Sumup from Oracle-l thread 
linux way
tail -f alert.log | grep pattern
Adrci
adrci exec="set home ${adrci_home} ; show alert -p \\\"(message_text like '%ORA-%' and originating_timestamp>=systimestamp-(1/48) \\\"" -term 
more explanation here 
Sql way
set lin 1000
col data form a20
col process_id form a12
col module_id form a40
col message_text form a200
select to_char(originating_timestamp,'dd/mm/rr hh24:mi:ss') dt_hour
  , process_id
  , module_id
  , message_text
from
  sys.x$dbgalertext
where
  originating_timestamp >trunc(sysdate)
order by originating_timestamp ;