MergingWithEmacs

Using Emacs as a merger program

[http://www.gnu.org/software/emacs/ Emacs] is bundled with an elisp program called Ediff which purpose is to help developers to visually apply patches. One of the various ediff commands is well suited to three way merging and can be used as a merger program with ["Mercurial"].

Wrapping Emacs+Ediff call in a script

So to use [http://www.gnu.org/software/emacs/ Emacs] as ["Mercurial"] merger program, 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
    
    Rm ()
    {
        if [ -f $BACKUP ]; then 
            rm  -f $BACKUP
        else
            :
        fi
    }

    Restore ()
    {
        cp $BACKUP $LOCAL 
    }

    ExitOK ()
    {
        Rm 
        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

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.

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)