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You can use username/password combination to add an on-premise site. User doesn’t need to be an administrator. Permission checks are done by JIRA’s Rest API on the server, not on the command line level. That means although you can try to run any command with any user, only the operations your account has permission to execute will be completed successfully. If you don’t have permission for an action you will receive “Not Permitted” (401/403) response from Jira.
For Server/DC instances you can optionally enable JMX Monitoring as explained here. When you enable JMX monitoring you can get more information about the server using ‘jira server-info:jmx’ command.