Checks CloudFormation templates for practices and behaviour that could potentially be improved
AWS CloudFormation Linter
Validate AWS CloudFormation yaml/json templates against the AWS CloudFormation Resource Specification and additional checks. Includes checking valid values for resource properties and best practices.
This is an attempt to provide validation for AWS CloudFormation templates properties and their values. For values things can get pretty complicated (mappings, joins, splits, conditions, and nesting those functions inside each other) so it's a best effort to validate those values but the promise is to not fail if we can't understand or translate all the things that could be going on.
Serverless Application Model
The Serverless Application Model (SAM) is supported by the linter. The template is transformed using AWS SAM before the linter processes the template.
To get information about the SAM Transformation, run the linter with
Python 2.7+ and 3.4+ are supported.
pip install cfn-lint. If pip is not available, run
python setup.py clean --all then
python setup.py install.
brew install cfn-lint
cfn-python-lint source tree:
docker build --tag cfn-python-lint:latest .
In repository to be linted:
docker run --rm -v `pwd`:/data cfn-python-lint:latest /data/template.yaml
There are IDE plugins available to get direct linter feedback from you favorite editor:
cfn-lint -t template.yaml
Multiple files can be linted by either specifying multiple specific files:
cfn-lint template1.yaml template2.yaml
cfn-lint -t template1.yaml template2.yaml
or by using wildcards (globbing):
yaml files in
yaml files in
path and all subdirectories (recursive):
Note: Glob in Python 3.5 supports recursive searching
**/*.yaml. If you are using an earlier version of Python you will have to handle this manually (
cfn-lint will return a non zero exit if there are any issues with your template. The value is dependent on the sevirity of the issues found. For each level of discovered error
cfn-lint will use bitwise OR to determine the final exit code. This will result in these possibilities.
- 0 is no issue was found
- 2 is an error
- 4 is a warning
- 6 is an error and a warning
- 8 is an informational
- 10 is an error and informational
- 12 is an warning and informational
- 14 is an error and a warning and an informational
Specifying the template as an input stream
The template to be linted can also be passed using standard input:
cat path/template.yaml | cfn-lint -
Specifying the template with other parameters
cfn-lint -r us-east-1 ap-south-1 -- template.yaml
cfn-lint -r us-east-1 ap-south-1 -t template.yaml
From a command prompt run
cfn-lint <path to template> to run standard linting of the template.
It will look for a configuration file in the following locations (by order of preference):
.cfnlintrc.ymlin the current working directory
~/.cfnlintrcfor the home directory
In that file you can specify settings from the parameter section below.
templates: - test/fixtures/templates/good/**/*.yaml ignore_templates: - codebuild.yaml include_checks: - I
|-h, --help||Get description of cfn-lint|
|-t, --template||filename||Alternative way to specify Template file path to the file that needs to be tested by cfn-lint|
|-f, --format||format||quiet, parseable, json, junit, pretty||Output format|
|-l, --list-rules||List all the rules|
|-r, --regions||regions||[REGIONS [REGIONS ...]], ALL_REGIONS||Test the template against many regions. Supported regions|
|-b, --ignore-bad-template||ignore_bad_template||Ignores bad template errors|
|--ignore-templates||IGNORE_TEMPLATES [IGNORE_TEMPLATES ...]||Ignore templates from being scanned|
|-a, --append-rules||append_rules||[RULESPATH [RULESPATH ...]]||Specify one or more rules paths using one or more --append-rules arguments. Each path can be either a directory containing python files, or an import path to a module.|
|-i, --ignore-checks||ignore_checks||[IGNORE_CHECKS [IGNORE_CHECKS ...]]||Only check rules whose ID do not match or prefix these values. Examples: - A value of
|-e, --include-experimental||include_experimental||Whether rules that still in an experimental state should be included in the checks|
|-c, --include-checks||INCLUDE_CHECKS [INCLUDE_CHECKS ...]||Include rules whose id match these values|
|-m, --mandatory-checks||Rules to check regardless of ignore configuration|
|-x, --configure-rule||CONFIGURE_RULES [CONFIGURE_RULES ...]||Provide configuration for a rule. Format RuleId:key=value. Example: E3012:strict=false|
|-D, --debug||Specify to enable debug logging. Debug logging outputs detailed information about rules processing, useful for debugging rules.|
|-I, --info||Specify to enable logging. Outputs additional information about the template processing.|
|-u, --update-specs||Update the CloudFormation Resource Specifications. You may need sudo to run this. You will need internet access when running this command|
|-o, --override-spec||filename||Spec-style file containing custom definitions. Can be used to override CloudFormation specifications. More info here|
|-g, --build-graph||Creates a file in the same directory as the template that models the template's resources in DOT format|
|-v, --version||Version of cfn-lint|
To maintain backwards compatibility
info rules are not included by default. To include these rules you will need to include
-c I or
Template Based Metadata
Inside the root level Metadata key you can configure cfn-lint using the supported parameters.
Metadata: cfn-lint: config: regions: - us-east-1 - us-east-2 ignore_checks: - E2530
Resource Based Metadata
Inside a resources Metadata key you can configure cfn-lint to ignore checks. This will filter out failures for the resource in which the Metadata belongs. Keep in mind that
AWS::Serverless resources may lose metadata during the Serverless transform
Resources: myInstance: Type: AWS::EC2::Instance Metadata: cfn-lint: config: ignore_checks: - E3030 Properties: InstanceType: nt.x4superlarge ImageId: ami-abc1234
cfn-lint applies configurations from several sources. The rules at lower levels are overridden by those at higher levels.
- cfnlintrc configurations
- Template Metadata configurations
- CLI parameters
Certain rules support configuration properties. You can configure these rules by using
From the command line the format is
RuleId:key=value, for example:
From the cfnlintrc or Metadata section the format is
Metadata: cfn-lint: config: configure_rules: RuleId: key: value
The configurable rules have a non-empty Config entry in the table here.
Getting Started Guides
There are getting started guides available in the documentation section to help with integrating
cfn-lint or creating rules.
This linter checks the AWS CloudFormation template by processing a collection of Rules, where every rule handles a specific function check or validation of the template.
This collection of rules can be extended with custom rules using the
More information describing how rules are set up and an overview of all the Rules that are applied by this linter are documented here.
The linter follows the AWS CloudFormation Resource Specifications by default. However, for your use case specific requirements might exist. For example, within your organisation it might be mandatory to use Tagging.
The linter provides the possibility to implement these customized specifications using the
More information about how this feature works is documented here
If you'd like cfn-lint to be run automatically when making changes to files in your Git repository, you can install pre-commit and add the following text to your repositories'
repos: - repo: https://github.com/aws-cloudformation/cfn-python-lint rev: v0.42.0 # The version of cfn-lint to use hooks: - id: cfn-python-lint files: path/to/cfn/dir/.*\.(json|yml|yaml)$
- If you exclude the
files:line above, every json/yml/yaml file will be checked.
- You can see available cfn-lint versions on the releases page.
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