tutorial-2.html - external/gitster/git-htmldocs - Git at Google

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 <title>A tutorial introduction to git: part two</title>
 </head>
 <body>
 <div id="header">
 <h1>A tutorial introduction to git: part two</h1>
 </div>
 <div id="preamble">
 <div class="sectionbody">
 <p>You should work through <a href="tutorial.html">A tutorial introduction to
 git</a> before reading this tutorial.</p>
 <p>The goal of this tutorial is to introduce two fundamental pieces of
 git's architecture&#8212;the object database and the index file&#8212;and to
 provide the reader with everything necessary to understand the rest
 of the git documentation.</p>
 </div>
 </div>
 <h2>The git object database</h2>
 <div class="sectionbody">
 <p>Let's start a new project and create a small amount of history:</p>
 <div class="listingblock">
 <div class="content">
 <pre><tt>$ mkdir test-project
 $ cd test-project
 $ git init-db
 defaulting to local storage area
 $ echo 'hello world' &gt; file.txt
 $ git add .
 $ git commit -a -m "initial commit"
 Committing initial tree 92b8b694ffb1675e5975148e1121810081dbdffe
 $ echo 'hello world!' &gt;file.txt
 $ git commit -a -m "add emphasis"</tt></pre>
 </div></div>
 <p>What are the 40 digits of hex that git responded to the first commit
 with?</p>
 <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
 such 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).</p>
 <p>We can ask git about this particular object with the cat-file
 command&#8212;just cut-and-paste from the reply to the initial commit, to
 save yourself typing all 40 hex digits:</p>
 <div class="listingblock">
 <div class="content">
 <pre><tt>$ git cat-file -t 92b8b694ffb1675e5975148e1121810081dbdffe
 tree</tt></pre>
 </div></div>
 <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 class="listingblock">
 <div class="content">
 <pre><tt>$ git ls-tree 92b8b694
 100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad file.txt</tt></pre>
 </div></div>
 <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 class="listingblock">
 <div class="content">
 <pre><tt>$ git cat-file -t 3b18e512
 blob</tt></pre>
 </div></div>
 <p>A "blob" is just file data, which we can also examine with cat-file:</p>
 <div class="listingblock">
 <div class="content">
 <pre><tt>$ git cat-file blob 3b18e512
 hello world</tt></pre>
 </div></div>
 <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>
 <p>All of these objects are stored under their SHA1 names inside the git
 directory:</p>
 <div class="listingblock">
 <div class="content">
 <pre><tt>$ 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</tt></pre>
 </div></div>
 <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. We've seen a blob and a tree now,
 so next we should look at a commit.</p>
 <p>The simplest commit to find is the HEAD commit, which we can find
 from .git/HEAD:</p>
 <div class="listingblock">
 <div class="content">
 <pre><tt>$ cat .git/HEAD
 ref: refs/heads/master</tt></pre>
 </div></div>
 <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 class="listingblock">
 <div class="content">
 <pre><tt>$ 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 &lt;bfields@puzzle.fieldses.org&gt; 1143418702 -0500
 committer J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143418702 -0500

 add emphasis</tt></pre>
 </div></div>
 <p>The "tree" object here refers to the new state of the tree:</p>
 <div class="listingblock">
 <div class="content">
 <pre><tt>$ git ls-tree d0492b36
 100644 blob a0423896973644771497bdc03eb99d5281615b51 file.txt
 $ git cat-file blob a0423896
 hello world!</tt></pre>
 </div></div>
 <p>and the "parent" object refers to the previous commit:</p>
 <div class="listingblock">
 <div class="content">
 <pre><tt>$ git-cat-file commit 54196cc2
 tree 92b8b694ffb1675e5975148e1121810081dbdffe
 author J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143414668 -0500
 committer J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143414668 -0500

 initial commit</tt></pre>
 </div></div>
 <p>The tree object is the tree we examined first, and this commit is
 unusual in that it lacks any parent.</p>
 <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>
 <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>
 <p>So now we know how git uses the object database to represent a
 project's history:</p>
 <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>
 <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&#8212;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.&#8212;and most such commands can accept any of these names.</p>
 <p>In command synopses, the word "tree-ish" is sometimes used to
 designate such an argument.</p>
 </div>
 <h2>The index file</h2>
 <div class="sectionbody">
 <p>The primary tool we've been using to create commits is "git commit
 -a", 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>
 <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>
 <p>Continuing with our test-project, let's modify file.txt again:</p>
 <div class="listingblock">
 <div class="content">
 <pre><tt>$ echo "hello world, again" &gt;&gt;file.txt</tt></pre>
 </div></div>
 <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 class="listingblock">
 <div class="content">
 <pre><tt>$ git diff
 --- a/file.txt
 +++ b/file.txt
 @@ -1 +1,2 @@
  hello world!
 +hello world, again
 $ git update-index file.txt
 $ git diff</tt></pre>
 </div></div>
 <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 class="listingblock">
 <div class="content">
 <pre><tt>$ 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</tt></pre>
 </div></div>
 <p>So "git diff" is comparing against something other than the head.
 The thing that it's comparing against is actually the index file,
 which is stored in .git/index in a binary format, but whose contents
 we can examine with ls-files:</p>
 <div class="listingblock">
 <div class="content">
 <pre><tt>$ git ls-files --stage
 100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt
 $ git cat-file -t 513feba2
 blob
 $ git cat-file blob 513feba2
 hello world!
 hello world, again</tt></pre>
 </div></div>
 <p>So what our "git update-index" did was store a new blob and then put
 a reference to it in the index file. If we modify the file again,
 we'll see that the new modifications are reflected in the "git-diff"
 output:</p>
 <div class="listingblock">
 <div class="content">
 <pre><tt>$ echo 'again?' &gt;&gt;file.txt
 $ git diff
 index 513feba..ba3da7b 100644
 --- a/file.txt
 +++ b/file.txt
 @@ -1,2 +1,3 @@
  hello world!
  hello world, again
 +again?</tt></pre>
 </div></div>
 <p>With the right arguments, git diff can also show us the difference
 between the working directory and the last commit, or between the
 index and the last commit:</p>
 <div class="listingblock">
 <div class="content">
 <pre><tt>$ git diff HEAD
 diff --git a/file.txt b/file.txt
 index a042389..ba3da7b 100644
 --- a/file.txt
 +++ b/file.txt
 @@ -1 +1,3 @@
  hello world!
 +hello world, again
 +again?
 $ git diff --cached
 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</tt></pre>
 </div></div>
 <p>At any time, we can create a new commit using "git commit" (without
 the -a option), and verify that the state committed only includes the
 changes stored in the index file, not the additional change that is
 still only in our working tree:</p>
 <div class="listingblock">
 <div class="content">
 <pre><tt>$ git commit -m "repeat"
 $ git diff HEAD
 diff --git a/file.txt b/file.txt
 index 513feba..ba3da7b 100644
 --- a/file.txt
 +++ b/file.txt
 @@ -1,2 +1,3 @@
  hello world!
  hello world, again
 +again?</tt></pre>
 </div></div>
 <p>So by default "git commit" uses the index to create the commit, not
 the working tree; the -a option to commit tells it to first update
 the index with all changes in the working tree.</p>
 <p>Finally, it's worth looking at the effect of "git add" on the index
 file:</p>
 <div class="listingblock">
 <div class="content">
 <pre><tt>$ echo "goodbye, world" &gt;closing.txt
 $ git add closing.txt</tt></pre>
 </div></div>
 <p>The effect of the "git add" was to add one entry to the index file:</p>
 <div class="listingblock">
 <div class="content">
 <pre><tt>$ git ls-files --stage
 100644 8b9743b20d4b15be3955fc8d5cd2b09cd2336138 0 closing.txt
 100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt</tt></pre>
 </div></div>
 <p>And, as you can see with cat-file, this new entry refers to the
 current contents of the file:</p>
 <div class="listingblock">
 <div class="content">
 <pre><tt>$ git cat-file blob a6b11f7a
 goodbye, word</tt></pre>
 </div></div>
 <p>The "status" command is a useful way to get a quick summary of the
 situation:</p>
 <div class="listingblock">
 <div class="content">
 <pre><tt>$ git status
 #
 # Updated but not checked in:
 # (will commit)
 #
 # new file: closing.txt
 #
 #
 # Changed but not updated:
 # (use git-update-index to mark for commit)
 #
 # modified: file.txt
 #</tt></pre>
 </div></div>
 <p>Since the current state of closing.txt is cached in the index file,
 it is listed as "updated but not checked in". Since file.txt has
 changes in the working directory that aren't reflected in the index,
 it is marked "changed but not updated". At this point, running "git
 commit" would create a commit that added closing.txt (with its new
 contents), but that didn't modify file.txt.</p>
 <p>Also, note that a bare "git diff" shows the changes to file.txt, but
 not the addition of closing.txt, because the version of closing.txt
 in the index file is identical to the one in the working directory.</p>
 <p>In addition to being the staging area for new commits, the index file
 is also populated from the object database when checking out a
 branch, and is used to hold the trees involved in a merge operation.
 See the <a href="core-tutorial.txt">core tutorial</a> and the relevant man
 pages for details.</p>
 </div>
 <h2>What next?</h2>
 <div class="sectionbody">
 <p>At this point you should know everything necessary to read the man
 pages for any of the git commands; one good place to start would be
 with the commands mentioned in <a href="everyday.html">Everyday git</a>. You
 should be able to find any unknown jargon in the
 <a href="glossary.html">Glossary</a>.</p>
 <p>The <a href="cvs-migration.html">CVS migration</a> document explains how to
 import a CVS repository into git, and shows how to use git in a
 CVS-like way.</p>
 <p>For some interesting examples of git use, see the
 <a href="howto-index.html">howtos</a>.</p>
 <p>For git developers, the <a href="core-tutorial.html">Core tutorial</a> goes
 into detail on the lower-level git mechanisms involved in, for
 example, creating a new commit.</p>
 </div>
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 <div id="footer-text">
 Last updated 10-Aug-2006 00:18:00 UTC
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	vertical-align: top;
	font-style: italic;
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	/* Workarounds for IE6's broken and incomplete CSS2. */

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	background: #ffffee;
	border: 1px solid silver;
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	</style>
	<title>A tutorial introduction to git: part two</title>
	</head>
	<body>
	<div id="header">
	<h1>A tutorial introduction to git: part two</h1>
	</div>
	<div id="preamble">
	<div class="sectionbody">
	<p>You should work through <a href="tutorial.html">A tutorial introduction to
	git</a> before reading this tutorial.</p>
	<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>
	<h2>The git object database</h2>
	<div class="sectionbody">
	<p>Let's start a new project and create a small amount of history:</p>
	<div class="listingblock">
	<div class="content">
	<pre><tt>$ mkdir test-project
	$ cd test-project
	$ git init-db
	defaulting to local storage area
	$ echo 'hello world' > file.txt
	$ git add .
	$ git commit -a -m "initial commit"
	Committing initial tree 92b8b694ffb1675e5975148e1121810081dbdffe
	$ echo 'hello world!' >file.txt
	$ git commit -a -m "add emphasis"</tt></pre>
	</div></div>
	<p>What are the 40 digits of hex that git responded to the first commit
	with?</p>
	<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
	such 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).</p>
	<p>We can ask git about this particular object with the cat-file
	command—just cut-and-paste from the reply to the initial commit, to
	save yourself typing all 40 hex digits:</p>
	<div class="listingblock">
	<div class="content">
	<pre><tt>$ git cat-file -t 92b8b694ffb1675e5975148e1121810081dbdffe
	tree</tt></pre>
	</div></div>
	<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 class="listingblock">
	<div class="content">
	<pre><tt>$ git ls-tree 92b8b694
	100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad file.txt</tt></pre>
	</div></div>
	<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 class="listingblock">
	<div class="content">
	<pre><tt>$ git cat-file -t 3b18e512
	blob</tt></pre>
	</div></div>
	<p>A "blob" is just file data, which we can also examine with cat-file:</p>
	<div class="listingblock">
	<div class="content">
	<pre><tt>$ git cat-file blob 3b18e512
	hello world</tt></pre>
	</div></div>
	<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>
	<p>All of these objects are stored under their SHA1 names inside the git
	directory:</p>
	<div class="listingblock">
	<div class="content">
	<pre><tt>$ 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</tt></pre>
	</div></div>
	<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. We've seen a blob and a tree now,
	so next we should look at a commit.</p>
	<p>The simplest commit to find is the HEAD commit, which we can find
	from .git/HEAD:</p>
	<div class="listingblock">
	<div class="content">
	<pre><tt>$ cat .git/HEAD
	ref: refs/heads/master</tt></pre>
	</div></div>
	<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 class="listingblock">
	<div class="content">
	<pre><tt>$ 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</tt></pre>
	</div></div>
	<p>The "tree" object here refers to the new state of the tree:</p>
	<div class="listingblock">
	<div class="content">
	<pre><tt>$ git ls-tree d0492b36
	100644 blob a0423896973644771497bdc03eb99d5281615b51 file.txt
	$ git cat-file blob a0423896
	hello world!</tt></pre>
	</div></div>
	<p>and the "parent" object refers to the previous commit:</p>
	<div class="listingblock">
	<div class="content">
	<pre><tt>$ 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</tt></pre>
	</div></div>
	<p>The tree object is the tree we examined first, and this commit is
	unusual in that it lacks any parent.</p>
	<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>
	<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>
	<p>So now we know how git uses the object database to represent a
	project's history:</p>
	<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>
	<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>
	<p>In command synopses, the word "tree-ish" is sometimes used to
	designate such an argument.</p>
	</div>
	<h2>The index file</h2>
	<div class="sectionbody">
	<p>The primary tool we've been using to create commits is "git commit
	-a", 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>
	<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>
	<p>Continuing with our test-project, let's modify file.txt again:</p>
	<div class="listingblock">
	<div class="content">
	<pre><tt>$ echo "hello world, again" >>file.txt</tt></pre>
	</div></div>
	<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 class="listingblock">
	<div class="content">
	<pre><tt>$ git diff
	--- a/file.txt
	+++ b/file.txt
	@@ -1 +1,2 @@
	hello world!
	+hello world, again
	$ git update-index file.txt
	$ git diff</tt></pre>
	</div></div>
	<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 class="listingblock">
	<div class="content">
	<pre><tt>$ 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</tt></pre>
	</div></div>
	<p>So "git diff" is comparing against something other than the head.
	The thing that it's comparing against is actually the index file,
	which is stored in .git/index in a binary format, but whose contents
	we can examine with ls-files:</p>
	<div class="listingblock">
	<div class="content">
	<pre><tt>$ git ls-files --stage
	100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt
	$ git cat-file -t 513feba2
	blob
	$ git cat-file blob 513feba2
	hello world!
	hello world, again</tt></pre>
	</div></div>
	<p>So what our "git update-index" did was store a new blob and then put
	a reference to it in the index file. If we modify the file again,
	we'll see that the new modifications are reflected in the "git-diff"
	output:</p>
	<div class="listingblock">
	<div class="content">
	<pre><tt>$ echo 'again?' >>file.txt
	$ git diff
	index 513feba..ba3da7b 100644
	--- a/file.txt
	+++ b/file.txt
	@@ -1,2 +1,3 @@
	hello world!
	hello world, again
	+again?</tt></pre>
	</div></div>
	<p>With the right arguments, git diff can also show us the difference
	between the working directory and the last commit, or between the
	index and the last commit:</p>
	<div class="listingblock">
	<div class="content">
	<pre><tt>$ git diff HEAD
	diff --git a/file.txt b/file.txt
	index a042389..ba3da7b 100644
	--- a/file.txt
	+++ b/file.txt
	@@ -1 +1,3 @@
	hello world!
	+hello world, again
	+again?
	$ git diff --cached
	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</tt></pre>
	</div></div>
	<p>At any time, we can create a new commit using "git commit" (without
	the -a option), and verify that the state committed only includes the
	changes stored in the index file, not the additional change that is
	still only in our working tree:</p>
	<div class="listingblock">
	<div class="content">
	<pre><tt>$ git commit -m "repeat"
	$ git diff HEAD
	diff --git a/file.txt b/file.txt
	index 513feba..ba3da7b 100644
	--- a/file.txt
	+++ b/file.txt
	@@ -1,2 +1,3 @@
	hello world!
	hello world, again
	+again?</tt></pre>
	</div></div>
	<p>So by default "git commit" uses the index to create the commit, not
	the working tree; the -a option to commit tells it to first update
	the index with all changes in the working tree.</p>
	<p>Finally, it's worth looking at the effect of "git add" on the index
	file:</p>
	<div class="listingblock">
	<div class="content">
	<pre><tt>$ echo "goodbye, world" >closing.txt
	$ git add closing.txt</tt></pre>
	</div></div>
	<p>The effect of the "git add" was to add one entry to the index file:</p>
	<div class="listingblock">
	<div class="content">
	<pre><tt>$ git ls-files --stage
	100644 8b9743b20d4b15be3955fc8d5cd2b09cd2336138 0 closing.txt
	100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt</tt></pre>
	</div></div>
	<p>And, as you can see with cat-file, this new entry refers to the
	current contents of the file:</p>
	<div class="listingblock">
	<div class="content">
	<pre><tt>$ git cat-file blob a6b11f7a
	goodbye, word</tt></pre>
	</div></div>
	<p>The "status" command is a useful way to get a quick summary of the
	situation:</p>
	<div class="listingblock">
	<div class="content">
	<pre><tt>$ git status
	#
	# Updated but not checked in:
	# (will commit)
	#
	# new file: closing.txt
	#
	#
	# Changed but not updated:
	# (use git-update-index to mark for commit)
	#
	# modified: file.txt
	#</tt></pre>
	</div></div>
	<p>Since the current state of closing.txt is cached in the index file,
	it is listed as "updated but not checked in". Since file.txt has
	changes in the working directory that aren't reflected in the index,
	it is marked "changed but not updated". At this point, running "git
	commit" would create a commit that added closing.txt (with its new
	contents), but that didn't modify file.txt.</p>
	<p>Also, note that a bare "git diff" shows the changes to file.txt, but
	not the addition of closing.txt, because the version of closing.txt
	in the index file is identical to the one in the working directory.</p>
	<p>In addition to being the staging area for new commits, the index file
	is also populated from the object database when checking out a
	branch, and is used to hold the trees involved in a merge operation.
	See the <a href="core-tutorial.txt">core tutorial</a> and the relevant man
	pages for details.</p>
	</div>
	<h2>What next?</h2>
	<div class="sectionbody">
	<p>At this point you should know everything necessary to read the man
	pages for any of the git commands; one good place to start would be
	with the commands mentioned in <a href="everyday.html">Everyday git</a>. You
	should be able to find any unknown jargon in the
	<a href="glossary.html">Glossary</a>.</p>
	<p>The <a href="cvs-migration.html">CVS migration</a> document explains how to
	import a CVS repository into git, and shows how to use git in a
	CVS-like way.</p>
	<p>For some interesting examples of git use, see the
	<a href="howto-index.html">howtos</a>.</p>
	<p>For git developers, the <a href="core-tutorial.html">Core tutorial</a> goes
	into detail on the lower-level git mechanisms involved in, for
	example, creating a new commit.</p>
	</div>
	<div id="footer">
	<div id="footer-text">
	Last updated 10-Aug-2006 00:18:00 UTC
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