why / as sysdba

to start up oracle:

->su -l oracle //login as oracle, password

->sqlplus //start sqlplus

->/ as sysdba //?? see below

->startup //start oracle

->exit //quit sqlplus

http://download.oracle.com/docs/cd/B10501_01/server.920/a90842/ch4.htm#1006252

The AS clause allows privileged connections by users who have been granted SYSOPER or SYSDBA system privileges. You can also use either of these privileged connections with / and /NOLOG.

If you use this option, you need to quote the command arguments on many operating systems, for example:

SQLPLUS "/ AS SYSDBA" SQLPLUS "SYSTEM/your_password AS SYSOPER"

http://ugweb.cs.ualberta.ca/~c391/manual/chapt4.html

Enter the command ``sqlplus /''.
Note: This example assumes that you have been given the same ORACLE user ID as your login. If you forget to add the ``/'' you will be asked to enter your name and password for your ORACLE user ID, in which case you should press $\wedge C$ (cntrl-C) and start over. Entering your system user ID and password will not get you access to SQL*Plus. If your ORACLE user ID differs from your system ID refer to the ``SQL*Plus User's Guide and Reference''.