#!/bin/bash

if [[ "$1" && "$2" ]]; then
    readonly USER1="$1" USER2="$2"
else
    echo $"Two users should be specified by the 1st and the 2nd arguments." > /dev/stderr
    exit 1
fi

set -ex # Exit on failures and echo what is done.

# Clean up:
rmdir rw-dir ||:

# Show perms for the working dir:
ls -ld .

# Prepare:
su -c 'mkdir -p rw-dir' "$USER1"
su -c 'chmod a+w rw-dir' "$USER1"
ls -ld rw-dir

# Try to modify:
su -c 'chmod a-w rw-dir' "$USER2"
ls -ld rw-dir