In this recipe, you'll learn how to set up an event handler script for Nagios Core. Event handlers are commands that are run on every state change for a host or service (whether for all hosts or services or just particular ones). They are defined in a similar way to notification commands and check commands for plugins.
In this example, we'll implement a simple event handler that writes the date, the host state, and the number of check attempts to a separate file for a single host. This is a simple example to demonstrate the concept; a more practical and complex application for the use of event handlers is given in the Setting up a redundant monitoring host recipe in Chapter 10, Security and Performance.