0f79665ea6e6bf1ffd4ff67f7aee2fcbddec0555
[git/git.git] / Documentation / git-merge.txt
1 git-merge(1)
2 ============
3
4 NAME
5 ----
6 git-merge - Grand Unified Merge Driver
7
8
9 SYNOPSIS
10 --------
11 [verse]
12 'git-merge' [-n] [--no-commit] [--squash] [-s <strategy>]...
13 -m=<msg> <remote> <remote>...
14
15 DESCRIPTION
16 -----------
17 This is the top-level interface to the merge machinery
18 which drives multiple merge strategy scripts.
19
20
21 OPTIONS
22 -------
23 include::merge-options.txt[]
24
25 <msg>::
26 The commit message to be used for the merge commit (in case
27 it is created). The `git-fmt-merge-msg` script can be used
28 to give a good default for automated `git-merge` invocations.
29
30 <head>::
31 Our branch head commit. This has to be `HEAD`, so new
32 syntax does not require it
33
34 <remote>::
35 Other branch head merged into our branch. You need at
36 least one <remote>. Specifying more than one <remote>
37 obviously means you are trying an Octopus.
38
39 include::merge-strategies.txt[]
40
41
42 If you tried a merge which resulted in a complex conflicts and
43 would want to start over, you can recover with
44 gitlink:git-reset[1].
45
46
47 HOW MERGE WORKS
48 ---------------
49
50 A merge is always between the current `HEAD` and one or more
51 remote branch heads, and the index file must exactly match the
52 tree of `HEAD` commit (i.e. the contents of the last commit) when
53 it happens. In other words, `git-diff --cached HEAD` must
54 report no changes.
55
56 [NOTE]
57 This is a bit of lie. In certain special cases, your index are
58 allowed to be different from the tree of `HEAD` commit. The most
59 notable case is when your `HEAD` commit is already ahead of what
60 is being merged, in which case your index can have arbitrary
61 difference from your `HEAD` commit. Otherwise, your index entries
62 are allowed have differences from your `HEAD` commit that match
63 the result of trivial merge (e.g. you received the same patch
64 from external source to produce the same result as what you are
65 merging). For example, if a path did not exist in the common
66 ancestor and your head commit but exists in the tree you are
67 merging into your repository, and if you already happen to have
68 that path exactly in your index, the merge does not have to
69 fail.
70
71 Otherwise, merge will refuse to do any harm to your repository
72 (that is, it may fetch the objects from remote, and it may even
73 update the local branch used to keep track of the remote branch
74 with `git pull remote rbranch:lbranch`, but your working tree,
75 `.git/HEAD` pointer and index file are left intact).
76
77 You may have local modifications in the working tree files. In
78 other words, `git-diff` is allowed to report changes.
79 However, the merge uses your working tree as the working area,
80 and in order to prevent the merge operation from losing such
81 changes, it makes sure that they do not interfere with the
82 merge. Those complex tables in read-tree documentation define
83 what it means for a path to "interfere with the merge". And if
84 your local modifications interfere with the merge, again, it
85 stops before touching anything.
86
87 So in the above two "failed merge" case, you do not have to
88 worry about loss of data --- you simply were not ready to do
89 a merge, so no merge happened at all. You may want to finish
90 whatever you were in the middle of doing, and retry the same
91 pull after you are done and ready.
92
93 When things cleanly merge, these things happen:
94
95 1. the results are updated both in the index file and in your
96 working tree,
97 2. index file is written out as a tree,
98 3. the tree gets committed, and
99 4. the `HEAD` pointer gets advanced.
100
101 Because of 2., we require that the original state of the index
102 file to match exactly the current `HEAD` commit; otherwise we
103 will write out your local changes already registered in your
104 index file along with the merge result, which is not good.
105 Because 1. involves only the paths different between your
106 branch and the remote branch you are pulling from during the
107 merge (which is typically a fraction of the whole tree), you can
108 have local modifications in your working tree as long as they do
109 not overlap with what the merge updates.
110
111 When there are conflicts, these things happen:
112
113 1. `HEAD` stays the same.
114
115 2. Cleanly merged paths are updated both in the index file and
116 in your working tree.
117
118 3. For conflicting paths, the index file records up to three
119 versions; stage1 stores the version from the common ancestor,
120 stage2 from `HEAD`, and stage3 from the remote branch (you
121 can inspect the stages with `git-ls-files -u`). The working
122 tree files have the result of "merge" program; i.e. 3-way
123 merge result with familiar conflict markers `<<< === >>>`.
124
125 4. No other changes are done. In particular, the local
126 modifications you had before you started merge will stay the
127 same and the index entries for them stay as they were,
128 i.e. matching `HEAD`.
129
130 After seeing a conflict, you can do two things:
131
132 * Decide not to merge. The only clean-up you need are to reset
133 the index file to the `HEAD` commit to reverse 2. and to clean
134 up working tree changes made by 2. and 3.; `git-reset` can
135 be used for this.
136
137 * Resolve the conflicts. `git-diff` would report only the
138 conflicting paths because of the above 2. and 3.. Edit the
139 working tree files into a desirable shape, `git-update-index`
140 them, to make the index file contain what the merge result
141 should be, and run `git-commit` to commit the result.
142
143
144 SEE ALSO
145 --------
146 gitlink:git-fmt-merge-msg[1], gitlink:git-pull[1]
147
148
149 Author
150 ------
151 Written by Junio C Hamano <junkio@cox.net>
152
153
154 Documentation
155 --------------
156 Documentation by Junio C Hamano and the git-list <git@vger.kernel.org>.
157
158 GIT
159 ---
160 Part of the gitlink:git[7] suite