Padding and show only the last N characters of a masked/password/secret variable's value

Question

I want to prompt a user to enter a secret value (for ex: password variable, credit card, some key/token) and would like to see if there's a way to show the last N (lets say 4 characters) of that variable's value.

Ex: Enter your credit card# : 1234567898765432

I can use wrappers for ex stty -echo/echo to hide what user will enter or use read -s variable and it'll hide the values but what I'd like is to show only last N (for ex: 4) characters of this variable i.e. if I say $ echo $mySecretVariable (not sure if I really have to write a function) then it will echo the value as: ************5432 or your token key value last 4 ends with *******************C5fx something like that where all the characters will be padded with a * and only the last N will be shown.

Answer 1

You can use this bash function for masking all but last 4 characters:

mask() {
   local r="${1?needs an argument}"
   if ((${#r} > 4)); then
      r="${r:0: -4}"
      echo "${r//?/*}${1: -4}"
   else
      echo "$r"
   fi
}

Then use it as:

mask 1234567898765432
************5432

mask 123
123

mask 123456
**3456