Jekyll的模板中可以使用Liquid语法进行取值，计算和数据处理，如果仅仅只为了使用Jekyll撰写博客文章，你可以不需要了解liquid, 但是如果你需要定制Jekyll的主题，灵活的控制你的站点，你确实有必要了解一些liquid相关的语法。
Liquid和大多数模板语言类似，也是比较简单的。
官方网址：
概览
Liquid可以分为objects, tags, and filters.
使用英文双大括号对objects进行取值。
获取页面的标题:
{{ page.title }}
标签，例如：条件控制，循环，都是包裹在{% 和 %}中的。
如果user不为空，则输出user.name:
{% if user %}
Hello {{ user.name }}!
{% endif %}
对于filter, 你可以理解为提供了一系列常用的方法，并且支持管道操作。
对url地址添加.html后缀：
{{ "/my/fancy/url" | append: ".html" }}
输出：
/my/fancy/url.html
首字母大写，然后在前面添加Hello :
{{ "adam!" | capitalize | prepend: "Hello " }}
输出：
Hello Adam!
是不是很easy?
操作符
Liquid包含如下操作符：
== |
equals |
!= |
does not equal |
> |
greater than |
< |
less than |
>= |
greater than or equal to |
<= |
less than or equal to |
or |
logical or |
and |
logical and |
举个栗子，你一看就懂得。
{% if product.title == "Awesome Shoes" %}
These shoes are awesome!
{% endif %}
{% if product.type == "Shirt" or product.type == "Shoes" %}
This is a shirt or a pair of shoes.
{% endif %}
另外还有一个操作符：contains, 它可以判断字符串的包含，还可以判断数组中的字符串包含关系（如果数组中是非字符串的，是不能判断出来的）。
{% if product.title contains "Pack" %}
This title of product contains the word Pack.
{% endif %}
{% if product.tags contains "Hello" %}
This product has been tagged with Hello.
{% endif %}
数据类型
和大多数语言一样，Liquid也有常用的数据类型。
Boolean
true or false
{% assign foo = true %}
{% assign bar = false %}
Number
统一都叫数字，不分整型，浮点型了
{% assign my_int = 25 %}
{% assign my_float = 39.756 %}
String
使用单引号或双引号包裹的字符串。
{% assign my_string = "Hello World!" %}
Nil
类似js的undefined.
Array
Liquid中无法单独定义出数组，不过你可以使用filter处理得到数组。
数组的取值采用普遍接受的索引取值方式：
{{ site.users[0] }}
{{ site.users[1] }}
{{ site.users[3] }}
对于数组比较常用的还有遍历操作：
{% for user in site.users %}
{{ user }}
{% endfor %}
Truthy and Falsy
这两个不叫数据类型，但是是条件判断的依据，他们叫：真 和 假。什么时候是真？什么时候是假？很简单，你只需要记着：Nil 和 false 是假，其他的都是真就对了，包括0，空字符串都是真。
| truthy | falsy | |
|---|---|---|
| true | • | |
| false | • | |
| nil | • | |
| string | • | |
| empty string | • | |
| 0 | • | |
| integer | • | |
| float | • | |
| array | • | |
| empty array | • | |
| page | • | |
| EmptyDrop | • |
Tags
Tags你可以理解为Liquid的关键词，如条件判断if, 循环语句for, Tag是 {% 和 %} 包裹起来的。
注释
注释是不会被Liquid引擎处理的。Liquid的注释既可以作用于单行，又可以作用于多行。
{% comment %} line comment {% endcomment %}
{% comment %}
block comment
{% endcomment %}
条件流程控制
直接看例子吧。
{% if product.title == 'Awesome Shoes' %}
These shoes are awesome!
{% endif %}
{% unless product.title == 'Awesome Shoes' %}
These shoes are not awesome.
{% endunless %}
equals
{% if product.title != 'Awesome Shoes' %}
These shoes are not awesome.
{% endif %}
{% if customer.name == 'kevin' %}
Hey Kevin!
{% elsif customer.name == 'anonymous' %}
Hey Anonymous!
{% else %}
Hi Stranger!
{% endif %}
{% 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 %}
循环遍历
{% for i in (1..5) %}
{% if i == 4 %}
{% break %}
{% else %}
{{ i }}
{% endif %}
{% endfor %}
{% for i in (1..5) %}
{% if i == 4 %}
{% continue %}
{% else %}
{{ i }}
{% endif %}
{% endfor %}
limit
<!-- if array = [1,2,3,4,5,6] -->
{% for item in array limit:2 %}
{{ item }}
{% endfor %}
1 2
offset
<!-- if array = [1,2,3,4,5,6] -->
{% for item in array offset:2 %}
{{ item }}
{% endfor %}
3 4 5 6
range
{% for i in (3..5) %}
{{ i }}
{% endfor %}
{% assign num = 4 %}
{% for i in (1..num) %}
{{ i }}
{% endfor %}
3 4 5
1 2 3 4
reversed
<!-- if array = [1,2,3,4,5,6] -->
{% for item in array reversed %}
{{ item }}
{% endfor %}
6 5 4 3 2 1
cycle, 周期循环的意思。
{% cycle 'one', 'two', 'three' %}
{% cycle 'one', 'two', 'three' %}
{% cycle 'one', 'two', 'three' %}
{% cycle 'one', 'two', 'three' %}
one
two
three
one
tablerow, 快速构建表格的行。
<table>
{% tablerow product in collection.products %}
{{ product.title }}
{% endtablerow %}
</table>
<table>
<tr class="row1">
<td class="col1">
Cool Shirt
</td>
<td class="col2">
Alien Poster
</td>
<td class="col3">
Batman Poster
</td>
<td class="col4">
Bullseye Shirt
</td>
<td class="col5">
Another Classic Vinyl
</td>
<td class="col6">
Awesome Jeans
</td>
</tr>
</table>
{% tablerow product in collection.products cols:2 %}
{{ product.title }}
{% endtablerow %}
<table>
<tr class="row1">
<td class="col1">
Cool Shirt
</td>
<td class="col2">
Alien Poster
</td>
</tr>
<tr class="row2">
<td class="col1">
Batman Poster
</td>
<td class="col2">
Bullseye Shirt
</td>
</tr>
<tr class="row3">
<td class="col1">
Another Classic Vinyl
</td>
<td class="col2">
Awesome Jeans
</td>
</tr>
</table>
{% tablerow product in collection.products cols:2 limit:3 %}
{{ product.title }}
{% endtablerow %}
{% tablerow product in collection.products cols:2 offset:3 %}
{{ product.title }}
{% endtablerow %}
<!--variable number example-->
{% assign num = 4 %}
<table>
{% tablerow i in (1..num) %}
{{ i }}
{% endtablerow %}
</table>
<!--literal number example-->
<table>
{% tablerow i in (3..5) %}
{{ i }}
{% endtablerow %}
</table>
变量
Liquid, 支持变量的定义和计算。
assign
{% assign my_variable = false %}
{% if my_variable != true %}
This statement is valid.
{% endif %}
{% assign foo = "bar" %}
{{ foo }}
capture
{% capture my_variable %}I am being captured.{% endcapture %}
{{ my_variable }}
I am being captured.
{% assign favorite_food = 'pizza' %}
{% assign age = 35 %}
{% capture about_me %}
I am {{ age }} and my favorite food is {{ favorite_food }}.
{% endcapture %}
{{ about_me }}
I am 35 and my favourite food is pizza.
Creates a new number variable, and increases its value by one every time it is called.
The initial value is 0.
{% increment my_counter %}
{% increment my_counter %}
{% increment my_counter %}
0
1
2
Variables created through the increment tag are independent from variables created through assign or capture.
{% assign var = 10 %}
{% increment var %}
{% increment var %}
{% increment var %}
{{ var }}
0
1
2
10
{% decrement variable %}
{% decrement variable %}
{% decrement variable %}
-1
-2
-3
Filters
Liquid提供了一些常用的过滤器（方法）。
abs
{{ -17 | abs }}
17
{{ "-19.86" | abs }}
19.86
append, prepend
{% assign filename = "/index.html" %}
{{ "website.com" | append: filename }}
website.com/index.html
capitalize
首字母大写。
{{ "title" | capitalize }}
Title
{{ "my great title" | capitalize }}
My great title
date
{{ article.published_at | date: "%a, %b %d, %y" }}
Fri, Jul 17, 15
{{ article.published_at | date: "%Y" }}
2015
{{ "March 14, 2016" | date: "%b %d, %y" }}
Mar 14, 16
To get the current time, pass the special word "now" (or "today") to date:
This page was last updated at {{ "now" | date: "%Y-%m-%d %H:%M" }}.
This page was last updated at 2017-06-29 14:45.
default
{{ product_price | default: 2.99 }}
2.99
{% assign product_price = 4.99 %}
{{ product_price | default: 2.99 }}
4.99
{% assign product_price = "" %}
{{ product_price | default: 2.99 }}
2.99
lstrip, rstrip, strip
去除句子首部的空白字符。
{{ " So much room for activities! " | lstrip }}
So much room for activities!
去除句子尾部的空白字符。
{{ " So much room for activities! " | rstrip }}
So much room for activities!
去除首尾的空白字符。
{{ " So much room for activities! " | strip }}
So much room for activities!
plus, minus, times, divided_by
加，减，乘，除运算。
{{ 16 | divided_by: 4 }}
4
{{ 5 | divided_by: 3 }}
1
{{ 20 | divided_by: 7.0 }}
2.857142857142857
{% assign my_integer = 7 %}
{% assign my_float = my_integer | times: 1.0 %}
{{ 20 | divided_by: my_float }}
2.857142857142857
remove, remove_first
{{ "I strained to see the train through the rain" | remove: "rain" }}
I sted to see the t through the
{{ "I strained to see the train through the rain" | remove_first: "rain" }}
I sted to see the train through the rain
replace, replace_first
{{ "Take my protein pills and put my helmet on" | replace: "my", "your" }}
Take your protein pills and put your helmet on
{% assign my_string = "Take my protein pills and put my helmet on" %}
{{ my_string | replace_first: "my", "your" }}
Take your protein pills and put my helmet on
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.
截取指定长度的字符串，第二个表示长度，第一个表示索引，从0开始，如果为负数，从后面往前数，从-1开始。
{{ "Liquid" | slice: 0 }}
L
{{ "Liquid" | slice: 2 }}
q
{{ "Liquid" | slice: 2, 5 }}
quid
{{ "Liquid" | slice: -3, 2 }}
ui
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.
分割字符串组成一个数组。
{% assign beatles = "John, Paul, George, Ringo" | split: ", " %}
{% for member in beatles %}
{{ member }}
{% endfor %}
John
Paul
George
Ringo
当然这里没有列举完所有的filter, 这些是相对比较常用的，记着这些，如果满足不了需求，再去官方文档上翻阅翻阅。
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