Access Process-Specific Terraform TF_VAR value - terraform

Question

I am attempting to grab a value within a Terraform/Python null_resource and make it available to other Terraform processes like so:
print_env.py
import os
# Create TF_VAR key value
param_tfvar = "TF_VAR_helloworld"
# Publish value through TF_VAR value
os.environ[param_tfvar] = "Hey Bob!"
print(param_tfvar, os.getenv(param_tfvar))
The Terraform *.tf files look like this:
variables.tf
variable "helloworld" {
description = "Display greeting"
type = string
default = ""
}
main.tf
resource "null_resource" "helloworld" {
provisioner "local-exec" {
command = <<EOC
python3 print_env.py
EOC
}
}
resource "null_resource" "echo_helloworld" {
provisioner "local-exec" {
command = "echo ${var.helloworld}"
}
depends_on = [
null_resource.helloworld
]
}
My issue is that the echo command does not display Hey Bob! but just a blank (the default value of helloworld defined in the variables.tf file.
I was looking at: https://www.terraform.io/language/values/variables#environment-variables on how to craft this solution.
Am I running into some type of scoping issue where the TF_VAR value published in the null_resource.helloworld block is not visible to the null_resource.echo_helloworld block?

Answer 1

Environment variables should be exported in the shell before you invoke terraform. Setting those at run-time like you are doing is not going to work.
Here's how I would solve your problem:
main.tf:
data "external" "get_greeting" {
program = ["python3", "get_env.py", var.greeting]
}
variable "greeting" {
description = "Display greeting"
type = string
default = ""
}
output "show_greeting" {
value = data.external.get_greeting.result.greeting
}
get_env.py:
import json
import sys
if len(sys.argv) > 1:
greeting = f"Hello {sys.argv[1]}"
else:
greeting = "Hello"
tf_dict = ({"greeting": greeting})
print(json.dumps(tf_dict))
And this is what you would see if you invoked it with no variable override:
$ terraform apply
outputs:
show_greeting = "Hello"
and with override:
$ terraform apply -var greeting="world"
outputs:
show_greeting = "Hello world"