If you are using Stardog free (the community edition), you might want to upgrade to the current version - 8.1 - and get the benefit of the new features and improvements. The default user for a free standalone version is admin, with password admin.
Thanks for your reply. I am not running free edition. I am running latest Stardog on premise installed on Linux. It maybe that instructions are not complete. So I am still awaiting for a Stardog engineer to chime in.
When you install Stardog via deb/rpm, the stardog user is created as a part of that process. If you look in your /etc/passwd file you should see an entry for this user:
stardog:x:1001:1001::/var/opt/stardog:/bin/sh
This is the user as which the stardog process runs, so it will need to be the owner of /var/opt/stardog or at least have read/write permissions on it. If you hadn't previously created that directory to use as the home directory, it will also have been created during the install with the proper permissions.
I should add that if you wish to NOT have a stardog user, you can manually edit the /etc/systemd/system/stardog.service file and change the user/group that runs it.
No, you can run the systemctl command with any user provided it has the correct permissions to do so. As defined in /etc/systemd/system/stardog.service, the server start command will be run as the stardog user.