How to make Git "forget" about a file that was tracked but is now in .gitignore?
.gitignore will prevent untracked files from being added (without an add -f) to the set of files tracked by git, however git will continue to track any files that are already being tracked.
To stop tracking a file you need to remove it from the index. This can be achieved with this command.
git rm --cached <file>
If you want to remove a whole folder, you need to remove all files in it recursively.
git rm -r --cached <folder>
The removal of the file from the head revision will happen on the next commit.
WARNING: While this will not remove the physical file from your local, it will remove the files from other developers machines on next git pull.
The series of commands below will remove all of the items from the Git Index (not from the working directory or local repo), and then updates the Git Index, while respecting git ignores. PS. Index = Cache
First:
git rm -r --cached .
git add .
Then:
git commit -am "Remove ignored files"
Or one-liner:
git rm -r --cached . && git add . && git commit -am "Remove ignored files"
git update-index does the job for me:
git update-index --assume-unchanged <file>
Note: This solution is actually independent on .gitignore as gitignore is only for untracked files.
Update, a better option
Since this answer was posted, a new option has been created and that should be preferred. You should use --skip-worktree which is for modified tracked files that the user don't want to commit anymore and keep --assume-unchanged for performance to prevent git to check status of big tracked files. See https://stackoverflow.com/a/13631525/717372 for more details...
git update-index --skip-worktree <file>