Verifying The Configuration From The Command Line
Once you've entered all the necessary data into the configuration files, its time to do a sanity check. Everyone make mistakes from time to time, so its best to verify what you've entered. Nagios automatically runs a "pre-flight check" before before it starts monitoring, but you also have the option of running this check manually before attempting to start Nagios. In order to do this, you must start Nagios with the -v command line argument as follows...
/usr/local/nagios/bin/nagios -v <main_config_file>
Note that you should be entering the path/filename of your main configuration file (i.e. /usr/local/nagios/etc/nagios.cfg) as the second argument. Nagios will read your main configuration file and all object configuration files and verify that they contain valid data.
Relationships Verified During The Pre-Flight Check
During the "pre-flight check", Nagios verifies that you have defined the data relationships necessary for monitoring. Objects are all related and need to be setup properly in order for things to run. This is a list of the basic things that Nagios attempts to check before it will start monitoring...
Fixing Configuration Errors
If you've forgotten to enter some critical data or just plain screwed things up, Nagios will spit out a warning or error message that should point you to the location of the problem. Error messages generally print out the line in the configuration file that seems to be the source of the problem. On errors, Nagios will often exit the pre-flight check and return to the command prompt after printing only the first error that it has encountered. This is done so that one error does not cascade into multiple errors as the remainder of the configuration data is verified. If you get any error messages you'll need to go and edit your configuration files to remedy the problem. Warning messages can generally be safely ignored, since they are only recommendations and not requirements.
Where To Go From Here
Once you've verified your configuration files and fixed any errors, you can be reasonably sure that Nagios will start monitoring the services you've specified. On to starting Nagios!