Facts & Variable Precedence

Variables covered where you define values. This page covers the variables Ansible provides automatically (facts and magic variables) and the rules for which value wins when the same name is defined in more than one place.

Facts — what Ansible discovers about a host

Before running tasks, Ansible connects to each host and gathers facts: OS, IP addresses, CPU, memory, disks, and more. These are available as ansible_* variables in tasks and templates.

# See every fact for a host
ansible ops-01.internal -m setup

# Filter to a subset
ansible ops-01.internal -m setup -a "filter=ansible_distribution*"
ansible ops-01.internal -m setup -a "filter=ansible_memtotal_mb"

Facts you'll reach for often:

They're used throughout this repo — e.g. the Docker role builds the apt repo line with {{ ansible_distribution_release }} so the same task works on Debian 12 or 13.

- hosts: homelab
  gather_facts: false

Magic variables — always available

These describe the run itself, not the machine:

- name: Show which groups this host is in
  ansible.builtin.debug:
    msg: "{{ inventory_hostname }} is in {{ group_names }}"

- name: Reference every app host
  ansible.builtin.debug:
    msg: "app servers: {{ groups['apps'] }}"

Setting variables at runtime — set_fact

Create or compute a variable mid-play:

- name: Compute half the RAM for a container limit
  ansible.builtin.set_fact:
    container_mem_mb: "{{ (ansible_memtotal_mb | int * 0.5) | int }}"

- name: Use it later
  ansible.builtin.debug:
    msg: "Limiting to {{ container_mem_mb }} MB"

register (from Writing Tasks) is the other runtime source — it captures a task's result.

Variable precedence (which value wins)

When the same variable name is defined in multiple places, Ansible picks by a fixed priority. From lowest to highest (higher wins), the parts you'll actually touch:

1. Role defaults (roles/x/defaults/main.yml) — the weakest, meant to be overridden
2. group_vars/all
3. group_vars/<specific group>
4. host_vars/<host>
5. Facts / set_fact / registered vars
6. -e / --extra-vars on the command line — always wins

graph LR
  A[role defaults] --> B[group_vars/all]
  B --> C[group_vars/group]
  C --> D[host_vars/host]
  D --> E[set_fact / register]
  E --> F["--extra-vars (-e)"]

Practical takeaways:

• Put safe fallbacks in roles/<role>/defaults/main.yml — anything can override them.
• Put shared settings in group_vars/all.yml.
• Host-specific overrides go in host_vars/<host>.yml — they beat group vars (that's why host_vars/nas.internal.yml can override docker_compose_dir).
• Force a one-off value with -e:

ansible-playbook playbooks/base.yml -e "timezone=UTC" --limit ops-01.internal

Ref: Ansible — Variable precedence