Using Emacs Ediff as Git Merge/Difftool

Overview
Objective
Implementation

Overview

Ediff is «a comprehensive visual interface to Unix diff and patch utilities»1 that comes with your Emacs. This document shows how to use Ediff with Git for resolving merge conflicts and inspecting differences between file versions.

The solution described here uses a wrapper-shell-script that is called by git mergetool and git difftool. Arguments for the script are provided by Git via the appropriate Git configuration.

The impatient may just download the wrapper-script for Ediff, possibly adapt it (see also: Ediff Script) and add the appropriate Git configuration values.

Objective

The objective is to integrate Ediff smoothly into the Git workflow. The following should be achieved:

Launch Ediff whenever git mergetool is invoked.
Launch Ediff whenever git difftool is invoked.
Prefer emacsclient over emacs.
Create a new Emacs frame when on X.
When merging, include ancestor (BASE) if available.
When merging, check resulting merged file for conflict markers.
When merging, use exit code to decide if merge was successful.

Implementation

As the Git mergetool and difftool help describes, it is possible to define custom merge and diff tool commands. Although it would be feasible to define an emacsclient command directly in the Git configuration, I prefer to use a shell script. This makes it easier to decide which Ediff function to execute and which exit code to return. The script is called with the appropriate arguments from git mergetool or git difftool as defined in the Git configuration.

Ediff Functions and Arguments

There are basically three different cases that the script must handle by executing a specific Ediff function with the proper arguments:

Git diff A simple diff, triggered by git difftool should execute ediff with the diff pre- and post-image as arguments.
Git merge without ancestor A merge without ancestors occurs for instance if you merge two branches and those two branches have created the same path/file independently, i.e. without a common base version. In this case ediff-merge-files should be executed with three arguments: the version of the file in the current branch, the version of the file in the branch to be merged from and the target file to write the merged version to.
Git merge with ancestor Often the conflicting versions of a file have a common earlier version, their ancestor or base. When an ancestor is available ediff-merge-files-with-ancestor should be executed with four arguments: the version of the file in the current branch, the version of the file in the branch to be merged from, the target file to write the merged version to and the base version of the file.

Ediff Script

Following below is the wrapper-script for Ediff which should be executable. The <a href="#sec-3-3">Git configuration</a> below assumes that the script is named <code>ediff.sh</code>.

<div class="org-src-container"> <pre class="src src-shell-script"> 1: #!/bin/bash 2: 3: # test args 4: if [ ! ${#} -ge 3 ]; then 5: echo 1>&2 "Usage: ${0} LOCAL REMOTE MERGED BASE" 6: echo 1>&2 " (LOCAL, REMOTE, MERGED, BASE can be provided by \`git mergetool'.)" 7: exit 1 8: fi 9: 10: # tools 11: _EMACSCLIENT=/usr/bin/emacsclient 12: _BASENAME=/bin/basename 13: _CP=/bin/cp 14: _EGREP=/bin/egrep 15: _MKTEMP=/bin/mktemp 16: 17: # args 18: _LOCAL=${1} 19: _REMOTE=${2} 20: _MERGED=${3} 21: if [ ${4} -a -r ${4} ] ; then 22: _BASE=${4} 23: _EDIFF=ediff-merge-files-with-ancestor 24: _EVAL="${_EDIFF} \"${_LOCAL}\" \"${_REMOTE}\" \"${_BASE}\" nil \"${_MERGED}\"" 25: elif [ ${_REMOTE} = ${_MERGED} ] ; then 26: _EDIFF=ediff 27: _EVAL="${_EDIFF} \"${_LOCAL}\" \"${_REMOTE}\"" 28: else 29: _EDIFF=ediff-merge-files 30: _EVAL="${_EDIFF} \"${_LOCAL}\" \"${_REMOTE}\" nil \"${_MERGED}\"" 31: fi 32: 33: # console vs. X 34: if [ "${TERM}" = "linux" ]; then 35: unset DISPLAY 36: _EMACSCLIENTOPTS="-t" 37: else 38: _EMACSCLIENTOPTS="-c" 39: fi 40: 41: # run emacsclient 42: ${_EMACSCLIENT} ${_EMACSCLIENTOPTS} -a "" -e "(${_EVAL})" 2>&1 43: 44: # check modified file 45: if [ ! $(egrep -c '^(<<<<<<<|=======|>>>>>>>|####### Ancestor)' ${_MERGED}) = 0 ]; then 46: _MERGEDSAVE=$(${_MKTEMP} --tmpdir `${_BASENAME} ${_MERGED}`.XXXXXXXXXX) 47: ${_CP} ${_MERGED} ${_MERGEDSAVE} 48: echo 1>&2 "Oops! Conflict markers detected in $_MERGED." 49: echo 1>&2 "Saved your changes to ${_MERGEDSAVE}" 50: echo 1>&2 "

Emacstool

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