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 735 <body class="manpage">
 736 <div id="header">
 737 <h1>
 738 git-merge-base(1) Manual Page
 739 </h1>
 740 <h2>NAME</h2>
 741 <div class="sectionbody">
 742 <p>git-merge-base -
 743    Find as good common ancestors as possible for a merge
 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"><em>git merge-base</em> [-a | --all] &lt;commit&gt; &lt;commit&gt;&#8230;
 753 <em>git merge-base</em> [-a | --all] --octopus &lt;commit&gt;&#8230;
 754 <em>git merge-base</em> --is-ancestor &lt;commit&gt; &lt;commit&gt;
 755 <em>git merge-base</em> --independent &lt;commit&gt;&#8230;
 756 <em>git merge-base</em> --fork-point &lt;ref&gt; [&lt;commit&gt;]</pre>
 757 <div class="attribution">
 758 </div></div>
 759 </div>
 760 </div>
 761 <div class="sect1">
 762 <h2 id="_description">DESCRIPTION</h2>
 763 <div class="sectionbody">
 764 <div class="paragraph"><p><em>git merge-base</em> finds best common ancestor(s) between two commits to use
 765 in a three-way merge.  One common ancestor is <em>better</em> than another common
 766 ancestor if the latter is an ancestor of the former.  A common ancestor
 767 that does not have any better common ancestor is a <em>best common
 768 ancestor</em>, i.e. a <em>merge base</em>.  Note that there can be more than one
 769 merge base for a pair of commits.</p></div>
 770 </div>
 771 </div>
 772 <div class="sect1">
 773 <h2 id="_operation_modes">OPERATION MODES</h2>
 774 <div class="sectionbody">
 775 <div class="paragraph"><p>As the most common special case, specifying only two commits on the
 776 command line means computing the merge base between the given two commits.</p></div>
 777 <div class="paragraph"><p>More generally, among the two commits to compute the merge base from,
 778 one is specified by the first commit argument on the command line;
 779 the other commit is a (possibly hypothetical) commit that is a merge
 780 across all the remaining commits on the command line.</p></div>
 781 <div class="paragraph"><p>As a consequence, the <em>merge base</em> is not necessarily contained in each of the
 782 commit arguments if more than two commits are specified. This is different
 783 from <a href="git-show-branch.html">git-show-branch(1)</a> when used with the <code>--merge-base</code> option.</p></div>
 784 <div class="dlist"><dl>
 785 <dt class="hdlist1">
 786 --octopus
 787 </dt>
 788 <dd>
 789 <p>
 790         Compute the best common ancestors of all supplied commits,
 791         in preparation for an n-way merge.  This mimics the behavior
 792         of <em>git show-branch --merge-base</em>.
 793 </p>
 794 </dd>
 795 <dt class="hdlist1">
 796 --independent
 797 </dt>
 798 <dd>
 799 <p>
 800         Instead of printing merge bases, print a minimal subset of
 801         the supplied commits with the same ancestors.  In other words,
 802         among the commits given, list those which cannot be reached
 803         from any other.  This mimics the behavior of <em>git show-branch
 804         --independent</em>.
 805 </p>
 806 </dd>
 807 <dt class="hdlist1">
 808 --is-ancestor
 809 </dt>
 810 <dd>
 811 <p>
 812         Check if the first &lt;commit&gt; is an ancestor of the second &lt;commit&gt;,
 813         and exit with status 0 if true, or with status 1 if not.
 814         Errors are signaled by a non-zero status that is not 1.
 815 </p>
 816 </dd>
 817 <dt class="hdlist1">
 818 --fork-point
 819 </dt>
 820 <dd>
 821 <p>
 822         Find the point at which a branch (or any history that leads
 823         to &lt;commit&gt;) forked from another branch (or any reference)
 824         &lt;ref&gt;. This does not just look for the common ancestor of
 825         the two commits, but also takes into account the reflog of
 826         &lt;ref&gt; to see if the history leading to &lt;commit&gt; forked from
 827         an earlier incarnation of the branch &lt;ref&gt; (see discussion
 828         on this mode below).
 829 </p>
 830 </dd>
 831 </dl></div>
 832 </div>
 833 </div>
 834 <div class="sect1">
 835 <h2 id="_options">OPTIONS</h2>
 836 <div class="sectionbody">
 837 <div class="dlist"><dl>
 838 <dt class="hdlist1">
 839 -a
 840 </dt>
 841 <dt class="hdlist1">
 842 --all
 843 </dt>
 844 <dd>
 845 <p>
 846         Output all merge bases for the commits, instead of just one.
 847 </p>
 848 </dd>
 849 </dl></div>
 850 </div>
 851 </div>
 852 <div class="sect1">
 853 <h2 id="_discussion">DISCUSSION</h2>
 854 <div class="sectionbody">
 855 <div class="paragraph"><p>Given two commits <em>A</em> and <em>B</em>, <code>git merge-base A B</code> will output a commit
 856 which is reachable from both <em>A</em> and <em>B</em> through the parent relationship.</p></div>
 857 <div class="paragraph"><p>For example, with this topology:</p></div>
 858 <div class="literalblock">
 859 <div class="content">
 860 <pre><code>         o---o---o---B
 861         /
 862 ---o---1---o---o---o---A</code></pre>
 863 </div></div>
 864 <div class="paragraph"><p>the merge base between <em>A</em> and <em>B</em> is <em>1</em>.</p></div>
 865 <div class="paragraph"><p>Given three commits <em>A</em>, <em>B</em> and <em>C</em>, <code>git merge-base A B C</code> will compute the
 866 merge base between <em>A</em> and a hypothetical commit <em>M</em>, which is a merge
 867 between <em>B</em> and <em>C</em>.  For example, with this topology:</p></div>
 868 <div class="literalblock">
 869 <div class="content">
 870 <pre><code>       o---o---o---o---C
 871       /
 872      /   o---o---o---B
 873     /   /
 874 ---2---1---o---o---o---A</code></pre>
 875 </div></div>
 876 <div class="paragraph"><p>the result of <code>git merge-base A B C</code> is <em>1</em>.  This is because the
 877 equivalent topology with a merge commit <em>M</em> between <em>B</em> and <em>C</em> is:</p></div>
 878 <div class="literalblock">
 879 <div class="content">
 880 <pre><code>       o---o---o---o---o
 881       /                 \
 882      /   o---o---o---o---M
 883     /   /
 884 ---2---1---o---o---o---A</code></pre>
 885 </div></div>
 886 <div class="paragraph"><p>and the result of <code>git merge-base A M</code> is <em>1</em>.  Commit <em>2</em> is also a
 887 common ancestor between <em>A</em> and <em>M</em>, but <em>1</em> is a better common ancestor,
 888 because <em>2</em> is an ancestor of <em>1</em>.  Hence, <em>2</em> is not a merge base.</p></div>
 889 <div class="paragraph"><p>The result of <code>git merge-base --octopus A B C</code> is <em>2</em>, because <em>2</em> is
 890 the best common ancestor of all commits.</p></div>
 891 <div class="paragraph"><p>When the history involves criss-cross merges, there can be more than one
 892 <em>best</em> common ancestor for two commits.  For example, with this topology:</p></div>
 893 <div class="literalblock">
 894 <div class="content">
 895 <pre><code>---1---o---A
 896     \ /
 897      X
 898     / \
 899 ---2---o---o---B</code></pre>
 900 </div></div>
 901 <div class="paragraph"><p>both <em>1</em> and <em>2</em> are merge-bases of A and B.  Neither one is better than
 902 the other (both are <em>best</em> merge bases).  When the <code>--all</code> option is not given,
 903 it is unspecified which best one is output.</p></div>
 904 <div class="paragraph"><p>A common idiom to check "fast-forward-ness" between two commits A
 905 and B is (or at least used to be) to compute the merge base between
 906 A and B, and check if it is the same as A, in which case, A is an
 907 ancestor of B.  You will see this idiom used often in older scripts.</p></div>
 908 <div class="literalblock">
 909 <div class="content">
 910 <pre><code>A=$(git rev-parse --verify A)
 911 if test "$A" = "$(git merge-base A B)"
 912 then
 913         ... A is an ancestor of B ...
 914 fi</code></pre>
 915 </div></div>
 916 <div class="paragraph"><p>In modern git, you can say this in a more direct way:</p></div>
 917 <div class="literalblock">
 918 <div class="content">
 919 <pre><code>if git merge-base --is-ancestor A B
 920 then
 921         ... A is an ancestor of B ...
 922 fi</code></pre>
 923 </div></div>
 924 <div class="paragraph"><p>instead.</p></div>
 925 </div>
 926 </div>
 927 <div class="sect1">
 928 <h2 id="_discussion_on_fork_point_mode">Discussion on fork-point mode</h2>
 929 <div class="sectionbody">
 930 <div class="paragraph"><p>After working on the <code>topic</code> branch created with <code>git switch -c
 931 topic origin/master</code>, the history of remote-tracking branch
 932 <code>origin/master</code> may have been rewound and rebuilt, leading to a
 933 history of this shape:</p></div>
 934 <div class="literalblock">
 935 <div class="content">
 936 <pre><code>                 o---B2
 937                 /
 938 ---o---o---B1--o---o---o---B (origin/master)
 939         \
 940          B0
 941           \
 942            D0---D1---D (topic)</code></pre>
 943 </div></div>
 944 <div class="paragraph"><p>where <code>origin/master</code> used to point at commits B0, B1, B2 and now it
 945 points at B, and your <code>topic</code> branch was started on top of it back
 946 when <code>origin/master</code> was at B0, and you built three commits, D0, D1,
 947 and D, on top of it.  Imagine that you now want to rebase the work
 948 you did on the topic on top of the updated origin/master.</p></div>
 949 <div class="paragraph"><p>In such a case, <code>git merge-base origin/master topic</code> would return the
 950 parent of B0 in the above picture, but B0^..D is <strong>not</strong> the range of
 951 commits you would want to replay on top of B (it includes B0, which
 952 is not what you wrote; it is a commit the other side discarded when
 953 it moved its tip from B0 to B1).</p></div>
 954 <div class="paragraph"><p><code>git merge-base --fork-point origin/master topic</code> is designed to
 955 help in such a case.  It takes not only B but also B0, B1, and B2
 956 (i.e. old tips of the remote-tracking branches your repository&#8217;s
 957 reflog knows about) into account to see on which commit your topic
 958 branch was built and finds B0, allowing you to replay only the
 959 commits on your topic, excluding the commits the other side later
 960 discarded.</p></div>
 961 <div class="paragraph"><p>Hence</p></div>
 962 <div class="literalblock">
 963 <div class="content">
 964 <pre><code>$ fork_point=$(git merge-base --fork-point origin/master topic)</code></pre>
 965 </div></div>
 966 <div class="paragraph"><p>will find B0, and</p></div>
 967 <div class="literalblock">
 968 <div class="content">
 969 <pre><code>$ git rebase --onto origin/master $fork_point topic</code></pre>
 970 </div></div>
 971 <div class="paragraph"><p>will replay D0, D1 and D on top of B to create a new history of this
 972 shape:</p></div>
 973 <div class="literalblock">
 974 <div class="content">
 975 <pre><code>                 o---B2
 976                 /
 977 ---o---o---B1--o---o---o---B (origin/master)
 978         \                   \
 979          B0                  D0'--D1'--D' (topic - updated)
 980           \
 981            D0---D1---D (topic - old)</code></pre>
 982 </div></div>
 983 <div class="paragraph"><p>A caveat is that older reflog entries in your repository may be
 984 expired by <code>git gc</code>.  If B0 no longer appears in the reflog of the
 985 remote-tracking branch <code>origin/master</code>, the <code>--fork-point</code> mode
 986 obviously cannot find it and fails, avoiding to give a random and
 987 useless result (such as the parent of B0, like the same command
 988 without the <code>--fork-point</code> option gives).</p></div>
 989 <div class="paragraph"><p>Also, the remote-tracking branch you use the <code>--fork-point</code> mode
 990 with must be the one your topic forked from its tip.  If you forked
 991 from an older commit than the tip, this mode would not find the fork
 992 point (imagine in the above sample history B0 did not exist,
 993 origin/master started at B1, moved to B2 and then B, and you forked
 994 your topic at origin/master^ when origin/master was B1; the shape of
 995 the history would be the same as above, without B0, and the parent
 996 of B1 is what <code>git merge-base origin/master topic</code> correctly finds,
 997 but the <code>--fork-point</code> mode will not, because it is not one of the
 998 commits that used to be at the tip of origin/master).</p></div>
 999 </div>
1000 </div>
1001 <div class="sect1">
1002 <h2 id="_see_also">See also</h2>
1003 <div class="sectionbody">
1004 <div class="paragraph"><p><a href="git-rev-list.html">git-rev-list(1)</a>,
1005 <a href="git-show-branch.html">git-show-branch(1)</a>,
1006 <a href="git-merge.html">git-merge(1)</a></p></div>
1007 </div>
1008 </div>
1009 <div class="sect1">
1010 <h2 id="_git">GIT</h2>
1011 <div class="sectionbody">
1012 <div class="paragraph"><p>Part of the <a href="git.html">git(1)</a> suite</p></div>
1013 </div>
1014 </div>
1015 </div>
1016 <div id="footnotes"><hr /></div>
1017 <div id="footer">
1018 <div id="footer-text">
1019 Last updated
1020  2022-10-28 11:53:56 PDT
1021 </div>
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