in unix,
to startup / shutdown multiple database in the same box, you have to set ORACLE_SID, ORALCE_HOME everytime and do the startup shutdown.
to avoid this pain, oracle for unix installations will carry a file called oratab.
just edit the file to add the appropriate entries in oratab, set ur TNS_ADMIN .
this oratab will be called from $ORACLE_HOME/bin/dbstart and $ORACLE_HOME/bin/dbshut scripts which will startup and shutdown all the oracle databases that have an entry in oratab file.