From Oracle FAQ
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.
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: # $ORACLE_SID:$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. # # orcl:/opt/oracle/product/10.2.0/db_1:Y
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.
- oraenv - utility used to change the Oracle environment