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Terraform

Terraform: Concatenate Lists with concat() Function

By Filip on 10/05/2024

Learn how to easily combine two lists in Terraform using the concat() function with practical examples and explanations.

Terraform: Concatenate Lists with concat() Function

Table of Contents

Introduction

In Terraform, combining two lists is a common task that can be achieved using the concat() function. This function allows you to merge the elements of multiple lists into a single new list.

Step-by-Step Guide

To combine two lists in Terraform, you can use the concat() function.

Let's say you have two lists:

variable "list1" {
  default = ["a", "b", "c"]
}

variable "list2" {
  default = ["d", "e", "f"]
}

You can create a new list that combines these two lists using concat() like this:

output "combined_list" {
  value = concat(var.list1, var.list2)
}

This will output a single list containing all the elements from both input lists: ["a", "b", "c", "d", "e", "f"].

Remember that concat() only combines lists. If you need to merge maps or perform more complex operations, you'll need to explore other functions and techniques in Terraform.

Code Example

The code defines two Terraform variables, "list1" and "list2", each containing a list of strings. It then combines the two lists using the "concat" function and outputs the result as a new variable called "combined_list".

variable "list1" {
  default = ["a", "b", "c"]
}

variable "list2" {
  default = ["d", "e", "f"]
}

output "combined_list" {
  value = concat(var.list1, var.list2)
}

Additional Notes

  • Purpose of concat(): The primary function of concat() is to merge multiple lists into a single, flat list. It's important to note that it doesn't perform any deduplication or sorting.

  • Data Types: While the example demonstrates string lists, concat() works with lists containing any Terraform data type (e.g., numbers, booleans, objects).

  • Order Preservation: concat() maintains the order of elements. The elements from the first list appear first in the output, followed by elements from the second list, and so on.

  • Empty Lists: Including empty lists ([]) within concat() has no impact on the output. The empty lists are essentially ignored.

  • Alternatives for Complex Merging:

    • merge(): Use the merge() function when you need to combine maps, giving priority to values from later maps in case of key collisions.
    • Loops and Conditionals: For more intricate list manipulations like merging based on specific conditions or creating nested structures, leverage for loops and conditional expressions (if/else) within your Terraform code.

  • Practical Applications:

    • Resource Creation: Combine lists of resource configurations dynamically based on input variables or module outputs.
    • Security Group Rules: Merge lists of ingress or egress rules from different sources to define comprehensive security policies.
    • Data Transformation: Restructure and combine data from various sources to fit the requirements of your infrastructure setup.

Summary

Action Function Description Example Output
Combine two lists concat(list1, list2) Combines two lists into a single list. concat(var.list1, var.list2) ["a", "b", "c", "d", "e", "f"]

Note: concat() only works with lists. For maps or more complex operations, use other Terraform functions.

Conclusion

The concat() function in Terraform is a simple yet powerful tool for combining lists. Its ability to merge lists of any data type makes it versatile for various tasks, from resource configuration to data manipulation. While concat() itself doesn't offer features like deduplication or conditional merging, it serves as a fundamental building block that, when combined with other Terraform functions and constructs, allows for complex list operations. Understanding the nuances of concat(), such as its order-preserving behavior and handling of empty lists, is crucial for writing effective and predictable Terraform code. For more intricate list manipulations, exploring functions like merge() or leveraging loops and conditional logic within your Terraform configurations will unlock a wider range of possibilities.

References

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