Wrapper around unionfs-fuse that uses a file-based branches configuration.

modfs.sh

#! /bin/bash

# This program is free software. It comes without any warranty, to
# the extent permitted by applicable law. You can redistribute it
# and/or modify it under the terms of the Do What The Fuck You Want
# To Public License, Version 2, as published by Sam Hocevar. See
# http://sam.zoy.org/wtfpl/COPYING for more details.

: ${UNIONFS:=unionfs}
readonly MODFS_BIN="$0"

modfs.usage() {
  cat << EOM
Usage: $MODFS_BIN [options] branches.d mountpoint

options:
  Any valid $UNIONFS options.

branches.d:
  Directory containing the configuration files describing the branches to mount.
  Configuration filenames must end with the .conf extension. Adding a new branch in a configuration files is done using the following syntax:
    MODFS_BRANCHES+=("/path/to/branch=mode")
  mode can be either RO (read-only) or RW (read-write).

mountpoint:
  Destination mountpoint.

EOM
}

modfs.main() {
  local -a options=()
  local branchesDir
  local mountpoint
  local -a MODFS_BRANCHES=()
  local i
  local branches=""
  
  # Read arguments
  (( $# < 2 )) && modfs.usage && return 1
  
  until (( $# == 2 )); do
    options+=("$1")
    shift
  done
  branchesDir="$1"
  mountpoint="$2"
  
  # Configuration
  for i in "$branchesDir"/*.conf; do
    [[ -f "$i" ]] && . "$i"
  done
  
  # Mount
  for i in "${MODFS_BRANCHES[@]}"; do
    [[ -z "$branches" ]] || branches+=":"
    branches+="$i"
  done
  
  $UNIONFS "${options[@]}" "$branches" "$mountpoint"
}

modfs.main "$@"

Requires unionfs-fuse