How do I flush pagespeed cache from the command line?
Are you using ee on your current setup ?
If you have followed this tutorial https://rtcamp.com/tutorials/nginx/using-pagespeed/ then your pagespeed cache files will be present under /var/ngx_pagespeed_cache/
You can empty that folder using rm command.
A better way is to purge specific file cache by hitting following url http://exmple.com/pagespeed_admin/cache?purge=/path/file.ext