You should have a Nagios Core 4.0 or newer server running with a few hosts and services configured already, and you should have a plugin installed for which you'd like to define a new command so that you can use it as part of a service definition. In this instance, we'll define a command for an installed check_rsync plugin.

We can define a new command in our configuration as follows:

The configuration we added to the preceding commands.cfg file defines a new command called check_rsync, which specifies a method for using the plugin of the same name to monitor a service. This enables us to use check_rsync as a value for the check_command directive in a service declaration, which might look like this:

define service {
    use                  generic-service
    host_name            troy.example.net
    service_description  RSYNC
    check_command        check_rsync
}

Only two directives are required for command definitions, and we've defined both:

This particular command line also uses two macros:

So, if we used the command in a service checking, the rsync(1) server on troy.example.net, the completed command might look like this:

$ /usr/local/nagios/libexec/check_rsync -H troy.example.net

We could run this straight from the command line as the nagios user to see what kind of results it returns:

$ /usr/local/nagios/libexec/check_rsync -H troy.example.net
OK: Rsync is up

A plugin can be used for more than one command. If we had a particular rsync(1) module that we wanted to check named backup, we could write another command called check_rsync_backup as follows, perhaps added to commands.cfg:

define command {
    command_name  check_rsync_backup
    command_line  $USER1$/check_rsync -H $HOSTADDRESS$ -m backup
}

If one or more of our rsync(1) servers were running on an alternate port, say port 5873, we could define a separate command check_rsync_altport for that:

define command {
    command_name  check_rsync_altport
    command_line  $USER1$/check_rsync -H $HOSTADDRESS$ -p 5873
}

Commands can thus be defined as precisely as we need them to be. We explore this in more detail in the Customizing an existing command recipe in this chapter.