Start cfn-init in Ubuntu instance with cloudformation (yaml)

I try to start the cfn-init with:

Fn::Base64: !Sub | 
  #!/bin/bash
  sudo apt-get -y install python-setuptools
  mkdir aws-cfn-bootstrap-latest
  curl https://s3.amazonaws.com/cloudformation-examples/aws-cfn-bootstrap-latest.tar.gz | tar xz -C aws-cfn-bootstrap-latest --strip-components 1
  sudo easy_install aws-cfn-bootstrap-latest
  sudo /usr/local/bin/cfn-init --stack !Ref 'AWS::StackName' --resource xxx --region !Ref 'AWS::Region'

The first steps work. I can access the instance and cfn-init is installed. When I execute the cfn-init --stack.. command inside my ec2 instance it works fine when I hardcode the values for stackname and region.

How to make it work in the yaml script? It seems it can not read the values for StackName and region.

If you are using !Sub you need to wrap your variables with ${} instead of using !Ref

Try this

Fn::Base64: !Sub | 
  #!/bin/bash
  sudo apt-get -y install python-setuptools
  mkdir aws-cfn-bootstrap-latest
  curl https://s3.amazonaws.com/cloudformation-examples/aws-cfn-bootstrap-latest.tar.gz | tar xz -C aws-cfn-bootstrap-latest --strip-components 1
  sudo easy_install aws-cfn-bootstrap-latest
  sudo /usr/local/bin/cfn-init --stack ${AWS::StackName} --resource xxx --region ${AWS::Region}

If you specify template parameter names or resource logical IDs, such as ${InstanceTypeParameter}, AWS CloudFormation returns the same values as if you used the Ref intrinsic function. If you specify resource attributes, such as ${MyInstance.PublicIp}, AWS CloudFormation returns the same values as if you used the Fn::GetAtt intrinsic function.

For more details check AWS - Fn::Sub