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| </head> | |
| <body class="manpage"> | |
| <div id="header"> | |
| <h1> | |
| git-merge-base(1) Manual Page | |
| </h1> | |
| <h2>NAME</h2> | |
| <div class="sectionbody"> | |
| <p>git-merge-base - | |
| Find as good common ancestors as possible for a merge | |
| </p> | |
| </div> | |
| </div> | |
| <div id="content"> | |
| <div class="sect1"> | |
| <h2 id="_synopsis">SYNOPSIS</h2> | |
| <div class="sectionbody"> | |
| <div class="verseblock"> | |
| <pre class="content"><em>git merge-base</em> [-a|--all] <commit> <commit>… | |
| <em>git merge-base</em> [-a|--all] --octopus <commit>… | |
| <em>git merge-base</em> --is-ancestor <commit> <commit> | |
| <em>git merge-base</em> --independent <commit>… | |
| <em>git merge-base</em> --fork-point <ref> [<commit>]</pre> | |
| <div class="attribution"> | |
| </div></div> | |
| </div> | |
| </div> | |
| <div class="sect1"> | |
| <h2 id="_description">DESCRIPTION</h2> | |
| <div class="sectionbody"> | |
| <div class="paragraph"><p><em>git merge-base</em> finds best common ancestor(s) between two commits to use | |
| in a three-way merge. One common ancestor is <em>better</em> than another common | |
| ancestor if the latter is an ancestor of the former. A common ancestor | |
| that does not have any better common ancestor is a <em>best common | |
| ancestor</em>, i.e. a <em>merge base</em>. Note that there can be more than one | |
| merge base for a pair of commits.</p></div> | |
| </div> | |
| </div> | |
| <div class="sect1"> | |
| <h2 id="_operation_modes">OPERATION MODES</h2> | |
| <div class="sectionbody"> | |
| <div class="paragraph"><p>As the most common special case, specifying only two commits on the | |
| command line means computing the merge base between the given two commits.</p></div> | |
| <div class="paragraph"><p>More generally, among the two commits to compute the merge base from, | |
| one is specified by the first commit argument on the command line; | |
| the other commit is a (possibly hypothetical) commit that is a merge | |
| across all the remaining commits on the command line.</p></div> | |
| <div class="paragraph"><p>As a consequence, the <em>merge base</em> is not necessarily contained in each of the | |
| commit arguments if more than two commits are specified. This is different | |
| from <a href="git-show-branch.html">git-show-branch(1)</a> when used with the <code>--merge-base</code> option.</p></div> | |
| <div class="dlist"><dl> | |
| <dt class="hdlist1"> | |
| --octopus | |
| </dt> | |
| <dd> | |
| <p> | |
| Compute the best common ancestors of all supplied commits, | |
| in preparation for an n-way merge. This mimics the behavior | |
| of <em>git show-branch --merge-base</em>. | |
| </p> | |
| </dd> | |
| <dt class="hdlist1"> | |
| --independent | |
| </dt> | |
| <dd> | |
| <p> | |
| Instead of printing merge bases, print a minimal subset of | |
| the supplied commits with the same ancestors. In other words, | |
| among the commits given, list those which cannot be reached | |
| from any other. This mimics the behavior of <em>git show-branch | |
| --independent</em>. | |
| </p> | |
| </dd> | |
| <dt class="hdlist1"> | |
| --is-ancestor | |
| </dt> | |
| <dd> | |
| <p> | |
| Check if the first <commit> is an ancestor of the second <commit>, | |
| and exit with status 0 if true, or with status 1 if not. | |
| Errors are signaled by a non-zero status that is not 1. | |
| </p> | |
| </dd> | |
| <dt class="hdlist1"> | |
| --fork-point | |
| </dt> | |
| <dd> | |
| <p> | |
| Find the point at which a branch (or any history that leads | |
| to <commit>) forked from another branch (or any reference) | |
| <ref>. This does not just look for the common ancestor of | |
| the two commits, but also takes into account the reflog of | |
| <ref> to see if the history leading to <commit> forked from | |
| an earlier incarnation of the branch <ref> (see discussion | |
| on this mode below). | |
| </p> | |
| </dd> | |
| </dl></div> | |
| </div> | |
| </div> | |
| <div class="sect1"> | |
| <h2 id="_options">OPTIONS</h2> | |
| <div class="sectionbody"> | |
| <div class="dlist"><dl> | |
| <dt class="hdlist1"> | |
| -a | |
| </dt> | |
| <dt class="hdlist1"> | |
| --all | |
| </dt> | |
| <dd> | |
| <p> | |
| Output all merge bases for the commits, instead of just one. | |
| </p> | |
| </dd> | |
| </dl></div> | |
| </div> | |
| </div> | |
| <div class="sect1"> | |
| <h2 id="_discussion">DISCUSSION</h2> | |
| <div class="sectionbody"> | |
| <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 | |
| which is reachable from both <em>A</em> and <em>B</em> through the parent relationship.</p></div> | |
| <div class="paragraph"><p>For example, with this topology:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code> o---o---o---B | |
| / | |
| ---o---1---o---o---o---A</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>the merge base between <em>A</em> and <em>B</em> is <em>1</em>.</p></div> | |
| <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 | |
| merge base between <em>A</em> and a hypothetical commit <em>M</em>, which is a merge | |
| between <em>B</em> and <em>C</em>. For example, with this topology:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code> o---o---o---o---C | |
| / | |
| / o---o---o---B | |
| / / | |
| ---2---1---o---o---o---A</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>the result of <code>git merge-base A B C</code> is <em>1</em>. This is because the | |
| equivalent topology with a merge commit <em>M</em> between <em>B</em> and <em>C</em> is:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code> o---o---o---o---o | |
| / \ | |
| / o---o---o---o---M | |
| / / | |
| ---2---1---o---o---o---A</code></pre> | |
| </div></div> | |
| <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 | |
| common ancestor between <em>A</em> and <em>M</em>, but <em>1</em> is a better common ancestor, | |
| because <em>2</em> is an ancestor of <em>1</em>. Hence, <em>2</em> is not a merge base.</p></div> | |
| <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 | |
| the best common ancestor of all commits.</p></div> | |
| <div class="paragraph"><p>When the history involves criss-cross merges, there can be more than one | |
| <em>best</em> common ancestor for two commits. For example, with this topology:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>---1---o---A | |
| \ / | |
| X | |
| / \ | |
| ---2---o---o---B</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>both <em>1</em> and <em>2</em> are merge-bases of A and B. Neither one is better than | |
| the other (both are <em>best</em> merge bases). When the <code>--all</code> option is not given, | |
| it is unspecified which best one is output.</p></div> | |
| <div class="paragraph"><p>A common idiom to check "fast-forward-ness" between two commits A | |
| and B is (or at least used to be) to compute the merge base between | |
| A and B, and check if it is the same as A, in which case, A is an | |
| ancestor of B. You will see this idiom used often in older scripts.</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>A=$(git rev-parse --verify A) | |
| if test "$A" = "$(git merge-base A B)" | |
| then | |
| ... A is an ancestor of B ... | |
| fi</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>In modern git, you can say this in a more direct way:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>if git merge-base --is-ancestor A B | |
| then | |
| ... A is an ancestor of B ... | |
| fi</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>instead.</p></div> | |
| </div> | |
| </div> | |
| <div class="sect1"> | |
| <h2 id="_discussion_on_fork_point_mode">Discussion on fork-point mode</h2> | |
| <div class="sectionbody"> | |
| <div class="paragraph"><p>After working on the <code>topic</code> branch created with <code>git checkout -b | |
| topic origin/master</code>, the history of remote-tracking branch | |
| <code>origin/master</code> may have been rewound and rebuilt, leading to a | |
| history of this shape:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code> o---B1 | |
| / | |
| ---o---o---B2--o---o---o---B (origin/master) | |
| \ | |
| B3 | |
| \ | |
| Derived (topic)</code></pre> | |
| </div></div> | |
| <div class="paragraph"><p>where <code>origin/master</code> used to point at commits B3, B2, B1 and now it | |
| points at B, and your <code>topic</code> branch was started on top of it back | |
| when <code>origin/master</code> was at B3. This mode uses the reflog of | |
| <code>origin/master</code> to find B3 as the fork point, so that the <code>topic</code> | |
| can be rebased on top of the updated <code>origin/master</code> by:</p></div> | |
| <div class="literalblock"> | |
| <div class="content"> | |
| <pre><code>$ fork_point=$(git merge-base --fork-point origin/master topic) | |
| $ git rebase --onto origin/master $fork_point topic</code></pre> | |
| </div></div> | |
| </div> | |
| </div> | |
| <div class="sect1"> | |
| <h2 id="_see_also">See also</h2> | |
| <div class="sectionbody"> | |
| <div class="paragraph"><p><a href="git-rev-list.html">git-rev-list(1)</a>, | |
| <a href="git-show-branch.html">git-show-branch(1)</a>, | |
| <a href="git-merge.html">git-merge(1)</a></p></div> | |
| </div> | |
| </div> | |
| <div class="sect1"> | |
| <h2 id="_git">GIT</h2> | |
| <div class="sectionbody"> | |
| <div class="paragraph"><p>Part of the <a href="git.html">git(1)</a> suite</p></div> | |
| </div> | |
| </div> | |
| </div> | |
| <div id="footnotes"><hr /></div> | |
| <div id="footer"> | |
| <div id="footer-text"> | |
| Last updated 2013-12-06 15:42:42 PST | |
| </div> | |
| </div> | |
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