Haskell Dockerfile Linter

A smarter Dockerfile linter that helps you build best practice Docker images. The linter is parsing the Dockerfile into an AST and performs rules on top of the AST. It is standing on the shoulders of ShellCheck to lint the Bash code inside RUN instructions.

🌐 Check the online version on hadolint.github.io/hadolint

How to use

You can run hadolint locally to lint your Dockerfile.

hadolint <Dockerfile>
hadolint --ignore DL3003 --ignore DL3006 <Dockerfile> # exclude specific rules
hadolint --trusted-registry my-company.com:500 <Dockerfile> # Warn when using untrusted FROM images

Docker comes to the rescue to provide an easy way how to run hadolint on most platforms. Just pipe your Dockerfile to docker run:

$ docker run --rm -i hadolint/hadolint < Dockerfile
# or
$ docker run --rm -i ghcr.io/hadolint/hadolint < Dockerfile

or if you are using Windows PowerShell:

> cat .\Dockerfile | docker run --rm -i hadolint/hadolint

Install

You can download prebuilt binaries for OSX, Windows and Linux from the latest release page. However, if it doesn't work for you, please fall back to Docker, brew or source installation.

If you are on OSX you can use brew to install hadolint.

brew install hadolint

On Windows you can use scoop to install hadolint.

scoop install hadolint

As shown before, hadolint is available as a Docker container:

$ docker pull hadolint/hadolint
# or
$ docker pull ghcr.io/hadolint/hadolint

If you need a Docker container with shell access, use the Debian or Alpine variants of the Docker image:

$ docker pull hadolint/hadolint:latest-debian
$ docker pull hadolint/hadolint:latest-alpine
# or
$ docker pull ghcr.io/hadolint/hadolint:latest-debian
$ docker pull ghcr.io/hadolint/hadolint:latest-alpine

You can also build hadolint locally. You need Haskell and the stack build tool to build the binary.

git clone https://github.com/hadolint/hadolint
cd hadolint
stack install

Configure

hadolint supports specifying the ignored rules using a configuration file. The configuration file should be in yaml format. This is one valid configuration file as an example:

ignored:
  - DL3000
  - SC1010

Additionally, hadolint can warn you when images from untrusted repositories are being used in Dockerfiles, you can append the trustedRegistries keys to the configuration file as shown below:

ignored:
  - DL3000
  - SC1010

trustedRegistries:
  - docker.io
  - my-company.com:5000

If you want to override the severity of specific rules, you can do that too:

override:
  error:
    - DL3001
    - DL3002
  warning:
    - DL3042
    - DL3033
  info:
    - DL3032
  style:
    - DL3015

Configuration files can be used globally or per project. By default, hadolint will look for a configuration file in the current directory with the name .hadolint.yaml or .hadolint.yml

The global configuration file should be placed in the folder specified by XDG_CONFIG_HOME, with the name hadolint.yaml or hadolint.yml. In summary, the following locations are valid for the configuration file, in order or preference:

• $PWD/.hadolint.yaml
• $XDG_CONFIG_HOME/hadolint.yaml
• ~/.config/hadolint.yaml

In windows, the %LOCALAPPDATA% environment variable is used instead of XDG_CONFIG_HOME

Additionally, you can pass a custom configuration file in the command line with the --config option

hadolint --config /path/to/config.yaml Dockerfile

To pass a custom configuration file (using relative or absolute path) to a container, use the following command:

$ docker run --rm -i -v /your/path/to/hadolint.yaml:/.config/hadolint.yaml hadolint/hadolint < Dockerfile
# or
$ docker run --rm -i -v /your/path/to/hadolint.yaml:/.config/hadolint.yaml ghcr.io/hadolint/hadolint < Dockerfile

Inline ignores

It is also possible to ignore rules by using a special comment directly above the Dockerfile instruction you want to make an exception for. Ignore rule comments look like # hadolint ignore=DL3001,SC1081. For example:

# hadolint ignore=DL3006
FROM ubuntu

# hadolint ignore=DL3003,SC1035
RUN cd /tmp && echo "hello!"

Inline ignores will only work if place directly above the instruction.

Linting Labels

Hadolint has the ability to check that specific labels be present and conform to a predefined label schema. First a label schema must be defined either via commandline:

$ hadolint --require-label author:text --require-label version:semver Dockerfile

or via config file:

label-schema:
  author: text
  contact: email
  created: rfc3339
  version: semver
  documentation: url
  git-revision: hash
  license: spdx

The value of a label can be either of text, url, semver, hash or rfc3339:

Schema	Description
text	Anything
rfc3339	A time, formatted according to RFC 3339
semver	A semantic version
url	A URI as described in RFC 3986
hash	Either a short or a long Git hash
spdx	An SPDX license identifier
email	An email address conforming to RFC 5322

By default, Hadolint ignores any label not specified in the label schema. To warn on such additional labels, turn on strict labels:

$ hadolint --strict-labels --require-label version:semver Dockerfile

or in the config file:

strict-labels: true

When strict labels is enabled, but no label schema has been specified, Hadolint will warn if any label is present.

Note on dealing with variables in labels

It is a common pattern to fill the value of a label not statically, but rather dynamically at build time by using a variable:

FROM debian:buster
ARG VERSION="du-jour"
LABEL version="${VERSION}"

To allow this, the label schema must specify text as value for that label:

label-schema:
  version: text

Integrations

To get most of hadolint it is useful to integrate it as a check to your CI or to your editor, or as a pre-commit hook, to lint your Dockerfile as you write it. See our Integration docs.

• Code Review Platform Integrations
• Continuous Integrations
• Editor Integrations
• Version Control Integrations

Rules

An incomplete list of implemented rules. Click on the error code to get more detailed information.

• Rules with the prefix DL originate from hadolint. Take a look at Rules.hs to find the implementation of the rules.

• Rules with the SC prefix originate from ShellCheck (Only the most common rules are listed, there are dozens more)

Please create an issue if you have an idea for a good rule.

Rule	Description
DL3000	Use absolute WORKDIR.
DL3001	For some bash commands it makes no sense running them in a Docker container like ssh, vim, shutdown, service, ps, free, top, kill, mount, ifconfig.
DL3002	Last user should not be root.
DL3003	Use WORKDIR to switch to a directory.
DL3004	Do not use sudo as it leads to unpredictable behavior. Use a tool like gosu to enforce root.
DL3005	Do not use apt-get dist-upgrade.
DL3006	Always tag the version of an image explicitly.
DL3007	Using latest is prone to errors if the image will ever update. Pin the version explicitly to a release tag.
DL3008	Pin versions in apt-get install.
DL3009	Delete the apt-get lists after installing something.
DL3010	Use ADD for extracting archives into an image.
DL3011	Valid UNIX ports range from 0 to 65535.
DL3012	Multiple HEALTHCHECK instructions.
DL3013	Pin versions in pip.
DL3014	Use the -y switch.
DL3015	Avoid additional packages by specifying --no-install-recommends.
DL3016	Pin versions in npm.
DL3017	Do not use apk upgrade.
DL3018	Pin versions in apk add. Instead of apk add <package> use apk add <package>=<version>.
DL3019	Use the --no-cache switch to avoid the need to use --update and remove /var/cache/apk/* when done installing packages.
DL3020	Use COPY instead of ADD for files and folders.
DL3021	COPY with more than 2 arguments requires the last argument to end with /
DL3022	COPY --from should reference a previously defined FROM alias
DL3023	COPY --from cannot reference its own FROM alias
DL3024	FROM aliases (stage names) must be unique
DL3025	Use arguments JSON notation for CMD and ENTRYPOINT arguments
DL3026	Use only an allowed registry in the FROM image
DL3027	Do not use apt as it is meant to be a end-user tool, use apt-get or apt-cache instead
DL3028	Pin versions in gem install. Instead of gem install <gem> use gem install <gem>:<version>
DL3029	Do not use --platform flag with FROM.
DL3030	Use the -y switch to avoid manual input yum install -y <package>
DL3031	Do not use yum update
DL3032	yum clean all missing after yum command.
DL3033	Specify version with yum install -y <package>-<version>
DL3034	Non-interactive switch missing from zypper command: zypper install -y
DL3035	Do not use zypper update.
DL3036	zypper clean missing after zypper use.
DL3037	Specify version with zypper install -y <package>[=]<version>.
DL3038	Use the -y switch to avoid manual input dnf install -y <package>
DL3039	Do not use dnf update
DL3040	dnf clean all missing after dnf command.
DL3041	Specify version with dnf install -y <package>-<version>
DL3042	Avoid cache directory with pip install --no-cache-dir <package>.
DL3043	ONBUILD, FROM or MAINTAINER triggered from within ONBUILD instruction.
DL3044	Do not refer to an environment variable within the same ENV statement where it is defined.
DL3045	COPY to a relative destination without WORKDIR set.
DL3046	useradd without flag -l and high UID will result in excessively large Image.
DL3047	wget without flag --progress will result in excessively bloated build logs when downloading larger files.
DL3048	Invalid Label Key
DL3049	Label <label> is missing.
DL3050	Superfluous label(s) present.
DL3051	Label <label> is empty.
DL3052	Label <label> is not a valid URL.
DL3053	Label <label> is not a valid time format - must be conform to RFC3339.
DL3054	Label <label> is not a valid SPDX license identifier.
DL3055	Label <label> is not a valid git hash.
DL3056	Label <label> does not conform to semantic versioning.
DL3057	HEALTHCHECK instruction missing.
DL3058	Label <label> is not a valid email format - must be conform to RFC5322.
DL3059	Multiple consecutive RUN instructions. Consider consolidation.
DL3060	yarn cache clean missing after yarn install was run.
DL4000	MAINTAINER is deprecated.
DL4001	Either use Wget or Curl but not both.
DL4003	Multiple CMD instructions found.
DL4004	Multiple ENTRYPOINT instructions found.
DL4005	Use SHELL to change the default shell.
DL4006	Set the SHELL option -o pipefail before RUN with a pipe in it
SC1000	$ is not used specially and should therefore be escaped.
SC1001	This \c will be a regular 'c' in this context.
SC1007	Remove space after = if trying to assign a value (or for empty string, use var='' ...).
SC1010	Use semicolon or linefeed before done (or quote to make it literal).
SC1018	This is a unicode non-breaking space. Delete it and retype as space.
SC1035	You need a space here
SC1045	It's not foo &; bar, just foo & bar.
SC1065	Trying to declare parameters? Don't. Use () and refer to params as $1, $2 etc.
SC1066	Don't use $ on the left side of assignments.
SC1068	Don't put spaces around the = in assignments.
SC1077	For command expansion, the tick should slant left (` vs ´).
SC1078	Did you forget to close this double-quoted string?
SC1079	This is actually an end quote, but due to next char, it looks suspect.
SC1081	Scripts are case sensitive. Use if, not If.
SC1083	This {/} is literal. Check expression (missing ;/\n?) or quote it.
SC1086	Don't use $ on the iterator name in for loops.
SC1087	Braces are required when expanding arrays, as in ${array[idx]}.
SC1095	You need a space or linefeed between the function name and body.
SC1097	Unexpected ==. For assignment, use =. For comparison, use [ .. ] or [[ .. ]].
SC1098	Quote/escape special characters when using eval, e.g. eval "a=(b)".
SC1099	You need a space before the #.
SC2002	Useless cat. Consider cmd < file | .. or cmd file | .. instead.
SC2015	Note that A && B || C is not if-then-else. C may run when A is true.
SC2026	This word is outside of quotes. Did you intend to 'nest '"'single quotes'"' instead'?
SC2028	echo won't expand escape sequences. Consider printf.
SC2035	Use ./*glob* or -- *glob* so names with dashes won't become options.
SC2039	In POSIX sh, something is undefined.
SC2046	Quote this to prevent word splitting
SC2086	Double quote to prevent globbing and word splitting.
SC2140	Word is in the form "A"B"C" (B indicated). Did you mean "ABC" or "A\"B\"C"?
SC2154	var is referenced but not assigned.
SC2155	Declare and assign separately to avoid masking return values.
SC2164	Use cd ... || exit in case cd fails.

Develop

If you are an experienced Haskeller we would be really thankful if you would tear our code apart in a review.

Setup

1. Clone repository

   git clone --recursive [email protected]:hadolint/hadolint.git

2. Install the dependencies

   stack install

REPL

The easiest way to try out the parser is using the REPL.

# start the repl
stack repl
# overload strings to be able to use Text
:set -XOverloadedStrings
# import parser library
import Language.Docker
# parse instruction and look at AST representation
parseText "FROM debian:jessie"

Tests

Run unit tests.

stack test

Run integration tests.

./integration_test.sh

AST

Dockerfile syntax is fully described in the Dockerfile reference. Just take a look at Syntax.hs in the language-docker project to see the AST definition.

Alternatives

• RedCoolBeans/dockerlint
• projectatomic/dockerfile_lint