How to read Alert and Listener logs from SQLPLUS

Before 11g, alert logs were found using the below option

show parameter BACKGROUND_DUMP_DEST

and from 11g, Location of alert logs are found by the below SQL, 

select * from   V$DIAG_INFO where name ='Diag Trace'
   INST_ID NAME                     VALUE
---------- ------------------------ ------------------------------------------
         1 Diag Trace               /database/diag/rdbms/orcl/ORCL/trace

Oracle 11G had made that a lot easier to read alert_log from SQLPLUS using X$DBGALERTEXT, which picks from XML version of the Alert log.
It contains vital information of the database in chronological order.

in 11gR2 and later

# Selecting from the Alert log :-
SQL> select ORIGINATING_TIMESTAMP, MESSAGE_TEXT
from V$DIAG_ALERT_EXT
WHERE ORIGINATING_TIMESTAMP > sysdate -1/24
and trim(COMPONENT_ID)='rdbms';

in 11gR1

Please note is
In our example, DBA_USER is a Read only user.
Steps to provide the access to read alert_log from SQLPLUS are as below:-
create view v_$alert_log as select * from x$dbgalertext;
create public synonym v$alert_log for sys.v_$alert_log;
grant select on v$alert_log to DBA_USER;


--Use the query below to find he error, timestamp and the criticality.
define time_min=60
select  ORIGINATING_TIMESTAMP , MESSAGE_LEVEL,message_text from v$alert_log where rownum <= 20
and MESSAGE_LEVEL =1
and cast(ORIGINATING_TIMESTAMP as DATE) > sysdate - &&time_min/1440
order by 1 desc;

--to check the count of such errors in a given time
define time_min=60
select substr(MESSAGE_TEXT, 1, 300) message_text, count(*) cnt
from X$DBGALERTEXT
where MESSAGE_LEVEL =1
and cast(ORIGINATING_TIMESTAMP as DATE) > sysdate - &&time_min/1440
group by substr(MESSAGE_TEXT, 1, 300);


Structure of the newly created view - v$alert_log
SQL> desc v$alert_log
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 ADDR                                               RAW(8)
 INDX                                               NUMBER
 INST_ID                                            NUMBER
 ORIGINATING_TIMESTAMP                              TIMESTAMP(3) WITH TIME ZONE
 NORMALIZED_TIMESTAMP                               TIMESTAMP(3) WITH TIME ZONE
 ORGANIZATION_ID                                    VARCHAR2(64)
 COMPONENT_ID                                       VARCHAR2(64)
 HOST_ID                                            VARCHAR2(64)
 HOST_ADDRESS                                       VARCHAR2(46)
 MESSAGE_TYPE                                       NUMBER
 MESSAGE_LEVEL                                      NUMBER
 MESSAGE_ID                                         VARCHAR2(64)
 MESSAGE_GROUP                                      VARCHAR2(64)
 CLIENT_ID                                          VARCHAR2(64)
 MODULE_ID                                          VARCHAR2(64)
 PROCESS_ID                                         VARCHAR2(32)
 THREAD_ID                                          VARCHAR2(64)
 USER_ID                                            VARCHAR2(64)
 INSTANCE_ID                                        VARCHAR2(64)
 DETAILED_LOCATION                                  VARCHAR2(160)
 PROBLEM_KEY                                        VARCHAR2(64)
 UPSTREAM_COMP_ID                                   VARCHAR2(100)
 DOWNSTREAM_COMP_ID                                 VARCHAR2(100)
 EXECUTION_CONTEXT_ID                               VARCHAR2(100)
 EXECUTION_CONTEXT_SEQUENCE                         NUMBER
 ERROR_INSTANCE_ID                                  NUMBER
 ERROR_INSTANCE_SEQUENCE                            NUMBER
 VERSION                                            NUMBER
 MESSAGE_TEXT                                       VARCHAR2(2048)
 MESSAGE_ARGUMENTS                                  VARCHAR2(128)
 SUPPLEMENTAL_ATTRIBUTES                            VARCHAR2(128)
 SUPPLEMENTAL_DETAILS                               VARCHAR2(128)
 PARTITION                                          NUMBER
 RECORD_ID                                          NUMBER

SQL>