Templates & Jinja2

Generate config files from data.

Most configuration is a file that differs slightly per host or environment, and the template module is how you generate it: you write a Jinja2 template with variables and logic, and Ansible renders it with the host’s variables and facts, then places the result. One template plus per-host variables replaces dozens of hand-maintained config files.

worker_processes {{ nginx_worker_processes }};
http {
  server {
    listen {{ app_port }};
    server_name {{ inventory_hostname }};
    {% if enable_tls | default(false) %}
    ssl_certificate {{ tls_cert_path }};
    {% endif %}
  }
}

- name: Render nginx config
  ansible.builtin.template:
    src: nginx.conf.j2
    dest: /etc/nginx/nginx.conf
    validate: nginx -t -c %s      # test the file before accepting it
  notify: reload nginx            # trigger a handler only if it changed

Jinja2 in practice

Jinja2 gives you variable interpolation ({{ }}), conditionals ({% if %}), loops ({% for %}), and filters (| default, | upper, | to_json) — enough to express real config logic without turning templates into programs. The discipline is to keep logic minimal: templates should render data, not compute it. Compute values in variables and tasks, and let the template mostly substitute them.