How to create the branch from specific commit in different branch

Question

I have made several commits in the master branch and then merged them to dev branch.

I want to create a branch from a specific commit in dev branch, which was first committed in master branch.

I used the commands:

git checkout dev
git branch  <branch name> <commit id>

However, this creates the branch from master branch, not the dev branch I expected. The commit id is same in master branch and dev branch. So, how can I distinguish same commit id in different branch?

PS: I made an example in github here https://github.com/RolandXu/test_for_branch

I used the commands:

git checkout dev
git branch test 07aeec983bfc17c25f0b0a7c1d47da8e35df7af8

What I expect is that the test branch contains aa.txt bb.txt cc.txt. However, the test branch only contains aa.txt and cc.txt. It most likely created the branch from the master branch.

Answer 1

If you are using this form of the branch command (with start point), it does not matter where your HEAD is.

What you are doing:

git checkout dev
git branch test 07aeec983bfc17c25f0b0a7c1d47da8e35df7af8

• First, you set your HEAD to the branch dev,

• Second, you start a new branch on commit 07aeec98. There is no bb.txt at this commit (according to your github repo).

If you want to start a new branch at the location you have just checked out, you can either run branch with no start point:

git branch test

or as other have answered, branch and checkout there in one operation:

git checkout -b test

---

I think that you might be confused by that fact that 07aeec98 is part of the branch dev. It is true that this commit is an ancestor of dev, its changes are needed to reach the latest commit in dev. However, they are other commits that are needed to reach the latest dev, and these are not necessarily in the history of 07aeec98.

8480e8ae (where you added bb.txt) is for example not in the history of 07aeec98. If you branch from 07aeec98, you won't get the changes introduced by 8480e8ae.

In other words: if you merge branch A and branch B into branch C, then create a new branch on a commit of A, you won't get the changes introduced in B.

Same here, you had two parallel branches master and dev, which you merged in dev. Branching out from a commit of master (older than the merge) won't provide you with the changes of dev.

---

If you want to permanently integrate new changes from master into your feature branches, you should merge master into them and go on. This will create merge commits in your feature branches, though.

If you have not published your feature branches, you can also rebase them on the updated master: git rebase master featureA. Be prepared to solve possible conflicts.

If you want a workflow where you can work on feature branches free of merge commits and still integrate with newer changes in master, I recommend the following:

• base every new feature branch on a commit of master
• create a dev branch on a commit of master
• when you need to see how your feature branch integrates with new changes in master, merge both master and the feature branch into dev.

Do not commit into dev directly, use it only for merging other branches.

For example, if you are working on feature A and B:

a---b---c---d---e---f---g -master
    \       \
     \       \-x -featureB
      \
       \-j---k -featureA

Merge branches into a dev branch to check if they work well with the new master:

a---b---c---d---e---f---g -master
    \       \            \
     \       \            \--x'---k' -dev
      \       \             /    /   
       \       \-x----------    /    -featureB
        \                      /
         \-j---k--------------- -featureA

You can continue working on your feature branches, and keep merging in new changes from both master and feature branches into dev regularly.

a---b---c---d---e---f---g---h---i----- -master
    \       \            \            \
     \       \            \--x'---k'---i'---l' -dev
      \       \             /    /         /
       \       \-x----------    /         /  -featureB
        \                      /         /  
         \-j---k-----------------l------ -featureA

When it is time to integrate the new features, merge the feature branches (not dev!) into master.