🐶
Terraform

Terraform Local Exec: Running Multiple Commands

By Ondřej Dolanský on 01/06/2025

Learn how to troubleshoot and resolve the common issue of Terraform's inability to execute multiple commands within a local-exec provisioner.

Terraform Local Exec: Running Multiple Commands

Table of Contents

Introduction

When working with Terraform, you might need to execute local commands on your system. The local-exec provisioner provides this functionality, but structuring multi-line commands within Terraform code requires a bit of care. Here are three common approaches to effectively execute multi-line commands using local-exec:

Step-by-Step Guide

  1. Use the \n character: Embed newline characters directly within your command string.

    resource "null_resource" "example" {
  provisioner "local-exec" {
    command = "echo 'Line 1'\necho 'Line 2'"
  }
}

  2. Use a heredoc string: This is useful for longer, multi-line commands.

    resource "null_resource" "example" {
  provisioner "local-exec" {
    command = <<EOT
      echo 'Line 1'
      echo 'Line 2'
    EOT
  }
}

  3. Store commands in a separate script: For complex scenarios, create a separate script file and invoke it using local-exec.

    resource "null_resource" "example" {
  provisioner "local-exec" {
    command = "./my_script.sh"
  }
}

Important Considerations:

  • Error Handling: local-exec runs commands in a subshell, so errors might not propagate as expected. Consider error checks within your commands or script.
  • Idempotency: Ensure your commands are idempotent; running them multiple times should have the same effect as running them once.
  • Security: Be cautious about storing sensitive information directly in your Terraform code. Consider using environment variables or secure storage mechanisms.

Code Example

This code provides examples of using Terraform's local-exec provisioner for running multi-line commands while handling errors and ensuring idempotency. It shows three methods: using \n for newlines, using heredoc for multi-line commands within the Terraform code, and executing a separate script file. Each example includes explanations and considerations for error handling and idempotency. The code emphasizes best practices for managing scripts and handling potential issues during execution.

This code example demonstrates the three methods of using multi-line commands with Terraform's local-exec provisioner, incorporating error handling and idempotency considerations.

1. Using \n for Newlines:

resource "null_resource" "newline_example" {
  provisioner "local-exec" {
    # Using \n for newlines and checking for errors
    command = <<EOT
      echo "Creating directory /tmp/example"
      mkdir -p /tmp/example || echo "Directory already exists"
      echo "Line 1" > /tmp/example/output.txt
      echo "Line 2" >> /tmp/example/output.txt
    EOT
  }
}

Explanation:

  • This example creates a directory /tmp/example if it doesn't exist, ensuring idempotency.
  • It then writes two lines to a file within that directory.
  • The || operator ensures that even if the directory creation fails (because it already exists), the script continues and echoes a message.

2. Using Heredoc for Multi-line Commands:

resource "null_resource" "heredoc_example" {
  provisioner "local-exec" {
    # Using heredoc for multi-line commands
    command = <<EOT
      #!/bin/bash
      # Check if file exists before overwriting
      if [ ! -f "/tmp/example/data.txt" ]; then
        echo "Creating and writing to file"
        echo "This is some data" > /tmp/example/data.txt
      else
        echo "File already exists, skipping"
      fi
    EOT
  }
}

Explanation:

  • This example uses a heredoc to define a bash script within the command attribute.
  • The script checks if a file exists before writing to it, ensuring idempotency.

3. Executing a Separate Script:

resource "null_resource" "script_example" {
  provisioner "local-exec" {
    # Executing a separate script
    command = "./scripts/my_script.sh"
  }
}

Content of scripts/my_script.sh:

#!/bin/bash
# Example script with error handling
set -e # Exit on any error

# Create a directory if it doesn't exist
mkdir -p /tmp/example || true

# Perform some action
echo "Executing script..."
echo "This is from the script" > /tmp/example/script_output.txt

# Example of error handling
if ! some_command; then
  echo "Error: some_command failed" >&2
  exit 1
fi

Explanation:

  • This example executes a separate bash script located at scripts/my_script.sh.
  • The script includes error handling using set -e to exit on any error.
  • It also demonstrates checking the success of a command and exiting with an error code if it fails.

Important Considerations:

  • Error Handling: The examples demonstrate different ways to handle errors within your scripts, ensuring that Terraform is aware of failures.
  • Idempotency: The examples ensure that running the local-exec provisioner multiple times has the same effect as running it once.
  • Security: Remember to avoid storing sensitive information directly in your scripts. Use environment variables or secure storage mechanisms when necessary.

Additional Notes

  • Alternatives to local-exec:
    • Terraform Functions: For simpler string manipulations or file operations, explore built-in Terraform functions before resorting to local-exec.
    • External Data Sources: If you need to fetch data dynamically during your Terraform run, external data sources offer a more robust and integrated approach.
    • Configuration Management Tools: For complex provisioning tasks or software installations, dedicated configuration management tools like Ansible or Puppet might be more suitable.
  • Debugging:
    • Verbose Output: Use the -verbose flag with Terraform to get more detailed output from local-exec commands.
    • Temporary Files: For debugging complex scripts, consider writing output to temporary files for inspection.
  • Best Practices:
    • Modularity: Break down large scripts into smaller, reusable modules for better organization and maintainability.
    • Version Control: Store your scripts in version control alongside your Terraform code to track changes and collaborate effectively.
    • Testing: Whenever possible, write tests for your scripts to ensure they function as expected and prevent regressions.

By understanding these nuances and following best practices, you can leverage the local-exec provisioner effectively while maintaining the reliability and maintainability of your Terraform codebase.

Summary

This document outlines three methods for executing multi-line commands using the local-exec provisioner in Terraform:

| Method | Description

Conclusion

Choosing the right approach depends on the complexity of your commands and your project's structure. For simple commands, using newline characters or heredoc strings might suffice. However, for more involved scripts, opting for separate script files enhances readability and maintainability. Always prioritize error handling and idempotency to ensure your Terraform code remains robust and predictable. By understanding these techniques and considering the security implications, you can effectively leverage the local-exec provisioner for various automation tasks within your infrastructure as code workflows.

References

Were You Able to Follow the Instructions?

😍Love it!
😊Yes
😐Meh-gical
😞No
🤮Clickbait

Related posts

Terraform Multi-AZ Deployments on AWS

Learn how to enhance the resilience and availability of your AWS infrastructure by deploying across multiple availability zones using Terraform.

Terraform Error: Accessing aws_alb_target_group Attributes

Learn how to resolve the Terraform error "aws_alb_target_group has 'count' set, its attributes must be accessed on specific instances" with this comprehensive guide.

Using Terragrunt with Terraform Cloud: A Guide

Learn how to leverage the power of Terragrunt's modularity and code reusability within the collaborative environment of Terraform Cloud.