Define input variables

4min
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Terraform

You now have enough Terraform knowledge to create useful configurations, but the examples so far have used hard-coded values. Terraform configurations can include variables to make your configuration more dynamic and flexible.

Prerequisites

After following previous tutorials, you will have a directory named learn-terraform-docker-container with the following configuration in a file called main.tf.

terraform {
  required_providers {
    docker = {
      source  = "kreuzwerker/docker"
      version = "~> 3.0.1"
    }
  }
}

provider "docker" {}

resource "docker_image" "nginx" {
  name         = "nginx:latest"
  keep_locally = false
}

resource "docker_container" "nginx" {
  image = docker_image.nginx.image_id
  name  = "tutorial"
  ports {
    internal = 80
    external = 8080
  }
}

terraform {
  required_providers {
    docker = {
      source  = "kreuzwerker/docker"
      version = "~> 3.0.1"
    }
  }
}

provider "docker" {
  host    = "npipe:////.//pipe//docker_engine"
}

resource "docker_image" "nginx" {
  name         = "nginx:latest"
  keep_locally = false
}

resource "docker_container" "nginx" {
  image = docker_image.nginx.image_id
  name  = "tutorial"
  ports {
    internal = 80
    external = 8080
  }
}

Ensure that your configuration matches this, and that you have run terraform init in the learn-terraform-docker-container directory.

Set the container name with a variable

The current configuration includes a number of hard-coded values. Terraform variables allow you to write configuration that is flexible and easier to re-use.

Add a variable to define the container name.

Create a new file called variables.tf with a block defining a new container_name variable.

variable "container_name" {
  description = "Value of the name for the Docker container"
  type        = string
  default     = "ExampleNginxContainer"
}

Note

Terraform loads all files in the current directory ending in .tf, so you can name your configuration files however you choose.

In main.tf, update the docker_container resource block to use the new variable. The container_name variable block will default to its default value ("ExampleNginxContainer") unless you declare a different value.

resource "docker_container" "nginx" {
  image = docker_image.nginx.image_id
- name  = "tutorial"
+ name  = var.container_name
  ports {
    internal = 80
    external = 8080
  }
}

Apply your configuration

Apply the configuration. Respond to the confirmation prompt with a yes.

$ terraform apply

docker_image.nginx: Refreshing state... [id=sha256:d1a364dc548d5357f0da3268c888e1971bbdb957ee3f028fe7194f1d61c6fdeenginx:latest]
docker_container.nginx: Refreshing state... [id=e1df62aa5ae5cf9e31e59066c9e686fe0980b1eed7712b417551bed6b04f364a]

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the
following symbols:
-/+ destroy and then create replacement

Terraform will perform the following actions:

  # docker_container.nginx must be replaced
-/+ resource "docker_container" "nginx" {
##...

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

Now apply the configuration again, this time overriding the default container name by passing in a variable using the -var flag. Terraform will update the container's name attribute with the new name. Respond to the confirmation prompt with yes.

$ terraform apply -var "container_name=YetAnotherName"
docker_image.nginx: Refreshing state... [id=sha256:d1a364dc548d5357f0da3268c888e1971bbdb957ee3f028fe7194f1d61c6fdeenginx:latest]
docker_container.nginx: Refreshing state... [id=ee653c84b8208dc141d0d79e1e4a276e3989a3359337ec5433e6a052279ed525]

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the
following symbols:
-/+ destroy and then create replacement

Terraform will perform the following actions:

  # docker_container.nginx must be replaced
-/+ resource "docker_container" "nginx" {
##...

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

Do you want to perform these actions?
  Terraform will perform the actions described above.
  Only 'yes' will be accepted to approve.

  Enter a value: yes

docker_container.nginx: Destroying... [id=ee653c84b8208dc141d0d79e1e4a276e3989a3359337ec5433e6a052279ed525]
docker_container.nginx: Destruction complete after 1s
docker_container.nginx: Creating...
docker_container.nginx: Creation complete after 2s [id=a7a6d308b8d4f77cda08ce20729f899530264d487cc1fd8ca92f9eb591ebcc6d]

Apply complete! Resources: 1 added, 0 changed, 1 destroyed.

Setting variables via the command-line will not save their values. Terraform supports many ways to use and set variables so you can avoid having to enter them repeatedly as you execute commands. To learn more, follow our in-depth tutorial, Customize Terraform Configuration with Variables.

Destroy

Outputs