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] [--summary] [--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 CONFIGURATION
  47 -------------
  48
  49 merge.summary::
  50         Whether to include summaries of merged commits in newly
  51         created merge commit. False by default.
  52
  53 merge.verbosity::
  54         Controls the amount of output shown by the recursive merge
  55         strategy.  Level 0 outputs nothing except a final error
  56         message if conflicts were detected. Level 1 outputs only
  57         conflicts, 2 outputs conflicts and file changes.  Level 5 and
  58         above outputs debugging information.  The default is level 2.
  59         Can be overriden by 'GIT_MERGE_VERBOSITY' environment variable.
  60
  61
  62 HOW MERGE WORKS
  63 ---------------
  64
  65 A merge is always between the current `HEAD` and one or more
  66 remote branch heads, and the index file must exactly match the
  67 tree of `HEAD` commit (i.e. the contents of the last commit) when
  68 it happens.  In other words, `git-diff --cached HEAD` must
  69 report no changes.
  70
  71 [NOTE]
  72 This is a bit of lie.  In certain special cases, your index are
  73 allowed to be different from the tree of `HEAD` commit.  The most
  74 notable case is when your `HEAD` commit is already ahead of what
  75 is being merged, in which case your index can have arbitrary
  76 difference from your `HEAD` commit.  Otherwise, your index entries
  77 are allowed have differences from your `HEAD` commit that match
  78 the result of trivial merge (e.g. you received the same patch
  79 from external source to produce the same result as what you are
  80 merging).  For example, if a path did not exist in the common
  81 ancestor and your head commit but exists in the tree you are
  82 merging into your repository, and if you already happen to have
  83 that path exactly in your index, the merge does not have to
  84 fail.
  85
  86 Otherwise, merge will refuse to do any harm to your repository
  87 (that is, it may fetch the objects from remote, and it may even
  88 update the local branch used to keep track of the remote branch
  89 with `git pull remote rbranch:lbranch`, but your working tree,
  90 `.git/HEAD` pointer and index file are left intact).
  91
  92 You may have local modifications in the working tree files.  In
  93 other words, `git-diff` is allowed to report changes.
  94 However, the merge uses your working tree as the working area,
  95 and in order to prevent the merge operation from losing such
  96 changes, it makes sure that they do not interfere with the
  97 merge. Those complex tables in read-tree documentation define
  98 what it means for a path to "interfere with the merge".  And if
  99 your local modifications interfere with the merge, again, it
 100 stops before touching anything.
 101
 102 So in the above two "failed merge" case, you do not have to
 103 worry about loss of data --- you simply were not ready to do
 104 a merge, so no merge happened at all.  You may want to finish
 105 whatever you were in the middle of doing, and retry the same
 106 pull after you are done and ready.
 107
 108 When things cleanly merge, these things happen:
 109
 110 1. the results are updated both in the index file and in your
 111    working tree,
 112 2. index file is written out as a tree,
 113 3. the tree gets committed, and
 114 4. the `HEAD` pointer gets advanced.
 115
 116 Because of 2., we require that the original state of the index
 117 file to match exactly the current `HEAD` commit; otherwise we
 118 will write out your local changes already registered in your
 119 index file along with the merge result, which is not good.
 120 Because 1. involves only the paths different between your
 121 branch and the remote branch you are pulling from during the
 122 merge (which is typically a fraction of the whole tree), you can
 123 have local modifications in your working tree as long as they do
 124 not overlap with what the merge updates.
 125
 126 When there are conflicts, these things happen:
 127
 128 1. `HEAD` stays the same.
 129
 130 2. Cleanly merged paths are updated both in the index file and
 131    in your working tree.
 132
 133 3. For conflicting paths, the index file records up to three
 134    versions; stage1 stores the version from the common ancestor,
 135    stage2 from `HEAD`, and stage3 from the remote branch (you
 136    can inspect the stages with `git-ls-files -u`).  The working
 137    tree files have the result of "merge" program; i.e. 3-way
 138    merge result with familiar conflict markers `<<< === >>>`.
 139
 140 4. No other changes are done.  In particular, the local
 141    modifications you had before you started merge will stay the
 142    same and the index entries for them stay as they were,
 143    i.e. matching `HEAD`.
 144
 145 After seeing a conflict, you can do two things:
 146
 147  * Decide not to merge.  The only clean-up you need are to reset
 148    the index file to the `HEAD` commit to reverse 2. and to clean
 149    up working tree changes made by 2. and 3.; `git-reset` can
 150    be used for this.
 151
 152  * Resolve the conflicts.  `git-diff` would report only the
 153    conflicting paths because of the above 2. and 3..  Edit the
 154    working tree files into a desirable shape, `git-add` or `git-rm`
 155    them, to make the index file contain what the merge result
 156    should be, and run `git-commit` to commit the result.
 157
 158
 159 SEE ALSO
 160 --------
 161 gitlink:git-fmt-merge-msg[1], gitlink:git-pull[1]
 162
 163
 164 Author
 165 ------
 166 Written by Junio C Hamano <junkio@cox.net>
 167
 168
 169 Documentation
 170 --------------
 171 Documentation by Junio C Hamano and the git-list <git@vger.kernel.org>.
 172
 173 GIT
 174 ---
 175 Part of the gitlink:git[7] suite