terraform variables from local environments to override default behavior

Working with Terraform sometimes requires different behaviors across environment stages (dev/prod/local lower environments).

To accomplish this we can use environment variables that force different behaviors depending on the local bash variable.

The syntax here to pay attention to is that the TF_VAR_<your variable name here> passes any environment variables to the underlying variable in Terraform.

Using an environment variable

We can set environment variables in a few different ways. To start, we'll need a variable to match it to. Either in our normal TF file or a variables.tf depending on code organization.

# variables.tf
variable "my_test_variable" {
  type        = string
  description = "My test variable"
}

Then we pass the variable value either with a local env variable or as part of our Terraform execution:

export TF_VAR_my_test_variable="something"

TF_VAR_my_test_variable="something" terraform plan

Complex examples with overrides

If we have a variable that has no default value we can check for its existence then provide a default value.

This checks for the TF_VAR_my_test_variable or the existence of a var.my_test_variable value passed in.

# main.tf
variable "my_test_variable" {
  type        = string
  description = "My test variable"
}

locals {
  my_var_or_default = (
    try(var.my_test_variable, false)
    ? var.my_test_variable
    : "default value"
  )
}

With a more complex variable we could check for the existence of other defaults, then coalesce them together:

# main.tf
locals {
  my_var_or_default = (
    try(var.my_test_variable, false) || var.my_other_test_variable != null
    ? coalesce(var.my_test_variable, var.my_other_test_variable)
    : "default value"
  )
}