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Revathi Joshi for AWS Community Builders

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How to fix a Terraform String Variable Interpolation Error

#terraform #ec2

In this article. I am going to show you how fix the string variable interpolation error.

Embedded within strings in Terraform, whether you're using the Terraform syntax or JSON syntax, you can interpolate other values. These interpolations are wrapped in ${}, such as ${var.name}.

The interpolation syntax is powerful and allows you to reference variables, attributes of resources, call functions, etc.

Use the var. prefix followed by the variable name. For example, ${var.name} will interpolate the name variable value.

Please visit my GitHub Repository for Terraform articles on various topics being updated on constant basis.

Let’s get started!

Objectives:

1. Login to AWS Management Console

2. Create infrastructure for resources block

3. Under terraform_files resources directory - Create 4 files - main.tf, variables.tf, outputs.tf and terrafprm.tfvars.

4. Interpolation Error

5. Fix Interpolation Error

Pre-requisites:

  • AWS user account with admin access, not a root account.
  • Cloud9 IDE with AWS CLI.

Resources Used:

Terraform documentation.
Terraform documentation for AMI.
Interpolation Syntax
Troubleshoot Terraform - Correct a variable interpolation error

Steps for implementation to this project:

1. Login to AWS Management Console

  • Make sure you're in the N. Virginia (us-east-1) region

2. Create infrastructure for resources block

  • Let’s create the following organizational structure as shown below.

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3. Under terraform_files resources directory - Create 4 files - main.tf, variables.tf, outputs.tf and terrafprm.tfvars.

  • 1. main.tf
 terraform { required_providers { aws = { source = "hashicorp/aws" version = "~> 4.23" } } required_version = ">= 0.14.9" } provider "aws" { region = var.region } data "aws_ami" "linux" { most_recent = true owners = ["amazon"] filter { name = "name" values = ["amzn2-ami-hvm-*-x86_64-gp2"] } filter { name = "virtualization-type" values = ["hvm"] } } resource "aws_instance" "web_server" { ami = data.aws_ami.linux.id instance_type = var.instance_type availability_zone = var.az_1a user_data = <<-EOF #!/bin/bash echo "Hello, World" > index.html nohup busybox httpd -f -p 8080 & EOF tags = { Name = $var.name-myweb } }
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  • 2. variables.tf
 variable "region" { description = "region" } variable "name" { description = "Value of the Name tag for the EC2 instance" } variable "az_1a" { description = "availability zone 1" type = string default = "us-east-1a" } variable "instance_type" { description = "Value of the Name tag for the EC2 instance type" type = string default = "t2.micro" }
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  • 3. outputs.tf
 output "instance_id" { description = "ID of the EC2 instance" value = aws_instance.web_server.id } output "instance_ip" { description = "Public IP address of the EC2 instance" value = aws_instance.web_server.public_ip } output "instance_name" { description = "Name of the EC2 instance" value = aws_instance.web_server.tags.Name }
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  • 4. terrafprm.tfvars
 name = "rev" region = "us-east-1"
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4. Interpolation Error

  • cd terraform-files

  • An error is expected after running the terraform fmt command.

 terraform fmt
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5. Fix Interpolation Error

  • main.tf

  • Edit and correct the tags block section by adding this:

  • From 

 tags = { Name = $var.name-myweb }
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  • To
 tags = { Name = "${var.name}-myweb"
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  • Check the format
 terraform fmt
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  • Initialize Terraform
 terraform init
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  • Check syntax
 terraform validate
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  • Preview changes
 terraform plan
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  • Execute changes
  • Type yes to confirm.
 terraform apply
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  • The apply should be completed successfully.

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Cleanup 

 terraform destroy
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What we have done so far

We have successfully fixed the string variable interpolation error and deployed our resources.

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