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chromium / external / gitster / git-htmldocs / b358e217ee50e22d66a3a5ababe7e84c41d553c4 / . / gittutorial-2.html
blob: b2f5a70e2a7aba198cf8a2cdc52475be3c6299c1 [file] [log] [blame]
Junio C Hamanob96f40a2024-08-01 00:57:25[diff] [blame]1<!DOCTYPE html>
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Junio C Hamanob96f40a2024-08-01 00:57:25[diff] [blame]8<title>gittutorial-2(7)</title>
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442<body class="manpage">
443<div id="header">
444<h1>gittutorial-2(7) Manual Page</h1>
445<h2 id="_name">NAME</h2>
446<div class="sectionbody">
447<p>gittutorial-2 - A tutorial introduction to Git: part two</p>
448</div>
449</div>
450<div id="content">
451<div class="sect1">
452<h2 id="_synopsis">SYNOPSIS</h2>
453<div class="sectionbody">
454<div class="verseblock">
455<pre class="content">git *</pre>
456</div>
457</div>
458</div>
459<div class="sect1">
460<h2 id="_description">DESCRIPTION</h2>
461<div class="sectionbody">
462<div class="paragraph">
463<p>You should work through <a href="gittutorial.html">gittutorial(7)</a> before reading this tutorial.</p>
464</div>
465<div class="paragraph">
466<p>The goal of this tutorial is to introduce two fundamental pieces of
467Git&#8217;s architecture&#8212;&#8203;the object database and the index file&#8212;&#8203;and to
468provide the reader with everything necessary to understand the rest
469of the Git documentation.</p>
470</div>
471</div>
472</div>
473<div class="sect1">
474<h2 id="_the_git_object_database">The Git object database</h2>
475<div class="sectionbody">
476<div class="paragraph">
477<p>Let&#8217;s start a new project and create a small amount of history:</p>
478</div>
479<div class="listingblock">
480<div class="content">
481<pre>$ mkdir test-project
482$ cd test-project
483$ git init
484Initialized empty Git repository in .git/
485$ echo 'hello world' &gt; file.txt
486$ git add .
487$ git commit -a -m "initial commit"
488[master (root-commit) 54196cc] initial commit
489 1 file changed, 1 insertion(+)
490 create mode 100644 file.txt
491$ echo 'hello world!' &gt;file.txt
492$ git commit -a -m "add emphasis"
493[master c4d59f3] add emphasis
494 1 file changed, 1 insertion(+), 1 deletion(-)</pre>
495</div>
496</div>
497<div class="paragraph">
498<p>What are the 7 digits of hex that Git responded to the commit with?</p>
499</div>
500<div class="paragraph">
501<p>We saw in part one of the tutorial that commits have names like this.
502It turns out that every object in the Git history is stored under
503a 40-digit hex name. That name is the SHA-1 hash of the object&#8217;s
504contents; among other things, this ensures that Git will never store
505the same data twice (since identical data is given an identical SHA-1
506name), and that the contents of a Git object will never change (since
507that would change the object&#8217;s name as well). The 7 char hex strings
508here are simply the abbreviation of such 40 character long strings.
509Abbreviations can be used everywhere where the 40 character strings
510can be used, so long as they are unambiguous.</p>
511</div>
512<div class="paragraph">
513<p>It is expected that the content of the commit object you created while
514following the example above generates a different SHA-1 hash than
515the one shown above because the commit object records the time when
516it was created and the name of the person performing the commit.</p>
517</div>
518<div class="paragraph">
519<p>We can ask Git about this particular object with the <code>cat-file</code>
520command. Don&#8217;t copy the 40 hex digits from this example but use those
521from your own version. Note that you can shorten it to only a few
522characters to save yourself typing all 40 hex digits:</p>
523</div>
524<div class="listingblock">
525<div class="content">
526<pre>$ git cat-file -t 54196cc2
527commit
528$ git cat-file commit 54196cc2
529tree 92b8b694ffb1675e5975148e1121810081dbdffe
530author J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143414668 -0500
531committer J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143414668 -0500
532
533initial commit</pre>
534</div>
535</div>
536<div class="paragraph">
537<p>A tree can refer to one or more "blob" objects, each corresponding to
538a file. In addition, a tree can also refer to other tree objects,
539thus creating a directory hierarchy. You can examine the contents of
540any tree using ls-tree (remember that a long enough initial portion
541of the SHA-1 will also work):</p>
542</div>
543<div class="listingblock">
544<div class="content">
545<pre>$ git ls-tree 92b8b694
546100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad file.txt</pre>
547</div>
548</div>
549<div class="paragraph">
550<p>Thus we see that this tree has one file in it. The SHA-1 hash is a
551reference to that file&#8217;s data:</p>
552</div>
553<div class="listingblock">
554<div class="content">
555<pre>$ git cat-file -t 3b18e512
556blob</pre>
557</div>
558</div>
559<div class="paragraph">
560<p>A "blob" is just file data, which we can also examine with cat-file:</p>
561</div>
562<div class="listingblock">
563<div class="content">
564<pre>$ git cat-file blob 3b18e512
565hello world</pre>
566</div>
567</div>
568<div class="paragraph">
569<p>Note that this is the old file data; so the object that Git named in
570its response to the initial tree was a tree with a snapshot of the
571directory state that was recorded by the first commit.</p>
572</div>
573<div class="paragraph">
574<p>All of these objects are stored under their SHA-1 names inside the Git
575directory:</p>
576</div>
577<div class="listingblock">
578<div class="content">
579<pre>$ find .git/objects/
580.git/objects/
581.git/objects/pack
582.git/objects/info
583.git/objects/3b
584.git/objects/3b/18e512dba79e4c8300dd08aeb37f8e728b8dad
585.git/objects/92
586.git/objects/92/b8b694ffb1675e5975148e1121810081dbdffe
587.git/objects/54
588.git/objects/54/196cc2703dc165cbd373a65a4dcf22d50ae7f7
589.git/objects/a0
590.git/objects/a0/423896973644771497bdc03eb99d5281615b51
591.git/objects/d0
592.git/objects/d0/492b368b66bdabf2ac1fd8c92b39d3db916e59
593.git/objects/c4
594.git/objects/c4/d59f390b9cfd4318117afde11d601c1085f241</pre>
595</div>
596</div>
597<div class="paragraph">
598<p>and the contents of these files is just the compressed data plus a
599header identifying their length and their type. The type is either a
600blob, a tree, a commit, or a tag.</p>
601</div>
602<div class="paragraph">
603<p>The simplest commit to find is the HEAD commit, which we can find
604from .git/HEAD:</p>
605</div>
606<div class="listingblock">
607<div class="content">
608<pre>$ cat .git/HEAD
609ref: refs/heads/master</pre>
610</div>
611</div>
612<div class="paragraph">
613<p>As you can see, this tells us which branch we&#8217;re currently on, and it
614tells us this by naming a file under the .git directory, which itself
615contains a SHA-1 name referring to a commit object, which we can
616examine with cat-file:</p>
617</div>
618<div class="listingblock">
619<div class="content">
620<pre>$ cat .git/refs/heads/master
621c4d59f390b9cfd4318117afde11d601c1085f241
622$ git cat-file -t c4d59f39
623commit
624$ git cat-file commit c4d59f39
625tree d0492b368b66bdabf2ac1fd8c92b39d3db916e59
626parent 54196cc2703dc165cbd373a65a4dcf22d50ae7f7
627author J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143418702 -0500
628committer J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143418702 -0500
629
630add emphasis</pre>
631</div>
632</div>
633<div class="paragraph">
634<p>The "tree" object here refers to the new state of the tree:</p>
635</div>
636<div class="listingblock">
637<div class="content">
638<pre>$ git ls-tree d0492b36
639100644 blob a0423896973644771497bdc03eb99d5281615b51 file.txt
640$ git cat-file blob a0423896
641hello world!</pre>
642</div>
643</div>
644<div class="paragraph">
645<p>and the "parent" object refers to the previous commit:</p>
646</div>
647<div class="listingblock">
648<div class="content">
649<pre>$ git cat-file commit 54196cc2
650tree 92b8b694ffb1675e5975148e1121810081dbdffe
651author J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143414668 -0500
652committer J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143414668 -0500
653
654initial commit</pre>
655</div>
656</div>
657<div class="paragraph">
658<p>The tree object is the tree we examined first, and this commit is
659unusual in that it lacks any parent.</p>
660</div>
661<div class="paragraph">
662<p>Most commits have only one parent, but it is also common for a commit
663to have multiple parents. In that case the commit represents a
664merge, with the parent references pointing to the heads of the merged
665branches.</p>
666</div>
667<div class="paragraph">
668<p>Besides blobs, trees, and commits, the only remaining type of object
669is a "tag", which we won&#8217;t discuss here; refer to <a href="git-tag.html">git-tag(1)</a>
670for details.</p>
671</div>
672<div class="paragraph">
673<p>So now we know how Git uses the object database to represent a
674project&#8217;s history:</p>
675</div>
676<div class="ulist">
677<ul>
678<li>
679<p>"commit" objects refer to "tree" objects representing the
680snapshot of a directory tree at a particular point in the
681history, and refer to "parent" commits to show how they&#8217;re
682connected into the project history.</p>
683</li>
684<li>
685<p>"tree" objects represent the state of a single directory,
686associating directory names to "blob" objects containing file
687data and "tree" objects containing subdirectory information.</p>
688</li>
689<li>
690<p>"blob" objects contain file data without any other structure.</p>
691</li>
692<li>
693<p>References to commit objects at the head of each branch are
694stored in files under .git/refs/heads/.</p>
695</li>
696<li>
697<p>The name of the current branch is stored in .git/HEAD.</p>
698</li>
699</ul>
700</div>
701<div class="paragraph">
702<p>Note, by the way, that lots of commands take a tree as an argument.
703But as we can see above, a tree can be referred to in many different
704ways&#8212;&#8203;by the SHA-1 name for that tree, by the name of a commit that
705refers to the tree, by the name of a branch whose head refers to that
706tree, etc.--and most such commands can accept any of these names.</p>
707</div>
708<div class="paragraph">
709<p>In command synopses, the word "tree-ish" is sometimes used to
710designate such an argument.</p>
711</div>
712</div>
713</div>
714<div class="sect1">
715<h2 id="_the_index_file">The index file</h2>
716<div class="sectionbody">
717<div class="paragraph">
Junio C Hamanodc392382024-10-11 18:57:29[diff] [blame]718<p>The primary tool we&#8217;ve been using to create commits is <code>git-commit</code>
719<code>-a</code>, which creates a commit including every change you&#8217;ve made to
Junio C Hamanob96f40a2024-08-01 00:57:25[diff] [blame]720your working tree. But what if you want to commit changes only to
721certain files? Or only certain changes to certain files?</p>
722</div>
723<div class="paragraph">
724<p>If we look at the way commits are created under the cover, we&#8217;ll see
725that there are more flexible ways creating commits.</p>
726</div>
727<div class="paragraph">
728<p>Continuing with our test-project, let&#8217;s modify file.txt again:</p>
729</div>
730<div class="listingblock">
731<div class="content">
732<pre>$ echo "hello world, again" &gt;&gt;file.txt</pre>
733</div>
734</div>
735<div class="paragraph">
736<p>but this time instead of immediately making the commit, let&#8217;s take an
737intermediate step, and ask for diffs along the way to keep track of
738what&#8217;s happening:</p>
739</div>
740<div class="listingblock">
741<div class="content">
742<pre>$ git diff
743--- a/file.txt
744+++ b/file.txt
745@@ -1 +1,2 @@
746 hello world!
747+hello world, again
748$ git add file.txt
749$ git diff</pre>
750</div>
751</div>
752<div class="paragraph">
753<p>The last diff is empty, but no new commits have been made, and the
754head still doesn&#8217;t contain the new line:</p>
755</div>
756<div class="listingblock">
757<div class="content">
758<pre>$ git diff HEAD
759diff --git a/file.txt b/file.txt
760index a042389..513feba 100644
761--- a/file.txt
762+++ b/file.txt
763@@ -1 +1,2 @@
764 hello world!
765+hello world, again</pre>
766</div>
767</div>
768<div class="paragraph">
769<p>So <em>git diff</em> is comparing against something other than the head.
770The thing that it&#8217;s comparing against is actually the index file,
771which is stored in .git/index in a binary format, but whose contents
772we can examine with ls-files:</p>
773</div>
774<div class="listingblock">
775<div class="content">
776<pre>$ git ls-files --stage
777100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt
778$ git cat-file -t 513feba2
779blob
780$ git cat-file blob 513feba2
781hello world!
782hello world, again</pre>
783</div>
784</div>
785<div class="paragraph">
786<p>So what our <em>git add</em> did was store a new blob and then put
787a reference to it in the index file. If we modify the file again,
788we&#8217;ll see that the new modifications are reflected in the <em>git diff</em>
789output:</p>
790</div>
791<div class="listingblock">
792<div class="content">
793<pre>$ echo 'again?' &gt;&gt;file.txt
794$ git diff
795index 513feba..ba3da7b 100644
796--- a/file.txt
797+++ b/file.txt
798@@ -1,2 +1,3 @@
799 hello world!
800 hello world, again
801+again?</pre>
802</div>
803</div>
804<div class="paragraph">
805<p>With the right arguments, <em>git diff</em> can also show us the difference
806between the working directory and the last commit, or between the
807index and the last commit:</p>
808</div>
809<div class="listingblock">
810<div class="content">
811<pre>$ git diff HEAD
812diff --git a/file.txt b/file.txt
813index a042389..ba3da7b 100644
814--- a/file.txt
815+++ b/file.txt
816@@ -1 +1,3 @@
817 hello world!
818+hello world, again
819+again?
820$ git diff --cached
821diff --git a/file.txt b/file.txt
822index a042389..513feba 100644
823--- a/file.txt
824+++ b/file.txt
825@@ -1 +1,2 @@
826 hello world!
827+hello world, again</pre>
828</div>
829</div>
830<div class="paragraph">
831<p>At any time, we can create a new commit using <em>git commit</em> (without
832the "-a" option), and verify that the state committed only includes the
833changes stored in the index file, not the additional change that is
834still only in our working tree:</p>
835</div>
836<div class="listingblock">
837<div class="content">
838<pre>$ git commit -m "repeat"
839$ git diff HEAD
840diff --git a/file.txt b/file.txt
841index 513feba..ba3da7b 100644
842--- a/file.txt
843+++ b/file.txt
844@@ -1,2 +1,3 @@
845 hello world!
846 hello world, again
847+again?</pre>
848</div>
849</div>
850<div class="paragraph">
851<p>So by default <em>git commit</em> uses the index to create the commit, not
852the working tree; the "-a" option to commit tells it to first update
853the index with all changes in the working tree.</p>
854</div>
855<div class="paragraph">
856<p>Finally, it&#8217;s worth looking at the effect of <em>git add</em> on the index
857file:</p>
858</div>
859<div class="listingblock">
860<div class="content">
861<pre>$ echo "goodbye, world" &gt;closing.txt
862$ git add closing.txt</pre>
863</div>
864</div>
865<div class="paragraph">
866<p>The effect of the <em>git add</em> was to add one entry to the index file:</p>
867</div>
868<div class="listingblock">
869<div class="content">
870<pre>$ git ls-files --stage
871100644 8b9743b20d4b15be3955fc8d5cd2b09cd2336138 0 closing.txt
872100644 513feba2e53ebbd2532419ded848ba19de88ba00 0 file.txt</pre>
873</div>
874</div>
875<div class="paragraph">
876<p>And, as you can see with cat-file, this new entry refers to the
877current contents of the file:</p>
878</div>
879<div class="listingblock">
880<div class="content">
881<pre>$ git cat-file blob 8b9743b2
882goodbye, world</pre>
883</div>
884</div>
885<div class="paragraph">
886<p>The "status" command is a useful way to get a quick summary of the
887situation:</p>
888</div>
889<div class="listingblock">
890<div class="content">
891<pre>$ git status
892On branch master
893Changes to be committed:
894 (use "git restore --staged &lt;file&gt;..." to unstage)
895
896 new file: closing.txt
897
898Changes not staged for commit:
899 (use "git add &lt;file&gt;..." to update what will be committed)
900 (use "git restore &lt;file&gt;..." to discard changes in working directory)
901
902 modified: file.txt</pre>
903</div>
904</div>
905<div class="paragraph">
906<p>Since the current state of closing.txt is cached in the index file,
907it is listed as "Changes to be committed". Since file.txt has
908changes in the working directory that aren&#8217;t reflected in the index,
909it is marked "changed but not updated". At this point, running "git
910commit" would create a commit that added closing.txt (with its new
911contents), but that didn&#8217;t modify file.txt.</p>
912</div>
913<div class="paragraph">
Junio C Hamanodc392382024-10-11 18:57:29[diff] [blame]914<p>Also, note that a bare <code>git</code> <code>diff</code> shows the changes to file.txt, but
Junio C Hamanob96f40a2024-08-01 00:57:25[diff] [blame]915not the addition of closing.txt, because the version of closing.txt
916in the index file is identical to the one in the working directory.</p>
917</div>
918<div class="paragraph">
919<p>In addition to being the staging area for new commits, the index file
920is also populated from the object database when checking out a
921branch, and is used to hold the trees involved in a merge operation.
922See <a href="gitcore-tutorial.html">gitcore-tutorial(7)</a> and the relevant man
923pages for details.</p>
924</div>
925</div>
926</div>
927<div class="sect1">
928<h2 id="_what_next">What next?</h2>
929<div class="sectionbody">
930<div class="paragraph">
931<p>At this point you should know everything necessary to read the man
932pages for any of the git commands; one good place to start would be
933with the commands mentioned in <a href="giteveryday.html">giteveryday(7)</a>. You
934should be able to find any unknown jargon in <a href="gitglossary.html">gitglossary(7)</a>.</p>
935</div>
936<div class="paragraph">
937<p>The <a href="user-manual.html">Git User&#8217;s Manual</a> provides a more
938comprehensive introduction to Git.</p>
939</div>
940<div class="paragraph">
941<p><a href="gitcvs-migration.html">gitcvs-migration(7)</a> explains how to
942import a CVS repository into Git, and shows how to use Git in a
943CVS-like way.</p>
944</div>
945<div class="paragraph">
946<p>For some interesting examples of Git use, see the
947<a href="howto-index.html">howtos</a>.</p>
948</div>
949<div class="paragraph">
950<p>For Git developers, <a href="gitcore-tutorial.html">gitcore-tutorial(7)</a> goes
951into detail on the lower-level Git mechanisms involved in, for
952example, creating a new commit.</p>
953</div>
954</div>
955</div>
956<div class="sect1">
957<h2 id="_see_also">SEE ALSO</h2>
958<div class="sectionbody">
959<div class="paragraph">
960<p><a href="gittutorial.html">gittutorial(7)</a>,
961<a href="gitcvs-migration.html">gitcvs-migration(7)</a>,
962<a href="gitcore-tutorial.html">gitcore-tutorial(7)</a>,
963<a href="gitglossary.html">gitglossary(7)</a>,
964<a href="git-help.html">git-help(1)</a>,
965<a href="giteveryday.html">giteveryday(7)</a>,
966<a href="user-manual.html">The Git User&#8217;s Manual</a></p>
967</div>
968</div>
969</div>
970<div class="sect1">
971<h2 id="_git">GIT</h2>
972<div class="sectionbody">
973<div class="paragraph">
974<p>Part of the <a href="git.html">git(1)</a> suite</p>
975</div>
976</div>
977</div>
978</div>
979<div id="footer">
980<div id="footer-text">
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984</body>
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