The problem I've noticed is that if I append the file with each of the 100k runs (one at a time), it can happen that two threads try to save to the file at the same time and some row (s) end up empty.

Ubuntu is a free computer system, like Windows or macOS, but it’s built by a community. It’s great for programmers because it’s very flexible and has lots of tools already built-in that help you code.