The official documentation is a good place to start:
https://assets.nagios.com/downloads/nag ... start.html

If there's a spot you get hung up on, you can ask for help here.

Why not use something like cron?
https://opensource.com/article/17/11/how-use-cron-linux

Nagios Core is for monitoring. If you just want to periodically run a script or command, cron is a much more lightweight and ubiquitous option for that particular use case IMO.

Can you tell if me if its possible to upgrade this to the current stable release? It is possible to upgrade the Nagios Core daemon in this state, yes. That doesn't mean 100% of what was built out on top of this 3.3.1 install ports nicely to a 4.x install. Some of your configuration files and Nagios...

Any way i can find what the password is? Kinda depends on how NSClient++ was set up. There's a few different ways to skin that cat. That's the source of truth for what a particular Windows machine's nscp service (process/service name for NSClient++) will accept as a password. It's pretty common to ...

Is there something preventing you from running check_http against the iframe's source directly? Rather than the page that attempts to render it in-line? I don't think check_http (or cURL for that matter) can be made to render iframes. Here's a test.html page I loaded up onto a test site of mine: <ht...

Here's a guide for getting started with monitoring Windows machines via NCPA, which interfaces with the check_ncpa plugin: https://assets.nagios.com/downloads/nagioscore/docs/nagioscore/4/en/monitoring-windows.html NSClient++ interfaces with the check_nrpe plugin. It looks like you're using the chec...

Here's the official NSClient++ documentation on wrapped scripts: https://docs.nsclient.org/howto/external_scripts/#wrapped-scripts The ps1 wrapper is the only "complex" one; It's checking to see if the script exists, printing an error if the script doesn't exist, else running the script an...

You can build the nagios-plugins project from source: https://support.nagios.com/kb/article/nagios-plugins-installing-nagios-plugins-from-source-569.html Those are the official instructions for getting those "common plugins" built. There's a check_disk plugins that is apart of that project...

As far as adding things to the GUI goes, I did something like this on the status page a few years ago:
viewtopic.php?f=7&t=44185&p=222987#p222987

You can define modules (custom queries) and pass args into them, yeah: https://edcint.co.nz/checkwmiplus/ini-file-documentation/ It's not super well documented, though. They're basically just templated WQL queries and should allow you to do all the above. I wrote this thing a while back if Python is...