Hi,
Just to be clear: Opal is not a database. Then if you want to store data using Opal you must declare a database (the location of the database does not matter). That is why the web interface redirects to the post-install page: you cannot do anything without a database in Opal’s backend.
Yes, you can use a command line to set the database: see how it is done in the Opal docker image startup script, where the database settings are sent to Opal server using the commands opal rest ... /system/databases ....
FYI the next release of Opal will introduce the concept of resources which will free Opal from requiring a database: Opal will provide data access control without storing the data in a managed database.
Regards
Yannick