Reg: sysdba not logging, system user and other user connecting - oracle

Question

Sysdba not logging other users are logging below, how to
fix it, able to login to other users successfully, please find below, only with sysdba not working
[oracle#dztupbin]$ ./sqlplus SYSTEM/"oracle"
SQL*Plus: Release 12.2.0.1.0 Production on Thu Apr 28 11:05:09 2022
Copyright (c) 1982, 2016, Oracle. All rights reserved.
Last Successful login time: Thu Apr 28 2022 11:05:00 +01:00
Connected to:
Oracle Database 12c Standard Edition Release 12.2.0.1.0 - 64bit Production
[oracle#dztup bin]$ ./sqlplus SYSTEM/"oracle" as sysdba
SQL*Plus: Release 12.2.0.1.0 Production on Thu Apr 28 11:05:18 2022
Copyright (c) 1982, 2016, Oracle. All rights reserved.
ERROR:
ORA-01017: invalid username/password; logon denied
Enter user-name:
sqlplus / as sysdba also giving same result:-
./sqlplus / as sysdba
ERROR:
ORA-01017: invalid username/password; logon denied
[oracle#dztup bin]$ ./sqlplus / as sysdba
SQL*Plus: Release 12.2.0.1.0 Production on Thu Apr 28 11:05:18 2022
Copyright (c) 1982, 2016, Oracle. All rights reserved.
ERROR:
ORA-01017: invalid username/password; logon denied
Enter user-name:
I connected to sys user now:-
[oracle#dztup bin]$ ./sqlplus system/"oracle"
SQL*Plus: Release 12.2.0.1.0 Production on Thu Apr 28 13:25:51 2022
Copyright (c) 1982, 2016, Oracle. All rights reserved.
Last Successful login time: Thu Apr 28 2022 13:25:15 +01:00
Connected to:
Oracle Database 12c Standard Edition Release 12.2.0.1.0 - 64bit Production
SQL> conn sys as sysdba
Enter password:
Connected.
SQL>
SQL>
SQL>
SQL> show user
USER is "SYS"
granted sysdba privilige to system user now able to connect:
but still sqlplus / as sysdba not working.
[oracle#dztup bin]$ ./sqlplus / as sysdba
SQL*Plus: Release 12.2.0.1.0 Production on Thu Apr 28 11:05:18 2022
Copyright (c) 1982, 2016, Oracle. All rights reserved.
ERROR:
ORA-01017: invalid username/password; logon denied

Answer 1

SYSDBA is a privilege, not a user. The SYSTEM account does not have SYSDBA privileges, by default. The only account with default SYSDBA privileges is SYS, which is usually authenticated locally by the operating system. e.g.:
sqlplus / as sysdba
If you have granted SYSDBA privileges to other users, then those users are authenticated against a separate password file when logging in with the SYSDBA role, and not the normal password hash. The password file is not updated automatically when the users' normal password is changed (as with ALTER USER x IDENTIFIED BY pass). To update the password in the password file you must re-grant SYSDBA privileges to the user; otherwise use the old password.