markdownlint/doc/Rules.md

# Rules

This document contains a description of all rules, what they are checking for,
as well as an examples of documents that break the rule and corrected
versions of the examples.

## MD001 - Header levels should only increment by one level at a time

Tags: headers

This rule is triggered when you skip header levels in a markdown document, for
example:

    # Header 1

    ### Header 3

    We skipped out a 2nd level header in this document

When using multiple header levels, nested headers should increase by only one
level at a time:

    # Header 1

    ## Header 2

    ### Header 3

    #### Header 4

    ## Another Header 2

    ### Another Header 3


## MD002 - First header should be a h1 header

Tags: headers

This rule is triggered when the first header in the document isn't a h1 header:

    ## This isn't a H1 header

    ### Another header

The first header in the document should be a h1 header:

    # Start with a H1 header

    ## Then use a H2 for subsections

## MD003 - Header style

Tags: headers

Parameters: style ("consistent", "atx", "atx_closed", "setext"; default "consistent")

This rule is triggered when different header styles (atx, setext, and 'closed'
atx) are used in the same document:

    # ATX style H1

    ## Closed ATX style H2 ##

    Setext style H1
    ===============

Be consistent with the style of header used in a document:

    # ATX style H1

    ## ATX style H2

Note: the configured header style can be a specific style to use (atx,
atx_closed, setext), or simply require that the usage be consistent within the
document.

## MD004 - Unordered list style

Tags: bullet, ul

Parameters: style ("consistent", "asterisk", "plus", "dash"; default "consistent")

This rule is triggered when the symbols used in the document for unordered
list items do not match the configured unordered list style:

    * Item 1
    + Item 2
    - Item 3

To fix this issue, use the configured style for list items throughout the
document:

    * Item 1
    * Item 2
    * Item 3

Note: the configured list style can be a specific symbol to use (asterisk,
plus, dash), or simply require that the usage be consistent within the
document.

## MD005 - Inconsistent indentation for list items at the same level

Tags: bullet, ul, indentation

This rule is triggered when list items are parsed as being at the same level,
but don't have the same indentation:

    * Item 1
        * Nested Item 1
        * Nested Item 2
       * A misaligned item

Usually this rule will be triggered because of a typo. Correct the indentation
for the list to fix it:

    * Item 1
      * Nested Item 1
      * Nested Item 2
      * Nested Item 3

## MD006 - Consider starting bulleted lists at the beginning of the line

Tags: bullet, ul, indentation

This rule is triggered when top level lists don't start at the beginning of a
line:

    Some text

      * List item
      * List item

To fix, ensure that top level list items are not indented:


    Some test

    * List item
    * List item

Rationale: Starting lists at the beginning of the line means that nested list
items can all be indented by the same amount when an editor's indent function
or the tab key is used to indent. Starting a list 1 space in means that the
indent of the first nested list is less than the indent of the second level (3
characters if you use 4 space tabs, or 1 character if you use 2 space tabs).

## MD007 - Unordered list indentation

Tags: bullet, ul, indentation

Parameters: indent (number; default 2)

This rule is triggered when list items are not indented by the configured
number of spaces (default: 2).

Example:

    * List item
       * Nested list item indented by 3 spaces

Corrected Example:

    * List item
      * Nested list item indented by 2 spaces

Rationale (2 space indent): indenting by 2 spaces allows the content of a
nested list to be in line with the start of the content of the parent list
when a single space is used after the list marker.

Rationale (4 space indent): Same indent as code blocks, simpler for editors to
implement. See
<http://www.cirosantilli.com/markdown-styleguide/#indented-lists> for more
information.

In addition, this is a compatibility issue with multi-markdown parsers, which
require a 4 space indents. See
<http://support.markedapp.com/discussions/problems/21-sub-lists-not-indenting>
for a description of the problem.

## MD009 - Trailing spaces

Tags: whitespace

This rule is triggered on any lines that end with whitespace. To fix this,
find the line that is triggered and remove any trailing spaces from the end.

## MD010 - Hard tabs

Tags: whitespace, hard_tab

This rule is triggered by any lines that contain hard tab characters instead
of using spaces for indentation. To fix this, replace any hard tab characters
with spaces instead.

Example:

    Some text

    	* hard tab character used to indent the list item

Corrected example:

    Some text

        * Spaces used to indent the list item instead

## MD011 - Reversed link syntax

Tags: links

This rule is triggered when text that appears to be a link is encountered, but
where the syntax appears to have been reversed (the `[]` and `()` are
reversed):

    (Incorrect link syntax)[http://www.example.com/]

To fix this, swap the `[]` and `()` around:

    [Correct link syntax](http://www.example.com/)

## MD012 - Multiple consecutive blank lines

Tags: whitespace, blank_lines

This rule is triggered when there are multiple consecutive blank lines in the
document:

    Some text here


    Some more text here

To fix this, delete the offending lines:

    Some text here

    Some more text here

Note: this rule will not be triggered if there are multiple consecutive blank
lines inside code blocks.

## MD013 - Line length

Tags: line_length

Parameters: line_length (number; default 80)

This rule is triggered when there are lines that are longer than the
configured line length (default: 80 characters). To fix this, split the line
up into multiple lines.

This rule has an exception where there is no whitespace beyond the configured
line length. This allows you to still include items such as long URLs without
being forced to break them in the middle.

## MD014 - Dollar signs used before commands without showing output

Tags: code

This rule is triggered when there are code blocks showing shell commands to be
typed, and the shell commands are preceded by dollar signs ($):

    $ ls
    $ cat foo
    $ less bar

The dollar signs are unnecessary in the above situation, and should not be
included:

    ls
    cat foo
    less bar

However, an exception is made when there is a need to distinguish between
typed commands and command output, as in the following example:

    $ ls
    foo bar
    $ cat foo
    Hello world
    $ cat bar
    baz

Rationale: it is easier to copy and paste and less noisy if the dollar signs
are omitted when they are not needed. See
<http://www.cirosantilli.com/markdown-styleguide/#dollar-signs-in-shell-code>
for more information.

## MD018 - No space after hash on atx style header

Tags: headers, atx, spaces

This rule is triggered when spaces are missing after the hash characters
in an atx style header:

    #Header 1

    ##Header 2

To fix this, separate the header text from the hash character by a single
space:

    # Header 1

    ## Header 2

## MD019 - Multiple spaces after hash on atx style header

Tags: headers, atx, spaces

This rule is triggered when more than one space is used to separate the
header text from the hash characters in an atx style header:

    #  Header 1

    ##  Header 2

To fix this, separate the header text from the hash character by a single
space:

    # Header 1

    ## Header 2

## MD020 - No space inside hashes on closed atx style header

Tags: headers, atx_closed, spaces

This rule is triggered when spaces are missing inside the hash characters
in a closed atx style header:

    #Header 1#

    ##Header 2##

To fix this, separate the header text from the hash character by a single
space:

    # Header 1 #

    ## Header 2 ##

Note: this rule will fire if either side of the header is missing spaces.

## MD021 - Multiple spaces inside hashes on closed atx style header

Tags: headers, atx_closed, spaces

This rule is triggered when more than one space is used to separate the
header text from the hash characters in a closed atx style header:

    #  Header 1  #

    ##  Header 2  ##

To fix this, separate the header text from the hash character by a single
space:

    # Header 1 #

    ## Header 2 ##

Note: this rule will fire if either side of the header contains multiple
spaces.

## MD022 - Headers should be surrounded by blank lines

Tags: headers, blank_lines

This rule is triggered when headers (any style) are either not preceded or not
followed by a blank line:

    # Header 1
    Some text

    Some more text
    ## Header 2

To fix this, ensure that all headers have a blank line both before and after
(except where the header is at the beginning or end of the document):

    # Header 1

    Some text

    Some more text

    ## Header 2

Rationale: Aside from aesthetic reasons, some parsers, including kramdown, will
not parse headers that don't have a blank line before, and will parse them as
regular text.

## MD023 - Headers must start at the beginning of the line

Tags: headers, spaces

This rule is triggered when a header is indented by one or more spaces:

    Some text

      # Indented header

To fix this, ensure that all headers start at the beginning of the line:

    Some text

    # Header

Rationale: Headers that don't start at the beginning of the line will not be
parsed as headers, and will instead appear as regular text.

## MD024 - Multiple headers with the same content

Tags: headers

This rule is triggered if there are multiple headers in the document that have
the same text:

    # Some text

    ## Some text

To fix this, ensure that the content of each header is different:

    # Some text

    ## Some more text

Rationale: Some markdown parses generate anchors for headers based on the
header name, and having headers with the same content can cause problems with
this.

## MD025 - Multiple top level headers in the same document

Tags: headers

This rule is triggered when a top level header is in use (the first line of
the file is a h1 header), and more than one h1 header is in use in the
document:

    # Top level header

    # Another top level header

To fix, structure your document so that there is a single h1 header that is
the title for the document, and all later headers are h2 or lower level
headers:

    # Title

    ## Header

    ## Another header

Rationale: A top level header is a h1 on the first line of the file, and
serves as the title for the document. If this convention is in use, then there
can not be more than one title for the document, and the entire document
should be contained within this header.

## MD026 - Trailing punctuation in header

Tags: headers

Parameters: punctuation (string; default ".,;:!?")

This rule is triggered on any header that has a punctuation character as the
last character in the line:

    # This is a header.

To fix this, remove any trailing punctuation:

    # This is a header

Note: The punctuation parameter can be used to specify what characters class
as punctuation at the end of the header. For example, you can set it to
`'.,;:!'` to allow headers with question marks in them, such as might be used
in an FAQ.

## MD027 - Multiple spaces after blockquote symbol

Tags: blockquote, whitespace, indentation

This rule is triggered when blockquotes have more than one space after the
blockquote (`>`) symbol:

    >  This is a block quote with bad indentation
    >  there should only be one.

To fix, remove any extraneous space:

    > This is a blockquote with correct
    > indentation.

## MD028 - Blank line inside blockquote

Tags: blockquote, whitespace

This rule is triggered when two blockquote blocks are separated by nothing
except for a blank line:

    > This is a blockquote
    > which is immediately followed by

    > this blockquote. Unfortunately
    > In some parsers, these are treated as the same blockquote.

To fix this, ensure that any blockquotes that are right next to each other
have some text in between:

    > This is a blockquote.

    And Jimmy also said:

    > This too is a blockquote.

Alternatively, if they are supposed to be the same quote, then add the
blockquote symbol at the beginning of the blank line:

    > This is a blockquote.
    >
    > This is the same blockquote.

Rationale: Some markdown parsers will treat two blockquotes separated by one
or more blank lines as the same blockquote, while others will treat them as
separate blockquotes.

## MD029 - Ordered list item prefix

Tags: ol

Parameters: style ("one", "ordered"; default "one")

This rule is triggered on ordered lists that do not either start with '1.' or
do not have a prefix that increases in numerical order (depending on the
configured style, which defaults to 'one').

Example valid list if the style is configured as 'one':

    1. Do this.
    1. Do that.
    1. Done.

Example valid list if the style is configured as 'ordered':

    1. Do this.
    2. Do that.
    3. Done.

## MD030 - Spaces after list markers

Tags: ol, ul, whitespace

Parameters: ul_single, ol_single, ul_multi, ol_multi (number, default 1)

This rule checks for the number of spaces between a list marker (e.g. '`-`',
'`*`', '`+`' or '`1.`') and the text of the list item.

The number of spaces checked for depends on the document style in use, but the
default is 1 space after any list marker:

    * Foo
    * Bar
    * Baz

    1. Foo
    1. Bar
    1. Baz

    1. Foo
       * Bar
    1. Baz

A document style may change the number of spaces after unordered list items
and ordered list items independently, as well as based on whether the content
of every item in the list consists of a single paragraph, or multiple
paragraphs (including sub-lists and code blocks).

For example, the style guide at
<http://www.cirosantilli.com/markdown-styleguide/#spaces-after-marker>
specifies that 1 space after the list marker should be used if every item in
the list fits within a single paragraph, but to use 2 or 3 spaces (for ordered
and unordered lists respectively) if there are multiple paragraphs of content
inside the list:

    * Foo
    * Bar
    * Baz

    vs.

    *   Foo

        Second paragraph

    *   Bar

    or

    1.  Foo

        Second paragraph

    1.  Bar

To fix this, ensure the correct number of spaces are used after list marker
for your selected document style.

## MD031 - Fenced code blocks should be surrounded by blank lines

Tags: code, blank_lines

This rule is triggered when fenced code blocks are either not preceded or not
followed by a blank line:

    Some text
    ```
    Code block
    ```

    ```
    Another code block
    ```
    Some more text

To fix this, ensure that all fenced code blocks have a blank line both before
and after (except where the block is at the beginning or end of the document):

    Some text

    ```
    Code block
    ```

    ```
    Another code block
    ```

    Some more text

Rationale: Aside from aesthetic reasons, some parsers, including kramdown, will
not parse fenced code blocks that don't have blank lines before and after them.

## MD032 - Lists should be surrounded by blank lines

Tags: bullet, ul, ol, blank_lines

This rule is triggered when lists (of any kind) are either not preceded or not
followed by a blank line:

    Some text
    * Some
    * List

    1. Some
    2. List
    Some text

To fix this, ensure that all lists have a blank line both before and after
(except where the block is at the beginning or end of the document):

    Some text

    * Some
    * List

    1. Some
    2. List

    Some text

Rationale: Aside from aesthetic reasons, some parsers, including kramdown, will
not parse lists that don't have blank lines before and after them.

Note: List items without hanging indents are a violation of this rule; list
items with hanging indents are okay:

    * This is
    not okay

    * This is
      okay