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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) # # 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="everyday.html">Everyday Git</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="everyday.html">Everyday Git</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 2013-06-10 20:01:55 UTC </div> </div> </body> </html>