#!/bin/bash

target="s3://your-AWS-bucket-here/"
file=""

# if you're looking for a specific file extension
# you can do something like this instead
# while [[ "$target" != *.tgz ]]; do

# while there is a slash at the end of the path
# i.e. it isn't a file
while [[ "$target" == */ ]]; do
        list=$(aws s3 ls "$target")

        # create an array of options using the
        # last "word" in each line of the
        # output from aws
        options=()
        while IFS= read; do
                options+=( $(echo "$REPLY" | awk '{print $(NF)}') )
        done <<< "$list"

        # select an option and append it
        select option in "${options[@]}"; do
                [ -z "$option" ] && exit 1

                target="$target$option"
                file="$option"
                break
        done
done

aws s3 cp $target $file

# do something with $file