Variables can be defined in a variable block. Let's say you have the following files on your Terraform server.

├── locals.tf
├── modules.tf
├── outputs.tf
├── provider.tf
├── terraform.tfstate
├── variables.tf
├── child (directory, child module)
│   ├── data.tf
│   ├── outputs.tf
│   ├── resources.tf

And let's say variables.tf in the same directory as your main root module (main.tf) has the following.

variable "foo" {
  type = string
  sensitive = false
}

variable "my-string" {
  type = string
  sensitive = false
  default = "Hello"
}

variable "my-integer" {
  type = number
  sensitive = false
  default = 1
}

variable "my-boolean" {
  type = bool
  sensitive = false
  default = true
}

And let's say outputs.tf in the same directory as your main root module (main.tf) has the following. In this example, output blocks are used to output the value of each variable.

output "my-text" {
  value = "World"
}

output "my-foo" {
  value = var.foo
}

output "string-out" {
  value = var.my-string
}

output "integer-out" {
  value = var.my-integer
}

output "boolean-out" {
  value = var.my-boolean
}

Or, like this.

output "my-foo" {
  value = ${var.foo}
}

The terraform refresh, terraform output, terrafrom plan, and terraform apply commands should now all show the output.

terraform refresh

Since the "foo" variable does not have a default value and no value was set, there will be a prompt like this.

var.foo
  Enter a value:

Which should return something like this.

string-out = "Hello"
my-foo = "My Value"
my-string = "World"
my-integer = 1
boolean-out = true

Let's say variables.tf in the same directory as your main root module (main.tf) has the following.

variable "username" {
  description = "prompt username"
  type = string
  sensitive = true
}

variable "password" {
  description = "prompt password"
  type = string
  sensitive = true
}

Here is an example of how you could use the username and password variables in a different .tf file as long as the .tf files are in the same directory.

terraform {
  required_providers {
    rabbitmq = {
      source  = "cyrilgdn/rabbitmq"
    }
  }
}

provider "rabbitmq" {
  endpoint = "http://rabbit.example.com:15672"
  username = var.username
  password = var.password
}

You will be prompted for the username and password.

var.username
  prompt username

  Enter a value:

var.password
  prompt password

  Enter a value:

The default key can be used if you do not want to be prompted.

variable "username" {
  description = "RabbitMQ username"
  type = string
  sensitive = true
  default = "john.doe"
}

variable "password" {
  description = "RabbitMQ password"
  type = string
  sensitive = true
  default = "itsasecret"
}

Or the -var command line option can be used.

terraform init -var="profile=my-profile" -var="region=us-east-1"