jsywulak

custom_function_arn=$(aws cloudformation describe-stacks --stack-name $create_stack --query Stacks[*].Outputs[?OutputKey==\'CustomFunctionArn\'].OutputValue --output text) 
aws cloudformation create-stack \ 
  ... 
  --parameters \
    ParameterKey="CustomFunctionArn",ParameterValue="$custom_function_arn" 

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Outputs:
  CustomFunctionArn:
    Description: The arn of the custom resource function.
    Value: !GetAtt 'CustomFunction.Arn'

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Resources:
  CustomFunction:
    Type: AWS::Lambda::Function
    Properties
      ...

CustomResource:
    Type: Custom::CustomResource
    Properties:
      ServiceToken: !GetAtt 'CustomFunction.Arn'

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 LambdaExecutionRole:
  Type: AWS::IAM::Role
  Properties:
    Policies:
      - PolicyName: root
        PolicyDocument:
          Version: '2012-10-17'
          Statement:
          ... 

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Resources:
  CustomFunction:
    Type: AWS::Lambda::Function
    Properties:
      Code:
        S3Bucket: !Ref 'S3Bucket'
        S3Key: !Ref 'S3Key'
        ...

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Parameters: 
    S3Bucket: 
    Description: The name of the bucket that contains your packaged source 
    Type: String 
  S3Key: 
    Description: The name of the ZIP package 
    Type: String 
    Default: custom-resource.zip 

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zip /tmp/custom-resource.zip customresource.js 
aws s3 cp /tmp/custom-resource.zip s3://your-custom-resource-code-bucket/custom-resource.zip