diff options
| author | Junio C Hamano <junio@hera.kernel.org> | 2008-05-26 01:16:14 +0000 |
|---|---|---|
| committer | Junio C Hamano <junio@hera.kernel.org> | 2008-05-26 01:16:14 +0000 |
| commit | dfccbb0719b88f4df05bf718c6008166dbadea44 (patch) | |
| tree | ddd5483c51044cab7a070039655f0b4074b1dfe6 /gittutorial-2.html | |
| parent | b713ff1cbf5b9979edaf2b85bfc568fbc37ec0d7 (diff) | |
| download | git-htmldocs-dfccbb0719b88f4df05bf718c6008166dbadea44.tar.gz | |
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+<title>gittutorial-2(7)</title>
+</head>
+<body>
+<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>
+<h2>SYNOPSIS</h2>
+<div class="sectionbody">
+<p>git *</p>
+</div>
+<h2>DESCRIPTION</h2>
+<div class="sectionbody">
+<p>You should work through <a href="gittutorial.html">gittutorial(7)</a>[A tutorial introduction to
+git] 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>
+<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
+Initialized empty Git repository in .git/
+$ echo 'hello world' > file.txt
+$ git add .
+$ git commit -a -m "initial commit"
+Created initial commit 54196cc2703dc165cbd373a65a4dcf22d50ae7f7
+ create mode 100644 file.txt
+$ echo 'hello world!' >file.txt
+$ git commit -a -m "add emphasis"
+Created commit c4d59f390b9cfd4318117afde11d601c1085f241</tt></pre>
+</div></div>
+<p>What are the 40 digits of hex that git responded to the 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>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>
+<p>We can ask git about this particular object with the cat-file
+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 class="listingblock">
+<div class="content">
+<pre><tt>$ 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</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.</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 add 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 add" 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 8b9743b2
+goodbye, world</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
+# On branch master
+# Changes to be committed:
+# (use "git reset HEAD <file>..." to unstage)
+#
+# new file: closing.txt
+#
+# Changed but not updated:
+# (use "git add <file>..." to update what will be committed)
+#
+# 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 "Changes to be committed". 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.html">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="user-manual.html">Git User's Manual</a> provides a more
+comprehensive introduction to git.</p>
+<p>The <a href="gitcvs-migration.html">gitcvs-migration(7)</a>[CVS migration] 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>
+<h2>SEE ALSO</h2>
+<div class="sectionbody">
+<p><a href="gittutorial.html">gittutorial(7)</a>,
+<a href="gitcvs-migration.html">gitcvs-migration(7)</a>,
+<a href="everyday.html">Everyday git</a>,
+<a href="user-manual.html">The Git User's Manual</a></p>
+</div>
+<h2>GIT</h2>
+<div class="sectionbody">
+<p>Part of the <a href="git.html">git(7)</a> suite.</p>
+</div>
+<div id="footer">
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