Question

I try to ignore file in a directory by relative path. E.g. I have several directories in working tree lib/file.f and I want all occurrences of this file to be ignored. I tried

lib/file.f

but this does not work.

Answer 1

Place

*/lib/file.f

in your .gitignore. This will cause git to ignore any file of the form project/<DIR>/lib/file.f, assuming .gitignore is in the project directory.
To make git ignore lib/file.f two directories down, you’d also have to add

*/*/lib/file.f

to the .gitignore, and so on.

Vastly simpler of course, would be to place

*file.f

in .gitignore to block all files named file.f, but your question seems to suggest there is at least one file by that name you wish not to ignore.