[xwdv]

At some point we must simply “know what we are doing” and do something. Software would be a tedious affair if you have to approach everything from the perspective that maybe you don’t really know what you’re doing and need to implement some way to verify.

[Kalium]

What are unit tests, linters, SAST tools, and more all for? Naively, it seems to me that we swim in a great sea of tools to verify that we do, in fact, know what we're doing.

[xwdv]

How do you know your tests are correctly testing what you need to test? Tests all the way down?

[Kalium]

An excellent question! One reflecting true wisdom.

The answer is that you never have true certainty. What you get is smaller error bars with each set of tools and tests until you find the error bars and effort involved both acceptable.

[ehsankia]

The other comment mentions tests, but I'd like to that to that. The reason why it's impossible for anyone, even the single greatest programmer on the planet, to not create bugs, is due to the high complexity of the software we write. It's not so much that you'll make a mistake when creating a new program. It's that over time, as you make more and more changes, it's impossible to actually trace out the exponential complexity that arises from your changes.

The purpose of unit tests is to catch these, and yes as you mention lower down even those aren't 100% infallible but they greatly help. That's why even with some of the greatest programmers and extensive testing, we still constantly see major bugs from every single top tech company. I don't think there exists a single completely bug-free software of non-negligible size out there.