YAML pitfalls & the quoting rule

Lesson 12 gave you a working playbook. This lesson covers the four ways YAML quietly wrecks the next one. None are Ansible bugs: a YAML parser reads your file before Ansible sees a single task, and applies YAML’s rules — not Ansible’s. Learn where those rules bite and you’ve pre-empted most beginner syntax errors.

1. Spaces, never tabs

In YAML, indentation is the structure — no braces mark where a block starts and ends, only depth. Two-space indent is the convention, and it must be spaces: one tab anywhere is a hard parse error. Configure your editor now (spaces for .yml, render whitespace) and this trap disappears.

2. The quoting rule

Ansible’s placeholder for “a variable’s value goes here” is {{ var }}. The full templating machinery is lesson 23; today it’s just a placeholder. Here’s the trap every newcomer hits:

- name: Deploy the config
  ansible.builtin.copy:
    src: {{ config_file }} # bare { — YAML chokes here
    dest: /etc/app.conf

Why: in YAML, { opens an inline flow mapping — {key: value}, the same brace syntax JSON uses. The parser reads {{ config_file }} as a dict inside a dict, finds no key: value shape, and dies with an error that never mentions quoting. The fix costs two characters:

    src: '{{ config_file }}'
    # mid-string needs no quotes — the value doesn't START with {
    dest: /etc/{{ app_name }}.conf

The rule is precise: a value that starts with {{ must be quoted. Braces mid-string open as plain text and parse fine. One preview so the rule doesn’t overreach: when: takes a bare expression with no {{ }} at all — that’s lesson 18.

3. The yes that became True

Unquoted yes, no, true and false all parse as booleans — not strings:

vars:
  enable_firewall: true # boolean — what you meant
  cleanup: yes # ALSO a boolean — same as true
  country_code: 'no' # quoted — stays the string "no"

The surprise cuts both ways. Older playbooks are full of become: yes — it works and means exactly true. The nastier direction is silent data loss: an unquoted no meant as a country code becomes false, with no error at all.

4. Colons and braces inside strings

The last trap: a colon-plus-space inside a value looks like a new key: value pair to the parser.

- name: Deploy: production release # second ": " = parse error
- name: 'Deploy: production release' # quoted — one string

Same family as the quoting rule: :, { and [ are live YAML syntax, so a string containing them needs quotes. When in doubt, quote — it is never wrong. Single quotes are fully literal; double quotes process escapes like \n, occasionally what you want and usually not.

That’s the whole minefield: tabs, leading {{, bare booleans, embedded colons. Each fails (or worse, succeeds) before a single module runs. That is why ansible-playbook --syntax-check is worth running the moment a playbook looks done. Next lesson: what separates a module from a plugin from a collection.