Playbooks & YAML basics

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Lesson 04 / 13

A playbook is where ad-hoc commands graduate into something repeatable and version-controllable. It's YAML describing one or more plays — each play targets a group of hosts and runs a list of tasks against them, in order.


# webserver.yml
- name: Configure web servers
  hosts: webservers
  become: true
  tasks:
    - name: Install nginx
      apt:
        name: nginx
        state: present

    - name: Copy homepage
      copy:
        src: files/index.html
        dest: /var/www/html/index.html

    - name: Ensure nginx is running
      service:
        name: nginx
        state: started
        enabled: true


ansible-playbook -i inventory.ini webserver.yml

# preview what would change, without changing anything
ansible-playbook -i inventory.ini webserver.yml --check --diff

YAML rule	Why it trips people up
Indentation is meaningful	Tabs are invalid — use spaces, consistently, or the file won't parse
`key: value` needs a space after the colon	`name:web` is not the same as `name: web`
Lists use a leading dash	Each task starts with `- name: ...`

Always run --check --diff on anything touching production for the first time. It shows you exactly what would change without changing it — the closest thing Ansible has to a dry run.

Try it yourselfSave the playbook above, run it with --check --diff first, read the proposed diff, then run it for real. Run it a second time right after — notice nothing reports as "changed" the second time. That's idempotency, and it's the whole next lesson.

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