How to escape "${}" in cloudformations "Fn::Sub"

Question

I want this resource to work with the !Sub (or Fn::Sub) intrinsic function

Resource: !Sub 'arn:aws:iam::${AWS::AccountId}:user/${aws:username}'

The aws:username is a pollicy variable that mustn't be replaced.

One solution would be to use Fn::Join instead and write a bit more boilerplate code.

Better: Can you escape the ${aws:username} so that !Sub will work here? Unfortunately, the documentation does not mention anything about escaping.

question from:https://stackoverflow.com/questions/44458304/how-to-escape-in-cloudformations-fnsub

Answer 1

You actually can escape $ characters with ${!}.

So your resource would look like this:

Resource: !Sub 'arn:aws:iam::${AWS::AccountId}:user/${!aws:username}'

It is mentioned in the docs under the string parameter section.

To write a dollar sign and curly braces (${}) literally, add an exclamation point (!) after the open curly brace, such as ${!Literal}. AWS CloudFormation resolves this text as ${Literal}.