To configure the database installation, Oracle provide several Environment variable / Registry Value.
If you start a database instance using spfile with an environment variable set, then its value is automatically stored in spfile. If you unset the environment variable subsequently and start the instance afresh, then database uses the parameter value of Oracle base stored in spfile.
In Windows, you can find all Oracle registry value in
HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE
To display the current value of an Linux environment variable, use the env command. For example, to display the value of the ORACLE_SID environment variable, run the following command:
$ env | grep ORACLE_SID
To display the current value of all environment variables, run the env command as follows:
$ env | more
Update the startup files of the oracle user and the operating system accounts of Oracle users, specifying the appropriate environment variables in the environment file.
You can use the oraenv or coraenv script to ensure that Oracle user accounts are updated.