如何通过cloudformation脚本将字符串传递到UserData中？

Parameters:
  UserDataString:
    Type: String
    Default: "default value"
    Description: The string to be passed to UserData

Resources:
  MyInstance:
    Type: AWS::EC2::Instance
    Properties:
      UserData:
        Fn::Base64: !Sub |
          #!/bin/bash
          echo ${UserDataString} > /path/to/output/file.txt

aws cloudformation create-stack --stack-name MyStack --template-body file://template.yaml --parameters ParameterKey=UserDataString,ParameterValue="my string value"