Oracle base /opt/oracle/product
tree -d

1) env | grep ORA
[oracle@ydcrh08 utility]$ env | grep ORA
ORACLE_SID=pdhptlt10g_vr
ORACLE_BASE=/opt/oracle/product
ORACLE_HOME=/opt/oracle/product/10.2.0.4.0.a
2) sqlplus '/ as sysdba'
SQL*Plus: Release 10.2.0.4.0 - Production on Fri Dec 10 13:28:40 2010
Copyright (c) 1982, 2007, Oracle. All Rights Reserved.
Connected to an idle instance.
SQL>
3) spfile
The default SPFILE (server parameter file) is located under $ORACLE_HOME/dbs. Oracle will use this SPFILE during startup, if you don’t specify PFILE.
Oracle will look for the parameter file in the following order under $ORACLE_HOME/dbs. If any one of them exist, it will use that particular parameter file.
   1. spfile$ORACLE_SID.ora
   2. spfile.ora
   3. init$ORACLE_SID.ora
/opt/oracle/product/10.2.0.4.0.a/dbs
ls -l
initpdhptlt10g_vr.ora -> /db/pdhptlt10g_vr/dbdump/pfile/initpdhptlt10g_vr.ora
ln -s /db/pdhptlt10g_vr/dbdump/pfile/initpdhptlt10g_vr.ora initpdhptlt10g_vr.ora
4) start database
SQL> startup
ORACLE instance started.
Total System Global Area 1677721600 bytes
Fixed Size                  2084424 bytes
Variable Size             603980216 bytes
Database Buffers         1056964608 bytes
Redo Buffers               14692352 bytes
Database mounted.
Database opened.
If you want to startup Oracle with PFILE, pass it as a parameter as shown below.
SQL> STARTUP PFILE=/u01/app/oracle/product/10.2.0/dbs/init.ora
note: kill oracle process by os level
pid=`ps -ef | grep _vr| grep -v grep |awk '{print $2}'`
for i in $pid
do
        echo kill $i
        kill -9 $i
done
5) shutdown oracle database
1. Normal Shutdown
2. Shutdown Immediate
3. Shutdown Abort
shutdown/shutdown immediate/shutdown abort
http://www.thegeekstuff.com/2009/01/oracle-database-startup-and-shutdown-procedure/