Configuration language

Configuration language#

The Terraform configuration language is designed to describe the infrastructure. This page considers the details and properties of different blocks.

Terraform#

The terraform block allows specifies the configuration of the application.

There are special block for defining providers.

Check more in the terraform block reference.

Providers#

The required_providers block inside terraform allows you to specify the set of providers to be used in this configuration.

The following code adds a random provider to the terraform configuration. And usses it’s random_id resource to ouput a random value.

rm -rf /tmp/terraform
mkdir /tmp/terraform
cat << EOF > /tmp/terraform/main.tf
terraform {
  required_providers {
    random = {source = "hashicorp/random"}
  }
}

resource "random_id" "rand_val" {
  byte_length = 8
}

output "variable_output" {
  value = random_id.rand_val.id
}
EOF
cd /tmp/terraform
terraform init &> /dev/null

The output of this terraform configuration is shown in the following cell

terraform output

variable_output = "O3XBEnZXo0I"

Resource#

The resource block defines a piece of infrastructure. Pieces of infrastructure really depends on the used provider, but there are common arguments. The generally syntax is following:

resource "<TYPE>" "<LABEL>" {
  <PROVIDER_ARGUMENTS>
  <COMMON_ARGUMENTS>
}

You have to define the type of resource in the TYPE field. The awailable types are defined by the choosen provider.

Check more in resource block reference.

The following cell applies the configuration tha defines the highly specific Nginx Docker image as the resouce.

rm -rf /tmp/terraform
mkdir /tmp/terraform
cat << EOF > /tmp/terraform/main.tf
terraform {
  required_providers {
    docker = {
      source = "kreuzwerker/docker"
      version = "~> 3.0.1"
    }
  }
}

resource "docker_image" "nginx" {
  name = "nginx:1.29.3-alpine3.22-perl"
  keep_locally = true
}
EOF
cd /tmp/terraform
terraform init &> /dev/null
terraform apply -auto-approve &> /dev/null

The specified Docker image appears in Docker runtime.

docker images | grep --color=None "1.29.3-alpine3.22-perl"

nginx                                                1.29.3-alpine3.22-perl       86f721376853   11 hours ago   91.2MB

Output#

The output block can be useful for experimentation purposes, as it allows you to easily print values to stdout.

Check the output block reference.

The definition of the "hello_world output is shown in the following cell.

rm -rf /tmp/terraform
mkdir /tmp/terraform
cat << EOF > /tmp/terraform/main.tf
output "hello_world" {
  value = "Hello, stdout!"
}
EOF
cd /tmp/terraform
terraform init &> /dev/null

In such a case, the terraform apply will have an Outputs section containing the relevant infromation.

terraform apply -auto-approve

Changes to Outputs:
  + hello_world = "Hello, stdout!"

You can apply this plan to save these new output values to the Terraform state,
without changing any real infrastructure.

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

Outputs:

hello_world = "Hello, stdout!"

You can also use the terraform output command to simply push the outputs to stdout.

terraform output

hello_world = "Hello, stdout!"

Variables#

The variable allows you to define the variables that can be set using an external configuration. This allows you to parameterise you configuration.

The following cell adds the variable parameter and prints its value alongside the output.

rm -rf /tmp/terraform
mkdir /tmp/terraform
cat << EOF > /tmp/terraform/main.tf
variable "variable" {
  description = "Value of the name for the Docker container"
  type        = string
  default     = "the value of the variable"
}

output "variable_output" {
  value = var.variable
}
EOF
cd /tmp/terraform
terraform init &> /dev/null

The following cell invokes the configuration output.

terraform output

variable_output = "the value of the variable"

Pass the value you like your configuration to run with to the terraform apply command with -var parameter.

terraform apply -auto-approve -var "variable=custom value"

Changes to Outputs:
  ~ variable_output = "the value of the variable" -> "custom value"

You can apply this plan to save these new output values to the Terraform state,
without changing any real infrastructure.

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

Outputs:

variable_output = "custom value"
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