今天在进行修改oracle_sid环境变量的时候，将相关的环境变量值去掉，从而不能进入sqlplus，报错如下：
[oracle@kel ~]$ sqlplus / as sysdba SQL*Plus: Release 10.2.0.1.0 - Production on Sun Jun 8 19:19:21 2014 Copyright (c) 1982, 2005, 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
查看错误信息：
[oracle@kel ~]$ oerr ora 12162 12162, 00000, "TNS:net service name is incorrectly specified" // *Cause: The connect descriptor corresponding to the net service name in // TNSNAMES.ORA or in the directory server (Oracle Internet Directory) is // incorrectly specified. // *Action: If using local naming make sure there are no syntax errors in // the corresponding connect descriptor in the TNSNAMES.ORA file. If using // directory naming check the information provided through the administration // used for directory naming.错误信息里描述的是：指定的连接字符串错误，服务名称出错，从而使用netmgr命令来检查连接字符串的正确性：
[oracle@kel ~]$ netmgr
在这里可以测试连接字符串是否出错，但是在这里我测试是失败的，从而查看1521端口是否启动监听：
[oracle@kel ~]$ netstat -tnlp (Not all processes could be identified, non-owned process info will not be shown, you would have to be root to see it all.) Active Internet connections (only servers) Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name tcp 0 0 0.0.0.0:37667 0.0.0.0:* LISTEN 5140/ora_d000_orcl tcp 0 0 0.0.0.0:111 0.0.0.0:* LISTEN -
从而可以看到1521端口未启动监听，从而查看监听的状态：
[oracle@kel ~]$ lsnrctl status LSNRCTL for Linux: Version 10.2.0.1.0 - Production on 08-JUN-2014 19:31:45 Copyright (c) 1991, 2005, Oracle. All rights reserved. Connecting to (ADDRESS=(PROTOCOL=tcp)(HOST=)(PORT=1521)) TNS-12541: TNS:no listener TNS-12560: TNS:protocol adapter error TNS-00511: No listener Linux Error: 111: Connection refused
在这里看到监听是未启动的状态，从而打开数据库监听：
[oracle@kel ~]$ lsnrctl start LSNRCTL for Linux: Version 10.2.0.1.0 - Production on 08-JUN-2014 19:31:49 Copyright (c) 1991, 2005, Oracle. All rights reserved. Starting /home/oracle/product/10.2.0/db_1/bin/tnslsnr: please wait... TNSLSNR for Linux: Version 10.2.0.1.0 - Production Log messages written to /home/oracle/product/10.2.0/db_1/network/log/listener.log Listening on: (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=kel)(PORT=1521))) Connecting to (ADDRESS=(PROTOCOL=tcp)(HOST=)(PORT=1521)) STATUS of the LISTENER ------------------------ Alias LISTENER Version TNSLSNR for Linux: Version 10.2.0.1.0 - Production Start Date 08-JUN-2014 19:31:50 Uptime 0 days 0 hr. 0 min. 0 sec Trace Level off Security ON: Local OS Authentication SNMP OFF Listener Log File /home/oracle/product/10.2.0/db_1/network/log/listener.log Listening Endpoints Summary... (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=kel)(PORT=1521))) The listener supports no services The command completed successfully
再次测试，发现是可以成功连接的，从而利用连接字符串来连接服务器：
[oracle@kel ~]$ sqlplus system/orcl@orcl SQL*Plus: Release 10.2.0.1.0 - Production on Sun Jun 8 19:34:53 2014 Copyright (c) 1982, 2005, Oracle. All rights reserved. Connected to: Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - 64bit Production With the Partitioning, OLAP and Data Mining options
发现使用连接字符串是可以连接数据库的，再次使用本地进行连接发现还是不可以的，如下：
[oracle@kel ~]$ sqlplus / as sysdba SQL*Plus: Release 10.2.0.1.0 - Production on Sun Jun 8 19:45:49 2014 Copyright (c) 1982, 2005, Oracle. All rights reserved. ERROR: ORA-12162: TNS:net service name is incorrectly specified最后查看环境变量ORACLE_SID的值：
[oracle@kel ~]$ echo $ORACLE_SID [oracle@kel ~]$
发现环境变量未设置，从而导致连接出错，将ORACLE_SID设置为数据库实例名称：
[oracle@kel ~]$ export ORACLE_SID=orcl [oracle@kel ~]$ sqlplus / as sysdba SQL*Plus: Release 10.2.0.1.0 - Production on Sun Jun 8 19:48:51 2014 Copyright (c) 1982, 2005, Oracle. All rights reserved. Connected to: Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - 64bit Production With the Partitioning, OLAP and Data Mining options SQL>
当将环境变量ORACLE_SID的值设置为正确的值的时候，发现已经可以连接。