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 735 <body class="article">
 736 <div id="header">
 737 <h1>How to rebase from an internal branch</h1>
 738 <span id="revdate">2023-06-26</span>
 739 </div>
 740 <div id="content">
 741 <div id="preamble">
 742 <div class="sectionbody">
 743 <div class="listingblock">
 744 <div class="content">
 745 <pre><code>Petr Baudis &lt;pasky@suse.cz&gt; writes:
 746
 747 &gt; Dear diary, on Sun, Aug 14, 2005 at 09:57:13AM CEST, I got a letter
 748 &gt; where Junio C Hamano &lt;junkio@cox.net&gt; told me that...
 749 &gt;&gt; Linus Torvalds &lt;torvalds@osdl.org&gt; writes:
 750 &gt;&gt;
 751 &gt;&gt; &gt; Junio, maybe you want to talk about how you move patches from your
 752 &gt;&gt; &gt; "seen" branch to the real branches.
 753 &gt;&gt;
 754 &gt; Actually, wouldn't this be also precisely for what StGIT is intended to?</code></pre>
 755 </div></div>
 756 <div class="paragraph"><p>Exactly my feeling.  I was sort of waiting for Catalin to speak
 757 up.  With its basing philosophical ancestry on quilt, this is
 758 the kind of task StGIT is designed to do.</p></div>
 759 <div class="paragraph"><p>I just have done a simpler one, this time using only the core
 760 Git tools.</p></div>
 761 <div class="paragraph"><p>I had a handful of commits that were ahead of master in <em>seen</em>, and I
 762 wanted to add some documentation bypassing my usual habit of
 763 placing new things in <em>seen</em> first.  At the beginning, the commit
 764 ancestry graph looked like this:</p></div>
 765 <div class="literalblock">
 766 <div class="content">
 767 <pre><code>                         *"seen" head
 768 master --&gt; #1 --&gt; #2 --&gt; #3</code></pre>
 769 </div></div>
 770 <div class="paragraph"><p>So I started from master, made a bunch of edits, and committed:</p></div>
 771 <div class="literalblock">
 772 <div class="content">
 773 <pre><code>$ git checkout master
 774 $ cd Documentation; ed git.txt ...
 775 $ cd ..; git add Documentation/*.txt
 776 $ git commit -s</code></pre>
 777 </div></div>
 778 <div class="paragraph"><p>After the commit, the ancestry graph would look like this:</p></div>
 779 <div class="literalblock">
 780 <div class="content">
 781 <pre><code>                          *"seen" head
 782 master^ --&gt; #1 --&gt; #2 --&gt; #3
 783       \
 784         \---&gt; master</code></pre>
 785 </div></div>
 786 <div class="paragraph"><p>The old master is now master^ (the first parent of the master).
 787 The new master commit holds my documentation updates.</p></div>
 788 <div class="paragraph"><p>Now I have to deal with "seen" branch.</p></div>
 789 <div class="paragraph"><p>This is the kind of situation I used to have all the time when
 790 Linus was the maintainer and I was a contributor, when you look
 791 at "master" branch being the "maintainer" branch, and "seen"
 792 branch being the "contributor" branch.  Your work started at the
 793 tip of the "maintainer" branch some time ago, you made a lot of
 794 progress in the meantime, and now the maintainer branch has some
 795 other commits you do not have yet.  And "git rebase" was written
 796 with the explicit purpose of helping to maintain branches like
 797 "seen".  You <em>could</em> merge master to <em>seen</em> and keep going, but if you
 798 eventually want to cherrypick and merge some but not necessarily
 799 all changes back to the master branch, it often makes later
 800 operations for <em>you</em> easier if you rebase (i.e. carry forward
 801 your changes) "seen" rather than merge.  So I ran "git rebase":</p></div>
 802 <div class="literalblock">
 803 <div class="content">
 804 <pre><code>$ git checkout seen
 805 $ git rebase master seen</code></pre>
 806 </div></div>
 807 <div class="paragraph"><p>What this does is to pick all the commits since the current
 808 branch (note that I now am on "seen" branch) forked from the
 809 master branch, and forward port these changes.</p></div>
 810 <div class="literalblock">
 811 <div class="content">
 812 <pre><code>master^ --&gt; #1 --&gt; #2 --&gt; #3
 813       \                                  *"seen" head
 814         \---&gt; master --&gt; #1' --&gt; #2' --&gt; #3'</code></pre>
 815 </div></div>
 816 <div class="paragraph"><p>The diff between master^ and #1 is applied to master and
 817 committed to create #1' commit with the commit information (log,
 818 author and date) taken from commit #1.  On top of that #2' and #3'
 819 commits are made similarly out of #2 and #3 commits.</p></div>
 820 <div class="paragraph"><p>Old #3 is not recorded in any of the .git/refs/heads/ file
 821 anymore, so after doing this you will have dangling commit if
 822 you ran fsck-cache, which is normal.  After testing "seen", you
 823 can run "git prune" to get rid of those original three commits.</p></div>
 824 <div class="paragraph"><p>While I am talking about "git rebase", I should talk about how
 825 to do cherrypicking using only the core Git tools.</p></div>
 826 <div class="paragraph"><p>Let&#8217;s go back to the earlier picture, with different labels.</p></div>
 827 <div class="paragraph"><p>You, as an individual developer, cloned upstream repository and
 828 made a couple of commits on top of it.</p></div>
 829 <div class="literalblock">
 830 <div class="content">
 831 <pre><code>                           *your "master" head
 832 upstream --&gt; #1 --&gt; #2 --&gt; #3</code></pre>
 833 </div></div>
 834 <div class="paragraph"><p>You would want changes #2 and #3 incorporated in the upstream,
 835 while you feel that #1 may need further improvements.  So you
 836 prepare #2 and #3 for e-mail submission.</p></div>
 837 <div class="literalblock">
 838 <div class="content">
 839 <pre><code>$ git format-patch master^^ master</code></pre>
 840 </div></div>
 841 <div class="paragraph"><p>This creates two files, 0001-XXXX.patch and 0002-XXXX.patch.  Send
 842 them out "To: " your project maintainer and "Cc: " your mailing
 843 list.  You could use contributed script git-send-email if
 844 your host has necessary perl modules for this, but your usual
 845 MUA would do as long as it does not corrupt whitespaces in the
 846 patch.</p></div>
 847 <div class="paragraph"><p>Then you would wait, and you find out that the upstream picked
 848 up your changes, along with other changes.</p></div>
 849 <div class="literalblock">
 850 <div class="content">
 851 <pre><code> where                      *your "master" head
 852 upstream --&gt; #1 --&gt; #2 --&gt; #3
 853   used   \
 854  to be     \--&gt; #A --&gt; #2' --&gt; #3' --&gt; #B --&gt; #C
 855                                               *upstream head</code></pre>
 856 </div></div>
 857 <div class="paragraph"><p>The two commits #2' and #3' in the above picture record the same
 858 changes your e-mail submission for #2 and #3 contained, but
 859 probably with the new sign-off line added by the upstream
 860 maintainer and definitely with different committer and ancestry
 861 information, they are different objects from #2 and #3 commits.</p></div>
 862 <div class="paragraph"><p>You fetch from upstream, but not merge.</p></div>
 863 <div class="literalblock">
 864 <div class="content">
 865 <pre><code>$ git fetch upstream</code></pre>
 866 </div></div>
 867 <div class="paragraph"><p>This leaves the updated upstream head in .git/FETCH_HEAD but
 868 does not touch your .git/HEAD or .git/refs/heads/master.
 869 You run "git rebase" now.</p></div>
 870 <div class="literalblock">
 871 <div class="content">
 872 <pre><code>$ git rebase FETCH_HEAD master</code></pre>
 873 </div></div>
 874 <div class="paragraph"><p>Earlier, I said that rebase applies all the commits from your
 875 branch on top of the upstream head.  Well, I lied.  "git rebase"
 876 is a bit smarter than that and notices that #2 and #3 need not
 877 be applied, so it only applies #1.  The commit ancestry graph
 878 becomes something like this:</p></div>
 879 <div class="literalblock">
 880 <div class="content">
 881 <pre><code> where                     *your old "master" head
 882 upstream --&gt; #1 --&gt; #2 --&gt; #3
 883   used   \                      your new "master" head*
 884  to be     \--&gt; #A --&gt; #2' --&gt; #3' --&gt; #B --&gt; #C --&gt; #1'
 885                                               *upstream
 886                                               head</code></pre>
 887 </div></div>
 888 <div class="paragraph"><p>Again, "git prune" would discard the disused commits #1-#3 and
 889 you continue on starting from the new "master" head, which is
 890 the #1' commit.</p></div>
 891 <div class="paragraph"><p>-jc</p></div>
 892 </div>
 893 </div>
 894 </div>
 895 <div id="footnotes"><hr /></div>
 896 <div id="footer">
 897 <div id="footer-text">
 898 Last updated
 899  2023-06-26 10:22:31 PDT
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