AWS CloudFormation: How to output a machine's PublicIP?

Assuming your have an EC2 instance resource in your template named Server:

"Server" : {
    "Type" : "AWS::EC2::Instance",
    "Properties" : {
    }
}

You output the public IP address referencing it's resource name:

"Outputs" : {
    "PublicIp" : {
      "Value" : { "Fn::GetAtt" : [ "Server", "PublicIp" ]},
      "Description" : "Server's PublicIp Address"
    }
}

As mentioned in the docs, the outputs can optionally be exported for cross-stack references. In case that is your use-case:

JSON:

"Outputs" : {
  "PublicIp" : {
    "Value" : { "Fn::GetAtt" : ["Server", "PublicIp"]},
    "Description" : "Server Public IP"
    "Export" : {
      "Name" : {"Fn::Sub": "${AWS::StackName}-PublicIP"}
    }
  }
}

YAML:

Outputs:
  PublicIp:
    Description: Server Public IP
    Value: !GetAtt Server.PublicIp
    Export:
      Name: !Sub "${AWS::StackName}-PublicIp"

See also:

• AWS::EC2::Instance properties and return values in the docs.