Documentation/git-merge.txt

git-merge(1)
============

NAME
----
git-merge - Join two or more development histories together


SYNOPSIS
--------
[verse]
'git-merge' [-n] [--no-commit] [--squash] [-s <strategy>]...
	[-m <msg>] <remote> <remote>...

DESCRIPTION
-----------
This is the top-level interface to the merge machinery
which drives multiple merge strategy scripts.


OPTIONS
-------
include::merge-options.txt[]

<msg>::
	The commit message to be used for the merge commit (in case
	it is created). The `git-fmt-merge-msg` script can be used
	to give a good default for automated `git-merge` invocations.

<head>::
	Our branch head commit.  This has to be `HEAD`, so new
	syntax does not require it

<remote>::
	Other branch head merged into our branch.  You need at
	least one <remote>.  Specifying more than one <remote>
	obviously means you are trying an Octopus.

include::merge-strategies.txt[]


If you tried a merge which resulted in a complex conflicts and
would want to start over, you can recover with
gitlink:git-reset[1].


HOW MERGE WORKS
---------------

A merge is always between the current `HEAD` and one or more
remote branch heads, and the index file must exactly match the
tree of `HEAD` commit (i.e. the contents of the last commit) when
it happens.  In other words, `git-diff --cached HEAD` must
report no changes.

[NOTE]
This is a bit of lie.  In certain special cases, your index are
allowed to be different from the tree of `HEAD` commit.  The most
notable case is when your `HEAD` commit is already ahead of what
is being merged, in which case your index can have arbitrary
difference from your `HEAD` commit.  Otherwise, your index entries
are allowed have differences from your `HEAD` commit that match
the result of trivial merge (e.g. you received the same patch
from external source to produce the same result as what you are
merging).  For example, if a path did not exist in the common
ancestor and your head commit but exists in the tree you are
merging into your repository, and if you already happen to have
that path exactly in your index, the merge does not have to
fail.

Otherwise, merge will refuse to do any harm to your repository
(that is, it may fetch the objects from remote, and it may even
update the local branch used to keep track of the remote branch
with `git pull remote rbranch:lbranch`, but your working tree,
`.git/HEAD` pointer and index file are left intact).

You may have local modifications in the working tree files.  In
other words, `git-diff` is allowed to report changes.
However, the merge uses your working tree as the working area,
and in order to prevent the merge operation from losing such
changes, it makes sure that they do not interfere with the
merge. Those complex tables in read-tree documentation define
what it means for a path to "interfere with the merge".  And if
your local modifications interfere with the merge, again, it
stops before touching anything.

So in the above two "failed merge" case, you do not have to
worry about loss of data --- you simply were not ready to do
a merge, so no merge happened at all.  You may want to finish
whatever you were in the middle of doing, and retry the same
pull after you are done and ready.

When things cleanly merge, these things happen:

1. the results are updated both in the index file and in your
   working tree,
2. index file is written out as a tree,
3. the tree gets committed, and 
4. the `HEAD` pointer gets advanced.

Because of 2., we require that the original state of the index
file to match exactly the current `HEAD` commit; otherwise we
will write out your local changes already registered in your
index file along with the merge result, which is not good.
Because 1. involves only the paths different between your
branch and the remote branch you are pulling from during the
merge (which is typically a fraction of the whole tree), you can
have local modifications in your working tree as long as they do
not overlap with what the merge updates.

When there are conflicts, these things happen:

1. `HEAD` stays the same.

2. Cleanly merged paths are updated both in the index file and
   in your working tree.

3. For conflicting paths, the index file records up to three
   versions; stage1 stores the version from the common ancestor,
   stage2 from `HEAD`, and stage3 from the remote branch (you
   can inspect the stages with `git-ls-files -u`).  The working
   tree files have the result of "merge" program; i.e. 3-way
   merge result with familiar conflict markers `<<< === >>>`.

4. No other changes are done.  In particular, the local
   modifications you had before you started merge will stay the
   same and the index entries for them stay as they were,
   i.e. matching `HEAD`.

After seeing a conflict, you can do two things:

 * Decide not to merge.  The only clean-up you need are to reset
   the index file to the `HEAD` commit to reverse 2. and to clean
   up working tree changes made by 2. and 3.; `git-reset` can
   be used for this.

 * Resolve the conflicts.  `git-diff` would report only the
   conflicting paths because of the above 2. and 3..  Edit the
   working tree files into a desirable shape, `git-update-index`
   them, to make the index file contain what the merge result
   should be, and run `git-commit` to commit the result.


SEE ALSO
--------
gitlink:git-fmt-merge-msg[1], gitlink:git-pull[1]


Author
------
Written by Junio C Hamano <junkio@cox.net>


Documentation
--------------
Documentation by Junio C Hamano and the git-list <git@vger.kernel.org>.

GIT
---
Part of the gitlink:git[7] suite