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<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---B2 | |
/ | |
---o---o---B1--o---o---o---B (origin/master) | |
\ | |
B0 | |
\ | |
D0---D1---D (topic)</code></pre> | |
</div></div> | |
<div class="paragraph"><p>where <code>origin/master</code> used to point at commits B0, B1, B2 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 B0, and you built three commits, D0, D1, | |
and D, on top of it. Imagine that you now want to rebase the work | |
you did on the topic on top of the updated origin/master.</p></div> | |
<div class="paragraph"><p>In such a case, <code>git merge-base origin/master topic</code> would return the | |
parent of B0 in the above picture, but B0^..D is <strong>not</strong> the range of | |
commits you would want to replay on top of B (it includes B0, which | |
is not what you wrote; it is a commit the other side discarded when | |
it moved its tip from B0 to B1).</p></div> | |
<div class="paragraph"><p><code>git merge-base --fork-point origin/master topic</code> is designed to | |
help in such a case. It takes not only B but also B0, B1, and B2 | |
(i.e. old tips of the remote-tracking branches your repository’s | |
reflog knows about) into account to see on which commit your topic | |
branch was built and finds B0, allowing you to replay only the | |
commits on your topic, excluding the commits the other side later | |
discarded.</p></div> | |
<div class="paragraph"><p>Hence</p></div> | |
<div class="literalblock"> | |
<div class="content"> | |
<pre><code>$ fork_point=$(git merge-base --fork-point origin/master topic)</code></pre> | |
</div></div> | |
<div class="paragraph"><p>will find B0, and</p></div> | |
<div class="literalblock"> | |
<div class="content"> | |
<pre><code>$ git rebase --onto origin/master $fork_point topic</code></pre> | |
</div></div> | |
<div class="paragraph"><p>will replay D0, D1 and D on top of B to create a new history of this | |
shape:</p></div> | |
<div class="literalblock"> | |
<div class="content"> | |
<pre><code> o---B2 | |
/ | |
---o---o---B1--o---o---o---B (origin/master) | |
\ \ | |
B0 D0'--D1'--D' (topic - updated) | |
\ | |
D0---D1---D (topic - old)</code></pre> | |
</div></div> | |
<div class="paragraph"><p>A caveat is that older reflog entries in your repository may be | |
expired by <code>git gc</code>. If B0 no longer appears in the reflog of the | |
remote-tracking branch <code>origin/master</code>, the <code>--fork-point</code> mode | |
obviously cannot find it and fails, avoiding to give a random and | |
useless result (such as the parent of B0, like the same command | |
without the <code>--fork-point</code> option gives).</p></div> | |
<div class="paragraph"><p>Also, the remote-tracking branch you use the <code>--fork-point</code> mode | |
with must be the one your topic forked from its tip. If you forked | |
from an older commit than the tip, this mode would not find the fork | |
point (imagine in the above sample history B0 did not exist, | |
origin/master started at B1, moved to B2 and then B, and you forked | |
your topic at origin/master^ when origin/master was B1; the shape of | |
the history would be the same as above, without B0, and the parent | |
of B1 is what <code>git merge-base origin/master topic</code> correctly finds, | |
but the <code>--fork-point</code> mode will not, because it is not one of the | |
commits that used to be at the tip of origin/master).</p></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 | |
2018-10-30 16:33:48 JST | |
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