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gitcore-tutorial.html
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</head>
<body class="manpage">
<div id="header">
<h1>gitcore-tutorial(7) Manual Page</h1>
<h2 id="_name">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 note">
<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>$ mkdir git-tutorial
$ cd git-tutorial
$ git init</pre>
</div>
</div>
<div class="paragraph">
<p>to which Git will reply</p>
</div>
<div class="listingblock">
<div class="content">
<pre>Initialized empty Git repository in .git/</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:</code> <code>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 note">
<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 \n 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 note">
<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>$ echo "Hello World" >hello
$ echo "Silly example" >example</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>$ git update-index --add hello example</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>$ ls .git/objects/??/*</pre>
</div>
</div>
<div class="paragraph">
<p>and see two files:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>.git/objects/55/7db03de997c86a4a028e1ebd3a1ceb225be238
.git/objects/f2/4c74a2e500f5ee1332c86b94199f52b1d1d962</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>$ git cat-file -t 557db03de997c86a4a028e1ebd3a1ceb225be238</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>$ git cat-file blob 557db03</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 note">
<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 note">
<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>$ echo "It's a new day for git" >>hello</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>$ git diff-files</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>$ 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</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</code> <code>diff-files</code> <code>-p</code> is to just write <code>git</code>
<code>diff</code>, which will do the same thing.</p>
</div>
<div class="listingblock">
<div class="content">
<pre>$ 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</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</code> <code>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>$ git write-tree</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>8988da15d077d4829fc51d8544c097def6644dbb</pre>
</div>
</div>
<div class="paragraph">
<p>which is another incomprehensible object name. Again, if you want to,
you can use <code>git</code> <code>cat-file</code> <code>-t</code> <code>8988d..</code>. to see that this time the object
is not a "blob" object, but a "tree" object (you can also use
<code>git</code> <code>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>$ tree=$(git write-tree)
$ commit=$(echo 'Initial commit' | git commit-tree $tree)
$ git update-ref HEAD $commit</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</code> <code>commit</code> that will do all of this for you. So
you could have just written <code>git</code> <code>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</code> <code>diff-files</code> <code>-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>$ git diff-index -p HEAD</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>$ git diff HEAD</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</code> <code>diff-index</code> <code>--cached</code> <code>-p</code> <code>HEAD</code> should thus return
an empty set of differences, and that’s exactly what it does.</p>
</div>
<div class="admonitionblock note">
<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