Terraform Multiline Resource Keys: A Complete Guide

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

In Terraform, managing lengthy strings, such as SSH keys or multi-line scripts, can be easily achieved using heredoc syntax within variables. This approach provides a clean and organized way to store and utilize such data. Let's explore how to define multi-line strings in Terraform using heredoc and understand its implications.

To define multi-line strings in Terraform, you can use the heredoc syntax ( <<EOF and EOF ). This approach is helpful for storing lengthy configuration data, like SSH keys or multi-line scripts, in variables.

For instance:

variable "long_key" {
  type = "string"
  default = <<EOF
This is a long key.
Running over several
lines.
EOF
}

In this example, the long_key variable stores a multi-line string. The <<EOF marks the beginning, and EOF marks the end of the string. Everything between these markers, including line breaks, is part of the string.

However, be mindful that certain Terraform resources might handle multi-line strings differently. For example, some resources might strip out newline characters. If you need to preserve newlines in such cases, you might need to explore alternative approaches like encoding or specific resource options.

This Terraform code demonstrates how to define a multi-line string variable using heredoc syntax. It shows that while some resources like 'local_file' preserve newline characters from the variable, others like 'null_resource' with 'local-exec' provisioner might strip them. Be mindful of this behavior and consider alternative approaches like encoding or specific resource options if preserving newlines is crucial.

This example demonstrates how to define a multi-line string in Terraform using the heredoc syntax and showcases potential issues with newline characters.

# Define a variable with a multi-line string
variable "long_key" {
  type = string
  default = <<EOF
This is a long key.
Running over several
lines.
EOF
}

# Resource that might strip newline characters (example)
resource "null_resource" "example" {
  provisioner "local-exec" {
    command = "echo '${var.long_key}'"
  }
}

# Resource that preserves newline characters (example)
resource "local_file" "example" {
  content = var.long_key
  filename = "output.txt"
}

Explanation:

1. variable "long_key": This defines a variable named long_key of type "string".
2. <<EOF and EOF: These markers define the beginning and end of the multi-line string. Everything between them, including line breaks, is part of the string.
3. null_resource "example": This resource demonstrates that some resources might strip newline characters from the multi-line string. In this case, the local-exec provisioner might output the string without line breaks.
4. local_file "example": This resource demonstrates that other resources, like local_file, preserve newline characters. The content of the file will include the line breaks from the long_key variable.

Important Note:

• Be aware of how different resources handle multi-line strings.
• If you need to preserve newlines in resources that might strip them, consider alternative approaches like encoding the string or using specific resource options.

• Alternatives to Heredoc: While heredoc is convenient, you can also define multi-line strings using:
  • Concatenation: Joining multiple single-line strings with the \ character.
  • Arrays: Defining an array of strings and then using join() to combine them.
• Indentation: Be mindful of indentation within heredoc blocks. Terraform will include leading whitespace in the resulting string. It's generally recommended to keep the closing EOF marker at the beginning of the line to avoid unexpected spaces.
• Security: Avoid placing sensitive data directly within your Terraform code. Use variables and consider storing secrets securely using tools like HashiCorp Vault.
• Templating: You can use string interpolation within heredoc blocks to dynamically construct multi-line strings based on variables and expressions.
• Debugging: Use terraform console and the output command to inspect the actual values of multi-line strings during development and troubleshooting.
• Best Practices:
  • Use descriptive variable names for clarity.
  • Add comments to explain the purpose and usage of multi-line strings.
  • Consider using a code editor with Terraform syntax highlighting for improved readability.

In conclusion, using heredoc syntax in Terraform simplifies the management of multi-line strings within variables, proving particularly useful for SSH keys and scripts. However, remember that certain resources might handle these strings differently, potentially stripping newline characters. If preserving newlines is crucial, explore alternative approaches like encoding or resource-specific options. By understanding these nuances, you can effectively leverage heredoc for cleaner, more organized configuration data in your Terraform projects.

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