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申请Let's Encrypt提供的免费证书，在根据说明使用certbot配置ssl证书时出现错误提示：

The nginx plugin is not working; there may be problems with your existing configuration.
The error was: NoInstallationError("Could not find a usable 'nginx' binary. 
Ensure nginx exists, the binary is executable, and your PATH is set correctly.")

需要指定nginx及配置的路径：

#仅下载证书
certbot certonly --nginx --nginx-ctl /usr/local/openresty/nginx/sbin/nginx --nginx-server-root /usr/local/openresty/nginx/conf
#下载并自动安装证书
certbot --nginx --nginx-ctl /usr/local/openresty/nginx/sbin/nginx --nginx-server-root /usr/local/openresty/nginx/conf