git-merge-base.html

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 442 <div id="header">
 443 <h1>git-merge-base(1) Manual Page</h1>
 444 <h2 id="_name">NAME</h2>
 445 <div class="sectionbody">
 446 <p>git-merge-base - Find as good common ancestors as possible for a merge</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"><em>git merge-base</em> [-a | --all] &lt;commit&gt; &lt;commit&gt;&#8230;&#8203;
 455 <em>git merge-base</em> [-a | --all] --octopus &lt;commit&gt;&#8230;&#8203;
 456 <em>git merge-base</em> --is-ancestor &lt;commit&gt; &lt;commit&gt;
 457 <em>git merge-base</em> --independent &lt;commit&gt;&#8230;&#8203;
 458 <em>git merge-base</em> --fork-point &lt;ref&gt; [&lt;commit&gt;]</pre>
 459 </div>
 460 </div>
 461 </div>
 462 <div class="sect1">
 463 <h2 id="_description">DESCRIPTION</h2>
 464 <div class="sectionbody">
 465 <div class="paragraph">
 466 <p><em>git merge-base</em> finds the best common ancestor(s) between two commits to use
 467 in a three-way merge.  One common ancestor is <em>better</em> than another common
 468 ancestor if the latter is an ancestor of the former.  A common ancestor
 469 that does not have any better common ancestor is a <em>best common
 470 ancestor</em>, i.e. a <em>merge base</em>.  Note that there can be more than one
 471 merge base for a pair of commits.</p>
 472 </div>
 473 </div>
 474 </div>
 475 <div class="sect1">
 476 <h2 id="_operation_modes">OPERATION MODES</h2>
 477 <div class="sectionbody">
 478 <div class="paragraph">
 479 <p>In the most common special case, specifying only two commits on the
 480 command line means computing the merge base between the given two commits.</p>
 481 </div>
 482 <div class="paragraph">
 483 <p>More generally, among the two commits to compute the merge base from,
 484 one is specified by the first commit argument on the command line;
 485 the other commit is a (possibly hypothetical) commit that is a merge
 486 across all the remaining commits on the command line.</p>
 487 </div>
 488 <div class="paragraph">
 489 <p>As a consequence, the <em>merge base</em> is not necessarily contained in each of the
 490 commit arguments if more than two commits are specified. This is different
 491 from <a href="git-show-branch.html">git-show-branch(1)</a> when used with the <code>--merge-base</code> option.</p>
 492 </div>
 493 <div class="dlist">
 494 <dl>
 495 <dt class="hdlist1">--octopus</dt>
 496 <dd>
 497 <p>Compute the best common ancestors of all supplied commits,
 498 in preparation for an n-way merge.  This mimics the behavior
 499 of <em>git show-branch --merge-base</em>.</p>
 500 </dd>
 501 <dt class="hdlist1">--independent</dt>
 502 <dd>
 503 <p>Instead of printing merge bases, print a minimal subset of
 504 the supplied commits with the same ancestors.  In other words,
 505 among the commits given, list those which cannot be reached
 506 from any other.  This mimics the behavior of <em>git show-branch
 507 --independent</em>.</p>
 508 </dd>
 509 <dt class="hdlist1">--is-ancestor</dt>
 510 <dd>
 511 <p>Check if the first &lt;commit&gt; is an ancestor of the second &lt;commit&gt;,
 512 and exit with status 0 if true, or with status 1 if not.
 513 Errors are signaled by a non-zero status that is not 1.</p>
 514 </dd>
 515 <dt class="hdlist1">--fork-point</dt>
 516 <dd>
 517 <p>Find the point at which a branch (or any history that leads
 518 to &lt;commit&gt;) forked from another branch (or any reference)
 519 &lt;ref&gt;. This does not just look for the common ancestor of
 520 the two commits, but also takes into account the reflog of
 521 &lt;ref&gt; to see if the history leading to &lt;commit&gt; forked from
 522 an earlier incarnation of the branch &lt;ref&gt; (see discussion
 523 of this mode below).</p>
 524 </dd>
 525 </dl>
 526 </div>
 527 </div>
 528 </div>
 529 <div class="sect1">
 530 <h2 id="_options">OPTIONS</h2>
 531 <div class="sectionbody">
 532 <div class="dlist">
 533 <dl>
 534 <dt class="hdlist1">-a</dt>
 535 <dt class="hdlist1">--all</dt>
 536 <dd>
 537 <p>Output all merge bases for the commits, instead of just one.</p>
 538 </dd>
 539 </dl>
 540 </div>
 541 </div>
 542 </div>
 543 <div class="sect1">
 544 <h2 id="_discussion">DISCUSSION</h2>
 545 <div class="sectionbody">
 546 <div class="paragraph">
 547 <p>Given two commits <em>A</em> and <em>B</em>, <code>git</code> <code>merge-base</code> <code>A</code> <code>B</code> will output a commit
 548 which is reachable from both <em>A</em> and <em>B</em> through the parent relationship.</p>
 549 </div>
 550 <div class="paragraph">
 551 <p>For example, with this topology:</p>
 552 </div>
 553 <div class="literalblock">
 554 <div class="content">
 555 <pre>         o---o---o---B
 556         /
 557 ---o---1---o---o---o---A</pre>
 558 </div>
 559 </div>
 560 <div class="paragraph">
 561 <p>the merge base between <em>A</em> and <em>B</em> is <em>1</em>.</p>
 562 </div>
 563 <div class="paragraph">
 564 <p>Given three commits <em>A</em>, <em>B</em>, and <em>C</em>, <code>git</code> <code>merge-base</code> <code>A</code> <code>B</code> <code>C</code> will compute the
 565 merge base between <em>A</em> and a hypothetical commit <em>M</em>, which is a merge
 566 between <em>B</em> and <em>C</em>.  For example, with this topology:</p>
 567 </div>
 568 <div class="literalblock">
 569 <div class="content">
 570 <pre>       o---o---o---o---C
 571       /
 572      /   o---o---o---B
 573     /   /
 574 ---2---1---o---o---o---A</pre>
 575 </div>
 576 </div>
 577 <div class="paragraph">
 578 <p>the result of <code>git</code> <code>merge-base</code> <code>A</code> <code>B</code> <code>C</code> is <em>1</em>.  This is because the
 579 equivalent topology with a merge commit <em>M</em> between <em>B</em> and <em>C</em> is:</p>
 580 </div>
 581 <div class="literalblock">
 582 <div class="content">
 583 <pre>       o---o---o---o---o
 584       /                 \
 585      /   o---o---o---o---M
 586     /   /
 587 ---2---1---o---o---o---A</pre>
 588 </div>
 589 </div>
 590 <div class="paragraph">
 591 <p>and the result of <code>git</code> <code>merge-base</code> <code>A</code> <code>M</code> is <em>1</em>.  Commit <em>2</em> is also a
 592 common ancestor between <em>A</em> and <em>M</em>, but <em>1</em> is a better common ancestor,
 593 because <em>2</em> is an ancestor of <em>1</em>.  Hence, <em>2</em> is not a merge base.</p>
 594 </div>
 595 <div class="paragraph">
 596 <p>The result of <code>git</code> <code>merge-base</code> <code>--octopus</code> <code>A</code> <code>B</code> <code>C</code> is <em>2</em>, because <em>2</em> is
 597 the best common ancestor of all commits.</p>
 598 </div>
 599 <div class="paragraph">
 600 <p>When the history involves criss-cross merges, there can be more than one
 601 <em>best</em> common ancestor for two commits.  For example, with this topology:</p>
 602 </div>
 603 <div class="literalblock">
 604 <div class="content">
 605 <pre>---1---o---A
 606     \ /
 607      X
 608     / \
 609 ---2---o---o---B</pre>
 610 </div>
 611 </div>
 612 <div class="paragraph">
 613 <p>both <em>1</em> and <em>2</em> are merge bases of A and B.  Neither one is better than
 614 the other (both are <em>best</em> merge bases).  When the <code>--all</code> option is not given,
 615 it is unspecified which best one is output.</p>
 616 </div>
 617 <div class="paragraph">
 618 <p>A common idiom to check "fast-forward-ness" between two commits A
 619 and B is (or at least used to be) to compute the merge base between
 620 A and B, and check if it is the same as A, in which case, A is an
 621 ancestor of B.  You will see this idiom used often in older scripts.</p>
 622 </div>
 623 <div class="literalblock">
 624 <div class="content">
 625 <pre>A=$(git rev-parse --verify A)
 626 if test "$A" = "$(git merge-base A B)"
 627 then
 628         ... A is an ancestor of B ...
 629 fi</pre>
 630 </div>
 631 </div>
 632 <div class="paragraph">
 633 <p>In modern git, you can say this in a more direct way:</p>
 634 </div>
 635 <div class="literalblock">
 636 <div class="content">
 637 <pre>if git merge-base --is-ancestor A B
 638 then
 639         ... A is an ancestor of B ...
 640 fi</pre>
 641 </div>
 642 </div>
 643 <div class="paragraph">
 644 <p>instead.</p>
 645 </div>
 646 </div>
 647 </div>
 648 <div class="sect1">
 649 <h2 id="_discussion_on_fork_point_mode">Discussion on fork-point mode</h2>
 650 <div class="sectionbody">
 651 <div class="paragraph">
 652 <p>After working on the <code>topic</code> branch created with <code>git</code> <code>switch</code> <code>-c</code>
 653 <code>topic</code> <code>origin/master</code>, the history of remote-tracking branch
 654 <code>origin/master</code> may have been rewound and rebuilt, leading to a
 655 history of this shape:</p>
 656 </div>
 657 <div class="literalblock">
 658 <div class="content">
 659 <pre>                 o---B2
 660                 /
 661 ---o---o---B1--o---o---o---B (origin/master)
 662         \
 663          B0
 664           \
 665            D0---D1---D (topic)</pre>
 666 </div>
 667 </div>
 668 <div class="paragraph">
 669 <p>where <code>origin/master</code> used to point at commits B0, B1, B2 and now it
 670 points at B, and your <code>topic</code> branch was started on top of it back
 671 when <code>origin/master</code> was at B0, and you built three commits, D0, D1,
 672 and D, on top of it.  Imagine that you now want to rebase the work
 673 you did on the topic on top of the updated origin/master.</p>
 674 </div>
 675 <div class="paragraph">
 676 <p>In such a case, <code>git</code> <code>merge-base</code> <code>origin/master</code> <code>topic</code> would return the
 677 parent of B0 in the above picture, but B0^..D is <strong>not</strong> the range of
 678 commits you would want to replay on top of B (it includes B0, which
 679 is not what you wrote; it is a commit the other side discarded when
 680 it moved its tip from B0 to B1).</p>
 681 </div>
 682 <div class="paragraph">
 683 <p><code>git</code> <code>merge-base</code> <code>--fork-point</code> <code>origin/master</code> <code>topic</code> is designed to
 684 help in such a case.  It takes not only B but also B0, B1, and B2
 685 (i.e. old tips of the remote-tracking branches your repository&#8217;s
 686 reflog knows about) into account to see on which commit your topic
 687 branch was built and finds B0, allowing you to replay only the
 688 commits on your topic, excluding the commits the other side later
 689 discarded.</p>
 690 </div>
 691 <div class="paragraph">
 692 <p>Hence</p>
 693 </div>
 694 <div class="literalblock">
 695 <div class="content">
 696 <pre>$ fork_point=$(git merge-base --fork-point origin/master topic)</pre>
 697 </div>
 698 </div>
 699 <div class="paragraph">
 700 <p>will find B0, and</p>
 701 </div>
 702 <div class="literalblock">
 703 <div class="content">
 704 <pre>$ git rebase --onto origin/master $fork_point topic</pre>
 705 </div>
 706 </div>
 707 <div class="paragraph">
 708 <p>will replay D0, D1, and D on top of B to create a new history of this
 709 shape:</p>
 710 </div>
 711 <div class="literalblock">
 712 <div class="content">
 713 <pre>                 o---B2
 714                 /
 715 ---o---o---B1--o---o---o---B (origin/master)
 716         \                   \
 717          B0                  D0'--D1'--D' (topic - updated)
 718           \
 719            D0---D1---D (topic - old)</pre>
 720 </div>
 721 </div>
 722 <div class="paragraph">
 723 <p>A caveat is that older reflog entries in your repository may be
 724 expired by <code>git</code> <code>gc</code>.  If B0 no longer appears in the reflog of the
 725 remote-tracking branch <code>origin/master</code>, the <code>--fork-point</code> mode
 726 obviously cannot find it and fails, avoiding to give a random and
 727 useless result (such as the parent of B0, like the same command
 728 without the <code>--fork-point</code> option gives).</p>
 729 </div>
 730 <div class="paragraph">
 731 <p>Also, the remote-tracking branch you use the <code>--fork-point</code> mode
 732 with must be the one your topic forked from its tip.  If you forked
 733 from an older commit than the tip, this mode would not find the fork
 734 point (imagine in the above sample history B0 did not exist,
 735 origin/master started at B1, moved to B2 and then B, and you forked
 736 your topic at origin/master^ when origin/master was B1; the shape of
 737 the history would be the same as above, without B0, and the parent
 738 of B1 is what <code>git</code> <code>merge-base</code> <code>origin/master</code> <code>topic</code> correctly finds,
 739 but the <code>--fork-point</code> mode will not, because it is not one of the
 740 commits that used to be at the tip of origin/master).</p>
 741 </div>
 742 </div>
 743 </div>
 744 <div class="sect1">
 745 <h2 id="_see_also">See also</h2>
 746 <div class="sectionbody">
 747 <div class="paragraph">
 748 <p><a href="git-rev-list.html">git-rev-list(1)</a>,
 749 <a href="git-show-branch.html">git-show-branch(1)</a>,
 750 <a href="git-merge.html">git-merge(1)</a></p>
 751 </div>
 752 </div>
 753 </div>
 754 <div class="sect1">
 755 <h2 id="_git">GIT</h2>
 756 <div class="sectionbody">
 757 <div class="paragraph">
 758 <p>Part of the <a href="git.html">git(1)</a> suite</p>
 759 </div>
 760 </div>
 761 </div>
 762 </div>
 763 <div id="footer">
 764 <div id="footer-text">
 765 Last updated 2023-10-23 14:43:46 -0700
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 768 </body>
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