The orabasetab file is used to define fundamental directories based on $ORACLE_HOME: ORACLE_BASE, ORACLE_BASE_HOME and ORACLE_BASE_CONFIG.

The orabasetab file resides in ORACLE_HOME/install/orabasetab and can be used to determine if an ORACLE_HOME is read-only or read/write. It also defines the ORACLE_BASE and the HOME_NAME of the Oracle home. HOME_NAME is the internal name for ORACLE_HOME.

The last line in the orabasetab file, which starts with $ORACLE_HOME, defines the directories for $ORACLE_HOME. The last line consists of four fields, each separate by a colon delimiter(:).

1. The first field matches the current $ORACLE_HOME.

2. The second field defines the ORACLE_BASE for the current ORACLE_HOME.

3. The third field defines the HOME_NAME which is used in constructing the ORACLE_BASE_HOME path in a read-only ORACLE_HOME.

4. The fourth field displays N in a read/write ORACLE_HOME and Y in a read-only ORACLE_HOME.

In a read-only ORACLE_HOME, the ORACLE_BASE_HOME path is ORACLE_BASE/homes/HOME_NAME and ORACLE_BASE_CONFIG is the same as ORACLE_BASE.

In a read/write ORACLE_HOME, ORACLE_HOME, ORACLE_BASE_HOME and ORACLE_BASE_CONFIG are all the same.