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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’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’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’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’re going to start a totally new repository from
scratch, with no pre-existing files, and we’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’s way of saying that you haven’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’t worry about the fact that the file that the <code>HEAD</code> link points to
doesn’t even exist yet — you haven’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’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’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’s
empty, that’s not very useful, so let’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’ll keep this simple and stupid, so we’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’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" >hello
$ echo "Silly example" >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’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’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’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’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’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’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’s not even check in the two files into Git yet, we’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" >>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’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’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’re now tying together all the different
filenames with their contents (and their permissions), and we’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’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’ll see
mainly a binary mess, so that’s less interesting).</p></div>
<div class="paragraph"><p>However — normally you’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’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 — 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’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’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’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’ll still see the same difference we saw last time: the index file
hasn’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’t do that, because we
didn’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’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’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’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’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’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-*</em> versions here.
After we’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’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’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’t an initial commit any more), but you’ve done that once
already, so let’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’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’ve now made your first real Git commit. And if you’re interested in
looking at what <code>git commit</code> really does, feel free to investigate:
it’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’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’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-*</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 <tagname></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 <tagname> 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 — 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’ll ignore that for now: it’s not
how normal projects work, and it’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’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’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’s no external repository, and there’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’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’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’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’t know <strong>what</strong> they’ve done and not yet checked in),
so usually you’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’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’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’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’t
actually have any of the working tree files to work on. To get
those, you’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’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’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’s just a convention, though,
and nothing enforces it.</p></div>
<div class="paragraph"><p>To show that as an example, let’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 switch -c 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 switch</em> what the base of the checkout would be.
In other words, if you have an earlier tag or branch, you’d just do</p></div>
<div class="listingblock">
<div class="content">
<pre><code>$ git switch -c 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 switch 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’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 <branchname> [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 — once you decide that you want to actually develop
on that branch — switch to that branch with a regular <em>git switch</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’s make sure we’re in
that branch, and do some work there.</p></div>
<div class="listingblock">
<div class="content">
<pre><code>$ git switch mybranch
$ echo "Work, work, work" >>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’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 switch 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’t contain the work we just did in <code>mybranch</code> — because that work
hasn’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" >>hello
$ echo "Lots of fun" >>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’ve got two branches, and you decide that you want to merge the
work done. Before we do that, let’s introduce a cool graphical tool that
helps you view what’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’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’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’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’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’ve ever used CVS, so let’s just
open <code>hello</code> in our editor (whatever that may be), and fix it up somehow.
I’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’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’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’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’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’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’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 switch 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’s usually much more common that you merge with somebody else than
merging with your own branches, so it’s worth pointing out that Git
makes that very easy too, and in fact, it’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 <remote-repository></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 — it’s such a common thing to <code>fetch</code> and then
immediately <code>merge</code>, that it’s called <code>git pull</code>, and you can
simply do</p></div>
<div class="listingblock">
<div class="content">
<pre><code>$ git pull <remote-repository></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’s config file
like this:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>$ git config remote.linus.url https://git.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’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’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’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 switch -C mybranch master^2
$ git switch 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’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’s Git directory is usually <code>.git</code>, but
your public repository is often named after the project name,
i.e. <code><project>.git</code>. Let’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 <public-host>:/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 & 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’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’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’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’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>
</div>
</div>
</div>
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