Poor man's COW

poor-cow.sh

#!/bin/bash

# Exit if something totally fails
set -e

source_directroy="$( readlink -e $1 )"
target_directory="$( readlink -e $2 )"
overlay_directory="$( readlink -e $3 )"

df --output=target | grep "$target_directory" | xargs -L1 umount -f

mount_point="$target_directory/.loopback"

mkdir -p $mount_point
    
rsync \
    --cvs-exclude \
    --verbose \
    --recursive \
    --hard-links \
    --link-dest="$source_directroy/" \
    "$source_directroy/" \
    "$target_directory/"

bindfs -o allow_recursion,noappledouble,noapplexattr,nobrowse,auto_cache,kill_on_unmount "$target_directory/" "$mount_point"
bindfs -o union,allow_recursion,noappledouble,noapplexattr,nobrowse,auto_cache,kill_on_unmount "$overlay_directory" "$target_directory"

    
    

This script has only been tested on OSX Sierra, for some reason mounting a directory on a nested mount point in your home directroy will hang-up bindfs. Make sure to install bindfs, rsync and coreutils before running this. ($ brew install bindfs rsync coreutils)

Usage: $ ./poor-cow.sh ./source-directory/ ./target-directory/ ./overlay-directory/