Gitignore: Excluding Everything Except a Specific Directory

I have the following directory structure:

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$ tree -aF src/
src/
├── github.com/
│   ├── git/
│   │   └── git/
│   │       └── file
│   └── realguess/
│       ├── git/
│       │   └── file
│       └── hello/
│           └── hello.go
└── .gitignore
6 directories, 4 files

My intention is to keep the projects in src/github.com/realguess/ only. So, I have added the following gitignore file:

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$ cat src/.gitignore
/github.com/
!/github.com/realguess/

But the directory is ignored:

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$ git check-ignore -v github.com/realguess/
src/.gitignore:1:/github.com/      github.com/realguess/

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.[^1]

Uh-oh! The directory github.com/ is excluded. Therefore, it is not possible to re-include a file github.com/realguess (directory is a file). To fix it, just need to add the wildcard to the end of the parent directory:

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$ cat src/.gitignore
/github.com/*
!/github.com/realguess/

By adding the wildcard character, it matches every file (including hidden ones) or directory under github.com/ directory, the directory itself as the parent of github.com/realguess/ is not excluded. Then, we use the negation pattern to re-include github.com/realguess/.

Double check it:

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$ git check-ignore -v github.com/realguess/
1

The debugging command returns non-zero status code, which indicates that the directory is not ignored.