Environment variables

ORACLE_BASE:
The Oracle Base directory is the top level directory that you can use to install the various oracle software products.

For example, /u01/app/oracle is an Oracle base directory created by the oracle user and /u01/app/applmgr is an Oracle base directory created by the applmgr user.

ORACLE_HOME: an environment variable pointing to the directory where the Oracle software is installed.

ORACLE_SID:

The Oracle System ID (SID) is used to uniquely identify a particular database on a system. For this reason, one cannot have more than one database with the same SID on a computer system.

When using RAC, all instances belonging to the same database must have unique SID's.

If you’re unfamiliar with the term SID or ORACLE_SID, a full definition is called for. The SID is a site identifier. It and ORACLE_HOME (where the Oracle software is installed) are hashed together in UNIX to create a unique key name for attaching an SGA. If your ORACLE_SID or ORACLE_HOME is not set correctly, you’ll get the ORACLE NOT AVAILABLE error, since you can’t attach to a shared memory segment that is identified by this unique key. On Windows, shared memory isn’t used in the same fashion as UNIX, but the SID is still
important. You can have more than one database on the same ORACLE_HOME, so you need a way to uniquely identify each one, along with their configuration files.