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To merge files Mercurial uses merge tools.
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A merge tool combines two different versions of a file into a merged
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file. Merge tools are given the two files and the greatest common
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ancestor of the two file versions, so they can determine the changes
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made on both branches.
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Merge tools are used both for :hg:`resolve`, :hg:`merge`, :hg:`update`,
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:hg:`backout` and in several extensions.
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Usually, the merge tool tries to automatically reconcile the files by
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combining all non-overlapping changes that occurred separately in
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the two different evolutions of the same initial base file. Furthermore, some
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interactive merge programs make it easier to manually resolve
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conflicting merges, either in a graphical way, or by inserting some
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conflict markers. Mercurial does not include any interactive merge
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programs but relies on external tools for that.
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Available merge tools
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=====================
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External merge tools and their properties are configured in the
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merge-tools configuration section - see hgrc(5) - but they can often just
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be named by their executable.
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A merge tool is generally usable if its executable can be found on the
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system and if it can handle the merge. The executable is found if it
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is an absolute or relative executable path or the name of an
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application in the executable search path. The tool is assumed to be
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able to handle the merge if it can handle symlinks if the file is a
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symlink, if it can handle binary files if the file is binary, and if a
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GUI is available if the tool requires a GUI.
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There are some internal merge tools which can be used. The internal
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merge tools are:
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.. internaltoolsmarker
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Internal tools are always available and do not require a GUI but will by default
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not handle symlinks or binary files.
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Choosing a merge tool
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=====================
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Mercurial uses these rules when deciding which merge tool to use:
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1. If a tool has been specified with the --tool option to merge or resolve, it
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is used. If it is the name of a tool in the merge-tools configuration, its
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configuration is used. Otherwise the specified tool must be executable by
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the shell.
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2. If the ``HGMERGE`` environment variable is present, its value is used and
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must be executable by the shell.
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3. If the filename of the file to be merged matches any of the patterns in the
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merge-patterns configuration section, the first usable merge tool
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corresponding to a matching pattern is used. Here, binary capabilities of the
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merge tool are not considered.
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4. If ui.merge is set it will be considered next. If the value is not the name
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of a configured tool, the specified value is used and must be executable by
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the shell. Otherwise the named tool is used if it is usable.
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5. If any usable merge tools are present in the merge-tools configuration
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section, the one with the highest priority is used.
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6. If a program named ``hgmerge`` can be found on the system, it is used - but
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it will by default not be used for symlinks and binary files.
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7. If the file to be merged is not binary and is not a symlink, then
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``internal:merge`` is used.
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8. The merge of the file fails and must be resolved before commit.
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.. note::
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After selecting a merge program, Mercurial will by default attempt
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to merge the files using a simple merge algorithm first. Only if it doesn't
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succeed because of conflicting changes Mercurial will actually execute the
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merge program. Whether to use the simple merge algorithm first can be
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controlled by the premerge setting of the merge tool. Premerge is enabled by
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default unless the file is binary or a symlink.
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See the merge-tools and ui sections of hgrc(5) for details on the
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configuration of merge tools.
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