Using Ref inside Fn::Sub in Cloudformation

Question

I am trying to use fn::sub with a Ref inside it.

I have a string "Comment xyz ${NAME}". This string comes as a parameter to the stack. Say the parameter name is "test".

I would now like to replace the ${NAME} in the string using fn::sub function inside the Cloudformation script.

fn::sub:[{"Ref":"test"},{"NAME":"balaji"}]

Expected output is "Comment xyz balaji".

However, this is the error I am getting:

Template validation error: Template error: One or more Fn::Sub intrinsic functions don't specify expected arguments. Specify a string as first argument, and an optional second argument to specify a mapping of values to replace in the string

Let me know what needs to be fixed here.

Answer 1

According to the docs, the first parameter in Fn::Sub must be:

A string with variables that AWS CloudFormation substitutes with their associated values at runtime. Write variables as ${MyVarName}. Variables can be template parameter names, resource logical IDs, resource attributes, or a variable in a key-value map. If you specify only template parameter names, resource logical IDs, and resource attributes, don't specify a key-value map.

(Emphasis added.)

Thus, it is (at the time of writing) impossible to do this.

If you had a parameter:

"test": {
  "Type": "String",
  "Default": "Comment xyz ${NAME}"
}

And then a substitution:

"Fn::Sub": ["${test}", {"NAME": "balaji"}]

The substitution would yield the literal string Comment xyz ${NAME}.