git-merge-base.html

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 306 </style>
 307 <title>git-merge-base(1)</title>
 308 </head>
 309 <body>
 310 <div id="header">
 311 <h1>
 312 git-merge-base(1) Manual Page
 313 </h1>
 314 <h2>NAME</h2>
 315 <div class="sectionbody">
 316 <p>git-merge-base -
 317    Find as good common ancestors as possible for a merge
 318 </p>
 319 </div>
 320 </div>
 321 <h2>SYNOPSIS</h2>
 322 <div class="sectionbody">
 323 <div class="para"><p><em>git merge-base</em> [-a|--all] &lt;commit&gt; &lt;commit&gt;&#8230;</p></div>
 324 </div>
 325 <h2 id="_description">DESCRIPTION</h2>
 326 <div class="sectionbody">
 327 <div class="para"><p><em>git merge-base</em> finds best common ancestor(s) between two commits to use
 328 in a three-way merge.  One common ancestor is <em>better</em> than another common
 329 ancestor if the latter is an ancestor of the former.  A common ancestor
 330 that does not have any better common ancestor is a <em>best common
 331 ancestor</em>, i.e. a <em>merge base</em>.  Note that there can be more than one
 332 merge base for a pair of commits.</p></div>
 333 <div class="para"><p>Among the two commits to compute the merge base from, one is specified by
 334 the first commit argument on the command line; the other commit is a
 335 (possibly hypothetical) commit that is a merge across all the remaining
 336 commits on the command line.  As the most common special case, specifying only
 337 two commits on the command line means computing the merge base between
 338 the given two commits.</p></div>
 339 <div class="para"><p>As a consequence, the <em>merge base</em> is not necessarily contained in each of the
 340 commit arguments if more than two commits are specified. This is different
 341 from <a href="git-show-branch.html">git-show-branch(1)</a> when used with the <tt>--merge-base</tt> option.</p></div>
 342 </div>
 343 <h2 id="_options">OPTIONS</h2>
 344 <div class="sectionbody">
 345 <div class="vlist"><dl>
 346 <dt>
 347 -a
 348 </dt>
 349 <dt>
 350 --all
 351 </dt>
 352 <dd>
 353 <p>
 354         Output all merge bases for the commits, instead of just one.
 355 </p>
 356 </dd>
 357 </dl></div>
 358 </div>
 359 <h2 id="_discussion">DISCUSSION</h2>
 360 <div class="sectionbody">
 361 <div class="para"><p>Given two commits <em>A</em> and <em>B</em>, <tt>git merge-base A B</tt> will output a commit
 362 which is reachable from both <em>A</em> and <em>B</em> through the parent relationship.</p></div>
 363 <div class="para"><p>For example, with this topology:</p></div>
 364 <div class="literalblock">
 365 <div class="content">
 366 <pre><tt>         o---o---o---B
 367         /
 368 ---o---1---o---o---o---A</tt></pre>
 369 </div></div>
 370 <div class="para"><p>the merge base between <em>A</em> and <em>B</em> is <em>1</em>.</p></div>
 371 <div class="para"><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
 372 merge base between <em>A</em> and a hypothetical commit <em>M</em>, which is a merge
 373 between <em>B</em> and <em>C</em>.  For example, with this topology:</p></div>
 374 <div class="literalblock">
 375 <div class="content">
 376 <pre><tt>       o---o---o---o---C
 377       /
 378      /   o---o---o---B
 379     /   /
 380 ---2---1---o---o---o---A</tt></pre>
 381 </div></div>
 382 <div class="para"><p>the result of <tt>git merge-base A B C</tt> is <em>1</em>.  This is because the
 383 equivalent topology with a merge commit <em>M</em> between <em>B</em> and <em>C</em> is:</p></div>
 384 <div class="literalblock">
 385 <div class="content">
 386 <pre><tt>       o---o---o---o---o
 387       /                 \
 388      /   o---o---o---o---M
 389     /   /
 390 ---2---1---o---o---o---A</tt></pre>
 391 </div></div>
 392 <div class="para"><p>and the result of <tt>git merge-base A M</tt> is <em>1</em>.  Commit <em>2</em> is also a
 393 common ancestor between <em>A</em> and <em>M</em>, but <em>1</em> is a better common ancestor,
 394 because <em>2</em> is an ancestor of <em>1</em>.  Hence, <em>2</em> is not a merge base.</p></div>
 395 <div class="para"><p>When the history involves criss-cross merges, there can be more than one
 396 <em>best</em> common ancestor for two commits.  For example, with this topology:</p></div>
 397 <div class="literalblock">
 398 <div class="content">
 399 <pre><tt>---1---o---A
 400     \ /
 401      X
 402     / \
 403 ---2---o---o---B</tt></pre>
 404 </div></div>
 405 <div class="para"><p>both <em>1</em> and <em>2</em> are merge-bases of A and B.  Neither one is better than
 406 the other (both are <em>best</em> merge bases).  When the <tt>--all</tt> option is not given,
 407 it is unspecified which best one is output.</p></div>
 408 </div>
 409 <h2 id="_author">Author</h2>
 410 <div class="sectionbody">
 411 <div class="para"><p>Written by Linus Torvalds &lt;torvalds@osdl.org&gt;</p></div>
 412 </div>
 413 <h2 id="_documentation">Documentation</h2>
 414 <div class="sectionbody">
 415 <div class="para"><p>Documentation by David Greaves, Junio C Hamano and the git-list &lt;git@vger.kernel.org&gt;.</p></div>
 416 </div>
 417 <h2 id="_git">GIT</h2>
 418 <div class="sectionbody">
 419 <div class="para"><p>Part of the <a href="git.html">git(1)</a> suite</p></div>
 420 </div>
 421 <div id="footer">
 422 <div id="footer-text">
 423 Last updated 2009-12-03 09:12:40 UTC
 424 </div>
 425 </div>
 426 </body>
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