use local env to replace variables in cloudformation template - amazon-ec2

Question

I have some variables i will like to replace in userdata of cloudformation template and i do not want to put these variables as parameters in cloudformation.
How can i do this?
Seems cloudformation wants one to always include any variable that need to be replaced as parameters but i feel this is not flexible enough. So not sure if someone else has figured out a way to do this.
Certain variables do not really need to tie to the infrastructure but there is need to replace those variables dynamically.
for example is i have this userdata
UserData:
"Fn::Base64":
!Sub |
#!/bin/bash -xe
cat >> /tmp/docker_compose.yaml << EOF
version: '3.5'
services:
ngnix:
container_name: nginx
image: nginx:$TAG
restart: always
ports:
- 80:80
environment:
SERVER_ID: $SERVER_ID
AWS_REGION: $AWS_REGION
EOF
and i want to be set the env variable values on the machine from where the cloudformation command will be ran
export TAG=1.9.9
export SERVER_ID=12
export AWS_REGION=us-east-1
How can i use these local env values to be replaced in the userdata without having those variables as parameters. I already tried all i can think of and i could not do this.
So wanted to tap into the power of internet if someone has thought of a way or hack.
Thanks

Answer 1

Here is one way of doing it via a script, there may be situations in which this script will give issues, but you'll have to test and see.
I don't want the environment variables being available outside of preparing my cloudformation script - so I've done everything inside one script file; loading the environment variables and substitution.
Note: You will need to install envsubst on your machine.
I have 3 files to start off with:
File one is my cloudformation script, in which I have a default value for each one of my parameters expressed as a bash variable:
cloudformation.yaml
Region
Default: $Region
InstanceType
Default: $InstanceType
Colour:
Default: $Colour
Then I have my variables file:
variables.txt
InstanceType=t2.micro
Colour=Blue
Region=eu-west-1
Then I have my script that does the substitution:
script.sh
#!/bin/bash
source variables.txt
export $(cut -d= -f1 variables.txt)
cat cloudformation.yaml | envsubst > subs_cloudformation.yaml
This is the contents of my folder:
cloudformation.yaml script.sh variables.txt
I make sure my script.sh has the correct permissions:
chmod +x script.sh
And run my script:
./script.sh
The contents of my folder is now:
cloudformation.yaml script.sh variables.txt subs_cloudformation.yaml
And if I view the contents of my subs_cloudformation.yaml file:
Region
Default: eu-west-1
InstanceType
Default: t2.micro
Colour:
Default: Blue
I can now run that cloudformation script, and cloudformation will do the job of substituting those defaults into my template - so all we're doing with the above script is giving cloudformation the defaults.
I've of course just given a snippet of the cloudformation template, you can further improve this by having a dev.txt, qa.txt, production.txt file of variables and substitute whichever one in.
Edit: It doesn't matter where in the file your variable is though, so it can be in userdata or parameters as default.. doesn't matter. You will also need to be careful, this won't check if you have a matching environment variable for every variable in your cloudformation file. If it isn't in your variable file, the substituted value will just be blank.