How do I setup a git driver to ignore a folder on merge?

I have the master branch, where all the main code resides, and the design branch, where the layout of the rails application is built by the design team. They have added a folder called "photoshop" to the public folder to keep their sources for the images also under version control. But I don't want this folder to be copied on merge to the master branch because, well, it is not needed.

Apparently, the way to do this is through a merge driver. So, I've created the "ignore" driver:

[merge "ignore"]
name = always ignore during merge
driver = ignore.sh %0 %A %B

And created the ignore.sh file on my $PATH:

exit 0

I've created the .gitattributes file inside public/, because the photoshop folder should be ignored in whole and it is going to appear under public/:

photoshop  merge=ignore
photoshop/ merge=ignore
photoshop/* merge=ignore
photoshop/**/* merge=ignore

As you can see, I've tried several different patterns to ignoring the whole folder, but it does not work. I believe this is because there is no folder on the master branch, so there's no conflict and so git doesn't use the ignore driver. Is there a way to achieve this without having to create a public/photoshop folder on master?

Solution

As suggested by my other answer, here goes the extended, generalized, industrial-strength version of the solution.

(yes, I was bored at home and had nothing else better to do :P)

This script will add a new, detached commit based on your local design branch, so it won't affect neither the design repository or your design branch. The commit will have all desired files removed. Then it performs the merge.

For those too lazy to read the full code, the "core" of these steps can be simplified as:

original=$(gitbranch HEAD)    # current branch name, or sha1 if not in a branch
branchsha=$(gitsha "$branch") # sha1 of a ref, to force detached commit

git checkout "$branchsha"   &&
git rm -rf "${files[@]}"    &&
git commit -m "$msgcommit"  &&
newsha=$(gitsha HEAD)       &&
git checkout "$original"    &&
git merge -m "$msgmerge" "${mergeopts[@]}" "$newsha"

And here is the full script:

git-strip-merge

#!/bin/bash
#
# git-strip-merge - a git-merge that delete files on branch before merging
#
#    Copyright (C) 2012 Rodrigo Silva (MestreLion) <linux@rodrigosilva.com>
#
#    This program is free software: you can redistribute it and/or modify
#    it under the terms of the GNU General Public License as published by
#    the Free Software Foundation, either version 3 of the License, or
#    (at your option) any later version.
#
#    This program is distributed in the hope that it will be useful,
#    but WITHOUT ANY WARRANTY; without even the implied warranty of
#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#    GNU General Public License for more details.
#
#    You should have received a copy of the GNU General Public License
#    along with this program. If not see <http://www.gnu.org/licenses/gpl.html>
#
# Answer for "How to setup a git driver to ignore a folder on merge?"
# See http://stackoverflow.com/questions/3111515

#Defaults:
msgcommit="remove files from '<branch>' before merge"
msgmerge="Merge stripped branch '<branch>'"
verbose=0
quiet=(--quiet)

usage() {
    cat <<- USAGE
    Usage: $myname [git-merge options] [-M <commitmsg>] <branch> FILE...
    USAGE
    if [[ "$1" ]] ; then
        cat >&2 <<- USAGE
        Try '$myname --help' for more information.
        USAGE
        exit 1
    fi
    cat <<-USAGE

    "git-merge that delete files on "foreign" <branch> before merging

    Useful for ignoring a folder in <branch> before merging it with
    current branch. Works by deleting FILE(S) in a detached commit based
    on <branch>, and then performing the merge of this new commit in the
    current branch. Note that <branch> is not changed by this procedure.
    Also note that <branch> may actually be any reference, like a tag,
    or a remote branch, or even a commit SHA.

    For more information, see <http://stackoverflow.com/questions/3111515>

    Options:
      -h, --help
         show this page.

      -v, --verbose
         do not use -q to supress normal output of internal steps from git
         checkout, rm, commit. By default, only git merge output is shown.
         Errors, however, are never supressed

      -M <message>, --msgcommit=<message>
         message for the removal commit in <branch>. Not to be confused
         with the message of the merge commit, which is set by -m. Default
         message is: "$msgcommit"

      -m <message>, --message=<message>
         message for the merge commit. Since we are not merging <branch>
         directly, but rather a detached commit based on it, we forge a
         message similar to git's default for a branch merge. Otherwise
         git would use in message the full and ugly SHA1 of our commit.
         Default message is: "$msgmerge"

      For both commit messages, the token "<branch>" is replaced for the
      actual <branch> name.

    Additional options are passed unchecked to git merge.

    All options must precede <branch> and FILE(s), except -h and --help
    that may appear anywhere on the command line.

    Example:
      $myname design "photoshop/*"

    Copyright (C) 2012 Rodrigo Silva (MestreLion) <linux@rodrigosilva.com>
    License: GPLv3 or later. See <http://www.gnu.org/licenses/gpl.html>"
    USAGE
    exit 0
}

# Helper functions
myname="${0##*/}"
argerr()  { printf "%s: %s\n" "${0##*/}" "${1:-error}" >&2 ; usage 1 ; }
invalid() { argerr "invalid option: $1" ; }
missing() { argerr "missing ${2:+$2 }operand${1:+ from $1}." ; }

# Option handling
files=()
mergeopts=()
for arg in "$@"; do case "$arg" in -h|--help) usage ;; esac; done
while (( $# )); do
    case "$1" in
    -v|--verbose  ) verbose=1            ;;
    -M            ) shift ; msgcommit=$1 ;;
    -m            ) shift ; msgmerge=$1  ;;
    --msgcommit=* ) msgcommit=${1#*=}    ;;
    --message=*   ) msgmerge=${1#*=}     ;;
    -*            ) mergeopts+=( "$1" )  ;;
    *             ) branch="$1"
                    shift ; break        ;;
    esac
    shift
done
files+=( "$@" )

# Argument handling

msgcommit=${msgcommit//<branch>/$branch}
msgmerge=${msgmerge//<branch>/$branch}

[[ "$msgcommit" ]]  || missing "msgcommit" "MSG"
[[ "$branch"   ]]   || missing ""          "<branch>"
(( ${#files[@]} ))  || missing ""          "FILE"

((verbose)) && quiet=()

# Here the fun begins...
gitsha()    { git rev-parse "$1" ; }
gitbranch() {
    git symbolic-ref "$1" 2> /dev/null | sed 's/refs\/heads\///' ||
    gitsha "$1"
}

original=$(gitbranch HEAD)
branchsha=$(gitsha "$branch")

trap 'git checkout --quiet "$original"' EXIT

git checkout "$branchsha"  "${quiet[@]}" &&
git rm -rf "${files[@]}"   "${quiet[@]}" &&
git commit -m "$msgcommit" "${quiet[@]}" &&
newsha=$(gitsha HEAD)                    &&
git checkout "$original"   "${quiet[@]}" &&
git merge -m "$msgmerge" "${mergeopts[@]}" "$newsha"

Before merge:

enter image description here

After merge:

enter image description here

Note that "design" branch tip wasn't affected at all, even being a local branch, thanks to the detached commit trick. Other than that, both commits (the removal and the merge) are regular commits, with suitable commit messages and parents. And "master" branch is clean of any undesired files.