defensive-bashisms

check for bash version 4+  (POSIX-compliant)  :
  [ -z "${BASH_VERSINFO}" ] || [ -z "${BASH_VERSINFO[0]}" ] || [ ${BASH_VERSINFO[0]} -lt 4 ]

path to script even if symlinked:
  SCRIPT_DIR="$( cd -P "$( dirname "$(readlink -f "${BASH_SOURCE[0]}")" )" >/dev/null 2>&1 && pwd )"

recursive-healing-chmod

#!/usr/bin/env bash
#
# Test bed for a recursive chmod one-liner
#
# 1. Generate a set of test directories and files using improper permissions
# 2. Run a recursive chmod to heal
# 3. Show before, fixes and after for inspection
#
# Cleaning up by diligently running `sudo rm -rf ./data` is left as an exercise for the reader
#

set -eu

declare -a modes=("400" "422" "600" "644" "755" "777")

# `apt install lsd`, see https://github.com/lsd-rs/lsd
tree() { lsd -l -g --tree "$@"; }


mkdir data
cd data

for dir_mode in "${modes[@]}"; do {
    dir="d${dir_mode}"
    mkdir "${dir}"

    cd "${dir}"
    for file_mode in "${modes[@]}"; do {
        file=f"${file_mode}"
        echo "${file_mode}" > "${file}"
        chmod "${file_mode}" "${file}"
    } done
    cd ..

    chmod "${dir_mode}" "${dir}"
} done

cd ..

# Before - fails to look inside directories not having the executable bit set
tree -a || true

# chmod
chmod -vR a=r-wx,u=wr,a+X data/

# After
tree -a || true