If you are like me, you want your templates to pass muster and therefore usually pass them through some type of verification tool beyond the human eye, which is a poor verification tool by the way. Especially without caffiene.

As much as I love hunting config files for missing comas and quotation marks, or an out of alignment issue. I would rather spend the time thinking about the content of the template, rather the formatting.

So have I ventured to find a few solutions to make the hunt easier.

Install cfn-lint

Installing cfn-lint is usually a few keystrokes away using your favourite package manager.

Linux

If you are on a Debian-based Linux

apt install cfn-lint

or if you run a Red Hat based system

yum install cfn-lint

or Arch Linux (umm, okay, I can see it)

sudo pacman -S python-cfn-lint

or Gentoo Linux (what the hell? why?)

ebuild dev-python/cfn-python-lint

macOS

brew install cfn-lint

FreeBSD

pkg install cfn-lint

This should get cfn-lint installed to your system. Verify.

$ cfn-lint -v
cfn-lint 0.48.2

Command line

Using this script you can check your CloudFormation templates until the cows come home. But, that is going to get old real quick. And you will want to either add an extension to your editor or at the very least add the pre-commit hooks to prevent commits with templates that don’t pass.

Issues can be output in different formats. There are parseable, json, junit, and pretty formats. For example, if you just want parse output in a script or command.

$ cfn-lint --format parseable template.yml -f parseable
template.yml:115:3:115:15:W3011:Both UpdateReplacePolicy and DeletionPolicy are needed to protect Resources/ConfigBucket from deletion

Or if you want JSON output

$ cfn-lint -t template.yml -f json
[
    {
        "Filename": "templates.yml",
        "Level": "Warning",
        "Location": {
            "End": {
                "ColumnNumber": 15,
                "LineNumber": 115
            },
            "Path": [
                "Resources",
                "ConfigBucket"
            ],
            "Start": {
                "ColumnNumber": 3,
                "LineNumber": 115
            }
        },
        "Message": "Both UpdateReplacePolicy and DeletionPolicy are needed to protect Resources/ConfigBucket from deletion",
        "Rule": {
            "Description": "Both UpdateReplacePolicy and DeletionPolicy are needed to protect resources from deletion",
            "Id": "W3011",
            "ShortDescription": "Check resources with UpdateReplacePolicy/DeletionPolicy have both",
            "Source": "https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-attribute-deletionpolicy.html"
        }
    }
]

Visual Studio Code & Codium

For the people that like to do things away from the command line, CloudFormation Linter is an extension that can add CodeLens details about template issues to VSC.

I couldn’t find it in the Codium marketplace, so I had to download the VISX file and install manually. VS Code should be a straight click Install of the extension from the Microsoft Visual Studio Code marketplace.

Git Pre Commit Hooks

Install

brew install pre-commit

Or from Python

pip install pre-commit

Repository Config

You can setup git repository pre-commit hooks to run cfn-lint against the file(s) in the commit. Of course, this requires that pre-commit be installed to the system.

# .pre-commit-config.yaml
repos:
# cfn-python-lint
- repo: https://github.com/aws-cloudformation/cfn-python-lint
  rev: v0.48.2
  hooks:
    - id: cfn-python-lint
      files: templates/.*\.(json|yml|yaml)$

The files entry uses regexp to math path and file names.

When you commit the pre-hook will run the templates through cfn-lint, preventing any templates that don’t pass cfn-lint.

$ git commit -m "Extremely nice commit message!"
AWS CloudFormation Linter................................................Passed

Happy Linting!