Opencart generates its sitemap on the fly and this is a problem in a big catalogs over 10.000 products. So I have modified the function to generate a static sitemap in an XML file.
When I access to my http://localhost/index.php?route=extension/feed/google_sitemap I generate a sitemap-google.xml file without problems and with a unlimited execution time.
I tried to add it in a cron in the development server each 4 hours
0 0,4,8,12,16,20 /usr/bin/php /path/to/index.php?route=extension/feed/google_sitemap
But I'm receiving a "command not found".
Can I execute on cli the "?params/etc"?