Home / server / Questions / 952351
Accepted
philolegein
Asked: 2019-02-06 02:31:55 +0800 CST

How to dynamically add users clause in CloudFormation::Init (or, how to programmatically set template keys)

  • 772

The AWS::CloudFormation::Init documentation allows the specification of users to be created on ec2 instances, thusly:

"users" : {
    "myUser" : {
        "groups" : ["groupOne", "groupTwo"],
        "uid" : "50",
        "homeDir" : "/tmp"
    }
}

What I'm trying to do is replace "myUser" programmatically. Using Ref: doesn't work:

"users": {
    "{ "Ref": "UserName" }": {
        "groups": [ "groupOne", "groupTwo" ]
    }
}

It says "Template format error: JSON not well-formed." on the line with the Ref. I've tried about 800 different ways to type that, with and without quotation marks, using different functions instead of Ref, and I'm not getting anywhere. Unfortunately, I can't figure out how to diagnose if this is really just a "your missing a comma" kind of problem, or if there's a bigger issue with trying to programmatically replace the "myUser". And if it's the latter, what the solution is.

Any ideas?

Thanks!

amazon-web-services

1 Answers

  1. Best Answer

    This is not doable with CloudFormation service directly. The intrinsic function Ref returns the value of the specified parameter or resource. The value can be then associated with the JSON/YAML key. But it can't be the key directly as JSON/YAML objects are written in key/value pairs.

    I have two ideas how you could resolve it:

    • supercharge CloudFormation templates written in YAML with Jinja2 templating, sceptre or stacks projects can easily do this
    • create users in EC2 user data directly, you can try something like this:
    
  "UserData"       : { "Fn::Base64" : { "Fn::Join" : ["", [
    "#!/bin/bash -xe\n",

    "useradd ",
    { "Ref": "UserName" },
    "\n",
  ]]}}
    • 0