I have searched Stack Overflow for a straight answer but have not found it. I noticed my install of nginx has three folders called
etc/nginx/sites-available
etc/nginx/sites-enabled
etc/nginx/conf.d
Do I really need these if I just want to work directly in the etc/nginx/nginx.conf file and remove the include lines that include these items in nginx.conf? Are these directories used for anything else that would mess things up if I delete them?
No, they are not needed if you define your server blocks properly in nginx.conf, but it's highly suggested. As you noticed, they are only used because of the include /etc/nginx/sites-enabled/*; in nginx.conf.
For curiosity, is there a reason why you do not want to use them? They are very useful; easier to add new sites, disabling sites, etc. Rather than having one large config file. This is a kind of a best practice of nginx folder layout.
Important information:
You should edit files only in sites-available directory.
Never edit files inside the sites-enabled directory, otherwise you can have problems if your editor runs out of memory or, for any reason, it receives a SIGHUP or SIGTERM.
For example: if you are using nano to edit the file sites-enabled/default and it runs out of memory or, for any reason, it receives a SIGHUP or SIGTERM, then nano will create an emergency file called default.save, inside the sites-enabled directory. So, there will be an extra file inside the sites-enabled directory. That will prevent Apache or NGINX to start. If your site was working, it will not be anymore. You will have a hard time until you find out, in the logs, something related to the default.save file and, then, remove it.
In the example above, if you were editing the file inside the sites-available directory, nothing bad would have happened. The file sites-available/default.save would have been created, but it wouldn't do any harm inside the sites-available directory.
It is not a must, but a best practise if you host more than one sites on your box.
It will be easier to manage by keep http context and common directives (such as ssl_dhparam, ssl_ciphers, or even gzip settings, etc.) on the nginx.conf so that it applied across all sites.
Keep site-specific server context (such as ssl-certificate, location directives, etc.) at etc/nginx/sites-available/ and name the configuration file as your-domain.conf. The file in etc/nginx/sites-enabled can be just a link to the file to the etc/nginx/sites-available.
