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<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 SHA-1 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 SHA-1 | |
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 SHA-1 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 SHA-1 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 SHA-1 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 SHA-1 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 SHA-1 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 SHA-1 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> | |
</div></div> | |
<div class="paragraph"><p>So <em>git diff</em> 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> | |
<div class="listingblock"> | |
<div class="content"> | |
<pre><code>$ 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</code></pre> | |
</div></div> | |
<div class="paragraph"><p>So what our <em>git add</em> 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 <em>git diff</em> | |
output:</p></div> | |
<div class="listingblock"> | |
<div class="content"> | |
<pre><code>$ 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?</code></pre> | |
</div></div> | |
<div class="paragraph"><p>With the right arguments, <em>git diff</em> can also show us the difference | |
between the working directory and the last commit, or between the | |
index and the last commit:</p></div> | |
<div class="listingblock"> | |
<div class="content"> | |
<pre><code>$ 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</code></pre> | |
</div></div> | |
<div class="paragraph"><p>At any time, we can create a new commit using <em>git commit</em> (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> | |
<div class="listingblock"> | |
<div class="content"> | |
<pre><code>$ 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?</code></pre> | |
</div></div> | |
<div class="paragraph"><p>So by default <em>git commit</em> 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></div> | |
<div class="paragraph"><p>Finally, it’s worth looking at the effect of <em>git add</em> on the index | |
file:</p></div> | |
<div class="listingblock"> | |
<div class="content"> | |
<pre><code>$ echo "goodbye, world" >closing.txt | |
$ git add closing.txt</code></pre> | |
</div></div> | |
<div class="paragraph"><p>The effect of the <em>git add</em> was to add one entry to the index file:</p></div> | |
<div class="listingblock"> | |
<div class="content"> | |
<pre><code>$ git ls-files --stage | |
100644 8b9743b20d4b15be3955fc8d5cd2b09cd2336138 0 closing.txt | |
100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt</code></pre> | |
</div></div> | |
<div class="paragraph"><p>And, as you can see with cat-file, this new entry refers to the | |
current contents of the file:</p></div> | |
<div class="listingblock"> | |
<div class="content"> | |
<pre><code>$ git cat-file blob 8b9743b2 | |
goodbye, world</code></pre> | |
</div></div> | |
<div class="paragraph"><p>The "status" command is a useful way to get a quick summary of the | |
situation:</p></div> | |
<div class="listingblock"> | |
<div class="content"> | |
<pre><code>$ git status | |
On branch master | |
Changes to be committed: | |
(use "git reset HEAD <file>..." to unstage) | |
new file: closing.txt | |
Changes not staged for commit: | |
(use "git add <file>..." to update what will be committed) | |
(use "git checkout -- <file>..." to discard changes in working directory) | |
modified: file.txt</code></pre> | |
</div></div> | |
<div class="paragraph"><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></div> | |
<div class="paragraph"><p>Also, note that a bare <code>git diff</code> 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></div> | |
<div class="paragraph"><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 <a href="gitcore-tutorial.html">gitcore-tutorial(7)</a> and the relevant man | |
pages for details.</p></div> | |
</div> | |
</div> | |
<div class="sect1"> | |
<h2 id="_what_next">What next?</h2> | |
<div class="sectionbody"> | |
<div class="paragraph"><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="giteveryday.html">giteveryday(7)</a>. You | |
should be able to find any unknown jargon in <a href="gitglossary.html">gitglossary(7)</a>.</p></div> | |
<div class="paragraph"><p>The <a href="user-manual.html">Git User’s Manual</a> provides a more | |
comprehensive introduction to Git.</p></div> | |
<div class="paragraph"><p><a href="gitcvs-migration.html">gitcvs-migration(7)</a> explains how to | |
import a CVS repository into Git, and shows how to use Git in a | |
CVS-like way.</p></div> | |
<div class="paragraph"><p>For some interesting examples of Git use, see the | |
<a href="howto-index.html">howtos</a>.</p></div> | |
<div class="paragraph"><p>For Git developers, <a href="gitcore-tutorial.html">gitcore-tutorial(7)</a> goes | |
into detail on the lower-level Git mechanisms involved in, for | |
example, creating a new commit.</p></div> | |
</div> | |
</div> | |
<div class="sect1"> | |
<h2 id="_see_also">SEE ALSO</h2> | |
<div class="sectionbody"> | |
<div class="paragraph"><p><a href="gittutorial.html">gittutorial(7)</a>, | |
<a href="gitcvs-migration.html">gitcvs-migration(7)</a>, | |
<a href="gitcore-tutorial.html">gitcore-tutorial(7)</a>, | |
<a href="gitglossary.html">gitglossary(7)</a>, | |
<a href="git-help.html">git-help(1)</a>, | |
<a href="giteveryday.html">giteveryday(7)</a>, | |
<a href="user-manual.html">The Git User’s Manual</a></p></div> | |
</div> | |
</div> | |
<div class="sect1"> | |
<h2 id="_git">GIT</h2> | |
<div class="sectionbody"> | |
<div class="paragraph"><p>Part of the <a href="git.html">git(1)</a> suite</p></div> | |
</div> | |
</div> | |
</div> | |
<div id="footnotes"><hr /></div> | |
<div id="footer"> | |
<div id="footer-text"> | |
Last updated | |
2018-01-27 08:11:04 JST | |
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