Remove files from history with filter-branch

Let's suppose we work on a project with a billion of files; at start we thought was fine versioning all of them so to not have problems, but after a while, since we have to share our work with others, we end with a repo of 1GB and more size that is not so nice to work with.

What to do? simple, use filter-branch: this command allows to modify the story of your repository, change commit messages, authorship and, more surgically, remove files. In my case I want to maintain only a subset of the original files so I started with obtaining all the files traced by git

$ git ls-files > to-remove.txt

Now I edit with my preferred editor (vim what else) the file obtained in order to remove the files I want to maintain under versioning and launch the following dangerous command

$ git filter-branch --tree-filter 'cat /absolute/path/to/to-remove.txt | \
    xargs rm -f ' HEAD

It's important to note that this takes several minutes to complete (it depends on repository size and its history of course) so take a coffee meanwhile. The absolute path is necessary in order to avoid "not found" error messages.

When the process is completed the repository is checkout-ed in the new rewritten branch and the original HEAD is referenced in the file .git/refs/original/refs/heads/master; if something went wrong read the original reference with

$ cat .git/refs/original/refs/heads/master 
382f89ae33a875d83507b276f0550ae315e408e1

and checkout that and repeat.