Scripting a menu with selection indicator in bash

Ok - I will do my best to help you (so you can learn something from this bash script) with comments and little bit of explanation to current code:

#!/bin/bash

# -- ASSIGNMENT OF ARRAY OF VALUES TO OPTIONS VARIABLE --
# This line below is basically array of options/strings
# assigned to variable "options" used later on.
# Try to copy paste this whole line into Linux Bash (shell)
# and then try to execute following command "echo ${options[0]}"
# without quotes. Then you can try also "echo ${options[1]}", etc.
options=("advanced-custom-fields-pro" "analytics360" "better-wp-security" "debug-bar" "debug-bar-console" "debug-bar-cron" "email-address-encoder" "force-regenerate-thumbnails" "gravityforms" "gravityforms-multilingual" "hansel-gretel" "log-deprecated-notices" "log-deprecated-notices-extender" "maintenance" "Maintenance-pro" "mshare" "post-duplicator" "responsive-lightbox" "sitepress-multilingual-cms" "what-the-file" "wordpress-seo" "wp-realtime-sitemap" "Exit")

# -- MENU FUNCTION DEFINITION --
# This "menu" function below is nothing more than printing
# of all array elements to your screen. You can try to type
# it manually to your shell. You will certainly find similarities
# in for loop and printf functions from HarvardX CS50X course :-)
# Or just copy paste whole "menu" function to your shell and then
# invoke menu function by typing "menu" into bash shell without quotes.
# Little bit description to "echo -e". Special characters
# "\033[1m...\033[0m" just says change font color between these two
# special values of "echo -e" argument to white".
# Just try to execute:  echo -e "\033[1mPlease select the plugin you want to install :\033[0m"
# and compare to:  echo -e "Please select the plugin you want to install :"
# Basically if you are not using this special formatting it is ok to
# use just: echo "Please select the plugin you want to install"
menu() {
    echo -e "\033[1mPlease select the plugin you want to install :\033[0m"
    for i in ${!options[@]}; do 
        printf "%3d%s) %s\n" $((i+1)) "${choices[i]:- }" "${options[i]}"
    done
    [[ "$msg" ]] && echo "$msg"; :
}

# -- CONDITION OF THE WHILE LOOP --
# This is little bit more complicated but first line only
# assign string to prompt variable named "prompt" - used later on.
# "While" line says (correct me if I am wrong) - do it again
# and again an again till return code of "menu" invoked function
# specified above and return code of "read -rp" program are "0" (ok)
# and also following [[ "$num" ]] tests if "num" variable has been
# assigned with a value - if it was it returns "0" (ok) if it wasn't
# it returns "1" (error) what will cause while loop to repeat.
# -- BODY OF THE WHILE LOOP --
# Body of the while loop is also bit complicated. First line is
# regular expression which checks if "num" variable contains only number
# AND second line checks if "num" is larger than 0 and less than or
# equal number of array elements from array "options". If
# these conditions are not met OR operation ensure that "msg" variable
# will contain error statement "Invalid option, etc.". "Continue" says
# to not interrupt while loop and continue execution of other statements
# from this while loop. Rest of this while body is about checking 
# and un-checking of choices. If you choose some option form menu
# value "+" is assigned to variable "choices[num]". If "choices[num]"
# contains value then "msg" contains "checked" AND choices[num]="+"    
# otherwise "unchecked" AND choices[num]="".
# You can try to execute "echo ${#options[@]}" whiteout quotes.
# You will see it holds number of array elements.
prompt="Enter the digit representing the plugin you need : "
while menu && read -rp "$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]="+"
done

# -- FOR LOOP --
# Just prints all checked options to the screen.
# Evaluation which one are printed and which are not is done by
# [[ "${choices[i]}" ]] which returns 0 if it contains value
# and returns 1 if it doesn't contain value.
# TEST NUMBER 1
# You can check return value e.g.:
# execute following: test=
# execute following: [[ $test ]]
# execute following: echo $?
# displayed is return value - hopefully 1 :)
# TEST NUMBER 2
# execute following: test=3
# execute following: [[ $test ]]
# execute following: echo $?
# displayed is return value - hopefully 0 :)
printf "You selected"; msg=" nothing"
for i in ${!options[@]}; do 
    [[ "${choices[i]}" ]] && { printf " %s" "${options[i]}"; msg=""; }
done
echo "$msg"