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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
 466 Git&#8217;s architecture&#8212;&#8203;the object database and the index file&#8212;&#8203;and to
 467 provide the reader with everything necessary to understand the rest
 468 of 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
 483 Initialized 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.
 501 It turns out that every object in the Git history is stored under
 502 a 40-digit hex name.  That name is the SHA-1 hash of the object&#8217;s
 503 contents; among other things, this ensures that Git will never store
 504 the same data twice (since identical data is given an identical SHA-1
 505 name), and that the contents of a Git object will never change (since
 506 that would change the object&#8217;s name as well). The 7 char hex strings
 507 here are simply the abbreviation of such 40 character long strings.
 508 Abbreviations can be used everywhere where the 40 character strings
 509 can 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
 513 following the example above generates a different SHA-1 hash than
 514 the one shown above because the commit object records the time when
 515 it 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>
 519 command. Don&#8217;t copy the 40 hex digits from this example but use those
 520 from your own version. Note that you can shorten it to only a few
 521 characters 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
 526 commit
 527 $ git cat-file commit 54196cc2
 528 tree 92b8b694ffb1675e5975148e1121810081dbdffe
 529 author J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143414668 -0500
 530 committer J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143414668 -0500
 531
 532 initial 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
 537 a file.  In addition, a tree can also refer to other tree objects,
 538 thus creating a directory hierarchy.  You can examine the contents of
 539 any tree using ls-tree (remember that a long enough initial portion
 540 of the SHA-1 will also work):</p>
 541 </div>
 542 <div class="listingblock">
 543 <div class="content">
 544 <pre>$ git ls-tree 92b8b694
 545 100644 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
 550 reference to that file&#8217;s data:</p>
 551 </div>
 552 <div class="listingblock">
 553 <div class="content">
 554 <pre>$ git cat-file -t 3b18e512
 555 blob</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
 564 hello 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
 569 its response to the initial tree was a tree with a snapshot of the
 570 directory 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
 574 directory:</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
 598 header identifying their length and their type.  The type is either a
 599 blob, 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
 603 from .git/HEAD:</p>
 604 </div>
 605 <div class="listingblock">
 606 <div class="content">
 607 <pre>$ cat .git/HEAD
 608 ref: 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
 613 tells us this by naming a file under the .git directory, which itself
 614 contains a SHA-1 name referring to a commit object, which we can
 615 examine with cat-file:</p>
 616 </div>
 617 <div class="listingblock">
 618 <div class="content">
 619 <pre>$ cat .git/refs/heads/master
 620 c4d59f390b9cfd4318117afde11d601c1085f241
 621 $ git cat-file -t c4d59f39
 622 commit
 623 $ git cat-file commit c4d59f39
 624 tree d0492b368b66bdabf2ac1fd8c92b39d3db916e59
 625 parent 54196cc2703dc165cbd373a65a4dcf22d50ae7f7
 626 author J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143418702 -0500
 627 committer J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143418702 -0500
 628
 629 add 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
 638 100644 blob a0423896973644771497bdc03eb99d5281615b51    file.txt
 639 $ git cat-file blob a0423896
 640 hello 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
 649 tree 92b8b694ffb1675e5975148e1121810081dbdffe
 650 author J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143414668 -0500
 651 committer J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143414668 -0500
 652
 653 initial 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
 658 unusual 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
 662 to have multiple parents.   In that case the commit represents a
 663 merge, with the parent references pointing to the heads of the merged
 664 branches.</p>
 665 </div>
 666 <div class="paragraph">
 667 <p>Besides blobs, trees, and commits, the only remaining type of object
 668 is a "tag", which we won&#8217;t discuss here; refer to <a href="git-tag.html">git-tag(1)</a>
 669 for details.</p>
 670 </div>
 671 <div class="paragraph">
 672 <p>So now we know how Git uses the object database to represent a
 673 project&#8217;s history:</p>
 674 </div>
 675 <div class="ulist">
 676 <ul>
 677 <li>
 678 <p>"commit" objects refer to "tree" objects representing the
 679 snapshot of a directory tree at a particular point in the
 680 history, and refer to "parent" commits to show how they&#8217;re
 681 connected into the project history.</p>
 682 </li>
 683 <li>
 684 <p>"tree" objects represent the state of a single directory,
 685 associating directory names to "blob" objects containing file
 686 data 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
 693 stored 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.
 702 But as we can see above, a tree can be referred to in many different
 703 ways&#8212;&#8203;by the SHA-1 name for that tree, by the name of a commit that
 704 refers to the tree, by the name of a branch whose head refers to that
 705 tree, 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
 709 designate 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</code>
 718 <code>-a</code>, which creates a commit including every change you&#8217;ve made to
 719 your working tree.  But what if you want to commit changes only to
 720 certain 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
 724 that 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
 736 intermediate step, and ask for diffs along the way to keep track of
 737 what&#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
 753 head still doesn&#8217;t contain the new line:</p>
 754 </div>
 755 <div class="listingblock">
 756 <div class="content">
 757 <pre>$ git diff HEAD
 758 diff --git a/file.txt b/file.txt
 759 index 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.
 769 The thing that it&#8217;s comparing against is actually the index file,
 770 which is stored in .git/index in a binary format, but whose contents
 771 we can examine with ls-files:</p>
 772 </div>
 773 <div class="listingblock">
 774 <div class="content">
 775 <pre>$ git ls-files --stage
 776 100644 513feba2e53ebbd2532419ded848ba19de88ba00 0       file.txt
 777 $ git cat-file -t 513feba2
 778 blob
 779 $ git cat-file blob 513feba2
 780 hello world!
 781 hello 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
 786 a reference to it in the index file.  If we modify the file again,
 787 we&#8217;ll see that the new modifications are reflected in the <em>git diff</em>
 788 output:</p>
 789 </div>
 790 <div class="listingblock">
 791 <div class="content">
 792 <pre>$ echo 'again?' &gt;&gt;file.txt
 793 $ git diff
 794 index 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
 805 between the working directory and the last commit, or between the
 806 index and the last commit:</p>
 807 </div>
 808 <div class="listingblock">
 809 <div class="content">
 810 <pre>$ git diff HEAD
 811 diff --git a/file.txt b/file.txt
 812 index 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
 820 diff --git a/file.txt b/file.txt
 821 index 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
 831 the "-a" option), and verify that the state committed only includes the
 832 changes stored in the index file, not the additional change that is
 833 still 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
 839 diff --git a/file.txt b/file.txt
 840 index 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
 851 the working tree; the "-a" option to commit tells it to first update
 852 the 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
 856 file:</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
 870 100644 8b9743b20d4b15be3955fc8d5cd2b09cd2336138 0       closing.txt
 871 100644 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
 876 current contents of the file:</p>
 877 </div>
 878 <div class="listingblock">
 879 <div class="content">
 880 <pre>$ git cat-file blob 8b9743b2
 881 goodbye, 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
 886 situation:</p>
 887 </div>
 888 <div class="listingblock">
 889 <div class="content">
 890 <pre>$ git status
 891 On branch master
 892 Changes to be committed:
 893   (use "git restore --staged &lt;file&gt;..." to unstage)
 894
 895         new file:   closing.txt
 896
 897 Changes 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,
 906 it is listed as "Changes to be committed".  Since file.txt has
 907 changes in the working directory that aren&#8217;t reflected in the index,
 908 it is marked "changed but not updated".  At this point, running "git
 909 commit" would create a commit that added closing.txt (with its new
 910 contents), but that didn&#8217;t modify file.txt.</p>
 911 </div>
 912 <div class="paragraph">
 913 <p>Also, note that a bare <code>git</code> <code>diff</code> shows the changes to file.txt, but
 914 not the addition of closing.txt, because the version of closing.txt
 915 in 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
 919 is also populated from the object database when checking out a
 920 branch, and is used to hold the trees involved in a merge operation.
 921 See <a href="gitcore-tutorial.html">gitcore-tutorial(7)</a> and the relevant man
 922 pages 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
 931 pages for any of the git commands; one good place to start would be
 932 with the commands mentioned in <a href="giteveryday.html">giteveryday(7)</a>.  You
 933 should 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
 937 comprehensive introduction to Git.</p>
 938 </div>
 939 <div class="paragraph">
 940 <p><a href="gitcvs-migration.html">gitcvs-migration(7)</a> explains how to
 941 import a CVS repository into Git, and shows how to use Git in a
 942 CVS-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
 950 into detail on the lower-level Git mechanisms involved in, for
 951 example, 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">
 980 Last updated 2020-03-10 15:02:33 -0700
 981 </div>
 982 </div>
 983 </body>
 984 </html>