If you look at the before you install manual for Linux there should be a section on getting databases to automatically startup. There should be a file in $ORACLE_HOME/bin called dbstart. It will look at the /var/optt/oracle/oratab file and determine which databases need to be started based on the third field.
Then there is a diectory in /etc, I don't remember which one off hand but you basically create a symbolic link there. It is all spelled out in the manual which you should read before installing Oracle on a new platform anyway in case there is something that you should know or change.
Not used Linux but I will assume you have the same scripts as those on Solaris, if so you can use the $ORACLE_HOME/bin/dbstart and $ORACLE_HOME/bin/dbstop scripts to start and stop all the instances on the host.
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Thanx for that .. i am able to use the script when i paste the same on the terminal window .. but when i save it as a shell script and execute it, its getting stopped after logging in as oracle.
and once i exit from the oracle user.. remaining commands are being executed.