Terragrunt Validator

Terragrunt Validator

Note: This skill is designed for Terragrunt 0.93+. For version compatibility details and command migration guidance, see references/version_compatibility.md.

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). See the "Documentation Lookup" section in the Validation Workflow below for the complete mandatory process.

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)

7-11. CLI Commands and Features

Terragrunt 0.93+ includes many new CLI commands and features. For detailed documentation on:

• Render configuration and info print
• Find and list units
• Run summary and reports
• Terragrunt Stacks (GA in v0.78.0+)
• Exec command for running arbitrary programs
• Feature flags for safe infrastructure changes
• Experiments for unstable features

See references/cli_commands.md for complete usage and examples.

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.

For custom providers:

• Option A - 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
• Option B - WebSearch: Search for provider documentation
  • Query format: "{provider_source} terraform provider documentation version {version}"
  • Example: "mongodb/mongodbatlas terraform provider documentation version 1.14.0"

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

Documentation lookup workflow:

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"

Validation Execution

1. Run comprehensive validation:

   bash scripts/validate_terragrunt.sh <target-directory>
   

2. 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.

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

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

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

For detailed troubleshooting guidance including debug mode, common error patterns, and resolution steps, see references/troubleshooting.md.

Output Interpretation

For detailed guidance on interpreting validation output, success/warning/error indicators, see references/output_interpretation.md.

Advanced Usage

For advanced topics including custom validation rules, security policies, and dependency graph analysis, see references/advanced_usage.md.

Anti-Patterns

NEVER validate only the current unit in isolation

• WHY: Terragrunt dependency chains mean a unit may validate cleanly on its own but fail when its dependencies have incompatible outputs or missing inputs; isolated validation misses integration errors.
• BAD: Run terragrunt validate in a single leaf unit directory and treat a clean result as full validation.
• GOOD: Use terragrunt run --all validate from the relevant parent directory to validate all units in the dependency graph together.

NEVER skip --terragrunt-log-level debug when diagnosing HCL function errors

• WHY: HCL function evaluation errors produce cryptic messages at default log levels; debug mode shows the evaluated function calls and helps pinpoint the exact source of the failure.
• BAD: Attempt to debug path_relative_to_include() or find_in_parent_folders() errors using the default log level output.
• GOOD: Add --terragrunt-log-level debug (or set TG_LOG=debug) to get the full HCL evaluation trace and locate the offending expression.

NEVER accept terragrunt validate passing as equivalent to terraform validate

• WHY: Terragrunt's validate wraps Terraform validate but may not catch all HCL parsing errors in terragrunt.hcl includes; the two tools validate different layers of the configuration stack.
• BAD: Skip terraform validate and rely solely on terragrunt validate as the complete validation signal.
• GOOD: Run terragrunt validate for Terragrunt-level HCL and input checks, then run terraform validate (or terragrunt init && terragrunt validate) for Terraform provider schema validation.

NEVER ignore dependency mock output warnings in plan output

• WHY: Mock output substitution warnings signal that the plan result is based on placeholder values; if mock types differ from actual outputs, the plan is unreliable and the apply will fail.
• BAD: Dismiss mock_outputs substitution warnings in terragrunt plan output as expected and harmless.
• GOOD: Verify that mock output types exactly match actual dependency outputs after the dependency has been applied at least once; update mocks if types change.

References

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
• references/version_compatibility.md - Terragrunt version compatibility and CLI migration
• references/cli_commands.md - Detailed CLI commands, Stacks, feature flags, experiments
• references/troubleshooting.md - Debug mode and common error patterns
• references/output_interpretation.md - Validation output indicators
• references/advanced_usage.md - Custom rules, security policies, dependency analysis

External Documentation

• Terragrunt Documentation
• Terraform Best Practices
• Terraform Registry