I built up most of my bikes from parts, so I know them pretty well. In this case the cable was four years old. I have not snapped a derailleur cable for decades, until yesterday. But it looked pretty corroded. I was sure I had bought stainless cables but yesterday I discovered that I had a few galvanized ones that snuck into my inventory. And the cable that broke was not stainless. Now there is a stainless cable on the bike.

Bar end shifters, being near sweaty hands are probably a place you want a stainless cable.

I do not carry spares like that on exercise rides near home, but on a multi-day tour, I bring spare cables, spare spokes, cassette tool, spoke wrench, etc. But I worked in a bike shop years ago, so I know how to use that stuff.