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Nagios XI is the easy-to-use, enterprise version of Nagios that features:
Every time you modify your configuration files, you should run a sanity check on them. It is important to do this before you (re)start Nagios Core, since it will shut down if your configuration contains errors.
In order to verify your configuration, run Nagios Core binary with the -v command line option like so:
/usr/local/nagios/bin/nagios -v /usr/local/nagios/etc/nagios.cfg
If you've forgotten to enter some critical data or misconfigured things, Nagios Core 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 Core 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.
Once you've verified your configuration files and fixed any errors you can go ahead and (re)start Nagios Core.