Jekyll 之修改 liquid 代码：去重、排序以及其它

遇到的小bug之一：相册分类重复

由于遍历相册分类时，有些相册的分类是相同的。这时生成的页面就会有多个相同的相册分类……（好像绕口令）

其实是当时设计修改相册功能时的一个小疏漏。改之。

原先代码：

{% assign groups = site.photos | group_by: "category" | sort: "name" %}
{% for group in groups %}
	{% for item in group.items %}				
		<a href="/photos/" data-filter=".f_{{ item.category }}">
			<span>{{ item.category }}</span>
		</a>
	{% endfor %}
{% endfor %}

// in post-sidebar.html，| first
{% for category in site.categories %}{% endfor %}

可以看到，其实上面犯了一个比较简单的错误，就是在遍历中遍历了item项。由于具有相同分类的页面会被记为同一分类的多个item，因而遍历出来的也会重复啦。

修改后代码：

{% assign groups = site.photos | group_by: "category" | sort: "name" %}
{% for group in groups %}			
	<a href="/photos/" data-filter=".f_{{ group.name }}">
	    <span>{{ group.name }}</span>
	</a>
{% endfor %}

遍历一下name标签即可。

遇到的小bug之二：category排序

强迫症实在不能忍按照时间顺序建立的tag标签树！必须要按首字母顺序啊，到时集齐了26个字母肯定好看！

还是先看代码：

{% for cat in site.categories %}
## {{ cat.first }}
<p><ol>
{% for post in site.categories[cat.first] %}
<li><a href="{{ post.url }}">{{ post.title }}</a> - {{ post.date | date: "%b %-d, %Y" }}</li>
{% endfor %}
</ol></p>
<div class="sep"><hr></div>
{% endfor %}

哎呀呀，这代码看得我真的很难受诶。非要从一个生成的category列表中一个个取出第一项，这要是以后生成规则变了还不全都挂掉？

还是用site.posts中选category项更保险吧！（虽然生成的时候会多花那么一丢丢时间）保持代码风格一致性还是很重要的（虽然并不知道自己在说什么）

修改后的代码：

{% assign groups = site.posts | group_by: "category" | sort: "name" %}
{% for group in groups %}
	## {{ group.name }}	
	<p><ol>	
		{% for post in group.items %}
			<li><a href="{{ post.url }}">{{ post.title }}</a> - {{ post.date | date: "%b %-d, %Y" }}</li>
		{% endfor %}
	</ol></p>
	<div class="sep"><hr></div>
{% endfor %}

对于tag也是同样的修改。另外记得把sidebar上的也给改掉啦。OK，安逸~

{% assign groups = site.posts | group_by: "category" | sort: "name" %}
{% for category in groups %}
<li><a href="/categories/#{{ category.name | downcase | replace: ' ', '-' }}">{{ category.name }}</a></li>
{% endfor %}

等等，不对吧……？！

遇到的小bug之三：tags排序

每篇文章有一个category，但是有多个tags啊，想当然就直接改了，最后的结果肯定是输出了一堆多元数组……

这倒是难不倒我。还是来分析一下。先看看结构：

{{ site.tags }}

的确，jekyll自动生成的tags列表和category是一个格式的：没有名称，直接把tags和链接怼在一起。这可难办了，刚才category用了另外的方法逃避了这个问题，这回没辙，只能硬上了。

弃了好麻烦

获得一份有name的数组除了自己生成和修改以外似乎没有别的办法了。

以下为手册内容

Liquid is an open-source template language created by Shopif that written in Ruby. Jekyll uses it to process templates and pages on your Jekyll site. With this powerful tool, you can get complex tasks done without additional plugins.

You can find Liquid documents at https://shopify.github.io/liquid/, and how Jekyll using it via https://jekyllrb.com/docs/templates/.

Liquid Basics

Liquid code can be categorized into objects, tags, and filters.

Objects

Objects tell Liquid where to show content on a page and denoted by double curly braces: {{ and }}.

Input:
{{ page.title }}

Output:
Jekyll 之修改 liquid 代码：去重、排序以及其它

Tags

Tags create the logic and control flow for templates. They are denoted by curly braces and percent signs: {% and %}.

The markup used in tags does not produce any visible text. This means that you can assign variables and create conditions and loops without showing any of the Liquid logic on the page.

Input:
{% if statement %}
  Hello, you are visiting {{ page.url | prepend: site.url }}.
{% endif %}

Output:
  Hello, you are visiting https://yo1995.github.io/html/jekyll-edit-liquid-1/.

Tags can be categorized into three types (we’ll talk about them later in details):

• Control flow
• Iteration
• Variable assignments

Filters

Filters change the output of a Liquid object. They are used within an output and are separated by a |.

Input:
{{ page.url | append: '.html' }}

Output:
/html/jekyll-edit-liquid-1/.html

Operators

Basic Operators

For example:

{% if product.title == "Awesome Shoes" %}
  These shoes are awesome!
{% endif %}

You can use multiple operators in a single tag:

{% if product.type == "Shirt" or product.type == "Shoes" %}
  This is a shirt or a pair of shoes.
{% endif %}

Contains

contains checks for the presence of a substring inside a string:

{% if page.title contains 'Pack' %}
  This page's title contains the word Pack.
{% endif %}

contains can also check for the presence of a string in an array of strings:

{% if page.tags contains 'Hello' %}
  This page has been tagged with 'Hello'.
{% endif %}

contains can only search strings. You cannot use it to check for an object in an array of objects.

Truthy and Falsy

Truthy

All values in Liquid are truthy except nil and false. In the example below, the string “Tobi” is not a boolean, but it is truthy in a conditional:

{% assign tobi = "Tobi" %}

{% if tobi %}
  This condition will always be true.
{% endif %}

strings, even when empty, are truthy.

Falsy

The falsy values in Liquid are nil and false.

Object Types

Liquid objects can have one of these types:

• string
• number
• boolean
• nil
• array

You can initialize Liquid variables with the assign or capture tags.

String

Declare a string by wrapping a variable’s value in single or double quotes:

{% assign my_string = "Hello World! This is Frank Lin." %}

Number

Numbers include floats and integers:

{% assign my_int = 25 %}
{% assign my_float = 39.756 %}

Boolean

Booleans are either true or false. No quotations are necessary when declaring a boolean:

{% assign foo = true %}
{% assign bar = false %}

Nil

Nil is a special empty value that is returned when Liquid code has no results. It is not a string with the characters “nil”. Nil is treated as false in the conditions of if blocks and other Liquid tags that check the truthfulness of a statement.

In the following example, the page.author does not exist (i.e. {{ page.author }} returns nil), Liquid will not print anything:

{% if page.author %}
  post by {{ page.author }}
{% endif %}
{% if page.title %}
 the title of this post: {{ page.title }}
{% endif %}

Output:

 the title of this post: Jekyll 之修改 liquid 代码：去重、排序以及其它

Array

Arrays hold lists of variables of any type. You cannot initialize arrays using only Liquid. You can, however, use the split filter to break a string into an array of substrings.

Accessing items in arrays:

Input:
{% for tag in page.tags %} {{ tag }} {% endfor %}

Output:
 technology  helloworld 

Accessing specific items in arrays

You can use square bracket [ ] notation to access a specific item in an array. Array indexing starts at zero.

Input:
{{ page.tags[1] }}

Output:
helloworld

Whitespace Control

In Liquid, you can include a hyphen in your tag syntax {{-, -}}, {%-, and -%} to strip whitespace from the left or right side of a rendered tag. Normally, even if it doesn’t output text, any line of Liquid in your template will still output a blank line in the rendered HTML:

Input:
{% assign my_variable = "tomato" %}
{{ my_variable }}

Output:

tomato
# notice the blank line before “tomato” in the rendered result

By including hyphens in your assign tag, you can strip the generated whitespace from the rendered template. But it seems not work when building with Jekyll 3.2.1 locally on my Windows PC.

Input:
{%- assign my_variable = "tomato" -%}
{{ my_variable }}

Output:
# jekyll 3.2.1 | Error:  Liquid syntax error: Tag '{%- assign my_variable = "tomato" -%}' was not properly terminated with regexp: /\%\}/

Liquid Tags

Control Flow

Control flow tags can change the information Liquid shows using programming logic.

case / when

Creates a switch statement to compare a variable with different values. case initializes the switch statement, and when compares its values.

Input:
{% assign handle = 'cake' %}
{% case handle %}
  {% when 'cake' %}
     This is a cake
  {% when 'cookie' %}
     This is a cookie
  {% else %}
     This is not a cake nor a cookie
{% endcase %}

Output:
This is a cake
# whitespaces were cleaned intentionally

if

Executes a block of code only if a certain condition is true.

Input:
{% if page.category == 'Web' %}
  This post is in Web.
{% endif %}

Output:

unless

The opposite of if – executes a block of code only if a certain condition is not met. This would be the equivalent by using if ... !=:

Input:
{% unless page.category == 'Awesome' %}
  This post is not awesome.
{% endunless %}

Output:
  This post is not awesome.

elsif / else

Adds more conditions within an if or unless block.

{% if customer.name == 'kevin' %}
  Hey Kevin!
{% elsif customer.name == 'anonymous' %}
  Hey Anonymous!
{% else %}
  Hi Stranger!
{% endif %}

Iteration

Iteration tags run blocks of code repeatedly.

for

Repeatedly executes a block of code. For a full list of attributes available within a for loop, click to see forloop (object).

Input:
{% for tag in page.tags %}
{% if forloop.first == true %}
    First tag
{% else %}
    Not the first tag.
{% endif %}
{% endfor %}

Output:
    First tag
    Not the first tag.

Input:
{% for tag in site.tags limit: 11 %}
    {{ forloop.index0 }}
{% endfor %}

Output:
 0  1  2  3  4  5  6  7  8  9  10 

Input:
{% for tag in site.tags limit: 11 %}
    {{ forloop.rindex }}
{% endfor %}

Output:
 11  10  9  8  7  6  5  4  3  2  1 

Input:
{% for item in page.array %}
  {{ item }}
{% endfor %}
# page.array = ["1", "2","3","4","5","6","7"]

Output:
 1  2  3  4  5  6  7 

break

Causes the loop to stop iterating when it encounters the break tag.

Input:
{% for i in (1..10) %}
  {% if i == 4 %}
    {% break %}
  {% elsif i == 9 %}
    {% break %}
  {% else %}
    {{ i }}
  {% endif %}
{% endfor %}

Output:
 1  2  3 

for (parameters)

limit

Limits the loop to the specified number of iterations.

offset

Begins the loop at the specified index.

Input:
{% for item in page.array offset:2 limit:4 %}
  {{ item }}
{% endfor %}

Output:
 3  4  5  6 

range

Defines a range of numbers to loop through. The range can be defined by both literal and variable numbers.

Input:
{% assign num = 4 %}
{% for i in (1..num) %}
  {{ i }}
{% endfor %}

Output:
 1  2  3  4 

reversed

Reverses the order of the loop.

Input:
{% for item in page.array reversed %}
  {{ item }}
{% endfor %}

Output:
 7  6  5  4  3  2  1 

cycle

Loops through a group of strings and outputs them in the order that they were passed as parameters. Each time cycle is called, the next string that was passed as a parameter is output. cycle must be used within a for loop block.

Input:
{% cycle 'one', 'two', 'three' %}
{% cycle 'one', 'two', 'three' %}
{% cycle 'one', 'two', 'three' %}
{% cycle 'one', 'two', 'three' %}

Output:
one
two
three
one

Uses for cycle include:

• applying odd/even classes to rows in a table
• applying a unique class to the last product thumbnail in a row

cycle (parameters)

cycle accepts a parameter called cycle group in cases where you need multiple cycle blocks in one template. If no name is supplied for the cycle group, then it is assumed that multiple calls with the same parameters are one group.

tablerow

Generates an HTML table. Must be wrapped in opening <table> and closing </table> HTML tags.

<table>
{% tablerow item in page.array %}
  {{ item }}
{% endtablerow %}
</table>

Output:
<tr class="row1">
<td class="col1">
1
</td><td class="col2">
2
</td><td class="col3">
3
</td><td class="col4">
4
</td><td class="col5">
5
</td><td class="col6">
6
</td><td class="col7">
7
</td></tr>

tablerow (parameters)

1. cols: defines how many columns the tables should have.
2. limit: dxits the tablerow after a specific index.
3. offset: starts the tablerow after a specific index.
4. range: defines a range of numbers to loop through. The range can be defined by both literal and variable numbers.

Variable

assign

Creates a new variable. Wrap a variable in quotations " to save it as a string as previous examples.

capture

Captures the string inside of the opening and closing tags and assigns it to a variable. Variables created through {% capture %} are strings.

Input:
{% capture my_variable %}I am being captured.{% endcapture %}
{{ my_variable }}

Output:
I am being captured.

increment

Creates a new number variable, and increases its value by one every time it is called. The initial value is 0.

Input:
{% increment my_counter %}
{% increment my_counter %}
{% increment my_counter %}

Output:
0
1
2

Variables created through the increment tag are independent from variables created through assign or capture.

Input:
{% assign var = 100 %}
{{ var }}
{% increment var %}
{% increment var %}
{% increment var %}
{{ var }}

Output:
100
0
1
2
100

decrement

Creates a new number variable, and decreases its value by one every time it is called. The initial value is -1.

Similar to increment, variables declared inside decrement are independent from variables created through assign or capture.

Liquid Filters

String

append / prepend

append concatenates two strings and returns the concatenated value, and also can be used with variables.

prepend adds the specified string to the beginning of another string.

Input:
{% assign filename = "/index.html" %}
{{ site.url | append: filename }}

{{ "apples, oranges, and bananas" | prepend: "Some fruit: " }}

Output:
https://yo1995.github.io/index.html
Some fruit: apples, oranges, and bananas

capitalize

Makes the first character of a string capitalized, and only capitalizes the first character of the string, so later words are not affected:

Input:
{{ "this is a great title" | capitalize }}

Output:
This is a great title

cgi_escape

Escape a string for use in a URL.

Input:
{{ "foo,bar;baz?" | cgi_escape }}

Output:
foo%2Cbar%3Bbaz%3F

default

Allows you to specify a fallback in case a value doesn’t exist. default will show its value if the left side is nil, false, or empty.

Input:
{% assign product_price = "" %}
{{ product_price | default: 2.99 }}

Output:
2.99

downcase / upcase

Makes each character in a string lowercase (or upcase). It has no effect on strings which are already all lowercase (or upcase).

Input:
{{ "Frank Lin" | downcase }}
{{ "Frank Lin" | upcase }}

Output:
frank lin
FRANK LIN

escape / escape_once

escape works to escapes a string by replacing characters with escape sequences (so that the string can be used in a URL, for example). It doesn’t change strings that don’t have anything to escape.

escape_once can be used to escapes a string without changing existing escaped entities.

Input:
{{ "Have you read 'James & the Giant Peach'?" | escape }}
{{ "1 < 2 & 3" | escape_once }}

Output:
Have you read &#39;James &amp; the Giant Peach&#39;?
1 &lt; 2 &amp; 3

lstrip / rstrip

lstrip removes all whitespaces (tabs, spaces, and newlines) from the beginning of a string. The filter does not affect spaces between words. rstrip removes all whitespace from the right side of a string.

Input:
{{ "          So much room for activities!          " | lstrip }}
{{ "          So much room for activities!          " | rstrip }}

Output:
So much room for activities!          
          So much room for activities!

markdownify

Convert a Markdown-formatted string into HTML.

Input:
{{ "Hello **Jekyll**" | markdownify }}

Output:
<p>Hello <strong>Jekyll</strong></p>

newline_to_br

Replaces every newline (\n) with an HTML line break (<br>).

Input:
{% capture string_with_newlines %}
Hello
there
{% endcapture %}
{{ string_with_newlines | newline_to_br }}

Output:
Hello<br />
there<br />

number_of_words

Count the number of words in a string.

Input:
{{ "Hello Jekyll!" | number_of_words }}

Output:
2

remove / remove_first

Use remove to remove every occurrence of the specified substring from a string.

remove_first only removes the first occurrence of the specified substring from a string.

Input:
{{ "I strained to see the train through the rain" | remove: "rain" }}
{{ "I strained to see the train through the rain" | remove_first: "rain" }}

Output:
I sted to see the t through the 
I sted to see the train through the rain

replace / replace_first

Use replace to replace every occurrence of an argument in a string with the second argument.

And replace_first only replaces the first occurrence of the first argument in a string with the second argument.

Input:
{{ "Take my protein pills and put my helmet on" | replace: "my", "your" }}
{% assign my_string = "Take my protein pills and put my helmet on" %}
{{ my_string | replace_first: "my", "your" }}

Output:
Take your protein pills and put your helmet on
Take your protein pills and put my helmet on

size

Returns the number of characters in a string or the number of items in an array. size can also be used with dot notation (for example, {{ my_string.size }}). This allows you to use size inside tags such as conditionals.

Input:
{{ "Ground control to Major Tom." | size }}
{% assign my_array = "apples, oranges, peaches, plums" | split: ", " %}
{{ my_array | size }}

{% if site.pages.size > 10 %}
  This site has more than 10 pages!
{% endif %}

Output:
28
4
This site has more than 10 pages!

slice

Returns a substring of 1 character beginning at the index specified by the argument passed in. An optional second argument specifies the length of the substring to be returned.

String indices are numbered starting from 0. If the first parameter is a negative number, the indices are counted from the end of the string:

Input:
{{ "Liquid" | slice: 0 }}
{{ "Liquid" | slice: 2 }}
{{ "Liquid" | slice: 2, 5 }}
{{ "Liquid" | slice: -3, 2 }}

Output:
L
q
quid
ui

slugify

Convert a string into a lowercase URL slug.

The slugify filter accepts an option:

• none: no characters
• raw: spaces
• default: spaces and non-alphaunmeric characters
• pretty: spaces and non-alphaunmeric characters except for ._~!$&'()+,;=@

Input:
{{ "The _config.yml file" | slugify }}
{{ "The _config.yml file" | slugify: 'pretty' }}

Output:
the-config-yml-file
the-_config.yml-file

smartify

Convert quotes into smart quotes with SmartyPants.

Input:
{{ 'Use "Jekyll" --- the static generator...' | smartify }}

Output:
Use “Jekyll” — the static generator…

split

Divides an input string into an array using the argument as a separator. split is commonly used to convert comma-separated items from a string to an array.

Input:
{{ "a~b" | split:"~" }}

Output:
ab

strip

Removes all whitespace (tabs, spaces, and newlines) from both the left and right side of a string. It does not affect spaces between words.

Input:
{{ "          So much room for activities!          " | strip }}

Output:
So much room for activities!

strip_html

Removes any HTML tags from a string.

strip_newlines

Removes any newline characters (line breaks) from a string.

truncate / truncatewords

truncate shortens a string down to the number of characters passed as a parameter. If the number of characters specified is less than the length of the string, an ellipsis (…) is appended to the string and is included in the character count.

Input:
{{ "Ground control to Major Tom." | truncate: 20 }}

Output:
Ground control to...

Custom Ellipsis

truncate takes an optional second parameter that specifies the sequence of characters to be appended to the truncated string. By default this is an ellipsis (…), but you can specify a different sequence. The length of the second parameter counts against the number of characters specified by the first parameter.

Input:
{{ "Ground control to Major Tom." | truncate: 25, ", and so on" }}

Output:
Ground control, and so on

truncatewords shortens a string down to the number of words passed as the argument. If the specified number of words is less than the number of words in the string, an ellipsis (…) is appended to the string. And Custom ellipsis can be also applied as truncate.

Input:
{{ "Ground control to Major Tom." | truncatewords: 3 }}

Output:
Ground control to...

uri_escape / xml_escape

Input:
{{ "foo, bar \baz?" | uri_escape }}
{{ "<p>Hi Jekyll</p>"| xml_escape }}

Output:
foo,%20bar%20%5Cbaz?
&lt;p&gt;Hi Jekyll&lt;/p&gt;

url_encode

Converts any URL-unsafe characters in a string into percent-encoded characters.

Input:
{{ "franklin@flinhong.com" | url_encode }}
{{ "Frank Lin" | url_encode }}

Output:
franklin%40flinhong.com
Frank+Lin

Integer

abs

Returns the absolute value of a number, and also work on a string if the string only contains a number.

Input:
{{ -17 | abs }}
{{ "-19.86" | abs }}

Output:
17
19.86

ceil

Rounds the input up to the nearest whole number. Liquid tries to convert the input to a number before the filter is applied.

Input:
{{ 183.357 | ceil }}
{{ "3.5" | ceil }}

Output:
184
4

divided_by / times

divided_by divides a number by the specified number. The result is rounded down to the nearest integer (that is, the floor) if the divisor is an integer.

divided_by produces a result of the same type as the divisor — that is, if you divide by an integer, the result will be an integer. If you divide by a float (a number with a decimal in it), the result will be a float.

times multiplies a number by another number.

Input:
{{ 16 | divided_by: 4 }}
{{ 5 | divided_by: 3 }}
{{ 20 | divided_by: 7.0 }}
{{ 3 | times: 2 }}
{{ 183.357 | times: 12 }}

Output:
4
1
2.857142857142857
6
2200.284

floor

Rounds a number down to the nearest whole number. Liquid tries to convert the input to a number before the filter is applied.

Input:
{{ 1.2 | floor }}
{{ "3.5" | floor }}
{{ 183.357 | floor }}

Output:
1
3
183

minus / plus

Subtracts (adds) a number from another number.

Input:
{{ 4 | minus: 2 }}
{{ 183.357 | minus: 12 }}
{{ 4 | plus: 1 }}

Output:
2
171.357
5

modulo

Returns the remainder of a division operation.

Input:
{{ 3 | modulo: 2 }}
{{ 183.357 | modulo: 12 }}

Output:
1
3.357

round

Rounds an input number to the nearest integer or, if a number is specified as an argument, to that number of decimal places.

Input:
{{ 1.2 | round }}
{{ 2.7 | round }}
{{ 183.357 | round: 2 }}

Output:
1
3
183.36

Array

array_to_sentence_string

Input:
{{ page.array | array_to_sentence_string }}

Output:
1, 2, 3, 4, 5, 6, and 7

first / last

Returns the first (or last) item of an array.

Input:
{% assign my_array = "apples, oranges, peaches, plums" | split: ", " %}
{{ my_array.first }}
{{ my_array.last }}

Output:
apples
plums

group_by

Group an array’s items by a given property.

join

Combines the items in an array into a single string using the argument as a separator.

Input:
{% assign beatles = "John, Paul, George, Ringo" | split: ", " %}
{{ beatles | join: " and " }}

Output:
John and Paul and George and Ringo

jsonify

Convert Hash or Array to JSON.

Input:
{{ page.array | jsonify }}

Output:
["1","2","3","4","5","6","7"]

map (extract)

Creates an array of values by extracting the values of a named property from another object.

{% assign all_categories = site.posts | map: "category" %}
{% for item in all_categories limit:3 %}
{{ item }}
{% endfor %}

push

Adds an object to a array.

Creates an array of values by extracting the values of a named property from another object.

Input:
{% assign my_array = "a,b,c" | split:"," %}
{% assign my_array = my_array | push: 'd' %}
{{ my_array | array_to_sentence_string }}

Output:
a, b, c, and d

reverse

Reverses the order of the items in an array. Although reverse cannot reverse a string, but you can split a string into an array, reverse the array, and rejoin it by chaining together filters:

Input:
{% assign my_array = "apples, oranges, peaches, plums" | split: ", " %}
{{ my_array | reverse | join: ", " }}

{{ "Ground control to Major Tom." | split: "" | reverse | join: "" }}
{{ "Ground control to Major Tom." | split: " " | reverse | join: " " }}

Output:
plums, peaches, oranges, apples

.moT rojaM ot lortnoc dnuorG
Tom. Major to control Ground

size

Return the size of an array or string.

sort

Sorts items in an array by a property of an item in the array. The order of the sorted array is case-sensitive.

Input:
{% assign my_array = "zebra, octopus, giraffe, Octopus" | split: ", " %}
{{ my_array | sort | join: ", " }}

Output:

Octopus, giraffe, octopus, zebra

uniq

Removes any duplicate elements in an array.

Input:
{% assign my_array = "ants, bugs, bees, bugs, ants" | split: ", " %}
{{ my_array | uniq | join: ", " }}

Output:
ants, bugs, bees

where

Select all the objects in an array where the key has the given value.

date

Converts a timestamp into another date format. The format for this syntax is the same as strftime. It also works on strings if they contain well-formatted dates:

Input:
{{ page.date | date: "%a, %b %d, %y" }}
{{ "August 20, 2016" | date: "%b %d, %y" }}

Output:
Wed, Aug 31, 16
Aug 20, 16

Besides, date_to_long_string convert a date to long format; date_to_long_rfc822 convert a Date into RFC-822 format; date_to_string convert a date to short format and date_to_xmlschema convert a Date into ISO 8601 format.

Input:
{{ page.date | date_to_long_string }}
{{ page.date | date_to_rfc822 }}
{{ page.date | date_to_string }}
{{ page.date | date_to_xmlschema }}

Output:
31 August 2016
Wed, 31 Aug 2016 00:00:00 -0700
31 Aug 2016
2016-08-31T00:00:00-07:00