Jinja2 Basics¶

What You'll Learn¶

• The {{ }} expression syntax and where it's evaluated
• Arithmetic, string concatenation, and comparisons inside expressions
• Why quoting matters when a value needs to stay a specific type

Mental Model¶

Every {{ expression }} in a playbook is rendered by the Jinja2 templating engine, after YAML parsing has already turned the file into Python data structures (see YAML → Python Mental Model). YAML has no idea Jinja2 exists — it just sees an ordinary string that happens to contain {{ }} characters; Ansible renders it in a separate, later pass.

Minimal Example¶

- name: Show a rendered string
  ansible.builtin.debug:
    msg: "Deploying to {{ inventory_hostname }} in {{ env_name }}"

Expressions Beyond Plain Substitution¶

vars:
  base_port: 8000
  instance_id: 3

tasks:
  - name: Compute a per-instance port
    ansible.builtin.debug:
      msg: "Port is {{ base_port + instance_id }}"   # 8003

  - name: String concatenation
    ansible.builtin.debug:
      msg: "{{ env_name ~ '-' ~ inventory_hostname }}"

  - name: Comparison
    ansible.builtin.debug:
      msg: "{{ 'production ready' if replica_count >= 3 else 'needs more replicas' }}"

~ concatenates as strings regardless of type (safer than +, which fails between a string and a number). The ternary x if cond else y form is the idiomatic Jinja2 conditional expression.

Keeping a Type When It Matters¶

# String "8080" — fine for most modules, which accept strings
port: "{{ base_port }}"

# Force an actual integer, e.g. for arithmetic or a module that validates type strictly
port: "{{ base_port | int }}"

Jinja2 renders every expression to a string by default unless the entire value is a single {{ }} expression — Ansible then attempts to preserve the underlying type (int, bool, list, dict) rather than stringify it. Mixing literal text with an expression ("Port: {{ base_port }}") always produces a string.

Common Mistakes¶

• Wrapping an already-Jinja2 field (like when:) in {{ }} — redundant, and non-idiomatic: when: "{{ x == 1 }}" works but should just be when: x == 1.
• Expecting + to concatenate a string and a number — use ~ for concatenation, reserve + for arithmetic between numbers.
• Assuming a value stays an integer after being wrapped in surrounding text — "Port: {{ port }}" is always a string, regardless of port's underlying type.

Interview Questions¶

• At what point does Jinja2 rendering happen relative to YAML parsing?
• Why would {{ a + b }} fail when a is a string and b is a number, and what's the fix?

Next¶

Continue to Filters and Tests.