First, run the following command to log in as an oracle user:
$ su - oracle
In this case, the user was a student and after running this command, the user will be oracle.
Now, log in to your Oracle Database by running the following command:
$ sqlplus sys as sysdba
Enter the password you set earlier when installing Oracle. After the successful execution of this command, you will get the SQL> prompt:
[oracle@localhost work]$ sqlplus sys as sysdba SQL*Plus: Release 220.127.116.11.0 Production on Mon Apr 30 12:02:53 2018 Copyright (c) 1982, 2009, Oracle. All rights reserved. Enter password: Connected to an idle instance. SQL>