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

NAME
----
git-merge-tree - Perform merge without touching index or working tree


SYNOPSIS
--------
[verse]
'git merge-tree' [--write-tree] [<options>] <branch1> <branch2>
'git merge-tree' [--trivial-merge] <base-tree> <branch1> <branch2> (deprecated)

[[NEWMERGE]]
DESCRIPTION
-----------

This command has a modern `--write-tree` mode and a deprecated
`--trivial-merge` mode.  With the exception of the
<<DEPMERGE,DEPRECATED DESCRIPTION>> section at the end, the rest of
this documentation describes modern `--write-tree` mode.

Performs a merge, but does not make any new commits and does not read
from or write to either the working tree or index.

The performed merge will use the same feature as the "real"
linkgit:git-merge[1], including:

  * three way content merges of individual files
  * rename detection
  * proper directory/file conflict handling
  * recursive ancestor consolidation (i.e. when there is more than one
    merge base, creating a virtual merge base by merging the merge bases)
  * etc.

After the merge completes, a new toplevel tree object is created.  See
`OUTPUT` below for details.

OPTIONS
-------

-z::
	Do not quote filenames in the <Conflicted file info> section,
	and end each filename with a NUL character rather than
	newline.  Also begin the messages section with a NUL character
	instead of a newline.  See <<OUTPUT>> below for more information.

--name-only::
	In the Conflicted file info section, instead of writing a list
	of (mode, oid, stage, path) tuples to output for conflicted
	files, just provide a list of filenames with conflicts (and
	do not list filenames multiple times if they have multiple
	conflicting stages).

--[no-]messages::
	Write any informational messages such as "Auto-merging <path>"
	or CONFLICT notices to the end of stdout.  If unspecified, the
	default is to include these messages if there are merge
	conflicts, and to omit them otherwise.

--allow-unrelated-histories::
	merge-tree will by default error out if the two branches specified
	share no common history.  This flag can be given to override that
	check and make the merge proceed anyway.

[[OUTPUT]]
OUTPUT
------

For a successful merge, the output from git-merge-tree is simply one
line:

	<OID of toplevel tree>

Whereas for a conflicted merge, the output is by default of the form:

	<OID of toplevel tree>
	<Conflicted file info>
	<Informational messages>

These are discussed individually below.

[[OIDTLT]]
OID of toplevel tree
~~~~~~~~~~~~~~~~~~~~

This is a tree object that represents what would be checked out in the
working tree at the end of `git merge`.  If there were conflicts, then
files within this tree may have embedded conflict markers.  This section
is always followed by a newline (or NUL if `-z` is passed).

[[CFI]]
Conflicted file info
~~~~~~~~~~~~~~~~~~~~

This is a sequence of lines with the format

	<mode> <object> <stage> <filename>

The filename will be quoted as explained for the configuration
variable `core.quotePath` (see linkgit:git-config[1]).  However, if
the `--name-only` option is passed, the mode, object, and stage will
be omitted.  If `-z` is passed, the "lines" are terminated by a NUL
character instead of a newline character.

[[IM]]
Informational messages
~~~~~~~~~~~~~~~~~~~~~~

This always starts with a blank line (or NUL if `-z` is passed) to
separate it from the previous sections, and then has free-form
messages about the merge, such as:

  * "Auto-merging <file>"
  * "CONFLICT (rename/delete): <oldfile> renamed...but deleted in..."
  * "Failed to merge submodule <submodule> (<reason>)"
  * "Warning: cannot merge binary files: <filename>"

Note that these free-form messages will never have a NUL character
in or between them, even if -z is passed.  It is simply a large block
of text taking up the remainder of the output.

EXIT STATUS
-----------

For a successful, non-conflicted merge, the exit status is 0.  When the
merge has conflicts, the exit status is 1.  If the merge is not able to
complete (or start) due to some kind of error, the exit status is
something other than 0 or 1 (and the output is unspecified).

USAGE NOTES
-----------

This command is intended as low-level plumbing, similar to
linkgit:git-hash-object[1], linkgit:git-mktree[1],
linkgit:git-commit-tree[1], linkgit:git-write-tree[1],
linkgit:git-update-ref[1], and linkgit:git-mktag[1].  Thus, it can be
used as a part of a series of steps such as:

       NEWTREE=$(git merge-tree --write-tree $BRANCH1 $BRANCH2)
       test $? -eq 0 || die "There were conflicts..."
       NEWCOMMIT=$(git commit-tree $NEWTREE -p $BRANCH1 -p $BRANCH2)
       git update-ref $BRANCH1 $NEWCOMMIT

Note that when the exit status is non-zero, `NEWTREE` in this sequence
will contain a lot more output than just a tree.

For conflicts, the output includes the same information that you'd get
with linkgit:git-merge[1]:

  * what would be written to the working tree (the
    <<OIDTLT,OID of toplevel tree>>)
  * the higher order stages that would be written to the index (the
    <<CFI,Conflicted file info>>)
  * any messages that would have been printed to stdout (the
    <<IM,Informational messages>>)

MISTAKES TO AVOID
-----------------

Do NOT look through the resulting toplevel tree to try to find which
files conflict; parse the <<CFI,Conflicted file info>> section instead.
Not only would parsing an entire tree be horrendously slow in large
repositories, there are numerous types of conflicts not representable by
conflict markers (modify/delete, mode conflict, binary file changed on
both sides, file/directory conflicts, various rename conflict
permutations, etc.)

Do NOT interpret an empty <<CFI,Conflicted file info>> list as a clean
merge; check the exit status.  A merge can have conflicts without having
individual files conflict (there are a few types of directory rename
conflicts that fall into this category, and others might also be added
in the future).

Do NOT attempt to guess or make the user guess the conflict types from
the <<CFI,Conflicted file info>> list.  The information there is
insufficient to do so.  For example: Rename/rename(1to2) conflicts (both
sides renamed the same file differently) will result in three different
file having higher order stages (but each only has one higher order
stage), with no way (short of the <<IM,Informational messages>> section)
to determine which three files are related.  File/directory conflicts
also result in a file with exactly one higher order stage.
Possibly-involved-in-directory-rename conflicts (when
"merge.directoryRenames" is unset or set to "conflicts") also result in
a file with exactly one higher order stage.  In all cases, the
<<IM,Informational messages>> section has the necessary info, though it
is not designed to be machine parseable.

Do NOT assume that each paths from <<CFI,Conflicted file info>>, and
the logical conflicts in the <<IM,Informational messages>> have a
one-to-one mapping, nor that there is a one-to-many mapping, nor a
many-to-one mapping.  Many-to-many mappings exist, meaning that each
path can have many logical conflict types in a single merge, and each
logical conflict type can affect many paths.

Do NOT assume all filenames listed in the <<IM,Informational messages>>
section had conflicts.  Messages can be included for files that have no
conflicts, such as "Auto-merging <file>".

AVOID taking the OIDS from the <<CFI,Conflicted file info>> and
re-merging them to present the conflicts to the user.  This will lose
information.  Instead, look up the version of the file found within the
<<OIDTLT,OID of toplevel tree>> and show that instead.  In particular,
the latter will have conflict markers annotated with the original
branch/commit being merged and, if renames were involved, the original
filename.  While you could include the original branch/commit in the
conflict marker annotations when re-merging, the original filename is
not available from the <<CFI,Conflicted file info>> and thus you would
be losing information that might help the user resolve the conflict.

[[DEPMERGE]]
DEPRECATED DESCRIPTION
----------------------

Per the <<NEWMERGE,DESCRIPTION>> and unlike the rest of this
documentation, this section describes the deprecated `--trivial-merge`
mode.

Other than the optional `--trivial-merge`, this mode accepts no
options.

This mode reads three tree-ish, and outputs trivial merge results and
conflicting stages to the standard output in a semi-diff format.
Since this was designed for higher level scripts to consume and merge
the results back into the index, it omits entries that match
<branch1>.  The result of this second form is similar to what
three-way 'git read-tree -m' does, but instead of storing the results
in the index, the command outputs the entries to the standard output.

This form not only has limited applicability (a trivial merge cannot
handle content merges of individual files, rename detection, proper
directory/file conflict handling, etc.), the output format is also
difficult to work with, and it will generally be less performant than
the first form even on successful merges (especially if working in
large repositories).

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