Simple usage of variables in Terraform

variables.tf:

variable "host_os" {
  type = string
}

For the next file, we will take the example of this EC2 instance provisioner main.tf. Modify line:

...
command = templatefile("linux-ssh-config.tpl", {
...

to:

...
command = templatefile("${var.host_os}-ssh-config.tpl", {
...

Now, run plan:

$ terraform plan
var.host_os
  Enter a value: linux
...

To not to be asked each time - use this variables.tf with default set up:

variable "host_os" {
  type = string
  default = "linux"
}