<Start of Console log>============================================

[oracle@oracledbserver dbs]$ ORACLE_SID=AG_PUB
[oracle@oracledbserver dbs]$ export ORACLE_SID
[oracle@oracledbserver dbs]$ sqlplus /nolog

SQL*Plus: Release 11.2.0.1.0 Production on Thu Apr 4 12:08:23 2013

Copyright (c) 1982, 2009, Oracle.  All rights reserved.

SQL> conn sys as sysdba
Enter password: 
ERROR:
ORA-09925: Unable to create audit trail file
Linux-x86_64 Error: 2: No such file or directory
Additional information: 9925
ORA-01075: you are currently logged on
SQL> exit

[oracle@oracledbserver dbs]$vi initAG_PUB.ora
db_name='AG_PUB'
processes = 150
audit_file_dest='/media/volume-01/u01/app/oracle/admin/oracl/adump'
audit_trail ='db'
db_block_size=8192
db_domain='nic.in'
diagnostic_dest='/media/volume-01/u01/app/oracle'
dispatchers='(PROTOCOL=TCP) (SERVICE=ORCLXDB)'
open_cursors=300
#remote_login_passwordfile='EXCLUSIVE'
#undo_tablespace='UNDOTBS1'
# You may want to ensure that control files are created on separate physical
# devices
control_files = (ora_control9, ora_control10)
compatible ='11.1.0'

[oracle@oracledbserver dbs]$ echo $ORACLE_BASE
/media/volume-01/u01/app/oracle
[oracle@oracledbserver dbs]$ 
[oracle@oracledbserver dbs]$ echo $ORACLE_HOME
/media/volume-01/u01/app/oracle/product/11.2.0/db_1
[oracle@oracledbserver dbs]$ 
[oracle@oracledbserver dbs]$ echo $ORACLE_SID 
AG_PUB
[oracle@oracledbserver dbs]$
< End of Console Log>=============================================

But in the same system , I can connect to another DB as sysdba.