求一條linux的命令解釋

cat /usr/local/var/log/nginx/access.log|sort -nrk9|head -2

我所疑惑的地方是sort -nrk9;
-nrk我知道是什么意思，這個(gè)9代表什么呢
下面是nginx log的格式

log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
                       '$upstream_response_time '
                       '$status $body_bytes_sent "$http_referer" '
                       '"$http_user_agent" "$http_x_forwarded_for"';