By Sebastian Stadil — 21 May 2025

Terraform Strings: How to Use Them

In the world of Infrastructure as Code (IaC) with Terraform, strings are more than just text—they're the dynamic glue holding your configurations together. From naming resources to crafting complex scripts, mastering string manipulation is essential. But as configurations grow, so does the complexity of managing these dynamic values.

Let's dive into the essentials of Terraform strings and touch upon how to keep them manageable.

The Basics: Literals and Interpolation

Terraform offers a couple of ways to define strings:

Heredoc Strings: Ideal for multiline content like scripts or embedded configuration files. They start with <<DELIMITER and end with DELIMITER on its own line. For cleaner indentation, <<-DELIMITER is your friend.

resource "aws_instance" "example" {
  # ... other configurations
  user_data = <<-EOF
    #!/bin/bash
    echo "Welcome to my new server!"
    echo "Provisioned at $(date)" > /etc/motd
  EOF
}

Quoted Strings: Simple, single-line strings enclosed in double quotes.

variable "greeting" {
  type    = string
  default = "Hello, Terraform User!"
}

String Interpolation (${...}) is where the magic happens, allowing you to embed expressions and variables directly into your strings:

variable "environment" {
  type    = string
  default = "dev"
}

output "instance_name" {
  value = "app-server-${var.environment}-01"
}
# Output: "app-server-dev-01"

Key String Functions for Your Toolkit

Terraform provides a rich set of built-in functions to manipulate strings. Here are a few common ones:

lower(string) / upper(string): Converts string case.

> lower("TerraForm")
"terraform"

format(spec, values...): Creates a formatted string, similar to printf.

> format("Instance ID: %s, Region: %s", "i-12345", "us-east-1")
"Instance ID: i-12345, Region: us-east-1"

replace(string, substring, replacement): Replaces occurrences of a substring.

> replace("app-server-01", "01", "02")
"app-server-02"

split(separator, string): Breaks a string into a list of strings.

> split(",", "alpha,beta,gamma")
["alpha", "beta", "gamma"]

join(separator, list): Combines a list of strings with a separator.

> join("-", ["web", "app", "prod"])
"web-app-prod"

Quick Summary: Terraform String Features

Feature	Description	Use Case Example
Quoted Strings	`"`Enclosed text`"`	Simple variable definitions, single-line attributes
Heredoc Strings	`<<EOF ... EOF` or `<<-EOF ... EOF` for multiline text	Embedding scripts, JSON/YAML snippets, long descriptions
Interpolation	`${var.name}` or `${resource.type.name.attribute}`	Dynamically inserting values into strings
`join()`	Concatenates list elements with a separator	Creating composite names (e.g., resource naming)
`split()`	Divides a string into a list based on a separator	Parsing comma-separated inputs
`replace()`	Replaces parts of a string	Modifying existing strings, templating
`format()`	`printf`-style string formatting	Complex string construction with multiple values
Template Directives	`%{ if bool }...%{ endif }`, `%{ for x in list }...%{ endfor }`	Conditional logic or loops within strings

Taming Complexity in String-Heavy Configurations

While Terraform's string manipulation capabilities are powerful, managing numerous dynamic strings across large projects and multiple teams can introduce challenges. Ensuring consistency, avoiding errors in complex interpolations, and maintaining readability become paramount.

This is where adopting a structured approach to your Terraform workflows, often facilitated by platforms like Scalr, can be highly beneficial. Scalr helps by providing robust environment management, version control integration, and policy enforcement (Open Policy Agent). This framework allows teams to collaborate more effectively and manage even string-heavy configurations with greater confidence, ensuring that dynamic values are generated and applied consistently according to organizational best practices. By centralizing configuration management and applying governance, the potential for misconfiguration arising from complex string logic is significantly reduced.

Conclusion

Strings are fundamental to leveraging Terraform's full potential for dynamic infrastructure. Understanding how to define, interpolate, and manipulate them using built-in functions is key. As your IaC practice matures, consider how a platform designed for Terraform collaboration and governance can help you manage the inherent complexities, ensuring your configurations remain robust, secure, and maintainable.