How to Build List of Lists with Terraform For Loop

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Terraform is a powerful infrastructure-as-code tool that allows you to define and provision cloud resources efficiently. One advanced technique is creating a list of lists using Terraform’s `for` loop. This approach helps in managing complex configurations dynamically.

You Should Know:

1. Basic List Creation in Terraform

Before diving into nested lists, let’s see how to create a simple list:

variable "simple_list" {
type = list(string)
default = ["item1", "item2", "item3"]
}
  1. Creating a List of Lists Using `for` Loop
    Harold Finch’s example demonstrates how to generate a nested list structure:

    output "list_of_lists" {
value = [for i in range(3) : [for j in range(2) : "item-${i}-${j}"]]
}

Expected Output:

[
["item-0-0", "item-0-1"],
["item-1-0", "item-1-1"],
["item-2-0", "item-2-1"]
]

3. Advanced Use Case: Dynamic Nested Lists

If you need to generate lists based on existing variables:

variable "regions" {
type = list(string)
default = ["us-east-1", "us-west-2"]
}

variable "services" {
type = list(string)
default = ["ec2", "s3"]
}

output "dynamic_nested_list" {
value = [for region in var.regions : [for service in var.services : "${region}-${service}"]]
}

Expected Output:

[
["us-east-1-ec2", "us-east-1-s3"],
["us-west-2-ec2", "us-west-2-s3"]
]

4. Flattening Nested Lists

To convert a list of lists into a single list, use flatten:

output "flattened_list" {
value = flatten([for i in range(3) : [for j in range(2) : "item-${i}-${j}"]])
}

Expected Output:

["item-0-0", "item-0-1", "item-1-0", "item-1-1", "item-2-0", "item-2-1"]

5. Real-World Example: Multi-Region AWS Deployment

locals {
regions = ["us-east-1", "eu-west-1"]
ami_ids = {
"us-east-1" = "ami-12345678"
"eu-west-1" = "ami-87654321"
}
}

resource "aws_instance" "example" {
for_each = { for idx, region in local.regions : region => idx }

ami = local.ami_ids[each.key]
instance_type = "t2.micro"
}

What Undercode Say:

Terraform’s `for` loops and list manipulations are essential for scalable infrastructure automation. Here are some related Linux/Windows commands for automation:

Linux Commands:

– `jq` (JSON processor for Terraform outputs):

terraform output -json | jq '.list_of_lists.value'

– `awk` for parsing Terraform state:

terraform show -json | awk '/"type":/ {print $2}'

Windows Commands:

  • PowerShell for Terraform automation:
    terraform plan -out tfplan | ConvertFrom-Json

AWS CLI Integration:

aws ec2 describe-instances --filters "Name=instance-type,Values=t2.micro" --query 'Reservations[].Instances[].InstanceId'

Expected Output:

Mastering Terraform’s `for` loops and nested lists enables dynamic infrastructure management. Combine these with scripting tools (jq, awk, AWS CLI) for full automation.

Reference:

How to build list of lists with terraform for loop

References:

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