Solution for [WARNING] ORA-01081: cannot start already-running ORACLE - shut it down first when running dbca

I received the following error multiple times, when executing dbca like below:

dbca -createDatabase -responsefile ./cdb.rsp -silent
 2023-09-14 15:05:45.435 CEST ] Prepare for db operation
DBCA_PROGRESS : 10%
[ 2023-09-14 15:05:45.505 CEST ] Copying database files
DBCA_PROGRESS : 12%
[ 2023-09-14 15:05:47.250 CEST ] [WARNING] ORA-01081: cannot start already-running ORACLE - shut it down first

DBCA_PROGRESS : 40%
DBCA_PROGRESS : 100%
[ 2023-09-14 15:05:47.616 CEST ] [FATAL] Recovery Manager failed to restore datafiles. Refer logs for details.
DBCA_PROGRESS : 10%
DBCA_PROGRESS : 0%

At first I thought it was because I had forgotten to terminate some processes owned by oracle. And I also made sure that /etc/oratab was empty before I started.

Reason:

Turns out, there were old memory segments left behind that dbca considers a part of a running instance. Even if

ps -fu oracle

gave no processes in the list.

Solution:

su - oracle
sqlplus / as sysdba
shutdown abort

Given that you've set ORACLE_SID to the same value you used in your response file, this will properly shut down your instance, and clean up the memory segments left behind from the previous run.