$ env|grep ORA
ORACLE_BASE=/u01
ORACLE_SID=test1
ORACLE_ADMIN=/u01/admin
ORACLE_HOME=/u01/OID

$ export ORACLE_SID=test1
$ sqlplus '/ as sysdba'

SQL*Plus: Release 10.1.0.5.0 - Production on Mon Dec 17 19:09:39 2007

Copyright (c) 1982, 2005, Oracle. All rights reserved.


Connected to:
Oracle Database 10g Enterprise Edition Release 10.1.0.5.0 - 64bit Production
With the Partitioning, OLAP and Data Mining options

SQL> show parameter instance

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
active_instance_count integer
cluster_database_instances integer 1
instance_groups string
instance_name string test1
instance_number integer 0
instance_type string RDBMS
open_links_per_instance integer 4
parallel_instance_group string
parallel_server_instances integer 1