Oracle SQL*Plus, always, first runs an sql script called "glogin.sql" when you try to connect to an Oracle database.
First take the backup of this script, if you need it later you can use the original version.
[oracle@srvdb01]:/oracle/orahome1/sqlplus/admin > ls -l total 40 -rw-r----- 1 oracle dba 1525 Aug 30 2004 glogin.sql drwxr-x--- 2 oracle dba 256 May 23 2008 help drwxr-x--- 3 oracle dba 256 May 23 2008 iplus -rw-r----- 1 oracle dba 5363 Mar 02 2008 libsqlplus.exp -rw-r----- 1 oracle dba 813 Mar 26 2006 plustrce.sql -rw-r----- 1 oracle dba 2118 Feb 16 2003 pupbld.sql [oracle@srvdb01]:/oracle/orahome1/sqlplus/admin > cp glogin.sql glogin.sql-201106141153
Add the following line to the end of "glogin.sql" script.
set sqlprompt "&_USER'@'&_CONNECT_IDENTIFIER &_PRIVILEGE> "
Now try to login to database by using SQL*Plus in a telnet session.
[oracle@srvdb01]:/oracle/orahome1/sqlplus/admin > sqlplus / as sysdba SQL*Plus: Release 10.2.0.4.0 - Production on Tue Jun 14 12:53:42 2011 Copyright (c) 1982, 2007, Oracle. All Rights Reserved. Connected to: Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bit Production With the Partitioning, OLAP, Data Mining and Real Application Testing options SYS@ORCL AS SYSDBA>
As you can see, we set the Oracle SQL*Plus sqlprompt to the username@instance_name the privilege like " as sysdba".
If you apply this in your Unix Oracle database servers, when you have multiple databases in the same server or when you have multiple telnet sessions connected to different database servers and you are always switching between them, you realize in which database you are connected to and it will help a lot not to write the wrong SQL in especially in production databases instead of development or test databases.