Ansible Variables: Define, Use, and Override Variables (Complete Guide)

Ansible Variables: Define, Use, and Override Variables (Complete Guide)

Variables are the backbone of flexible Ansible playbooks. Define them in inventory, playbooks, roles, files, or the command line — then reference them anywhere with {{ variable_name }}.

See also: Ansible Playbook: Write and Run Your First Playbook (Complete Guide)

Define Variables

In a Playbook

- hosts: webservers
  vars:
    http_port: 80
    app_name: mywebapp
    packages:
      - nginx
      - python3
      - redis
    database:
      host: db.example.com
      port: 5432
      name: appdb
tasks:
    - name: Show app config
      ansible.builtin.debug:
        msg: "{{ app_name }} on port {{ http_port }}, DB: {{ database.host }}:{{ database.port }}"

In Variable Files

# vars/production.yml
http_port: 443
ssl_enabled: true
app_replicas: 3

- hosts: webservers
  vars_files:
    - vars/common.yml
    - "vars/{{ env }}.yml"

In Inventory

# inventory.ini
[webservers]
web01 ansible_host=10.0.0.1 http_port=8080
web02 ansible_host=10.0.0.2
[webservers:vars]
app_name=mywebapp
env=production
[all:vars]
ansible_user=deploy

In Group/Host Vars Directories

inventory/
├── hosts.ini
├── group_vars/
│   ├── all.yml          # All hosts
│   ├── webservers.yml   # Webserver group
│   └── databases.yml    # Database group
└── host_vars/
    ├── web01.yml         # Specific host
    └── db01.yml

From Command Line (Extra Vars)

# String variable
ansible-playbook site.yml -e "app_version=2.1.0"
# Multiple variables
ansible-playbook site.yml -e "app_version=2.1.0 env=staging"
# JSON
ansible-playbook site.yml -e '{"app_version": "2.1.0", "debug": true}'
# From file
ansible-playbook site.yml -e @vars/deploy.yml

Extra vars (-e) have the highest precedence — they override everything.

Register Variables

Capture task output for use in later tasks.

- name: Check if app is running
  ansible.builtin.command: systemctl is-active myapp
  register: app_status
  ignore_errors: true
  changed_when: false
- name: Start app if not running
  ansible.builtin.service:
    name: myapp
    state: started
  when: app_status.rc != 0
- name: Show status
  ansible.builtin.debug:
    msg: "App status: {{ app_status.stdout }}"

Common Register Fields

result.stdout        # Standard output (string)
result.stdout_lines  # Output as list of lines
result.stderr        # Standard error
result.rc            # Return code
result.changed       # Whether task made changes
result.failed        # Whether task failed
result.skipped       # Whether task was skipped

See also: Configuring Ansible for VMware: Complete Setup Guide & Playbook

set_fact — Runtime Variables

- name: Calculate config values
  ansible.builtin.set_fact:
    worker_count: "{{ ansible_processor_vcpus * 2 }}"
    cache_size: "{{ (ansible_memtotal_mb * 0.25) | int }}m"
    deploy_timestamp: "{{ ansible_date_time.iso8601 }}"
- name: Use calculated values
  ansible.builtin.template:
    src: app.conf.j2
    dest: /etc/myapp/app.conf

Variable Precedence (Low → High)

Command line values (-u, etc.) Role defaults (roles/x/defaults/main.yml) Inventory file group vars Inventory group_vars/all Inventory group_vars/ Inventory file host vars Inventory host_vars/ Host facts / registered vars Play vars: Play vars_prompt: Play vars_files: Role vars (roles/x/vars/main.yml) Block vars Task vars include_vars set_fact / registered vars Role params include params Extra vars (-e) ← highest

See also: Creating Custom Ansible Plugins to Fetch API Data Easily

Variable Data Types

vars:
  # String
  app_name: "myapp"
# Integer
  http_port: 8080
# Boolean
  ssl_enabled: true
# List
  packages:
    - nginx
    - redis
    - postgresql
# Dictionary
  database:
    host: db.example.com
    port: 5432
    name: appdb
# Multi-line string
  welcome_message: |
    Welcome to {{ app_name }}
    Running on {{ ansible_hostname }}

Access Variables

# Simple
"{{ app_name }}"
# Dictionary (dot notation)
"{{ database.host }}"
# Dictionary (bracket notation — safer for special chars)
"{{ database['host'] }}"
# List by index
"{{ packages[0] }}"
# Default value if undefined
"{{ optional_var | default('fallback') }}"
# Mandatory (fail if undefined)
"{{ required_var | mandatory }}"

Variable Filters

# String manipulation
"{{ name | upper }}"           # MYAPP
"{{ name | lower }}"           # myapp
"{{ name | capitalize }}"      # Myapp
"{{ name | replace('-','_') }}" # my_app
# Math
"{{ port | int + 1000 }}"     # 9080
"{{ size | float }}"          # 1024.0
# Lists
"{{ packages | length }}"      # 3
"{{ packages | join(',') }}"   # nginx,redis,postgresql
"{{ packages | first }}"       # nginx
"{{ packages | sort }}"        # sorted list
# Defaults
"{{ var | default(omit) }}"    # Skip parameter if undefined

Encrypted Variables (Vault)

# Encrypt a variable file
ansible-vault encrypt vars/secrets.yml
# Encrypt a string
ansible-vault encrypt_string 'SuperSecret123' --name 'vault_db_password'

# Use encrypted variables like any other
vars_files:
  - vars/secrets.yml
tasks:
  - name: Configure database
    ansible.builtin.template:
      src: db.conf.j2
      dest: /etc/myapp/db.conf
    # vault_db_password is decrypted automatically

FAQ

How do I define variables in Ansible?

Define variables in playbook vars: section, vars_files:, inventory files, group_vars/ and host_vars/ directories, role defaults/vars, or via -e on the command line. Each location has different precedence.

What is Ansible variable precedence?

Ansible has 22 levels of variable precedence. Extra vars (-e) have the highest. Role defaults have the lowest. The most common override order: role defaults < inventory < playbook vars < set_fact < extra vars.

How do I use registered variables in Ansible?

Use register: varname on a task to capture its output. Access fields like varname.stdout, varname.rc, varname.changed. Use in conditionals: when: varname.rc == 0.

What is the difference between vars and set_fact?

vars are defined at play/task level and scoped to that context. set_fact creates variables at runtime that persist for the host across plays. set_fact can use values from previous tasks.

How do I set a default value for a variable?

Use the default filter: {{ my_var | default('fallback_value') }}. Use default(omit) to skip an optional module parameter when the variable is undefined.

Conclusion

Variables make Ansible playbooks flexible and reusable. Understand the precedence order, use group_vars and host_vars for inventory-specific values, and encrypt sensitive data with Vault.

Related Articles

• Ansible Variable Precedence: Complete Order • Ansible set_fact: Create Runtime Variables • Ansible Vault: Encrypt Sensitive Data • Ansible Extra Vars: Override Variables from CLI