git update-index does the job for me:

git update-index --assume-unchanged <file>

Note: This solution is actually independent of .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>

To cancel

git update-index --no-skip-worktree <file>

The copy/paste answer is:

git rm --cached -r .; git add .; git status; git commit -m "Ignore unwanted files"

This command will NOT change the content of the .gitignore file. It will just ignore the files that have already been committed to a Git repository but now we have added them to .gitignore.

The command git status; is just to review the changes and could be dropped.

In the end, it will immediately commit the changes with the message "Ignore unwanted files".

If you don't want to commit the changes, drop the last part of the command (git commit -m "Ignore unwanted files")

Use this when:

1. You want to untrack a lot of files, or
2. You updated your .gitignore file

Source: Untrack files already added to Git repository based on .gitignore

Let’s say you have already added/committed some files to your Git repository and you then add them to your .gitignore file; these files will still be present in your repository index. This article we will see how to get rid of them.

Step 1: Commit all your changes

Before proceeding, make sure all your changes are committed, including your .gitignore file.

Step 2: Remove everything from the repository

To clear your repository, use:

git rm -r --cached .

• rm is the remove command
• -r will allow recursive removal
• –cached will only remove files from the index. Your files will still be there.

The rm command can be unforgiving. If you wish to try what it does beforehand, add the -n or --dry-run flag to test things out.

Step 3: Readd everything

git add .

Step 4: Commit

git commit -m ".gitignore fix"

Your repository is clean :)

Push the changes to your remote to see the changes effective there as well.

Do the following steps for a file/folder:

Remove a File:

1. need to add that file to .gitignore.
2. need to remove that file using the command (git rm --cached file name).
3. need to run (git add .).
4. need to (commit -m) "file removed".
5. and finally, (git push).

For example:

I want to delete the test.txt file. I accidentally pushed to GitHub and want to remove it. Commands will be as follows:

First, add "test.txt" in file .gitignore

git rm --cached test.txt
git add .
git commit -m "test.txt removed"
git push

Remove Folder:

1. need to add that folder to file .gitignore.
2. need to remove that folder using the command (git rm -r --cached folder name).
3. need to run (git add .).
4. need to (commit -m) "folder removed".
5. and finally, (git push).

For example:

I want to delete the .idea folder/directory. I accidentally pushed to GitHub and want to remove it. The commands will be as follows:

First, add .idea in file .gitignore

git rm -r --cached .idea
git add .
git commit -m ".idea removed"
git push

Do the following steps serially, and you will be fine.

1. Remove the mistakenly added files from the directory/storage. You can use the "rm -r" (for Linux) command or delete them by browsing the directories. Or move them to another location on your PC. (You maybe need to close the IDE if running for moving/removing.)

2. Add the files / directories to the .gitignore file now and save it.

3. Now remove them from the Git cache by using these commands (if there is more than one directory, remove them one by one by repeatedly issuing this command)

    git rm -r --cached path-to-those-files
   

4. Now do a commit and push by using the following commands. This will remove those files from Git remote and make Git stop tracking those files.

    git add .
    git commit -m "removed unnecessary files from Git"
    git push origin
   

Using the git rm --cached command does not answer the original question:

How do you force git to completely forget about [a file]?

In fact, this solution will cause the file to be deleted in every other instance of the repository when executing a git pull!

The correct way to force Git to forget about a file is documented by GitHub here.

I recommend reading the documentation, but basically:

git fetch --all
git filter-branch --force --index-filter 'git rm --cached --ignore-unmatch full/path/to/file' --prune-empty --tag-name-filter cat -- --all
git push origin --force --all
git push origin --force --tags
git for-each-ref --format='delete %(refname)' refs/original | git update-ref --stdin
git reflog expire --expire=now --all
git gc --prune=now

Just replace full/path/to/file with the full path of the file. Make sure you've added the file to your .gitignore file.

You'll also need to (temporarily) allow non-fast-forward pushes to your repository, since you're changing your Git history.

git rm --cached -r <YOUR_files_or_folders>

--cached | only remove files from the index

In my case I needed to put ".envrc" in the .gitignore file.

And then I used:

git update-index --skip-worktree .envrc
git rm --cached .envrc

And the file was removed.

Then I committed again, telling that the file was removed.

But when I used the command git log -p, the content of the file (which was secret credentials of the Amazon S3) was showing the content which was removed and I don't want to show this content ever on the history of the Git repository.

Then I used this command:

git filter-branch --index-filter 'git rm --cached --ignore-unmatch .envrc' HEAD

And I don't see the content again.

The accepted answer does not "make Git "forget" about a file..." (historically). It only makes Git ignore the file in the present/future.

This method makes Git completely forget ignored files (past/present/future), but it does not delete anything from the working directory (even when re-pulled from remote).

This method requires usage of file /.git/info/exclude (preferred) or a pre-existing .gitignore in all the commits that have files to be ignored/forgotten. ¹

All methods of enforcing Git ignore behavior after-the-fact effectively rewrite history and thus have significant ramifications for any public/shared/collaborative repositories that might be pulled after this process. ²

General advice: start with a clean repository - everything committed, nothing pending in working directory or index, and make a backup!

Also, the comments/revision history of this answer (and revision history of this question) may be useful/enlightening.

#Commit up-to-date .gitignore (if not already existing)
#This command must be run on each branch

git add .gitignore
git commit -m "Create .gitignore"

#Apply standard Git ignore behavior only to the current index, not the working directory (--cached)
#If this command returns nothing, ensure /.git/info/exclude AND/OR .gitignore exist
#This command must be run on each branch

git ls-files -z --ignored --exclude-standard | xargs -0 git rm --cached

#Commit to prevent working directory data loss!
#This commit will be automatically deleted by the --prune-empty flag in the following command
#This command must be run on each branch

git commit -m "ignored index"

#Apply standard git ignore behavior RETROACTIVELY to all commits from all branches (--all)
#This step WILL delete ignored files from working directory UNLESS they have been dereferenced from the index by the commit above
#This step will also delete any "empty" commits.  If deliberate "empty" commits should be kept, remove --prune-empty and instead run git reset HEAD^ immediately after this command

git filter-branch --tree-filter 'git ls-files -z --ignored --exclude-standard | xargs -0 git rm -f --ignore-unmatch' --prune-empty --tag-name-filter cat -- --all

#List all still-existing files that are now ignored properly
#If this command returns nothing, it's time to restore from backup and start over
#This command must be run on each branch

git ls-files --other --ignored --exclude-standard

Finally, follow the rest of this GitHub guide (starting at step 6) which includes important warnings/information about the commands below.

git push origin --force --all
git push origin --force --tags
git for-each-ref --format="delete %(refname)" refs/original | git update-ref --stdin
git reflog expire --expire=now --all
git gc --prune=now

Other developers that pull from the now-modified remote repository should make a backup and then:

#fetch modified remote

git fetch --all

#"Pull" changes WITHOUT deleting newly-ignored files from working directory
#This will overwrite local tracked files with remote - ensure any local modifications are backed-up/stashed

git reset FETCH_HEAD

Footnotes

¹ Because /.git/info/exclude can be applied to all historical commits using the instructions above, perhaps details about getting a .gitignore file into the historical commit(s) that need it is beyond the scope of this answer. I wanted a proper .gitignore file to be in the root commit, as if it was the first thing I did. Others may not care since /.git/info/exclude can accomplish the same thing regardless where the .gitignore file exists in the commit history, and clearly rewriting history is a very touchy subject, even when aware of the ramifications.

FWIW, potential methods may include git rebase or a git filter-branch that copies an external .gitignore into each commit, like the answers to this question.

² Enforcing Git ignore behavior after-the-fact by committing the results of a stand-alone git rm --cached command may result in newly-ignored file deletion in future pulls from the force-pushed remote. The --prune-empty flag in the following git filter-branch command avoids this problem by automatically removing the previous "delete all ignored files" index-only commit. Rewriting Git history also changes commit hashes, which will wreak havoc on future pulls from public/shared/collaborative repositories. Please understand the ramifications fully before doing this to such a repository. This GitHub guide specifies the following:

Tell your collaborators to rebase, not merge, any branches they created off of your old (tainted) repository history. One merge commit could reintroduce some or all of the tainted history that you just went to the trouble of purging.

Alternative solutions that do not affect the remote repository are git update-index --assume-unchanged </path/file> or git update-index --skip-worktree <file>, examples of which can be found here.

I liked JonBrave's answer, but I have messy enough working directories that commit -a scares me a bit, so here's what I've done:

git config --global alias.exclude-ignored '!git ls-files -z --ignored --exclude-standard | xargs -0 git rm -r --cached &&  git ls-files -z --ignored --exclude-standard | xargs -0 git stage &&  git stage .gitignore && git commit -m "new gitignore and remove ignored files from index"'

Breaking it down:

git ls-files -z --ignored --exclude-standard | xargs -0 git rm -r --cached
git ls-files -z --ignored --exclude-standard | xargs -0 git stage
git stage .gitignore
git commit -m "new gitignore and remove ignored files from index"

• remove ignored files from the index
• stage .gitignore and the files you just removed
• commit

This is no longer an issue in the latest Git (v2.17.1 at the time of writing).

The .gitignore file finally ignores tracked-but-deleted files. You can test this for yourself by running the following script. The final git status statement should report "nothing to commit".

# Create an empty repository
mkdir gitignore-test
cd gitignore-test
git init

# Create a file and commit it
echo "hello" > file
git add file
git commit -m initial

# Add the file to gitignore and commit
echo "file" > .gitignore
git add .gitignore
git commit -m gitignore

# Remove the file and commit
git rm file
git commit -m "removed file"

# Reintroduce the file and check status.
# .gitignore is now respected - status reports "nothing to commit".
echo "hello" > file
git status

This is how I solved my issue:

git filter-branch --tree-filter 'rm -rf path/to/your/file' HEAD
git push

In this, we are basically trying to rewrite the history of that particular file in previous commits also.

For more information, you can refer to the man page of filter-branch here.

Source: Removing sensitive data from a repository - using filter-branch

Source: Git: How to remove a big file wrongly committed

In case of already committed DS_Store:

find . -name .DS_Store -print0 | xargs -0 git rm --ignore-unmatch

Ignore them by:

echo ".DS_Store" >> ~/.gitignore_global
echo "._.DS_Store" >> ~/.gitignore_global
echo "**/.DS_Store" >> ~/.gitignore_global
echo "**/._.DS_Store" >> ~/.gitignore_global
git config --global core.excludesfile ~/.gitignore_global

Finally, make a commit!

Especially for the IDE-based files, I use this:

For instance, for the slnx.sqlite file, I just got rid off it completely like the following:

git rm {PATH_OF_THE_FILE}/slnx.sqlite -f
git commit -m "remove slnx.sqlite"

Just keep that in mind that some of those files store some local user settings and preferences for projects (like what files you had open). So every time you navigate or do some changes in your IDE, that file is changed and therefore it checks it out and show as uncommitted changes.

If anyone is having a hard time on Windows and you want to ignore the entire folder, go to the desired 'folder' on file explorer, right click and do 'Git Bash Here' (Git for Windows should have been installed).

Run this command:

git ls-files -z | xargs -0 git update-index --assume-unchanged

In my case here, I had several .lock files in several directories that I needed to remove. I ran the following and it worked without having to go into each directory to remove them:

git rm -r --cached **/*.lock

Doing this went into each folder under the 'root' of where I was at and excluded all files that matched the pattern.

if you created gitignore file using command like echo node_modules >> .gitignore, It won't work.

The windows terminal saves the file in UCS-2 LE BOM and git doesn't seem to accept that.

You can fix this by opening in Notepad and resave it with UTF-8 encoding

It Works now.

I think they need to fix this since doing echo "filetoignore" >> .gitignore actually seems very handy

I am aware that this answer is many years late, however I decided I would like to contribute after noticing that there could be different answer depending on user's desired output.

How do you force Git to completely forget about it?

That depends which of the following is the desired outcome:

1. Stop tracking file and remove it from the repository, or
2. Stop tracking file without removing it from the repository (maybe for other collaborators to download)

For (1), the solution would be like Matt's answer above. For (2), refer to Konstantin's answer instead.

Source: https://stackabuse.com/git-stop-tracking-file-after-adding-to-gitignore/

If none of these questions work for you, you'll want to confirm that your ".gitignore" file is actually a .gitignore file, and not a text file, JSON file, or the like.

Make sure it appears like this.