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Docker Linter

Dockerfile Linter (written in Node.js)

Description

A Dockerfile linter that you can use to quickly check if your Dockerfile follows the best practices for building efficient Docker images. The tool uses ShellCheck analysis tool to lint code in RUN instructions. Some of the rules were inspired by Hadolint and Dockerfile reference documentation.

Installation

The linter requires Node.js to run:

npm install --global dockerlinter

Application

CLI

The linter can be used directly from the CLI:

dockerfilelinter -f <path to Dockerfile>
dockerfilelinter -f <path to Dockerfile> -s bash #default sh
dockerfilelinter -f <path to Dockerfile> -s none #disable shellcheck
dockerfilelinter -f <path to Dockerfile> -i ER0012,ER0015 #coma separated list of ignored rules
dockerfilelinter -f <path to Dockerfile> -e #return error code 1 for any error
dockerfilelinter -f <path to Dockerfile> -e warning #return error code 1 for errors with level 'warning' or higher(available levels: info, warning, error)

Docker

Docker allows you to run the linter on any type of platform. To mount the file, use the -v parameter:

docker build . -t imagename
docker run -v /tmp/files/dockerfile:/dockerfilelinter/dockerfile imagename linter -f dockerfile

Inline ignores

You can ignore rules for a specific instruction block in the Dockerfile by commenting it. The ignore comment must be applied above the instruction as # linter ignore=EF0003. The exceptions are ED and EL rules. Example:

# linter ignore=EF0003,EF0004
FROM node

YAML file with ignores

You can create YAML file "your dockerfile name".linter.yaml with list of ignored rules for specific Dockerfile or for all in folder dockerfilelinter.yaml. If you put file in the same folder where linting dockerfile is it will be auto-detected, but you can also use flag -y/--yaml to specify a path. Example:

ignored:
 - ER0012
 - ER0015

Rules

The list of rules implemented.

Rules with the E prefix come from dockerfilelinter. Implementation can be found in lib/lints.js.
Rules with the SC prefix come from ShellCheck. You can find their description in the tool's Wiki. For a more detailed description, use the Pages search tool.

Legend

ED - Error Directives EI - Error Instructions ER - Error RUN EC - Error COPY EU - Error USER EF - Error FROM EW - Error WORKDIR EE - Error EXPORT EL - Error Lines EA - Error ADD EJ - Error JSON

Rules	Description
EL0001	Invalid line
ED0001	All parser directives must be at the very top of a Dockerfile.
ED0002	Directive appears more then once.
ED0003	Directives should be lowercase.
ED0004	Parser directive will be treated as a comment.
ED0005	Missing value for directive.
ER0001	Set the `SHELL` option `-o` (`-eo` for Alpine image) pipefail before `RUN` with a pipe in.
EU0001	Last user should not be `root`.
EI0001	There can only be one instruction like `(CMD, HEALTHCHECK, ENTRYPOINT)`.
EI0002	`FROM` may only be preceded by one or more `ARG`.
EF0001	Missing `FROM`.
EC0001	`COPY --from` cannot reference its own `FROM` alias.
EC0002	`COPY --from` should reference a previously defined `FROM` alias.
EI0003	`MAINTAINER` is deprecated, instead use `LABEL`.
EJ0001	You must use double-quotes (") in JSON array.
EJ0002	`CMD` and `ENTRYPOINT` should be written in JSON form.
EJ0003	`SHELL` must be written in JSON form.
EF0002	`FROM` aliases must be unique.
EF0003	Using `latest` is prone to errors if the image will ever update.
EF0004	Always tag the version of an image explicitly.
ER0002	Delete the apt-get lists after installing something.
ER0003	Use `WORKDIR` to switch to a directory.
ER0004	Do not use sudo, consider using gosu.
ER0005	Command `(ssh, vim, shutdown, service, ps, free, top, kill, mount, ifconfig)` does not make sense in a container.
ER0006	Using `(apt-get upgrade, dist-upgrade, apk upgrade, apt install)` is not recommended.
EA0001	Use `curl` or `wget` instead, and delete files when no longer needed.
EC0003	Use `ADD` for extracting archives into a image.
ER0007	Either use `wget` or `curl`, but not both.
ER0008	Use `SHELL` to change the default shell.
ER0009	Use the `-y` switch.
ER0010	Avoid additional packages by specifying `--no-install-recommends`.
EA0002	Use `COPY` instead of `ADD` for files and folders.
EC0004	`COPY` with more then 2 arguments requires the last argument to end with `/`.
ER0011	Use the `--no-cache` switch.
ER0012	Pin versions in `apt get install`.
ER0013	Pin versions in `pip install`.
ER0014	Pin versions in `npm install`.
ER0015	Pin versions in `apk add`.
ER0016	Pin versions in `gem install`.
EI0004	Don't use `(ONBUILD,FROM,MAINTAINTER)` in `ONBUILD`.
EW0001	Use absolute `WORKDIR`.
EE0001	Valid UNIX ports range from 0 to 65535.
EI0005	Instructions should be uppercase.

Development

You can help us develop linter by suggesting new rules and reporting bugs.

Tests

To run unit tests, use the command below:

npm run test

Maintainer

Docker Linter was created and developed by Buddy, creators of delivery automation tools for web and software developers.

CI/CD

The linter was created to validate the Dockerfile syntax in Continuous Integration and Delivery processes and can be used in any CI/CE tool. Buddy natively supports the linter, allowing you to create a pipeline that will check the syntax, build the Docker image and push it to the registry in a few simple steps:

First, define when and for what refs (branchs / tags / pull requests) should the pipeline execute:

Docker Linter

Next, add the Lint Dockerfile action and select the Dockerfile to validate:

Docker Linter

Last, add the Docker build action that will build the image and push it to the registry.

Docker Linter

You can extend the pipeline to automate other processes in your workflow. For example, automatically update the image on your K8s cluster and notify the QA team in case something goes wrong:

Docker Linter