判斷密碼是否正確

#!/bin/bash
str="yhg123"
for ((i=1;i<=3;i++))
do
read -p "請輸入密碼: " yhg
if [ ${nb}a = {$str}a ];then
    echo "密碼輸入正確"
    break
else
    echo "密碼輸入錯誤。第i次機會"
fi
done