Change Oracle Prompt at SQL*PLUS

PRE10G:
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Enter following commands in glogin.sql file located at $ORACLE_HOME/sqlplus/admin directory or execute them one by one at SQL Prompt.

col username new_value username
col dbname new_value dbname
set termout off
SELECT lower(user) username,
       substr(global_name, 1, instr(global_name, '.')-1) dbname
FROM   global_name
/
set termout on
set sqlprompt '&&username@&&dbname> '

POST 10G:

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Below is an example to change the SQL*Plus prompt (10g and up), simple and yet very useful.
SET SQLPROMPT command is used to change the default SQL> prompt

1) Display username

set sqlprompt '_user>' 2) Display database name
set sqlprompt '_connect_identifier_privilege>'
Step 2 and 3 can be combined together to display username and database name together.
3) Display username and database name (e.g. apps@dbname> )
set sqlprompt "_USER'@'_CONNECT_IDENTIFIER _PRIVILEGE>" 4) To set the time at sqlprompt
set time on
Now the best part is to avoid typing this command everytime you open a new SQL*Plus session, edit glogin.sql file located at $ORACLE_HOME/sqlplus/admin directory as follows.
set sqlprompt "_USER'@'_CONNECT_IDENTIFIER _PRIVILEGE>" set time on