Documentation: git fmt-merge-msg does not have to be a script
[git/git.git] / Documentation / git-merge.txt
1 git-merge(1)
2 ============
3
4 NAME
5 ----
6 git-merge - Join two or more development histories together
7
8
9 SYNOPSIS
10 --------
11 [verse]
12 'git merge' [-n] [--stat] [--no-commit] [--squash] [-s <strategy>]...
13 [-m <msg>] <remote>...
14 'git merge' <msg> HEAD <remote>...
15
16 DESCRIPTION
17 -----------
18 This is the top-level interface to the merge machinery
19 which drives multiple merge strategy scripts.
20
21 The second syntax (<msg> `HEAD` <remote>) is supported for
22 historical reasons. Do not use it from the command line or in
23 new scripts. It is the same as `git merge -m <msg> <remote>`.
24
25
26 OPTIONS
27 -------
28 include::merge-options.txt[]
29
30 -m <msg>::
31 Set the commit message to be used for the merge commit (in
32 case one is created). The 'git fmt-merge-msg' command can be
33 used to give a good default for automated 'git merge'
34 invocations.
35
36 <remote>...::
37 Other branch heads to merge into our branch. You need at
38 least one <remote>. Specifying more than one <remote>
39 obviously means you are trying an Octopus.
40
41 include::merge-strategies.txt[]
42
43
44 If you tried a merge which resulted in complex conflicts and
45 want to start over, you can recover with 'git-reset'.
46
47 CONFIGURATION
48 -------------
49 include::merge-config.txt[]
50
51 branch.<name>.mergeoptions::
52 Sets default options for merging into branch <name>. The syntax and
53 supported options are equal to that of 'git-merge', but option values
54 containing whitespace characters are currently not supported.
55
56 HOW MERGE WORKS
57 ---------------
58
59 A merge is always between the current `HEAD` and one or more
60 commits (usually, branch head or tag), and the index file must
61 match the tree of `HEAD` commit (i.e. the contents of the last commit)
62 when it starts out. In other words, `git diff --cached HEAD` must
63 report no changes. (One exception is when the changed index
64 entries are already in the same state that would result from
65 the merge anyway.)
66
67 Three kinds of merge can happen:
68
69 * The merged commit is already contained in `HEAD`. This is the
70 simplest case, called "Already up-to-date."
71
72 * `HEAD` is already contained in the merged commit. This is the
73 most common case especially when invoked from 'git pull':
74 you are tracking an upstream repository, have committed no local
75 changes and now you want to update to a newer upstream revision.
76 Your `HEAD` (and the index) is updated to point at the merged
77 commit, without creating an extra merge commit. This is
78 called "Fast-forward".
79
80 * Both the merged commit and `HEAD` are independent and must be
81 tied together by a merge commit that has both of them as its parents.
82 The rest of this section describes this "True merge" case.
83
84 The chosen merge strategy merges the two commits into a single
85 new source tree.
86 When things merge cleanly, this is what happens:
87
88 1. The results are updated both in the index file and in your
89 working tree;
90 2. Index file is written out as a tree;
91 3. The tree gets committed; and
92 4. The `HEAD` pointer gets advanced.
93
94 Because of 2., we require that the original state of the index
95 file matches exactly the current `HEAD` commit; otherwise we
96 will write out your local changes already registered in your
97 index file along with the merge result, which is not good.
98 Because 1. involves only those paths differing between your
99 branch and the remote branch you are pulling from during the
100 merge (which is typically a fraction of the whole tree), you can
101 have local modifications in your working tree as long as they do
102 not overlap with what the merge updates.
103
104 When there are conflicts, the following happens:
105
106 1. `HEAD` stays the same.
107
108 2. Cleanly merged paths are updated both in the index file and
109 in your working tree.
110
111 3. For conflicting paths, the index file records up to three
112 versions; stage1 stores the version from the common ancestor,
113 stage2 from `HEAD`, and stage3 from the remote branch (you
114 can inspect the stages with `git ls-files -u`). The working
115 tree files contain the result of the "merge" program; i.e. 3-way
116 merge results with familiar conflict markers `<<< === >>>`.
117
118 4. No other changes are done. In particular, the local
119 modifications you had before you started merge will stay the
120 same and the index entries for them stay as they were,
121 i.e. matching `HEAD`.
122
123 HOW CONFLICTS ARE PRESENTED
124 ---------------------------
125
126 During a merge, the working tree files are updated to reflect the result
127 of the merge. Among the changes made to the common ancestor's version,
128 non-overlapping ones (that is, you changed an area of the file while the
129 other side left that area intact, or vice versa) are incorporated in the
130 final result verbatim. When both sides made changes to the same area,
131 however, git cannot randomly pick one side over the other, and asks you to
132 resolve it by leaving what both sides did to that area.
133
134 By default, git uses the same style as that is used by "merge" program
135 from the RCS suite to present such a conflicted hunk, like this:
136
137 ------------
138 Here are lines that are either unchanged from the common
139 ancestor, or cleanly resolved because only one side changed.
140 <<<<<<< yours:sample.txt
141 Conflict resolution is hard;
142 let's go shopping.
143 =======
144 Git makes conflict resolution easy.
145 >>>>>>> theirs:sample.txt
146 And here is another line that is cleanly resolved or unmodified.
147 ------------
148
149 The area where a pair of conflicting changes happened is marked with markers
150 `<<<<<<<`, `=======`, and `>>>>>>>`. The part before the `=======`
151 is typically your side, and the part afterwards is typically their side.
152
153 The default format does not show what the original said in the conflicting
154 area. You cannot tell how many lines are deleted and replaced with
155 Barbie's remark on your side. The only thing you can tell is that your
156 side wants to say it is hard and you'd prefer to go shopping, while the
157 other side wants to claim it is easy.
158
159 An alternative style can be used by setting the "merge.conflictstyle"
160 configuration variable to "diff3". In "diff3" style, the above conflict
161 may look like this:
162
163 ------------
164 Here are lines that are either unchanged from the common
165 ancestor, or cleanly resolved because only one side changed.
166 <<<<<<< yours:sample.txt
167 Conflict resolution is hard;
168 let's go shopping.
169 |||||||
170 Conflict resolution is hard.
171 =======
172 Git makes conflict resolution easy.
173 >>>>>>> theirs:sample.txt
174 And here is another line that is cleanly resolved or unmodified.
175 ------------
176
177 In addition to the `<<<<<<<`, `=======`, and `>>>>>>>` markers, it uses
178 another `|||||||` marker that is followed by the original text. You can
179 tell that the original just stated a fact, and your side simply gave in to
180 that statement and gave up, while the other side tried to have a more
181 positive attitude. You can sometimes come up with a better resolution by
182 viewing the original.
183
184
185 HOW TO RESOLVE CONFLICTS
186 ------------------------
187
188 After seeing a conflict, you can do two things:
189
190 * Decide not to merge. The only clean-ups you need are to reset
191 the index file to the `HEAD` commit to reverse 2. and to clean
192 up working tree changes made by 2. and 3.; 'git-reset --hard' can
193 be used for this.
194
195 * Resolve the conflicts. Git will mark the conflicts in
196 the working tree. Edit the files into shape and
197 'git-add' them to the index. Use 'git-commit' to seal the deal.
198
199 You can work through the conflict with a number of tools:
200
201 * Use a mergetool. 'git mergetool' to launch a graphical
202 mergetool which will work you through the merge.
203
204 * Look at the diffs. 'git diff' will show a three-way diff,
205 highlighting changes from both the HEAD and remote versions.
206
207 * Look at the diffs on their own. 'git log --merge -p <path>'
208 will show diffs first for the HEAD version and then the
209 remote version.
210
211 * Look at the originals. 'git show :1:filename' shows the
212 common ancestor, 'git show :2:filename' shows the HEAD
213 version and 'git show :3:filename' shows the remote version.
214
215 SEE ALSO
216 --------
217 linkgit:git-fmt-merge-msg[1], linkgit:git-pull[1],
218 linkgit:gitattributes[5],
219 linkgit:git-reset[1],
220 linkgit:git-diff[1], linkgit:git-ls-files[1],
221 linkgit:git-add[1], linkgit:git-rm[1],
222 linkgit:git-mergetool[1]
223
224 Author
225 ------
226 Written by Junio C Hamano <gitster@pobox.com>
227
228
229 Documentation
230 --------------
231 Documentation by Junio C Hamano and the git-list <git@vger.kernel.org>.
232
233 GIT
234 ---
235 Part of the linkgit:git[1] suite