在安裝oracle時，安裝向導會提示你創建一個oracle用戶，來管理oracle。這個用戶一般都叫‘oracle’。
root下無法登陸oracle，需要進入oracle用戶，才能連接數據庫
su - oracle
sqlplus / as sysdba --以管理員身份登陸oracle

[root@oel51 ~]# export ORACLE_HOME=/u01/app/oracle/product/11.2.0/dbhome_1
[root@oel51 ~]# /u01/app/oracle/product/11.2.0/dbhome_1/bin/sqlplus dexter/[email protected]:1521/ggdb2

SQL*Plus: Release 11.2.0.4.0 Production on Tue Jun 30 13:38:18 2015

Copyright (c) 1982, 2013, Oracle. All rights reserved.

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL>

方法一：
[root@ogg1 ~]$ cat /home/oracle/.bash_profile |grep SID
export ORACLE_SID=ogg1
方法二：
查看oracle任意一個后臺進程，例如lgwr,ora_lgwr_ogg1其中ogg1就是sid
[oracle@ogg1 ~]$ ps -ef|grep lgwr
oracle 4864 1 0 11:30 ? 00:00:00 ora_lgwr_ogg1
oracle 5442 2329 0 12:18 pts/0 00:00:00 grep lgwr