"Error: Could not read host and service status information" in Nagios

    In the previous article, we have learned - How To : Install Nagios Core 4 on Ubuntu Linux. After installing Nagios Core 4 on your Ubuntu system, when you try to search for the hosts that are being monitored, by clicking on the "Hosts" link on the left panel, or when you try to monitor the services running on the added hosts by clicking on "Services" link, you might face an error page that displays:

Error: Could not read host and service status information!

Solution:
First of all, make sure that the nagios service is running. You can check this by running the following command:

service nagios status

OR

/etc/init.d/nagios status

The output should resemble:

nagios start/running, process 1234

If the service is not running, you will see an output as follows:

nagios stop/waiting

To start the nagios service, run the following command:

service nagios start

Now, start the nagios daemon using the command:

/usr/local/nagios/bin/nagios -d /usr/local/nagios/etc/nagios.cfg

Now, try clicking on "Hosts" or "Services" link. Error should have been eliminated, at least until the next reboot.