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Oratab is a colon-delimited text file on Unix and Linux systems that associates ORACLE_SID and ORACLE_HOME values.

The last field contains a "Y" or "N" for database startup when the machine boots up. One can also use hashes (#) to start comment lines.

This file is either found in the /var/opt/oracle or the /etc directories.

Scripts like oraenv, dbhome, dbstart and dbshut use the oratab file.

[edit] Sample oratab file

# 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:
# 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.

The above oratab file describes an Oracle instance called orcl (don't confuse this with the database name!) that uses directory /opt/oracle/product/10.2.0/db_1 as its Oracle home. The Y indicates that this database should be started when the system boots up.

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