oracle/host_2676003343_107
3: diag/tnslsnr/o71/listener
Q: to quit