When you install Oracle Software for the first time in a machine, you would need to specify a folder that would act as a base for all the Oracle related softwares and folders.
And every subsequent folders will be organized and referred using this base folder. This folder is called ORACLE_BASE
For example, if you configure C:\app\oracle as your ORACLE BASE, then every folders will be created beneath it in an uniform structure.
If you install, Oracle 12c software in Windows, it will by default, be installed in,
Because Oracle_BASE is
And, if you install, SQL Developer software, it will be installed in
So, it is very important to setup the ORACLE_BASE environment variable before installing the Oracle software.
To do that, first create the folder that you would want to designate as ORACLE_BASE.
Then set the environment variable ORACLE_BASE
What if ORACLE_BASE wasn’t set before invoking the Oracle Universal Installer?
It will search for the first writable directory from /u01 to /u09
Let us say, it finds /u02 as the first writable directory and you have logged in as the OS user ‘oracle’
Then it will create a folder called /u02/app/oracle and will make it as the ORACLE_BASE
If it cannot find any writable area from /u01 to /u09, then it will create the app/oracle directory under the user’s home directory.
It will search for the disk drive that has the most amount of free space
Let us say, it finds D drive as the one and you are logged in as the OS user Oracle
Then it will create a folder called D:\app\oracle and will make it as the ORACLE_BASE