How to refer SSMParameter value in AWS::SSM::Association?

Question

Hi there, I am stuck with the following problem:-
I am creating an SSMParameter like this:-

```
CustomParameter:
    Type: 'AWS::SSM::Parameter'
    Properties:
      Name:
        Fn::Sub: '/${AWS::StackName}/name'
      Type: String
      Value: !Ref Name
```
Now, I want to get the value of this parameter in my **AWS::SSM::Association** resource:-

```
RunCommandAssociation1:
    Type: 'AWS::SSM::Association'
    Properties:
      Name: 'AWS-RunShellScript'
      Targets:
        - Key: InstanceIds
          Values:
            - !Ref Instance1
      Parameters:
        commands:
          - availability_zone=$(curl -s http://169.254.169.254/latest/meta-data/placement/availability-zone)
          - region=${availability_zone::-1}
          - name=$(aws ssm get-parameter --name /${AWS::StackName}/name --query 'Parameter.Value' --output text --region "$region")
```
I believe we cannot directly reference **${AWS::StackName}** in above commands.
Can anyone help me how can I get this work done. I don't want to use ec2 userdata here.
I only need the solution using **'AWS::SSM::Association'**.

Thanks in advance!

Answer

Hello Confidential,

To reference the SSM Parameter value with the stack name in your AWS CloudFormation template, you can use the `Fn::Sub` function to create the association document dynamically. Here's how you can achieve this:

```yaml
Resources:
  CustomParameter:
    Type: 'AWS::SSM::Parameter'
    Properties:
      Name:
        Fn::Sub: '/${AWS::StackName}/name'
      Type: String
      Value: !Ref Name

RunCommandAssociation1:
    Type: 'AWS::SSM::Association'
    Properties:
      Name: 'AWS-RunShellScript'
      Targets:
        - Key: InstanceIds
          Values:
            - !Ref Instance1
      Parameters:
        commands:
          - Fn::Sub:
              - availability_zone=$(curl -s http://169.254.169.254/latest/meta-data/placement/availability-zone)
                region=${availability_zone::-1}
                name=$(aws ssm get-parameter --name /${StackName}/name --query 'Parameter.Value' --output text --region "${region}")
              - StackName: !Ref "AWS::StackName"
```

In this updated template:

1. We use `Fn::Sub` to create a string where `${StackName}` is substituted with the value of `AWS::StackName`. This dynamically generates the `/stack-name/name` parameter name.

2. Inside the `commands`, you can access this dynamic parameter name and fetch the value accordingly.

This way, you can reference the SSM Parameter value that includes the stack name in your AWS::SSM::Association without hardcoding the stack name.

**Please give a thumbs up if my suggestion helps**

Answer

Hi,

On the proposal, you should replace `!Ref "AWS::StackName"` with `${AWS::StackName"}`

See section "Fn::Sub without a mapping" of https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference-sub.html for more details

Best,

Didier

How to refer SSMParameter value in AWS::SSM::Association?

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