How can I ignore everything in a git repository subdirectory except a single file?

Question

I have a repository with a subdirectory called mod/. I want this subdirectory to be included in the repository along with a README file within it, but I do not want other subdirectories within mod/ to be included. I have tried several fixes proposed here using .gitignore and committing changes, but git status still shows everything in mod/ is being tracked. Currently, my .gitignore contains:

mod/* # Ignore everything in 'mod'...
!mod/README.md # ... except this.

But I have also tried setting the first line to:

/mod
mod/
./mod
./mod/*

and a few other variations...

To 'apply' these settings each time I edit .gitignore, I run:

git rm -r --cached .  
git add .gitignore
git add mod/README.md
git commit -m "banging my head on the wall"
git status

and the status continues to show untracked files in mod/.

Answer 1

There are three intuitive options I can think of:

1. Add a separate .gitignore to mod:

   *
   !README.md
   

2. Use your root level .gitignore, with only either a rule or comment on each line, but not both:

   ./mod/*
   !./mod/README.md
   

   I use ./ prefix here for emphasis, but it is not strictly necessary. Since the paths all contain a non-trailing slash, they will be interpreted relative to the directory of the .gitignore regardless.

3. Don't use .gitignore for the positive rule: ignore mod, but then do

   git add -f mod/README.md
   

   Git keeps tracking any files that were added using the -f/--force flag.