Track file inside ignored directory

Question

Some time ago I set up my .gitignore file to not track a folder my_folder with:

my_folder/ 

Now I want to track only a given file inside said folder, named my_file.md. After making .gitignore look like this:

my_folder/ !my_folder/my_file.md 

and checking:

git status 

the file does not appear as a change to be committed.

What am I doing wrong?

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Add

I tried changing my .gitignore file to:

my_folder/* !my_folder/my_file.md 

as advised but the file is still not showing up as a change to commit after a git status. Do I need to reset something?

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Add 2

Attempting to add the file with git add my_folder/my_file.md returns:

The following paths are ignored by one of your .gitignore files: my_folder/my_file.md Use -f if you really want to add them. fatal: no files added 

The command git check-ignore -v my_folder/my_file.md gives:

.gitignore:1:my_folder/*    my_folder/my_file.md 

Answer 1

To add to ".gitignore exclude folder but include specific subfolder", one good way to debug those .gitignore file is to use git check-ignore (Git 1.8.4+):

git check-ignore -v my_folder/my_file.md 

You would see it is still ignored because of the my_folder/ rule.

That is because it is not possible to re-include a file if a parent directory of that file is excluded.(*)
(*: unless certain conditions are met in git 2.?+, see below)

That is why ignoring the files within that folder (my_folder/*, instead of the folder itself) allows you to exclude one.

Of course, you can force adding a file ignored (git add -f my_folder/my_file.md), but that is not the point of this answer.
The point is to explain why adding !my_folder/my_file.md in .gitignore doesn't work with git 2.6 or less.

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Note that with git 2.9.x/2.10 (mid 2016?), it might be possible to re-include a file if a parent directory of that file is excluded if there is no wildcard in the path re-included.

Nguyễn Thái Ngọc Duy (pclouds) is trying to add this feature:

• commit 506d8f1 for git v2.7.0, reverted in commit 76b620d git v2.8.0-rc0
• commit 5e57f9c git v2.8.0-rc0,... reverted(!) in commit 5cee3493 git 2.8.0-rc4.

So here, with git 2.8+, this would work:

/my_folder !my_folder/my_file.md