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Hi everyone — this lesson covers linting and validating AWS CloudFormation templates using cfn-lint. Follow the steps below to catch errors early, enforce best practices, and integrate checks into your editor and CI/CD pipelines.

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What is linting?

  • Linting scans code or templates for errors, bad practices, and formatting issues — think of it as a spell-checker for CloudFormation.
  • It improves readability, enforces consistency, and reduces the chance of deployment-time failures.

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What is validation?

  • Validation checks that a template follows AWS rules: correct structure, required fields, and correct value types.
  • CloudFormation performs server-side validation when you upload a template, but that only happens at deployment time.
  • Use local validation and linting (cfn-lint) to find issues earlier — before uploading or deploying.

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About cfn-lint

  • cfn-lint is an AWS-supported linter for CloudFormation templates (YAML and JSON).
  • It runs spec-driven checks against current AWS resource specifications and provides more extensive validation and best-practice rules than CloudFormation’s server-side checks.
  • Project and docs: https://github.com/aws-cloudformation/cfn-lint

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Key advantages

AdvantageWhat it helps with
Resource/property validationIdentifies missing or invalid properties for resource types
Parameter and value checksVerifies parameter types, allowed values, and intrinsic function usage
Deprecation and spec checksFlags deprecated resources and properties against current specs
Best-practice guidanceEnforces conventions and recommended patterns
A cfn-lint branded slide showing a lint roller icon with a refresh badge. A checklist lists features: identifies missing or incorrect properties, verifies parameter types/allowed values and intrinsic functions, flags deprecated resources, and ensures alignment with AWS standards.

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Install cfn-lint

Recommended installation via pip: 
pip install cfn-lint
If your environment uses pip3: 
pip3 install cfn-lint
Confirm installation: 
cfn-lint --version

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Update AWS resource specifications

cfn-lint stores local copies of AWS resource specs. Update periodically to get the latest checks: 
cfn-lint --update-specs

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Basic usage examples

Lint individual files, directories, or multiple inputs:
ActionCommand
Lint a single templatecfn-lint template.yml
Lint a directorycfn-lint templates/
Lint multiple files/directoriescfn-lint file1.yml file2.json templates/
You can also customize rules via configuration files (.cfnlintrc) and ignore specific rule IDs when necessary.

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What to expect from cfn-lint output

  • Output typically includes: filename, line/column (when available), severity (ERROR/WARN), and a descriptive message.
  • Example scenarios: misspelled property names, invalid property values, wrong parameter types, deprecated resource usage.
Example: a simple template with a misspelled property 
AWSTemplateFormatVersion: '2010-09-09'
Resources:
  MyBucket:
    Type: AWS::S3::Bucket
    Properties:
      BucketNamee: my-bucket   # <-- misspelled property
Run: 
cfn-lint my-template.yml
Expected behavior:
  • cfn-lint reports an error pointing to the misspelled property and explains that the property is not valid for AWS::S3::Bucket. The error includes file and line information to help you correct the template quickly.

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Integration tips (editors, pre-commit, CI)

  • Visual Studio Code: install the “AWS CloudFormation Linter (cfn-lint)” extension for inline diagnostics while you edit templates.
  • Pre-commit: add cfn-lint to pre-commit hooks so templates are validated before commits.
  • CI/CD: run cfn-lint as an early pipeline stage to block invalid templates from progressing toward deployment.
  • Local practice: run cfn-lint before attempting to deploy CloudFormation stacks to catch issues early.
Example .pre-commit-config.yaml snippet: 
repos:
  - repo: https://github.com/awslabs/cfn-python-lint
    rev: v0.70.0  # use an appropriate, pinned version
    hooks:
      - id: cfn-lint
        args: ['--ignore-checks', 'E3012']  # optional
Run cfn-lint regularly (locally, in editors, and in CI) to catch syntax, property, and best-practice issues before deploying templates to AWS.

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Validation vs. linting — a quick comparison

  • CloudFormation server-side validation: ensures templates are syntactically valid and meet service constraints at deployment time.
  • cfn-lint: augments that validation with local, spec-driven checks and best-practice rules so you can detect and fix issues before deployment.

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Next steps

  • Install the VS Code extension and test cfn-lint on real templates.
  • Add cfn-lint to your pre-commit configuration and CI pipelines.
  • Explore customizing rules and creating project-specific ignore lists or rules configurations.

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