Merge Tools

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

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:

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".
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.
Uses the local version of files as the merged version.
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.
Uses the other version of files as the merged version.
Asks the user which of the local or the other version to keep as the merged version.
Uses the internal tag merge algorithm (experimental).

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

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.


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.