NagiosPi: Installing Nagios on Raspian

Log into your RaspberryPi

sudo apt-get install /y nagios3

Much selecting and unpacking follows, then setting up, then creating config files, then setting up.

at last, you'll be presented with a dialog asking you to create a password for the nagiosadmin user.  Choose, enter then confirm and you're on your way to...some more setting up.

Finally, you'll end up at a command prompt.  Not much reward for all that waiting.