gitcore-tutorial.html - pub/scm/git/git-htmldocs - Git at Google

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 <body class="manpage">
 <div id="header">
 <h1>
 gitcore-tutorial(7) Manual Page
 </h1>
 <h2>NAME</h2>
 <div class="sectionbody">
 <p>gitcore-tutorial -
    A Git core tutorial for developers
 </p>
 </div>
 </div>
 <div id="content">
 <div class="sect1">
 <h2 id="_synopsis">SYNOPSIS</h2>
 <div class="sectionbody">
 <div class="paragraph"><p>git *</p></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_description">DESCRIPTION</h2>
 <div class="sectionbody">
 <div class="paragraph"><p>This tutorial explains how to use the "core" Git commands to set up and
 work with a Git repository.</p></div>
 <div class="paragraph"><p>If you just need to use Git as a revision control system you may prefer
 to start with "A Tutorial Introduction to Git" (<a href="gittutorial.html">gittutorial(7)</a>) or
 <a href="user-manual.html">the Git User Manual</a>.</p></div>
 <div class="paragraph"><p>However, an understanding of these low-level tools can be helpful if
 you want to understand Git&#8217;s internals.</p></div>
 <div class="paragraph"><p>The core Git is often called "plumbing", with the prettier user
 interfaces on top of it called "porcelain". You may not want to use the
 plumbing directly very often, but it can be good to know what the
 plumbing does when the porcelain isn&#8217;t flushing.</p></div>
 <div class="paragraph"><p>Back when this document was originally written, many porcelain
 commands were shell scripts. For simplicity, it still uses them as
 examples to illustrate how plumbing is fit together to form the
 porcelain commands. The source tree includes some of these scripts in
 contrib/examples/ for reference. Although these are not implemented as
 shell scripts anymore, the description of what the plumbing layer
 commands do is still valid.</p></div>
 <div class="admonitionblock">
 <table><tr>
 <td class="icon">
 <div class="title">Note</div>
 </td>
 <td class="content">Deeper technical details are often marked as Notes, which you can
 skip on your first reading.</td>
 </tr></table>
 </div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_creating_a_git_repository">Creating a Git repository</h2>
 <div class="sectionbody">
 <div class="paragraph"><p>Creating a new Git repository couldn&#8217;t be easier: all Git repositories start
 out empty, and the only thing you need to do is find yourself a
 subdirectory that you want to use as a working tree - either an empty
 one for a totally new project, or an existing working tree that you want
 to import into Git.</p></div>
 <div class="paragraph"><p>For our first example, we&#8217;re going to start a totally new repository from
 scratch, with no pre-existing files, and we&#8217;ll call it <em>git-tutorial</em>.
 To start up, create a subdirectory for it, change into that
 subdirectory, and initialize the Git infrastructure with <em>git init</em>:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ mkdir git-tutorial
 $ cd git-tutorial
 $ git init</code></pre>
 </div></div>
 <div class="paragraph"><p>to which Git will reply</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>Initialized empty Git repository in .git/</code></pre>
 </div></div>
 <div class="paragraph"><p>which is just Git&#8217;s way of saying that you haven&#8217;t been doing anything
 strange, and that it will have created a local <code>.git</code> directory setup for
 your new project. You will now have a <code>.git</code> directory, and you can
 inspect that with <em>ls</em>. For your new empty project, it should show you
 three entries, among other things:</p></div>
 <div class="ulist"><ul>
 <li>
 <p>
 a file called <code>HEAD</code>, that has <code>ref: refs/heads/master</code> in it.
    This is similar to a symbolic link and points at
    <code>refs/heads/master</code> relative to the <code>HEAD</code> file.
 </p>
 <div class="paragraph"><p>Don&#8217;t worry about the fact that the file that the <code>HEAD</code> link points to
 doesn&#8217;t even exist yet&#8201;&#8212;&#8201;you haven&#8217;t created the commit that will
 start your <code>HEAD</code> development branch yet.</p></div>
 </li>
 <li>
 <p>
 a subdirectory called <code>objects</code>, which will contain all the
    objects of your project. You should never have any real reason to
    look at the objects directly, but you might want to know that these
    objects are what contains all the real <em>data</em> in your repository.
 </p>
 </li>
 <li>
 <p>
 a subdirectory called <code>refs</code>, which contains references to objects.
 </p>
 </li>
 </ul></div>
 <div class="paragraph"><p>In particular, the <code>refs</code> subdirectory will contain two other
 subdirectories, named <code>heads</code> and <code>tags</code> respectively. They do
 exactly what their names imply: they contain references to any number
 of different <em>heads</em> of development (aka <em>branches</em>), and to any
 <em>tags</em> that you have created to name specific versions in your
 repository.</p></div>
 <div class="paragraph"><p>One note: the special <code>master</code> head is the default branch, which is
 why the <code>.git/HEAD</code> file was created points to it even if it
 doesn&#8217;t yet exist. Basically, the <code>HEAD</code> link is supposed to always
 point to the branch you are working on right now, and you always
 start out expecting to work on the <code>master</code> branch.</p></div>
 <div class="paragraph"><p>However, this is only a convention, and you can name your branches
 anything you want, and don&#8217;t have to ever even <em>have</em> a <code>master</code>
 branch. A number of the Git tools will assume that <code>.git/HEAD</code> is
 valid, though.</p></div>
 <div class="admonitionblock">
 <table><tr>
 <td class="icon">
 <div class="title">Note</div>
 </td>
 <td class="content">An <em>object</em> is identified by its 160-bit SHA-1 hash, aka <em>object name</em>,
 and a reference to an object is always the 40-byte hex
 representation of that SHA-1 name. The files in the <code>refs</code>
 subdirectory are expected to contain these hex references
 (usually with a final <code>\n</code> at the end), and you should thus
 expect to see a number of 41-byte files containing these
 references in these <code>refs</code> subdirectories when you actually start
 populating your tree.</td>
 </tr></table>
 </div>
 <div class="admonitionblock">
 <table><tr>
 <td class="icon">
 <div class="title">Note</div>
 </td>
 <td class="content">An advanced user may want to take a look at <a href="gitrepository-layout.html">gitrepository-layout(5)</a>
 after finishing this tutorial.</td>
 </tr></table>
 </div>
 <div class="paragraph"><p>You have now created your first Git repository. Of course, since it&#8217;s
 empty, that&#8217;s not very useful, so let&#8217;s start populating it with data.</p></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_populating_a_git_repository">Populating a Git repository</h2>
 <div class="sectionbody">
 <div class="paragraph"><p>We&#8217;ll keep this simple and stupid, so we&#8217;ll start off with populating a
 few trivial files just to get a feel for it.</p></div>
 <div class="paragraph"><p>Start off with just creating any random files that you want to maintain
 in your Git repository. We&#8217;ll start off with a few bad examples, just to
 get a feel for how this works:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ echo "Hello World" &gt;hello
 $ echo "Silly example" &gt;example</code></pre>
 </div></div>
 <div class="paragraph"><p>you have now created two files in your working tree (aka <em>working directory</em>),
 but to actually check in your hard work, you will have to go through two steps:</p></div>
 <div class="ulist"><ul>
 <li>
 <p>
 fill in the <em>index</em> file (aka <em>cache</em>) with the information about your
    working tree state.
 </p>
 </li>
 <li>
 <p>
 commit that index file as an object.
 </p>
 </li>
 </ul></div>
 <div class="paragraph"><p>The first step is trivial: when you want to tell Git about any changes
 to your working tree, you use the <em>git update-index</em> program. That
 program normally just takes a list of filenames you want to update, but
 to avoid trivial mistakes, it refuses to add new entries to the index
 (or remove existing ones) unless you explicitly tell it that you&#8217;re
 adding a new entry with the <code>--add</code> flag (or removing an entry with the
 <code>--remove</code>) flag.</p></div>
 <div class="paragraph"><p>So to populate the index with the two files you just created, you can do</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git update-index --add hello example</code></pre>
 </div></div>
 <div class="paragraph"><p>and you have now told Git to track those two files.</p></div>
 <div class="paragraph"><p>In fact, as you did that, if you now look into your object directory,
 you&#8217;ll notice that Git will have added two new objects to the object
 database. If you did exactly the steps above, you should now be able to do</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ ls .git/objects/??/*</code></pre>
 </div></div>
 <div class="paragraph"><p>and see two files:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>.git/objects/55/7db03de997c86a4a028e1ebd3a1ceb225be238
 .git/objects/f2/4c74a2e500f5ee1332c86b94199f52b1d1d962</code></pre>
 </div></div>
 <div class="paragraph"><p>which correspond with the objects with names of <code>557db...</code> and
 <code>f24c7...</code> respectively.</p></div>
 <div class="paragraph"><p>If you want to, you can use <em>git cat-file</em> to look at those objects, but
 you&#8217;ll have to use the object name, not the filename of the object:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git cat-file -t 557db03de997c86a4a028e1ebd3a1ceb225be238</code></pre>
 </div></div>
 <div class="paragraph"><p>where the <code>-t</code> tells <em>git cat-file</em> to tell you what the "type" of the
 object is. Git will tell you that you have a "blob" object (i.e., just a
 regular file), and you can see the contents with</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git cat-file blob 557db03</code></pre>
 </div></div>
 <div class="paragraph"><p>which will print out "Hello World". The object <code>557db03</code> is nothing
 more than the contents of your file <code>hello</code>.</p></div>
 <div class="admonitionblock">
 <table><tr>
 <td class="icon">
 <div class="title">Note</div>
 </td>
 <td class="content">Don&#8217;t confuse that object with the file <code>hello</code> itself. The
 object is literally just those specific <strong>contents</strong> of the file, and
 however much you later change the contents in file <code>hello</code>, the object
 we just looked at will never change. Objects are immutable.</td>
 </tr></table>
 </div>
 <div class="admonitionblock">
 <table><tr>
 <td class="icon">
 <div class="title">Note</div>
 </td>
 <td class="content">The second example demonstrates that you can
 abbreviate the object name to only the first several
 hexadecimal digits in most places.</td>
 </tr></table>
 </div>
 <div class="paragraph"><p>Anyway, as we mentioned previously, you normally never actually take a
 look at the objects themselves, and typing long 40-character hex
 names is not something you&#8217;d normally want to do. The above digression
 was just to show that <em>git update-index</em> did something magical, and
 actually saved away the contents of your files into the Git object
 database.</p></div>
 <div class="paragraph"><p>Updating the index did something else too: it created a <code>.git/index</code>
 file. This is the index that describes your current working tree, and
 something you should be very aware of. Again, you normally never worry
 about the index file itself, but you should be aware of the fact that
 you have not actually really "checked in" your files into Git so far,
 you&#8217;ve only <strong>told</strong> Git about them.</p></div>
 <div class="paragraph"><p>However, since Git knows about them, you can now start using some of the
 most basic Git commands to manipulate the files or look at their status.</p></div>
 <div class="paragraph"><p>In particular, let&#8217;s not even check in the two files into Git yet, we&#8217;ll
 start off by adding another line to <code>hello</code> first:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ echo "It's a new day for git" &gt;&gt;hello</code></pre>
 </div></div>
 <div class="paragraph"><p>and you can now, since you told Git about the previous state of <code>hello</code>, ask
 Git what has changed in the tree compared to your old index, using the
 <em>git diff-files</em> command:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git diff-files</code></pre>
 </div></div>
 <div class="paragraph"><p>Oops. That wasn&#8217;t very readable. It just spit out its own internal
 version of a <em>diff</em>, but that internal version really just tells you
 that it has noticed that "hello" has been modified, and that the old object
 contents it had have been replaced with something else.</p></div>
 <div class="paragraph"><p>To make it readable, we can tell <em>git diff-files</em> to output the
 differences as a patch, using the <code>-p</code> flag:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git diff-files -p
 diff --git a/hello b/hello
 index 557db03..263414f 100644
 --- a/hello
 +++ b/hello
 @@ -1 +1,2 @@
  Hello World
 +It's a new day for git</code></pre>
 </div></div>
 <div class="paragraph"><p>i.e. the diff of the change we caused by adding another line to <code>hello</code>.</p></div>
 <div class="paragraph"><p>In other words, <em>git diff-files</em> always shows us the difference between
 what is recorded in the index, and what is currently in the working
 tree. That&#8217;s very useful.</p></div>
 <div class="paragraph"><p>A common shorthand for <code>git diff-files -p</code> is to just write <code>git
 diff</code>, which will do the same thing.</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git diff
 diff --git a/hello b/hello
 index 557db03..263414f 100644
 --- a/hello
 +++ b/hello
 @@ -1 +1,2 @@
  Hello World
 +It's a new day for git</code></pre>
 </div></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_committing_git_state">Committing Git state</h2>
 <div class="sectionbody">
 <div class="paragraph"><p>Now, we want to go to the next stage in Git, which is to take the files
 that Git knows about in the index, and commit them as a real tree. We do
 that in two phases: creating a <em>tree</em> object, and committing that <em>tree</em>
 object as a <em>commit</em> object together with an explanation of what the
 tree was all about, along with information of how we came to that state.</p></div>
 <div class="paragraph"><p>Creating a tree object is trivial, and is done with <em>git write-tree</em>.
 There are no options or other input: <code>git write-tree</code> will take the
 current index state, and write an object that describes that whole
 index. In other words, we&#8217;re now tying together all the different
 filenames with their contents (and their permissions), and we&#8217;re
 creating the equivalent of a Git "directory" object:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git write-tree</code></pre>
 </div></div>
 <div class="paragraph"><p>and this will just output the name of the resulting tree, in this case
 (if you have done exactly as I&#8217;ve described) it should be</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>8988da15d077d4829fc51d8544c097def6644dbb</code></pre>
 </div></div>
 <div class="paragraph"><p>which is another incomprehensible object name. Again, if you want to,
 you can use <code>git cat-file -t 8988d...</code> to see that this time the object
 is not a "blob" object, but a "tree" object (you can also use
 <code>git cat-file</code> to actually output the raw object contents, but you&#8217;ll see
 mainly a binary mess, so that&#8217;s less interesting).</p></div>
 <div class="paragraph"><p>However&#8201;&#8212;&#8201;normally you&#8217;d never use <em>git write-tree</em> on its own, because
 normally you always commit a tree into a commit object using the
 <em>git commit-tree</em> command. In fact, it&#8217;s easier to not actually use
 <em>git write-tree</em> on its own at all, but to just pass its result in as an
 argument to <em>git commit-tree</em>.</p></div>
 <div class="paragraph"><p><em>git commit-tree</em> normally takes several arguments&#8201;&#8212;&#8201;it wants to know
 what the <em>parent</em> of a commit was, but since this is the first commit
 ever in this new repository, and it has no parents, we only need to pass in
 the object name of the tree. However, <em>git commit-tree</em> also wants to get a
 commit message on its standard input, and it will write out the resulting
 object name for the commit to its standard output.</p></div>
 <div class="paragraph"><p>And this is where we create the <code>.git/refs/heads/master</code> file
 which is pointed at by <code>HEAD</code>. This file is supposed to contain
 the reference to the top-of-tree of the master branch, and since
 that&#8217;s exactly what <em>git commit-tree</em> spits out, we can do this
 all with a sequence of simple shell commands:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ tree=$(git write-tree)
 $ commit=$(echo 'Initial commit' | git commit-tree $tree)
 $ git update-ref HEAD $commit</code></pre>
 </div></div>
 <div class="paragraph"><p>In this case this creates a totally new commit that is not related to
 anything else. Normally you do this only <strong>once</strong> for a project ever, and
 all later commits will be parented on top of an earlier commit.</p></div>
 <div class="paragraph"><p>Again, normally you&#8217;d never actually do this by hand. There is a
 helpful script called <code>git commit</code> that will do all of this for you. So
 you could have just written <code>git commit</code>
 instead, and it would have done the above magic scripting for you.</p></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_making_a_change">Making a change</h2>
 <div class="sectionbody">
 <div class="paragraph"><p>Remember how we did the <em>git update-index</em> on file <code>hello</code> and then we
 changed <code>hello</code> afterward, and could compare the new state of <code>hello</code> with the
 state we saved in the index file?</p></div>
 <div class="paragraph"><p>Further, remember how I said that <em>git write-tree</em> writes the contents
 of the <strong>index</strong> file to the tree, and thus what we just committed was in
 fact the <strong>original</strong> contents of the file <code>hello</code>, not the new ones. We did
 that on purpose, to show the difference between the index state, and the
 state in the working tree, and how they don&#8217;t have to match, even
 when we commit things.</p></div>
 <div class="paragraph"><p>As before, if we do <code>git diff-files -p</code> in our git-tutorial project,
 we&#8217;ll still see the same difference we saw last time: the index file
 hasn&#8217;t changed by the act of committing anything. However, now that we
 have committed something, we can also learn to use a new command:
 <em>git diff-index</em>.</p></div>
 <div class="paragraph"><p>Unlike <em>git diff-files</em>, which showed the difference between the index
 file and the working tree, <em>git diff-index</em> shows the differences
 between a committed <strong>tree</strong> and either the index file or the working
 tree. In other words, <em>git diff-index</em> wants a tree to be diffed
 against, and before we did the commit, we couldn&#8217;t do that, because we
 didn&#8217;t have anything to diff against.</p></div>
 <div class="paragraph"><p>But now we can do</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git diff-index -p HEAD</code></pre>
 </div></div>
 <div class="paragraph"><p>(where <code>-p</code> has the same meaning as it did in <em>git diff-files</em>), and it
 will show us the same difference, but for a totally different reason.
 Now we&#8217;re comparing the working tree not against the index file,
 but against the tree we just wrote. It just so happens that those two
 are obviously the same, so we get the same result.</p></div>
 <div class="paragraph"><p>Again, because this is a common operation, you can also just shorthand
 it with</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git diff HEAD</code></pre>
 </div></div>
 <div class="paragraph"><p>which ends up doing the above for you.</p></div>
 <div class="paragraph"><p>In other words, <em>git diff-index</em> normally compares a tree against the
 working tree, but when given the <code>--cached</code> flag, it is told to
 instead compare against just the index cache contents, and ignore the
 current working tree state entirely. Since we just wrote the index
 file to HEAD, doing <code>git diff-index --cached -p HEAD</code> should thus return
 an empty set of differences, and that&#8217;s exactly what it does.</p></div>
 <div class="admonitionblock">
 <table><tr>
 <td class="icon">
 <div class="title">Note</div>
 </td>
 <td class="content">
 <div class="paragraph"><p><em>git diff-index</em> really always uses the index for its
 comparisons, and saying that it compares a tree against the working
 tree is thus not strictly accurate. In particular, the list of
 files to compare (the "meta-data") <strong>always</strong> comes from the index file,
 regardless of whether the <code>--cached</code> flag is used or not. The <code>--cached</code>
 flag really only determines whether the file <strong>contents</strong> to be compared
 come from the working tree or not.</p></div>
 <div class="paragraph"><p>This is not hard to understand, as soon as you realize that Git simply
 never knows (or cares) about files that it is not told about
 explicitly. Git will never go <strong>looking</strong> for files to compare, it
 expects you to tell it what the files are, and that&#8217;s what the index
 is there for.</p></div>
 </td>
 </tr></table>
 </div>
 <div class="paragraph"><p>However, our next step is to commit the <strong>change</strong> we did, and again, to
 understand what&#8217;s going on, keep in mind the difference between "working
 tree contents", "index file" and "committed tree". We have changes
 in the working tree that we want to commit, and we always have to
 work through the index file, so the first thing we need to do is to
 update the index cache:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git update-index hello</code></pre>
 </div></div>
 <div class="paragraph"><p>(note how we didn&#8217;t need the <code>--add</code> flag this time, since Git knew
 about the file already).</p></div>
 <div class="paragraph"><p>Note what happens to the different <em>git diff-&#42;</em> versions here.
 After we&#8217;ve updated <code>hello</code> in the index, <code>git diff-files -p</code> now shows no
 differences, but <code>git diff-index -p HEAD</code> still <strong>does</strong> show that the
 current state is different from the state we committed. In fact, now
 <em>git diff-index</em> shows the same difference whether we use the <code>--cached</code>
 flag or not, since now the index is coherent with the working tree.</p></div>
 <div class="paragraph"><p>Now, since we&#8217;ve updated <code>hello</code> in the index, we can commit the new
 version. We could do it by writing the tree by hand again, and
 committing the tree (this time we&#8217;d have to use the <code>-p HEAD</code> flag to
 tell commit that the HEAD was the <strong>parent</strong> of the new commit, and that
 this wasn&#8217;t an initial commit any more), but you&#8217;ve done that once
 already, so let&#8217;s just use the helpful script this time:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git commit</code></pre>
 </div></div>
 <div class="paragraph"><p>which starts an editor for you to write the commit message and tells you
 a bit about what you have done.</p></div>
 <div class="paragraph"><p>Write whatever message you want, and all the lines that start with <em>#</em>
 will be pruned out, and the rest will be used as the commit message for
 the change. If you decide you don&#8217;t want to commit anything after all at
 this point (you can continue to edit things and update the index), you
 can just leave an empty message. Otherwise <code>git commit</code> will commit
 the change for you.</p></div>
 <div class="paragraph"><p>You&#8217;ve now made your first real Git commit. And if you&#8217;re interested in
 looking at what <code>git commit</code> really does, feel free to investigate:
 it&#8217;s a few very simple shell scripts to generate the helpful (?) commit
 message headers, and a few one-liners that actually do the
 commit itself (<em>git commit</em>).</p></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_inspecting_changes">Inspecting Changes</h2>
 <div class="sectionbody">
 <div class="paragraph"><p>While creating changes is useful, it&#8217;s even more useful if you can tell
 later what changed. The most useful command for this is another of the
 <em>diff</em> family, namely <em>git diff-tree</em>.</p></div>
 <div class="paragraph"><p><em>git diff-tree</em> can be given two arbitrary trees, and it will tell you the
 differences between them. Perhaps even more commonly, though, you can
 give it just a single commit object, and it will figure out the parent
 of that commit itself, and show the difference directly. Thus, to get
 the same diff that we&#8217;ve already seen several times, we can now do</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git diff-tree -p HEAD</code></pre>
 </div></div>
 <div class="paragraph"><p>(again, <code>-p</code> means to show the difference as a human-readable patch),
 and it will show what the last commit (in <code>HEAD</code>) actually changed.</p></div>
 <div class="admonitionblock">
 <table><tr>
 <td class="icon">
 <div class="title">Note</div>
 </td>
 <td class="content">
 <div class="paragraph"><p>Here is an ASCII art by Jon Loeliger that illustrates how
 various <em>diff-&#42;</em> commands compare things.</p></div>
 <div class="literalblock">
 <div class="content">
 <pre><code>            diff-tree
              +----+
              |    |
              |    |
              V    V
           +-----------+
           | Object DB |
           |  Backing  |
           |   Store   |
           +-----------+
             ^    ^
             |    |
             |    |  diff-index --cached
             |    |
 diff-index  |    V
             |  +-----------+
             |  |   Index   |
             |  |  "cache"  |
             |  +-----------+
             |    ^
             |    |
             |    |  diff-files
             |    |
             V    V
           +-----------+
           |  Working  |
           | Directory |
           +-----------+</code></pre>
 </div></div>
 </td>
 </tr></table>
 </div>
 <div class="paragraph"><p>More interestingly, you can also give <em>git diff-tree</em> the <code>--pretty</code> flag,
 which tells it to also show the commit message and author and date of the
 commit, and you can tell it to show a whole series of diffs.
 Alternatively, you can tell it to be "silent", and not show the diffs at
 all, but just show the actual commit message.</p></div>
 <div class="paragraph"><p>In fact, together with the <em>git rev-list</em> program (which generates a
 list of revisions), <em>git diff-tree</em> ends up being a veritable fount of
 changes.  You can emulate <code>git log</code>, <code>git log -p</code>, etc. with a trivial
 script that pipes the output of <code>git rev-list</code> to <code>git diff-tree --stdin</code>,
 which was exactly how early versions of <code>git log</code> were implemented.</p></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_tagging_a_version">Tagging a version</h2>
 <div class="sectionbody">
 <div class="paragraph"><p>In Git, there are two kinds of tags, a "light" one, and an "annotated tag".</p></div>
 <div class="paragraph"><p>A "light" tag is technically nothing more than a branch, except we put
 it in the <code>.git/refs/tags/</code> subdirectory instead of calling it a <code>head</code>.
 So the simplest form of tag involves nothing more than</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git tag my-first-tag</code></pre>
 </div></div>
 <div class="paragraph"><p>which just writes the current <code>HEAD</code> into the <code>.git/refs/tags/my-first-tag</code>
 file, after which point you can then use this symbolic name for that
 particular state. You can, for example, do</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git diff my-first-tag</code></pre>
 </div></div>
 <div class="paragraph"><p>to diff your current state against that tag which at this point will
 obviously be an empty diff, but if you continue to develop and commit
 stuff, you can use your tag as an "anchor-point" to see what has changed
 since you tagged it.</p></div>
 <div class="paragraph"><p>An "annotated tag" is actually a real Git object, and contains not only a
 pointer to the state you want to tag, but also a small tag name and
 message, along with optionally a PGP signature that says that yes,
 you really did
 that tag. You create these annotated tags with either the <code>-a</code> or
 <code>-s</code> flag to <em>git tag</em>:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git tag -s &lt;tagname&gt;</code></pre>
 </div></div>
 <div class="paragraph"><p>which will sign the current <code>HEAD</code> (but you can also give it another
 argument that specifies the thing to tag, e.g., you could have tagged the
 current <code>mybranch</code> point by using <code>git tag &lt;tagname&gt; mybranch</code>).</p></div>
 <div class="paragraph"><p>You normally only do signed tags for major releases or things
 like that, while the light-weight tags are useful for any marking you
 want to do&#8201;&#8212;&#8201;any time you decide that you want to remember a certain
 point, just create a private tag for it, and you have a nice symbolic
 name for the state at that point.</p></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_copying_repositories">Copying repositories</h2>
 <div class="sectionbody">
 <div class="paragraph"><p>Git repositories are normally totally self-sufficient and relocatable.
 Unlike CVS, for example, there is no separate notion of
 "repository" and "working tree". A Git repository normally <strong>is</strong> the
 working tree, with the local Git information hidden in the <code>.git</code>
 subdirectory. There is nothing else. What you see is what you got.</p></div>
 <div class="admonitionblock">
 <table><tr>
 <td class="icon">
 <div class="title">Note</div>
 </td>
 <td class="content">You can tell Git to split the Git internal information from
 the directory that it tracks, but we&#8217;ll ignore that for now: it&#8217;s not
 how normal projects work, and it&#8217;s really only meant for special uses.
 So the mental model of "the Git information is always tied directly to
 the working tree that it describes" may not be technically 100%
 accurate, but it&#8217;s a good model for all normal use.</td>
 </tr></table>
 </div>
 <div class="paragraph"><p>This has two implications:</p></div>
 <div class="ulist"><ul>
 <li>
 <p>
 if you grow bored with the tutorial repository you created (or you&#8217;ve
    made a mistake and want to start all over), you can just do simple
 </p>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ rm -rf git-tutorial</code></pre>
 </div></div>
 <div class="paragraph"><p>and it will be gone. There&#8217;s no external repository, and there&#8217;s no
 history outside the project you created.</p></div>
 </li>
 <li>
 <p>
 if you want to move or duplicate a Git repository, you can do so. There
    is <em>git clone</em> command, but if all you want to do is just to
    create a copy of your repository (with all the full history that
    went along with it), you can do so with a regular
    <code>cp -a git-tutorial new-git-tutorial</code>.
 </p>
 <div class="paragraph"><p>Note that when you&#8217;ve moved or copied a Git repository, your Git index
 file (which caches various information, notably some of the "stat"
 information for the files involved) will likely need to be refreshed.
 So after you do a <code>cp -a</code> to create a new copy, you&#8217;ll want to do</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git update-index --refresh</code></pre>
 </div></div>
 <div class="paragraph"><p>in the new repository to make sure that the index file is up to date.</p></div>
 </li>
 </ul></div>
 <div class="paragraph"><p>Note that the second point is true even across machines. You can
 duplicate a remote Git repository with <strong>any</strong> regular copy mechanism, be it
 <em>scp</em>, <em>rsync</em> or <em>wget</em>.</p></div>
 <div class="paragraph"><p>When copying a remote repository, you&#8217;ll want to at a minimum update the
 index cache when you do this, and especially with other peoples'
 repositories you often want to make sure that the index cache is in some
 known state (you don&#8217;t know <strong>what</strong> they&#8217;ve done and not yet checked in),
 so usually you&#8217;ll precede the <em>git update-index</em> with a</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git read-tree --reset HEAD
 $ git update-index --refresh</code></pre>
 </div></div>
 <div class="paragraph"><p>which will force a total index re-build from the tree pointed to by <code>HEAD</code>.
 It resets the index contents to <code>HEAD</code>, and then the <em>git update-index</em>
 makes sure to match up all index entries with the checked-out files.
 If the original repository had uncommitted changes in its
 working tree, <code>git update-index --refresh</code> notices them and
 tells you they need to be updated.</p></div>
 <div class="paragraph"><p>The above can also be written as simply</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git reset</code></pre>
 </div></div>
 <div class="paragraph"><p>and in fact a lot of the common Git command combinations can be scripted
 with the <code>git xyz</code> interfaces.  You can learn things by just looking
 at what the various git scripts do.  For example, <code>git reset</code> used to be
 the above two lines implemented in <em>git reset</em>, but some things like
 <em>git status</em> and <em>git commit</em> are slightly more complex scripts around
 the basic Git commands.</p></div>
 <div class="paragraph"><p>Many (most?) public remote repositories will not contain any of
 the checked out files or even an index file, and will <strong>only</strong> contain the
 actual core Git files. Such a repository usually doesn&#8217;t even have the
 <code>.git</code> subdirectory, but has all the Git files directly in the
 repository.</p></div>
 <div class="paragraph"><p>To create your own local live copy of such a "raw" Git repository, you&#8217;d
 first create your own subdirectory for the project, and then copy the
 raw repository contents into the <code>.git</code> directory. For example, to
 create your own copy of the Git repository, you&#8217;d do the following</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ mkdir my-git
 $ cd my-git
 $ rsync -rL rsync://rsync.kernel.org/pub/scm/git/git.git/ .git</code></pre>
 </div></div>
 <div class="paragraph"><p>followed by</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git read-tree HEAD</code></pre>
 </div></div>
 <div class="paragraph"><p>to populate the index. However, now you have populated the index, and
 you have all the Git internal files, but you will notice that you don&#8217;t
 actually have any of the working tree files to work on. To get
 those, you&#8217;d check them out with</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git checkout-index -u -a</code></pre>
 </div></div>
 <div class="paragraph"><p>where the <code>-u</code> flag means that you want the checkout to keep the index
 up to date (so that you don&#8217;t have to refresh it afterward), and the
 <code>-a</code> flag means "check out all files" (if you have a stale copy or an
 older version of a checked out tree you may also need to add the <code>-f</code>
 flag first, to tell <em>git checkout-index</em> to <strong>force</strong> overwriting of any old
 files).</p></div>
 <div class="paragraph"><p>Again, this can all be simplified with</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git clone git://git.kernel.org/pub/scm/git/git.git/ my-git
 $ cd my-git
 $ git checkout</code></pre>
 </div></div>
 <div class="paragraph"><p>which will end up doing all of the above for you.</p></div>
 <div class="paragraph"><p>You have now successfully copied somebody else&#8217;s (mine) remote
 repository, and checked it out.</p></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_creating_a_new_branch">Creating a new branch</h2>
 <div class="sectionbody">
 <div class="paragraph"><p>Branches in Git are really nothing more than pointers into the Git
 object database from within the <code>.git/refs/</code> subdirectory, and as we
 already discussed, the <code>HEAD</code> branch is nothing but a symlink to one of
 these object pointers.</p></div>
 <div class="paragraph"><p>You can at any time create a new branch by just picking an arbitrary
 point in the project history, and just writing the SHA-1 name of that
 object into a file under <code>.git/refs/heads/</code>. You can use any filename you
 want (and indeed, subdirectories), but the convention is that the
 "normal" branch is called <code>master</code>. That&#8217;s just a convention, though,
 and nothing enforces it.</p></div>
 <div class="paragraph"><p>To show that as an example, let&#8217;s go back to the git-tutorial repository we
 used earlier, and create a branch in it. You do that by simply just
 saying that you want to check out a new branch:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git checkout -b mybranch</code></pre>
 </div></div>
 <div class="paragraph"><p>will create a new branch based at the current <code>HEAD</code> position, and switch
 to it.</p></div>
 <div class="admonitionblock">
 <table><tr>
 <td class="icon">
 <div class="title">Note</div>
 </td>
 <td class="content">
 <div class="paragraph"><p>If you make the decision to start your new branch at some
 other point in the history than the current <code>HEAD</code>, you can do so by
 just telling <em>git checkout</em> what the base of the checkout would be.
 In other words, if you have an earlier tag or branch, you&#8217;d just do</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git checkout -b mybranch earlier-commit</code></pre>
 </div></div>
 <div class="paragraph"><p>and it would create the new branch <code>mybranch</code> at the earlier commit,
 and check out the state at that time.</p></div>
 </td>
 </tr></table>
 </div>
 <div class="paragraph"><p>You can always just jump back to your original <code>master</code> branch by doing</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git checkout master</code></pre>
 </div></div>
 <div class="paragraph"><p>(or any other branch-name, for that matter) and if you forget which
 branch you happen to be on, a simple</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ cat .git/HEAD</code></pre>
 </div></div>
 <div class="paragraph"><p>will tell you where it&#8217;s pointing.  To get the list of branches
 you have, you can say</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git branch</code></pre>
 </div></div>
 <div class="paragraph"><p>which used to be nothing more than a simple script around <code>ls .git/refs/heads</code>.
 There will be an asterisk in front of the branch you are currently on.</p></div>
 <div class="paragraph"><p>Sometimes you may wish to create a new branch <em>without</em> actually
 checking it out and switching to it. If so, just use the command</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git branch &lt;branchname&gt; [startingpoint]</code></pre>
 </div></div>
 <div class="paragraph"><p>which will simply <em>create</em> the branch, but will not do anything further.
 You can then later&#8201;&#8212;&#8201;once you decide that you want to actually develop
 on that branch&#8201;&#8212;&#8201;switch to that branch with a regular <em>git checkout</em>
 with the branchname as the argument.</p></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_merging_two_branches">Merging two branches</h2>
 <div class="sectionbody">
 <div class="paragraph"><p>One of the ideas of having a branch is that you do some (possibly
 experimental) work in it, and eventually merge it back to the main
 branch. So assuming you created the above <code>mybranch</code> that started out
 being the same as the original <code>master</code> branch, let&#8217;s make sure we&#8217;re in
 that branch, and do some work there.</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git checkout mybranch
 $ echo "Work, work, work" &gt;&gt;hello
 $ git commit -m "Some work." -i hello</code></pre>
 </div></div>
 <div class="paragraph"><p>Here, we just added another line to <code>hello</code>, and we used a shorthand for
 doing both <code>git update-index hello</code> and <code>git commit</code> by just giving the
 filename directly to <code>git commit</code>, with an <code>-i</code> flag (it tells
 Git to <em>include</em> that file in addition to what you have done to
 the index file so far when making the commit).  The <code>-m</code> flag is to give the
 commit log message from the command line.</p></div>
 <div class="paragraph"><p>Now, to make it a bit more interesting, let&#8217;s assume that somebody else
 does some work in the original branch, and simulate that by going back
 to the master branch, and editing the same file differently there:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git checkout master</code></pre>
 </div></div>
 <div class="paragraph"><p>Here, take a moment to look at the contents of <code>hello</code>, and notice how they
 don&#8217;t contain the work we just did in <code>mybranch</code>&#8201;&#8212;&#8201;because that work
 hasn&#8217;t happened in the <code>master</code> branch at all. Then do</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ echo "Play, play, play" &gt;&gt;hello
 $ echo "Lots of fun" &gt;&gt;example
 $ git commit -m "Some fun." -i hello example</code></pre>
 </div></div>
 <div class="paragraph"><p>since the master branch is obviously in a much better mood.</p></div>
 <div class="paragraph"><p>Now, you&#8217;ve got two branches, and you decide that you want to merge the
 work done. Before we do that, let&#8217;s introduce a cool graphical tool that
 helps you view what&#8217;s going on:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ gitk --all</code></pre>
 </div></div>
 <div class="paragraph"><p>will show you graphically both of your branches (that&#8217;s what the <code>--all</code>
 means: normally it will just show you your current <code>HEAD</code>) and their
 histories. You can also see exactly how they came to be from a common
 source.</p></div>
 <div class="paragraph"><p>Anyway, let&#8217;s exit <em>gitk</em> (<code>^Q</code> or the File menu), and decide that we want
 to merge the work we did on the <code>mybranch</code> branch into the <code>master</code>
 branch (which is currently our <code>HEAD</code> too). To do that, there&#8217;s a nice
 script called <em>git merge</em>, which wants to know which branches you want
 to resolve and what the merge is all about:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git merge -m "Merge work in mybranch" mybranch</code></pre>
 </div></div>
 <div class="paragraph"><p>where the first argument is going to be used as the commit message if
 the merge can be resolved automatically.</p></div>
 <div class="paragraph"><p>Now, in this case we&#8217;ve intentionally created a situation where the
 merge will need to be fixed up by hand, though, so Git will do as much
 of it as it can automatically (which in this case is just merge the <code>example</code>
 file, which had no differences in the <code>mybranch</code> branch), and say:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>        Auto-merging hello
         CONFLICT (content): Merge conflict in hello
         Automatic merge failed; fix conflicts and then commit the result.</code></pre>
 </div></div>
 <div class="paragraph"><p>It tells you that it did an "Automatic merge", which
 failed due to conflicts in <code>hello</code>.</p></div>
 <div class="paragraph"><p>Not to worry. It left the (trivial) conflict in <code>hello</code> in the same form you
 should already be well used to if you&#8217;ve ever used CVS, so let&#8217;s just
 open <code>hello</code> in our editor (whatever that may be), and fix it up somehow.
 I&#8217;d suggest just making it so that <code>hello</code> contains all four lines:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>Hello World
 It's a new day for git
 Play, play, play
 Work, work, work</code></pre>
 </div></div>
 <div class="paragraph"><p>and once you&#8217;re happy with your manual merge, just do a</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git commit -i hello</code></pre>
 </div></div>
 <div class="paragraph"><p>which will very loudly warn you that you&#8217;re now committing a merge
 (which is correct, so never mind), and you can write a small merge
 message about your adventures in <em>git merge</em>-land.</p></div>
 <div class="paragraph"><p>After you&#8217;re done, start up <code>gitk --all</code> to see graphically what the
 history looks like. Notice that <code>mybranch</code> still exists, and you can
 switch to it, and continue to work with it if you want to. The
 <code>mybranch</code> branch will not contain the merge, but next time you merge it
 from the <code>master</code> branch, Git will know how you merged it, so you&#8217;ll not
 have to do <em>that</em> merge again.</p></div>
 <div class="paragraph"><p>Another useful tool, especially if you do not always work in X-Window
 environment, is <code>git show-branch</code>.</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git show-branch --topo-order --more=1 master mybranch
 * [master] Merge work in mybranch
  ! [mybranch] Some work.
 --
 -  [master] Merge work in mybranch
 *+ [mybranch] Some work.
 *  [master^] Some fun.</code></pre>
 </div></div>
 <div class="paragraph"><p>The first two lines indicate that it is showing the two branches
 with the titles of their top-of-the-tree commits, you are currently on
 <code>master</code> branch (notice the asterisk <code>*</code> character), and the first
 column for the later output lines is used to show commits contained in the
 <code>master</code> branch, and the second column for the <code>mybranch</code>
 branch. Three commits are shown along with their titles.
 All of them have non blank characters in the first column (<code>*</code>
 shows an ordinary commit on the current branch, <code>-</code> is a merge commit), which
 means they are now part of the <code>master</code> branch. Only the "Some
 work" commit has the plus <code>+</code> character in the second column,
 because <code>mybranch</code> has not been merged to incorporate these
 commits from the master branch.  The string inside brackets
 before the commit log message is a short name you can use to
 name the commit.  In the above example, <em>master</em> and <em>mybranch</em>
 are branch heads.  <em>master^</em> is the first parent of <em>master</em>
 branch head.  Please see <a href="gitrevisions.html">gitrevisions(7)</a> if you want to
 see more complex cases.</p></div>
 <div class="admonitionblock">
 <table><tr>
 <td class="icon">
 <div class="title">Note</div>
 </td>
 <td class="content">Without the <em>--more=1</em> option, <em>git show-branch</em> would not output the
 <em>[master^]</em> commit, as <em>[mybranch]</em> commit is a common ancestor of
 both <em>master</em> and <em>mybranch</em> tips.  Please see <a href="git-show-branch.html">git-show-branch(1)</a>
 for details.</td>
 </tr></table>
 </div>
 <div class="admonitionblock">
 <table><tr>
 <td class="icon">
 <div class="title">Note</div>
 </td>
 <td class="content">If there were more commits on the <em>master</em> branch after the merge, the
 merge commit itself would not be shown by <em>git show-branch</em> by
 default.  You would need to provide <code>--sparse</code> option to make the
 merge commit visible in this case.</td>
 </tr></table>
 </div>
 <div class="paragraph"><p>Now, let&#8217;s pretend you are the one who did all the work in
 <code>mybranch</code>, and the fruit of your hard work has finally been merged
 to the <code>master</code> branch. Let&#8217;s go back to <code>mybranch</code>, and run
 <em>git merge</em> to get the "upstream changes" back to your branch.</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git checkout mybranch
 $ git merge -m "Merge upstream changes." master</code></pre>
 </div></div>
 <div class="paragraph"><p>This outputs something like this (the actual commit object names
 would be different)</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>Updating from ae3a2da... to a80b4aa....
 Fast-forward (no commit created; -m option ignored)
  example | 1 +
  hello   | 1 +
  2 files changed, 2 insertions(+)</code></pre>
 </div></div>
 <div class="paragraph"><p>Because your branch did not contain anything more than what had
 already been merged into the <code>master</code> branch, the merge operation did
 not actually do a merge. Instead, it just updated the top of
 the tree of your branch to that of the <code>master</code> branch. This is
 often called <em>fast-forward</em> merge.</p></div>
 <div class="paragraph"><p>You can run <code>gitk --all</code> again to see how the commit ancestry
 looks like, or run <em>show-branch</em>, which tells you this.</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git show-branch master mybranch
 ! [master] Merge work in mybranch
  * [mybranch] Merge work in mybranch
 --
 -- [master] Merge work in mybranch</code></pre>
 </div></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_merging_external_work">Merging external work</h2>
 <div class="sectionbody">
 <div class="paragraph"><p>It&#8217;s usually much more common that you merge with somebody else than
 merging with your own branches, so it&#8217;s worth pointing out that Git
 makes that very easy too, and in fact, it&#8217;s not that different from
 doing a <em>git merge</em>. In fact, a remote merge ends up being nothing
 more than "fetch the work from a remote repository into a temporary tag"
 followed by a <em>git merge</em>.</p></div>
 <div class="paragraph"><p>Fetching from a remote repository is done by, unsurprisingly,
 <em>git fetch</em>:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git fetch &lt;remote-repository&gt;</code></pre>
 </div></div>
 <div class="paragraph"><p>One of the following transports can be used to name the
 repository to download from:</p></div>
 <div class="dlist"><dl>
 <dt class="hdlist1">
 SSH
 </dt>
 <dd>
 <p>
         <code>remote.machine:/path/to/repo.git/</code> or
 </p>
 <div class="paragraph"><p><code>ssh://remote.machine/path/to/repo.git/</code></p></div>
 <div class="paragraph"><p>This transport can be used for both uploading and downloading,
 and requires you to have a log-in privilege over <code>ssh</code> to the
 remote machine.  It finds out the set of objects the other side
 lacks by exchanging the head commits both ends have and
 transfers (close to) minimum set of objects.  It is by far the
 most efficient way to exchange Git objects between repositories.</p></div>
 </dd>
 <dt class="hdlist1">
 Local directory
 </dt>
 <dd>
 <p>
         <code>/path/to/repo.git/</code>
 </p>
 <div class="paragraph"><p>This transport is the same as SSH transport but uses <em>sh</em> to run
 both ends on the local machine instead of running other end on
 the remote machine via <em>ssh</em>.</p></div>
 </dd>
 <dt class="hdlist1">
 Git Native
 </dt>
 <dd>
 <p>
         <code>git://remote.machine/path/to/repo.git/</code>
 </p>
 <div class="paragraph"><p>This transport was designed for anonymous downloading.  Like SSH
 transport, it finds out the set of objects the downstream side
 lacks and transfers (close to) minimum set of objects.</p></div>
 </dd>
 <dt class="hdlist1">
 HTTP(S)
 </dt>
 <dd>
 <p>
         <code>http://remote.machine/path/to/repo.git/</code>
 </p>
 <div class="paragraph"><p>Downloader from http and https URL
 first obtains the topmost commit object name from the remote site
 by looking at the specified refname under <code>repo.git/refs/</code> directory,
 and then tries to obtain the
 commit object by downloading from <code>repo.git/objects/xx/xxx...</code>
 using the object name of that commit object.  Then it reads the
 commit object to find out its parent commits and the associate
 tree object; it repeats this process until it gets all the
 necessary objects.  Because of this behavior, they are
 sometimes also called <em>commit walkers</em>.</p></div>
 <div class="paragraph"><p>The <em>commit walkers</em> are sometimes also called <em>dumb
 transports</em>, because they do not require any Git aware smart
 server like Git Native transport does.  Any stock HTTP server
 that does not even support directory index would suffice.  But
 you must prepare your repository with <em>git update-server-info</em>
 to help dumb transport downloaders.</p></div>
 </dd>
 </dl></div>
 <div class="paragraph"><p>Once you fetch from the remote repository, you <code>merge</code> that
 with your current branch.</p></div>
 <div class="paragraph"><p>However&#8201;&#8212;&#8201;it&#8217;s such a common thing to <code>fetch</code> and then
 immediately <code>merge</code>, that it&#8217;s called <code>git pull</code>, and you can
 simply do</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git pull &lt;remote-repository&gt;</code></pre>
 </div></div>
 <div class="paragraph"><p>and optionally give a branch-name for the remote end as a second
 argument.</p></div>
 <div class="admonitionblock">
 <table><tr>
 <td class="icon">
 <div class="title">Note</div>
 </td>
 <td class="content">You could do without using any branches at all, by
 keeping as many local repositories as you would like to have
 branches, and merging between them with <em>git pull</em>, just like
 you merge between branches. The advantage of this approach is
 that it lets you keep a set of files for each <code>branch</code> checked
 out and you may find it easier to switch back and forth if you
 juggle multiple lines of development simultaneously. Of
 course, you will pay the price of more disk usage to hold
 multiple working trees, but disk space is cheap these days.</td>
 </tr></table>
 </div>
 <div class="paragraph"><p>It is likely that you will be pulling from the same remote
 repository from time to time. As a short hand, you can store
 the remote repository URL in the local repository&#8217;s config file
 like this:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git config remote.linus.url http://www.kernel.org/pub/scm/git/git.git/</code></pre>
 </div></div>
 <div class="paragraph"><p>and use the "linus" keyword with <em>git pull</em> instead of the full URL.</p></div>
 <div class="paragraph"><p>Examples.</p></div>
 <div class="olist arabic"><ol class="arabic">
 <li>
 <p>
 <code>git pull linus</code>
 </p>
 </li>
 <li>
 <p>
 <code>git pull linus tag v0.99.1</code>
 </p>
 </li>
 </ol></div>
 <div class="paragraph"><p>the above are equivalent to:</p></div>
 <div class="olist arabic"><ol class="arabic">
 <li>
 <p>
 <code>git pull http://www.kernel.org/pub/scm/git/git.git/ HEAD</code>
 </p>
 </li>
 <li>
 <p>
 <code>git pull http://www.kernel.org/pub/scm/git/git.git/ tag v0.99.1</code>
 </p>
 </li>
 </ol></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_how_does_the_merge_work">How does the merge work?</h2>
 <div class="sectionbody">
 <div class="paragraph"><p>We said this tutorial shows what plumbing does to help you cope
 with the porcelain that isn&#8217;t flushing, but we so far did not
 talk about how the merge really works.  If you are following
 this tutorial the first time, I&#8217;d suggest to skip to "Publishing
 your work" section and come back here later.</p></div>
 <div class="paragraph"><p>OK, still with me?  To give us an example to look at, let&#8217;s go
 back to the earlier repository with "hello" and "example" file,
 and bring ourselves back to the pre-merge state:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git show-branch --more=2 master mybranch
 ! [master] Merge work in mybranch
  * [mybranch] Merge work in mybranch
 --
 -- [master] Merge work in mybranch
 +* [master^2] Some work.
 +* [master^] Some fun.</code></pre>
 </div></div>
 <div class="paragraph"><p>Remember, before running <em>git merge</em>, our <code>master</code> head was at
 "Some fun." commit, while our <code>mybranch</code> head was at "Some
 work." commit.</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git checkout mybranch
 $ git reset --hard master^2
 $ git checkout master
 $ git reset --hard master^</code></pre>
 </div></div>
 <div class="paragraph"><p>After rewinding, the commit structure should look like this:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git show-branch
 * [master] Some fun.
  ! [mybranch] Some work.
 --
 *  [master] Some fun.
  + [mybranch] Some work.
 *+ [master^] Initial commit</code></pre>
 </div></div>
 <div class="paragraph"><p>Now we are ready to experiment with the merge by hand.</p></div>
 <div class="paragraph"><p><code>git merge</code> command, when merging two branches, uses 3-way merge
 algorithm.  First, it finds the common ancestor between them.
 The command it uses is <em>git merge-base</em>:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ mb=$(git merge-base HEAD mybranch)</code></pre>
 </div></div>
 <div class="paragraph"><p>The command writes the commit object name of the common ancestor
 to the standard output, so we captured its output to a variable,
 because we will be using it in the next step.  By the way, the common
 ancestor commit is the "Initial commit" commit in this case.  You can
 tell it by:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git name-rev --name-only --tags $mb
 my-first-tag</code></pre>
 </div></div>
 <div class="paragraph"><p>After finding out a common ancestor commit, the second step is
 this:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git read-tree -m -u $mb HEAD mybranch</code></pre>
 </div></div>
 <div class="paragraph"><p>This is the same <em>git read-tree</em> command we have already seen,
 but it takes three trees, unlike previous examples.  This reads
 the contents of each tree into different <em>stage</em> in the index
 file (the first tree goes to stage 1, the second to stage 2,
 etc.).  After reading three trees into three stages, the paths
 that are the same in all three stages are <em>collapsed</em> into stage
 0.  Also paths that are the same in two of three stages are
 collapsed into stage 0, taking the SHA-1 from either stage 2 or
 stage 3, whichever is different from stage 1 (i.e. only one side
 changed from the common ancestor).</p></div>
 <div class="paragraph"><p>After <em>collapsing</em> operation, paths that are different in three
 trees are left in non-zero stages.  At this point, you can
 inspect the index file with this command:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git ls-files --stage
 100644 7f8b141b65fdcee47321e399a2598a235a032422 0       example
 100644 557db03de997c86a4a028e1ebd3a1ceb225be238 1       hello
 100644 ba42a2a96e3027f3333e13ede4ccf4498c3ae942 2       hello
 100644 cc44c73eb783565da5831b4d820c962954019b69 3       hello</code></pre>
 </div></div>
 <div class="paragraph"><p>In our example of only two files, we did not have unchanged
 files so only <em>example</em> resulted in collapsing.  But in real-life
 large projects, when only a small number of files change in one commit,
 this <em>collapsing</em> tends to trivially merge most of the paths
 fairly quickly, leaving only a handful of real changes in non-zero
 stages.</p></div>
 <div class="paragraph"><p>To look at only non-zero stages, use <code>--unmerged</code> flag:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git ls-files --unmerged
 100644 557db03de997c86a4a028e1ebd3a1ceb225be238 1       hello
 100644 ba42a2a96e3027f3333e13ede4ccf4498c3ae942 2       hello
 100644 cc44c73eb783565da5831b4d820c962954019b69 3       hello</code></pre>
 </div></div>
 <div class="paragraph"><p>The next step of merging is to merge these three versions of the
 file, using 3-way merge.  This is done by giving
 <em>git merge-one-file</em> command as one of the arguments to
 <em>git merge-index</em> command:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git merge-index git-merge-one-file hello
 Auto-merging hello
 ERROR: Merge conflict in hello
 fatal: merge program failed</code></pre>
 </div></div>
 <div class="paragraph"><p><em>git merge-one-file</em> script is called with parameters to
 describe those three versions, and is responsible to leave the
 merge results in the working tree.
 It is a fairly straightforward shell script, and
 eventually calls <em>merge</em> program from RCS suite to perform a
 file-level 3-way merge.  In this case, <em>merge</em> detects
 conflicts, and the merge result with conflict marks is left in
 the working tree..  This can be seen if you run <code>ls-files
 --stage</code> again at this point:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git ls-files --stage
 100644 7f8b141b65fdcee47321e399a2598a235a032422 0       example
 100644 557db03de997c86a4a028e1ebd3a1ceb225be238 1       hello
 100644 ba42a2a96e3027f3333e13ede4ccf4498c3ae942 2       hello
 100644 cc44c73eb783565da5831b4d820c962954019b69 3       hello</code></pre>
 </div></div>
 <div class="paragraph"><p>This is the state of the index file and the working file after
 <em>git merge</em> returns control back to you, leaving the conflicting
 merge for you to resolve.  Notice that the path <code>hello</code> is still
 unmerged, and what you see with <em>git diff</em> at this point is
 differences since stage 2 (i.e. your version).</p></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_publishing_your_work">Publishing your work</h2>
 <div class="sectionbody">
 <div class="paragraph"><p>So, we can use somebody else&#8217;s work from a remote repository, but
 how can <strong>you</strong> prepare a repository to let other people pull from
 it?</p></div>
 <div class="paragraph"><p>You do your real work in your working tree that has your
 primary repository hanging under it as its <code>.git</code> subdirectory.
 You <strong>could</strong> make that repository accessible remotely and ask
 people to pull from it, but in practice that is not the way
 things are usually done. A recommended way is to have a public
 repository, make it reachable by other people, and when the
 changes you made in your primary working tree are in good shape,
 update the public repository from it. This is often called
 <em>pushing</em>.</p></div>
 <div class="admonitionblock">
 <table><tr>
 <td class="icon">
 <div class="title">Note</div>
 </td>
 <td class="content">This public repository could further be mirrored, and that is
 how Git repositories at <code>kernel.org</code> are managed.</td>
 </tr></table>
 </div>
 <div class="paragraph"><p>Publishing the changes from your local (private) repository to
 your remote (public) repository requires a write privilege on
 the remote machine. You need to have an SSH account there to
 run a single command, <em>git-receive-pack</em>.</p></div>
 <div class="paragraph"><p>First, you need to create an empty repository on the remote
 machine that will house your public repository. This empty
 repository will be populated and be kept up to date by pushing
 into it later. Obviously, this repository creation needs to be
 done only once.</p></div>
 <div class="admonitionblock">
 <table><tr>
 <td class="icon">
 <div class="title">Note</div>
 </td>
 <td class="content"><em>git push</em> uses a pair of commands,
 <em>git send-pack</em> on your local machine, and <em>git-receive-pack</em>
 on the remote machine. The communication between the two over
 the network internally uses an SSH connection.</td>
 </tr></table>
 </div>
 <div class="paragraph"><p>Your private repository&#8217;s Git directory is usually <code>.git</code>, but
 your public repository is often named after the project name,
 i.e. <code>&lt;project&gt;.git</code>. Let&#8217;s create such a public repository for
 project <code>my-git</code>. After logging into the remote machine, create
 an empty directory:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ mkdir my-git.git</code></pre>
 </div></div>
 <div class="paragraph"><p>Then, make that directory into a Git repository by running
 <em>git init</em>, but this time, since its name is not the usual
 <code>.git</code>, we do things slightly differently:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ GIT_DIR=my-git.git git init</code></pre>
 </div></div>
 <div class="paragraph"><p>Make sure this directory is available for others you want your
 changes to be pulled via the transport of your choice. Also
 you need to make sure that you have the <em>git-receive-pack</em>
 program on the <code>$PATH</code>.</p></div>
 <div class="admonitionblock">
 <table><tr>
 <td class="icon">
 <div class="title">Note</div>
 </td>
 <td class="content">Many installations of sshd do not invoke your shell as the login
 shell when you directly run programs; what this means is that if
 your login shell is <em>bash</em>, only <code>.bashrc</code> is read and not
 <code>.bash_profile</code>. As a workaround, make sure <code>.bashrc</code> sets up
 <code>$PATH</code> so that you can run <em>git-receive-pack</em> program.</td>
 </tr></table>
 </div>
 <div class="admonitionblock">
 <table><tr>
 <td class="icon">
 <div class="title">Note</div>
 </td>
 <td class="content">If you plan to publish this repository to be accessed over http,
 you should do <code>mv my-git.git/hooks/post-update.sample
 my-git.git/hooks/post-update</code> at this point.
 This makes sure that every time you push into this
 repository, <code>git update-server-info</code> is run.</td>
 </tr></table>
 </div>
 <div class="paragraph"><p>Your "public repository" is now ready to accept your changes.
 Come back to the machine you have your private repository. From
 there, run this command:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git push &lt;public-host&gt;:/path/to/my-git.git master</code></pre>
 </div></div>
 <div class="paragraph"><p>This synchronizes your public repository to match the named
 branch head (i.e. <code>master</code> in this case) and objects reachable
 from them in your current repository.</p></div>
 <div class="paragraph"><p>As a real example, this is how I update my public Git
 repository. Kernel.org mirror network takes care of the
 propagation to other publicly visible machines:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git push master.kernel.org:/pub/scm/git/git.git/</code></pre>
 </div></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_packing_your_repository">Packing your repository</h2>
 <div class="sectionbody">
 <div class="paragraph"><p>Earlier, we saw that one file under <code>.git/objects/??/</code> directory
 is stored for each Git object you create. This representation
 is efficient to create atomically and safely, but
 not so convenient to transport over the network. Since Git objects are
 immutable once they are created, there is a way to optimize the
 storage by "packing them together". The command</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git repack</code></pre>
 </div></div>
 <div class="paragraph"><p>will do it for you. If you followed the tutorial examples, you
 would have accumulated about 17 objects in <code>.git/objects/??/</code>
 directories by now. <em>git repack</em> tells you how many objects it
 packed, and stores the packed file in the <code>.git/objects/pack</code>
 directory.</p></div>
 <div class="admonitionblock">
 <table><tr>
 <td class="icon">
 <div class="title">Note</div>
 </td>
 <td class="content">You will see two files, <code>pack-*.pack</code> and <code>pack-*.idx</code>,
 in <code>.git/objects/pack</code> directory. They are closely related to
 each other, and if you ever copy them by hand to a different
 repository for whatever reason, you should make sure you copy
 them together. The former holds all the data from the objects
 in the pack, and the latter holds the index for random
 access.</td>
 </tr></table>
 </div>
 <div class="paragraph"><p>If you are paranoid, running <em>git verify-pack</em> command would
 detect if you have a corrupt pack, but do not worry too much.
 Our programs are always perfect ;-).</p></div>
 <div class="paragraph"><p>Once you have packed objects, you do not need to leave the
 unpacked objects that are contained in the pack file anymore.</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git prune-packed</code></pre>
 </div></div>
 <div class="paragraph"><p>would remove them for you.</p></div>
 <div class="paragraph"><p>You can try running <code>find .git/objects -type f</code> before and after
 you run <code>git prune-packed</code> if you are curious.  Also <code>git
 count-objects</code> would tell you how many unpacked objects are in
 your repository and how much space they are consuming.</p></div>
 <div class="admonitionblock">
 <table><tr>
 <td class="icon">
 <div class="title">Note</div>
 </td>
 <td class="content"><code>git pull</code> is slightly cumbersome for HTTP transport, as a
 packed repository may contain relatively few objects in a
 relatively large pack. If you expect many HTTP pulls from your
 public repository you might want to repack &amp; prune often, or
 never.</td>
 </tr></table>
 </div>
 <div class="paragraph"><p>If you run <code>git repack</code> again at this point, it will say
 "Nothing new to pack.". Once you continue your development and
 accumulate the changes, running <code>git repack</code> again will create a
 new pack, that contains objects created since you packed your
 repository the last time. We recommend that you pack your project
 soon after the initial import (unless you are starting your
 project from scratch), and then run <code>git repack</code> every once in a
 while, depending on how active your project is.</p></div>
 <div class="paragraph"><p>When a repository is synchronized via <code>git push</code> and <code>git pull</code>
 objects packed in the source repository are usually stored
 unpacked in the destination.
 While this allows you to use different packing strategies on
 both ends, it also means you may need to repack both
 repositories every once in a while.</p></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_working_with_others">Working with Others</h2>
 <div class="sectionbody">
 <div class="paragraph"><p>Although Git is a truly distributed system, it is often
 convenient to organize your project with an informal hierarchy
 of developers. Linux kernel development is run this way. There
 is a nice illustration (page 17, "Merges to Mainline") in
 <a href="https://web.archive.org/web/20120915203609/http://www.xenotime.net/linux/mentor/linux-mentoring-2006.pdf">Randy Dunlap&#8217;s presentation</a>.</p></div>
 <div class="paragraph"><p>It should be stressed that this hierarchy is purely <strong>informal</strong>.
 There is nothing fundamental in Git that enforces the "chain of
 patch flow" this hierarchy implies. You do not have to pull
 from only one remote repository.</p></div>
 <div class="paragraph"><p>A recommended workflow for a "project lead" goes like this:</p></div>
 <div class="olist arabic"><ol class="arabic">
 <li>
 <p>
 Prepare your primary repository on your local machine. Your
    work is done there.
 </p>
 </li>
 <li>
 <p>
 Prepare a public repository accessible to others.
 </p>
 <div class="paragraph"><p>If other people are pulling from your repository over dumb
 transport protocols (HTTP), you need to keep this repository
 <em>dumb transport friendly</em>.  After <code>git init</code>,
 <code>$GIT_DIR/hooks/post-update.sample</code> copied from the standard templates
 would contain a call to <em>git update-server-info</em>
 but you need to manually enable the hook with
 <code>mv post-update.sample post-update</code>.  This makes sure
 <em>git update-server-info</em> keeps the necessary files up to date.</p></div>
 </li>
 <li>
 <p>
 Push into the public repository from your primary
    repository.
 </p>
 </li>
 <li>
 <p>
 <em>git repack</em> the public repository. This establishes a big
    pack that contains the initial set of objects as the
    baseline, and possibly <em>git prune</em> if the transport
    used for pulling from your repository supports packed
    repositories.
 </p>
 </li>
 <li>
 <p>
 Keep working in your primary repository. Your changes
    include modifications of your own, patches you receive via
    e-mails, and merges resulting from pulling the "public"
    repositories of your "subsystem maintainers".
 </p>
 <div class="paragraph"><p>You can repack this private repository whenever you feel like.</p></div>
 </li>
 <li>
 <p>
 Push your changes to the public repository, and announce it
    to the public.
 </p>
 </li>
 <li>
 <p>
 Every once in a while, <em>git repack</em> the public repository.
    Go back to step 5. and continue working.
 </p>
 </li>
 </ol></div>
 <div class="paragraph"><p>A recommended work cycle for a "subsystem maintainer" who works
 on that project and has an own "public repository" goes like this:</p></div>
 <div class="olist arabic"><ol class="arabic">
 <li>
 <p>
 Prepare your work repository, by running <em>git clone</em> on the public
    repository of the "project lead". The URL used for the
    initial cloning is stored in the remote.origin.url
    configuration variable.
 </p>
 </li>
 <li>
 <p>
 Prepare a public repository accessible to others, just like
    the "project lead" person does.
 </p>
 </li>
 <li>
 <p>
 Copy over the packed files from "project lead" public
    repository to your public repository, unless the "project
    lead" repository lives on the same machine as yours.  In the
    latter case, you can use <code>objects/info/alternates</code> file to
    point at the repository you are borrowing from.
 </p>
 </li>
 <li>
 <p>
 Push into the public repository from your primary
    repository. Run <em>git repack</em>, and possibly <em>git prune</em> if the
    transport used for pulling from your repository supports
    packed repositories.
 </p>
 </li>
 <li>
 <p>
 Keep working in your primary repository. Your changes
    include modifications of your own, patches you receive via
    e-mails, and merges resulting from pulling the "public"
    repositories of your "project lead" and possibly your
    "sub-subsystem maintainers".
 </p>
 <div class="paragraph"><p>You can repack this private repository whenever you feel
 like.</p></div>
 </li>
 <li>
 <p>
 Push your changes to your public repository, and ask your
    "project lead" and possibly your "sub-subsystem
    maintainers" to pull from it.
 </p>
 </li>
 <li>
 <p>
 Every once in a while, <em>git repack</em> the public repository.
    Go back to step 5. and continue working.
 </p>
 </li>
 </ol></div>
 <div class="paragraph"><p>A recommended work cycle for an "individual developer" who does
 not have a "public" repository is somewhat different. It goes
 like this:</p></div>
 <div class="olist arabic"><ol class="arabic">
 <li>
 <p>
 Prepare your work repository, by <em>git clone</em> the public
    repository of the "project lead" (or a "subsystem
    maintainer", if you work on a subsystem). The URL used for
    the initial cloning is stored in the remote.origin.url
    configuration variable.
 </p>
 </li>
 <li>
 <p>
 Do your work in your repository on <em>master</em> branch.
 </p>
 </li>
 <li>
 <p>
 Run <code>git fetch origin</code> from the public repository of your
    upstream every once in a while. This does only the first
    half of <code>git pull</code> but does not merge. The head of the
    public repository is stored in <code>.git/refs/remotes/origin/master</code>.
 </p>
 </li>
 <li>
 <p>
 Use <code>git cherry origin</code> to see which ones of your patches
    were accepted, and/or use <code>git rebase origin</code> to port your
    unmerged changes forward to the updated upstream.
 </p>
 </li>
 <li>
 <p>
 Use <code>git format-patch origin</code> to prepare patches for e-mail
    submission to your upstream and send it out. Go back to
    step 2. and continue.
 </p>
 </li>
 </ol></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_working_with_others_shared_repository_style">Working with Others, Shared Repository Style</h2>
 <div class="sectionbody">
 <div class="paragraph"><p>If you are coming from a CVS background, the style of cooperation
 suggested in the previous section may be new to you. You do not
 have to worry. Git supports the "shared public repository" style of
 cooperation you are probably more familiar with as well.</p></div>
 <div class="paragraph"><p>See <a href="gitcvs-migration.html">gitcvs-migration(7)</a> for the details.</p></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_bundling_your_work_together">Bundling your work together</h2>
 <div class="sectionbody">
 <div class="paragraph"><p>It is likely that you will be working on more than one thing at
 a time.  It is easy to manage those more-or-less independent tasks
 using branches with Git.</p></div>
 <div class="paragraph"><p>We have already seen how branches work previously,
 with "fun and work" example using two branches.  The idea is the
 same if there are more than two branches.  Let&#8217;s say you started
 out from "master" head, and have some new code in the "master"
 branch, and two independent fixes in the "commit-fix" and
 "diff-fix" branches:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git show-branch
 ! [commit-fix] Fix commit message normalization.
  ! [diff-fix] Fix rename detection.
   * [master] Release candidate #1
 ---
  +  [diff-fix] Fix rename detection.
  +  [diff-fix~1] Better common substring algorithm.
 +   [commit-fix] Fix commit message normalization.
   * [master] Release candidate #1
 ++* [diff-fix~2] Pretty-print messages.</code></pre>
 </div></div>
 <div class="paragraph"><p>Both fixes are tested well, and at this point, you want to merge
 in both of them.  You could merge in <em>diff-fix</em> first and then
 <em>commit-fix</em> next, like this:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git merge -m "Merge fix in diff-fix" diff-fix
 $ git merge -m "Merge fix in commit-fix" commit-fix</code></pre>
 </div></div>
 <div class="paragraph"><p>Which would result in:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git show-branch
 ! [commit-fix] Fix commit message normalization.
  ! [diff-fix] Fix rename detection.
   * [master] Merge fix in commit-fix
 ---
   - [master] Merge fix in commit-fix
 + * [commit-fix] Fix commit message normalization.
   - [master~1] Merge fix in diff-fix
  +* [diff-fix] Fix rename detection.
  +* [diff-fix~1] Better common substring algorithm.
   * [master~2] Release candidate #1
 ++* [master~3] Pretty-print messages.</code></pre>
 </div></div>
 <div class="paragraph"><p>However, there is no particular reason to merge in one branch
 first and the other next, when what you have are a set of truly
 independent changes (if the order mattered, then they are not
 independent by definition).  You could instead merge those two
 branches into the current branch at once.  First let&#8217;s undo what
 we just did and start over.  We would want to get the master
 branch before these two merges by resetting it to <em>master~2</em>:</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git reset --hard master~2</code></pre>
 </div></div>
 <div class="paragraph"><p>You can make sure <code>git show-branch</code> matches the state before
 those two <em>git merge</em> you just did.  Then, instead of running
 two <em>git merge</em> commands in a row, you would merge these two
 branch heads (this is known as <em>making an Octopus</em>):</p></div>
 <div class="listingblock">
 <div class="content">
 <pre><code>$ git merge commit-fix diff-fix
 $ git show-branch
 ! [commit-fix] Fix commit message normalization.
  ! [diff-fix] Fix rename detection.
   * [master] Octopus merge of branches 'diff-fix' and 'commit-fix'
 ---
   - [master] Octopus merge of branches 'diff-fix' and 'commit-fix'
 + * [commit-fix] Fix commit message normalization.
  +* [diff-fix] Fix rename detection.
  +* [diff-fix~1] Better common substring algorithm.
   * [master~1] Release candidate #1
 ++* [master~2] Pretty-print messages.</code></pre>
 </div></div>
 <div class="paragraph"><p>Note that you should not do Octopus just because you can.  An octopus
 is a valid thing to do and often makes it easier to view the
 commit history if you are merging more than two independent
 changes at the same time.  However, if you have merge conflicts
 with any of the branches you are merging in and need to hand
 resolve, that is an indication that the development happened in
 those branches were not independent after all, and you should
 merge two at a time, documenting how you resolved the conflicts,
 and the reason why you preferred changes made in one side over
 the other.  Otherwise it would make the project history harder
 to follow, not easier.</p></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_see_also">SEE ALSO</h2>
 <div class="sectionbody">
 <div class="paragraph"><p><a href="gittutorial.html">gittutorial(7)</a>,
 <a href="gittutorial-2.html">gittutorial-2(7)</a>,
 <a href="gitcvs-migration.html">gitcvs-migration(7)</a>,
 <a href="git-help.html">git-help(1)</a>,
 <a href="giteveryday.html">giteveryday(7)</a>,
 <a href="user-manual.html">The Git User&#8217;s Manual</a></p></div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="_git">GIT</h2>
 <div class="sectionbody">
 <div class="paragraph"><p>Part of the <a href="git.html">git(1)</a> suite</p></div>
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 Last updated
  2018-01-27 08:11:04 JST
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