Comprehensive guide for writing and running Terraform tests. Use when creating test files (.tftest.hcl), writing test scenarios with run blocks, validating infrastructure behavior with assertions, mocking providers and data sources, testing module outputs and resource configurations, or troubleshooting Terraform test syntax and execution.
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Comprehensive guide for writing and running Terraform tests. Use when creating test files (.tftest.hcl), writing test scenarios with run blocks, validating infrastructure behavior with assertions, mocking providers and data sources, testing module outputs and resource configurations, or troubleshooting Terraform test syntax and execution.
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{"lifecycle-status":"active","copyright":"Copyright IBM Corp. 2026","version":"0.0.2"}
Terraform Test
Terraform's built-in testing framework validates that configuration updates don't introduce breaking changes. Tests run against temporary resources, protecting existing infrastructure and state files.
Reference Files
references/MOCK_PROVIDERS.md — Mock provider syntax, common defaults, when to use mocks (Terraform 1.7.0+ only — skip if the user's version is below 1.7)
references/CI_CD.md — GitHub Actions and GitLab CI pipeline examples
references/EXAMPLES.md — Complete example test suite (unit, integration, and mock tests for a VPC module)
Read the relevant reference file when the user asks about mocking, CI/CD integration, or wants a full example.
Core Concepts
Test file (.tftest.hcl / .tftest.json): Contains run blocks that validate your configuration
Run block: A single test scenario with optional variables, providers, and assertions
Assert block: Conditions that must be true for the test to pass
Mock provider: Simulates provider behavior without real infrastructure (Terraform 1.7.0+)
Test modes: apply (default, creates real resources) or plan (validates logic only)
File Structure
my-module/
├── main.tf
├── variables.tf
├── outputs.tf
└── tests/
├── defaults_unit_test.tftest.hcl # plan mode — fast, no resources
├── validation_unit_test.tftest.hcl # plan mode
└── full_stack_integration_test.tftest.hcl # apply mode — creates real resources
Use *_unit_test.tftest.hcl for plan-mode tests and *_integration_test.tftest.hcl for apply-mode tests so they can be filtered separately in CI.
Test File Structure
# Optional: test-wide settings
test {
parallel = true # Enable parallel execution for all run blocks (default: false)
}
# Optional: file-level variables (highest precedence, override all other sources)
variables {
aws_region = "us-west-2"
instance_type = "t2.micro"
}
# Optional: provider configuration
provider "aws" {
region = var.aws_region
}
# Required: at least one run block
run "test_default_configuration" {
command = plan
assert {
condition = aws_instance.example.instance_type == "t2.micro"
error_message = "Instance type should be t2.micro by default"
}
}
Run Block
run "test_name" {
command = plan # or apply (default)
parallel = true # optional, since v1.9.0
# Override file-level variables
variables {
instance_type = "t3.large"
}
# Reference a specific module
module {
source = "./modules/vpc" # local or registry only (not git/http)
version = "5.0.0" # registry modules only
}
# Control state isolation
state_key = "shared_state" # since v1.9.0
# Plan behavior
plan_options {
mode = refresh-only # or normal (default)
refresh = true
replace = [aws_instance.example]
target = [aws_instance.example]
}
# Assertions
assert {
condition = aws_instance.example.id != ""
error_message = "Instance should have a valid ID"
}
# Expected failures (test passes if these fail)
expect_failures = [
var.instance_count
]
}
Common Test Patterns
Validate outputs
run "test_outputs" {
command = plan
assert {
condition = output.vpc_id != null
error_message = "VPC ID output must be defined"
}
assert {
condition = can(regex("^vpc-", output.vpc_id))
error_message = "VPC ID should start with 'vpc-'"
}
}
Conditional resources
run "test_nat_gateway_disabled" {
command = plan
variables {
create_nat_gateway = false
}
assert {
condition = length(aws_nat_gateway.main) == 0
error_message = "NAT gateway should not be created when disabled"
}
}
run "test_resource_tags" {
command = plan
variables {
common_tags = {
Environment = "production"
ManagedBy = "Terraform"
}
}
assert {
condition = aws_instance.example.tags["Environment"] == "production"
error_message = "Environment tag should be set correctly"
}
assert {
condition = aws_instance.example.tags["ManagedBy"] == "Terraform"
error_message = "ManagedBy tag should be set correctly"
}
}
Data sources
run "test_data_source_lookup" {
command = plan
assert {
condition = data.aws_ami.ubuntu.id != ""
error_message = "Should find a valid Ubuntu AMI"
}
assert {
condition = can(regex("^ami-", data.aws_ami.ubuntu.id))
error_message = "AMI ID should be in correct format"
}
}
Validation rules
run "test_invalid_environment" {
command = plan
variables {
environment = "invalid"
}
expect_failures = [
var.environment
]
}
Sequential tests with dependencies
run "setup_vpc" {
command = apply
assert {
condition = output.vpc_id != ""
error_message = "VPC should be created"
}
}
run "test_subnet_in_vpc" {
command = plan
variables {
vpc_id = run.setup_vpc.vpc_id
}
assert {
condition = aws_subnet.example.vpc_id == run.setup_vpc.vpc_id
error_message = "Subnet should be in the VPC from setup_vpc"
}
}
Plan options (refresh-only, targeted)
run "test_refresh_only" {
command = plan
plan_options {
mode = refresh-only
}
assert {
condition = aws_instance.example.tags["Environment"] == "production"
error_message = "Tags should be refreshed correctly"
}
}
run "test_specific_resource" {
command = plan
plan_options {
target = [aws_instance.example]
}
assert {
condition = aws_instance.example.instance_type == "t2.micro"
error_message = "Targeted resource should be planned"
}
}
Parallel modules
run "test_networking_module" {
command = plan
parallel = true
module {
source = "./modules/networking"
}
assert {
condition = output.vpc_id != ""
error_message = "VPC should be created"
}
}
run "test_compute_module" {
command = plan
parallel = true
module {
source = "./modules/compute"
}
assert {
condition = output.instance_id != ""
error_message = "Instance should be created"
}
}
State key sharing
run "create_foundation" {
command = apply
state_key = "foundation"
assert {
condition = aws_vpc.main.id != ""
error_message = "Foundation VPC should be created"
}
}
run "create_application" {
command = apply
state_key = "foundation"
variables {
vpc_id = run.create_foundation.vpc_id
}
assert {
condition = aws_instance.app.vpc_id == run.create_foundation.vpc_id
error_message = "Application should use foundation VPC"
}
}
Cleanup ordering (S3 objects before bucket)
run "create_bucket" {
command = apply
assert {
condition = aws_s3_bucket.example.id != ""
error_message = "Bucket should be created"
}
}
run "add_objects" {
command = apply
assert {
condition = length(aws_s3_object.files) > 0
error_message = "Objects should be added"
}
}
# Cleanup destroys in reverse: objects first, then bucket
Multiple aliased providers
provider "aws" {
alias = "primary"
region = "us-west-2"
}
provider "aws" {
alias = "secondary"
region = "us-east-1"
}
run "test_with_specific_provider" {
command = plan
providers = {
aws = provider.aws.secondary
}
assert {
condition = aws_instance.example.availability_zone == "us-east-1a"
error_message = "Instance should be in us-east-1 region"
}
}
Complex conditions
assert {
condition = alltrue([
for subnet in aws_subnet.private :
can(regex("^10\\.0\\.", subnet.cidr_block))
])
error_message = "All private subnets should use 10.0.0.0/8 CIDR range"
}
Cleanup
Resources are destroyed in reverse run block order after test completion. This matters for dependencies (e.g., S3 objects before bucket). Use terraform test -no-cleanup to skip cleanup for debugging.
Running Tests
terraform test# all tests
terraform test tests/defaults.tftest.hcl # specific file
terraform test -filter=test_vpc_configuration # by run block name
terraform test -test-directory=integration-tests # custom directory
terraform test -verbose # detailed output
terraform test -no-cleanup # skip resource cleanup
Best Practices
Naming: *_unit_test.tftest.hcl for plan mode, *_integration_test.tftest.hcl for apply mode
Test naming: Use descriptive run block names that explain the scenario being tested
Default to plan: Use command = plan unless you need to test real resource behavior
Use mocks for external dependencies — faster and no credentials needed (see references/MOCK_PROVIDERS.md)
Error messages: Make them specific enough to diagnose failures without running the test again
Negative tests: Use expect_failures to verify validation rules reject bad inputs
Variable coverage: Test different variable combinations to validate all code paths — test variables have the highest precedence and override all other sources
Module sources: Test files only support local paths and registry modules — not git or HTTP URLs
Parallel execution: Use parallel = true for independent tests with different state files
Cleanup: Integration tests destroy resources in reverse run block order automatically; use -no-cleanup for debugging
CI/CD: Run unit tests on every PR, integration tests on merge (see references/CI_CD.md)
Troubleshooting
Issue
Solution
Assertion failures
Use -verbose to see actual vs expected values
Missing credentials
Use mock providers for unit tests
Unsupported module source
Convert git/HTTP sources to local modules
Tests interfering
Use state_key or separate modules for isolation
Slow tests
Use command = plan and mocks; run integration tests separately
