This one actually works. HT https://github.com/gma/bundler-exec/blob/master/bundler-exec.sh

Auto-Bundle-Exec

bundler-installed()
{
    which bundle > /dev/null 2>&1
}

within-bundled-project()
{
    local dir="$(pwd)"
    while [ "$(dirname $dir)" != "/" ]; do
        [ -f "$dir/Gemfile" ] && return
        dir="$(dirname $dir)"
    done
    false
}

run-with-bundler()
{
    if bundler-installed && within-bundled-project; then
        bundle exec "$@"
    else
        "$@"
    fi
}

## Main program

BUNDLED_COMMANDS="${BUNDLED_COMMANDS:-
berks
cap
capify
chefspec
chef-apply
chef-client
chef-shell
chef-solo
cucumber
foodcritic
foreman
guard
haml
html2haml
jasmine
kitchen
knife
pry
rackup
rake
rake2thor
rspec
ruby
sass
sass-convert
serve
shotgun
spec
spork
strainer
thin
thor
tilt
tt
turn
tux
unicorn
unicorn_rails
}"

for CMD in $BUNDLED_COMMANDS; do
    if [[ $CMD != "bundle" && $CMD != "gem" ]]; then
        alias $CMD="run-with-bundler $CMD"
    fi
done