git-reference/branching/index.html

---
layout: reference
---

<div class="box">
  <h2>
    <span class="docs">
      <a target="new" href="http://progit.org/book/ch2-2.html">book</a>
    </span>
    Branching and Merging
  </h2>
  <div class="block">
    <p>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 <i>contexts</i> 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.
    </p>

    <p class="nutshell">
    <b>In a nutshell</b> you can create a branch with 
    <code>git branch (branchname)</code>, switch into that context with
    <code>git checkout (branchname)</code>, 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 
    <code>git merge</code>.  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.
    </p>

  </div>
</div>

<div class="box">
  <h2>
    <span class="docs">
      <a target="new" href="http://www.kernel.org/pub/software/scm/git/docs/git-branch.html">docs</a> &nbsp;
      <a target="new" href="http://progit.org/book/">book</a>
    </span>
    <a name="branch">git branch</a>
    <span class="desc">list, create and manage working contexts</span>
  </h2>

  <br/>

  <h2>
    <span class="docs">
      <a target="new" href="http://www.kernel.org/pub/software/scm/git/docs/git-checkout.html">docs</a> &nbsp;
      <a target="new" href="http://progit.org/book/">book</a>
    </span>
    <a name="checkout">git checkout</a>
    <span class="desc">switch to a new branch context</span>
  </h2>

  <div class="block">
    <p>The <code>git branch</code> 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 <code>git checkout</code> here which
    switches you between your branches.
    </p>

    <h4>
      git branch
      <small>list your available branches</small>
    </h4>

    <p>Without arguments, <code>git branch</code> 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 
    <a href="http://progit.org/book/ch7-1.html#colors_in_git">coloring turned on</a>, 
    will show the current branch in green.
    </p>

<pre>
$ git branch
* <span class="green">master</span>
</pre>

    <p>This means that we have a 'master' branch and we are currently on it.
    When you run <code>git init</code> 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.
    </p>

    <h4>
      git branch (branchname)
      <small>create a new branch</small>
    </h4>

    <p>So let's start by creating a new branch and switching to it.  You can do
    that by running <code>git branch (branchname)</code>.

<pre>
$ git branch testing
$ git branch
* <span class="green">master</span>
  testing
</pre>

    <p>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 <code>git checkout (branch)</code> to switch the
    branch we're currently on.
    </p>

<pre>
<b>$ ls</b>
README   hello.rb
<b>$ echo 'test content' > test.txt</b>
<b>$ echo 'more content' > more.txt</b>
<b>$ git add *.txt</b>
<b>$ git commit -m 'added two files'</b>
[master 8bd6d8b] added two files
 2 files changed, 2 insertions(+), 0 deletions(-)
 create mode 100644 more.txt
 create mode 100644 test.txt
<b>$ ls</b>
README   hello.rb more.txt test.txt
<b>$ git checkout testing</b>
Switched to branch 'testing'
<b>$ ls</b>
README   hello.rb
</pre>

    <p>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.</p>

<pre>
<b>$ ls</b>
README   hello.rb
<b>$ git checkout master</b>
Switched to branch 'master'
<b>$ ls</b>
README   hello.rb more.txt test.txt
</pre>

    <h4>
      git checkout -b (branchname)
      <small>create and immediately switch to a branch</small>
    </h4>

    <p>
    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
    <code>git branch newbranch; git checkout newbranch</code>, but Git gives
    you a shortcut for this: <code>git checkout -b newbranch</code>.
    </p>

<pre>
<b>$ git branch</b>
* master
<b>$ ls</b>
README   hello.rb more.txt test.txt
<b>$ git checkout -b removals</b>
Switched to a new branch 'removals'
<b>$ git rm more.txt </b>
rm 'more.txt'
<b>$ git rm test.txt </b>
rm 'test.txt'
<b>$ ls</b>
README   hello.rb
<b>$ git commit -am 'removed useless files'</b>
[removals 8f7c949] removed useless files
 2 files changed, 0 insertions(+), 2 deletions(-)
 delete mode 100644 more.txt
 delete mode 100644 test.txt
<b>$ git checkout master</b>
Switched to branch 'master'
<b>$ ls</b>
README   hello.rb more.txt test.txt
</pre>

    <p>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.</p>

    <p>
    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.</p>
      
    <h4>
      git branch -d (branchname)
      <small>delete a branch</small>
    </h4>

    <p>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 <code>git branch -d (branch)</code> to remove it.

<pre>
<b>$ git branch</b>
* <span class="green">master</span>
  testing
<b>$ git branch -d testing</b>
Deleted branch testing (was 78b2670).
<b>$ git branch</b>
* <span class="green">master</span>
</pre>

    <p class="nutshell">
    <b>In a nutshell</b> you use <code>git branch</code> to list your 
    current branches, create new branches and delete unnecessary or 
    already merged branches.
    </p>

  </div>
</div>

<div class="box">
  <h2>
    <span class="docs">
      <a target="new" href="http://www.kernel.org/pub/software/scm/git/docs/git-merge.html">docs</a> &nbsp;
      <a target="new" href="http://progit.org/book/">book</a>
    </span>
    <a name="merge">git merge</a>
    <span class="desc">merge a branch context into your current one</span>
  </h2>

  <div class="block">
    <p>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 <code>git merge</code> 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.
    </p>

<pre>
<b>$ git branch</b>
* <span class="green">master</span>
  removals
<b>$ ls</b>
README   hello.rb more.txt test.txt
<b>$ git merge removals</b>
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
<b>$ ls</b>
<span class="hl">README   hello.rb</span>
</pre>

    <h4>
      more complex merges
    </h4>

    <p>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.
    </p>

<pre>
<b>$ git branch</b>
* master
<b>$ cat hello.rb </b>
class HelloWorld
  def self.hello
    puts "Hello World"
  end
end

HelloWorld.hello
</pre>

    <p>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'.</p>

<pre>
<b>$ git checkout -b change_class</b>
M hello.rb
Switched to a new branch 'change_class'
<b>$ vim hello.rb </b>
<b>$ head -1 hello.rb </b>
class HiWorld
<b>$ git commit -am 'changed the class name'</b>
[change_class 3467b0a] changed the class name
 1 files changed, 2 insertions(+), 4 deletions(-)
</pre>
    
    <p>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 <code>hello.rb</code> to <code>ruby.rb</code>.
      </b>

<pre>
<b>$ git checkout master</b>
Switched to branch 'master'
<b>$ git mv hello.rb ruby.rb</b>
<b>$ vim ruby.rb </b>
<b>$ git diff</b>
<span class="umber">diff --git a/ruby.rb b/ruby.rb
index 2aabb6e..bf64b17 100644
--- a/ruby.rb
+++ b/ruby.rb</span>
<span class="lblue">@@ -1,7 +1,7 @@</span>
 class HelloWorld
 
   def self.hello
<span class="red">-    puts "Hello World"</span>
<span class="green">+    puts "Hello World from Ruby"</span>
   end
 
 end
<b>$ git commit -am 'added from ruby'</b>
[master b7ae93b] added from ruby
 1 files changed, 1 insertions(+), 1 deletions(-)
 rename hello.rb => ruby.rb (65%)
</pre>

    <p>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?</p>

<pre>
<b>$ git branch</b>
  change_class
* master
<b>$ git merge change_class</b>
Renaming hello.rb => ruby.rb
Auto-merging ruby.rb
Merge made by recursive.
 ruby.rb |    6 ++----
 1 files changed, 2 insertions(+), 4 deletions(-)
<b>$ cat ruby.rb</b>
class HiWorld
  def self.hello
    puts "Hello World from Ruby"
  end
end

HiWorld.hello
</pre>

    <p>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.</p>

    <h4>
      merge conflicts
    </h4>

    <p>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.
    <p> 

<pre>
<b>$ git branch</b>
* master
<b>$ git checkout -b fix_readme</b>
Switched to a new branch 'fix_readme'
<b>$ vim README </b>
<b>$ git commit -am 'fixed readme title'</b>
[fix_readme 3ac015d] fixed readme title
 1 files changed, 1 insertions(+), 1 deletions(-)
</pre>

    <p>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.</p>

<pre>
<b>$ git checkout master</b>
Switched to branch 'master'
<b>$ vim README </b>
<b>$ git commit -am 'fixed readme title differently'</b>
[master 3cbb6aa] fixed readme title differently
 1 files changed, 1 insertions(+), 1 deletions(-)
</pre>

    <p>Now is the fun part - we will merge the first branch into our master
      branch, causing a merge conflict.</p>

<pre>
<b>$ git merge fix_readme</b>
Auto-merging README
CONFLICT (content): Merge conflict in README
Automatic merge failed; fix conflicts and then commit the result.
<b>$ cat README </b>
<<<<<<< HEAD
Many Hello World Examples
=======
Hello World Lang Examples
>>>>>>> fix_readme

This project has examples of hello world in
nearly every programming language.
</pre>

    <p>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
      <a href="http://www.kernel.org/pub/software/scm/git/docs/git-mergetool.html">git mergetool</a>
      if you want Git to fire up a graphical mergetool 
      (like kdiff3, emerge, p4merge, etc) instead.
    </p>

<pre>
<b>$ vim README </b>  <span class="exp"># here I'm fixing the conflict</span>
<b>$ git diff</b>
<span class="umber">diff --cc README
index 9103e27,69cad1a..0000000
--- a/README
+++ b/README</span>
<span class="lblue">@@@ -1,4 -1,4 +1,4 @@@</span>
<span class="red">- Many Hello World Examples</span>
 <span class="red">-Hello World Lang Examples</span>
<span class="green">++Many Hello World Lang Examples</span>
  
  This project has examples of hello world in
</pre>

    <p>A cool tip in doing merge conflict resolution in Git is that if you
    run <code>git diff</code>, 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 <code>git add</code> - 
    to tell Git the file has been resolved, you have to stage it.</p>

<pre>
<b>$ git status -s</b>
UU README
<b>$ git add README </b>
<b>$ git status -s</b>
M  README
<b>$ git commit </b>
[master 8d585ea] Merge branch 'fix_readme'
</pre>
    
    <p>And now we've successfully resolved our merge conflict and committed
    the result.</p>

    <p class="nutshell">
    <b>In a nutshell</b> you use <code>git merge</code> to combine another
    branch context into your current branch.  It automatically figures out
    how to best combine the different snapshots into a new snapshot with the
    unique work of both.
    </p>

  </div>
</div>

<div class="box">
  <h2>
    <span class="docs">
      <a target="new" href="http://www.kernel.org/pub/software/scm/git/docs/git-log.html">docs</a> &nbsp;
      <a target="new" href="http://progit.org/book/">book</a>
    </span>
    <a name="log">git log</a>
    <span class="desc">show commit history of a branch</span>
  </h2>

  <div class="block">

    <p>So far we have been committing snapshots of your project and switching
    between different isolated contexts, but what if we've forgotten how we've
    got to where we are?  Or what if we want to know how one branch differs
    from another?  Git provides a tool that shows you all the commit messages
    that have lead up to the snapshot you are currently on, which is called
    <code>git log</code>.</p>

    <p>To understand the log command, you have to understand what information
    is stored when you run the <code>git commit</code> command to store a 
    snapshot.  In addition to the manifest of files and commit message and 
    information about the person who committed it, Git also stores the commit
    that you based this snapshot on.  That is, if you clone a project, what was
    the snapshot that you modified to get to the snapshot that you saved?  This
    is helpful to give context to how the project got to where it is and allows
    Git to figure out who changed what.  If Git has the snapshot you save and
    the one you based it on, then it can automatically figure out what you
    changed.  The commit that a new commit was based on is called the "parent".
    </p>

    <p>To see a chronological list of the parents of any branch, you can run
    <code>git log</code> when you are in that branch.  For example, if we run
    <code>git log</code> in the Hello World project that we have been working 
    on in this section, we'll see all the commit messages that we've done. 

<pre>
<b>$ git log</b>
<span class="yellow">commit 8d585ea6faf99facd39b55d6f6a3b3f481ad0d3d</span>
Merge: 3cbb6aa 3ac015d
Author: Scott Chacon &lt;schacon@gmail.com>
Date:   Fri Jun 4 12:59:47 2010 +0200

    Merge branch 'fix_readme'
    
    Conflicts:
        README

<span class="yellow">commit 3cbb6aae5c0cbd711c098e113ae436801371c95e</span>
Author: Scott Chacon &lt;schacon@gmail.com>
Date:   Fri Jun 4 12:58:53 2010 +0200

    fixed readme title differently

<span class="yellow">commit 3ac015da8ade34d4c7ebeffa2053fcac33fb495b</span>
Author: Scott Chacon &lt;schacon@gmail.com>
Date:   Fri Jun 4 12:58:36 2010 +0200

    fixed readme title

<span class="yellow">commit 558151a95567ba4181bab5746bc8f34bd87143d6</span>
Merge: b7ae93b 3467b0a
Author: Scott Chacon &lt;schacon@gmail.com>
Date:   Fri Jun 4 12:37:05 2010 +0200

    Merge branch 'change_class'
...
</pre>

    <p>To see a more compact version of the same history, we can use the 
    <code>--oneline</code> option.</p>

<pre>
<b>$ git log --oneline</b>
8d585ea Merge branch 'fix_readme'
3cbb6aa fixed readme title differently
3ac015d fixed readme title
558151a Merge branch 'change_class'
b7ae93b added from ruby
3467b0a changed the class name
17f4acf first commit
</pre>

    <p>What this is telling us is that this is the history of the development
    of this project.  If the commit messages are descriptive, this can inform
    us as to what all changes have been applied or have influenced the current
    state of the snapshot and thus what is in it.</p>

    <p>We can also use it to see when the history was branched and merged with
    the very helpful <code>--graph</code> option.  Here is the same command
    but with the topology graph turned on:</p>

<pre>
<b>$ git log --oneline --graph</b>
*   8d585ea Merge branch 'fix_readme'
|\  
| * 3ac015d fixed readme title
* | 3cbb6aa fixed readme title differently
|/  
*   558151a Merge branch 'change_class'
|\  
| * 3467b0a changed the class name
* | b7ae93b added from ruby
|/  
* 17f4acf first commit
</pre>

    <p>Now we can more clearly see when effort diverged and then was merged
    back together.  This is very nice for seeing what has happened or what 
    changes are applied, but
    it is also incredibly useful for managing your branches.  Let's create a new
    branch, do some work in it and then switch back and do some work in our 
    master branch, then see how the <code>log</code> command can help us figure
    out what is happening on each.</p>

    <p>First we'll create a new branch to add the Erlang programming language
    Hello World example - we want to do this in a branch so that we don't 
    muddy up our stable branch with code that may not work for a while so we
    can cleanly switch in and out of it.</p>

<pre>
<b>$ git checkout -b erlang</b>
Switched to a new branch 'erlang'
<b>$ vim erlang_hw.erl</b>
<b>$ git add erlang_hw.erl </b>
<b>$ git commit -m 'added erlang'</b>
[erlang ab5ab4c] added erlang
 1 files changed, 5 insertions(+), 0 deletions(-)
 create mode 100644 erlang_hw.erl
</pre>

    <p>Since we're having fun playing in functional programming languages we
    get caught up in it and also add a Haskell example program while still in
    the branch named 'erlang'.</p>

<pre>
<b>$ vim haskell.hs</b>
<b>$ git add haskell.hs </b>
<b>$ git commit -m 'added haskell'</b>
[erlang 1834130] added haskell
 1 files changed, 4 insertions(+), 0 deletions(-)
 create mode 100644 haskell.hs
</pre>

    <p>Finally, we decide that we want to change the class name of our Ruby
    program back to the way it was.  So, we can go back to the master branch
    and change that and we decide to just commit it directly in the master
    branch instead of creating another branch.</p>

<pre>
<b>$ git checkout master</b>
Switched to branch 'master'
<b>$ ls</b>
README  ruby.rb
<b>$ vim ruby.rb </b>
<b>$ git commit -am 'reverted to old class name'</b>
[master 594f90b] reverted to old class name
 1 files changed, 2 insertions(+), 2 deletions(-)
</pre>

    <p>So, now say we don't work on the project for a while, we have other
    things to do.  When we come back we want to know what the 'erlang' branch
    is all about and where we've left off on the master branch.  Just by looking
    at the branch name, we can't know that we made Haskell changes in there, but
    using <code>git log</code> we easily can.  If you give Git a branch name, 
    it will show you just the commits that are "reachable" in the history of 
    that branch, that is the commits that influenced the final snapshot.</p>

<pre>
<b>$ git log --oneline erlang</b>
<span class="hl">1834130 added haskell</span>
ab5ab4c added erlang
8d585ea Merge branch 'fix_readme'
3cbb6aa fixed readme title differently
3ac015d fixed readme title
558151a Merge branch 'change_class'
b7ae93b added from ruby
3467b0a changed the class name
17f4acf first commit
</pre>

    <p>This way, it's pretty easy to see that we have Haskell code included in
      the branch (as I've highlighted). What is even cooler is that we can 
      easily tell Git that we only are interested in the commits that are
      reachable in one branch that are not reachable in another, in other words
      which commits are unique to a branch in comparison to another.
    </p>

    <p> 
      In this case if we are interested in merging in the 'erlang' branch we 
      want to see what commits are going to effect our snapshot when we do 
      that merge.  The way we tell Git that is by putting a <code>^</code> in
      front of the branch that we don't want to see.  For instance, if we want
      to see the commits that are in the 'erlang' branch that are not in the
      'master' branch, we can do <code>erlang ^master</code>, or vice versa.
    </p>

<pre>
<b>$ git log --oneline erlang ^master</b>
1834130 added haskell
ab5ab4c added erlang
<b>$ git log --oneline master ^erlang</b>
594f90b reverted to old class name
</pre>

    <p>This gives us a nice, simple branch management tool. It allows us to
      easily see what commits are unique to which branches so we know what
      we're missing and what we would be merging in if we were to do a merge.
    </p>

    <p class="nutshell">
    <b>In a nutshell</b> you use <code>git log</code> to list out the commit
    history or list of changes people have made that have lead to the snapshot
    at the tip of the branch.  This allows you to see how the project in that
    context got to the state that it is currently in.
    </p>
    
  </div>
</div>

<p><a href="/remotes">On to Sharing and Updating Projects &#187;</a></p>