archive: handle commits with an empty tree

[git/jnareb-git.git] / Documentation / git-merge.txt

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

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


SYNOPSIS
--------
[verse]
'git merge' [-n] [--stat] [--no-commit] [--squash] [--[no-]edit]
        [-s <strategy>] [-X <strategy-option>]
        [--[no-]rerere-autoupdate] [-m <msg>] [<commit>...]
'git merge' <msg> HEAD <commit>...
'git merge' --abort

DESCRIPTION
-----------
Incorporates changes from the named commits (since the time their
histories diverged from the current branch) into the current
branch.  This command is used by 'git pull' to incorporate changes
from another repository and can be used by hand to merge changes
from one branch into another.

Assume the following history exists and the current branch is
"`master`":

------------
          A---B---C topic
         /
    D---E---F---G master
------------

Then "`git merge topic`" will replay the changes made on the
`topic` branch since it diverged from `master` (i.e., `E`) until
its current commit (`C`) on top of `master`, and record the result
in a new commit along with the names of the two parent commits and
a log message from the user describing the changes.

------------
          A---B---C topic
         /         \
    D---E---F---G---H master
------------

The second syntax (<msg> `HEAD` <commit>...) is supported for
historical reasons.  Do not use it from the command line or in
new scripts.  It is the same as `git merge -m <msg> <commit>...`.

The third syntax ("`git merge --abort`") can only be run after the
merge has resulted in conflicts. 'git merge --abort' will abort the
merge process and try to reconstruct the pre-merge state. However,
if there were uncommitted changes when the merge started (and
especially if those changes were further modified after the merge
was started), 'git merge --abort' will in some cases be unable to
reconstruct the original (pre-merge) changes. Therefore:

*Warning*: Running 'git merge' with uncommitted changes is
discouraged: while possible, it leaves you in a state that is hard to
back out of in the case of a conflict.


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

-m <msg>::
        Set the commit message to be used for the merge commit (in
        case one is created).
+
If `--log` is specified, a shortlog of the commits being merged
will be appended to the specified message.
+
The 'git fmt-merge-msg' command can be
used to give a good default for automated 'git merge'
invocations.

--rerere-autoupdate::
--no-rerere-autoupdate::
        Allow the rerere mechanism to update the index with the
        result of auto-conflict resolution if possible.

--abort::
        Abort the current conflict resolution process, and
        try to reconstruct the pre-merge state.
+
If there were uncommitted worktree changes present when the merge
started, 'git merge --abort' will in some cases be unable to
reconstruct these changes. It is therefore recommended to always
commit or stash your changes before running 'git merge'.
+
'git merge --abort' is equivalent to 'git reset --merge' when
`MERGE_HEAD` is present.

<commit>...::
        Commits, usually other branch heads, to merge into our branch.
        Specifying more than one commit will create a merge with
        more than two parents (affectionately called an Octopus merge).
+
If no commit is given from the command line, and if `merge.defaultToUpstream`
configuration variable is set, merge the remote-tracking branches
that the current branch is configured to use as its upstream.
See also the configuration section of this manual page.


PRE-MERGE CHECKS
----------------

Before applying outside changes, you should get your own work in
good shape and committed locally, so it will not be clobbered if
there are conflicts.  See also linkgit:git-stash[1].
'git pull' and 'git merge' will stop without doing anything when
local uncommitted changes overlap with files that 'git pull'/'git
merge' may need to update.

To avoid recording unrelated changes in the merge commit,
'git pull' and 'git merge' will also abort if there are any changes
registered in the index relative to the `HEAD` commit.  (One
exception is when the changed index entries are in the state that
would result from the merge already.)

If all named commits are already ancestors of `HEAD`, 'git merge'
will exit early with the message "Already up-to-date."

FAST-FORWARD MERGE
------------------

Often the current branch head is an ancestor of the named commit.
This is the most common case especially when invoked from 'git
pull': you are tracking an upstream repository, you have committed
no local changes, and now you want to update to a newer upstream
revision.  In this case, a new commit is not needed to store the
combined history; instead, the `HEAD` (along with the index) is
updated to point at the named commit, without creating an extra
merge commit.

This behavior can be suppressed with the `--no-ff` option.

TRUE MERGE
----------

Except in a fast-forward merge (see above), the branches to be
merged must be tied together by a merge commit that has both of them
as its parents.

A merged version reconciling the changes from all branches to be
merged is committed, and your `HEAD`, index, and working tree are
updated to it.  It is possible to have modifications in the working
tree as long as they do not overlap; the update will preserve them.

When it is not obvious how to reconcile the changes, the following
happens:

1. The `HEAD` pointer stays the same.
2. The `MERGE_HEAD` ref is set to point to the other branch head.
3. Paths that merged cleanly are updated both in the index file and
   in your working tree.
4. For conflicting paths, the index file records up to three
   versions: stage 1 stores the version from the common ancestor,
   stage 2 from `HEAD`, and stage 3 from `MERGE_HEAD` (you
   can inspect the stages with `git ls-files -u`).  The working
   tree files contain the result of the "merge" program; i.e. 3-way
   merge results with familiar conflict markers `<<<` `===` `>>>`.
5. No other changes are made.  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`.

If you tried a merge which resulted in complex conflicts and
want to start over, you can recover with `git merge --abort`.

HOW CONFLICTS ARE PRESENTED
---------------------------

During a merge, the working tree files are updated to reflect the result
of the merge.  Among the changes made to the common ancestor's version,
non-overlapping ones (that is, you changed an area of the file while the
other side left that area intact, or vice versa) are incorporated in the
final result verbatim.  When both sides made changes to the same area,
however, git cannot randomly pick one side over the other, and asks you to
resolve it by leaving what both sides did to that area.

By default, git uses the same style as the one used by the "merge" program
from the RCS suite to present such a conflicted hunk, like this:

------------
Here are lines that are either unchanged from the common
ancestor, or cleanly resolved because only one side changed.
<<<<<<< yours:sample.txt
Conflict resolution is hard;
let's go shopping.
=======
Git makes conflict resolution easy.
>>>>>>> theirs:sample.txt
And here is another line that is cleanly resolved or unmodified.
------------

The area where a pair of conflicting changes happened is marked with markers
`<<<<<<<`, `=======`, and `>>>>>>>`.  The part before the `=======`
is typically your side, and the part afterwards is typically their side.

The default format does not show what the original said in the conflicting
area.  You cannot tell how many lines are deleted and replaced with
Barbie's remark on your side.  The only thing you can tell is that your
side wants to say it is hard and you'd prefer to go shopping, while the
other side wants to claim it is easy.

An alternative style can be used by setting the "merge.conflictstyle"
configuration variable to "diff3".  In "diff3" style, the above conflict
may look like this:

------------
Here are lines that are either unchanged from the common
ancestor, or cleanly resolved because only one side changed.
<<<<<<< yours:sample.txt
Conflict resolution is hard;
let's go shopping.
|||||||
Conflict resolution is hard.
=======
Git makes conflict resolution easy.
>>>>>>> theirs:sample.txt
And here is another line that is cleanly resolved or unmodified.
------------

In addition to the `<<<<<<<`, `=======`, and `>>>>>>>` markers, it uses
another `|||||||` marker that is followed by the original text.  You can
tell that the original just stated a fact, and your side simply gave in to
that statement and gave up, while the other side tried to have a more
positive attitude.  You can sometimes come up with a better resolution by
viewing the original.


HOW TO RESOLVE CONFLICTS
------------------------

After seeing a conflict, you can do two things:

 * Decide not to merge.  The only clean-ups 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 merge --abort`
   can be used for this.

 * Resolve the conflicts.  Git will mark the conflicts in
   the working tree.  Edit the files into shape and
   'git add' them to the index.  Use 'git commit' to seal the deal.

You can work through the conflict with a number of tools:

 * Use a mergetool.  `git mergetool` to launch a graphical
   mergetool which will work you through the merge.

 * Look at the diffs.  `git diff` will show a three-way diff,
   highlighting changes from both the `HEAD` and `MERGE_HEAD`
   versions.

 * Look at the diffs from each branch. `git log --merge -p <path>`
   will show diffs first for the `HEAD` version and then the
   `MERGE_HEAD` version.

 * Look at the originals.  `git show :1:filename` shows the
   common ancestor, `git show :2:filename` shows the `HEAD`
   version, and `git show :3:filename` shows the `MERGE_HEAD`
   version.


EXAMPLES
--------

* Merge branches `fixes` and `enhancements` on top of
  the current branch, making an octopus merge:
+
------------------------------------------------
$ git merge fixes enhancements
------------------------------------------------

* Merge branch `obsolete` into the current branch, using `ours`
  merge strategy:
+
------------------------------------------------
$ git merge -s ours obsolete
------------------------------------------------

* Merge branch `maint` into the current branch, but do not make
  a new commit automatically:
+
------------------------------------------------
$ git merge --no-commit maint
------------------------------------------------
+
This can be used when you want to include further changes to the
merge, or want to write your own merge commit message.
+
You should refrain from abusing this option to sneak substantial
changes into a merge commit.  Small fixups like bumping
release/version name would be acceptable.


include::merge-strategies.txt[]

CONFIGURATION
-------------
include::merge-config.txt[]

branch.<name>.mergeoptions::
        Sets default options for merging into branch <name>. The syntax and
        supported options are the same as those of 'git merge', but option
        values containing whitespace characters are currently not supported.

SEE ALSO
--------
linkgit:git-fmt-merge-msg[1], linkgit:git-pull[1],
linkgit:gitattributes[5],
linkgit:git-reset[1],
linkgit:git-diff[1], linkgit:git-ls-files[1],
linkgit:git-add[1], linkgit:git-rm[1],
linkgit:git-mergetool[1]

GIT
---
Part of the linkgit:git[1] suite