Dynamic Files with Jinja2

Lesson 12 of 12 · 11 min · Advanced

Jinja2 is a powerful templating engine used by Ansible. Instead of creating and maintaining dozens of static configuration files for different servers, you create one template, and Ansible dynamically injects the correct variables during execution.

Why do we need Jinja2?

Suppose you manage three different environments (Dev, Test, Prod). Each requires a configuration file with slightly different ports and debug modes.

The Static Nightmare

You would need to create and maintain three completely separate files (dev.conf, test.conf, prod.conf). As the number of servers scales up to 100+, maintaining multiple static files becomes impossible.

Server Env	Port	Debug Mode
Dev	8080	true
Test	8081	true
Prod	9090	false

How Jinja2 Works

Step 1: Create a Template (config.j2)

# The values inside {{ }} are variables
PORT={{ app_port }}
DEBUG={{ debug }}

Step 2: Define Variables (Inventory)

[dev]
devserver app_port=8080 debug=true

[prod]
prodserver app_port=9090 debug=false

Step 3: Use the Template Module

---
- hosts: all
  tasks:
    - name: Generate configuration
      template:
        src: config.j2
        dest: /tmp/config.conf

The Output

The same template generates completely different files because Ansible provides different variable values for each server during execution.

Dev Output PORT=8080
DEBUG=true
Prod Output PORT=9090
DEBUG=false

Where do the values come from?

Jinja2 does not read values directly from the server. It relies entirely on variables passed to it by Ansible. These variables can come from:

Inventory files
group_vars and host_vars directories
vars: section inside a playbook
Extra variables passed via CLI (-e)
Ansible Facts

Ansible Facts in Templates

# Facts are system data automatically collected by Ansible
$ Hostname: {{ ansible_hostname }}
$ IP Address: {{ ansible_default_ipv4.address }}
$ OS: {{ ansible_distribution }}
# Example Output rendered on the server:
Hostname: prod-server
IP Address: 192.168.1.10
OS: Ubuntu

Jinja2 Syntax & Logic

Variables

Employee Name: {{ empname }}

If Conditions

{% if environment == "production" %}
Debug=false
{% else %}
Debug=true
{% endif %}

For Loops

Users:
{% for user in users %}
- {{ user }}
{% endfor %}

Rendered Loop Output

Users:
- alice
- bob
- charlie

Copy Module vs Template Module

Feature	Copy Module	Template Module (Jinja2)
Function	Copies the exact same static file to every server.	Generates a different, dynamic file for each server based on variables.
Source File	Usually a standard text file (`.conf`, `.txt`)	A Jinja2 template file (`.j2`)
Variables	Does not process variables inside the file.	Parses and replaces `{{ variables }}` and executes `{% logic %}`.
Code Example	`- copy: src: config.conf dest: /tmp/config.conf`	`- template: src: config.j2 dest: /tmp/config.conf`

Real-Time Usage & Interview Prep

Common Use Cases

Jinja2 is heavily used in production environments to generate:

Application configuration files (Nginx, Apache)
Environment-specific properties (DB connections)
System configuration files
Custom shell scripts populated with server info