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 571 <div id="header">
 572 <h1>
 573 git-merge-base(1) Manual Page
 574 </h1>
 575 <h2>NAME</h2>
 576 <div class="sectionbody">
 577 <p>git-merge-base -
 578    Find as good common ancestors as possible for a merge
 579 </p>
 580 </div>
 581 </div>
 582 <div id="content">
 583 <h2 id="_synopsis">SYNOPSIS</h2>
 584 <div class="sectionbody">
 585 <div class="verseblock">
 586 <div class="verseblock-content"><em>git merge-base</em> [-a|--all] &lt;commit&gt; &lt;commit&gt;&#8230;
 587 <em>git merge-base</em> [-a|--all] --octopus &lt;commit&gt;&#8230;
 588 <em>git merge-base</em> --independent &lt;commit&gt;&#8230;</div>
 589 <div class="verseblock-attribution">
 590 </div></div>
 591 </div>
 592 <h2 id="_description">DESCRIPTION</h2>
 593 <div class="sectionbody">
 594 <div class="paragraph"><p><em>git merge-base</em> finds best common ancestor(s) between two commits to use
 595 in a three-way merge.  One common ancestor is <em>better</em> than another common
 596 ancestor if the latter is an ancestor of the former.  A common ancestor
 597 that does not have any better common ancestor is a <em>best common
 598 ancestor</em>, i.e. a <em>merge base</em>.  Note that there can be more than one
 599 merge base for a pair of commits.</p></div>
 600 </div>
 601 <h2 id="_operation_mode">OPERATION MODE</h2>
 602 <div class="sectionbody">
 603 <div class="paragraph"><p>As the most common special case, specifying only two commits on the
 604 command line means computing the merge base between the given two commits.</p></div>
 605 <div class="paragraph"><p>More generally, among the two commits to compute the merge base from,
 606 one is specified by the first commit argument on the command line;
 607 the other commit is a (possibly hypothetical) commit that is a merge
 608 across all the remaining commits on the command line.</p></div>
 609 <div class="paragraph"><p>As a consequence, the <em>merge base</em> is not necessarily contained in each of the
 610 commit arguments if more than two commits are specified. This is different
 611 from <a href="git-show-branch.html">git-show-branch(1)</a> when used with the <tt>--merge-base</tt> option.</p></div>
 612 <div class="dlist"><dl>
 613 <dt class="hdlist1">
 614 --octopus
 615 </dt>
 616 <dd>
 617 <p>
 618         Compute the best common ancestors of all supplied commits,
 619         in preparation for an n-way merge.  This mimics the behavior
 620         of <em>git show-branch --merge-base</em>.
 621 </p>
 622 </dd>
 623 <dt class="hdlist1">
 624 --independent
 625 </dt>
 626 <dd>
 627 <p>
 628         Instead of printing merge bases, print a minimal subset of
 629         the supplied commits with the same ancestors.  In other words,
 630         among the commits given, list those which cannot be reached
 631         from any other.  This mimics the behavior of <em>git show-branch
 632         --independent</em>.
 633 </p>
 634 </dd>
 635 </dl></div>
 636 </div>
 637 <h2 id="_options">OPTIONS</h2>
 638 <div class="sectionbody">
 639 <div class="dlist"><dl>
 640 <dt class="hdlist1">
 641 -a
 642 </dt>
 643 <dt class="hdlist1">
 644 --all
 645 </dt>
 646 <dd>
 647 <p>
 648         Output all merge bases for the commits, instead of just one.
 649 </p>
 650 </dd>
 651 </dl></div>
 652 </div>
 653 <h2 id="_discussion">DISCUSSION</h2>
 654 <div class="sectionbody">
 655 <div class="paragraph"><p>Given two commits <em>A</em> and <em>B</em>, <tt>git merge-base A B</tt> will output a commit
 656 which is reachable from both <em>A</em> and <em>B</em> through the parent relationship.</p></div>
 657 <div class="paragraph"><p>For example, with this topology:</p></div>
 658 <div class="literalblock">
 659 <div class="content">
 660 <pre><tt>         o---o---o---B
 661         /
 662 ---o---1---o---o---o---A</tt></pre>
 663 </div></div>
 664 <div class="paragraph"><p>the merge base between <em>A</em> and <em>B</em> is <em>1</em>.</p></div>
 665 <div class="paragraph"><p>Given three commits <em>A</em>, <em>B</em> and <em>C</em>, <tt>git merge-base A B C</tt> will compute the
 666 merge base between <em>A</em> and a hypothetical commit <em>M</em>, which is a merge
 667 between <em>B</em> and <em>C</em>.  For example, with this topology:</p></div>
 668 <div class="literalblock">
 669 <div class="content">
 670 <pre><tt>       o---o---o---o---C
 671       /
 672      /   o---o---o---B
 673     /   /
 674 ---2---1---o---o---o---A</tt></pre>
 675 </div></div>
 676 <div class="paragraph"><p>the result of <tt>git merge-base A B C</tt> is <em>1</em>.  This is because the
 677 equivalent topology with a merge commit <em>M</em> between <em>B</em> and <em>C</em> is:</p></div>
 678 <div class="literalblock">
 679 <div class="content">
 680 <pre><tt>       o---o---o---o---o
 681       /                 \
 682      /   o---o---o---o---M
 683     /   /
 684 ---2---1---o---o---o---A</tt></pre>
 685 </div></div>
 686 <div class="paragraph"><p>and the result of <tt>git merge-base A M</tt> is <em>1</em>.  Commit <em>2</em> is also a
 687 common ancestor between <em>A</em> and <em>M</em>, but <em>1</em> is a better common ancestor,
 688 because <em>2</em> is an ancestor of <em>1</em>.  Hence, <em>2</em> is not a merge base.</p></div>
 689 <div class="paragraph"><p>The result of <tt>git merge-base --octopus A B C</tt> is <em>2</em>, because <em>2</em> is
 690 the best common ancestor of all commits.</p></div>
 691 <div class="paragraph"><p>When the history involves criss-cross merges, there can be more than one
 692 <em>best</em> common ancestor for two commits.  For example, with this topology:</p></div>
 693 <div class="literalblock">
 694 <div class="content">
 695 <pre><tt>---1---o---A
 696     \ /
 697      X
 698     / \
 699 ---2---o---o---B</tt></pre>
 700 </div></div>
 701 <div class="paragraph"><p>both <em>1</em> and <em>2</em> are merge-bases of A and B.  Neither one is better than
 702 the other (both are <em>best</em> merge bases).  When the <tt>--all</tt> option is not given,
 703 it is unspecified which best one is output.</p></div>
 704 </div>
 705 <h2 id="_see_also">See also</h2>
 706 <div class="sectionbody">
 707 <div class="paragraph"><p><a href="git-rev-list.html">git-rev-list(1)</a>,
 708 <a href="git-show-branch.html">git-show-branch(1)</a>,
 709 <a href="git-merge.html">git-merge(1)</a></p></div>
 710 </div>
 711 <h2 id="_git">GIT</h2>
 712 <div class="sectionbody">
 713 <div class="paragraph"><p>Part of the <a href="git.html">git(1)</a> suite</p></div>
 714 </div>
 715 </div>
 716 <div id="footnotes"><hr /></div>
 717 <div id="footer">
 718 <div id="footer-text">
 719 Last updated 2011-09-21 23:01:14 PDT
 720 </div>
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