判读密码是否争正确

#!/bin/bash
str="ccc666"
for ((i=1;i<=3;i++))
do
read -p "请输入密码: " nb
if [ ${nb}a = {$str}a ];then
    echo "密码输入正确"
    break
else
    echo "密码输入错误。第i次机会"
fi
done