How to Append to a List in Terraform?

TLDR

To append elements to a list in Terraform, use the concat function or dynamic blocks for more complex scenarios. These methods allow you to manage lists dynamically and efficiently.

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Terraform provides multiple ways to append elements to a list, depending on your use case. This guide explains how to achieve this with practical examples.

Step 1: Use the concat Function

The concat function is the simplest way to append elements to a list.

Example

variable "example_list" {
  default = ["apple", "banana"]
}

locals {
  updated_list = concat(var.example_list, ["cherry"])
}

output "result" {
  value = local.updated_list
}

Explanation

• concat(var.example_list, ["cherry"]): Appends the element cherry to the existing list.
• local.updated_list: Stores the updated list.

Step 2: Use Dynamic Blocks for Complex Scenarios

Dynamic blocks can be used to append elements to a list when working with resources or modules.

Example

variable "example_list" {
  default = ["apple", "banana"]
}

resource "null_resource" "example" {
  count = length(var.example_list)

  triggers = {
    item = var.example_list[count.index]
  }
}

output "result" {
  value = [for r in null_resource.example : r.triggers.item] ++ ["cherry"]
}

Explanation

• null_resource: Iterates over the existing list.
• ++ ["cherry"]: Appends the element cherry to the list.

Step 3: Combine with Conditional Logic

You can conditionally append elements to a list based on specific criteria.

Example

variable "example_list" {
  default = ["apple", "banana"]
}

locals {
  updated_list = var.example_list
  final_list   = var.example_list == [] ? ["default"] : concat(local.updated_list, ["cherry"])
}

output "result" {
  value = local.final_list
}

Explanation

• var.example_list == []: Checks if the list is empty.
• concat(local.updated_list, ["cherry"]): Appends cherry if the list is not empty.

Best Practices

• Use Descriptive Variable Names: Clearly indicate the purpose of each variable.
• Validate Inputs: Ensure lists are in the expected format before appending elements.
• Document Logic: Provide clear comments for complex expressions.

By following these steps, you can effectively append elements to a list in Terraform, enabling dynamic and flexible configurations.