AWS Lambda String interpolation not working - aws-lambda

Question

So i was trying to over ride the lambda environment, i used string interpolation, but there is a small thing i am unable to understand, so basically following is my Lambda, if you see the function name it has a place holder for Environment. But when i deploy it like this
aws cloudformation deploy --template-file build/output.yaml --stack-name test-stack --capabilities CAPABILITY_IAM --parameter-overrides Environment=de
v
The placeholder doesn't update the following code
Parameters:
Environment:
Type: String
Resources:
HelloWorldFunction:
Type: AWS::Serverless::Function # More info about Function Resource: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#awsserverlessfunction
Properties:
CodeUri: src
Handler: index.lambda_handler
Runtime: python3.6
FunctionName: HelloLambda-${Environment}
MemorySize: 128
Timeout: 30
Policies:
- AWSLambdaBasicExecutionRole
But if i do same like this
Parameters:
Environment:
Type: String
Resources:
HelloWorldFunction:
Type: AWS::Serverless::Function # More info about Function Resource: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#awsserverlessfunction
Properties:
CodeUri: src
Handler: index.lambda_handler
Runtime: python3.6
FunctionName: !Sub HelloLambda-${Environment}
MemorySize: 128
Timeout: 30
Policies:
- AWSLambdaBasicExecutionRole
The above execution works, so whats the difference between FunctionName: !Sub HelloLambda-${Environment} and FunctionName: HelloLambda-${Environment}

Answer 1

By having !Sub in the front, you are invoking Sub-function with cloud formation. It takes the template parameters and applies the replacement where needed.
More documentation at,
https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference-sub.html
!Sub
HelloLambda-${Environment}
takes the Environment variable and replace with the value specified, thus you get different functions based on the environment variable.