Terraform: Append String to List Elements

• Introduction
• Step-by-Step Guide
• Code Example
• Additional Notes
• Summary
• Conclusion
• References

In Terraform, you can easily append a string to each element of a list using a for expression combined with string interpolation. This technique allows you to dynamically modify list elements, such as adding a suffix or prefix to each item. Let's explore how to achieve this with a simple example.

To append a string to each element of a list in Terraform, you can use a for expression with string interpolation.

variable "names" {
  default = ["Alice", "Bob", "Charlie"]
}

output "greetings" {
  value = [for name in var.names : "${name}!"]
}

This code will output a list of strings where each element is the corresponding name from the names variable with an exclamation mark appended to it:

greetings = [
  "Alice!",
  "Bob!",
  "Charlie!",
]

Here's how it works:

1. for name in var.names: This iterates over each element in the names list, assigning each element to the name variable in turn.
2. ${name}!: This performs string interpolation. It takes the current value of the name variable and concatenates it with an exclamation mark.
3. [...]: The square brackets enclose the entire expression, creating a new list from the results of each iteration.

This approach leverages Terraform's built-in string interpolation and list manipulation capabilities to efficiently modify each element in a list.

This Terraform code defines a variable with a list of names and outputs a new list where each name is followed by an exclamation mark.

# Define a variable with a list of names
variable "names" {
  default = ["Alice", "Bob", "Charlie"]
}

# Append a string to each element in the list
output "greetings" {
  value = [for name in var.names : "${name}!"]
}

Explanation:

1. variable "names": This line defines a variable named "names" and sets its default value to a list containing three strings: "Alice", "Bob", and "Charlie".

2. output "greetings": This line defines an output variable named "greetings". Output variables are used to display values after Terraform has applied the configuration.

3. value = [for name in var.names : "${name}!"]: This line uses a for expression to iterate over each element in the var.names list.

   • for name in var.names: This part iterates through each element in the var.names list, assigning each element to the temporary variable name in each iteration.
   • "${name}!": This part performs string interpolation. It takes the current value of the name variable (which represents each name from the list) and concatenates it with an exclamation mark ("!").
   • [ ... ]: The square brackets enclose the entire expression, creating a new list from the results of each iteration.

Output:

When you run terraform apply, the output will show the following:

greetings = [
  "Alice!",
  "Bob!",
  "Charlie!",
]

This demonstrates that the code successfully appended an exclamation mark to each element in the original names list.

• Flexibility: This approach isn't limited to just exclamation marks. You can append any string you want, making it useful for adding prefixes, suffixes, or even inserting text within each element.
• Dynamic Appending: You can use variables or other expressions within the appended string to make it dynamic. For example, you could append a different string based on some condition.
• Alternative to concat: While concat can combine lists, it doesn't modify individual elements. This for expression approach is specifically for modifying each element in a list.
• Readability: This method is generally more readable than nested function calls, especially for simple appends.
• Error Handling: If any element in the list is not a string, Terraform will throw an error during the interpolation. Ensure your list contains only strings before applying this method.
• Terraform 0.12 and above: This syntax is only available in Terraform versions 0.12 and above. For older versions, you might need to explore alternative approaches.
• Use Cases: This technique is valuable for various scenarios, such as:
  • Constructing resource names with dynamic prefixes or suffixes.
  • Formatting output strings for display or logging.
  • Creating lists of URLs by appending paths to base URLs.

This document summarizes how to append a string to each element of a list in Terraform using a for expression and string interpolation.

Explanation:

1. Iteration: The for name in var.names part loops through each element (name) in the input list (var.names).
2. String Interpolation: For each name, ${name}! combines the name with "!" using string interpolation.
3. List Creation: The entire expression is enclosed in [...], creating a new list with the modified elements.

This method effectively appends a string to each element of a list in Terraform, providing a concise and readable solution for various use cases. By understanding for expressions and string interpolation, you can leverage this technique to manipulate lists dynamically and efficiently in your Terraform configurations.

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