How do you create custom Nagios plugins

[bob]

You start by figuring out exactly what you need to watch on the system. I always ask myself what metric matters most right now. Then you sketch a quick plan in your head before touching any code. But you keep it simple at first so nothing gets too messy later. Perhaps you test the idea with a basic tool you already have handy. Now you pick a language you feel comfortable writing in fast. I prefer something quick like a script that runs without extra installs. You check the output format Nagios expects so your results show up clean. And you make sure the exit codes match what you want for each state.
Or you tinker around with the logic until the numbers look right every time. I run small tests on my own machine before putting anything live. You watch how the check behaves under load too. Maybe you add some error handling to catch weird cases that pop up. Then you tweak the thresholds based on what your setup actually needs. But you avoid overcomplicating it because that slows you down later. I share early versions with you so we can spot issues together. You document the steps in a note file for future reference. Perhaps you loop back and refine the whole thing after a week of real use.
Also you think about how often the check should fire without overloading the host. I usually start with longer intervals and shorten them if needed. You monitor the performance data it spits out to catch trends early. But you keep the script lightweight so it does not hog resources. Now you consider permissions because the check might need special access. You test it under the same user Nagios runs as. I have seen checks fail just from that one detail. Perhaps you add logging inside the script for troubleshooting. Then you verify the results appear correctly in the monitoring dashboard.
You build on what works from other checks you already use. I grab ideas from simple existing ones and twist them my way. But you focus on one thing at a time to stay clear. Or you experiment with different ways to pull the data you need. You make sure the output stays under the line length limit so nothing gets cut off. Perhaps you run it manually first to confirm the states trigger right. Then you automate the deployment so it lands on all your servers smoothly. I check the return values repeatedly during development. You adjust for edge cases like missing files or slow responses.
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