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 735 <body class="manpage">
 736 <div id="header">
 737 <h1>
 738 gittutorial-2(7) Manual Page
 739 </h1>
 740 <h2>NAME</h2>
 741 <div class="sectionbody">
 742 <p>gittutorial-2 -
 743    A tutorial introduction to Git: part two
 744 </p>
 745 </div>
 746 </div>
 747 <div id="content">
 748 <div class="sect1">
 749 <h2 id="_synopsis">SYNOPSIS</h2>
 750 <div class="sectionbody">
 751 <div class="verseblock">
 752 <pre class="content">git *</pre>
 753 <div class="attribution">
 754 </div></div>
 755 </div>
 756 </div>
 757 <div class="sect1">
 758 <h2 id="_description">DESCRIPTION</h2>
 759 <div class="sectionbody">
 760 <div class="paragraph"><p>You should work through <a href="gittutorial.html">gittutorial(7)</a> before reading this tutorial.</p></div>
 761 <div class="paragraph"><p>The goal of this tutorial is to introduce two fundamental pieces of
 762 Git&#8217;s architecture&#8212;the object database and the index file&#8212;and to
 763 provide the reader with everything necessary to understand the rest
 764 of the Git documentation.</p></div>
 765 </div>
 766 </div>
 767 <div class="sect1">
 768 <h2 id="_the_git_object_database">The Git object database</h2>
 769 <div class="sectionbody">
 770 <div class="paragraph"><p>Let&#8217;s start a new project and create a small amount of history:</p></div>
 771 <div class="listingblock">
 772 <div class="content">
 773 <pre><code>$ mkdir test-project
 774 $ cd test-project
 775 $ git init
 776 Initialized empty Git repository in .git/
 777 $ echo 'hello world' &gt; file.txt
 778 $ git add .
 779 $ git commit -a -m "initial commit"
 780 [master (root-commit) 54196cc] initial commit
 781  1 file changed, 1 insertion(+)
 782  create mode 100644 file.txt
 783 $ echo 'hello world!' &gt;file.txt
 784 $ git commit -a -m "add emphasis"
 785 [master c4d59f3] add emphasis
 786  1 file changed, 1 insertion(+), 1 deletion(-)</code></pre>
 787 </div></div>
 788 <div class="paragraph"><p>What are the 7 digits of hex that Git responded to the commit with?</p></div>
 789 <div class="paragraph"><p>We saw in part one of the tutorial that commits have names like this.
 790 It turns out that every object in the Git history is stored under
 791 a 40-digit hex name.  That name is the SHA-1 hash of the object&#8217;s
 792 contents; among other things, this ensures that Git will never store
 793 the same data twice (since identical data is given an identical SHA-1
 794 name), and that the contents of a Git object will never change (since
 795 that would change the object&#8217;s name as well). The 7 char hex strings
 796 here are simply the abbreviation of such 40 character long strings.
 797 Abbreviations can be used everywhere where the 40 character strings
 798 can be used, so long as they are unambiguous.</p></div>
 799 <div class="paragraph"><p>It is expected that the content of the commit object you created while
 800 following the example above generates a different SHA-1 hash than
 801 the one shown above because the commit object records the time when
 802 it was created and the name of the person performing the commit.</p></div>
 803 <div class="paragraph"><p>We can ask Git about this particular object with the <code>cat-file</code>
 804 command. Don&#8217;t copy the 40 hex digits from this example but use those
 805 from your own version. Note that you can shorten it to only a few
 806 characters to save yourself typing all 40 hex digits:</p></div>
 807 <div class="listingblock">
 808 <div class="content">
 809 <pre><code>$ git cat-file -t 54196cc2
 810 commit
 811 $ git cat-file commit 54196cc2
 812 tree 92b8b694ffb1675e5975148e1121810081dbdffe
 813 author J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143414668 -0500
 814 committer J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143414668 -0500
 815
 816 initial commit</code></pre>
 817 </div></div>
 818 <div class="paragraph"><p>A tree can refer to one or more "blob" objects, each corresponding to
 819 a file.  In addition, a tree can also refer to other tree objects,
 820 thus creating a directory hierarchy.  You can examine the contents of
 821 any tree using ls-tree (remember that a long enough initial portion
 822 of the SHA-1 will also work):</p></div>
 823 <div class="listingblock">
 824 <div class="content">
 825 <pre><code>$ git ls-tree 92b8b694
 826 100644 blob 3b18e512dba79e4c8300dd08aeb37f8e728b8dad    file.txt</code></pre>
 827 </div></div>
 828 <div class="paragraph"><p>Thus we see that this tree has one file in it.  The SHA-1 hash is a
 829 reference to that file&#8217;s data:</p></div>
 830 <div class="listingblock">
 831 <div class="content">
 832 <pre><code>$ git cat-file -t 3b18e512
 833 blob</code></pre>
 834 </div></div>
 835 <div class="paragraph"><p>A "blob" is just file data, which we can also examine with cat-file:</p></div>
 836 <div class="listingblock">
 837 <div class="content">
 838 <pre><code>$ git cat-file blob 3b18e512
 839 hello world</code></pre>
 840 </div></div>
 841 <div class="paragraph"><p>Note that this is the old file data; so the object that Git named in
 842 its response to the initial tree was a tree with a snapshot of the
 843 directory state that was recorded by the first commit.</p></div>
 844 <div class="paragraph"><p>All of these objects are stored under their SHA-1 names inside the Git
 845 directory:</p></div>
 846 <div class="listingblock">
 847 <div class="content">
 848 <pre><code>$ find .git/objects/
 849 .git/objects/
 850 .git/objects/pack
 851 .git/objects/info
 852 .git/objects/3b
 853 .git/objects/3b/18e512dba79e4c8300dd08aeb37f8e728b8dad
 854 .git/objects/92
 855 .git/objects/92/b8b694ffb1675e5975148e1121810081dbdffe
 856 .git/objects/54
 857 .git/objects/54/196cc2703dc165cbd373a65a4dcf22d50ae7f7
 858 .git/objects/a0
 859 .git/objects/a0/423896973644771497bdc03eb99d5281615b51
 860 .git/objects/d0
 861 .git/objects/d0/492b368b66bdabf2ac1fd8c92b39d3db916e59
 862 .git/objects/c4
 863 .git/objects/c4/d59f390b9cfd4318117afde11d601c1085f241</code></pre>
 864 </div></div>
 865 <div class="paragraph"><p>and the contents of these files is just the compressed data plus a
 866 header identifying their length and their type.  The type is either a
 867 blob, a tree, a commit, or a tag.</p></div>
 868 <div class="paragraph"><p>The simplest commit to find is the HEAD commit, which we can find
 869 from .git/HEAD:</p></div>
 870 <div class="listingblock">
 871 <div class="content">
 872 <pre><code>$ cat .git/HEAD
 873 ref: refs/heads/master</code></pre>
 874 </div></div>
 875 <div class="paragraph"><p>As you can see, this tells us which branch we&#8217;re currently on, and it
 876 tells us this by naming a file under the .git directory, which itself
 877 contains a SHA-1 name referring to a commit object, which we can
 878 examine with cat-file:</p></div>
 879 <div class="listingblock">
 880 <div class="content">
 881 <pre><code>$ cat .git/refs/heads/master
 882 c4d59f390b9cfd4318117afde11d601c1085f241
 883 $ git cat-file -t c4d59f39
 884 commit
 885 $ git cat-file commit c4d59f39
 886 tree d0492b368b66bdabf2ac1fd8c92b39d3db916e59
 887 parent 54196cc2703dc165cbd373a65a4dcf22d50ae7f7
 888 author J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143418702 -0500
 889 committer J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143418702 -0500
 890
 891 add emphasis</code></pre>
 892 </div></div>
 893 <div class="paragraph"><p>The "tree" object here refers to the new state of the tree:</p></div>
 894 <div class="listingblock">
 895 <div class="content">
 896 <pre><code>$ git ls-tree d0492b36
 897 100644 blob a0423896973644771497bdc03eb99d5281615b51    file.txt
 898 $ git cat-file blob a0423896
 899 hello world!</code></pre>
 900 </div></div>
 901 <div class="paragraph"><p>and the "parent" object refers to the previous commit:</p></div>
 902 <div class="listingblock">
 903 <div class="content">
 904 <pre><code>$ git cat-file commit 54196cc2
 905 tree 92b8b694ffb1675e5975148e1121810081dbdffe
 906 author J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143414668 -0500
 907 committer J. Bruce Fields &lt;bfields@puzzle.fieldses.org&gt; 1143414668 -0500
 908
 909 initial commit</code></pre>
 910 </div></div>
 911 <div class="paragraph"><p>The tree object is the tree we examined first, and this commit is
 912 unusual in that it lacks any parent.</p></div>
 913 <div class="paragraph"><p>Most commits have only one parent, but it is also common for a commit
 914 to have multiple parents.   In that case the commit represents a
 915 merge, with the parent references pointing to the heads of the merged
 916 branches.</p></div>
 917 <div class="paragraph"><p>Besides blobs, trees, and commits, the only remaining type of object
 918 is a "tag", which we won&#8217;t discuss here; refer to <a href="git-tag.html">git-tag(1)</a>
 919 for details.</p></div>
 920 <div class="paragraph"><p>So now we know how Git uses the object database to represent a
 921 project&#8217;s history:</p></div>
 922 <div class="ulist"><ul>
 923 <li>
 924 <p>
 925 "commit" objects refer to "tree" objects representing the
 926     snapshot of a directory tree at a particular point in the
 927     history, and refer to "parent" commits to show how they&#8217;re
 928     connected into the project history.
 929 </p>
 930 </li>
 931 <li>
 932 <p>
 933 "tree" objects represent the state of a single directory,
 934     associating directory names to "blob" objects containing file
 935     data and "tree" objects containing subdirectory information.
 936 </p>
 937 </li>
 938 <li>
 939 <p>
 940 "blob" objects contain file data without any other structure.
 941 </p>
 942 </li>
 943 <li>
 944 <p>
 945 References to commit objects at the head of each branch are
 946     stored in files under .git/refs/heads/.
 947 </p>
 948 </li>
 949 <li>
 950 <p>
 951 The name of the current branch is stored in .git/HEAD.
 952 </p>
 953 </li>
 954 </ul></div>
 955 <div class="paragraph"><p>Note, by the way, that lots of commands take a tree as an argument.
 956 But as we can see above, a tree can be referred to in many different
 957 ways&#8212;by the SHA-1 name for that tree, by the name of a commit that
 958 refers to the tree, by the name of a branch whose head refers to that
 959 tree, etc.--and most such commands can accept any of these names.</p></div>
 960 <div class="paragraph"><p>In command synopses, the word "tree-ish" is sometimes used to
 961 designate such an argument.</p></div>
 962 </div>
 963 </div>
 964 <div class="sect1">
 965 <h2 id="_the_index_file">The index file</h2>
 966 <div class="sectionbody">
 967 <div class="paragraph"><p>The primary tool we&#8217;ve been using to create commits is <code>git-commit
 968 -a</code>, which creates a commit including every change you&#8217;ve made to
 969 your working tree.  But what if you want to commit changes only to
 970 certain files?  Or only certain changes to certain files?</p></div>
 971 <div class="paragraph"><p>If we look at the way commits are created under the cover, we&#8217;ll see
 972 that there are more flexible ways creating commits.</p></div>
 973 <div class="paragraph"><p>Continuing with our test-project, let&#8217;s modify file.txt again:</p></div>
 974 <div class="listingblock">
 975 <div class="content">
 976 <pre><code>$ echo "hello world, again" &gt;&gt;file.txt</code></pre>
 977 </div></div>
 978 <div class="paragraph"><p>but this time instead of immediately making the commit, let&#8217;s take an
 979 intermediate step, and ask for diffs along the way to keep track of
 980 what&#8217;s happening:</p></div>
 981 <div class="listingblock">
 982 <div class="content">
 983 <pre><code>$ git diff
 984 --- a/file.txt
 985 +++ b/file.txt
 986 @@ -1 +1,2 @@
 987  hello world!
 988 +hello world, again
 989 $ git add file.txt
 990 $ git diff</code></pre>
 991 </div></div>
 992 <div class="paragraph"><p>The last diff is empty, but no new commits have been made, and the
 993 head still doesn&#8217;t contain the new line:</p></div>
 994 <div class="listingblock">
 995 <div class="content">
 996 <pre><code>$ git diff HEAD
 997 diff --git a/file.txt b/file.txt
 998 index a042389..513feba 100644
 999 --- a/file.txt
1000 +++ b/file.txt
1001 @@ -1 +1,2 @@
1002  hello world!
1003 +hello world, again</code></pre>
1004 </div></div>
1005 <div class="paragraph"><p>So <em>git diff</em> is comparing against something other than the head.
1006 The thing that it&#8217;s comparing against is actually the index file,
1007 which is stored in .git/index in a binary format, but whose contents
1008 we can examine with ls-files:</p></div>
1009 <div class="listingblock">
1010 <div class="content">
1011 <pre><code>$ git ls-files --stage
1012 100644 513feba2e53ebbd2532419ded848ba19de88ba00 0       file.txt
1013 $ git cat-file -t 513feba2
1014 blob
1015 $ git cat-file blob 513feba2
1016 hello world!
1017 hello world, again</code></pre>
1018 </div></div>
1019 <div class="paragraph"><p>So what our <em>git add</em> did was store a new blob and then put
1020 a reference to it in the index file.  If we modify the file again,
1021 we&#8217;ll see that the new modifications are reflected in the <em>git diff</em>
1022 output:</p></div>
1023 <div class="listingblock">
1024 <div class="content">
1025 <pre><code>$ echo 'again?' &gt;&gt;file.txt
1026 $ git diff
1027 index 513feba..ba3da7b 100644
1028 --- a/file.txt
1029 +++ b/file.txt
1030 @@ -1,2 +1,3 @@
1031  hello world!
1032  hello world, again
1033 +again?</code></pre>
1034 </div></div>
1035 <div class="paragraph"><p>With the right arguments, <em>git diff</em> can also show us the difference
1036 between the working directory and the last commit, or between the
1037 index and the last commit:</p></div>
1038 <div class="listingblock">
1039 <div class="content">
1040 <pre><code>$ git diff HEAD
1041 diff --git a/file.txt b/file.txt
1042 index a042389..ba3da7b 100644
1043 --- a/file.txt
1044 +++ b/file.txt
1045 @@ -1 +1,3 @@
1046  hello world!
1047 +hello world, again
1048 +again?
1049 $ git diff --cached
1050 diff --git a/file.txt b/file.txt
1051 index a042389..513feba 100644
1052 --- a/file.txt
1053 +++ b/file.txt
1054 @@ -1 +1,2 @@
1055  hello world!
1056 +hello world, again</code></pre>
1057 </div></div>
1058 <div class="paragraph"><p>At any time, we can create a new commit using <em>git commit</em> (without
1059 the "-a" option), and verify that the state committed only includes the
1060 changes stored in the index file, not the additional change that is
1061 still only in our working tree:</p></div>
1062 <div class="listingblock">
1063 <div class="content">
1064 <pre><code>$ git commit -m "repeat"
1065 $ git diff HEAD
1066 diff --git a/file.txt b/file.txt
1067 index 513feba..ba3da7b 100644
1068 --- a/file.txt
1069 +++ b/file.txt
1070 @@ -1,2 +1,3 @@
1071  hello world!
1072  hello world, again
1073 +again?</code></pre>
1074 </div></div>
1075 <div class="paragraph"><p>So by default <em>git commit</em> uses the index to create the commit, not
1076 the working tree; the "-a" option to commit tells it to first update
1077 the index with all changes in the working tree.</p></div>
1078 <div class="paragraph"><p>Finally, it&#8217;s worth looking at the effect of <em>git add</em> on the index
1079 file:</p></div>
1080 <div class="listingblock">
1081 <div class="content">
1082 <pre><code>$ echo "goodbye, world" &gt;closing.txt
1083 $ git add closing.txt</code></pre>
1084 </div></div>
1085 <div class="paragraph"><p>The effect of the <em>git add</em> was to add one entry to the index file:</p></div>
1086 <div class="listingblock">
1087 <div class="content">
1088 <pre><code>$ git ls-files --stage
1089 100644 8b9743b20d4b15be3955fc8d5cd2b09cd2336138 0       closing.txt
1090 100644 513feba2e53ebbd2532419ded848ba19de88ba00 0       file.txt</code></pre>
1091 </div></div>
1092 <div class="paragraph"><p>And, as you can see with cat-file, this new entry refers to the
1093 current contents of the file:</p></div>
1094 <div class="listingblock">
1095 <div class="content">
1096 <pre><code>$ git cat-file blob 8b9743b2
1097 goodbye, world</code></pre>
1098 </div></div>
1099 <div class="paragraph"><p>The "status" command is a useful way to get a quick summary of the
1100 situation:</p></div>
1101 <div class="listingblock">
1102 <div class="content">
1103 <pre><code>$ git status
1104 On branch master
1105 Changes to be committed:
1106   (use "git restore --staged &lt;file&gt;..." to unstage)
1107
1108         new file:   closing.txt
1109
1110 Changes not staged for commit:
1111   (use "git add &lt;file&gt;..." to update what will be committed)
1112   (use "git restore &lt;file&gt;..." to discard changes in working directory)
1113
1114         modified:   file.txt</code></pre>
1115 </div></div>
1116 <div class="paragraph"><p>Since the current state of closing.txt is cached in the index file,
1117 it is listed as "Changes to be committed".  Since file.txt has
1118 changes in the working directory that aren&#8217;t reflected in the index,
1119 it is marked "changed but not updated".  At this point, running "git
1120 commit" would create a commit that added closing.txt (with its new
1121 contents), but that didn&#8217;t modify file.txt.</p></div>
1122 <div class="paragraph"><p>Also, note that a bare <code>git diff</code> shows the changes to file.txt, but
1123 not the addition of closing.txt, because the version of closing.txt
1124 in the index file is identical to the one in the working directory.</p></div>
1125 <div class="paragraph"><p>In addition to being the staging area for new commits, the index file
1126 is also populated from the object database when checking out a
1127 branch, and is used to hold the trees involved in a merge operation.
1128 See <a href="gitcore-tutorial.html">gitcore-tutorial(7)</a> and the relevant man
1129 pages for details.</p></div>
1130 </div>
1131 </div>
1132 <div class="sect1">
1133 <h2 id="_what_next">What next?</h2>
1134 <div class="sectionbody">
1135 <div class="paragraph"><p>At this point you should know everything necessary to read the man
1136 pages for any of the git commands; one good place to start would be
1137 with the commands mentioned in <a href="giteveryday.html">giteveryday(7)</a>.  You
1138 should be able to find any unknown jargon in <a href="gitglossary.html">gitglossary(7)</a>.</p></div>
1139 <div class="paragraph"><p>The <a href="user-manual.html">Git User&#8217;s Manual</a> provides a more
1140 comprehensive introduction to Git.</p></div>
1141 <div class="paragraph"><p><a href="gitcvs-migration.html">gitcvs-migration(7)</a> explains how to
1142 import a CVS repository into Git, and shows how to use Git in a
1143 CVS-like way.</p></div>
1144 <div class="paragraph"><p>For some interesting examples of Git use, see the
1145 <a href="howto-index.html">howtos</a>.</p></div>
1146 <div class="paragraph"><p>For Git developers, <a href="gitcore-tutorial.html">gitcore-tutorial(7)</a> goes
1147 into detail on the lower-level Git mechanisms involved in, for
1148 example, creating a new commit.</p></div>
1149 </div>
1150 </div>
1151 <div class="sect1">
1152 <h2 id="_see_also">SEE ALSO</h2>
1153 <div class="sectionbody">
1154 <div class="paragraph"><p><a href="gittutorial.html">gittutorial(7)</a>,
1155 <a href="gitcvs-migration.html">gitcvs-migration(7)</a>,
1156 <a href="gitcore-tutorial.html">gitcore-tutorial(7)</a>,
1157 <a href="gitglossary.html">gitglossary(7)</a>,
1158 <a href="git-help.html">git-help(1)</a>,
1159 <a href="giteveryday.html">giteveryday(7)</a>,
1160 <a href="user-manual.html">The Git User&#8217;s Manual</a></p></div>
1161 </div>
1162 </div>
1163 <div class="sect1">
1164 <h2 id="_git">GIT</h2>
1165 <div class="sectionbody">
1166 <div class="paragraph"><p>Part of the <a href="git.html">git(1)</a> suite</p></div>
1167 </div>
1168 </div>
1169 </div>
1170 <div id="footnotes"><hr /></div>
1171 <div id="footer">
1172 <div id="footer-text">
1173 Last updated
1174  2020-03-10 15:02:33 PDT
1175 </div>
1176 </div>
1177 </body>
1178 </html>