Merge Program
A merge program combines two different versions of a file into a merged file.
Usually, the program tries to do so automatically, by combining all the 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.
By default, Mercurial will attempt to do a classic 3-way merge on text files internally before trying to use an external tool.
How Mercurial decides which merge program to use
If the HGMERGE environment variable is present, it is used.
Otherwise, if the filename of the file to be merged matches any of the patterns in the merge-patterns configuration section, then the corresponding merge tool is used. (See MergeToolConfiguration for details.)
Otherwise, if the ui.merge config setting is set, that is used. (The next section explains how to configure this.)
Otherwise, if any merge tools are present in the merge-tools configuration section, and any of the tools can be found on the system, the priority settings are used to determine which one to use. (MergeToolConfiguration explains how to configure this.)
Otherwise, if a program named hgmerge exists on the system, that is used.
Otherwise, if the file to be merged is not binary and is not a symlink, then internal:merge is used.
- Otherwise, the merge fails.
However: after selecting a merge program, Mercurial will usually attempt to merge the files using a simple merge algorithm first, to see if they can be merged without conflicts. Only if there are conflicting changes will hg actually execute the merge program. (If the file to be merged is binary or a symlink, then hg doesn't bother with the simple merge algorithm. If the selected merge tool is internal:fail, internal:local, or internal:other, then hg skips the simple merge algorithm, as the user has specifically requested that no merging take place.)
Before Mercurial 1.0, a script named hgmerge was installed by default. As of Mercurial 1.0, hgmerge is no longer installed (and it should not be distributed with Mercurial), but hg still uses it, if it's present on the system and no other merge tools are configured (see hgmerge)
Choosing and configuring a merge program
Some merge programs (roughly sorted by "relevance") include:
KDiff3 - feature-rich merge tool, uses QT and is thus cross-platform.
Meld - 3-way editor, uses GTK / Gnome and thus Unix only.
gPyFm - merge tool from OpenSolaris, cross-platform.
diff3 - non-interactive merge tool, standard on most Unix/Linux installs, available from GnuWin32 for Windows. See MergingManuallyInEditor for a wrapper script that uses it with your favorite editor.
tkdiff - known to work under Windows.
TortoiseMerge - for Windows, part of TortoiseSVN.
WinMerge - Windows only, as the name suggests.
DiffMerge - Works on Windows, OS X and Linux.
"vim -d" - can be used wherever the popular "vim" editor can be found.
Emacs thanks to a little wrapper script to spawn ediff.
FileMerge - Native Mac OS X file merging application, see MacOSXFileMerge .
Changes - alternative native Mac OS X file merging application. (proprietary)
P4Merge - Works on Windows, OS X and Linux. (proprietary, free?)
If you have a preferred merge program, you can set the merge entry in the ui section of your hgrc file.
[ui] merge = your-merge-program
In Mercurial 1.0 you can add the following to .hgrc:
[merge-tools] kdiff3.args = $base $local $other -o $output