Jinja2 Basics - KodeKloud

In this lesson we cover the essentials of Jinja2: what templating is, how templates are used in IT automation and web development, and the core syntax for substitutions, filters, loops, and conditionals. What is templating? Think of an invitation letter: the body (event details, venue, signature) stays the same for all recipients, while the addressee and family member names change. The invitation format is the template and the names are variables. A templating engine takes a template plus variables and renders the final personalized output — one email or hundreds.

The image displays four invitation templates with differing text colors for different recipients, titled "Templating Engine." Each template invites the recipient and their family to a company celebration.

Templates are widely used in web pages (HTML templates -> rendered HTML) and in automation tools like Ansible (templates -> configuration or files on remote hosts). Jinja2 is the templating engine commonly used in Python projects and many automation frameworks.

Templating helps you reuse structure and separate content/data from presentation. You can render the same template with different data to produce many customized outputs.

Basic HTML example (Jinja2 template)

Template:

<!DOCTYPE html>
<html>
<head>
  <title>&#123;&#123; title &#125;&#125;</title>
</head>
<body>
  &#123;&#123; msg &#125;&#125;
</body>
</html>

Variables (YAML):

title: Our Site
msg: Welcome!

Rendered HTML:

<!DOCTYPE html>
<html>
<head>
  <title>Our Site</title>
</head>
<body>
  Welcome!
</body>
</html>

Templating in Ansible

Ansible uses Jinja2 extensively for templates and variable interpolation. Example task that touches a file whose path is provided as a variable:

- hosts: web1
  tasks:
    - file:
        path: "&#123;&#123; file &#125;&#125;"
        state: touch

If you supply file: /tmp/1.txt, the task will operate on /tmp/1.txt. Templates are also used to generate configuration files. Example Jinja2 template for a MySQL configuration:

[mysqld]
innodb_buffer_pool_size=&#123;&#123; pool_size &#125;&#125;
datadir=&#123;&#123; datadir &#125;&#125;
user=&#123;&#123; mysql &#125;&#125;

When pool_size, datadir, and mysql are provided, the template renders into a ready-to-use configuration file for the target host. For more on Ansible templating, see the Ansible documentation and tutorials such as the KodeKloud course linked below.

What is Jinja2?

Jinja2 is a full-featured templating engine for Python. It supports:

Variable substitution
Filters to transform values
Control structures: loops and conditionals
Blocks and template inheritance
Macros for reusable snippets

Example using a block and a loop:

<title>&#123;% block title &#37;&#125;&#123;% endblock &#37;&#125;</title>
<ul>
&#123;% for user in users &#37;&#125;
  <li><a href="&#123;&#123; user.url &#125;&#125;">&#123;&#123; user.username &#125;&#125;</a></li>
&#123;% endfor &#37;&#125;
</ul>

Variables and Filters

A simple substitution uses {{ variable }}. Filters modify values using the pipe | syntax. Examples:

The name is &#123;&#123; my_name &#125;&#125;             => The name is Bond
The name is &#123;&#123; my_name | upper &#125;&#125;    => The name is BOND
The name is &#123;&#123; my_name | lower &#125;&#125;    => The name is bond
The name is &#123;&#123; my_name | title &#125;&#125;    => The name is Bond
The name is &#123;&#123; my_name | replace("Bond", "Bourne") &#125;&#125; => The name is Bourne
The name is &#123;&#123; first_name | default("James") &#125;&#125; &#123;&#123; my_name &#125;&#125; => The name is James Bond

Undefined variables can cause unexpected output. Use default(...) or enable strict undefined checks during rendering to catch missing values early.

Common filters (overview)

Filter	Purpose	Example
`upper`, `lower`, `title`	Change string case	`{{ my_name	upper }}`
`replace(old, new)`	Replace substrings	`{{ my_name	replace(“Bond”, “Bourne”) }}`
`default(value)`	Provide fallback for undefined values	`{{ first_name	default(“James”) }}`
`join(sep)`	Join list items into a string	`{{ my_list	join(”, ”) }}`
`random`	Select a random element	`{{ my_list	random }}`
`unique`	Remove duplicates from a list	`{{ [1,2,2,3]	unique }}`
`min`, `max`	Minimum/maximum of a list	`{{ [1,2,3]	min }}`/`{{ [1,2,3]	max }}`
`union(list2)`	Merge lists and remove duplicates	`{{ [1,2]	union([2,3]) }}`

List and set-based filters (examples)

&#123;&#123; [1, 2, 3] | min &#125;&#125;         => 1
&#123;&#123; [1, 2, 3] | max &#125;&#125;         => 3
&#123;&#123; [1, 2, 3, 2] | unique &#125;&#125;   => [1, 2, 3]
&#123;&#123; [1, 2, 3, 4] | union([4, 5]) &#125;&#125;    => [1, 2, 3, 4, 5]

Control structures: loops and conditionals

Jinja2 control structures use {% ... %} tags. Common constructs:

For loop:

&#123;% for number in [0, 1, 2, 3, 4] %&#125;
&#123;&#123; number &#125;&#125;
&#123;% endfor %&#125;

Rendered:

Loop with conditional (print only number equal to 2):

&#123;% for number in [0, 1, 2, 3, 4] %&#125;
  &#123;% if number == 2 %&#125;
&#123;&#123; number &#125;&#125;
  &#123;% endif %&#125;
&#123;% endfor %&#125;

Rendered:

If/elif/else example:

&#123;% if users | length == 0 %&#125;
  No users found.
&#123;% elif users | length == 1 %&#125;
  One user found.
&#123;% else %&#125;
  Multiple users found.
&#123;% endif %&#125;

Summary

Jinja2 templates mix static text with placeholders, filters, loops, and conditionals to produce dynamic outputs.
Use filters to transform values and control structures to repeat or conditionally render content.
Templating is invaluable in automation (Ansible), configuration management, and web development for reusing structure while rendering variable data.

​Basic HTML example (Jinja2 template)

​Templating in Ansible

​What is Jinja2?

​Variables and Filters

​Common filters (overview)

​List and set-based filters (examples)

​Control structures: loops and conditionals

​Summary

Watch Video

Basic HTML example (Jinja2 template)

Templating in Ansible

What is Jinja2?

Variables and Filters

Common filters (overview)

List and set-based filters (examples)

Control structures: loops and conditionals

Summary