export a environment variable for terraform does not work?

Question

i am trying to set a environment variable for terraform

using export TF_VAR_instanceType="t2.nano"

my tf file looks something like this:-

resource "aws_instance" "myFirstEc2Instance" {

  ami           = "ami-0ca285d4c2cda3300"
  instance_type = var.instanceType
}

Note there is terraform.tfvars as well as variables.tf files too, i am trying to see how exporting environment variables work in Mac. It is using value present terraform.tfvars file.