Markdown is a lightweight markup language with plain text formatting syntax designed so that it can be converted to HTML and many other formats using a tool by the same name. Markdown is often used to format readme files, for writing messages in online discussion forums, and to create rich text using a plain text editor.
John Gruber created the Markdown language in 2004, with significant collaboration with Aaron Swartz on the syntax, with the goal of enabling people "to write using an easy-to-read, easy-to-write plain text format, and optionally convert it to structurally valid XHTML (or HTML)”
The key design goal is readability – that the language be readable as-is, without looking like it has been marked up with tags or formatting instructions, unlike text formatted with a markup language, such as Rich Text Format (RTF) or HTML, which have obvious tags and formatting instructions. To this end, its main inspiration is the existing conventions for marking up plain text in email, though it also draws from earlier markup languages, notably setext, atx (by Aaron Swartz), Textile, reStructuredText, Grutatext, and EtText.
Gruber wrote a Perl script, Markdown.pl, which converts marked-up text input to valid, well-formed XHTML or HTML and replaces left-pointing angle brackets ('<') and ampersands with their corresponding character entity references. It can be used as a standalone script, as a plugin for Blosxom or Movable Type, or as a text filter for BBEdit.
Markdown has since been re-implemented by others as a Perl module available on CPAN (Text::Markdown), and in a variety of other programming languages. It is distributed under a BSD-style license and is included with, or available as a plugin for, several content-management systems.
Sites such as GitHub, reddit, Diaspora, Stack Exchange, OpenStreetMap, and SourceForge use variants of Markdown to facilitate discussion between users. A Markdown dialect is also used by the instant messaging system Slack.
There is no clearly defined Markdown standard, apart from the original writeup and implementation by John Gruber, which some consider to be abandonware. This has led to fragmentation as different vendors write their own variants of the language to correct flaws or add missing features.
From 2012 through 2014, a group of people including Jeff Atwood launched what Atwood characterized as a standardization effort. A community website now aims to "document various tools and resources available to document authors and developers, as well as implementors of the various markdown implementations". In September 2014, Gruber objected to the usage of "Markdown" in the name of this effort and it was rebranded as a new dialect named CommonMark.
A tool (named Babelmark2) is also available to "[compare] the output of various implementations" to "promote discussion of how and whether certain vague aspects of the markdown spec should be clarified". Gruber has argued that complete standardisation would be mistaken: "Different sites (and people) have different needs. No one syntax would make all happy."
A standards effort for the Internet media type text/markdown (independent of the syntax) is underway in the IETF. The Internet-Draft, draft-ietf-appsawg-text-markdown, is planned to be submitted to the IESG in April 2015. The Markdown community is invited to submit use cases for Markdown, which will inform the media type standardization process.
A number of other lightweight markup languages extend Markdown by implementing added features (such as tables, footnotes, definition lists, and Markdown inside HTML blocks) not available with plain Markdown syntax. Among these are Markdown Extra, MultiMarkdown, and the Pandoc Markdown extension. In some cases, this is in order to enable conversion into more formats than HTML, e.g. LaTeX, RTF, and DocBook.
Many implementations also intentionally omit support for middle word emphasis. The original Markdown implementation interprets constructs like my_long_variable as a request to emphasize "long" in the middle of a word. Many users found this confusing, so many later implementations such as PHP Markdown and Python Markdown do not implement middle word emphasis by default.
The essay "Thoughts on Markdown" stated that Markdown's original developer, John Gruber, has not responded to discussions about extensions to Markdown, and that "Markdown is changing, with or without him".
|text using Markdown syntax||the corresponding HTML produced by a Markdown processor||the text viewed in a browser|
Heading ======= Sub-heading ----------- ### Another deeper heading Paragraphs are separated by a blank line. Leave 2 spaces at the end of a line to do a line break Text attributes *italic*, **bold**, `monospace`, ~~strikethrough~~ . A [link](http://example.com). Shopping list: * apples * oranges * pears Numbered list: 1. apples 2. oranges 3. pears The rain---not the reign---in Spain.
<h1>Heading</h1> <h2>Sub-heading</h2> <h3>Another deeper heading</h3> <p>Paragraphs are separated by a blank line.</p> <p>Leave 2 spaces at the end of a line to do a<br /> line break</p> <p>Text attributes <em>italic</em>, <strong>bold</strong>, <code>monospace</code>, <s>strikethrough</s>.</p> <p>A <a href="http://example.com">link</a>.</p> <p>Shopping list:</p> <ul> <li>apples</li> <li>oranges</li> <li>pears</li> </ul> <p>Numbered list:</p> <ol> <li>apples</li> <li>oranges</li> <li>pears</li> </ol> <p>The rain—not the reign—in Spain.</p>
Another deeper heading
Paragraphs are separated by a blank line.
Leave 2 spaces at the end of a line to do a line break
Text attributes italic, bold,
The rain—not the reign—in Spain.
While Markdown is a minimal markup language and is easily read and edited with a normal text editor, there are specially designed editors that preview the files with styles. There are a variety of such editors available for all major platforms. There are syntax highlighting plugins for Markdown built into emacs, gedit, and vim.
Implementations of Markdown are available for many different frameworks, platforms and languages.
- The sourcecode documentation generator Doxygen supports Markdown with extra features.
- RStudio, an IDE for R provides a C++ wrapper function for the markdown implementation sundown.
- IntelliJ IDEA, an IDE for Java, provides a Markdown plugin
- MultiMarkdown, a format and program with more syntax features and export options than traditional Markdown
- PageDown, a parser for StackExchange's Markdown syntax
- GitHub Flavored Markdown (GFM) adds syntax highlighting, task lists, and tables. It has several implementations:
- "Grip - Github Readme Instant Preview" (Joe Esposito) - Python
- "Vim-Flavored-Markdown" (Jeff Tratner) - Vim scripting
- "GitHub-Flavored Markdown Comments" (Evan Solomon) - PHP WordPress plugin
- "Markdown on Save" (Mark Jaquith) - PHP WordPress plugin
- "Parsedown" (Emanuil Rusev) - PHP
- Pandoc adds tables, line blocks, definition lists, superscripts and subscripts, can generate table of contents.
- Discount - a C implementation.
- Md2doc - a XSLT 2.0 implementation.
- Visual Studio Web Essentials - an extension library for Microsoft Visual Studio with live markdown preview
- Markdown Text Editor
- Markdown Test Suite
There are many more open-source implementations of Markdown available online.