如何将变量传递到 AWS Cloudformation cfn-init 文件内容

Question

I want to change my Instances UserData to MetaData:->AWS::CloudFormation::Init:->files:->content.

My difficulty is with my existing UserData Bash script because it has variables and I want to have the variables in the cfn-init files content section.

Here is the existing UserData and it is the last 6 lines I want to transfer to the cfn-init. How are variables used in the content section?

UserData: 
    Fn::Base64: |
      #!/bin/bash -xe
      yum update -y
      yum install -y httpd
      systemctl start httpd
      systemctl enable httpd
      ID=`curl -s http://169.254.169.254/latest/meta-data/instance-id`
      AVAIL_ZONE=`curl -s http://169.254.169.254/latest/meta-data/placement/availability-zone`
      REGION=`curl http://169.254.169.254/latest/dynamic/instance-identity/document|grep region|awk -F\" '{print $4}'`
      echo "This is Server * $ID * in AWS Region $REGION in AZ $AVAIL_ZONE<br>" > /var/www/html/index.html
      EC2_AVAIL_ZONE=$(curl -s http://169.254.169.254/latest/meta-data/placement/availability-zone)
      echo "<h1>Hello World from $(hostname -f) in AZ $EC2_AVAIL_ZONE </h1><br>" >> /var/www/html/index.html

Answer 1

This can be a bit finicky to get right, but you can use a Fn::Sub to interpolate parameters into the user data:

UserData:
  Fn::Base64:
    Fn::Sub: |
      #!/bin/bash -xe
      echo "${ThisIsACloudFormationParameter}"

The reason it's finicky is that CFN and Bash can both use the ${…} style for variable substitution, so make sure to use that syntax only for CFN parameters.

Note that you can use the !Base64 and !Sub shorthand for one of the functions, but not both, CFN doesn't support nested shorthand function calls.