Branching in Git is one of my favorite features. If you have used other version control systems, it's probably helpful to forget most of what you think about branches - in fact, it may be more helpful to think of them practically as contexts since that is how you will most often be using them. When you checkout different branches, you change contexts that you are working in and you can quickly context-switch back and forth between several different branches.
In a nutshell you can create a branch with
git branch (branchname), switch into that context with
git checkout (branchname), record commit snapshots while
in that context, then can switch back and forth easily. When you switch
branches, Git replaces your working directory with the snapshot of the
latest commit on that branch so you don't have to have multiple directories
for multiple branches. You merge branches together with
git merge. You can easily merge multiple times from the same
branch over time, or alternately you can choose to delete a branch
immediately after merging it.
The git branch command is a general branch management tool
for Git and can do several different things. We'll cover the basic ones
that you'll use most - listing branches, creating branches and deleting
branches. We will also cover basic git checkout here which
switches you between your branches.
Without arguments, git branch will list out the local
branches that you have. The branch that you are currently working on will
have a star next to it and if you have
coloring turned on,
will show the current branch in green.
$ git branch
* master
This means that we have a 'master' branch and we are currently on it.
When you run git init it will automatically create a 'master'
branch for you by default, however there is nothing special about the name -
you don't actually have to have a 'master' branch but since it's the default
that is created, most projects do.
So let's start by creating a new branch and switching to it. You can do
that by running git branch (branchname).
$ git branch testing
$ git branch
* master
testing
Now we can see that we have a new branch. When you create a branch this
way it creates the branch at your last commit so if you record some commits
at this point and then switch to 'testing', it will revert your working
directory context back to when you created the branch in the first place -
you can think of it like a bookmark for where you currently are. Let's see
this in action - we use git checkout (branch) to switch the
branch we're currently on.
$ ls README hello.rb $ echo 'test content' > test.txt $ echo 'more content' > more.txt $ git add *.txt $ git commit -m 'added two files' [master 8bd6d8b] added two files 2 files changed, 2 insertions(+), 0 deletions(-) create mode 100644 more.txt create mode 100644 test.txt $ ls README hello.rb more.txt test.txt $ git checkout testing Switched to branch 'testing' $ ls README hello.rb
So now we can see that when we switch to the 'testing' branch, our new files were removed. We could switch back to the 'master' branch and see them re-appear.
$ ls README hello.rb $ git checkout master Switched to branch 'master' $ ls README hello.rb more.txt test.txt
In most cases you will be wanting to switch to the branch immediately, so
you can do work in it and then merging into a branch that only contains
stable work (such as 'master') at a later point when the work in your new
context branch is stable. You can do this pretty easily with
git branch newbranch; git checkout newbranch, but Git gives
you a shortcut for this: git checkout -b newbranch.
$ git branch * master $ ls README hello.rb more.txt test.txt $ git checkout -b removals Switched to a new branch 'removals' $ git rm more.txt rm 'more.txt' $ git rm test.txt rm 'test.txt' $ ls README hello.rb $ git commit -am 'removed useless files' [removals 8f7c949] removed useless files 2 files changed, 0 insertions(+), 2 deletions(-) delete mode 100644 more.txt delete mode 100644 test.txt $ git checkout master Switched to branch 'master' $ ls README hello.rb more.txt test.txt
You can see there how we created a branch, removed some of our files while in the context of that branch, then switched back to our main branch and we see the files return. Branching safely isolates work that we do into contexts we can switch between.
If you start on work it is very useful to always start it in a branch (because it's fast and easy to do) and then merge it in and delete the branch when you're done. That way if what you're working on doesn't work out you can easily discard it and if you're forced to switch back to a more stable context your work in progress is easy to put aside and then come back to.
If we want to delete a branch (such as the 'testing' branch in the
previous example, since there is no unique work on it),
we can run git branch -d (branch) to remove it.
$ git branch * master testing $ git branch -d testing Deleted branch testing (was 78b2670). $ git branch * master
In a nutshell you use git branch to list your
current branches, create new branches and delete unnecessary or
already merged branches.
Once you have work isolated in a branch, you will eventually want to
incorporate it into your main branch. You can merge any branch into your
current branch with the git merge command. Let's take as a
simple example the 'removals' branch from above. If we create a branch
and remove files in it and commit our removals to that branch, it is
isolated from our main ('master', in this case) branch. To include those
deletions in your 'master' branch, you can just merge in the 'removals'
branch.
$ git branch * master removals $ ls README hello.rb more.txt test.txt $ git merge removals Updating 8bd6d8b..8f7c949 Fast-forward more.txt | 1 - test.txt | 1 - 2 files changed, 0 insertions(+), 2 deletions(-) delete mode 100644 more.txt delete mode 100644 test.txt $ ls README hello.rb
Of course, this doesn't just work for simple file additions and deletions. Git will merge file modifications as well - in fact, it's very good at it. For example, let's see what happens when we edit a file in one branch and in another branch we rename it and then edit it and then merge these branches together. Chaos, you say? Let's see.
$ git branch
* master
$ cat hello.rb
class HelloWorld
def self.hello
puts "Hello World"
end
end
HelloWorld.hello
So first we're going to create a new branch named 'change_class' and switch to it so your class renaming changes are isolated. I'm going to change each instance of 'HelloWorld' to 'HiWorld'.
$ git checkout -b change_class M hello.rb Switched to a new branch 'change_class' $ vim hello.rb $ head -1 hello.rb class HiWorld $ git commit -am 'changed the class name' [change_class 3467b0a] changed the class name 1 files changed, 2 insertions(+), 4 deletions(-)
So now I've committed the class renaming changes to the 'change_class'
branch. If I now switch back to the 'master' branch my class name will
revert to what it was before I switched branches. Here I can change
something different (in this case the printed output) and at the same
time rename the file from hello.rb to ruby.rb.
$ git checkout master Switched to branch 'master' $ git mv hello.rb ruby.rb $ vim ruby.rb $ git diff diff --git a/ruby.rb b/ruby.rb index 2aabb6e..bf64b17 100644 --- a/ruby.rb +++ b/ruby.rb @@ -1,7 +1,7 @@ class HelloWorld def self.hello - puts "Hello World" + puts "Hello World from Ruby" end end $ git commit -am 'added from ruby' [master b7ae93b] added from ruby 1 files changed, 1 insertions(+), 1 deletions(-) rename hello.rb => ruby.rb (65%)
Now those changes are recorded in my 'master' branch. Notice that the class name is back to 'HelloWorld', not 'HiWorld'. Now I want to incorporate the 'HiWorld' change so I can just merge in my 'change_class' branch. However, I've changed the name of the file since I branched, what will Git do?
$ git branch
change_class
* master
$ git merge change_class
Renaming hello.rb => ruby.rb
Auto-merging ruby.rb
Merge made by recursive.
ruby.rb | 6 ++----
1 files changed, 2 insertions(+), 4 deletions(-)
$ cat ruby.rb
class HiWorld
def self.hello
puts "Hello World from Ruby"
end
end
HiWorld.hello
Well, it will just figure it out. Notice that I had no merge conflicts and the file that had been renamed now has the 'HiWorld' class name change that was done in the other branch. Pretty cool.
So, Git merges are magical, we never ever have to deal with merge conflicts again, right? Not quite. In situations where the same block of code is edited in different branches there is no way for a computer to figure it out, so it's up to us. Let's see another example of changing the same line in two branches.
$ git branch * master $ git checkout -b fix_readme Switched to a new branch 'fix_readme' $ vim README $ git commit -am 'fixed readme title' [fix_readme 3ac015d] fixed readme title 1 files changed, 1 insertions(+), 1 deletions(-)
Now we have committed a change to one line in our README file in a branch. Now let's change the same line in a different way back on our 'master' branch.
$ git checkout master Switched to branch 'master' $ vim README $ git commit -am 'fixed readme title differently' [master 3cbb6aa] fixed readme title differently 1 files changed, 1 insertions(+), 1 deletions(-)
Now is the fun part - we will merge the first branch into our master branch, causing a merge conflict.
$ git merge fix_readme Auto-merging README CONFLICT (content): Merge conflict in README Automatic merge failed; fix conflicts and then commit the result. $ cat README <<<<<<< HEAD Many Hello World Examples ======= Hello World Lang Examples >>>>>>> fix_readme This project has examples of hello world in nearly every programming language.
You can see that Git inserts standard merge conflict markers, much like Subversion, into files when it gets a merge conflict. Now it's up to us to resolve them. We will do it manually here, but check out git mergetool if you want Git to fire up a graphical mergetool (like kdiff3, emerge, p4merge, etc) instead.
$ vim README # here I'm fixing the conflict $ git diff diff --cc README index 9103e27,69cad1a..0000000 --- a/README +++ b/README @@@ -1,4 -1,4 +1,4 @@@ - Many Hello World Examples -Hello World Lang Examples ++Many Hello World Lang Examples This project has examples of hello world in
A cool tip in doing merge conflict resolution in Git is that if you
run git diff, it will show you both sides of the conflict
and how you've resolved it as I've shown here. Now it's time to mark
the file as resolved. In Git we do that with git add -
to tell Git the file has been resolved, you have to stage it.
$ git status -s UU README $ git add README $ git status -s M README $ git commit [master 8d585ea] Merge branch 'fix_readme'
And now we've successfully resolved our merge conflict and committed the result.