Mastering Ansible Playbook Variables: Priorities, Definitions, and Best Practices

Variable Priority

extra vars

(passed with -e) – highest priority

Connection variables defined in inventory (e.g., ansible_ssh_user) – second priority

Other variables such as command‑line overrides, play vars, include vars, role vars – third priority

Variables defined directly in the inventory file – fourth priority

Automatically discovered facts – fifth priority

Role default variables – lowest priority

In an inventory list, a variable defined for a single host overrides the same variable defined for a group.

YAML Trap

When a value starts with {{ foo }}, the entire line must be quoted to avoid being interpreted as a YAML dictionary.

|---
| hosts: app_servers
| vars:
|   app_path: "{{ base_path }}/data/web"

Methods to Define Variables

1. Define in Inventory Host List

Variables can be added directly to /etc/ansible/hosts and apply only to the specified host or group.

| $ egrep -v "^#|^$" /etc/ansible/hosts
| 10.4.7.101 key=20180101
| 10.4.7.102 key="niubility"
|---
| - hosts: all
|   gather_facts: False
|   tasks:
|   - name: show key
|     debug: msg="the {{ inventory_hostname }} value is {{ key }}"

2. Use host_vars and group_vars Directories

Create host_vars and group_vars under the Ansible configuration directory ( /etc/ansible/ or /usr/local/etc/ansible/) and place variable files there.

# /etc/ansible/host_vars/10.4.7.101
---
user: root
pass: root@123

# /etc/ansible/group_vars/test
---
user: work
pass: work@123

3. Define with vars_files

# vars.yml
---
key: jiayou

# bo.yml
---
- hosts: all
  gather_facts: False
  vars_files:
    - vars.yml
  tasks:
    - name: display
      debug: msg="the {{ inventory_hostname }} value is {{ key }}"

4. Interactive Prompt with vars_prompt

# prom.yml
---
- hosts: test
  remote_user: root
  vars_prompt:
    - name: "var1"
      prompt: "please input your name"
      private: no
    - name: "var2"
      prompt: "please input your age"
      private: yes
      default: 18
  tasks:
    - name: display var1
      debug: msg="your name is {{ var1 }}"
    - name: display var2
      debug: msg="your age is {{ var2 }}"

5. Pass Variables via Command Line ( --extra-vars )

# exap.yml
---
- hosts: "{{ hosts }}"
  remote_user: "{{ user }}"
  tasks:
    - name: test
      debug: msg="your hosts is {{ hosts }}, user is {{ user }}"

# Run
ansible-playbook exap.yml -e "hosts=test user=root"

6. Define Variables Inside Playbook ( vars Section)

# playbook.yml
---
- hosts: test
  remote_user: root
  vars:
    dir1: /root/Ansible
    dir2: /root/Ansible/test1
    dir3: /root/Ansible/test2
  tasks:
    - name: create dir1
      file: name={{ dir1 }} state=directory
    - name: create dir2
      file: name={{ dir2 }} state=directory
    - name: create dir3
      file: name={{ dir3 }} state=directory

7. Define Variables in Roles

# roles/test/vars/main.yml
---
var1: value1
var2: value2
var3: value3

8. Use Automatically Collected Facts

Facts provide system information such as IP address, OS version, hostname, etc.

# ansible test -m setup | grep "ansible_python_version"
"ansible_python_version": "2.7.5",
# ansible test -m setup | grep "ansible_nodename"
"ansible_nodename": "template",

Facts can be disabled with gather_facts: no to speed up execution.

9. Register Variable from Task Output

# register.yml
---
- hosts: test
  remote_user: root
  tasks:
    - name: get IP
      shell: hostname -I
      register: info
    - name: display IP
      debug: msg="this host ip is {{ info.stdout }}"

10. Access Variables of Other Hosts with hostvars

# test.yml
---
- hosts: test
  remote_user: root
  gather_facts: False
  tasks:
    - name: show addr from host 10.4.7.101
      debug: msg="She comes from {{ hostvars['10.4.7.101']['addr'] }}"
    - name: show user and age from host 10.4.7.102
      debug: msg="I am {{ hostvars['10.4.7.102']['user'] }}, age {{ hostvars['10.4.7.102']['age'] }}"

11. List, Loop, and Dictionary Variables

Variables can be lists and used with loops.

# test.yml
---
- hosts: test
  remote_user: root
  gather_facts: False
  vars:
    list: [1,2,3]
  tasks:
    - name: echo list
      debug: msg="{{ list }}"
    - name: loop over list
      debug: msg="{{ item }}"
      with_items: "{{ list }}"