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Using Emacs as a merge program

Emacs is bundled with an elisp program called Ediff whose purpose is to help developers to visually apply patches. One of the ediff commands is well-suited to three-way merging and can be used as a merge program with Mercurial.

Note that emacs comes with a version control wrapper that works with mercurial and other systems. See particularly its special dired mode for operations on the whole repository, rather than individual files. C-x v m does a merge.

Mercurial 1.0 and later

Add the following to your ~/.hgrc file (see MergeToolConfiguration):

[ui]
merge = emacs

[merge-tools]
emacs.args = -q --eval "(ediff-merge-with-ancestor \""$local"\" \""$other"\" \""$base"\" nil \""$output"\")"

If you want to use the plain emerge tool in emacs, the following line works instead in the merge-tools section:

emacs.args = -q --eval "(emerge-files-with-ancestor nil \""$local"\" \""$other"\" \""$base"\" \"$output\" nil 'kill-emacs)"

A more elaborate version that puts the ediff control window inside the main frame:

emacs.args = -q --eval "(require 'ediff)" --eval "(setq ediff-window-setup-function 'ediff-setup-windows-plain)" --eval "(add-hook 'ediff-quit-hook 'save-buffers-kill-emacs)" --eval "(ediff-merge-with-ancestor \""$local"\" \""$other"\" \""$base"\" nil \""$output"\")"

Mercurial 0.9.5 and earlier: Wrap Emacs+Ediff call in a script

Dump the following content into a file in your PATH (don't forget to turn on the execute bit):

 #!/bin/sh

 set -e # bail out quickly on failure

 LOCAL="$1"
 BASE="$2"
 OTHER="$3"

 BACKUP="$LOCAL.orig"

 Restore ()
 {
   cp "$BACKUP" "$LOCAL"
 }

 ExitOK ()
 {
   exit $?
 }

 # Back up our file
 cp "$LOCAL" "$BACKUP"

 # Attempt to do a non-interactive merge
 if which merge > /dev/null 2>&1 ; then
   if merge "$LOCAL" "$BASE" "$OTHER" 2> /dev/null; then
       # success!
       ExitOK
   fi
   Restore
 elif which diff3 > /dev/null 2>&1 ; then
   if diff3 -m "$BACKUP" "$BASE" "$OTHER" > "$LOCAL" ; then
       # success
       ExitOK
   fi
   Restore
 fi

 if emacs -q --no-site-file --eval "(ediff-merge-with-ancestor \"$BACKUP\" \"$OTHER\" \"$BASE\" nil \"$LOCAL\")"
 then
   ExitOK
 fi

 echo "emacs-merge: failed to merge files"
 exit 1

 # End of file

Considering Mercurial is now able to premerge before running the merge tool and even does it by default (see hgrc(5)), I feel that attempts of merge with merge and diff3 are not needed anymore.

0.1. How the script works

This script tries first to automatically merge the files using the RCS merge program or the diff3 program. If the automatic merger fails merging the files because of a conflict or, neither merge nor diff3 are available on the system, then emacs is launched to let the developer resolve the conflicts.

0.2. Enabling the script usage

Don't forget to add an entry in your hgrc file (either ~/.hgrc or the local working copy .hg/hgrc) to point Mercurial at your merge command (let's call it emacs-merge)

[ui]
merge = emacs-merge

All Mercurial versions: using emacsclient

You may want to use 'emacsclient' to reuse an already running Emacs session.

1. With emacs 24 or later

This works on a Linux system under a shell running in emacs. Windows users will have to adjust.

1. In .emacs.d/init_bash.sh include

export EDITOR=emacsclient
export HGMERGE=emacsclient

The first line is not necessary for merges, but will help with, e.g., commit messages.

See the emacs help for the shell command on alternative ways to set the environment variables. If your shell is not bash, you will need to change the name of the startup file.

2. In ~/.hgrc:

[merge-tools]
emacsclient.args = --eval "(ediff-merge-with-ancestor \"$local\" \"$other\" \"$base\" nil \"$outp\
ut\")"

3. Start the emacs server. You can do this by executing start-server once you have started emacs, or you can put (start-server) in your emacs initialization file.

4. In the shell, do an hg merge. This will launch you into an ediff session. Adjust the C buffer until it is OK and then save it and exit the ediff session. The C-x # command doesn't seem to work in its usual way for emacsclient.

2. With emacs 23 or earlier

Until the trunk was patched (on 02 Oct 2010), emacsclient did not relay errors from the server process properly (it always exited with status 0), so hg had no way to know if an error occurred, and would happily commit the merge, possibly without you ever noticing something is amiss.

As a workaround if the fix isn't in the version of emacs you use, you'll need a wrapper script like the one below to supply the error handling.

(save it as e. g. 'emacsclient-merge' and configure it like this)

[ui]
merge = emacsclient-merge

 #!/bin/bash

 if [ $# -lt 1 ]; then
    echo 1>&2 "Usage: $0 local other base output"
    exit 1
 fi

 local=$1
 other=$2
 base=$3
 output=$4

 OUTPUT=`emacsclient --no-wait --eval "(ediff-merge-with-ancestor \"$local\" \"$other\" \"$base\" nil \"$output\")" 2>&1`
 echo $OUTPUT | grep -v "Ediff Control Panel"

 if echo "$OUTPUT" | grep -q '^*ERROR*'; then
    exit 1
 fi

Q (28 Jun, 2011): This currently does not work. Seems that others are having the same problem (https://identi.ca/notice/71698204). The error message is: *ERROR*: The merge buffer file ~/path must not be a directory

A (July, 2013): The issue appears to be with how the arguments are passed to the merge. I found that the following .hgrc stanza fixed the issue:

[ui]
merge = emacsclient-merge
[merge-tools]
emacsclient-merge.args=$local $other $base $output

MergingWithEmacs (last edited 2015-08-18 22:49:33 by mpm)