/
gittutorial-2.html
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gittutorial-2.html
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
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<body class="manpage">
<div id="header">
<h1>
gittutorial-2(7) Manual Page
</h1>
<h2>NAME</h2>
<div class="sectionbody">
<p>gittutorial-2 -
A tutorial introduction to git: part two
</p>
</div>
</div>
<div id="content">
<div class="sect1">
<h2 id="_synopsis">SYNOPSIS</h2>
<div class="sectionbody">
<div class="verseblock">
<pre class="content">git *</pre>
<div class="attribution">
</div></div>
</div>
</div>
<div class="sect1">
<h2 id="_description">DESCRIPTION</h2>
<div class="sectionbody">
<div class="paragraph"><p>You should work through <a href="gittutorial.html">gittutorial(7)</a> before reading this tutorial.</p></div>
<div class="paragraph"><p>The goal of this tutorial is to introduce two fundamental pieces of
git’s architecture—the object database and the index file—and to
provide the reader with everything necessary to understand the rest
of the git documentation.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_the_git_object_database">The git object database</h2>
<div class="sectionbody">
<div class="paragraph"><p>Let’s start a new project and create a small amount of history:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>$ mkdir test-project
$ cd test-project
$ git init
Initialized empty Git repository in .git/
$ echo 'hello world' > file.txt
$ git add .
$ git commit -a -m "initial commit"
[master (root-commit) 54196cc] initial commit
1 file changed, 1 insertion(+)
create mode 100644 file.txt
$ echo 'hello world!' >file.txt
$ git commit -a -m "add emphasis"
[master c4d59f3] add emphasis
1 file changed, 1 insertion(+), 1 deletion(-)</code></pre>
</div></div>
<div class="paragraph"><p>What are the 7 digits of hex that git responded to the commit with?</p></div>
<div class="paragraph"><p>We saw in part one of the tutorial that commits have names like this.
It turns out that every object in the git history is stored under
a 40-digit hex name. That name is the SHA1 hash of the object’s
contents; among other things, this ensures that git will never store
the same data twice (since identical data is given an identical SHA1
name), and that the contents of a git object will never change (since
that would change the object’s name as well). The 7 char hex strings
here are simply the abbreviation of such 40 character long strings.
Abbreviations can be used everywhere where the 40 character strings
can be used, so long as they are unambiguous.</p></div>
<div class="paragraph"><p>It is expected that the content of the commit object you created while
following the example above generates a different SHA1 hash than
the one shown above because the commit object records the time when
it was created and the name of the person performing the commit.</p></div>
<div class="paragraph"><p>We can ask git about this particular object with the <code>cat-file</code>
command. Don’t copy the 40 hex digits from this example but use those
from your own version. Note that you can shorten it to only a few
characters to save yourself typing all 40 hex digits:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>$ git cat-file -t 54196cc2
commit
$ git cat-file commit 54196cc2
tree 92b8b694ffb1675e5975148e1121810081dbdffe
author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500
committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500
initial commit</code></pre>
</div></div>
<div class="paragraph"><p>A tree can refer to one or more "blob" objects, each corresponding to
a file. In addition, a tree can also refer to other tree objects,
thus creating a directory hierarchy. You can examine the contents of
any tree using ls-tree (remember that a long enough initial portion
of the SHA1 will also work):</p></div>
<div class="listingblock">
<div class="content">
<pre><code>$ git ls-tree 92b8b694
100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad file.txt</code></pre>
</div></div>
<div class="paragraph"><p>Thus we see that this tree has one file in it. The SHA1 hash is a
reference to that file’s data:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>$ git cat-file -t 3b18e512
blob</code></pre>
</div></div>
<div class="paragraph"><p>A "blob" is just file data, which we can also examine with cat-file:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>$ git cat-file blob 3b18e512
hello world</code></pre>
</div></div>
<div class="paragraph"><p>Note that this is the old file data; so the object that git named in
its response to the initial tree was a tree with a snapshot of the
directory state that was recorded by the first commit.</p></div>
<div class="paragraph"><p>All of these objects are stored under their SHA1 names inside the git
directory:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>$ find .git/objects/
.git/objects/
.git/objects/pack
.git/objects/info
.git/objects/3b
.git/objects/3b/18e512dba79e4c8300dd08aeb37f8e728b8dad
.git/objects/92
.git/objects/92/b8b694ffb1675e5975148e1121810081dbdffe
.git/objects/54
.git/objects/54/196cc2703dc165cbd373a65a4dcf22d50ae7f7
.git/objects/a0
.git/objects/a0/423896973644771497bdc03eb99d5281615b51
.git/objects/d0
.git/objects/d0/492b368b66bdabf2ac1fd8c92b39d3db916e59
.git/objects/c4
.git/objects/c4/d59f390b9cfd4318117afde11d601c1085f241</code></pre>
</div></div>
<div class="paragraph"><p>and the contents of these files is just the compressed data plus a
header identifying their length and their type. The type is either a
blob, a tree, a commit, or a tag.</p></div>
<div class="paragraph"><p>The simplest commit to find is the HEAD commit, which we can find
from .git/HEAD:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>$ cat .git/HEAD
ref: refs/heads/master</code></pre>
</div></div>
<div class="paragraph"><p>As you can see, this tells us which branch we’re currently on, and it
tells us this by naming a file under the .git directory, which itself
contains a SHA1 name referring to a commit object, which we can
examine with cat-file:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>$ cat .git/refs/heads/master
c4d59f390b9cfd4318117afde11d601c1085f241
$ git cat-file -t c4d59f39
commit
$ git cat-file commit c4d59f39
tree d0492b368b66bdabf2ac1fd8c92b39d3db916e59
parent 54196cc2703dc165cbd373a65a4dcf22d50ae7f7
author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500
committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143418702 -0500
add emphasis</code></pre>
</div></div>
<div class="paragraph"><p>The "tree" object here refers to the new state of the tree:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>$ git ls-tree d0492b36
100644 blob a0423896973644771497bdc03eb99d5281615b51 file.txt
$ git cat-file blob a0423896
hello world!</code></pre>
</div></div>
<div class="paragraph"><p>and the "parent" object refers to the previous commit:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>$ git cat-file commit 54196cc2
tree 92b8b694ffb1675e5975148e1121810081dbdffe
author J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500
committer J. Bruce Fields <bfields@puzzle.fieldses.org> 1143414668 -0500
initial commit</code></pre>
</div></div>
<div class="paragraph"><p>The tree object is the tree we examined first, and this commit is
unusual in that it lacks any parent.</p></div>
<div class="paragraph"><p>Most commits have only one parent, but it is also common for a commit
to have multiple parents. In that case the commit represents a
merge, with the parent references pointing to the heads of the merged
branches.</p></div>
<div class="paragraph"><p>Besides blobs, trees, and commits, the only remaining type of object
is a "tag", which we won’t discuss here; refer to <a href="git-tag.html">git-tag(1)</a>
for details.</p></div>
<div class="paragraph"><p>So now we know how git uses the object database to represent a
project’s history:</p></div>
<div class="ulist"><ul>
<li>
<p>
"commit" objects refer to "tree" objects representing the
snapshot of a directory tree at a particular point in the
history, and refer to "parent" commits to show how they’re
connected into the project history.
</p>
</li>
<li>
<p>
"tree" objects represent the state of a single directory,
associating directory names to "blob" objects containing file
data and "tree" objects containing subdirectory information.
</p>
</li>
<li>
<p>
"blob" objects contain file data without any other structure.
</p>
</li>
<li>
<p>
References to commit objects at the head of each branch are
stored in files under .git/refs/heads/.
</p>
</li>
<li>
<p>
The name of the current branch is stored in .git/HEAD.
</p>
</li>
</ul></div>
<div class="paragraph"><p>Note, by the way, that lots of commands take a tree as an argument.
But as we can see above, a tree can be referred to in many different
ways—by the SHA1 name for that tree, by the name of a commit that
refers to the tree, by the name of a branch whose head refers to that
tree, etc.--and most such commands can accept any of these names.</p></div>
<div class="paragraph"><p>In command synopses, the word "tree-ish" is sometimes used to
designate such an argument.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_the_index_file">The index file</h2>
<div class="sectionbody">
<div class="paragraph"><p>The primary tool we’ve been using to create commits is <code>git-commit
-a</code>, which creates a commit including every change you’ve made to
your working tree. But what if you want to commit changes only to
certain files? Or only certain changes to certain files?</p></div>
<div class="paragraph"><p>If we look at the way commits are created under the cover, we’ll see
that there are more flexible ways creating commits.</p></div>
<div class="paragraph"><p>Continuing with our test-project, let’s modify file.txt again:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>$ echo "hello world, again" >>file.txt</code></pre>
</div></div>
<div class="paragraph"><p>but this time instead of immediately making the commit, let’s take an
intermediate step, and ask for diffs along the way to keep track of
what’s happening:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>$ git diff
--- a/file.txt
+++ b/file.txt
@@ -1 +1,2 @@
hello world!
+hello world, again
$ git add file.txt
$ git diff</code></pre>
</div></div>
<div class="paragraph"><p>The last diff is empty, but no new commits have been made, and the
head still doesn’t contain the new line:</p></div>
<div class="listingblock">
<div class="content">
<pre><code>$ git diff HEAD
diff --git a/file.txt b/file.txt
index a042389..513feba 100644
--- a/file.txt
+++ b/file.txt
@@ -1 +1,2 @@
hello world!
+hello world, again</code></pre>