Terragrunt Validator

Terragrunt Validator

Overview

This skill provides comprehensive validation, linting, and testing capabilities for Terragrunt configurations. Terragrunt is a thin wrapper for Terraform/OpenTofu that provides extra tools for keeping configurations DRY (Don't Repeat Yourself), working with multiple modules, and managing remote state.

Use this skill when:

• Validating Terragrunt HCL files (*.hcl, terragrunt.hcl, terragrunt.stack.hcl)
• Working with Terragrunt Stacks (unit/stack blocks, terragrunt stack generate/run)
• Performing dry-run testing with terragrunt plan
• Linting Terragrunt/Terraform code for best practices
• Detecting and researching custom providers or modules
• Debugging Terragrunt configuration issues
• Checking dependency graphs
• Formatting HCL files
• Running security scans on infrastructure code (Trivy, Checkov)
• Generating run reports and summaries

Terragrunt Version Compatibility

This skill is designed for Terragrunt 0.93+ which includes the new CLI redesign.

CLI Command Migration Reference

Key Changes in 0.93+:

• terragrunt run --all replaces terragrunt run-all for multi-module operations
• terragrunt dag graph replaces terragrunt graph-dependencies for dependency visualization
• terragrunt hcl validate --inputs replaces validate-inputs for input validation
• HCL syntax validation via terragrunt hcl fmt --check or terragrunt hcl validate
• Full validation requires terragrunt init && terragrunt validate

If using an older Terragrunt version, some commands may need adjustment.

Core Capabilities

1. Comprehensive Validation Suite

Run the comprehensive validation script to perform all checks at once:

bash scripts/validate_terragrunt.sh [TARGET_DIR]

What it validates:

• HCL formatting (terragrunt hcl fmt --check)
• HCL input validation (terragrunt hcl validate --inputs)
• Terragrunt configuration syntax
• Terraform configuration validation
• Linting with tflint
• Security scanning with Trivy (or legacy tfsec)
• Dependency graph validation
• Dry-run planning

Environment variables:

• SKIP_PLAN=true - Skip terragrunt plan step
• SKIP_SECURITY=true - Skip security scanning (Trivy/tfsec)
• SKIP_LINT=true - Skip tflint linting
• TG_STRICT_MODE=true - Enable strict mode (errors on deprecated features)

Example usage:

# Full validation
bash scripts/validate_terragrunt.sh ./infrastructure/prod

# Skip plan generation (faster)
SKIP_PLAN=true bash scripts/validate_terragrunt.sh ./infrastructure

# Only validate, skip linting and security
SKIP_LINT=true SKIP_SECURITY=true bash scripts/validate_terragrunt.sh

2. Custom Provider and Module Detection

Use the detection script to identify custom providers and modules that may require documentation lookup:

python3 scripts/detect_custom_resources.py [DIRECTORY] [--format text|json]

What it detects:

• Custom Terraform providers (non-HashiCorp)
• Remote modules (Git, Terraform Registry, HTTP)
• Provider versions
• Module versions and sources

Output formats:

• text - Human-readable report with search recommendations
• json - Machine-readable format for automation

When custom resources are detected:

CRITICAL: You MUST look up documentation for EVERY detected custom resource (both providers AND modules). Do NOT skip any. This is mandatory, not optional.

1. For custom providers:

   • Option A - WebSearch: Search for provider documentation
     • Query format: "{provider_source} terraform provider documentation version {version}"
     • Example: "mongodb/mongodbatlas terraform provider documentation version 1.14.0"
   • Option B - Context7 MCP (Preferred): Use Context7 for structured documentation lookup
     • Step 1: Resolve library ID: mcp__context7__resolve-library-id with provider name (e.g., "datadog terraform provider")
     • Step 2: REQUIRED - Fetch documentation: mcp__context7__get-library-docs with the resolved library ID
     • Use topic: "authentication" or topic: "configuration" for targeted docs

2. For custom modules (EQUALLY IMPORTANT - DO NOT SKIP):

   • Terraform Registry modules:
     • Use Context7: mcp__context7__resolve-library-id with module name (e.g., "terraform-aws-modules vpc")
     • Then fetch docs with mcp__context7__get-library-docs
     • Or visit https://registry.terraform.io/modules/{source}/{version}
   • Git modules: Use WebSearch with the repository URL to find README or documentation
   • HTTP modules: Investigate the source URL for documentation
   • Pay attention to version compatibility with your Terraform/Terragrunt version

3. Documentation lookup workflow (MANDATORY for ALL detected resources):

   a) Run detect_custom_resources.py
   b) For EACH custom provider/module:
      - Note the exact version
      - Use Context7 MCP:
        1. mcp__context7__resolve-library-id with libraryName: "{provider/module name}"
        2. mcp__context7__get-library-docs with:
           - context7CompatibleLibraryID: "{resolved ID}"
           - topic: "authentication" (for auth requirements)
           - topic: "configuration" (for setup requirements)
      - OR use WebSearch with version-specific queries
      - Review documentation for:
        * Required configuration blocks
        * Authentication requirements (API keys, credentials)
        * Available resources/data sources
        * Known issues or breaking changes in the version
   c) Apply learnings to validation/troubleshooting
   d) Document findings if issues are encountered
   

Example using Context7 MCP:

# 1. Detect custom resources
python3 scripts/detect_custom_resources.py ./infrastructure
# Output: Provider: datadog/datadog, Version: 3.30.0

# 2. Resolve library ID
mcp__context7__resolve-library-id with libraryName: "datadog terraform provider"
# Result: /datadog/terraform-provider-datadog

# 3. Fetch authentication docs (REQUIRED)
mcp__context7__get-library-docs with:
  context7CompatibleLibraryID: "/datadog/terraform-provider-datadog"
  topic: "authentication"

# 4. Fetch configuration docs
mcp__context7__get-library-docs with:
  context7CompatibleLibraryID: "/datadog/terraform-provider-datadog"
  topic: "configuration"

Example using WebSearch:

# Detect custom resources
python3 scripts/detect_custom_resources.py ./infrastructure

# Then search for documentation:
# WebSearch: "datadog terraform provider 3.30.0 authentication configuration"
# WebSearch: "datadog terraform provider api_key app_key setup"

3. Step-by-Step Validation

For manual or granular validation, use these individual commands:

Format Validation

cd <target-directory>
terragrunt hcl fmt --check

# To auto-fix formatting
terragrunt hcl fmt

Configuration Validation

# Check HCL syntax and formatting
terragrunt hcl fmt --check

# Note: In Terragrunt 0.93+, for deeper configuration validation,
# initialize and validate (requires actual resources/credentials):
# terragrunt init && terragrunt validate

Terraform Validation

# Initialize if needed
terragrunt init

# Validate
terragrunt validate

Linting with tflint

# Initialize tflint (if .tflint.hcl exists)
tflint --init

# Run linting
tflint --recursive

Security Scanning with Trivy (Recommended)

Note: tfsec has been merged into Trivy and is no longer actively maintained. Use Trivy for all new projects.

# Using Trivy (recommended)
trivy config . --severity HIGH,CRITICAL

# With tfvars file
trivy config --tf-vars terraform.tfvars .

# Exclude downloaded modules
trivy config --tf-exclude-downloaded-modules .

# Legacy: Using tfsec (deprecated)
tfsec . --soft-fail

Alternative: Security Scanning with Checkov

# Scan directory
checkov -d . --framework terraform

# Scan with specific checks
checkov -d . --check CKV_AWS_21

# Output as JSON
checkov -d . --output json

Dependency Graph Validation

# Note: graph-dependencies command replaced with 'dag graph' in Terragrunt 0.93+
# Validate and display dependency graph
terragrunt dag graph

# Visualize dependencies (requires graphviz)
terragrunt dag graph | dot -Tpng > dependencies.png

Dry-Run Planning

# Single module
terragrunt plan

# All modules (new syntax - Terragrunt 0.93+)
terragrunt run --all plan

# Legacy syntax (deprecated)
# terragrunt run-all plan

4. Multi-Module Operations

For projects with multiple Terragrunt modules, use run --all (replaces deprecated run-all):

# Validate all modules
terragrunt run --all validate

# Plan all modules
terragrunt run --all plan

# Apply all modules
terragrunt run --all apply

# Destroy all modules
terragrunt run --all destroy

# Format all HCL files
terragrunt hcl fmt

# With parallelism
terragrunt run --all plan --parallelism 4

# With strict mode (errors on deprecated features)
terragrunt --strict-mode run --all plan

# Or via environment variable
TG_STRICT_MODE=true terragrunt run --all plan

5. HCL Input Validation (New in 0.93+)

Validate that all required inputs are set and no unused inputs exist:

# Validate inputs
terragrunt hcl validate --inputs

# Show paths of invalid files
terragrunt hcl validate --show-config-path

# Combine with run --all to exclude invalid files
terragrunt run --all plan --queue-excludes-file <(terragrunt hcl validate --show-config-path || true)

6. Strict Mode

Enable strict mode to catch deprecated features early:

# Via CLI flag
terragrunt --strict-mode run --all plan

# Via environment variable (recommended for CI/CD)
export TG_STRICT_MODE=true
terragrunt run --all plan

# Check available strict controls
terragrunt info strict

Specific Strict Controls:

For finer-grained control, use --strict-control to enable specific controls:

# Enable specific strict controls
terragrunt run --all plan --strict-control cli-redesign --strict-control deprecated-commands

# Via environment variable (comma-separated)
TG_STRICT_CONTROL='cli-redesign,deprecated-commands' terragrunt run --all plan

# Available strict controls:
# - cli-redesign: Errors on deprecated CLI syntax
# - deprecated-commands: Errors on deprecated commands (run-all, hclfmt, etc.)
# - root-terragrunt-hcl: Errors when using root terragrunt.hcl (use root.hcl instead)
# - skip-dependencies-inputs: Improves performance by not reading dependency inputs
# - bare-include: Errors on bare include blocks (use named includes)

7. New CLI Commands (0.93+)

Render Configuration

# Render configuration to JSON
terragrunt render --json

# Render and write to file
terragrunt render --json --write

# Output goes to terragrunt.rendered.json

Info Print (replaces terragrunt-info)

# Get contextual information about current configuration
terragrunt info print

# Output includes:
# - config_path
# - download_dir
# - terraform_binary
# - working_dir

Find and List Units

# Find all units/stacks in directory
terragrunt find

# Output as JSON
terragrunt find --json

# Include dependency information
terragrunt find --json --dag

# List units (simpler output)
terragrunt list

Run Summary and Reports

# Run with summary output (default in newer versions)
terragrunt run --all plan

# Disable summary output
terragrunt run --all plan --summary-disable

# Generate detailed report file
terragrunt run --all plan --report-file=report.json

# CSV format report
terragrunt run --all plan --report-file=report.csv

8. Terragrunt Stacks (GA in v0.78.0+)

Terragrunt Stacks provide declarative infrastructure generation using terragrunt.stack.hcl files.

Stack File Structure

# terragrunt.stack.hcl
locals {
  environment = "dev"
  aws_region  = "us-east-1"
}

# Define a unit (generates a single terragrunt.hcl)
unit "vpc" {
  source = "git::git@github.com:acme/infra-catalog.git//units/vpc?ref=v0.0.1"
  path   = "vpc"

  values = {
    environment = local.environment
    cidr        = "10.0.0.0/16"
  }
}

unit "database" {
  source = "git::git@github.com:acme/infra-catalog.git//units/database?ref=v0.0.1"
  path   = "database"

  values = {
    environment = local.environment
    vpc_path    = "../vpc"
  }
}

# Include reusable stacks
stack "monitoring" {
  source = "git::git@github.com:acme/infra-catalog.git//stacks/monitoring?ref=v0.0.1"
  path   = "monitoring"

  values = {
    environment = local.environment
  }
}

Stack Commands

# Generate stack (creates .terragrunt-stack directory)
terragrunt stack generate

# Generate stack without validation
terragrunt stack generate --no-stack-validate

# Run command on all stack units
terragrunt stack run plan
terragrunt stack run apply

# Clean generated stack directories
terragrunt stack clean

# Get stack outputs
terragrunt stack output

Stack Validation Control

Use no_validation attribute to skip validation for specific units:

unit "experimental" {
  source = "git::git@github.com:acme/infra-catalog.git//units/experimental?ref=v0.0.1"
  path   = "experimental"

  # Skip validation for this unit (useful for incomplete/experimental units)
  no_validation = true

  values = {
    environment = local.environment
  }
}

Benefits of Stacks

• Clean working directory: Generated code in hidden .terragrunt-stack directory
• Reusable patterns: Define infrastructure patterns once, deploy many times
• Version pinning: Different environments can pin different versions
• Atomic updates: Easy rollbacks of both modules and configurations

9. Exec Command (Run Arbitrary Programs)

The exec command allows you to run arbitrary programs against units with Terragrunt context. This is useful for integrating other tools like tflint, checkov, or AWS CLI with Terragrunt's configuration.

# Run tflint with unit context (TF_VAR_ env vars available)
terragrunt exec -- tflint

# Run checkov against specific unit
terragrunt exec -- checkov -d .

# Run AWS CLI with unit's configuration
terragrunt exec -- aws s3 ls s3://my-bucket

# Run custom scripts with Terragrunt context
terragrunt exec -- ./scripts/validate.sh

# Run across all units
terragrunt run --all exec -- tflint

Key Features:

• Terragrunt loads the inputs for the unit and makes them available as TF_VAR_ prefixed environment variables
• Works with any program that can use environment variables
• Integrates with Terragrunt's authentication context (e.g., AWS profiles)
• Can be combined with run --all for multi-unit operations

Use Cases:

• Running security scanners (checkov, trivy) with unit context
• Executing linters (tflint) per unit
• Running operational commands (AWS CLI) with correct credentials
• Custom validation scripts that need Terragrunt inputs

10. Feature Flags (Production Feature)

Terragrunt supports first-class Feature Flags for safe infrastructure changes. Feature flags allow you to integrate incomplete work without risk, decouple release from deployment, and codify IaC evolution.

Defining Feature Flags

# terragrunt.hcl
feature "enable_monitoring" {
  default = false
}

feature "use_new_vpc" {
  default = true
}

inputs = {
  monitoring_enabled = feature.enable_monitoring.value
  vpc_version       = feature.use_new_vpc.value ? "v2" : "v1"
}

Using Feature Flags via CLI

# Enable a feature flag
terragrunt plan --feature enable_monitoring=true

# Enable multiple feature flags
terragrunt plan --feature enable_monitoring=true --feature use_new_vpc=false

# Via environment variable
TG_FEATURE='enable_monitoring=true' terragrunt plan

Feature Flags with run --all

# Apply feature flag across all units
terragrunt run --all plan --feature enable_monitoring=true

Benefits:

• Safe rollouts: Test changes on subset of infrastructure
• Gradual migrations: Enable new features incrementally
• A/B testing: Compare infrastructure configurations
• Emergency rollbacks: Quickly disable problematic features

11. Experiments (Opt-in Unstable Features)

Terragrunt provides an experiments system for trying unstable features before they're GA:

# Enable all experiments (not recommended for production)
terragrunt --experiment-mode run --all plan

# Enable specific experiment
terragrunt --experiment symlinks run --all plan

# Enable CAS (Content Addressable Storage) for faster cloning
terragrunt --experiment cas run --all plan

Available Experiments:

• symlinks - Support symlink resolution for Terragrunt units
• cas - Content Addressable Storage for faster Git/module cloning
• filter-flag - Advanced filtering capabilities (coming in 1.0)

Validation Workflow

Follow this workflow when validating Terragrunt configurations:

Step 0: Read Best Practices Reference (MANDATORY FIRST STEP)

You MUST read the best practices reference file BEFORE starting validation. This is not optional.

# Read the best practices reference file first
cat references/best_practices.md

This ensures you understand the patterns, anti-patterns, and checklists you will verify.

Initial Assessment

1. Understand the structure:

   tree -L 3 <infrastructure-directory>
   

2. Identify Terragrunt files:

   find . -name "*.hcl" -o -name "terragrunt.hcl"
   

3. Detect custom resources:

   python3 scripts/detect_custom_resources.py .
   

Documentation Lookup (MANDATORY for ALL detected custom resources)

CRITICAL: If ANY custom providers or modules are detected, you MUST look up documentation for EACH ONE. Do not skip any.

4. For EACH detected custom provider - look up documentation:

   • Use Context7 MCP (preferred):
     1. mcp__context7__resolve-library-id with provider name
     2. mcp__context7__get-library-docs with topic: "authentication"
     3. mcp__context7__get-library-docs with topic: "configuration"
   • OR use WebSearch: "{provider} terraform provider {version} documentation"

5. For EACH detected custom module - look up documentation:

   • Use Context7 MCP for Terraform Registry modules:
     1. mcp__context7__resolve-library-id with module name (e.g., "terraform-aws-modules vpc")
     2. mcp__context7__get-library-docs with relevant topic
   • For Git modules: Use WebSearch with repository URL
   • For HTTP modules: Investigate source URL for documentation

6. Document findings for each resource:

   • Required configuration blocks
   • Authentication requirements
   • Known issues or breaking changes in the version

Validation Execution

7. Run comprehensive validation:

   bash scripts/validate_terragrunt.sh <target-directory>
   

8. Review output for errors:

   • Format errors → Fix with terragrunt hcl fmt
   • Configuration errors → Check terragrunt.hcl syntax and inputs
   • Terraform validation errors → Check .tf files or generated configs
   • Linting issues → Review tflint output and fix
   • Security issues → Review tfsec output and address
   • Dependency errors → Check dependency blocks and paths
   • Plan errors → Review Terraform configuration and provider setup

Best Practices Check (REQUIRED - Must Complete All Checklists)

You MUST verify each checklist item below and document the result (✅ pass or ❌ fail). Incomplete verification is not acceptable.

9. Perform explicit best practices verification using references/best_practices.md:

   Configuration Pattern Checklist - verify each item:

   [ ] Include blocks: Child modules use `include "root" { path = find_in_parent_folders("root.hcl") }`
   [ ] Named includes: All include blocks have names (not bare `include {}`)
   [ ] Root file naming: Root config is named `root.hcl` (not `terragrunt.hcl`)
   [ ] Environment configs: Environment-level configs named `env.hcl` (not `terragrunt.hcl`)
   [ ] Common variables: Shared variables in `common.hcl` read via `read_terragrunt_config()`
   

   Dependency Management Checklist:

   [ ] Mock outputs: ALL dependency blocks have mock_outputs for validation
   [ ] Mock allowed commands: mock_outputs_allowed_terraform_commands includes ["validate", "plan", "init"]
   [ ] Explicit paths: Dependency config_path uses relative paths ("../vpc" not absolute)
   [ ] No circular deps: Run `terragrunt dag graph` to verify no cycles
   

   Security Checklist:

   [ ] State encryption: remote_state config has `encrypt = true`
   [ ] State locking: DynamoDB table configured for S3 backend
   [ ] No hardcoded credentials: Search for patterns like "AKIA", "password =", account IDs
   [ ] Sensitive variables: Passwords/keys use `sensitive = true` in variable blocks
   [ ] IAM roles: Provider uses assume_role instead of static credentials
   

   DRY Principle Checklist:

   [ ] Generate blocks: Provider and backend configs use `generate` blocks
   [ ] Version constraints: terragrunt_version_constraint and terraform_version_constraint set
   [ ] Reusable locals: Common values in shared files, not duplicated
   [ ] if_exists: Generate blocks use appropriate if_exists strategy
   

   Quick grep checks to run:

   # Check for hardcoded AWS account IDs
   grep -r "[0-9]\{12\}" --include="*.hcl" . | grep -v mock
   
   # Check for potential credentials
   grep -ri "password\s*=" --include="*.hcl" .
   grep -ri "api_key\s*=" --include="*.hcl" .
   
   # Check for dependencies without mock_outputs
   grep -l "dependency\s" --include="*.hcl" -r . | xargs grep -L "mock_outputs"
   
   # Check for terragrunt.hcl files in non-module directories (anti-pattern)
   find . -name "terragrunt.hcl" -not -path "*/.terragrunt-cache/*" | head -20
   

Troubleshooting

10. Common issues and resolutions:

Issue: Module not found

rm -rf .terragrunt-cache
terragrunt init

Issue: Provider authentication errors

• Check provider configuration in generated files
• Verify environment variables or credentials
• Review provider documentation from WebSearch

Issue: Dependency errors

• Check dependency paths are correct
• Ensure mock_outputs are provided for validation
• Review dependency graph with terragrunt dag graph

Issue: State locking errors

terragrunt force-unlock <LOCK_ID>

Issue: Unknown provider or module parameters

• Re-run custom resource detection
• Use WebSearch to look up current documentation
• Check version compatibility

Issue: Generate block conflicts (file already exists)

ERROR: The file path ./versions.tf already exists and was not generated by terragrunt.
Can not generate terraform file: ./versions.tf already exists

Solution: This occurs when static .tf files exist that conflict with Terragrunt's generate blocks. Either:

• Remove the conflicting static files (versions.tf, provider.tf, backend.tf)
• Or use if_exists = "skip" in the generate block to not overwrite existing files

# Remove conflicting files
rm -f versions.tf provider.tf backend.tf
rm -rf .terragrunt-cache

Issue: Root terragrunt.hcl anti-pattern warning

WARN: Using `terragrunt.hcl` as the root of Terragrunt configurations is an anti-pattern

Solution: In Terragrunt 0.93+, the root configuration file should be named root.hcl instead of terragrunt.hcl. Rename the file:

mv terragrunt.hcl root.hcl
# Update include blocks in child modules to reference root.hcl

Best Practices Integration

Reference the comprehensive best practices guide for detailed recommendations:

# Read the best practices reference
cat references/best_practices.md

Key best practices to check:

• ✅ Use include for shared configuration
• ✅ Provide mock_outputs for dependencies
• ✅ Use generate blocks for provider config
• ✅ Enable state encryption and locking
• ✅ Use environment variables for dynamic values
• ✅ Specify version constraints
• ✅ Avoid hardcoded values
• ✅ Use meaningful directory structure
• ✅ Enable security features (encryption, IAM roles)

When validating, check for anti-patterns:

• ❌ Hardcoded credentials or account IDs
• ❌ Missing mock outputs
• ❌ Overly deep directory nesting
• ❌ Duplicated configuration across modules
• ❌ Missing version constraints
• ❌ Unencrypted state

Refer to references/best_practices.md for complete examples and detailed guidance.

Tool Requirements

Required:

• terragrunt (>= 0.93.0 recommended for new CLI)
• terraform or opentofu (>= 1.6.0 recommended)

Optional but recommended:

• tflint - HCL linting
• trivy - Security scanning (replaces tfsec)
• checkov - Alternative security scanner (750+ built-in policies)
• graphviz (dot) - Dependency visualization
• jq - JSON parsing
• python3 - For custom resource detection script

Deprecated tools:

• tfsec - Merged into Trivy, no longer actively maintained

Installation commands:

# macOS
brew install terragrunt terraform tflint trivy graphviz jq

# Install Trivy (recommended security scanner)
brew install trivy

# Install Checkov (alternative security scanner)
pip3 install checkov

# Legacy tfsec (deprecated - use trivy instead)
# brew install tfsec

# Linux - Trivy
curl -sfL https://raw.githubusercontent.com/aquasecurity/trivy/main/contrib/install.sh | sh -s -- -b /usr/local/bin

# Linux - Checkov
pip3 install checkov

# Verify installations
terragrunt --version
trivy --version
checkov --version

Integration with Context7 MCP

If Context7 MCP is available, use it for provider/module documentation lookup:

1. Resolve library ID:

   mcp__context7__resolve-library-id with libraryName: "mongodb/mongodbatlas"
   

2. Get documentation:

   mcp__context7__get-library-docs with context7CompatibleLibraryID: "/mongodb/mongodbatlas"
   

This provides version-aware documentation directly, as an alternative to WebSearch.

Automated Workflows

CI/CD Integration

Example validation in CI/CD pipeline:

#!/bin/bash
# ci-validate.sh

set -e

echo "Installing dependencies..."
# Install terragrunt, terraform, tflint, tfsec

echo "Detecting custom resources..."
python3 scripts/detect_custom_resources.py . --format json > custom_resources.json

# Could integrate with automated documentation lookup here

echo "Running validation suite..."
SKIP_PLAN=true bash scripts/validate_terragrunt.sh .

echo "Validation complete!"

Pre-commit Hook

Example pre-commit hook for local development:

#!/bin/bash
# .git/hooks/pre-commit

# Format check
terragrunt hcl fmt --check || {
    echo "HCL formatting issues found. Run: terragrunt hcl fmt"
    exit 1
}

# Quick HCL syntax validation (Terragrunt 0.93+)
# Note: For full validation, use: terragrunt init && terragrunt validate
# But that requires credentials. HCL format check catches syntax errors.

echo "Pre-commit validation passed!"

Troubleshooting Guide

Debug Mode

Enable debug output for troubleshooting:

# Terragrunt debug
TERRAGRUNT_DEBUG=1 terragrunt plan

# Terraform trace
TF_LOG=TRACE terragrunt plan

Common Error Patterns

"Error: Module not found"

• Clear cache: rm -rf .terragrunt-cache
• Re-initialize: terragrunt init

"Error: Provider not found"

• Check provider configuration
• Run custom resource detection
• Use WebSearch to find correct provider source and version
• Verify required_providers block

"Error: Invalid function call"

• Check Terragrunt version compatibility
• Review function syntax in documentation

"Cycle detected in dependency graph"

• Review dependency chains
• Consider refactoring into single module
• Use data sources instead of dependencies

"Error acquiring state lock"

• Check if another process is running
• Verify DynamoDB table (for S3 backend)
• Force unlock if safe: terragrunt force-unlock <LOCK_ID>

"Error: unknown command" (Terragrunt 0.93+)

• Terragrunt 0.93+ has a new CLI with breaking changes
• Commands like render-json, validate-inputs are deprecated
• Use terragrunt run -- <command> for custom/unsupported commands
• Replace graph-dependencies with dag graph
• See: https://terragrunt.gruntwork.io/docs/migrate/cli-redesign/

Output Interpretation

Success Indicators

✅ All checks passing:

• All HCL files properly formatted
• Inputs are valid
• Terraform configuration is valid
• No linting issues
• No critical security issues
• Valid dependency graph
• Plan generated successfully

Warning Indicators

⚠️ Review needed:

• Security warnings from tfsec (non-critical)
• Linting suggestions (best practices)
• Deprecated provider features
• Missing recommended configurations

Error Indicators

✗ Must fix:

• Format errors
• Invalid inputs
• Terraform validation failures
• Circular dependencies
• Provider authentication failures
• State locking errors

Advanced Usage

Custom Validation Rules

Create custom tflint rules by adding .tflint.hcl:

plugin "terraform" {
  enabled = true
  preset  = "recommended"
}

plugin "aws" {
  enabled = true
  version = "0.27.0"
  source  = "github.com/terraform-linters/tflint-ruleset-aws"
}

rule "terraform_naming_convention" {
  enabled = true
}

Custom Security Policies

Create custom tfsec policies by adding .tfsec/config.yml:

minimum_severity: MEDIUM
exclude:
  - AWS001  # Example: exclude specific rules

Dependency Graph Analysis

Analyze complex dependency chains:

# Generate detailed graph (Terragrunt 0.93+ syntax)
terragrunt dag graph > graph.dot

# Convert to visual format
dot -Tpng graph.dot > graph.png
dot -Tsvg graph.dot > graph.svg

# Analyze for circular dependencies
grep -A5 "cycle" <(terragrunt dag graph 2>&1)

Resources

Scripts

• scripts/validate_terragrunt.sh - Comprehensive validation suite
• scripts/detect_custom_resources.py - Custom provider/module detector

References

• references/best_practices.md - Comprehensive best practices guide covering:
  • Directory structure patterns
  • DRY principles and configuration sharing
  • Dependency management
  • Security best practices
  • Testing and validation workflows
  • Common anti-patterns to avoid
  • Troubleshooting guides

External Documentation

• Terragrunt Documentation
• Terraform Best Practices
• Terraform Registry