[oracle@usha /]$ cat /etc/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:<N|Y>:
#
# 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.
#
#
testdb:/u01/app/oracle/product/11.2.0/db_1:Y

[oracle@usha /]$

Now lets derive OTACLE_HOME, ORACLE_SID, AUTO_START from the oratab using UNIX commands.

[oracle@usha /]$ cat /etc/oratab |egrep ':N|:Y'
testdb:/u01/app/oracle/product/11.2.0/db_1:Y

[oracle@usha /]$ cat /etc/oratab |egrep ':N|:Y'|grep -v \*
testdb:/u01/app/oracle/product/11.2.0/db_1:Y

[oracle@usha /]$ cat /etc/oratab |egrep ':N|:Y'|cut -f1 -d':'
testdb

[oracle@usha /]$ cat /etc/oratab |egrep ':N|:Y'|cut -f2 -d':'
/u01/app/oracle/product/11.2.0/db_1

[oracle@usha /]$ cat /etc/oratab |egrep ':N|:Y'|cut -f3 -d':'
Y

[oracle@usha /]$ ORACLE_SID=`cat /etc/oratab |egrep ':N|:Y'|cut -f1 -d':'`
[oracle@usha /]$ echo $ORACLE_SID
testdb

[oracle@usha /]$ ORACLE_HOME=`cat /etc/oratab |egrep ':N|:Y'|cut -f2 -d':'`
[oracle@usha /]$ echo $ORACLE_HOME
/u01/app/oracle/product/11.2.0/db_1

[oracle@usha /]$ AUTO_START=`cat /etc/oratab |egrep ':N|:Y'|cut -f3 -d':'`
[oracle@usha /]$ echo $AUTO_START
Y
[oracle@usha /]$