terraform plan/apply with "-var-file=<custom_value>"still loads terraform.tfvars if present

Terraform Version

Terraform v0.12.25
+ provider.null v2.1.2

Terraform Configuration Files

main.tf:
resource null_resource test_resource {
  triggers = {
    val = var.test_val
  }
  provisioner "local-exec" {
    command = "echo ${var.test_val}"
  }
}

variables.tf:
variable test_val {
  type = string
}

custom.tfvars:
test_val1="custom"

terraform.tfvars:
test_val="default"

Debug Output

Crash Output

Expected Behavior

When running terraform plan or apply and providing custom value of -var-file and -state, the expectation is that only these specified files will be loaded.

Actual Behavior

According to my experiment if terraform.tfvars exists in the same directory and some value is missing in "custom" provided tfvar files, it will gets loaded from terrafrom.tfvars when such file should be not read at all.

Steps to Reproduce

$ terraform init $ terraform plan -var-file=./custom.tfvars -state=./custom.tfstate Refreshing Terraform state in-memory prior to plan... The refreshed state will be used to calculate this plan, but will not be persisted to local or remote state storage.

An execution plan has been generated and is shown below. Resource actions are indicated with the following symbols:

create

Terraform will perform the following actions:

null_resource.test_resource will be created

resource "null_resource" "test_resource" {
- id = (known after apply)
- triggers = {
  - "val" = "default" } }

Plan: 1 to add, 0 to change, 0 to destroy.

Now if I delete the terraform.tfvars I get the expected behavior - asking for missing value: $ rm terraform.tfvars $ terraform plan -var-file=./custom.tfvars -state=./custom.tfstate var.test_val Enter a value:

hashicorp / terraform