Git rebase and Git rebase –onto

Git rebase and Git rebase –onto

Lately I?ve been doing the Githug tutorial to improve my Git skills. I?ve learned a lot of new Git commands while solving the problems or answering to the questions that you are asked in the tutorial (some of which rather hard!)

The toughest question I had to deal with ? and which took me some time to solve ? was about the command git rebase –onto. I?ve just lately started to become familiar with the command git rebase <branch> but the option –onto added to that same command has complicated things further.

That is why I decided to learn more in depth about the command rebase –onto, but also to enhance my knowledge of the basic command git rebase as I felt it wasn?t clear enough to me, since I had to struggle so much to get the –onto option.

How git rebase works

In a nutshell, git rebase takes the commits of a branch and appends them to the commits of a different branch. The commits to rebase are previously saved into a temporary area and then reapplied to the new branch, one by one, in order. Let?s look at a practical example.

In the case of two different branches below:

Image for post

where other_branch is based on master, we want to copy the commits of other_branch and add them after the last commit of master in order to achieve this result:

Image for post

other_branch now includes all the commits of master. In order to achieve this we have to use the following Git command:

git rebase master other_branch

or just

git rebase master

Both above commands lead to the same result. The only difference is that for the latter example you are in other_branch branch already, whereas in the former example Git first performs an automatic git checkout other_branch before rebasing other_branch to master. After rebasing you automatically are in other_branch branch.

The technical syntax for the Git command rebase is:

git rebase [<upstream>] [<branch>]

In case of conflict, git rebase will stop at the first problematic commit and leave conflict markers in the tree. You can use git diff to locate the markers (<<<<<<) and make edits to resolve the conflict. For each file you edit, you need to tell Git that the conflict has been resolved with the following commands:

git add <file-name>git rebase –continue

Git rebase –onto

The syntax for Git command git rebase ?-onto is:

git rebase [–onto <new base>] [<upstream> [<branch>]]

Let?s say we had a situation like this with three branches:

Image for post

branch_two is based on branch_one but we need to fork it from master. The reason for this could be, for example, that a functionality on which branch_two depends was merged into master and branch_two needs to include the commits master assimilitated after the fork.

We need to achieve something like this:

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The commits of branch_two are rebased on master while the commits of branch_one stay untouched. To achieve this we use the command:

git rebase –onto master branch_one branch_two

Githug exercise

The exercise from Githug that I struggled to figure out is the following:

Image for postImage for postReproduction of Githug problem

other_branch is based on wrong_branch. We want to rebase it to master so that the commits of wrong_branch are not included in its commits.

We want to achieve this:

Image for postHow the Githug problem should be resolved

The Git command to solve the above Githug problem is:

git rebase –onto master wrong_branch other_branch

or, if you?re already in other_branch, just:

git rebase –onto master wrong_branch

master (aka <new base>) becomes the new base for <branch> other_branch which was previously based on wrong_branch (aka <upstream>).

I think the reason why I was confused about this command at the beginning lies in the <upstream> part: I couldn?t figure out why I would have to put wrong_branch in the whole command syntax (as a matter of fact, what I tried to do at the beginning was something like git rebase –onto master other_branch, or the other way round.)

The above command makes more sense now that I was able to define the concept of upstream and the part it plays in the command as a whole, although I feel I have to look more into it and understand its meaning better.

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