请教SHELL中函数调用如何不成功?

不知为什么Prompt函数调用总是显示错误:
./l.sh: line 4: Prompt: command not found
代码如下:
#!/bin/sh
username=""
password=""
Prompt
read username
while [ $username != "Q" ]
do
if [ $username != "" ]
then
  echo -n "password for $username:"
  stty -echo
  read password
  stty echo
  case $username in
  root)
        echo "hello root.your password is $password, "
        ;;
  *)
        echo "hello,$username,your password is $password"
        ;;
  esac
  echo "welcome to the system"
fi
Prompt
read username
done
Prompt ( )
{
echo "Login Emulation ('Q' to quit)"
echo -n "login:"
}　　　　　　

函数要先定义再引用　　　　　　

哦,改成如下可正常执行,谢谢啦!呵呵...
#!/bin/sh
username=""
password=""
Prompt ( )
{
echo "Login Emulation ('Q' to quit)"
echo -n "login:"
}
Prompt
read username
while [ $username != "Q" ]
do
if [ $username != "" ]
then
echo -n "password for $username:"
stty -echo
read password
stty echo
case $username in
root)
echo "hello root.your password is $password, "
;;
*)
echo "hello,$username,your password is $password"
;;
esac
echo "welcome to the system"
fi
Prompt
read username
done　　　　　　

哦,改成如下可正常执行,谢谢啦!呵呵...
#!/bin/sh
username=""
password=""
Prompt ( )
{
echo "Login Emulation ('Q' to quit)"
echo -n "login:"
}
Prompt
read username
while [ $username != "Q" ]
do
if [ $username != "" ]
then
echo -n "password for $username:"
stty -echo
read password
stty echo
case $username in
root)
echo "hello root.your password is $password, "
;;
*)
echo "hello,$username,your password is $password"
;;
esac
echo "welcome to the system"
fi
Prompt
read username
done