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Terraform

Terraform Output to File: Save and Share Results

By Filip on 11/10/2024

Learn how to streamline your Terraform workflow by saving the output of your infrastructure deployments to a file for easy access and automation.

Terraform Output to File: Save and Share Results

Table of Contents

Introduction

Terraform is a powerful tool for infrastructure as code, but it doesn't directly support saving output values to a file during the terraform apply process. However, there are several workarounds to achieve this. This article explores three common methods: using terraform output with shell redirection, leveraging the local_file resource with templates, and employing scripting languages like Bash or Python. We'll also touch upon important considerations such as security and alternative tools for enhanced output management.

Step-by-Step Guide

Terraform itself doesn't have a direct way to save output values to a file during terraform apply.

Workarounds:

  1. Using terraform output and shell redirection:

    • After terraform apply, run: 
      terraform output my_output_value > output.txt
    • This redirects the output value to output.txt.

  2. Using local_file resource with templatefile or template_cloudinit_config:

    • Define a local_file resource and use templatefile or template_cloudinit_config to populate its content with output values. 
      resource "local_file" "output_file" {
  content = templatefile("${path.module}/output.tpl", {
    my_value = module.example.my_output_value
  })
  filename = "output.txt"
}

  3. Scripting (e.g., Bash, Python):

    • Use a scripting language to parse terraform output and write to a file. 
      #!/bin/bash
terraform output -json > output.json
# Process output.json and write desired values to a file

Important Considerations:

  • Security: Be cautious when storing sensitive data (e.g., passwords, keys) in output files.
  • Plan Files: While you can save terraform plan output to a file using terraform plan -out=planfile, treat these files as potentially sensitive.
  • Alternative Tools: Consider tools like terragrunt or CI/CD systems for more advanced output handling and automation.

Code Example

This code provides three examples of how to save Terraform output values to a file. The first example uses shell redirection with the terraform output command. The second example uses the local_file resource with the templatefile function to dynamically generate the file content. The third example uses a Python script to execute the terraform output command, parse the JSON output, and write the desired value to a file.

This document provides code examples for the workarounds mentioned in the article about saving Terraform output values to a file.

1. Using terraform output and shell redirection:

# After running terraform apply
terraform output my_output_value > output.txt

This command will redirect the value of the output variable my_output_value to a file named output.txt.

2. Using local_file resource with templatefile:

# Define the output variable in your module
output "my_output_value" {
  value = "This is the output value"
}

# Create a local file resource
resource "local_file" "output_file" {
  content = templatefile("${path.module}/output.tpl", {
    my_value = module.example.my_output_value
  })
  filename = "output.txt"
}

output.tpl:

My output value is: ${my_value}

This code defines a local_file resource that uses the templatefile function to populate the content of the file. The templatefile function takes a template file (output.tpl) and a map of variables. The template file can then use these variables to dynamically generate the content of the output file.

3. Using Python Scripting:

import json
import subprocess

# Execute terraform output command and capture the output
output = subprocess.check_output(["terraform", "output", "-json"])

# Parse the JSON output
data = json.loads(output)

# Extract the desired output value
my_value = data['my_output_value']['value']

# Write the output value to a file
with open("output.txt", "w") as f:
  f.write(f"My output value is: {my_value}")

This script uses the subprocess module to execute the terraform output -json command and capture its output. It then uses the json module to parse the JSON output and extract the desired value. Finally, it writes the value to a file named output.txt.

Remember:

  • Replace my_output_value with the actual name of your output variable.
  • Choose the method that best suits your needs and security requirements.
  • Be mindful of storing sensitive information in output files.

Additional Notes

General Considerations:

  • Idempotency: Ensure your methods for saving output are idempotent, meaning they produce the same result even when run multiple times. This is crucial for predictable infrastructure management.
  • Error Handling: Implement error handling in your scripts to gracefully manage situations where terraform output might fail or return unexpected data.
  • Output Formatting: Consider how you want to format the output in the file (plain text, JSON, YAML, etc.) based on how it will be consumed later.

Specific to local_file Resource:

  • Lifecycle Management: Be aware of the lifecycle of the local_file resource. Changes to the template or output values will trigger a recreation of the file.
  • Alternative Provisioners: While not directly related to output, consider using provisioners like file or remote-exec with the local_file resource for more complex file operations after creation.

Security Best Practices:

  • Avoid Storing Secrets: Never store sensitive information like API keys, passwords, or access tokens directly in output files. Use secure storage mechanisms like HashiCorp Vault or environment variables.
  • File Permissions: Set appropriate file permissions on output files to restrict access and prevent unauthorized modifications.

Beyond Workarounds:

  • Terraform Modules: Design reusable Terraform modules that expose relevant output values, promoting consistency and reducing the need for manual output extraction.
  • Configuration Management Tools: Integrate Terraform with configuration management tools like Ansible, Chef, or Puppet to utilize output values for further infrastructure provisioning and management.

Example Use Cases:

  • Dynamic Inventory: Generate inventory files for configuration management tools based on Terraform-provisioned resources.
  • Application Configuration: Create configuration files for applications deployed on top of Terraform-managed infrastructure.
  • Monitoring and Logging: Extract relevant output values (e.g., IP addresses, hostnames) to configure monitoring and logging systems.

Summary

While Terraform doesn't directly support saving output values to files during terraform apply, here are three common workarounds:

1. Shell Redirection:

  • Simplest approach: Use terraform output my_output_value > output.txt after running terraform apply.

2. Terraform Resources:

  • More integrated: Define a local_file resource and populate its content using templatefile or template_cloudinit_config with desired output values.

3. Scripting:

  • Flexible and powerful: Parse terraform output using scripts (e.g., Bash, Python) and write extracted values to a file.

Key Considerations:

  • Security: Avoid storing sensitive information in output files.
  • Plan Files: Handle terraform plan output files (created with terraform plan -out=planfile) with care due to potential sensitivity.
  • Advanced Options: Explore tools like terragrunt or CI/CD systems for enhanced output management and automation.

Conclusion

In conclusion, while Terraform doesn't natively support saving output values to files during terraform apply, you can employ workarounds like shell redirection, the local_file resource with templates, or scripting to achieve this. When choosing a method, prioritize security by avoiding storing sensitive data in output files and setting appropriate file permissions. For more robust output management and automation, consider integrating Terraform with tools like terragrunt or CI/CD systems. By understanding these techniques and considerations, you can effectively leverage Terraform's output values for various use cases, such as dynamic inventory generation, application configuration, and monitoring setup.

References

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