Using variables in terraform backend "s3" - terraform

Question

I have written below backend configuration in terraform:
terraform {
backend "s3" {
bucket = "${var.application_name}"
region = "${var.AWS_REGION}"
key = "tf-scripts/${var.application_name}-tfstate"
}
}
while running terraform init, I am getting below error message:
terraform init
Initializing the backend...
╷
│ Error: Variables not allowed
│
│ on backend.tf line 4, in terraform:
│ 4: bucket = "${var.application_name}"
│
│ Variables may not be used here.
╵
╷
│ Error: Variables not allowed
│
│ on backend.tf line 5, in terraform:
│ 5: region = "${var.AWS_REGION}"
│
│ Variables may not be used here.
╵
╷
│ Error: Variables not allowed
│
│ on backend.tf line 6, in terraform:
│ 6: key = "tf-scripts/${var.application_name}-tfstate"
│
│ Variables may not be used here.
Can anyone assist me on achieving the same?

Answer 1

If you want to pass variables you could do something like this:
echo "yes" | terraform init -backend-config="${backend_env}" -backend-config="key=global/state/${backend_config}/terraform.tfstate" -backend-config="region=us-east-1" -backend-config="encrypt=true" -backend-config="kms_key_id=${kmykeyid}" -backend-config="dynamodb_table=iha-test-platform-db-${backend_config}"
Now the trick here is that when you initialize it has to be done at the command line level. Terraform can not do this as mentioned already by other community members, it's just the way it is. that said you can modify the commands for initializing and pass it through as environment variables on your host or pull in variables from another source.
in this example, I declared the variables using a container through AWS Codebuild but you can use any method as long as the variable is defined prior to initialization. Let me know if you need help with this, the documentation isn't very clear and the folks on StackOverflow have been amazing at addressing this but for beginners, it's been hard to understand how this all comes together.