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"

Answer 2

You can do this with a CloudFormation macro.

Parameters:
  InputString:
    Default: "This is a test input string"
    Type: String
Resources:
  S3Bucket:
    Type: "AWS::S3::Bucket"
    Properties:
      Tags:
        - Key: Upper
          Value:
            'Fn::Transform':
             - Name: 'String'
               Parameters:
                 InputString: !Ref InputString
                 Operation: Upper

https://github.com/awslabs/aws-cloudformation-templates/tree/master/aws/services/CloudFormation/MacrosExamples/StringFunctions

Answer 3

Mr. Young is correct, that is the syntax you need to use to invoke the macros.

HOWEVER, the key factor which both they and the documentation failed to mention is that in order to invoke the transform macros that you need to deploy this stack into your accounts BEFORE you can use the functions listed in the ReadMe.

https://github.com/awslabs/aws-cloudformation-templates/blob/master/aws/services/CloudFormation/MacrosExamples/StringFunctions/string.yaml

I think the docs could be clarified in this regard, I'll see if I can PR a clarification

Answer 4

I got the answer of this question. For this I have used Mappings JSON in which I have added values like If Selected value is DEV then use dev, If QA then qa like this, and used below JSON which used Fn:FindInMap

[ { "Fn::FindInMap": [ "Enviroment", "PlatformName", { "Ref": "selectedEnv" } ] }, "clientname" ]

Below is the Mappings JSON:

"Mappings" : { "Enviroment" : { "PlatformName" : { "DEV" : "dev", "QA" : "qa", "UAT" : "uat", "PROD" : "prod" } } }