*nix: How to make a "virtual" file?

Question

linux

Is there an out-of-the-box mechanism in linux in which, for example, when accessing the /test/file.txt file, it would be possible to do it on the fly? (for example, run a script that returns "test-string")
so when the command is cat /test/file.txt, the user will see test-string

Answer 1

https://unix.stackexchange.com/a/66992/61540