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Branching and Merging
# master -> D234254; HEAD -> master
git branch abranch # master -> abranch -> D234254; HEAD -> master
Creates a new handle for referencing a commit-tree. By default, it will point to the same one your current working branch is pointing to.
A branch allows us to isolate changes and still have a reference to our master branch or any other important set of changes. We can quickly switch out and work on one set of changes and move back and work on that with ease.
git checkout abranch # HEAD -> abranch
Replaces your folder's contents with the contents of the commit-tree that abranch references.
Git's HEAD now points to this branch.
Note: any untracked files will remain and any staged files remain in the file cache.
# master -> D254644; HEAD -> master
git checkout -b abranch # abranch -> master -> D254644; HEAD -> abranch
Creates a new branch and makes it the working branch.
git add . # G343657 commit-tree created; G343657.parent -> D254644
git commit -m "Fixes for feature #35" # abranch -> G343657
Here the same thing happens os any other commit. All the file and tree objects that were cached by the add command are pointed to by the branch file simply pointing to the commit-tree object. The only difference here is that HEAD points to abranch rather than master and it is abranch that is updating its hash and not master.
# master -> D143424; HEAD -> master
git checkout -b abranch # abranch -> master -> D143424; HEAD -> abranch
git add . # G154547 commit-tree created; G154547.parent -> D143424
git commit -m "abranch work" # abranch -> G154547
git checkout master # HEAD -> master
git merge abranch # master -> G154547
When the master branch has not changed since we branched from it, we simply change master branch to point to the commit-tree that abranch points to.
master -> A143421; HEAD -> master
abranch -> master -> A143421
# master -> D143424; HEAD -> master
git checkout -b abranch # abranch -> master -> D143424; HEAD -> abranch
git add . # G154547 commit-tree created; G154547.parent -> D143424
git commit -m "abranch work" # abranch -> G154547
git checkout master # HEAD -> master
git add . # J154510 commit-tree created; J154510.parent -> D143424
git commit -m "master work" # master -> J154510
git merge abranch # N124512 commit-tree created; N124512.parents -> J154510, G154547; master -> N124512
When the master branch has changed since we branched from it. Git finds the common ancestor between master and abranch, then applies changes to it.
D143424 common ancestor commit-tree
C143423 tree
hello.txt A153531
goodbye.txt B153532
``G154547`` abranch work commit-tree
F143426 tree
hello.txt E153535 hello is updated
goodbye.txt B153532
``J154510`` abranch work commit-tree
K143409 tree
hello.txt A153531
goodbye.txt I153548 goodbye is updated
``N124512`` merge work commit-tree
L143411 tree
hello.txt E153535 hello is updated
goodbye.txt I153548 goodbye is updated
The only thing the merge had to do here was switch out hash-references. There are more complex conflict-less merge possibilities, like if there were changes in the same file. But this gives a nice taste about how merges work.
If there are conflicts, the process is much the same. Files with conflicts use a standard merge text within the file where there are conflicts.
Typical markup:
hello.txt
<<<<<<< HEAD:hello.txt
Hi.
=======
Hello.
>>>>>>> abranch:hello.txt
There are merge tools you can use.
git mergetool
or hand edit the file.
git merge abranch --squash # N124512 commit-tree created; N124512.parent -> D143424; master (still) -> D143424
This will apply changes from abranch into your working branch, but not commit them. When you do commit, it will not include the history of commits you see in abranch.
git branch
Lists your branches
git branch -v
Lists your branches along with information on each branch's last commit
git branch --merged
Lists which branches have been merged into the current working branch already