Squashing or rebasing while branching

Question

I'm starting a new branch from my master branch. But I do not want the whole history on the new branch. I would like only 1 commit to start the branch.

I have created the new branch and tried to rebase all the commits but it does not work since I don't have a previous commit. I don't understand that stuff "previous commit"...

Maybe it would be better squashing the commits. But my new branch is already merged with the master when created.

I don't know how to do that. Do you have an idea?

Answer 1

You can do that using a little plumbing:

tree=$(git log -1 --format=%T master)
commit=$(git commit-tree $tree -m "initial commit")
git checkout -b new_branch $commit

Explanation:

1. Extract only the tree (data) from the current commit on master, because that's what you're really interested in. The SHA1 of the tree object is printed by git log -1 --format=%T, and saved to $tree

2. Create a commit out of that tree that has no parents. This is done with the plumbing command git commit-tree, which takes a tree object and a commit message, and creates a commit object. It normally also takes the parent commit(s), but in this case we don't want a parent, so we don't give one. The command outputs the SHA1 of the resulting commit, which we save to $commit.

3. The third, easiest step, is to create a branch with the new commit as a starting point, done with the usual git checkout -b.

EDIT: A much simpler way to do this is using git checkout --orphan new_branch master followed by git commit -m "Initial commit", with exactly the same results. I'm leaving the above explanation for educational purposes because researching it taught me a few details about git. Also, do note that git makes this hard to do for a reason - history tracing is highly encouraged - so if you're doing this, be sure to have a good reason.

Answer 2

As I see you don't understand what a branch is. A branch just a pointer to a commit. When you create a branch from master you don't create any commits, the new branch just points exactly at the same commit as the master, they both represent the same. So, there is nothing to squash or rebase yet, and there is no history yet.

Only after you do some changes and commit them on a branch, then your question will make a sense: how to merge all changes made in a branch as one commit into master? Just use git merge --squash branch.