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chromium / external / gitster / git-htmldocs / b96f40a0cb74a86e910bb0f415c4df5182ea5ccf / . / gittutorial-2.html
blob: efedc1fbbb463123c41423d444f4cf568c26f1e3 [file] [log] [blame]
Junio C Hamanob96f40a2024-08-01 00:57:25[diff] [blame^]1<!DOCTYPE html>
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441<body class="manpage">
442<div id="header">
443<h1>gittutorial-2(7) Manual Page</h1>
444<h2 id="_name">NAME</h2>
445<div class="sectionbody">
446<p>gittutorial-2 - A tutorial introduction to Git: part two</p>
447</div>
448</div>
449<div id="content">
450<div class="sect1">
451<h2 id="_synopsis">SYNOPSIS</h2>
452<div class="sectionbody">
453<div class="verseblock">
454<pre class="content">git *</pre>
455</div>
456</div>
457</div>
458<div class="sect1">
459<h2 id="_description">DESCRIPTION</h2>
460<div class="sectionbody">
461<div class="paragraph">
462<p>You should work through <a href="gittutorial.html">gittutorial(7)</a> before reading this tutorial.</p>
463</div>
464<div class="paragraph">
465<p>The goal of this tutorial is to introduce two fundamental pieces of
466Git&#8217;s architecture&#8212;&#8203;the object database and the index file&#8212;&#8203;and to
467provide the reader with everything necessary to understand the rest
468of the Git documentation.</p>
469</div>
470</div>
471</div>
472<div class="sect1">
473<h2 id="_the_git_object_database">The Git object database</h2>
474<div class="sectionbody">
475<div class="paragraph">
476<p>Let&#8217;s start a new project and create a small amount of history:</p>
477</div>
478<div class="listingblock">
479<div class="content">
480<pre>$ mkdir test-project
481$ cd test-project
482$ git init
483Initialized empty Git repository in .git/
484$ echo 'hello world' &gt; file.txt
485$ git add .
486$ git commit -a -m "initial commit"
487[master (root-commit) 54196cc] initial commit
488 1 file changed, 1 insertion(+)
489 create mode 100644 file.txt
490$ echo 'hello world!' &gt;file.txt
491$ git commit -a -m "add emphasis"
492[master c4d59f3] add emphasis
493 1 file changed, 1 insertion(+), 1 deletion(-)</pre>
494</div>
495</div>
496<div class="paragraph">
497<p>What are the 7 digits of hex that Git responded to the commit with?</p>
498</div>
499<div class="paragraph">
500<p>We saw in part one of the tutorial that commits have names like this.
501It turns out that every object in the Git history is stored under
502a 40-digit hex name. That name is the SHA-1 hash of the object&#8217;s
503contents; among other things, this ensures that Git will never store
504the same data twice (since identical data is given an identical SHA-1
505name), and that the contents of a Git object will never change (since
506that would change the object&#8217;s name as well). The 7 char hex strings
507here are simply the abbreviation of such 40 character long strings.
508Abbreviations can be used everywhere where the 40 character strings
509can be used, so long as they are unambiguous.</p>
510</div>
511<div class="paragraph">
512<p>It is expected that the content of the commit object you created while
513following the example above generates a different SHA-1 hash than
514the one shown above because the commit object records the time when
515it was created and the name of the person performing the commit.</p>
516</div>
517<div class="paragraph">
518<p>We can ask Git about this particular object with the <code>cat-file</code>
519command. Don&#8217;t copy the 40 hex digits from this example but use those
520from your own version. Note that you can shorten it to only a few
521characters to save yourself typing all 40 hex digits:</p>
522</div>
523<div class="listingblock">
524<div class="content">
525<pre>$ git cat-file -t 54196cc2
526commit
527$ git cat-file commit 54196cc2
528tree 92b8b694ffb1675e5975148e1121810081dbdffe
529author J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143414668 -0500
530committer J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143414668 -0500
531
532initial commit</pre>
533</div>
534</div>
535<div class="paragraph">
536<p>A tree can refer to one or more "blob" objects, each corresponding to
537a file. In addition, a tree can also refer to other tree objects,
538thus creating a directory hierarchy. You can examine the contents of
539any tree using ls-tree (remember that a long enough initial portion
540of the SHA-1 will also work):</p>
541</div>
542<div class="listingblock">
543<div class="content">
544<pre>$ git ls-tree 92b8b694
545100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad file.txt</pre>
546</div>
547</div>
548<div class="paragraph">
549<p>Thus we see that this tree has one file in it. The SHA-1 hash is a
550reference to that file&#8217;s data:</p>
551</div>
552<div class="listingblock">
553<div class="content">
554<pre>$ git cat-file -t 3b18e512
555blob</pre>
556</div>
557</div>
558<div class="paragraph">
559<p>A "blob" is just file data, which we can also examine with cat-file:</p>
560</div>
561<div class="listingblock">
562<div class="content">
563<pre>$ git cat-file blob 3b18e512
564hello world</pre>
565</div>
566</div>
567<div class="paragraph">
568<p>Note that this is the old file data; so the object that Git named in
569its response to the initial tree was a tree with a snapshot of the
570directory state that was recorded by the first commit.</p>
571</div>
572<div class="paragraph">
573<p>All of these objects are stored under their SHA-1 names inside the Git
574directory:</p>
575</div>
576<div class="listingblock">
577<div class="content">
578<pre>$ find .git/objects/
579.git/objects/
580.git/objects/pack
581.git/objects/info
582.git/objects/3b
583.git/objects/3b/18e512dba79e4c8300dd08aeb37f8e728b8dad
584.git/objects/92
585.git/objects/92/b8b694ffb1675e5975148e1121810081dbdffe
586.git/objects/54
587.git/objects/54/196cc2703dc165cbd373a65a4dcf22d50ae7f7
588.git/objects/a0
589.git/objects/a0/423896973644771497bdc03eb99d5281615b51
590.git/objects/d0
591.git/objects/d0/492b368b66bdabf2ac1fd8c92b39d3db916e59
592.git/objects/c4
593.git/objects/c4/d59f390b9cfd4318117afde11d601c1085f241</pre>
594</div>
595</div>
596<div class="paragraph">
597<p>and the contents of these files is just the compressed data plus a
598header identifying their length and their type. The type is either a
599blob, a tree, a commit, or a tag.</p>
600</div>
601<div class="paragraph">
602<p>The simplest commit to find is the HEAD commit, which we can find
603from .git/HEAD:</p>
604</div>
605<div class="listingblock">
606<div class="content">
607<pre>$ cat .git/HEAD
608ref: refs/heads/master</pre>
609</div>
610</div>
611<div class="paragraph">
612<p>As you can see, this tells us which branch we&#8217;re currently on, and it
613tells us this by naming a file under the .git directory, which itself
614contains a SHA-1 name referring to a commit object, which we can
615examine with cat-file:</p>
616</div>
617<div class="listingblock">
618<div class="content">
619<pre>$ cat .git/refs/heads/master
620c4d59f390b9cfd4318117afde11d601c1085f241
621$ git cat-file -t c4d59f39
622commit
623$ git cat-file commit c4d59f39
624tree d0492b368b66bdabf2ac1fd8c92b39d3db916e59
625parent 54196cc2703dc165cbd373a65a4dcf22d50ae7f7
626author J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143418702 -0500
627committer J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143418702 -0500
628
629add emphasis</pre>
630</div>
631</div>
632<div class="paragraph">
633<p>The "tree" object here refers to the new state of the tree:</p>
634</div>
635<div class="listingblock">
636<div class="content">
637<pre>$ git ls-tree d0492b36
638100644 blob a0423896973644771497bdc03eb99d5281615b51 file.txt
639$ git cat-file blob a0423896
640hello world!</pre>
641</div>
642</div>
643<div class="paragraph">
644<p>and the "parent" object refers to the previous commit:</p>
645</div>
646<div class="listingblock">
647<div class="content">
648<pre>$ git cat-file commit 54196cc2
649tree 92b8b694ffb1675e5975148e1121810081dbdffe
650author J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143414668 -0500
651committer J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143414668 -0500
652
653initial commit</pre>
654</div>
655</div>
656<div class="paragraph">
657<p>The tree object is the tree we examined first, and this commit is
658unusual in that it lacks any parent.</p>
659</div>
660<div class="paragraph">
661<p>Most commits have only one parent, but it is also common for a commit
662to have multiple parents. In that case the commit represents a
663merge, with the parent references pointing to the heads of the merged
664branches.</p>
665</div>
666<div class="paragraph">
667<p>Besides blobs, trees, and commits, the only remaining type of object
668is a "tag", which we won&#8217;t discuss here; refer to <a href="git-tag.html">git-tag(1)</a>
669for details.</p>
670</div>
671<div class="paragraph">
672<p>So now we know how Git uses the object database to represent a
673project&#8217;s history:</p>
674</div>
675<div class="ulist">
676<ul>
677<li>
678<p>"commit" objects refer to "tree" objects representing the
679snapshot of a directory tree at a particular point in the
680history, and refer to "parent" commits to show how they&#8217;re
681connected into the project history.</p>
682</li>
683<li>
684<p>"tree" objects represent the state of a single directory,
685associating directory names to "blob" objects containing file
686data and "tree" objects containing subdirectory information.</p>
687</li>
688<li>
689<p>"blob" objects contain file data without any other structure.</p>
690</li>
691<li>
692<p>References to commit objects at the head of each branch are
693stored in files under .git/refs/heads/.</p>
694</li>
695<li>
696<p>The name of the current branch is stored in .git/HEAD.</p>
697</li>
698</ul>
699</div>
700<div class="paragraph">
701<p>Note, by the way, that lots of commands take a tree as an argument.
702But as we can see above, a tree can be referred to in many different
703ways&#8212;&#8203;by the SHA-1 name for that tree, by the name of a commit that
704refers to the tree, by the name of a branch whose head refers to that
705tree, etc.--and most such commands can accept any of these names.</p>
706</div>
707<div class="paragraph">
708<p>In command synopses, the word "tree-ish" is sometimes used to
709designate such an argument.</p>
710</div>
711</div>
712</div>
713<div class="sect1">
714<h2 id="_the_index_file">The index file</h2>
715<div class="sectionbody">
716<div class="paragraph">
717<p>The primary tool we&#8217;ve been using to create commits is <code>git-commit
718-a</code>, which creates a commit including every change you&#8217;ve made to
719your working tree. But what if you want to commit changes only to
720certain files? Or only certain changes to certain files?</p>
721</div>
722<div class="paragraph">
723<p>If we look at the way commits are created under the cover, we&#8217;ll see
724that there are more flexible ways creating commits.</p>
725</div>
726<div class="paragraph">
727<p>Continuing with our test-project, let&#8217;s modify file.txt again:</p>
728</div>
729<div class="listingblock">
730<div class="content">
731<pre>$ echo "hello world, again" &gt;&gt;file.txt</pre>
732</div>
733</div>
734<div class="paragraph">
735<p>but this time instead of immediately making the commit, let&#8217;s take an
736intermediate step, and ask for diffs along the way to keep track of
737what&#8217;s happening:</p>
738</div>
739<div class="listingblock">
740<div class="content">
741<pre>$ git diff
742--- a/file.txt
743+++ b/file.txt
744@@ -1 +1,2 @@
745 hello world!
746+hello world, again
747$ git add file.txt
748$ git diff</pre>
749</div>
750</div>
751<div class="paragraph">
752<p>The last diff is empty, but no new commits have been made, and the
753head still doesn&#8217;t contain the new line:</p>
754</div>
755<div class="listingblock">
756<div class="content">
757<pre>$ git diff HEAD
758diff --git a/file.txt b/file.txt
759index a042389..513feba 100644
760--- a/file.txt
761+++ b/file.txt
762@@ -1 +1,2 @@
763 hello world!
764+hello world, again</pre>
765</div>
766</div>
767<div class="paragraph">
768<p>So <em>git diff</em> is comparing against something other than the head.
769The thing that it&#8217;s comparing against is actually the index file,
770which is stored in .git/index in a binary format, but whose contents
771we can examine with ls-files:</p>
772</div>
773<div class="listingblock">
774<div class="content">
775<pre>$ git ls-files --stage
776100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt
777$ git cat-file -t 513feba2
778blob
779$ git cat-file blob 513feba2
780hello world!
781hello world, again</pre>
782</div>
783</div>
784<div class="paragraph">
785<p>So what our <em>git add</em> did was store a new blob and then put
786a reference to it in the index file. If we modify the file again,
787we&#8217;ll see that the new modifications are reflected in the <em>git diff</em>
788output:</p>
789</div>
790<div class="listingblock">
791<div class="content">
792<pre>$ echo 'again?' &gt;&gt;file.txt
793$ git diff
794index 513feba..ba3da7b 100644
795--- a/file.txt
796+++ b/file.txt
797@@ -1,2 +1,3 @@
798 hello world!
799 hello world, again
800+again?</pre>
801</div>
802</div>
803<div class="paragraph">
804<p>With the right arguments, <em>git diff</em> can also show us the difference
805between the working directory and the last commit, or between the
806index and the last commit:</p>
807</div>
808<div class="listingblock">
809<div class="content">
810<pre>$ git diff HEAD
811diff --git a/file.txt b/file.txt
812index a042389..ba3da7b 100644
813--- a/file.txt
814+++ b/file.txt
815@@ -1 +1,3 @@
816 hello world!
817+hello world, again
818+again?
819$ git diff --cached
820diff --git a/file.txt b/file.txt
821index a042389..513feba 100644
822--- a/file.txt
823+++ b/file.txt
824@@ -1 +1,2 @@
825 hello world!
826+hello world, again</pre>
827</div>
828</div>
829<div class="paragraph">
830<p>At any time, we can create a new commit using <em>git commit</em> (without
831the "-a" option), and verify that the state committed only includes the
832changes stored in the index file, not the additional change that is
833still only in our working tree:</p>
834</div>
835<div class="listingblock">
836<div class="content">
837<pre>$ git commit -m "repeat"
838$ git diff HEAD
839diff --git a/file.txt b/file.txt
840index 513feba..ba3da7b 100644
841--- a/file.txt
842+++ b/file.txt
843@@ -1,2 +1,3 @@
844 hello world!
845 hello world, again
846+again?</pre>
847</div>
848</div>
849<div class="paragraph">
850<p>So by default <em>git commit</em> uses the index to create the commit, not
851the working tree; the "-a" option to commit tells it to first update
852the index with all changes in the working tree.</p>
853</div>
854<div class="paragraph">
855<p>Finally, it&#8217;s worth looking at the effect of <em>git add</em> on the index
856file:</p>
857</div>
858<div class="listingblock">
859<div class="content">
860<pre>$ echo "goodbye, world" &gt;closing.txt
861$ git add closing.txt</pre>
862</div>
863</div>
864<div class="paragraph">
865<p>The effect of the <em>git add</em> was to add one entry to the index file:</p>
866</div>
867<div class="listingblock">
868<div class="content">
869<pre>$ git ls-files --stage
870100644 8b9743b20d4b15be3955fc8d5cd2b09cd2336138 0 closing.txt
871100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt</pre>
872</div>
873</div>
874<div class="paragraph">
875<p>And, as you can see with cat-file, this new entry refers to the
876current contents of the file:</p>
877</div>
878<div class="listingblock">
879<div class="content">
880<pre>$ git cat-file blob 8b9743b2
881goodbye, world</pre>
882</div>
883</div>
884<div class="paragraph">
885<p>The "status" command is a useful way to get a quick summary of the
886situation:</p>
887</div>
888<div class="listingblock">
889<div class="content">
890<pre>$ git status
891On branch master
892Changes to be committed:
893 (use "git restore --staged &lt;file&gt;..." to unstage)
894
895 new file: closing.txt
896
897Changes not staged for commit:
898 (use "git add &lt;file&gt;..." to update what will be committed)
899 (use "git restore &lt;file&gt;..." to discard changes in working directory)
900
901 modified: file.txt</pre>
902</div>
903</div>
904<div class="paragraph">
905<p>Since the current state of closing.txt is cached in the index file,
906it is listed as "Changes to be committed". Since file.txt has
907changes in the working directory that aren&#8217;t reflected in the index,
908it is marked "changed but not updated". At this point, running "git
909commit" would create a commit that added closing.txt (with its new
910contents), but that didn&#8217;t modify file.txt.</p>
911</div>
912<div class="paragraph">
913<p>Also, note that a bare <code>git diff</code> shows the changes to file.txt, but
914not the addition of closing.txt, because the version of closing.txt
915in the index file is identical to the one in the working directory.</p>
916</div>
917<div class="paragraph">
918<p>In addition to being the staging area for new commits, the index file
919is also populated from the object database when checking out a
920branch, and is used to hold the trees involved in a merge operation.
921See <a href="gitcore-tutorial.html">gitcore-tutorial(7)</a> and the relevant man
922pages for details.</p>
923</div>
924</div>
925</div>
926<div class="sect1">
927<h2 id="_what_next">What next?</h2>
928<div class="sectionbody">
929<div class="paragraph">
930<p>At this point you should know everything necessary to read the man
931pages for any of the git commands; one good place to start would be
932with the commands mentioned in <a href="giteveryday.html">giteveryday(7)</a>. You
933should be able to find any unknown jargon in <a href="gitglossary.html">gitglossary(7)</a>.</p>
934</div>
935<div class="paragraph">
936<p>The <a href="user-manual.html">Git User&#8217;s Manual</a> provides a more
937comprehensive introduction to Git.</p>
938</div>
939<div class="paragraph">
940<p><a href="gitcvs-migration.html">gitcvs-migration(7)</a> explains how to
941import a CVS repository into Git, and shows how to use Git in a
942CVS-like way.</p>
943</div>
944<div class="paragraph">
945<p>For some interesting examples of Git use, see the
946<a href="howto-index.html">howtos</a>.</p>
947</div>
948<div class="paragraph">
949<p>For Git developers, <a href="gitcore-tutorial.html">gitcore-tutorial(7)</a> goes
950into detail on the lower-level Git mechanisms involved in, for
951example, creating a new commit.</p>
952</div>
953</div>
954</div>
955<div class="sect1">
956<h2 id="_see_also">SEE ALSO</h2>
957<div class="sectionbody">
958<div class="paragraph">
959<p><a href="gittutorial.html">gittutorial(7)</a>,
960<a href="gitcvs-migration.html">gitcvs-migration(7)</a>,
961<a href="gitcore-tutorial.html">gitcore-tutorial(7)</a>,
962<a href="gitglossary.html">gitglossary(7)</a>,
963<a href="git-help.html">git-help(1)</a>,
964<a href="giteveryday.html">giteveryday(7)</a>,
965<a href="user-manual.html">The Git User&#8217;s Manual</a></p>
966</div>
967</div>
968</div>
969<div class="sect1">
970<h2 id="_git">GIT</h2>
971<div class="sectionbody">
972<div class="paragraph">
973<p>Part of the <a href="git.html">git(1)</a> suite</p>
974</div>
975</div>
976</div>
977</div>
978<div id="footer">
979<div id="footer-text">
980Last updated 2020-03-10 15:02:33 -0700
981</div>
982</div>
983</body>
984</html>