Format, Validate, and Terraform Test

Overview

Gate infrastructure changes with terraform fmt, terraform validate, terraform test, and static analysis (tflint) before any plan reaches production apply.

Never discover broken modules at apply time. CI should format-check, validate, lint, and run module tests on every pull request. terraform test exercises plan/apply-style scenarios with assertions; Terratest covers deeper Go-based integration when you need real cloud smoke checks. Policy validation sits beside these gates so unsafe plans fail before merge.

This is a core tutorial in Module 14 · Testing & Validation of the REBASH Academy Terraform for Cloud & DevOps Engineers series — written for Cloud, DevOps, Platform, and SRE engineers.

Prerequisites

• Terraform Cloud and HCP Terraform

Learning Objectives

By the end of this tutorial, you will be able to:

• Use terraform fmt -check and terraform validate correctly
• Author a *.tftest.hcl with run and assert
• Outline Terratest vs native terraform test
• Run tflint as static analysis
• Place policy validation in the same PR gate

Architecture

This topic’s control points and relationships are shown below.

Theory

What it is

Testing and validation for Terraform means proving configuration is consistent and modules behave as contracted before privileged apply. Core layers:

Gate	What it catches
terraform fmt	Style drift
terraform validate	Invalid references / types (after init)
tflint	Provider-aware smells, naming, deprecated patterns
terraform test	Module contract via plan/apply + asserts
Terratest	Go tests that often hit real APIs
Policy (OPA/Sentinel)	Organisational “must not” rules on plans

fmt rewrites HCL to canonical style; CI uses -check -recursive so unformatted files fail the build. validate needs providers installed — always init first. Native terraform test uses *.tftest.hcl with run blocks (command = plan or apply) and assert conditions. Terratest is a Go library for richer integration suites. Policy validation evaluates planned JSON against rules; it complements unit-style module tests rather than replacing them.

Why it matters

Template typos and wrong module outputs should fail in CI, not on Friday deploy. A pipeline that formats, validates, lints, and tests turns Terraform PRs into reviewable artefacts. Static analysis catches classes of mistakes validate ignores (for example AWS resource naming or unused declarations via plugins). Together these gates reduce mean time to detect packaging defects and protect remote state from merges that would destroy or misconfigure production.

How it works

Recommended gate order:

1. terraform fmt -check -recursive — fail on style drift.
2. terraform init -backend=false (or full init) then terraform validate.
3. tflint --recursive (with appropriate plugins) — fail on rule violations.
4. terraform test in module directories — assert outputs and planned behaviour.
5. Optional: Terratest in a separate job for integration against a sandbox account.
6. Policy check on terraform show -json plan.tfplan before apply approval.

Treat plan JSON and test failure messages as review surfaces: reviewers skim destroys, replaces, and failed asserts the same way they review application test output.

Key concepts and comparisons

Layer	Offline / local?	Cloud credentials?
fmt / validate	Yes (after provider download)	Usually no
tflint	Yes	No
terraform test (local provider)	Yes	No
terraform test (cloud resources)	No	Often yes
Terratest integration	No	Yes (sandbox)
Policy on plan	Needs a plan	Depends on backend

Common pitfalls

• Relying only on fmt — formatting never proved a module correct.
• Running validate before init — schema checks need providers.
• Assuming terraform test replaces policy — tests assert your contract; policy asserts org rules.
• Flaky Terratest against shared accounts without cleanup or locking.
• Skipping lint for “tiny” variable renames that break call sites.

Hands-on Lab

Create a workspace for this tutorial.

mkdir -p ~/rebash-terraform/module-14/modules/hello && cd ~/rebash-terraform/module-14/modules/hello

Focus: Run fmt, validate, and a simple terraform test

Step 1 – Create configuration and a test file

cat > main.tf <<'EOF'
terraform {
  required_providers {
    local = { source = "hashicorp/local", version = "~> 2.5" }
  }
}
variable "prefix" {
  type = string
}
resource "local_file" "out" {
  filename = "${path.module}/${var.prefix}.txt"
  content  = var.prefix
}
EOF
mkdir -p tests
cat > tests/basic.tftest.hcl <<'EOF'
run "applies_with_prefix" {
  command = apply
  variables {
    prefix = "ok"
  }
  assert {
    condition     = local_file.out.content == "ok"
    error_message = "content should match prefix"
  }
}
EOF
terraform init
terraform fmt
terraform validate

Step 2 – Execute terraform test and a normal apply

terraform test
terraform apply -auto-approve -var=prefix=lab
cat lab.txt

Final step – Cleanup note

terraform destroy -auto-approve
# Workspace kept for notes; remove with: rm -rf "$(pwd)" when finished

Validation

• Lab commands run under ~/rebash-terraform/module-14/modules/hello/
• You can explain each Theory section in your own words
• You used modern tooling where it applies to this topic
• You can describe one production failure mode for this topic

Code Walkthrough

Production practice for Format, Validate, and Terraform Test always combines:

1. Inspect before you change (status, plan, logs, dry-run)
2. Prefer reversible, documented changes (Git, IaC, drop-ins, version pins)
3. Capture evidence (command output, pipeline logs) for handovers
4. Prefer current tools and APIs over legacy shortcuts
5. Least privilege — escalate credentials only when required

Keep runbooks short enough to follow under pressure. Automate checks; keep humans for judgement.

Security Considerations

• Treat credentials and tokens for terraform as privileged — never commit them
• Prefer short-lived auth (OIDC, roles, SSO) over long-lived keys
• Validate blast radius before apply/deploy/delete operations
• Restrict who can approve production changes
• Collect audit logs; limit who can read sensitive traces

Common Mistakes

Relying only on fmt — formatting never proved a module correct.

Validate assumptions against the Theory section and official docs before changing production.

Running validate before init — schema checks need providers.

Lab shortcuts (open security groups, admin roles, skip approvals) must not ship unchanged.

Changing production without a rollback path

Always know how to revert (previous artefact, prior release, state rollback, DNS failback).

Best Practices

• Encode Format, Validate, and Terraform Test changes as code and review them in pull requests
• Pin versions (images, modules, actions, provider plugins)
• Separate environments with clear promotion gates
• Alert on symptoms with runbooks attached
• Destroy lab resources; tag everything with owner and expiry where possible

Troubleshooting

Symptom	Likely cause	Fix
Auth / permission denied	Wrong identity, policy, or scope	Check caller identity, roles, and least-privilege policies
Timeout / no route	Network, DNS, security group, or endpoint	Trace path, DNS, and allow-lists before retrying
Drift / unexpected plan	Manual change or wrong state/workspace	Reconcile desired vs actual; avoid click-ops on managed resources
Pipeline/job red	Flaky step, cache, or missing secret	Read failing step logs; bisect recent workflow/config changes
Cost spike	Idle load balancer, NAT, oversized compute	Inventory billable resources; stop/delete labs promptly

Summary

Format, Validate, and Terraform Test is essential for Cloud and DevOps engineers working with terraform. Practise the lab until the inspection and change path is muscle memory, then continue the track.

Interview Questions

1. What does terraform fmt guarantee?
2. How does terraform test differ from only running plan in CI?
3. What belongs in a minimal module test?
4. Why should format and validate gate merges?
5. What cannot validate catch that plan still might reveal?

Sample answer — question 2

terraform test can apply assertions against real or mocked runs, catching behavioural regressions beyond syntax. Plan alone may miss output/contract mistakes.

Sample answer — question 4

fmt/validate stop noise and basic errors early. Skipping them slows reviews and lets broken modules reach later, costlier pipeline stages.

Related Tutorials

• Course overview
• Terraform Security and Secrets

References

• terraform test · tflint · Terratest