sqlplus登录提示：ORA-12162：TNS:net service name is incorrectly specified错误

[root@localhost ~]# su - oracle
[oracle@localhost ~]$ sqlplus '/as sysdba'

SQL*Plus: Release 11.2.0.1.0 Production on Tue Apr 22 17:10:52 2014


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


ERROR:
ORA-12162: TNS:net service name is incorrectly specified

Enter user-name: 
ERROR:
ORA-12162: TNS:net service name is incorrectly specified

Enter user-name: 
ERROR:
ORA-12162: TNS:net service name is incorrectly specified

SP2-0157: unable to CONNECT to ORACLE after 3 attempts, exiting SQL*Plus
[oracle@localhost ~]$ export ORACLE_SID=PROD    //向环境变量中加入实例名。说明需手动设置
[oracle@localhost ~]$ sqlplus '/as sysdba'

SQL*Plus: Release 11.2.0.1.0 Production on Tue Apr 22 17:11:16 2014

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

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL> select status,instance_name from v$instance;  //登录后查询状态,正常

STATUS       INSTANCE_NAME
------------ ----------------
OPEN         PROD