LUHN Check in Bash

isLUHNValid.sh

#!/usr/bin/env bash

. library.sh

pan="$1"

isLUHNValid "$pan"

library.sh

isLUHNValid() {
  local pan="$1"
  local panlen="${#pan}"
  local sum=0

  for ((i = panlen - 1; i >= 0; i--)); do
    local digit="${pan:$i:1}"
    if (((panlen-i) % 2 == 0)); then
       #even
       ((digit*=2))
       ((${#digit} == 2)) && digit=$((${digit:0:1}+${digit:1:1}))
    fi
    ((sum+=digit))
  done

  ((sum % 10 == 0))
}