Jinja2 in Ansible

In our previous guide, we explored the basic features of Jinja2. In this document, we focus on how Ansible enhances Jinja2 with custom filters specifically designed for managing infrastructure. For more details on Jinja2 filters, refer to the Jinja2 documentation.

Ansible extends the built-in Jinja2 filters with additional options to handle practical tasks such as converting YAML and JSON, managing file names and directory paths across Linux and Windows, and processing passwords and regular expressions. Here, we emphasize file-related filters.

File-Related Filters

To extract the file name from a complete path on a Linux system, you use the basename filter. For example, given the path /etc/hosts, applying the basename filter returns hosts. However, note that this method does not work with Windows paths—which use backslashes. Instead, use the win_basename filter for Windows paths.

To separate the drive letter from the rest of a Windows path, use the win_splitdrive filter. This filter returns an array where the first element is the drive letter and the second element is the remaining path. If you need only the drive letter, simply chain the first filter to the result.

Examples of File-Related Filters

Below are examples demonstrating how to use these filters:

{{ "/etc/hosts" | basename }}
{{ "c:\windows\hosts" | win_basename }}
{{ "c:\windows\hosts" | win_splitdrive }}

To extract only the drive letter from a Windows path, chain the filters as follows:

{{ "c:\windows\hosts" | win_splitdrive | first }}

Similarly, if you need just the path without the drive letter, chain the last filter:

{{ "c:\windows\hosts" | win_splitdrive | last }}

Integrating Jinja2 with Ansible Playbooks

Before executing an Ansible playbook, Ansible processes the file through the Jinja2 templating engine. This step replaces variables with actual values from the inventory, ensuring the final playbook is ready for execution.

Consider the example below, which includes a simple inventory file and a playbook that updates the DNS server settings:

# /etc/ansible/hosts
web1 ansible_host=172.20.1.100 dns_server=10.5.5.4
web2 ansible_host=172.20.1.101 dns_server=10.5.5.4
web3 ansible_host=172.20.1.102 dns_server=10.5.5.4

---

- name: Update DNS server
  hosts: all
  tasks:
    - nsupdate:
        server: '{{ dns_server }}'

Hands-On Exercises

The first exercise introduces you to an emulated environment where you can practice working with Jinja2 independently. After gaining some experience, you will transition into an Ansible lab exercise designed to cement your understanding of integrating Jinja2 filters with Ansible playbooks.

Good luck with the exercises and enjoy exploring the powerful combination of Jinja2 and Ansible!