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