AWS CloudFormation Linter Custom Rules

While diving deep on AWS CloudFormation Linter (cfn-lint), I discovered Custom Rules support.

The linter supports the creation of custom one-line rules which compare any resource with a property using pre-defined operators.

To give it a try, I created a simple rule that enforced a naming standard for my Python Lambda functions by creating cfn_custom_rules.txt containing my custom rule.

AWS::Lambda::Function Handler EQUALS "app.lambda_handler" WARN "Lambda function handler should be app.lambda_handler"

Then when I run cfn-lint, I can include evaluation of this custom rule file.

$ cfn-lint template.yaml --custom-rules cfn_custom_rules.txt

Of course, I need this in my buildspec.yml so AWS CodePipeline and AWS CodeBuild would enforce this.

  pre_build:
    commands:
      - cfn-lint template.yaml --custom-rules cfn_custom_rules.txt
      - status=$?
      - |
        if [ $status -ne 0 ]; then 
          exit 1;
        fi

An alternative to the --custom-rules CLI option is to add a section to my .cfnlintrc.yaml to automatically enable this option and allows for easy integration with the Visual Studio Code vscode-cfn-lint extension.

# https://github.com/aws-cloudformation/cfn-lint#configuration
custom_rules: cfn_custom_rules.txt