You should have a working Nagios Core 3.0 or better server running.

We can verify the Nagios Core configuration as follows:

The configuration is parsed as though Nagios Core were about to start up, to check that the configuration all makes sense. It will run basic checks such as looking for syntax errors, and will also check things like having at least one host and service to monitor. Some of the things it reports are warnings, meaning that they're not necessarily problems; examples include hosts not having any services monitored, or not having any contacts defined.

This is the quickest way to get an idea of whether the Nagios Core configuration is sane and will work correctly. Whenever there's trouble restarting the Nagios Core server, it's a good idea to check the output of this command first. In fact, it's a good habit to check the configuration before restarting, particularly if we're unsure about the configuration changes, or if the monitoring server is checking something very important! This means if it turns out that our configuration is broken, then the Nagios Core daemon will keep running with the configuration from the point before we changed it, and we can fix things before we restart.

The program at /usr/local/nagios/bin/nagios is actually the same program that runs the Nagios Core server, but the -v part of the command is a switch for the program that verifies the configuration instead, and shows any problems with it. The second path is to the configuration file with which Nagios Core starts, which in turn imports the configuration files for objects, such as contact, host, and service definitions.