# suggestion for a simple addition: git update-ref --ff-only

3 messages from 2013-03-01 to 2013-03-02. Participants: David Madore, Junio C Hamano, Jeff King.
Thread: https://gitlist.dev/t/33052

## David Madore, 2013-03-01 23:18

Subject: suggestion for a simple addition: git update-ref --ff-only
Message-ID: <20130301231859.GA334@achernar.madore.org>
URL: https://gitlist.dev/e/20130301231859.GA334%40achernar.madore.org

```
Hi list,

I'd like to suggest a very simple, but IMHO quite useful, additional
option to git-update-ref: an option --ff-only which would cause the
command to refuse unless the current ref is an ancestor of the new
one.

The reason I think it would be useful: I occasionally wish to perform
a trivial (i.e., fast-forward) merge of some branch into another
(e.g., after a git-fetch) without checking it out.  Now git-update-ref
is perfect for that, but there is always the possibility of getting
something wrong (which one would not have with git merge --ff-only,
but the latter requires checking out the branch), and this option
would avoid tedious verifications.

Happy hacking,

-- 
     David A. Madore
   ( http://www.madore.org/~david/ )

```

## Junio C Hamano, 2013-03-01 23:33

Subject: Re: suggestion for a simple addition: git update-ref --ff-only
Message-ID: <7vehfyelvg.fsf@alter.siamese.dyndns.org>
URL: https://gitlist.dev/e/7vehfyelvg.fsf%40alter.siamese.dyndns.org
In-Reply-To: <20130301231859.GA334@achernar.madore.org>

```
David Madore <david+news@madore.org> writes:

> Hi list,
>
> I'd like to suggest a very simple, but IMHO quite useful, additional
> option to git-update-ref: an option --ff-only which would cause the
> command to refuse unless the current ref is an ancestor of the new
> one.

Mild NAK.

What you want may be "git push . new_commit:refs/heads/foo", by the
way.

The "update-ref" plumbing does not want any such frill.

```

## Jeff King, 2013-03-02 01:10

Subject: Re: suggestion for a simple addition: git update-ref --ff-only
Message-ID: <20130302011049.GA16458@sigill.intra.peff.net>
URL: https://gitlist.dev/e/20130302011049.GA16458%40sigill.intra.peff.net
In-Reply-To: <20130301231859.GA334@achernar.madore.org>

```
On Sat, Mar 02, 2013 at 12:18:59AM +0100, David Madore wrote:

> I'd like to suggest a very simple, but IMHO quite useful, additional
> option to git-update-ref: an option --ff-only which would cause the
> command to refuse unless the current ref is an ancestor of the new
> one.
> 
> The reason I think it would be useful: I occasionally wish to perform
> a trivial (i.e., fast-forward) merge of some branch into another
> (e.g., after a git-fetch) without checking it out.  Now git-update-ref
> is perfect for that, but there is always the possibility of getting
> something wrong (which one would not have with git merge --ff-only,
> but the latter requires checking out the branch), and this option
> would avoid tedious verifications.

The update-ref command is plumbing, which is supposed to do one small,
well-defined job. But you can compose many plumbing commands to do what
you want:

  # input
  ref=refs/heads/master
  new=$some_sha1

  # where are we now?
  old=`git rev-parse --verify $ref` || exit 1

  # is it a fast-forward?
  if ! git merge-base --is-ancestor $old $new; then
    echo >&2 "Not a fast-forward"
    exit 1
  fi

  # update it; we do not have to worry about race conditions because
  # update-ref will abort if somebody touched the ref in the meantime
  git update-ref $ref $new $old

Yes, it's three commands instead of one, but it's much more flexible
(you get to write your own message, you can use the same "merge-base" to
handle the "already up to date" case, etc).

-Peff

```