Access Process-Specific Terraform TF_VAR value

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"