How to configure an existing git repo to be shared by a UNIX group

I have an existing git repo (a bare one) which has up to this point only been writable by me. I want to open it up to some UNIX user group, foo, so that all members of foo can push to it. I'm aware that I can easily set up a new git repo with:

git init --bare --shared=group repodir
chgrp -R foo repodir

But I need the equivalent operation for an existing repo dir.

Try this to make an existing repository in repodir work for users in group foo:

chgrp -R foo repodir                 # set the group
chmod -R g+rw repodir                # allow the group to read/write
chmod g+s `find repodir -type d`     # new files get group id of directory
git init --bare --shared=all repodir # sets some important variables in repodir/config ("core.sharedRepository=2" and "receive.denyNonFastforwards=true")

Merging @David Underhill and @kixorz answers, I made my own (definitive) solution.

It is for bare repos and non-bare repos. There are only little differences between them, but in this way is clearer.

BARE REPOSITORY

cd <repo.git>/                            # Enter inside the git repo
git config core.sharedRepository group    # Update the git's config
chgrp -R <group-name> .                   # Change files and directories' group
chmod -R g+w .                            # Change permissions
chmod g-w objects/pack/*                  # Git pack files should be immutable
find -type d -exec chmod g+s {} +         # New files get directory's group id

where:

• <repo.git> is the bare repository directory, typically on the server (e.g. my_project.git/).
• <group-name> is the group name for git users (e.g. users).

---

NON-BARE REPOSITORY

cd <project_dir>/                         # Enter inside the project directory
git config core.sharedRepository group    # Update the git's config
chgrp -R <group-name> .                   # Change files and directories' group
chmod -R g+w .                            # Change permissions
chmod g-w .git/objects/pack/*             # Git pack files should be immutable
find -type d -exec chmod g+s {} +         # New files get directory's group id

where:

• <project_dir> is the project directory containing the .git folder.
• <group-name> is the group name for git users (e.g. users).

In the repo dir execute following commands:

git config core.sharedRepository group
chgrp -R foo repodir
chmod -R g+w repodir

Edit: To address frequent confusion, group is an actual keyword, you're not supposed to replace this with the name of the group.

This is probably not necessary, but it's worth pointing out that git init --bare --shared also sets the denyNonFastForwards option.

git config receive.denyNonFastForwards true

The meaning of this option is as follows:

> receive.denyNonFastForwards > > If you rebase commits that you’ve already pushed and then try to push > again, or otherwise try to push a commit to a remote branch that > doesn’t contain the commit that the remote branch currently points to, > you’ll be denied. This is generally good policy; but in the case of > the rebase, you may determine that you know what you’re doing and can > force-update the remote branch with a -f flag to your push command.

(from http://git-scm.com/book/en/v2/Customizing-Git-Git-Configuration)

In addition to the above answers of allowing a group to read/write you also need to add the user to the group (say "foo").

sudo usermod -a -G [groupname] [username]

Note: you will have to first create user if it doesn't exist