Ignore files that have already been committed to a Git repository

To untrack a single file that has already been added/initialized to your repository, i.e., stop tracking the file but not delete it from your system use: git rm --cached filename

To untrack every file that is now in your .gitignore:

First commit any outstanding code changes, and then, run this command:

git rm -r --cached .

This removes any changed files from the index(staging area), then just run:

git add .

Commit it:

git commit -m ".gitignore is now working"

To undo git rm --cached filename, use git add filename.

Make sure to commit all your important changes before running git add . Otherwise, you will lose any changes to other files.

Please be careful, when you push this to a repository and pull from somewhere else into a state where those files are still tracked, the files will be DELETED

Yes - .gitignore system only ignores files not currently under version control from git.

I.e. if you've already added a file called test.txt using git-add, then adding test.txt to .gitignore will still cause changes to test.txt to be tracked.

You would have to git rm test.txt first and commit that change. Only then will changes to test.txt be ignored.

If the files are already in version control you need to remove them manually.