checkboxes in bash

#!/bin/bash

# Lazy "checkboxes" in bash script
# (for those who dont want to use whiptail)

# My most complex (and cryptic... and useless) pure bash script so far
# answers http://serverfault.com/questions/144939

options=("AAA" "BBB" "CCC" "DDD")

menu() {
    echo "Avaliable options:"
    for i in ${!options[@]}; do
        printf "%3d%s) %s\n" $((i+1)) "${choices[i]:- }" "${options[i]}"
    done; [[ "$msg" ]] && echo "$msg"; :
}

prompt="Check an option (again to uncheck, ENTER when done): "
while menu && read -rp "$prompt" num && [[ "$num" ]]; do
    [[ "$num" != *[![:digit:]]* ]] && (( num > 0 && num <= ${#options[@]} )) || {
        msg="Invalid option: $num"; continue
    }
    ((num--)); msg="${options[num]} was ${choices[num]:+un}checked"
    [[ "${choices[num]}" ]] && choices[num]="" || choices[num]="+"
done

printf "You selected"; msg=" nothing"
for i in ${!options[@]}; do
    [[ "${choices[i]}" ]] && { printf " %s" "${options[i]}"; msg=""; }
done; echo "$msg"