Terraform Variables: Types and Uses for Beginners

In this blog post, we cover Terraform variable types of input, and output variables, and how to define them in terraform configuration.

If you want to know more about Terraform certification please go through our previous blog on Terraform Associate.

Index:

• Terraform Input Variables
• Types of Input Variables in Terraform
• Use Input Variables
• Assign Values to Input Variables
• Define Terraform Output Variables

Terraform Input Variables

Terraform input variables are used as parameters to input values at run time to customize our deployments. Input terraform variables can be defined in the main.tf configuration file but it is a best practice to define them in a separate variable.tf file to provide better readability and organization.

A variable is defined by using a variable block with a label. The label is the name of the variable and must be unique among all the variables in the same configuration.

The variable declaration can optionally include three arguments:

• description: briefly explain the purpose of the variable and what kind of value is expected.
• type: specifies the type of value such as string, number, bool, map, list, etc.
• default: If present, the variable is considered to be optional and if no value is set, the default value is used.

Check out: How to Install Terraform in Linux, Mac, Windows

Terraform Input Variables Types

The type argument in a variable block allows you to enforce type constraints on the variables a user passes in. Terraform supports several types, including string, number, bool, list, map, set, object, tuple, and any.

If a type isn’t specified, then Terraform assumes the type is any. Now, let’s have a look at some of these terraform variables types and their classification.

Primitive Types

A primitive type is a simple type that isn’t made from any other type. The available primitive types are:

• string: a sequence of characters representing some text, such as “hello”.
• number: a numeric value. The number type can represent both whole numbers like 15 and fractional values such as 6.28318.
• bool: either true or false.

Also Read: Terraform Workflow. 

Complex Types

A complex type is a type that groups multiple values into a single value. These values could be of a similar type or different types.

1. List: A Terraform list variable is a sequence of similar values indexed by numbers (starting with 0). It accepts any type of value as long as they are all of the same types. Lists can be defined either implicitly or explicitly.
   
2. Map: A map is a collection of values where each value is identified by a string label.
   In the example below is a map variable named managed_disk_type to define the type of storage we want to use based on the region in Azure. In the default block, there are two string values, “Premium_LRS” and “Standard_LRS” with a named label to identify each one westus2 and eastus.
   
3. Object: An object is a structural type that can contain different types of values, unlike a map, or list. It is a collection of named attributes that each have their type.
   In the below example, we have declared an object type variable os for the os image that can be used to deploy a VM.
   

Lists, maps, and objects are the three most common complex variable types. They all can be used for their specific use cases.

Also read: Step-by-step guide on Terraform Certification

Use Input Variables

After declaration, we can use the variables in our main.tf file by calling that variable in the var.<name> format. The value to the variables is assigned during terraform apply.

Also, Check Our blog post on Terraform Cheat Sheet. Click here

Assign Values To Input Variables

There are multiple ways to assign values to variables. The following is the descending order of precedence in which variables are considered.

1. Command-line flags

The most simple way to assign value to a variable is using the -var option in the command line when running the terraform plan and terraform apply commands.

$ terraform apply -var="resourceGroupName=terraformdemo-rg" -var="location=eastus"

2. Variable Definition (.tfvars) Files

If there are many variable values to input, we can define them in a variable definition file. Terraform also automatically loads several variable definitions files if they are present:

• Files named exactly terraform.tfvars or terraform.tfvars.json
• Any files with names ending in .auto.tfvars or .auto.tfvars.json

If the file is named something else, then use the -var-file flag directly to specify a file.

$ terraform apply -var-file="testing.tfvars"

3. Terraform Environment Variables

Terraform searches the environment of its process for environment variables named TF_VAR_<var-name> followed by the name of a declared variable. Terraform scans all variables starting with TF_VAR and uses those as variable values for Terraform.

$ export TF_VAR_location=eastus

This can be useful when running Terraform in automation, or when running a sequence of Terraform commands in succession with the same Terraform variables.

Read More: About Hashicorp Terraform. Click here