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1 From: Junio C Hamano <gitster@pobox.com>
2 Subject: Separating topic branches
3 Abstract: In this article, JC describes how to separate topic branches.
4 Content-type: text/asciidoc
6 How to separate topic branches
7 ==============================
9 This text was originally a footnote to a discussion about the
10 behaviour of the git diff commands.
12 Often I find myself doing that [running diff against something other
13 than HEAD] while rewriting messy development history.  For example, I
14 start doing some work without knowing exactly where it leads, and end
15 up with a history like this:
17             "master"
18         o---o
19              \                    "topic"
20               o---o---o---o---o---o
22 At this point, "topic" contains something I know I want, but it
23 contains two concepts that turned out to be completely independent.
24 And often, one topic component is larger than the other.  It may
25 contain more than two topics.
27 In order to rewrite this mess to be more manageable, I would first do
28 "diff master..topic", to extract the changes into a single patch, start
29 picking pieces from it to get logically self-contained units, and
30 start building on top of "master":
32         $ git diff master..topic >P.diff
33         $ git checkout -b topicA master
34         ... pick and apply pieces from P.diff to build
35         ... commits on topicA branch.
37               o---o---o
38              /        "topicA"
39         o---o"master"
40              \                    "topic"
41               o---o---o---o---o---o
43 Before doing each commit on "topicA" HEAD, I run "diff HEAD"
44 before update-index the affected paths, or "diff --cached HEAD"
45 after.  Also I would run "diff --cached master" to make sure
46 that the changes are only the ones related to "topicA".  Usually
47 I do this for smaller topics first.
49 After that, I'd do the remainder of the original "topic", but
50 for that, I do not start from the patchfile I extracted by
51 comparing "master" and "topic" I used initially.  Still on
52 "topicA", I extract "diff topic", and use it to rebuild the
53 other topic:
55         $ git diff -R topic >P.diff ;# --cached also would work fine
56         $ git checkout -b topicB master
57         ... pick and apply pieces from P.diff to build
58         ... commits on topicB branch.
60                                 "topicB"
61                o---o---o---o---o
62               /
63              /o---o---o
64             |/        "topicA"
65         o---o"master"
66              \                    "topic"
67               o---o---o---o---o---o
69 After I am done, I'd try a pretend-merge between "topicA" and
70 "topicB" in order to make sure I have not missed anything:
72         $ git pull . topicA ;# merge it into current "topicB"
73         $ git diff topic
74                                 "topicB"
75                o---o---o---o---o---* (pretend merge)
76               /                   /
77              /o---o---o----------'
78             |/        "topicA"
79         o---o"master"
80              \                    "topic"
81               o---o---o---o---o---o
83 The last diff better not to show anything other than cleanups
84 for crufts.  Then I can finally clean things up:
86         $ git branch -D topic
87         $ git reset --hard HEAD^ ;# nuke pretend merge
89                                 "topicB"
90                o---o---o---o---o
91               /
92              /o---o---o
93             |/        "topicA"
94         o---o"master"