To merge files Mercurial uses merge tools.

A merge tool combines two different versions of a file into a merged

file. Merge tools are given the two files and the greatest common

ancestor of the two file versions, so they can determine the changes

made on both branches.

Merge tools are used both for :hg:`resolve`, :hg:`merge`, :hg:`update`,

:hg:`backout` and in several extensions.

Usually, the merge tool tries to automatically reconcile the files by

combining all non-overlapping changes that occurred separately in

the two different evolutions of the same initial base file. Furthermore, some

interactive merge programs make it easier to manually resolve

conflicting merges, either in a graphical way, or by inserting some

conflict markers. Mercurial does not include any interactive merge

programs but relies on external tools for that.

Available merge tools

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External merge tools and their properties are configured in the

merge-tools configuration section - see hgrc(5) - but they can often just

be named by their executable.

A merge tool is generally usable if its executable can be found on the

system and if it can handle the merge. The executable is found if it

is an absolute or relative executable path or the name of an

application in the executable search path. The tool is assumed to be

able to handle the merge if it can handle symlinks if the file is a

symlink, if it can handle binary files if the file is binary, and if a

GUI is available if the tool requires a GUI.

There are some internal merge tools which can be used. The internal

merge tools are:

``internal:merge``

   Uses the internal non-interactive simple merge algorithm for merging

   files. It will fail if there are any conflicts and leave markers in

   the partially merged file.

``internal:fail``

   Rather than attempting to merge files that were modified on both

   branches, it marks them as unresolved. The resolve command must be

   used to resolve these conflicts.

``internal:local``

   Uses the local version of files as the merged version.

``internal:other``

   Uses the other version of files as the merged version.

``internal:prompt``

   Asks the user which of the local or the other version to keep as

   the merged version.

``internal:dump``

   Creates three versions of the files to merge, containing the

   contents of local, other and base. These files can then be used to

   perform a merge manually. If the file to be merged is named

   ``a.txt``, these files will accordingly be named ``a.txt.local``,

   ``a.txt.other`` and ``a.txt.base`` and they will be placed in the

   same directory as ``a.txt``.

Internal tools are always available and do not require a GUI but will by default

not handle symlinks or binary files.

Choosing a merge tool

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Mercurial uses these rules when deciding which merge tool to use:

1. If a tool has been specified with the --tool option to merge or resolve, it

   is used.  If it is the name of a tool in the merge-tools configuration, its

   configuration is used. Otherwise the specified tool must be executable by

   the shell.

2. If the ``HGMERGE`` environment variable is present, its value is used and

   must be executable by the shell.

3. If the filename of the file to be merged matches any of the patterns in the

   merge-patterns configuration section, the first usable merge tool

   corresponding to a matching pattern is used. Here, binary capabilities of the

   merge tool are not considered.

4. If ui.merge is set it will be considered next. If the value is not the name

   of a configured tool, the specified value is used and must be executable by

   the shell. Otherwise the named tool is used if it is usable.

5. If any usable merge tools are present in the merge-tools configuration

   section, the one with the highest priority is used.

6. If a program named ``hgmerge`` can be found on the system, it is used - but

   it will by default not be used for symlinks and binary files.

7. If the file to be merged is not binary and is not a symlink, then

   ``internal:merge`` is used.

8. The merge of the file fails and must be resolved before commit.

.. note::

   After selecting a merge program, Mercurial will by default attempt

   to merge the files using a simple merge algorithm first. Only if it doesn't

   succeed because of conflicting changes Mercurial will actually execute the

   merge program. Whether to use the simple merge algorithm first can be

   controlled by the premerge setting of the merge tool. Premerge is enabled by

   default unless the file is binary or a symlink.

See the merge-tools and ui sections of hgrc(5) for details on the

configuration of merge tools.