shell编程-case语句中遇到问题

#!/bin/bash
echo "Hit a key, then hit return."
read Keypress
case "$Keypress" in
       [A-Z] ) echo "Uppercase letter";;
       [a-z] ) echo "Lowercase letter";;
       [0-9] ) echo "Digit";;
       * ) echo "Punctuation, whitespace, or other";;
esac

 

运行结果：

[root@localhost shell实验]# ./s10
Hit a key, then hit return.
a                                                     //输入小写字母
Lowercase letter                            //显示小写字母

 

[root@localhost shell实验]# ./s10
Hit a key, then hit return.
b                                                  //输入小写字母
Uppercase letter                         //显示大写字母？？？？？？

  

[root@localhost shell实验]# ./s10
Hit a key, then hit return.
d                                                  //再次验证：输入小写字母
Uppercase letter                           //显示大写字母？？？？？？

[root@localhost shell实验]# ./s10
Hit a key, then hit return.
D
Uppercase letter

 

真是邪门，上面代码到底出了什么问题，但是当输入小写a字母时，他能够正确检测，但是当输入小写字母b、d.....时，就不能显示出小写字母了，咋为事呢？？？？？？？？？？？？？