validpan

#!/bin/bash
# check if a given credit card number (var1) passes mod10 checksum

pan=$1
panlen=${#pan}

for i in $(seq $((panlen - 1)) -1 0); do
  digit=${pan:$i:1}
  if [ $(((panlen-i) % 2)) -eq 0 ]; then
     #even
     ((digit*=2))
     [ ${#digit} -eq 2 ] && digit=$((${digit:0:1}+${digit:1:1}))
  fi
  ((sum+=digit))
done

[ $((sum % 10)) -eq 0 ] || echo "luhn Mod10 check Fails" && exit 1
echo "Luhn Mod10 Check Passes"