linux shell

Why not use chown, and chmod to have the script that starts the db run under the credentials of the user you want?
Script
Chown ora12 oracledbstart.sh
Chmod 2711 oracledbstart.sh

As an example.

No matter which user runs it,

I might be solving a problem ...

Echo $username might be what you are after. The environment variable is the loged in reference.

Issue with id, if you are within an suid script,Id will return the owner of the script.


Back to your question

try
If test "$username" = "${USER1}; then

Su ? Are you running as root? Sudo ...

the question is not clear, what you want to do ?

What do you mean by 'detect' exactly? Are you trying to find the userId that has started the Oracle database? Or are you trying to find if the current logged on user is the correct user to start the Oracle database?

If it is about starting the database and you want a certain user to allow - then I agree with arnold - configure sudo to allow that user to start the database. Then you would allow /tmp/detect.oracle.ksk to be started by that certain user.

As @Gerwin mentioned, knowing what you're trying to accomplish (question context) is essential for how you accomplish what you're asking.

As asked, the simple answer is...

echo "$(date): $USER" >> /var/log/oracle-startup-users.log

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I search to detect which user linux start oracle database.
Thank you
Best regards 

Detect means you are to replace what the users run to detect.

there is an environment variable $username that survives sudo.
Su - on the other hand would reset.

Oracle provides the scripts to startup and shutdown Oracle instances in a Linux OS.

Any reason you aren't using them?

https://docs.oracle.com/en/database/oracle/oracle-database/19/unxar/stopping-and-starting-oracle-software.html#GUID-EFE15D61-4BDC-4A9B-B8E4-46A7325C2409