"oratab" is a file created by Oracle in the /etc or /var/opt/oracle directory when installing database software. It is used to list the databases and software versions installed on a server.

 This file may contain comments staring with a pound signs (#) in column one, and data lines consisting of entries in the following format:

 database_sid:oracle_home_dir:Y|N

- database_sid is the system id (SID) of an Oracle instances on the server.

 - Oracle_home_dir is the ORACLE_HOME directory associated with this instance.

 - The Y|N flags indicate if the instance should automatically start at boot time (Y=yes, N=no).

 Besides acting as a registry for what databases and software versions are installed on the server, ORATAB is also used for the following purposes:

 Oracle's "dbstart" and "dbshut" scripts use this file to figure out which instances are to be start up or shut down (using the third field, Y or N).

 The "oraenv" utility uses ORATAB to set the correct environment variables.

One can also write Unix shell scripts that cycle through multiple instances using the information in the oratab file.