[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                            <----有参数文件pfile或者spfile就可以，分配内存，只启动实例，不加载数据库。启动了后台进程，打开跟踪文件，警告文件
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;                      <----根据参数文件中定义的控制文件路径，读入内存。只加载数据库，不打开数据库。修改静态参数要重启数据库，在这一步改。

Database altered.

SQL> alter database open;                       <-----打开数据库。这三步等同于startup.

Database altered.