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<title>A git core tutorial for developers</title>
</head>
<body>
<div id="header">
<h1>A git core tutorial for developers</h1>
</div>
<h2>Introduction</h2>
<div class="sectionbody">
<p>This is trying to be a short tutorial on setting up and using a git
repository, mainly because being hands-on and using explicit examples is
often the best way of explaining what is going on.</p>
<p>In normal life, most people wouldn't use the "core" git programs
directly, but rather script around them to make them more palatable.
Understanding the core git stuff may help some people get those scripts
done, though, and it may also be instructive in helping people
understand what it is that the higher-level helper scripts are actually
doing.</p>
<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 for when the porcelain isn't flushing.</p>
<p>The material presented here often goes deep describing how things
work internally. If you are mostly interested in using git as a
SCM, you can skip them during your first pass.</p>
<div class="admonitionblock">
<table><tr>
<td class="icon">
<div class="title">Note</div>
</td>
<td class="content">And those "too deep" descriptions are often marked as Note.</td>
</tr></table>
</div>
<div class="admonitionblock">
<table><tr>
<td class="icon">
<div class="title">Note</div>
</td>
<td class="content">If you are already familiar with another version control system,
like CVS, you may want to take a look at
<a href="everyday.html">Everyday GIT in 20 commands or so</a> first
before reading this.</td>
</tr></table>
</div>
</div>
<h2>Creating a git repository</h2>
<div class="sectionbody">
<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>
<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 <tt>git-tutorial</tt>.
To start up, create a subdirectory for it, change into that
subdirectory, and initialize the git infrastructure with <tt>git-init</tt>:</p>
<div class="listingblock">
<div class="content">
<pre><tt>$ mkdir git-tutorial
$ cd git-tutorial
$ git-init</tt></pre>
</div></div>
<p>to which git will reply</p>
<div class="listingblock">
<div class="content">
<pre><tt>Initialized empty Git repository in .git/</tt></pre>
</div></div>
<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 <tt>.git</tt> directory setup for
your new project. You will now have a <tt>.git</tt> directory, and you can
inspect that with <tt>ls</tt>. For your new empty project, it should show you
three entries, among other things:</p>
<ul>
<li>
<p>
a file called <tt>HEAD</tt>, that has <tt>ref: refs/heads/master</tt> in it.
This is similar to a symbolic link and points at
<tt>refs/heads/master</tt> relative to the <tt>HEAD</tt> file.
</p>
<p>Don't worry about the fact that the file that the <tt>HEAD</tt> link points to
doesn't even exist yet — you haven't created the commit that will
start your <tt>HEAD</tt> development branch yet.</p>
</li>
<li>
<p>
a subdirectory called <tt>objects</tt>, 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 <tt>refs</tt>, which contains references to objects.
</p>
</li>
</ul>
<p>In particular, the <tt>refs</tt> subdirectory will contain two other
subdirectories, named <tt>heads</tt> and <tt>tags</tt> 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>
<p>One note: the special <tt>master</tt> head is the default branch, which is
why the <tt>.git/HEAD</tt> file was created points to it even if it
doesn't yet exist. Basically, the <tt>HEAD</tt> 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 <tt>master</tt> branch.</p>
<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 <tt>master</tt>
branch. A number of the git tools will assume that <tt>.git/HEAD</tt> is
valid, though.</p>
<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 SHA1 hash, aka <em>object name</em>,
and a reference to an object is always the 40-byte hex
representation of that SHA1 name. The files in the <tt>refs</tt>
subdirectory are expected to contain these hex references
(usually with a final <tt>'\n'</tt> at the end), and you should thus
expect to see a number of 41-byte files containing these
references in these <tt>refs</tt> 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 the
<a href="repository-layout.html">repository layout</a> document
after finishing this tutorial.</td>
</tr></table>
</div>
<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>
<h2>Populating a git repository</h2>
<div class="sectionbody">
<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>
<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 class="listingblock">
<div class="content">
<pre><tt>$ echo "Hello World" >hello
$ echo "Silly example" >example</tt></pre>
</div></div>
<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>
<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>
<p>The first step is trivial: when you want to tell git about any changes
to your working tree, you use the <tt>git-update-index</tt> 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 <tt>--add</tt> flag (or removing an entry with the
<tt>--remove</tt>) flag.</p>
<p>So to populate the index with the two files you just created, you can do</p>
<div class="listingblock">
<div class="content">
<pre><tt>$ git-update-index --add hello example</tt></pre>
</div></div>
<p>and you have now told git to track those two files.</p>
<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 class="listingblock">
<div class="content">
<pre><tt>$ ls .git/objects/??/*</tt></pre>
</div></div>
<p>and see two files:</p>
<div class="listingblock">
<div class="content">
<pre><tt>.git/objects/55/7db03de997c86a4a028e1ebd3a1ceb225be238
.git/objects/f2/4c74a2e500f5ee1332c86b94199f52b1d1d962</tt></pre>
</div></div>
<p>which correspond with the objects with names of <tt>557db…</tt> and
<tt>f24c7…</tt> respectively.</p>
<p>If you want to, you can use <tt>git-cat-file</tt> to look at those objects, but
you'll have to use the object name, not the filename of the object:</p>
<div class="listingblock">
<div class="content">
<pre><tt>$ git-cat-file -t 557db03de997c86a4a028e1ebd3a1ceb225be238</tt></pre>
</div></div>
<p>where the <tt>-t</tt> tells <tt>git-cat-file</tt> 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 class="listingblock">
<div class="content">
<pre><tt>$ git-cat-file "blob" 557db03</tt></pre>
</div></div>
<p>which will print out "Hello World". The object <tt>557db03</tt> is nothing
more than the contents of your file <tt>hello</tt>.</p>
<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 <tt>hello</tt> itself. The
object is literally just those specific <strong>contents</strong> of the file, and
however much you later change the contents in file <tt>hello</tt>, 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>
<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 <tt>git-update-index</tt> did something magical, and
actually saved away the contents of your files into the git object
database.</p>
<p>Updating the index did something else too: it created a <tt>.git/index</tt>
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>
<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>
<p>In particular, let's not even check in the two files into git yet, we'll
start off by adding another line to <tt>hello</tt> first:</p>
<div class="listingblock">
<div class="content">
<pre><tt>$ echo "It's a new day for git" >>hello</tt></pre>
</div></div>
<p>and you can now, since you told git about the previous state of <tt>hello</tt>, ask
git what has changed in the tree compared to your old index, using the
<tt>git-diff-files</tt> command:</p>
<div class="listingblock">
<div class="content">
<pre><tt>$ git-diff-files</tt></pre>
</div></div>
<p>Oops. That wasn't very readable. It just spit out its own internal
version of a <tt>diff</tt>, 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>
<p>To make it readable, we can tell git-diff-files to output the
differences as a patch, using the <tt>-p</tt> flag:</p>
<div class="listingblock">
<div class="content">
<pre><tt>$ 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</tt></pre>
</div></div>
<p>i.e. the diff of the change we caused by adding another line to <tt>hello</tt>.</p>
<p>In other words, <tt>git-diff-files</tt> 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>
<p>A common shorthand for <tt>git-diff-files -p</tt> is to just write <tt>git
diff</tt>, which will do the same thing.</p>
<div class="listingblock">
<div class="content">
<pre><tt>$ 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</tt></pre>
</div></div>
</div>
<h2>Committing git state</h2>
<div class="sectionbody">
<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>
<p>Creating a tree object is trivial, and is done with <tt>git-write-tree</tt>.
There are no options or other input: git-write-tree 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 class="listingblock">
<div class="content">
<pre><tt>$ git-write-tree</tt></pre>
</div></div>
<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 class="listingblock">
<div class="content">
<pre><tt>8988da15d077d4829fc51d8544c097def6644dbb</tt></pre>
</div></div>
<p>which is another incomprehensible object name. Again, if you want to,
you can use <tt>git-cat-file -t 8988d...</tt> to see that this time the object
is not a "blob" object, but a "tree" object (you can also use
<tt>git-cat-file</tt> to actually output the raw object contents, but you'll see
mainly a binary mess, so that's less interesting).</p>
<p>However — normally you'd never use <tt>git-write-tree</tt> on its own, because
normally you always commit a tree into a commit object using the
<tt>git-commit-tree</tt> command. In fact, it's easier to not actually use
<tt>git-write-tree</tt> on its own at all, but to just pass its result in as an
argument to <tt>git-commit-tree</tt>.</p>
<p><tt>git-commit-tree</tt> 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, <tt>git-commit-tree</tt> 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>
<p>And this is where we create the <tt>.git/refs/heads/master</tt> file
which is pointed at by <tt>HEAD</tt>. This file is supposed to contain
the reference to the top-of-tree of the master branch, and since
that's exactly what <tt>git-commit-tree</tt> spits out, we can do this
all with a sequence of simple shell commands:</p>
<div class="listingblock">
<div class="content">
<pre><tt>$ tree=$(git-write-tree)
$ commit=$(echo 'Initial commit' | git-commit-tree $tree)
$ git-update-ref HEAD $commit</tt></pre>
</div></div>
<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>
<p>Again, normally you'd never actually do this by hand. There is a
helpful script called <tt>git commit</tt> that will do all of this for you. So
you could have just written <tt>git commit</tt>
instead, and it would have done the above magic scripting for you.</p>
</div>
<h2>Making a change</h2>
<div class="sectionbody">
<p>Remember how we did the <tt>git-update-index</tt> on file <tt>hello</tt> and then we
changed <tt>hello</tt> afterward, and could compare the new state of <tt>hello</tt> with the
state we saved in the index file?</p>
<p>Further, remember how I said that <tt>git-write-tree</tt> 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 <tt>hello</tt>, 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>
<p>As before, if we do <tt>git-diff-files -p</tt> 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:
<tt>git-diff-index</tt>.</p>
<p>Unlike <tt>git-diff-files</tt>, which showed the difference between the index
file and the working tree, <tt>git-diff-index</tt> shows the differences
between a committed <strong>tree</strong> and either the index file or the working
tree. In other words, <tt>git-diff-index</tt> 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>
<p>But now we can do</p>
<div class="listingblock">
<div class="content">
<pre><tt>$ git-diff-index -p HEAD</tt></pre>
</div></div>
<p>(where <tt>-p</tt> has the same meaning as it did in <tt>git-diff-files</tt>), 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>
<p>Again, because this is a common operation, you can also just shorthand
it with</p>
<div class="listingblock">
<div class="content">
<pre><tt>$ git diff HEAD</tt></pre>
</div></div>
<p>which ends up doing the above for you.</p>
<p>In other words, <tt>git-diff-index</tt> normally compares a tree against the
working tree, but when given the <tt>--cached</tt> 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 <tt>git-diff-index --cached -p HEAD</tt> should thus return
an empty set of differences, and that's exactly what it does.</p>
<div class="admonitionblock">
<table><tr>
<td class="icon">
<div class="title">Note</div>
</td>
<td class="content">
<p><tt>git-diff-index</tt> 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 <tt>--cached</tt> flag is used or not. The <tt>--cached</tt>
flag really only determines whether the file <strong>contents</strong> to be compared
come from the working tree or not.</p>
<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>
</td>
</tr></table>
</div>
<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 class="listingblock">
<div class="content">
<pre><tt>$ git-update-index hello</tt></pre>
</div></div>
<p>(note how we didn't need the <tt>--add</tt> flag this time, since git knew
about the file already).</p>
<p>Note what happens to the different <tt>git-diff-*</tt> versions here. After
we've updated <tt>hello</tt> in the index, <tt>git-diff-files -p</tt> now shows no
differences, but <tt>git-diff-index -p HEAD</tt> still *does* show that the
current state is different from the state we committed. In fact, now
<tt>git-diff-index</tt> shows the same difference whether we use the <tt>—cached</tt>
flag or not, since now the index is coherent with the working tree.</p>
<p>Now, since we've updated <tt>hello</tt> 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 <tt>-p HEAD</tt> 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 class="listingblock">
<div class="content">
<pre><tt>$ git commit</tt></pre>
</div></div>
<p>which starts an editor for you to write the commit message and tells you
a bit about what you have done.</p>
<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 <tt>git commit</tt> will commit
the change for you.</p>
<p>You've now made your first real git commit. And if you're interested in
looking at what <tt>git commit</tt> 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 (<tt>git-commit</tt>).</p>
</div>
<h2>Inspecting Changes</h2>
<div class="sectionbody">
<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
<tt>diff</tt> family, namely <tt>git-diff-tree</tt>.</p>
<p><tt>git-diff-tree</tt> 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 class="listingblock">
<div class="content">
<pre><tt>$ git-diff-tree -p HEAD</tt></pre>
</div></div>
<p>(again, <tt>-p</tt> means to show the difference as a human-readable patch),
and it will show what the last commit (in <tt>HEAD</tt>) actually changed.</p>
<div class="admonitionblock">
<table><tr>
<td class="icon">
<div class="title">Note</div>
</td>
<td class="content">
<p>Here is an ASCII art by Jon Loeliger that illustrates how
various diff-* commands compare things.</p>
<div class="literalblock">
<div class="content">
<pre><tt> diff-tree
+----+
| |
| |
V V
+-----------+
| Object DB |
| Backing |
| Store |
+-----------+
^ ^
| |
| | diff-index --cached
| |
diff-index | V
| +-----------+
| | Index |
| | "cache" |
| +-----------+
| ^
| |
| | diff-files
| |
V V
+-----------+
| Working |
| Directory |
+-----------+</tt></pre>
</div></div>
</td>
</tr></table>
</div>
<p>More interestingly, you can also give <tt>git-diff-tree</tt> the <tt>—pretty</tt> 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>
<p>In fact, together with the <tt>git-rev-list</tt> program (which generates a
list of revisions), <tt>git-diff-tree</tt> ends up being a veritable fount of
changes. A trivial (but very useful) script called <tt>git-whatchanged</tt> is
included with git which does exactly this, and shows a log of recent
activities.</p>
<p>To see the whole history of our pitiful little git-tutorial project, you
can do</p>
<div class="listingblock">
<div class="content">
<pre><tt>$ git log</tt></pre>
</div></div>
<p>which shows just the log messages, or if we want to see the log together
with the associated patches use the more complex (and much more
powerful)</p>
<div class="listingblock">
<div class="content">
<pre><tt>$ git-whatchanged -p --root</tt></pre>
</div></div>
<p>and you will see exactly what has changed in the repository over its
short history.</p>
<div class="admonitionblock">
<table><tr>
<td class="icon">
<div class="title">Note</div>
</td>
<td class="content">The <tt>--root</tt> flag is a flag to <tt>git-diff-tree</tt> to tell it to
show the initial aka <em>root</em> commit too. Normally you'd probably not
want to see the initial import diff, but since the tutorial project
was started from scratch and is so small, we use it to make the result
a bit more interesting.</td>
</tr></table>
</div>
<p>With that, you should now be having some inkling of what git does, and
can explore on your own.</p>
<div class="admonitionblock">
<table><tr>
<td class="icon">
<div class="title">Note</div>
</td>
<td class="content">Most likely, you are not directly using the core
git Plumbing commands, but using Porcelain like Cogito on top
of it. Cogito works a bit differently and you usually do not
have to run <tt>git-update-index</tt> yourself for changed files (you
do tell underlying git about additions and removals via
<tt>cg-add</tt> and <tt>cg-rm</tt> commands). Just before you make a commit
with <tt>cg-commit</tt>, Cogito figures out which files you modified,
and runs <tt>git-update-index</tt> on them for you.</td>
</tr></table>
</div>
</div>
<h2>Tagging a version</h2>
<div class="sectionbody">
<p>In git, there are two kinds of tags, a "light" one, and an "annotated tag".</p>
<p>A "light" tag is technically nothing more than a branch, except we put
it in the <tt>.git/refs/tags/</tt> subdirectory instead of calling it a <tt>head</tt>.
So the simplest form of tag involves nothing more than</p>
<div class="listingblock">
<div class="content">
<pre><tt>$ git tag my-first-tag</tt></pre>
</div></div>
<p>which just writes the current <tt>HEAD</tt> into the <tt>.git/refs/tags/my-first-tag</tt>
file, after which point you can then use this symbolic name for that
particular state. You can, for example, do</p>
<div class="listingblock">
<div class="content">
<pre><tt>$ git diff my-first-tag</tt></pre>
</div></div>
<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>
<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 <tt>-a</tt> or
<tt>-s</tt> flag to <tt>git tag</tt>:</p>
<div class="listingblock">
<div class="content">
<pre><tt>$ git tag -s <tagname></tt></pre>
</div></div>
<p>which will sign the current <tt>HEAD</tt> (but you can also give it another
argument that specifies the thing to tag, i.e., you could have tagged the
current <tt>mybranch</tt> point by using <tt>git tag <tagname> mybranch</tt>).</p>
<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>
<h2>Copying repositories</h2>
<div class="sectionbody">
<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 <tt>.git</tt>
subdirectory. There is nothing else. What you see is what you got.</p>
<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>
<p>This has two implications:</p>
<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><tt>$ rm -rf git-tutorial</tt></pre>
</div></div>
<p>and it will be gone. There's no external repository, and there's no
history outside the project you created.</p>
</li>
<li>
<p>
if you want to move or duplicate a git repository, you can do so. There
is <tt>git clone</tt> 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
<tt>cp -a git-tutorial new-git-tutorial</tt>.
</p>
<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 <tt>cp -a</tt> to create a new copy, you'll want to do</p>
<div class="listingblock">
<div class="content">
<pre><tt>$ git-update-index --refresh</tt></pre>
</div></div>
<p>in the new repository to make sure that the index file is up-to-date.</p>
</li>
</ul>
<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
<tt>scp</tt>, <tt>rsync</tt> or <tt>wget</tt>.</p>
<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 <tt>git-update-index</tt> with a</p>
<div class="listingblock">
<div class="content">
<pre><tt>$ git-read-tree --reset HEAD
$ git-update-index --refresh</tt></pre>
</div></div>
<p>which will force a total index re-build from the tree pointed to by <tt>HEAD</tt>.
It resets the index contents to <tt>HEAD</tt>, and then the <tt>git-update-index</tt>
makes sure to match up all index entries with the checked-out files.
If the original repository had uncommitted changes in its
working tree, <tt>git-update-index —refresh</tt> notices them and
tells you they need to be updated.</p>
<p>The above can also be written as simply</p>
<div class="listingblock">
<div class="content">
<pre><tt>$ git reset</tt></pre>
</div></div>
<p>and in fact a lot of the common git command combinations can be scripted
with the <tt>git xyz</tt> interfaces. You can learn things by just looking
at what the various git scripts do. For example, <tt>git reset</tt> is the
above two lines implemented in <tt>git-reset</tt>, but some things like
<tt>git status</tt> and <tt>git commit</tt> are slightly more complex scripts around
the basic git commands.</p>
<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
<tt>.git</tt> subdirectory, but has all the git files directly in the
repository.</p>
<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 <tt>.git</tt> directory. For example, to
create your own copy of the git repository, you'd do the following</p>
<div class="listingblock">
<div class="content">
<pre><tt>$ mkdir my-git
$ cd my-git
$ rsync -rL rsync://rsync.kernel.org/pub/scm/git/git.git/ .git</tt></pre>
</div></div>
<p>followed by</p>
<div class="listingblock">
<div class="content">
<pre><tt>$ git-read-tree HEAD</tt></pre>
</div></div>
<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 class="listingblock">
<div class="content">
<pre><tt>$ git-checkout-index -u -a</tt></pre>
</div></div>
<p>where the <tt>-u</tt> 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
<tt>-a</tt> 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 <tt>-f</tt>
flag first, to tell git-checkout-index to <strong>force</strong> overwriting of any old
files).</p>
<p>Again, this can all be simplified with</p>
<div class="listingblock">
<div class="content">
<pre><tt>$ git clone rsync://rsync.kernel.org/pub/scm/git/git.git/ my-git
$ cd my-git
$ git checkout</tt></pre>
</div></div>
<p>which will end up doing all of the above for you.</p>
<p>You have now successfully copied somebody else's (mine) remote
repository, and checked it out.</p>
</div>
<h2>Creating a new branch</h2>
<div class="sectionbody">
<p>Branches in git are really nothing more than pointers into the git
object database from within the <tt>.git/refs/</tt> subdirectory, and as we