How to connect to an existing EC2 Instance and execute a shell file using CloudFormation Template

Question

I know that it's possible to use a CloudFormation template to launch a new EC2 instance in AWS and install any packages using user data.

But is there any way to connect to an existing instance and execute a shell file using the CloudFormation template?

Answer 1

Sadly, this is currently not possible in pure CloudFormatoin. To enable this, you would have to develope a [custom resource][1] in CloudFormation.

The resource would be in the form of a lambda function which would use AWS SDK to run SSM Run Command on your instance, provided that it was configured to work with SSM.

Alternatively, you could use tools such as Paraminko to ssh into the instance from the lambda function in your custom resource.

Answer 2

If you want to do this on an existing instance and you are forced to use cloudformation.

You can create a SystemManager Document to run on the instance with cloudformation i.e.

document: 
  Type: AWS::SSM::Document
  Properties:
    Content:
      schemaVersion: '2.2'
      description: 'Run a script on Linux instances.'
      parameters:
        commands:
          type: String
          description: "(Required) The commands to run or the path to an existing script
    on the instance."
          default: 'echo Hello World'
      mainSteps:
      - action: aws:runShellScript
        name: runCommands
        inputs:
          timeoutSeconds: '60'
          runCommand:
           - "{{ commands }}"
    DocumentType: Command
    Name: 'CFN_2.2_command_example'