How to make 'git not ignore' certain file extension in nested folder structure?

Question

Say I have a folder structure like this:

├── a
│   ├── b
│   │   ├── c.xml
│   │   └── d.c
│   ├── v
│       ├── e.class
│       └── g.c
|
├── m
│   ├── p
│   │   ├── w.out
│   │   └── x.c
│   ├── q
│   │   ├── y.mp3
│   │   └── z.c

And I only want to not ignore *.c.

I tried

*                                                                         
!*.c 

in .gitignore, but that seems only work in the current level.

What I want is to set something like !*/*/*.c, since there may be other directories added to the git repo later, I can't hard-code the .gitignore file.

Answer 1

Add !*/ to your .gitignore to allow directories:

*
!*/
!*.c

From gitignore man page (emphasis mine):

An optional prefix "!" which negates the pattern; any matching file excluded by a previous pattern will become included again. It is not possible to re-include a file if a parent directory of that file is excluded. Git doesn't list excluded directories for performance reasons, so any patterns on contained files have no effect, no matter where they are defined. Put a backslash ("\") in front of the first "!" for patterns that begin with a literal "!", for example, "!important!.txt".

That's why you have to exclude the directories.

Answer 2

From GITIGNORE(5):

An optional prefix "!" which negates the pattern; any matching file excluded by a previous pattern will become included again. It is not possible to re-include a file if a parent directory of that file is excluded.

So what is happening here is that /example.c becomes unignored, but /a/b/d.c is still ignored because /a and /a/b are ignored.

The solution is to unignore all directories by putting this in your .gitignore:

*
!*/
!*.c