About oratab File

oratab file can be used to set oracle homes for multiple instances, and also has options to start the database at system boot time. This file is located at /var/opt on Unix box.

[oracle@localhost etc]$ vi oratab

# This file is used by ORACLE utilities. It is created by root.sh
# and updated by the Database Configuration Assistant when creating a database.

# A colon, ':', is used as the field terminator. A new line terminates
# the entry. Lines beginning with a pound sign, '#', are comments.
#
# Entries are of the form:
# $ORACLE_SID:$ORACLE_HOME::
#
# The first and second fields are the system identifier and home
# directory of the database respectively. The third filed indicates
# to the dbstart utility that the database should , "Y", or should not,
# "N", be brought up at system boot time.
#
# Multiple entries with the same $ORACLE_SID are not allowed.
#

Example Entry:
vinay:/u01/app/oracle/product/10.2.0/db_1:N