[oracle@aa ~]$ export $ORACLE_SID=orcl

[oracle@aa ~]$ echo $ORACLE_SID

orcl

[oracle@aa ~]$ rlwrap sqlplus / as sysdba

SQL*Plus: Release 10.2.0.1.0 - Production on Thu Jun 23 17:14:36 2011

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

Connected to an idle instance.

SQL> startup nomount

ORACLE instance started.

Total System Global Area 524288000 bytes

Fixed Size 1220384 bytes

Variable Size 306184416 bytes

Database Buffers 209715200 bytes

Redo Buffers 7168000 bytes

SQL> alter database mount

Database altered.

SQL> alter database open

Database altered.