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[git/git.git] / Documentation / howto / rebase-from-internal-branch.txt
1 From: Junio C Hamano <junkio@cox.net>
2 To: git@vger.kernel.org
3 Cc: Petr Baudis <pasky@suse.cz>, Linus Torvalds <torvalds@osdl.org>
4 Subject: Re: sending changesets from the middle of a git tree
5 Date: Sun, 14 Aug 2005 18:37:39 -0700
6 Abstract: In this article, JC talks about how he rebases the
7 public "pu" branch using the core GIT tools when he updates
8 the "master" branch, and how "rebase" works. Also discussed
9 is how this applies to individual developers who sends patches
10 upstream.
11
12 Petr Baudis <pasky@suse.cz> writes:
13
14 > Dear diary, on Sun, Aug 14, 2005 at 09:57:13AM CEST, I got a letter
15 > where Junio C Hamano <junkio@cox.net> told me that...
16 >> Linus Torvalds <torvalds@osdl.org> writes:
17 >>
18 >> > Junio, maybe you want to talk about how you move patches from your "pu"
19 >> > branch to the real branches.
20 >>
21 > Actually, wouldn't this be also precisely for what StGIT is intended to?
22
23 Exactly my feeling. I was sort of waiting for Catalin to speak
24 up. With its basing philosophical ancestry on quilt, this is
25 the kind of task StGIT is designed to do.
26
27 I just have done a simpler one, this time using only the core
28 GIT tools.
29
30 I had a handful commits that were ahead of master in pu, and I
31 wanted to add some documentation bypassing my usual habit of
32 placing new things in pu first. At the beginning, the commit
33 ancestry graph looked like this:
34
35 *"pu" head
36 master --> #1 --> #2 --> #3
37
38 So I started from master, made a bunch of edits, and committed:
39
40 $ git checkout master
41 $ cd Documentation; ed git.txt ...
42 $ cd ..; git add Documentation/*.txt
43 $ git commit -s
44
45 After the commit, the ancestry graph would look like this:
46
47 *"pu" head
48 master^ --> #1 --> #2 --> #3
49 \
50 \---> master
51
52 The old master is now master^ (the first parent of the master).
53 The new master commit holds my documentation updates.
54
55 Now I have to deal with "pu" branch.
56
57 This is the kind of situation I used to have all the time when
58 Linus was the maintainer and I was a contributor, when you look
59 at "master" branch being the "maintainer" branch, and "pu"
60 branch being the "contributor" branch. Your work started at the
61 tip of the "maintainer" branch some time ago, you made a lot of
62 progress in the meantime, and now the maintainer branch has some
63 other commits you do not have yet. And "git rebase" was written
64 with the explicit purpose of helping to maintain branches like
65 "pu". You _could_ merge master to pu and keep going, but if you
66 eventually want to cherrypick and merge some but not necessarily
67 all changes back to the master branch, it often makes later
68 operations for _you_ easier if you rebase (i.e. carry forward
69 your changes) "pu" rather than merge. So I ran "git rebase":
70
71 $ git checkout pu
72 $ git rebase master pu
73
74 What this does is to pick all the commits since the current
75 branch (note that I now am on "pu" branch) forked from the
76 master branch, and forward port these changes.
77
78 master^ --> #1 --> #2 --> #3
79 \ *"pu" head
80 \---> master --> #1' --> #2' --> #3'
81
82 The diff between master^ and #1 is applied to master and
83 committed to create #1' commit with the commit information (log,
84 author and date) taken from commit #1. On top of that #2' and #3'
85 commits are made similarly out of #2 and #3 commits.
86
87 Old #3 is not recorded in any of the .git/refs/heads/ file
88 anymore, so after doing this you will have dangling commit if
89 you ran fsck-cache, which is normal. After testing "pu", you
90 can run "git prune" to get rid of those original three commits.
91
92 While I am talking about "git rebase", I should talk about how
93 to do cherrypicking using only the core GIT tools.
94
95 Let's go back to the earlier picture, with different labels.
96
97 You, as an individual developer, cloned upstream repository and
98 made a couple of commits on top of it.
99
100 *your "master" head
101 upstream --> #1 --> #2 --> #3
102
103 You would want changes #2 and #3 incorporated in the upstream,
104 while you feel that #1 may need further improvements. So you
105 prepare #2 and #3 for e-mail submission.
106
107 $ git format-patch master^^ master
108
109 This creates two files, 0001-XXXX.txt and 0002-XXXX.txt. Send
110 them out "To: " your project maintainer and "Cc: " your mailing
111 list. You could use contributed script git-send-email if
112 your host has necessary perl modules for this, but your usual
113 MUA would do as long as it does not corrupt whitespaces in the
114 patch.
115
116 Then you would wait, and you find out that the upstream picked
117 up your changes, along with other changes.
118
119 where *your "master" head
120 upstream --> #1 --> #2 --> #3
121 used \
122 to be \--> #A --> #2' --> #3' --> #B --> #C
123 *upstream head
124
125 The two commits #2' and #3' in the above picture record the same
126 changes your e-mail submission for #2 and #3 contained, but
127 probably with the new sign-off line added by the upsteam
128 maintainer and definitely with different committer and ancestry
129 information, they are different objects from #2 and #3 commits.
130
131 You fetch from upstream, but not merge.
132
133 $ git fetch upstream
134
135 This leaves the updated upstream head in .git/FETCH_HEAD but
136 does not touch your .git/HEAD nor .git/refs/heads/master.
137 You run "git rebase" now.
138
139 $ git rebase FETCH_HEAD master
140
141 Earlier, I said that rebase applies all the commits from your
142 branch on top of the upstream head. Well, I lied. "git rebase"
143 is a bit smarter than that and notices that #2 and #3 need not
144 be applied, so it only applies #1. The commit ancestry graph
145 becomes something like this:
146
147 where *your old "master" head
148 upstream --> #1 --> #2 --> #3
149 used \ your new "master" head*
150 to be \--> #A --> #2' --> #3' --> #B --> #C --> #1'
151 *upstream
152 head
153
154 Again, "git prune" would discard the disused commits #1-#3 and
155 you continue on starting from the new "master" head, which is
156 the #1' commit.
157
158 -jc
159
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