Controlling the Nginx service

The next step is obviously run Nginx for the first time. However, before doing so, it's important to understand the nature of this application. There are two types of computer applications—those that require immediate user input, thus running in the foreground, and those that do not, thus running in the background. Nginx is of the latter type, often referred to as daemon. Daemon names usually come with a trailing "d" and a couple of examples can be mentioned here—httpd, the HTTP server daemon, is the name given to Apache under several Linux distributions; named, the name server daemon; or crond the task scheduler—although, as you will notice, this is not the case for Nginx. When started from the command line, a daemon immediately returns the prompt, and in most cases, does not even bother outputting data to the terminal.

Consequently, when starting Nginx, you will not see any text appear on the screen, and the prompt will return immediately. While this might seem startling, it is, on the contrary, a good sign. It means the daemon was started correctly and the configuration did not contain any errors.

It is of utmost importance to understand the process architecture of Nginx, and particularly the user and groups under which its various processes run. A very common source of trouble when setting up Nginx is invalid file access permissions—due to a user or group misconfiguration, you often end up getting 403 Forbidden HTTP errors because Nginx cannot access the requested files.

There are two levels of processes with possibly different permission sets:

The Nginx binary accepts command-line arguments to perform various operations, among which is controlling the background processes. To get the full list of commands, you may invoke the help screen using the following commands:

[[email protected] ~]$ cd /usr/local/nginx/sbin
[[email protected] sbin]$ ./nginx -h

The next few sections will describe the purpose of these switches. Some allow you to control the daemon and some let you perform various operations on the application configuration.

You can start Nginx by running the Nginx binary without any switches. If the daemon is already running, a message will show up indicating that a socket is already listening on the specified port:

[emerg]: bind() to 0.0.0.0:80 failed (98: Address already in use) […] [emerg]: still could not bind().

Beyond this point, you may control the daemon by stopping it, restarting it, or simply reloading its configuration. Controlling is done by sending signals to the process using the nginx -s command.

Note that when starting the daemon, stopping it, or performing any of the preceding operations, the configuration file is first parsed and verified. If the configuration is invalid, whatever command you have submitted will fail, even when trying to stop the daemon. In other words, in some cases you will not be able to even stop Nginx if the configuration file is invalid.

An alternate way to terminate the process, in desperate cases only, is to use the kill or killall commands with root privileges:

[[email protected] ~]# killall nginx

As you can imagine, testing the validity of your configuration will become crucial if you constantly tweak your server setup . The slightest mistake in any of the configuration files can result in a loss of control over the service—you will then be unable to stop it via regular init control commands, and obviously, it will refuse to start again.

Consequently, the following command will be useful to you in many occasions; it allows you to check the syntax, validity, and integrity of your configuration:

[[email protected] ~]$ /usr/local/nginx/sbin/nginx –t

The –t switch stands for test configuration. Nginx will parse the configuration anew and let you know whether it is valid or not. A valid configuration file does not necessarily mean Nginx will start, though, as there might be additional problems such as socket issues, invalid paths, or incorrect access permissions.

Obviously, manipulating your configuration files while your server is in production is a dangerous thing to do and should be avoided when possible. The best practice, in this case, is to place your new configuration into a separate temporary file and run the test on that file. Nginx makes it possible by offering the –c switch:

[[email protected] sbin]$ ./nginx –t –c /home/alex/test.conf

This command will parse /home/alex/test.conf and make sure it is a valid Nginx configuration file. When you are done, after making sure that your new file is valid, proceed to replacing your current configuration file and reload the server configuration:

[[email protected] sbin]$ cp -i /home/alex/test.conf /usr/local/nginx/conf/nginx.conf
cp: erase 'nginx.conf' ? yes
[[email protected] sbin]$ ./nginx –s reload

Another switch that might come in handy in many situations is –V. Not only does it tell you the current Nginx build version, but more importantly, it also reminds you about the arguments that you used during the configuration step—in other words, the command switches that you passed to the configure script before compilation.

[[email protected] sbin]$ ./nginx -V
nginx version: nginx/1.8.0 (Ubuntu)
built by gcc 4.8.4 (Ubuntu 4.8.4-2ubuntu1~14.04)
TLS SNI support enabled
configure arguments: --with-http_ssl_module

In this case, Nginx was configured with the --with-http_ssl_module switch only.

Why is this so important? Well, if you ever try to use a module that was not included with the configure script during the precompilation process, the directive enabling the module will result in a configuration error. Your first reaction will be to wonder where the syntax error comes from. Your second reaction will be to wonder if you even built the module in the first place! Running nginx –V will answer this question.

Additionally, the –g option lets you specify additional configuration directives in case they were not included in the configuration file:

[[email protected] sbin]$ ./nginx –g "timer_resolution 200ms";