How to make next git commit to exactly represent project state from another commit without merge?

Question

I have two git branches: Alpha and Beta and I need to create next commit on the branch Alpha so its project state would be exactly the same like on branch Beta. Without merge.

I understand that every git commit - on a logical level - is a complete and immutable project state + some metadata. So I just need branch Alpha to have the same state of files like Beta does.

It would be another commit (with different hash), and one branch tip wouldn't reference another.

You may read the manual of [git-format-patch](https://git-scm.com/docs/git-format-patch) and [git-apply](https://git-scm.com/docs/git-apply) to achieve your goal without merge — TravorLZH, Aug 18 '18 at 06:44
@TravorLiu, yes its possible to create a patch based on their diff, and the apply it. But it I believe there should be a simpler and more elegant way. Thanks! — Adam, Aug 18 '18 at 07:02

score 6 · Accepted Answer · answered Aug 18 '18 at 07:07

6

c=$(git commit-tree Beta^{tree} -p Alpha -m 'foo')
test $c && git update-ref refs/heads/Alpha $c

git commit-tree creates a new commit, whose tree is the same with Beta's tip's and whose parent is Alpha's tip.

git update-ref makes Alpha point at the new commit.

answered Aug 18 '18 at 07:07

ElpieKay

27,194
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Exactly what I needed! Just one-line command `git update-ref refs/heads/Alpha $(git commit-tree Beta^{tree} -p Alpha -m 'foo')` – Adam Aug 18 '18 at 07:50
2

That's exactly the right solution. (And it shows that it helps learning the Git object model, [which is really not that complicated](https://stackoverflow.com/a/44872194/2988).) A commit is a pointer to a tree, a set of pointers to parent commits, and some metadata. You already have the tree, all you need is to construct the commit. – Jörg W Mittag Aug 18 '18 at 08:14

score 3 · Answer 2 · answered Aug 18 '18 at 07:06

But it I believe there should be a simpler and more elegant way.

There is a way which does not involve patches: reset --hard followed by reset --soft.

First, mark the current HEAD of your branch: we will need to move that HEAD without modifying Alpha, so let's create a new temporary branch called 'tmp' where Alpha is.
```
git checkout Alpha
git checkout -b tmp
```
Then we go back to an Beta HEAD commit with the right content.
```
git reset --hard Beta 
```

That resets the index (and working tree) to the right content, but that also moves HEAD. However, that moves tmp HEAD, not Alpha HEAD.

move back tmp HEAD to where mAlphaaster is, but without modifying the index or the working tree (which are representing what we need for a new commit)
```
git reset --soft Alpha
```
make a new commit, on top of Alpha/tmp HEAD, which represents the right content (Beta HEAD commit content).
```
git commit -m "new commit, image of an old one"
```

Finally, force Alpha to be where tmp is: one new commit later.

git branch -M tmp Alpha
git checkout Alpha
git branch -d tmp

Now a regular git push is enough, any collaborator can simply pull as usual, and still get the old reset content.

git push

How to make next git commit to exactly represent project state from another commit without merge?

2 Answers2

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