Event handlers are commands that are triggered whenever the state of a host or service changes. They offer functionalities similar to those of notifications. The main difference is that the event handlers are called for each type of change and even for each soft state change. This provides the ability to react to a problem before Nagios notifies it as a hard state and sends out notifications about it. Another difference is what the event handlers do: instead of notifying users that there is a problem, event handlers carry out actions automatically.

For example, if a service is defined with max_check_attempts set to 4, the retry_interval set to 1, and check_interval set to 5, then the following example illustrates when event handlers will be triggered and with what values for $SERVICESTATE$, $SERVICESTATETYPE$, and $SERVICEATTEMP$ macro definitions:

Event handlers are triggered for each state change, for example, in minutes, 10, 23, 28, and 29. When writing an event...