Logo Questions Linux Laravel Mysql Ubuntu Git Menu
 

Finding a branch point with Git?

Tags:

git

branch

I have a repository with branches master and A and lots of merge activity between the two. How can I find the commit in my repository when branch A was created based on master?

My repository basically looks like this:

-- X -- A -- B -- C -- D -- F  (master)            \     /   \     /            \   /     \   /              G -- H -- I -- J  (branch A) 

I'm looking for revision A, which is not what git merge-base (--all) finds.

like image 248
Jochen Avatar asked Oct 06 '09 18:10

Jochen


People also ask

Where do git branches point?

A branch in Git is simply a lightweight movable pointer to one of these commits. The default branch name in Git is master . As you start making commits, you're given a master branch that points to the last commit you made. Every time you commit, the master branch pointer moves forward automatically.

How do you check from which branch a branch is created git?

You can use git branch --contains to list all the branches descended from the tip of develop , then use grep to make sure feature is among them. If it is among them, it will print " feature" to standard output and have a return code of 0.

How do you find the branch of a commit?

It is based on "Find merge commit which include a specific commit". Find when a commit was merged into one or more branches. Find the merge commit that brought COMMIT into the specified BRANCH(es). Specifically, look for the oldest commit on the first-parent history of BRANCH that contains the COMMIT as an ancestor.


2 Answers

I was looking for the same thing, and I found this question. Thank you for asking it!

However, I found that the answers I see here don't seem to quite give the answer you asked for (or that I was looking for) -- they seem to give the G commit, instead of the A commit.

So, I've created the following tree (letters assigned in chronological order), so I could test things out:

A - B - D - F - G   <- "master" branch (at G)      \   \     /       C - E --'     <- "topic" branch (still at E) 

This looks a little different than yours, because I wanted to make sure that I got (referring to this graph, not yours) B, but not A (and not D or E). Here are the letters attached to SHA prefixes and commit messages (my repo can be cloned from here, if that's interesting to anyone):

G: a9546a2 merge from topic back to master F: e7c863d commit on master after master was merged to topic E: 648ca35 merging master onto topic D: 37ad159 post-branch commit on master C: 132ee2a first commit on topic branch B: 6aafd7f second commit on master before branching A: 4112403 initial commit on master 

So, the goal: find B. Here are three ways that I found, after a bit of tinkering:


1. visually, with gitk:

You should visually see a tree like this (as viewed from master):

gitk screen capture from master

or here (as viewed from topic):

gitk screen capture from topic

in both cases, I've selected the commit that is B in my graph. Once you click on it, its full SHA is presented in a text input field just below the graph.


2. visually, but from the terminal:

git log --graph --oneline --all

(Edit/side-note: adding --decorate can also be interesting; it adds an indication of branch names, tags, etc. Not adding this to the command-line above since the output below doesn't reflect its use.)

which shows (assuming git config --global color.ui auto):

output of git log --graph --oneline --all

Or, in straight text:

 *   a9546a2 merge from topic back to master |\   | *   648ca35 merging master onto topic | |\   | * | 132ee2a first commit on topic branch * | | e7c863d commit on master after master was merged to topic | |/   |/|    * | 37ad159 post-branch commit on master |/   * 6aafd7f second commit on master before branching * 4112403 initial commit on master 

in either case, we see the 6aafd7f commit as the lowest common point, i.e. B in my graph, or A in yours.


3. With shell magic:

You don't specify in your question whether you wanted something like the above, or a single command that'll just get you the one revision, and nothing else. Well, here's the latter:

diff -u <(git rev-list --first-parent topic) \              <(git rev-list --first-parent master) | \      sed -ne 's/^ //p' | head -1 6aafd7ff98017c816033df18395c5c1e7829960d 

Which you can also put into your ~/.gitconfig as (note: trailing dash is important; thanks Brian for bringing attention to that):

[alias]     oldest-ancestor = !zsh -c 'diff -u <(git rev-list --first-parent "${1:-master}") <(git rev-list --first-parent "${2:-HEAD}") | sed -ne \"s/^ //p\" | head -1' - 

Which could be done via the following (convoluted with quoting) command-line:

git config --global alias.oldest-ancestor '!zsh -c '\''diff -u <(git rev-list --first-parent "${1:-master}") <(git rev-list --first-parent "${2:-HEAD}") | sed -ne "s/^ //p" | head -1'\'' -' 

Note: zsh could just as easily have been bash, but sh will not work -- the <() syntax doesn't exist in vanilla sh. (Thank you again, @conny, for making me aware of it in a comment on another answer on this page!)

Note: Alternate version of the above:

Thanks to liori for pointing out that the above could fall down when comparing identical branches, and coming up with an alternate diff form which removes the sed form from the mix, and makes this "safer" (i.e. it returns a result (namely, the most recent commit) even when you compare master to master):

As a .git-config line:

[alias]     oldest-ancestor = !zsh -c 'diff --old-line-format='' --new-line-format='' <(git rev-list --first-parent "${1:-master}") <(git rev-list --first-parent "${2:-HEAD}") | head -1' - 

From the shell:

git config --global alias.oldest-ancestor '!zsh -c '\''diff --old-line-format='' --new-line-format='' <(git rev-list --first-parent "${1:-master}") <(git rev-list --first-parent "${2:-HEAD}") | head -1'\'' -' 

So, in my test tree (which was unavailable for a while, sorry; it's back), that now works on both master and topic (giving commits G and B, respectively). Thanks again, liori, for the alternate form.


So, that's what I [and liori] came up with. It seems to work for me. It also allows an additional couple of aliases that might prove handy:

git config --global alias.branchdiff '!sh -c "git diff `git oldest-ancestor`.."' git config --global alias.branchlog '!sh -c "git log `git oldest-ancestor`.."' 

Happy git-ing!

like image 129
lindes Avatar answered Sep 18 '22 14:09

lindes


You may be looking for git merge-base:

git merge-base finds best common ancestor(s) between two commits to use in a three-way merge. One common ancestor is better than another common ancestor if the latter is an ancestor of the former. A common ancestor that does not have any better common ancestor is a best common ancestor, i.e. a merge base. Note that there can be more than one merge base for a pair of commits.

like image 33
Greg Hewgill Avatar answered Sep 18 '22 14:09

Greg Hewgill