Markdown linting and style checking for Visual Studio Code
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README.md

markdownlint

Markdown/CommonMark linting and style checking for Visual Studio Code

Intro

The Markdown markup language is designed to be easy to read, write, and understand. It succeeds - and its flexibility is both a benefit and a drawback. Many styles are possible, so formatting can be inconsistent. Some constructs don't work well in all parsers and should be avoided. For example, here are some common/troublesome Markdown constructs.

markdownlint is a Visual Studio Code extension that includes a library of rules to encourage standards and consistency for Markdown files. It is powered by markdownlint for Node.js which is based on markdownlint for Ruby.

Install

  1. Open Visual Studio Code
  2. Press Ctrl+P to open the Quick Open dialog
  3. Type ext install markdownlint to find the extension
  4. Click the Install button, then the Enable button

OR

  1. Press Ctrl+Shift+X to open the Extensions tab
  2. Type markdownlint to find the extension
  3. Click the Install button, then the Enable button

OR

  1. Open a command-line prompt
  2. Run code --install-extension DavidAnson.vscode-markdownlint

Use

When editing a Markdown file in Code with markdownlint installed, any lines that violate one of markdownlint's rules (see below) will trigger a Warning in the editor. Warnings are indicated by a wavy green underline and can also be seen by pressing Ctrl+Shift+M to open the Errors and Warnings dialog. Hover the mouse pointer over a green line to see the warning or press F8 and Shift+F8 to cycle through all the warnings (markdownlint warnings all begin with MD###). For more information about a markdownlint warning, place the cursor on a line and click the light bulb icon or press Ctrl+. to open the code action dialog. Clicking one of the warnings in the dialog will display that rule's help entry in the default web browser.

For a tutorial, please see Build an Amazing Markdown Editor Using Visual Studio Code and Pandoc by Dave Johnson.

Rules

  • MD001 heading-increment/header-increment - Heading levels should only increment by one level at a time
  • MD002 first-heading-h1/first-header-h1 - First heading should be a top level heading
  • MD003 heading-style/header-style - Heading style
  • MD004 ul-style - Unordered list style
  • MD005 list-indent - Inconsistent indentation for list items at the same level
  • MD006 ul-start-left - Consider starting bulleted lists at the beginning of the line
  • MD007 ul-indent - Unordered list indentation
  • MD009 no-trailing-spaces - Trailing spaces
  • MD010 no-hard-tabs - Hard tabs
  • MD011 no-reversed-links - Reversed link syntax
  • MD012 no-multiple-blanks - Multiple consecutive blank lines
  • MD013 line-length - Line length
  • MD014 commands-show-output - Dollar signs used before commands without showing output
  • MD018 no-missing-space-atx - No space after hash on atx style heading
  • MD019 no-multiple-space-atx - Multiple spaces after hash on atx style heading
  • MD020 no-missing-space-closed-atx - No space inside hashes on closed atx style heading
  • MD021 no-multiple-space-closed-atx - Multiple spaces inside hashes on closed atx style heading
  • MD022 blanks-around-headings/blanks-around-headers - Headings should be surrounded by blank lines
  • MD023 heading-start-left/header-start-left - Headings must start at the beginning of the line
  • MD024 no-duplicate-heading/no-duplicate-header - Multiple headings with the same content
  • MD025 single-h1 - Multiple top level headings in the same document
  • MD026 no-trailing-punctuation - Trailing punctuation in heading
  • MD027 no-multiple-space-blockquote - Multiple spaces after blockquote symbol
  • MD028 no-blanks-blockquote - Blank line inside blockquote
  • MD029 ol-prefix - Ordered list item prefix
  • MD030 list-marker-space - Spaces after list markers
  • MD031 blanks-around-fences - Fenced code blocks should be surrounded by blank lines
  • MD032 blanks-around-lists - Lists should be surrounded by blank lines
  • MD033 no-inline-html - Inline HTML
  • MD034 no-bare-urls - Bare URL used
  • MD035 hr-style - Horizontal rule style
  • MD036 no-emphasis-as-heading/no-emphasis-as-header - Emphasis used instead of a heading
  • MD037 no-space-in-emphasis - Spaces inside emphasis markers
  • MD038 no-space-in-code - Spaces inside code span elements
  • MD039 no-space-in-links - Spaces inside link text
  • MD040 fenced-code-language - Fenced code blocks should have a language specified
  • MD041 first-line-h1 - First line in file should be a top level heading
  • MD042 no-empty-links - No empty links
  • MD043 required-headings/required-headers - Required heading structure
  • MD044 proper-names - Proper names should have the correct capitalization
  • MD045 no-alt-text - Images should have alternate text (alt text)

See markdownlint's Rules.md file for more details.

The following rules can be automatically fixed by moving the cursor to a rule violation (green underlined text) and typing Ctrl+. or clicking the light bulb icon.

  • MD006 ul-start-left
  • MD009 no-trailing-spaces
  • MD010 no-hard-tabs
  • MD011 no-reversed-links
  • MD018 no-missing-space-atx
  • MD019 no-multiple-space-atx
  • MD020 no-missing-space-closed-atx
  • MD021 no-multiple-space-closed-atx
  • MD023 heading-start-left
  • MD027 no-multiple-space-blockquote
  • MD028 no-blanks-blockquote
  • MD034 no-bare-urls
  • MD037 no-space-in-emphasis
  • MD038 no-space-in-code
  • MD039 no-space-in-links

Fixes can be reverted by Edit|Undo or Ctrl+Z.

Configure

markdownlint.config

The default rule configuration disables MD013/line-length because many files include lines longer than the conventional 80 character limit:

{
    "MD013": false
}

Rules can be enabled, disabled, and customized by creating a JSON file named .markdownlint.json or a YAML file named .markdownlint.yaml in any directory of a project. The rules defined by .markdownlint.{json,yaml} apply to Markdown files in the same directory and any sub-directories without their own .markdownlint.{json,yaml}.

Note: .markdownlint.{json,yaml} is used only if a project has been opened. When no folder is open or a file is not part of the current project, normal user and workspace settings apply (see below). If both .markdownlint.json and .markdownlint.yaml are present in the same directory, only .markdownlint.json is used.

A custom configuration is often defined by a .markdownlint.json file in the root of the project:

{
    "default": true,
    "MD003": { "style": "atx_closed" },
    "MD007": { "indent": 4 },
    "no-hard-tabs": false
}

To extend another configuration file, any configuration file can use the extends property to provide a relative path:

{
    "extends": "../.markdownlint.json",
    "no-hard-tabs": true
}

Files referenced via extends do not need to be part of the current project (but usually are).

Rules can also be configured using Code's support for user and workspace settings.

The earlier configuration might look like the following in Code's user settings:

{
    "editor.someSetting": true,
    "markdownlint.config": {
        "default": true,
        "MD003": { "style": "atx_closed" },
        "MD007": { "indent": 4 },
        "no-hard-tabs": false
    }
}

Rule locations have the following precedence (in decreasing order):

  • .markdownlint.{json,yaml} file in the same directory
  • .markdownlint.{json,yaml} file in a parent directory
  • .markdownlint.{json,yaml} file in the root of the project
  • Visual Studio Code user/workspace settings
  • Default configuration (see above)

Changes saved to any of these locations take effect immediately. Files referenced via extends are not monitored for changes. Only the bottom two locations apply to files outside the project.

See markdownlint's options.config section for more information about rule configuration.

markdownlint.ignore

If a workspace contains generated content or other Markdown files that trigger warnings but cannot be fixed, it may be helpful to ignore (skip) those files when linting. This can be done by updating the user/workspace configuration with a glob expression matching the relevant file names.

An example from Code's workspace settings might be:

{
    "editor.someSetting": true,
    "markdownlint.ignore": [
        "ignore.md",
        "directory/ignore.md",
        "**/ignore.md",
        "**/*.md"
    ]
}

The globbing library used for matching file names is minimatch with the dot and nocomment options enabled. Matching is case-sensitive and paths are resolved relative to the root of the workspace. The directory separator is /, even on Windows.

markdownlint.run

By default, linting is performed as you type or edit a document. Linting is fast and efficient and should not interfere with typical workflows.

If you find this distracting, linting can be configured to run only when the document is saved. This looks like the following in Code's user settings:

{
    "editor.someSetting": true,
    "markdownlint.run": "onSave"
}

Note: When configured to run onSave, the list of reported issues will become outdated while the document is edited and will update when the document is saved.

markdownlint.customRules

Custom rules can be specified in Code's user/workspace configuration and used to apply additional linting beyond the default set of rules. Custom rules are identified by the path to a JavaScript file or the path to an npm package exporting the rule. Paths are relative to the root of the current workspace (or the Code install directory if no folder is open) and can be absolute.

When adding custom rules to a workspace, consider committing those rules under the .vscode directory where they will be separate from other files in the workspace and available to anyone who clones the repository.

An example of Code's workspace settings might look like the following:

{
    "editor.someSetting": true,
    "markdownlint.customRules": [
        ".vscode/my-custom-rule.js",
        ".vscode/rule-as-npm-package",
        "c:\\absolute\\path\\to\\rule.js"
    ]
}

To troubleshoot issues loading or running custom rules, please refer to diagnostic messages from the extension in Code's Output window.

For information about authoring custom rules, see markdownlint/CustomRules.

Suppress

Individual warnings can be suppressed with inline comments:

<!-- markdownlint-disable MD037 -->
deliberate space * in * emphasis
<!-- markdownlint-enable MD037 -->

See markdownlint's configuration section for more details.

History

See CHANGELOG.md.