while应用问题

#!/bin/bash
clear
declare raining=10
while [  $raining -ge 1 ]
     do
       echo "Still raining"
       let $raining=" $raining - 1"
done
出错如下:

./while: line 7: let: 10= 10 - 1: attempted assignment to non-variable (error token is "= 10 - 1")
Still raining
./while: line 7: let: 10= 10 - 1: attempted assignment to non-variable (error token is "= 10 - 1")
Still raining
./while: line 7: let: 10= 10 - 1: attempted assignment to non-variable (error token is "= 10 - 1")
Still raining
./while: line 7: let: 10= 10 - 1: attempted assignment to non-variable (error token is "= 10 - 1")
Still raining
./while: line 7: let: 10= 10 - 1: attempted assignment to non-variable (error token is "= 10 - 1")
Still raining
./while: line 7: let: 10= 10 - 1: attempted assignment to non-variable (error token is "= 10 - 1")
Still raining

　　　　　　

哈哈,我已经知道为什么了!!!!