What is the difference between Parameters and Outputs?

Parameters accept input values when you deploy the stack, letting you reuse the same template for different environments. Outputs export values from the stack that other stacks or scripts can reference.

What does !Ref do in CloudFormation?

!Ref is a CloudFormation intrinsic function that returns the value of a Parameter or the physical ID of a Resource, letting you pass values between resource definitions.

Can I use JSON instead of YAML for CloudFormation?

Yes. CloudFormation accepts both formats. YAML is preferred because it supports comments and is easier to read, but both produce identical stacks.

Validate a CloudFormation Template

AWS CloudFormation templates define cloud infrastructure as code and must be valid YAML before deployment. This example provisions an S3 bucket with versioning and a lifecycle policy. CloudFormation returns cryptic errors for invalid YAML and does not show which line failed. Validate and format the template before uploading to the AWS console or running aws cloudformation deploy.

Example

AWSTemplateFormatVersion: '2010-09-09'
Description: S3 bucket with versioning

Parameters:
  BucketName:
    Type: String
    Default: my-app-bucket

Resources:
  AppBucket:
    Type: AWS::S3::Bucket
    Properties:
      BucketName: !Ref BucketName
      VersioningConfiguration:
        Status: Enabled
      LifecycleConfiguration:
        Rules:
          - Id: DeleteOldVersions
            Status: Enabled
            NoncurrentVersionExpirationInDays: 30

Outputs:
  BucketArn:
    Value: !GetAtt AppBucket.Arn

[ open in YAML Validator → ]

FAQ

What is the difference between Parameters and Outputs?: Parameters accept input values when you deploy the stack, letting you reuse the same template for different environments. Outputs export values from the stack that other stacks or scripts can reference.
What does !Ref do in CloudFormation?: !Ref is a CloudFormation intrinsic function that returns the value of a Parameter or the physical ID of a Resource, letting you pass values between resource definitions.
Can I use JSON instead of YAML for CloudFormation?: Yes. CloudFormation accepts both formats. YAML is preferred because it supports comments and is easier to read, but both produce identical stacks.

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