How to merge a branch to another branch in GIT?

Question

Let me explain the problem in detail.

I have a main git branch on which I created a new side branch bug10101010, now I wan't to merge the bug10101010 to main. So far everything is good. Now I have a different branch of the same product, named legacy. I wan't to merge the bug10101010 to the legacy branch in GIT.

Any ideas?

I can't just merge it directly, as the branch bug10101010 is spin off from the main branch and in the legacy I need only the diff between the branch bug10101010 and its parent branch.

Answer 1

Use git-diff and then git-apply?

Answer 2

This is hard to do. Git saves merge history, and if you "cherrypick" and point at a commit in bug10101010 as a parent (indicating you have done a merge) Git will assume that all commits before that (back to the point where they split) as been merged as well. Giving you problems when you want to do a "real" merge.

On the other hand you can just manually generate a patch from that (and only that) specific commit. But that will also give you problems when you later do the "real" merge, since it tries to apply your manually handled commit twice.

But then again, since one branch is named "Legacy" I suspect that you dont plan to do that real merge anyway, in which case youre pretty much free to do it anyway you please.

Heres an interesting blog post on the topic.

Answer 3

You should use git rebase --onto here, and specify a range.
(see git rebase man page:

transplant a topic branch based on one branch to another, to pretend that you forked the topic branch from the latter branch, using rebase --onto.

).

Of course, this would move your bug10 branch on top of the legacy branch, which is not what you want/need.

So, one workaround would be to do that rebase in a cloned repo, then merge that 'enhanced' legacy branch (the one in the clone repo, with the bug10 modifications on top of it) to the local and current legacy branch (the one you want to modify, while leaving bug10 alone).

Now:

• this involves an extra repo (which can lead to disk space limitation)
• all in all, this is fairly equivalent to define a patch and apply it to legacy branch, so the other answers (patch) are valid (and simpler).
• the only advantage I see in this method is the opportunity to define a legacy environment in which you rebase what you want (like the bug10 commits), before pushing only that branch legacy to your original repo (you would not push bug10, since its history would have been entirely rewritten!)

I just wanted to see if it works, so... Let's test that approach.
(Git1.6.5.1, on a old XP SP2, with a Powershell 1.0 session because of the Start-Transcript command)

---

PS D:\> mkdir git
PS D:\> cd git
PS D:\git> mkdir tests
PS D:\git> cd tests
PS D:\git\tests> git init mainRepo

^{I like how I do not have anymore to make the git repo directory first, then type in it git init! Since 1.6.5:}

"git init" learned to mkdir/chdir into a directory when given an extra argument (i.e. "git init this").

^{This is GREAT!}

Let's create 3 files, for 3 different purposes.
(For the sake of example, I will keep the file modifications separate per branch: no conflict during merge or rebase here.)

PS D:\git\tests> cd mainRepo
PS D:\git\tests\mainRepo> echo mainFile > mainFile.txt
PS D:\git\tests\mainRepo> echo contentToBeFixed > toBeFixedFile.txt
PS D:\git\tests\mainRepo> echo legacyContent > legacy.txt
PS D:\git\tests\mainRepo> git add -A
PS D:\git\tests\mainRepo> git ci -m "first commit"
PS D:\git\tests\mainRepo> echo firstMainEvol >> mainFile.txt
PS D:\git\tests\mainRepo> git ci -a -m "first evol, for making 1.0"
PS D:\git\tests\mainRepo> git tag -m "1.0 legacy content" 1.0

At this point, a git log --graph --oneline --branches returns:

* b68c1f5 first evol, for making 1.0
* 93f9f7c first commit

Let's build a legacybranch

PS D:\git\tests\mainRepo> git co -b legacy
PS D:\git\tests\mainRepo> echo aFirstLegacyEvol >> legacy.txt
PS D:\git\tests\mainRepo> git ci -a -m "a first legacy evolution"

We return to master, make another commit, which we will tag "2.0" (a release which will need some bug-fixing!)

PS D:\git\tests\mainRepo> git co -b master
PS D:\git\tests\mainRepo> git co master
PS D:\git\tests\mainRepo> echo aMainEvol >> mainFile.txt
PS D:\git\tests\mainRepo> git ci -a -m "a main evol"
PS D:\git\tests\mainRepo> echo aSecondMainEvolFor2.0 >> mainFile.txt
PS D:\git\tests\mainRepo> git ci -a -m "a second evol for 2.0"
PS D:\git\tests\mainRepo> git tag -m "main 2.0 before bugfix" 2.0

We have:

* e727105 a second evol for 2.0
* 473d44e a main evol
| * dbcc7aa a first legacy evolution
|/
* b68c1f5 first evol, for making 1.0
* 93f9f7c first commit

Now we do a bug10 bug-fixing branch:

PS D:\git\tests\mainRepo> git co -b bug10
PS D:\git\tests\mainRepo> echo aFirstBug10Fix >> toBeFixedFile.txt
PS D:\git\tests\mainRepo> git ci -a -m "a first bug10 fix"
PS D:\git\tests\mainRepo> echo aSecondBug10Fix >> toBeFixedFile.txt
PS D:\git\tests\mainRepo> git ci -a -m "a second bug10 fix"

Let's add a final commit on the main branch

PS D:\git\tests\mainRepo> git co master
PS D:\git\tests\mainRepo> echo anotherMainEvol >> mainFile.txt
PS D:\git\tests\mainRepo> git ci -a -m "another main evol"

Final state of our main repo:

* 55aac85 another main evol
| * 47e6ee1 a second bug10 fix
| * 8183707 a first bug10 fix
|/
* e727105 a second evol for 2.0
* 473d44e a main evol
| * dbcc7aa a first legacy evolution
|/
* b68c1f5 first evol, for making 1.0
* 93f9f7c first commit

At this stage, I will not make any further manipulation in mainRepo. I will only clone it to make some tests. If those fails, I can always get back to this repo and clone it again.

The first clone is actually mandatory, in order to perform our git rebase --onto

PS D:\git\tests\mainRepo> cd ..
PS D:\git\tests> git clone mainRepo rebaseRepo
PS D:\git\tests> cd rebaseRepo

We need two of the mainRepo branches in our cloned repo:

PS D:\git\tests\rebaseRepo> git co -b bug10 origin/bug10
PS D:\git\tests\rebaseRepo> git co -b legacy origin/legacy

Let's rebase only bug10 (that is all commits after 2.0 tag up to HEAD of bug10 branch):

PS D:\git\tests\rebaseRepo> git co bug10
PS D:\git\tests\rebaseRepo> git rebase --onto legacy 2.0
First, rewinding head to replay your work on top of it...
Applying: a first bug10 fix
Applying: a second bug10 fix

At this point bug10 has been replayed on top of legacy without all the other intermediate commits.
We can now fast-forward HEADof legacy to the top of the replayed bug10 branch.

PS D:\git\tests\rebaseRepo> git co legacy
Switched to branch 'legacy'
PS D:\git\tests\rebaseRepo> git merge bug10
Updating dbcc7aa..cf02bfc
Fast forward
 toBeFixedFile.txt |  Bin 38 -> 104 bytes
 1 files changed, 0 insertions(+), 0 deletions(-)

The content follow what we need:

• We do have all the legacy content:

 

PS D:\git\tests\rebaseRepo> type legacy.txt
legacyContent
aFirstLegacyEvol

 

• the content for the main branch is there only up to 1.0 tag (root for legacy branch), and not any further.

 

PS D:\git\tests\rebaseRepo> type mainFile.txt
mainFile
firstMainEvol

 

• and the bug10 fixes are here:

 

PS D:\git\tests\rebaseRepo> type toBeFixedFile.txt
contentToBeFixed
aFirstBug10Fix
aSecondBug10Fix

 

That's it.
The idea is to to pull that 'enhanced' legacy branch in your original repo, which will still have its bug10 unchanged (i.e. still starting from the 2.0 tag, and not replayed anywhere like we did on the rebaseRepo.
In this cloned repo, I track the origin/legacy branch, in order to merge on it the legacy branch of another remote source: the rebaseRepo.

PS D:\git\tests\rebaseRepo> cd ..
PS D:\git\tests> git clone mainRepo finalRepo
PS D:\git\tests> cd finalRepo

PS D:\git\tests\finalRepo> git co -b legacy origin/legacy

In this original repo (I only cloned it to not mess with the state of the mainRepo, in case I had some other experiments to do), I will declare rebaseRepo as a remote, and fetch its branches.

PS D:\git\tests\finalRepo> git remote add rebasedRepo D:/git/tests/rebaseRepo
PS D:\git\tests\finalRepo> type D:\git\tests\finalRepo\.git\config
[remote "origin"]
    fetch = +refs/heads/*:refs/remotes/origin/*
    url = D:/git/tests/mainRepo
[branch "master"]
    remote = origin
    merge = refs/heads/master
[branch "legacy"]
    remote = origin
    merge = refs/heads/legacy
[remote "rebasedRepo"]
    url = D:/git/tests/rebaseRepo
    fetch = +refs/heads/*:refs/remotes/rebasedRepo/*

PS D:\git\tests\finalRepo> git fetch rebasedRepo
remote: Counting objects: 8, done.
remote: Compressing objects: 100% (6/6), done.
remote: Total 6 (delta 3), reused 0 (delta 0)
Unpacking objects: 100% (6/6), done.
From D:/git/tests/rebaseRepo
 * [new branch]      bug10      -> rebasedRepo/bug10
 * [new branch]      legacy     -> rebasedRepo/legacy
 * [new branch]      master     -> rebasedRepo/master

We can now update legacy without touching to bug10:

PS D:\git\tests\finalRepo> git merge rebasedRepo/legacy
Updating dbcc7aa..4919b68
Fast forward
 toBeFixedFile.txt |  Bin 38 -> 104 bytes
 1 files changed, 0 insertions(+), 0 deletions(-)

You can repeat the process as many time as you want, whenever new bug10 commits need to be replayed on top of an old legacy branch, without including all the intermediate commits.