One of the most common areas where Nagios can be suited to fit your needs is that of active checks . These are the checks that are scheduled and run by the Nagios daemon. This functionality is described in more detail in Chapter 2, Installing Nagios 4.
Nagios has a project that ships the commonly-used plugins and comes with a large variety of checks that can be performed. Before thinking about writing anything on your own, it is best to check for the standard plugins (described in detail in Chapter 4, Using the Nagios Plugins).
The Nagios Exchange (http://exchange.nagios.org) website contains multiple ready-to-use plugins for performing active checks. It is recommended that you check whether somebody has already written a similar plugin for your needs.
The reason for this is that even though active checks are quite easy to implement, sometimes a complete implementation that handles errors and command line options parsing is not very easy to create. Typically...