How To: Branches: merge versus rebase
related commands:
merge rebase
I know, there is a big debate around this question. But if we look at how works these commands, the difference is big and there usages are clear. Let’s see it.
Here is the schematic of the repository before merging / rebasing:
e --- f --- g sandbox
/
a --- b --- c--- d master
Merge
When you apply the $ git merge sandbox at the master HEAD, the result will be:
e --- f --- g sandbox
/ \
a --- b --- c --- d --- h master
The h commit will contain changes from d and g, and the commit history is preserved.
In the facts, Git will apply e, f and g to d and store the result in a new commit h.
Rebase
When you apply the $ git rebase master at the sandbox HEAD, the result will be:
e' -- f' -- g' sandbox
/
a --- b --- c --- d master
At this point, g' will contain changes from g and d, but the commits e, f and g doesn’t exist anymore.
In the facts, Git will apply c and d to e and create e', to f and create f' then to g to create g'
Differences
So yes, g' and h will be the same if we look the source codes, but there positions in history are tottally different!
As abstract, we can tell:
mergemerges branches into a new commit
rebasechange the start point of a branch
Usages
Let’s come back to the previous post.
The repository can have two schematics: Schematic A:
e --- f --- g sandbox
/
a --- b --- c--- d master
Schematic B:
e --- f --- g sandbox
/
a --- b master
If you want to keep the branch in the logs, merge is the command to use. But with schematic B, a simple merge will destroy the branch: it’s the fast-forward method.
So, to be sure to keep it in both case, use the option --no-ff:
$ git checkout master
$ git merge --no-ff sandbox
Result A:
e --- f --- g sandbox
/ \
a --- b --- c --- d --- h master
Result B:
e --- f --- g sandbox
/ \
a --- b --------------- c master
If you don’t want to keep the branch in the log, you mustn’t use --no-ff. But with schematic A, a simple merge will keep the branch.
So here are the solutions:
A:
$ git checkout sandbox
$ git rebase master sandbox
e' -- f' -- g' sandbox
/
a --- b --- c --- d master
$ git checkout master
$ git merge sandbox
e' -- f' -- g' sandbox
X
a --- b --- c --- d --- e' --- f' --- g' master
B:
$ git checkout master
$ git merge sandbox
e --- f --- g sandbox
X
a --- b --- e --- f --- g master
In both cases, ‘sandbox’ branches will be abandonned. A $ git gc command will delete them.
Reminder
merge
Merge <branch> in current branch
$ git merge <branch>
Merge <branch> in current branch with ensure that it will remain in logs
$ git merge --no-ff <branch>
Merge <branch> in current branch with ensure that it will remain in logs with a message for the merge commit
$ git merge --no-ff <branch> -m "message"
rebase
Apply commits from <branch> to current branch
$ git rebase <branch>
Modify commits history from the commit <hash>
$ git rebase -i <hash>^