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:
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: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: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: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 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.
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.
