How to subtract lines of one textbook from another?

Question

Text Processing Automation

I couldn't find exactly what I need on Google. There are two txt files, both with about 4 million lines. Task: subtract from the first file the lines that are in the second file, save the result in the third file, or save the result in the first. The whole difficulty is that there are a lot of lines and all sorts of online services do not cope with the task.

Answer 1

generally classic
https://shapeshed.com/unix-uniq/
https://www.geeksforgeeks.org/diff-command-linux-e...