N-digit guess number game

guess_number.sh

#!/bin/bash
# Guess number~
# 2013/11/20 Jason YiZhang Chen

ANSWER=1288
LENGTH=$(expr length $ANSWER)

declare -i a

until [ "$a" == $LENGTH ]; do 

  a=0 && b=0 && list=""
  
  while [ 1 ]; do
    # Read user input
    read -p "Hello what's your number?" num
    
    if [[ $num =~ "^[0-9]{${LENGTH}}$" ]]; then
      break
    else
      echo "Please entering ${LENGTH}-digit number!"
    fi
  done
  
  # compare to the answer
  for (( i = 0; i < LENGTH; i++ )); do 
    if [ ${num:$i:1} == ${ANSWER:i:1} ]; then
      a=$((a+1))
    else
      list="$list $i"
    fi   
  done

  for i in $list; do
    for j in $list; do
      if [ ${num:$i:1} == ${ANSWER:j:1} ]; then
        b=$((b+1))
        break
      fi
    done
  done

  # Show hit
  echo "${a}A${b}B"
 
done

echo "Right!"