AWS Cloudformation- How to do string Uppercase or lowercase in json/yaml template

Question

I am working on AWS CloudFormation and I created one template in which I asked user to select Environment.

On the basis of selected value I created the resources.

Answer 1

The accepted answer suggested using a CloudFormation macro, and another answer suggesting using FindInMap.

FindInMap is not very useful here, since it would only work with hardcoded values.

The macro suggestion will work, but requires quite a bit of setup (declare the macro in a separate stack, ensure your deployer role has permission to invoke the Lambda, and your CloudFormation stack is deployed with CAPABILITY_AUTO_EXPAND, and so on).

Declaring a custom resource within the template will work and IMO involves less work than relying on the macro. Here's a CFN snippet, adapting the S3 bucket resource you were asking about, demonstrating the use of a custom resource which will lowercase an arbitrary S3 bucket name:

  # Custom resource to transform input to lowercase.                                             
  LowerCaseLambda:
    Type: 'AWS::Lambda::Function'
    Properties:
      Description: Returns the lowercase version of a string
      MemorySize: 256
      Runtime: python3.8
      Handler: index.lambda_handler
      Role: !GetAtt LowerCaseLambdaRole.Arn
      Timeout: 30
      Code:
        ZipFile: |
          import cfnresponse

          def lambda_handler(event, context):                                                    
              output = event['ResourceProperties'].get('InputString', '').lower()                
              responseData = {'OutputString': output}                                            
              cfnresponse.send(event, context, cfnresponse.SUCCESS, responseData)                

  LowerCaseLambdaRole:
    Type: AWS::IAM::Role
    Properties:
      AssumeRolePolicyDocument:
        Version: "2012-10-17"
        Statement:
          - Effect: "Allow"
            Principal:
              Service:
                - "lambda.amazonaws.com"
            Action:
              - "sts:AssumeRole"
      Policies:
        - PolicyName: "lambda-write-logs"
          PolicyDocument:
            Version: "2012-10-17"
            Statement:
              - Effect: "Allow"
                Action:
                  - "logs:CreateLogGroup"
                  - "logs:CreateLogStream"
                  - "logs:PutLogEvents"
                Resource: "arn:aws:logs:*:*"


  S3BucketName:
    Type: Custom::Lowercase
    Properties:
      ServiceToken: !GetAtt LowerCaseLambda.Arn
      InputString: !Ref selectedEnv

  S3Bucket:
    BucketName: !Join
      - ''
      - - !GetAtt S3BucketName.OutputString
        - "-deployment.companyname.com"