Configure nginx to serve a 503 if a file exists

Solution 1:

From this page: https://calomel.org/nginx.html

## System Maintenance (Service Unavailable) 
if (-f $document_root/system_maintenance.html ) {
    error_page 503 /system_maintenance.html;
    return 503;
}

The important part being the '-f', I believe - it tests to see if the file exists.