Jinja2 Basics

Templating lets you separate fixed structure (the template) from changing content (the variables). For example, a company’s CEO may want to send a personalized invitation to every employee: the letter layout remains the same (the template) while names and family members vary (the variables). A templating engine takes a template plus a set of variables and renders one or many final outputs (one email or hundreds).

A dark presentation slide titled "Templating Engine" showing four colored
text boxes, each containing a sample invitation letter addressed to different
people. The letters have different colored monospaced text (yellow, orange,
blue, green) on black
panels.

In IT, templating is widely used to generate HTML pages, configuration files, or inputs for automation tools such as Ansible. Keep a template file and feed variables to produce valid output automatically.

Simple HTML templating example

Template:

<!-- template.html -->
<!DOCTYPE html>
<html>
  <head>
    <title>{{ title }}</title>
  </head>
  <body>
    {{ msg }}
  </body>
</html>

Variables:

# variables.yml
title: Our Site
msg: Welcome!

Rendered HTML:

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

Using Jinja2 in Ansible

Ansible uses Jinja2 templating everywhere: playbooks, task parameters, templates, and inventory. Example: using a variable for a file path in a task. Template playbook:

# playbook.yml (template)
- hosts: web1
  tasks:
    - name: Create a file at a path from a variable
      file:
        path: "{{ file }}"
        state: touch

Variables:

# vars.yml
file: /tmp/1.txt

Rendered task after substitution:

- hosts: web1
  tasks:
    - name: Create a file at a path from a variable
      file:
        path: /tmp/1.txt
        state: touch

You can template configuration files too. Example MySQL configuration template and result. Template:

# my.cnf.j2 (template)
[mysqld]
innodb-buffer-pool-size={{ pool_size }}
datadir={{ datadir }}
user={{ mysql_user }}

Variables:

# vars.yml
pool_size: 128M
datadir: /var/lib/mysql
mysql_user: mysql

Rendered configuration:

[mysqld]
innodb-buffer-pool-size=128M
datadir=/var/lib/mysql
user=mysql

What is Jinja2?

Jinja2 is a full-featured, powerful templating engine for Python. It provides a compact syntax for:

Expressions that output values
Control structures (loops, conditionals)
Template inheritance (blocks)
Filters to transform values
Tests to check values (defined, none, etc.)

Use Jinja2 to create reusable templates for web pages, configuration files, cloud templates, and automation playbooks.

Common Jinja2 constructs

Expressions: {{ ... }} — evaluate and output a value.
Statements: {% ... %} — control structures like for/if or block definitions.
Comments: {# ... #} — template comments not rendered in output.

Blocks and template inheritance

Blocks let a base template define placeholders that child templates can override. Example:

<title>{% block title %}{% endblock %}</title>
<ul>
  {% for user in users %}
  <li><a href="{{ user.url }}">{{ user.username }}</a></li>
  {% endfor %}
</ul>

Filters: transform values inline

Filters use the pipe (|) syntax and let you transform a variable before rendering.

Strings: upper, lower, title, replace, default
Sequences: min, max, unique, union, intersect, random, join

Examples:

The name is {{ my_name }}                      => The name is Bond
The name is {{ my_name | upper }}              => The name is BOND
The name is {{ my_name | lower }}              => The name is bond
The name is {{ my_name | title }}              => The name is Bond
The name is {{ my_name | replace("Bond","Bourne") }} => The name is Bourne
{{ first_name | default("James") }} {{ my_name }}     => James Bond   # if first_name undefined

Use filters to transform values inline. The default filter prevents undefined-variable errors by providing a fallback.

Table: selected filters, use cases, and examples | Filter | Use case | Example | | ------------------- | --------------------------------- | ---------------- | ----------------------------------- | | upper, lower, title | Normalize string case | {{ name | upper }} -> “BOND” | | replace | Replace substrings | {{ text | replace("a","b") }} | | default | Fallback when undefined | {{ value | default("N/A") }} | | join | Join list elements into a string | {{ words | join(" ") }} -> “The name is Bond” | | unique | Remove duplicates from a sequence | {{ [1,2,2] | unique }} -> [1,2] | | union, intersect | Combine or intersect sequences | {{ [1,2] | union([2,3]) }} -> [1,2,3] | | random | Pick a random element | {{ range(1,201) | random }} -> random 1–200 | Selected examples (sequence filters):

{{ [1, 2, 3] | min }}                => 1
{{ [1, 2, 3] | max }}                => 3
{{ [1, 2, 3, 2] | unique }}          => [1, 2, 3]
{{ [1, 2, 3, 4] | union([4, 5]) }}   => [1, 2, 3, 4, 5]
{{ [1, 2, 3, 4] | intersect([4, 5]) }} => [4]
{{ range(1, 201) | random }}         => Random number between 1 and 200
{{ ["The", "name", "is", "Bond"] | join(" ") }} => The name is Bond

Blocks vs expressions

Use {{ ... }} when you want to output a value or the result of an expression.
Use {% ... %} for control flow, including loops, conditionals, and block definitions.

Loops and conditionals

Example template that prints a message for numbers 0–4 and highlights when the number equals 2:

{% for number in [0, 1, 2, 3, 4] %}
  {% if number == 2 %}
    Number is two
  {% else %}
    Number is not two
  {% endif %}
{% endfor %}

Rendered output:

Number is not two
Number is not two
Number is two
Number is not two
Number is not two

When to use Jinja2 templates

Resource Type	Use Case
HTML pages	Render dynamic web pages with consistent layouts
Configuration files	Generate valid configs for services and daemons
Automation scripts (Ansible, Terraform templates)	Inject variables into tasks and manifests
Email or document generation	Produce personalized documents at scale

Next steps and references

Jinja2 provides many more filters, tests, and extension points. Combine filters, tests, template inheritance, and macros to build clean, maintainable templates for web apps, automation, and infrastructure. Links and references:

Jinja2 documentation: https://jinja.palletsprojects.com/
Ansible templating: https://docs.ansible.com/ansible/latest/user_guide/playbooks_templating.html
Ansible Advanced Course: https://learn.kodekloud.com/user/courses/ansible-advanced-course

Introduction

Core Components

Install and Configure

Ansible Modules

Variables and Jinja2

Include and Roles

Other Topics

Playbook Flow

Simple HTML templating example

Using Jinja2 in Ansible

What is Jinja2?

Common Jinja2 constructs

Blocks and template inheritance

Filters: transform values inline

Blocks vs expressions

Loops and conditionals

When to use Jinja2 templates

Next steps and references

Watch Video

​Simple HTML templating example

​Using Jinja2 in Ansible

​What is Jinja2?

​Common Jinja2 constructs

​Blocks and template inheritance

​Filters: transform values inline

​Blocks vs expressions

​Loops and conditionals

​When to use Jinja2 templates

​Next steps and references

Watch Video

Simple HTML templating example

Using Jinja2 in Ansible

What is Jinja2?

Common Jinja2 constructs

Blocks and template inheritance

Filters: transform values inline

Blocks vs expressions

Loops and conditionals

When to use Jinja2 templates

Next steps and references