Master Ansible Playbooks: Syntax, Variables, Handlers & Advanced Tips

What is a Playbook?

A playbook is essentially a collection of tasks written in YAML, similar to a shell script, but it can run on multiple servers. Each task corresponds to a command, and the playbook specifies which hosts to target.

Playbook Syntax

Playbooks use YAML. The file starts with three dashes (---) and uses indentation to define blocks. A simple example from the official documentation:

---
- hosts: webservers
  vars:
    http_port: 80
    max_clients: 200
  remote_user: root
  tasks:
  - name: ensure apache is at the latest version
    yum: pkg=httpd state=latest
  - name: write the apache config file
    template: src=/srv/httpd.j2 dest=/etc/httpd.conf

Indentation works like Python to separate code blocks.

Example: Check MySQL Status

---
- hosts: all
  remote_user: root
  gather_facts: no
  tasks:
    - name: check the mysql status
      service: name=mysqld state=running

Running the playbook produces output showing the status of each host.

Limiting Host Execution

Use the --limit option to run a playbook only on a subset of hosts defined in the inventory.

[all]
10.0.102.212
10.0.102.200
10.0.102.162

[dbservers]
10.0.102.162

Running with --limit dbservers restricts execution to the dbservers group.

Listing Target Hosts

ansible-playbook test.yml --list-hosts

Shows which hosts the playbook will affect.

Playbook Variables

Extra Vars

ansible-playbook test.yml --extra-vars "test_var=test" -v

The variable test_var is passed to the playbook and can be echoed.

Vars Block

---
- hosts: all
  remote_user: root
  gather_facts: no
  vars:
    test_var: Hello World
  tasks:
    - name: test playbook variables
      command: echo {{ test_var }}

Vars Files

---
- hosts: all
  remote_user: root
  gather_facts: no
  vars_files:
    - vars.yml
  tasks:
    - name: test playbook variables
      command: echo {{ test_var }}

Where vars.yml contains:

---
test_var: Hello World

Host‑Specific Variables

Define variables per host directly in the inventory:

[all]
10.0.102.212 test_var=212
10.0.102.200 test_var=200
10.0.102.162 test_var=162

Host and Group Variable Files

Create /etc/ansible/host_vars/ and /etc/ansible/group_vars/ directories with files named after hosts or groups.

# /etc/ansible/host_vars/10.0.102.162
---
test_var: 162

Group variables example ( group_vars/all):

---
test_group_var: from group

Facts Variables

Facts are automatically gathered system information. You can view them with the setup module.

ansible 10.0.102.162 -m setup

Common facts include ansible_os_family, ansible_hostname, etc., and can be used in when statements.

Register Variables

Use register to capture a task’s result.

- name: test the register variables
  shell: uptime
  register: results

- name: print the register result
  debug: msg="{{ results.stdout }}"

Conditional Execution (when)

- name: shut down the db server
  service: name=mysqld state=stopped
  when: ansible_eth0.ipv4.address == "10.0.102.162"

Task Delegation

Run a specific task on a different host using delegate_to or local_action.

- name: stop the db server
  service: name=mysqld state=stopped
  delegate_to: 10.0.102.162

Pausing Execution

Use the wait_for module to pause until a condition is met.

- name: wait for webserver to start
  local_action:
    module: wait_for
    host: webserver1
    port: 80
    delay: 10
    timeout: 300
    state: started

Interactive Prompts (vars_prompt)

---
- hosts: all
  remote_user: root
  vars_prompt:
    - name: share_user
      prompt: "what is your network username?"
      private: no
    - name: share_pass
      prompt: "what is your network password"
      private: no

Tags

Tag tasks, roles, or entire playbooks to run or skip specific parts.

- name: Notify on completion
  local_action:
    module: osx_say
    msg: "{{inventory_hostname}} is finished"
    voice: Zarvox
  tags:
    - notifications
    - say

Blocks, Rescue & Always

Group tasks with block and handle errors with rescue and always.

tasks:
  - block:
      - name: risky command
        shell: ./do_something.sh
    rescue:
      - debug: msg="There was an error in the block"
    always:
      - debug: msg="This always executes"

Source: https://www.cnblogs.com/wxzhe/p/10386649.html