Terraform: Convert List to String

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

In Terraform, converting a list to a formatted string is a common task, especially when you need to pass data to resources or outputs that expect a specific string format. This article will guide you through various techniques to achieve this, from basic string concatenation to advanced formatting options. We'll cover how to use functions like join, handle quotes within strings, convert lists to JSON format, and leverage for expressions for custom formatting. Along the way, we'll highlight important considerations regarding data types and encourage you to experiment with these techniques in your own Terraform projects.

Let's break down how to convert a list in Terraform to a formatted string, covering various scenarios and techniques.

1. Basic String Concatenation with join

The most common way to combine list elements into a single string is with the join function.

• You provide a delimiter string (e.g., ", ") and the list you want to merge.

• Terraform inserts the delimiter between each list element in the output.

  my_list = ["apple", "banana", "cherry"]
  output = join(", ", my_list) 

  This would output: "apple, banana, cherry"

2. Handling Quotes within the String

If you need literal quotes within your final string, you'll need to escape them using backslashes (\).

my_list = ["a", "b", "c"]
output = join("\", \"", my_list) 

This results in: "a", "b", "c". The \" sequences ensure the quotes are treated literally.

3. Converting to JSON Format

When you need the list represented in JSON syntax, the jsonencode function is your go-to.

names = ["Alice", "Bob"]
json_output = jsonencode(names)

This produces: ["Alice", "Bob"], a valid JSON array representation.

4. Advanced Formatting within the String

For more complex formatting where you need control over each element's representation, consider using a for expression in conjunction with join.

my_list = ["apple", "banana", "cherry"]
formatted_output = join(", ", [for fruit in my_list : "Fruit: ${fruit}"])

This would output: "Fruit: apple, Fruit: banana, Fruit: cherry". The for expression iterates through the list, allowing you to apply custom formatting to each item.

Important Considerations

• Type Awareness: Terraform is strict about types. If a function expects a string and you provide a list, you'll encounter an error. Always be mindful of the data types you're working with.
• Implicit vs. Explicit Conversion: While Terraform often handles type conversions automatically, there are cases where you might need to explicitly convert a value using functions like tostring or tolist.
• Experimentation: The best way to solidify your understanding is to experiment with these techniques in your own Terraform configurations.

This Terraform code demonstrates various string manipulation techniques, particularly focusing on converting lists into formatted strings. It showcases basic concatenation using the join function, handling quotes within strings, converting lists to JSON format with jsonencode, and advanced formatting using a for expression to embed list elements within a string. The output is stored in a variable named "results" for easy access.

# 1. Basic String Concatenation with `join`
my_list = ["apple", "banana", "cherry"]
output = join(", ", my_list)

# Output: "apple, banana, cherry"

# 2. Handling Quotes within the String
quoted_list = ["a", "b", "c"]
quoted_output = join("\", \"", quoted_list)

# Output: "a", "b", "c"

# 3. Converting to JSON Format
names = ["Alice", "Bob"]
json_output = jsonencode(names)

# Output: ["Alice", "Bob"]

# 4. Advanced Formatting within the String
fruits = ["apple", "banana", "cherry"]
formatted_output = join(", ", [for fruit in fruits : "Fruit: ${fruit}"])

# Output: "Fruit: apple, Fruit: banana, Fruit: cherry"

output "results" {
  value = {
    basic_concatenation = output
    quoted_string       = quoted_output
    json_representation = json_output
    advanced_formatting = formatted_output
  }
}

Explanation:

• Basic Concatenation: The join function combines the elements of my_list using ", " as the separator.
• Handling Quotes: We escape the double quotes using backslashes (\) within the join function to include them literally in the output.
• JSON Conversion: jsonencode takes the names list and directly converts it into a valid JSON array string.
• Advanced Formatting: The for expression iterates over each fruit in the fruits list. For each fruit, it creates a string "Fruit: [fruit name]" and then uses join to combine these formatted strings.

This code demonstrates the different ways to convert a list to a formatted string in Terraform, providing practical examples for each scenario. Remember to run terraform apply to execute this code and see the output.

• Real-world Application: Imagine provisioning virtual machines in Terraform. You might have a list of hostnames and need to format them into a comma-separated string for a load balancer configuration.
• Debugging: Use the terraform console to experiment with these functions and see how they behave with different inputs. This is invaluable for understanding how Terraform handles type conversions and string manipulation.
• Alternative Formatting: While join is versatile, explore other functions like format and formatlist for more specialized string formatting needs.
• Error Handling: Be prepared to handle situations where your list might be empty. Using join on an empty list results in an empty string, which might not be the desired behavior in all cases.
• Readability: For complex formatting logic, consider breaking down the transformation into smaller, more manageable steps using local variables. This improves code readability and maintainability.
• Performance: While these string manipulations are generally fast, be mindful of excessive looping or complex operations within large configurations, as they might impact performance.
• Terraform Version Compatibility: Ensure the functions and syntax you use are compatible with your Terraform version. Refer to the official Terraform documentation for your specific version.
• Security: Avoid embedding sensitive information directly within strings. Use variables and secrets management solutions to handle sensitive data securely.
• Best Practices: Prioritize clarity and maintainability in your code. Use comments to explain the purpose of your string manipulations, especially when the logic is complex.

This table summarizes various techniques for converting lists to formatted strings in Terraform:

Key Considerations:

• Type Awareness: Be mindful of data types when using functions.
• Implicit vs. Explicit Conversion: Terraform often handles type conversions automatically, but explicit conversion might be needed in some cases.
• Experimentation: Practice these techniques in your own Terraform configurations to solidify your understanding.

Mastering list-to-string conversions in Terraform is crucial for crafting effective infrastructure configurations. By understanding the nuances of functions like join, jsonencode, and the power of for expressions, you gain precise control over how data is presented to resources and outputs. Remember to prioritize clarity and maintainability in your code, especially as your configurations grow in complexity. Experimentation is key – leverage Terraform's debugging tools and explore the rich ecosystem of string manipulation functions to solidify your understanding and unlock the full potential of Terraform's flexibility.

• Converting a list to a string | Nov 21, 2016 ... You received this message because you are subscribed to the Google Groups "Terraform" group. To unsubscribe from this group and stop receiving ...
• Turn list into literal string with quotes - Terraform - HashiCorp Discuss | Hello, I’m trying to turn a list into its literal correspondent, so basically the same thing without the square brackets: ["a", "b", "c"] should be turned into: "a", "b", "c" There’s no use case here, but I’d like to understand better how terraform works in these cases. What I’ve tried is: > "${join("", "", ["a", "b", "c"])}" "a", "b", "c" As you can see, the slashes themselves are output too. This is rather weird to me, because on the one hand, they do work as escaping characters ...
• Formatting Terraform Lists as Strings - Stark & Wayne | I’m deep in the weeds again. I’m inside of a YAML document inlined inside of a heredoc inside of a bash script that exists as a string inside of a heredoc inside of a Terraform configuration file written in HCL. It’s not my finest work, but it’s up there. To complete this masterpiece, I simply
• Terraform: Convert List To String (join Function) | Build5Nines | HashiCorp Terraform is a versatile infrastructure-as-code tool that empowers users to define and provision infrastructure resources with ease using a
• join - Functions - Configuration Language | Terraform | HashiCorp ... | The join function produces a string by concatenating the elements of a list with a given delimiter ... Type Conversion Functions. Terraform-specific ...
• Terraform Types: Use of Boolean, Map,List, Data types | Use of Boolean, Map,List, Data types
• tostring - Functions - Configuration Language | Terraform ... | tostring converts its argument to a string value. Explicit type conversions are rarely necessary in Terraform because it will convert types automatically where ...
• [Question] How to convert string to list or map · Issue #16868 ... | Hi there, Terraform Version Terraform v0.10.7 Question I have a json string, like this: { "key1":"value1", "key2":["value2-1","value2-2"], "key3":[{ "subkey1":"subvalue1"},{"subkey2","subvalue2"},{...
• tolist - Functions - Configuration Language | Terraform | HashiCorp ... | tolist converts its argument to a list value. Explicit type conversions are rarely necessary in Terraform because it will convert types automatically where ...