ansible.builtin.set_fact Module: Set Variables Dynamically (Complete Guide)

By Luca Berton · Published 2026-04-03 · Category: linux-administration

Complete guide to ansible.builtin.set_fact module. Set variables dynamically during playbook execution, cache facts across plays, use with conditionals.

The ansible.builtin.set_fact module creates or modifies variables (facts) at runtime. Unlike vars, facts set with set_fact persist across tasks and can be cached across playbook runs.

Basic Usage

- name: Set a simple variable
  ansible.builtin.set_fact:
    app_version: "2.5.1"
    deploy_path: "/opt/myapp"

- name: Use the variable
  ansible.builtin.debug:
    msg: "Deploying version {{ app_version }} to {{ deploy_path }}"

Set Facts from Registered Variables

- name: Get current user
  ansible.builtin.command: whoami
  register: whoami_result

- name: Store as fact
  ansible.builtin.set_fact:
    current_user: "{{ whoami_result.stdout }}"

Conditional set_fact

- name: Set environment-specific config
  ansible.builtin.set_fact:
    db_host: "{{ 'db-prod.example.com' if env == 'production' else 'db-dev.example.com' }}"
    db_port: "{{ 5432 if env == 'production' else 5433 }}"

- name: Set fact based on OS
  ansible.builtin.set_fact:
    package_manager: apt
  when: ansible_os_family == 'Debian'

- name: Set fact for RedHat
  ansible.builtin.set_fact:
    package_manager: dnf
  when: ansible_os_family == 'RedHat'

Complex Data Structures

Set a dictionary

- name: Set database config
  ansible.builtin.set_fact:
    db_config:
      host: db.example.com
      port: 5432
      name: myapp
      user: appuser

Set a list

- name: Build server list dynamically
  ansible.builtin.set_fact:
    server_list: "{{ server_list | default([]) + [item] }}"
  loop: "{{ groups['webservers'] }}"

Merge dictionaries

- name: Merge with existing config
  ansible.builtin.set_fact:
    app_config: "{{ app_config | default({}) | combine({'new_key': 'new_value'}) }}"

Cacheable Facts

Persist facts across playbook runs using fact caching:

- name: Set cacheable fact
  ansible.builtin.set_fact:
    last_deploy_time: "{{ ansible_date_time.iso8601 }}"
    cacheable: true

Configure fact caching in ansible.cfg:

[defaults]
fact_caching = jsonfile
fact_caching_connection = /tmp/ansible_facts_cache
fact_caching_timeout = 86400

set_fact vs vars vs register

Feature	set_fact	vars	register
Scope	Host-level, persists across plays	Play/block/task level	Task-level
Dynamic	Yes — computed at runtime	Limited with Jinja2	Output only
Cacheable	Yes (with cacheable: true)	No	No
Overrides	Higher precedence than vars	Lower precedence	N/A

Practical Examples

Calculate derived values

- name: Get filesystem usage
  ansible.builtin.command: df -BG / --output=avail
  register: disk_result

- name: Parse available disk space
  ansible.builtin.set_fact:
    disk_available_gb: "{{ disk_result.stdout_lines[1] | regex_search('([0-9]+)') | int }}"
    disk_low: "{{ (disk_result.stdout_lines[1] | regex_search('([0-9]+)') | int) < 10 }}"

Build configuration from inventory

- name: Build upstream server list for nginx
  ansible.builtin.set_fact:
    upstream_servers: "{{ groups['appservers'] | map('extract', hostvars, 'ansible_host') | list }}"

FAQ

When should I use set_fact vs register?

Use register to capture command output. Use set_fact to create computed/derived variables or transform registered data.

Do set_fact variables survive between plays?

Yes! Facts set with set_fact persist for the host across all plays in the same playbook run. With cacheable: true, they survive across runs.

How do I unset a fact?

You can't directly unset a fact, but you can set it to omit or an empty value: my_fact: ""

Basic set_fact

- name: Set a variable
  ansible.builtin.set_fact:
    app_version: "2.5.0"
    deploy_path: "/opt/myapp-2.5.0"

- debug:
    msg: "Deploying v{{ app_version }} to {{ deploy_path }}"

Set Facts from Task Results

- name: Get hostname
  ansible.builtin.command: hostname -f
  register: hostname_result
  changed_when: false

- name: Store as fact
  ansible.builtin.set_fact:
    server_fqdn: "{{ hostname_result.stdout }}"

Conditional Facts

- name: Set environment-specific config
  ansible.builtin.set_fact:
    db_host: "{{ 'db.prod.com' if env == 'production' else 'db.dev.com' }}"
    debug_mode: "{{ env != 'production' }}"

Computed Facts

- name: Calculate values
  ansible.builtin.set_fact:
    total_memory_mb: "{{ ansible_memtotal_mb }}"
    java_heap_mb: "{{ (ansible_memtotal_mb * 0.75) | int }}"
    worker_count: "{{ ansible_processor_vcpus * 2 }}"

Build Complex Data Structures

- name: Build server config
  ansible.builtin.set_fact:
    server_config:
      hostname: "{{ inventory_hostname }}"
      ip: "{{ ansible_default_ipv4.address }}"
      os: "{{ ansible_distribution }}"
      roles: "{{ group_names }}"

- name: Build list incrementally
  ansible.builtin.set_fact:
    all_ips: "{{ all_ips | default([]) + [hostvars[item].ansible_host] }}"
  loop: "{{ groups['webservers'] }}"

Cacheable Facts (Persistent)

- name: Set persistent fact
  ansible.builtin.set_fact:
    last_deploy_time: "{{ ansible_date_time.iso8601 }}"
    cacheable: true  # Survives between plays

Requires fact_caching in ansible.cfg:

[defaults]
fact_caching = jsonfile
fact_caching_connection = /tmp/ansible_facts

set_fact vs vars vs register

Method	Scope	When Set
`set_fact`	Host, rest of play	Runtime (dynamic)
`vars`	Task/play/role	Before execution (static)
`register`	Host, rest of play	From task output
`group_vars`	Group hosts	Inventory load time
`include_vars`	Host, rest of play	File load time

Common Patterns

Default values

- set_fact:
    config: "{{ user_config | default(default_config) | combine({'timestamp': now}) }}"

Cross-host facts

- set_fact:
    primary_ip: "{{ hostvars[groups['primary'][0]].ansible_host }}"

FAQ

Does set_fact persist between playbook runs?

Only with cacheable: true and fact_caching configured. Otherwise, facts reset each run.

Can I unset a fact?

No direct way. Set it to "", null, or omit:

- set_fact:
    my_var: null

set_fact in a loop — does it accumulate?

Each iteration replaces the fact. To accumulate, build on the previous value:

- set_fact:
    items: "{{ items | default([]) + [item] }}"
  loop: [a, b, c]

Basic set_fact

- set_fact:
    app_version: "2.5.0"
    deploy_time: "{{ ansible_date_time.iso8601 }}"
    full_name: "{{ first_name }} {{ last_name }}"

From Command Output

- command: git rev-parse --short HEAD
  register: git_output
  changed_when: false

- set_fact:
    git_commit: "{{ git_output.stdout }}"
    deploy_tag: "deploy-{{ git_output.stdout }}-{{ ansible_date_time.epoch }}"

Computed Values

- set_fact:
    worker_count: "{{ ansible_processor_vcpus * 2 }}"
    memory_limit: "{{ (ansible_memtotal_mb * 0.75) | int }}m"
    is_production: "{{ env == 'production' }}"
    app_url: "{{ 'https' if enable_ssl else 'http' }}://{{ server_name }}:{{ app_port }}"

Build Lists and Dicts

# Append to list
- set_fact:
    server_list: "{{ server_list | default([]) + [inventory_hostname] }}"

# Build dict
- set_fact:
    host_info:
      hostname: "{{ inventory_hostname }}"
      ip: "{{ ansible_default_ipv4.address }}"
      os: "{{ ansible_distribution }}"
      cpu: "{{ ansible_processor_vcpus }}"

Conditional set_fact

- set_fact:
    package_manager: apt
  when: ansible_os_family == "Debian"

- set_fact:
    package_manager: dnf
  when: ansible_os_family == "RedHat"

# Ternary (one-liner)
- set_fact:
    db_host: "{{ 'db.prod.com' if env == 'production' else 'localhost' }}"

Cacheable Facts (Persist Across Plays)

- set_fact:
    discovered_version: "2.5.0"
    cacheable: true  # Survives between plays when fact_caching is enabled

# ansible.cfg (required for cacheable)
[defaults]
gathering = smart
fact_caching = jsonfile
fact_caching_connection = /tmp/ansible-facts
fact_caching_timeout = 86400

Transform Data

# Filter list
- set_fact:
    active_hosts: "{{ all_hosts | selectattr('status', 'eq', 'active') | list }}"

# Extract field
- set_fact:
    hostnames: "{{ servers | map(attribute='name') | list }}"

# Merge dicts
- set_fact:
    final_config: "{{ default_config | combine(env_config) | combine(host_config) }}"

Loop with set_fact

# Build a dict from loop
- set_fact:
    service_ports: "{{ service_ports | default({}) | combine({item.name: item.port}) }}"
  loop:
    - { name: web, port: 80 }
    - { name: api, port: 8080 }
    - { name: db, port: 5432 }
# Result: { web: 80, api: 8080, db: 5432 }

set_fact vs vars vs register

Method	Scope	Timing	Use Case
`set_fact`	Host, persists in play	Runtime	Dynamic values
`vars`	Task/play/role	Compile time	Static values
`register`	Host, current play	After task	Capture output
`group_vars`	Group hosts	Load time	Inventory config

FAQ

Does set_fact persist across plays?

By default, yes — within the same playbook run. For cross-playbook persistence, use cacheable: true with fact caching enabled.

Can I unset a fact?

No direct way. Set it to null or "", or use meta: clear_facts to clear all facts.

set_fact vs vars?

set_fact creates host-level facts at runtime. vars are evaluated at parse time. Use set_fact when the value depends on runtime data (command output, conditionals).

Basic set_fact

- ansible.builtin.set_fact:
    app_url: "https://{{ inventory_hostname }}:{{ app_port }}"
    full_name: "{{ first_name }} {{ last_name }}"

Conditional set_fact

- set_fact:
    package_manager: apt
  when: ansible_os_family == "Debian"

- set_fact:
    package_manager: yum
  when: ansible_os_family == "RedHat"

From Registered Results

- command: cat /opt/myapp/VERSION
  register: version_output
  changed_when: false

- set_fact:
    app_version: "{{ version_output.stdout | trim }}"

Complex Data Structures

- set_fact:
    server_config:
      host: "{{ ansible_default_ipv4.address }}"
      port: "{{ app_port }}"
      workers: "{{ ansible_processor_vcpus * 2 }}"
      env: "{{ 'production' if is_prod else 'staging' }}"

Cacheable Facts

# Persists across plays (with fact caching enabled)
- set_fact:
    deployment_timestamp: "{{ ansible_date_time.iso8601 }}"
    cacheable: true

Append to List

- set_fact:
    my_list: "{{ my_list | default([]) + [new_item] }}"

# Build list from loop
- set_fact:
    server_ips: "{{ server_ips | default([]) + [hostvars[item]['ansible_default_ipv4']['address']] }}"
  loop: "{{ groups['webservers'] }}"

Merge Dictionaries

- set_fact:
    config: "{{ config | default({}) | combine({'new_key': 'new_value'}) }}"

set_fact vs vars

Feature	`set_fact`	`vars`
Scope	Per-host, persists across plays	Per-play or per-task
Dynamic	✅ Computed at runtime	❌ Static (evaluated once)
Conditional	✅ Can use `when`	❌ Always set
Cacheable	✅ With `cacheable: true`	❌

Cross-Host Access

# set_fact on web1
- set_fact:
    app_version: "2.5.0"
  when: inventory_hostname == "web1"

# Access from any host
- debug:
    msg: "Web1 runs version {{ hostvars['web1']['app_version'] }}"

FAQ

set_fact persists between plays?

Yes — set_fact values persist for the host across all plays in the same playbook run. With cacheable: true and fact caching, they persist across runs.

How to unset a fact?

You can't truly unset it, but you can set it to undefined: this isn't directly supported. Set it to empty or use default() pattern.

set_fact in a loop?

Each iteration overwrites the previous value. To accumulate, use the append pattern: "{{ list + [item] }}".

Category: linux-administration

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About Luca Berton

ansible.builtin.set_fact Module: Set Variables Dynamically (Complete Guide)

Basic Usage

Set Facts from Registered Variables

Conditional set_fact

Complex Data Structures

Set a dictionary

Set a list

Merge dictionaries

Cacheable Facts

set_fact vs vars vs register

Practical Examples

Calculate derived values

Build configuration from inventory

FAQ

When should I use set_fact vs register?

Do set_fact variables survive between plays?

How do I unset a fact?

Basic set_fact

Set Facts from Task Results

Conditional Facts

Computed Facts

Build Complex Data Structures

Cacheable Facts (Persistent)

set_fact vs vars vs register

Common Patterns

Default values

Cross-host facts

FAQ

Does set_fact persist between playbook runs?

Can I unset a fact?

set_fact in a loop — does it accumulate?

Basic set_fact

From Command Output

Computed Values

Build Lists and Dicts

Conditional set_fact

Cacheable Facts (Persist Across Plays)

Transform Data

Loop with set_fact

set_fact vs vars vs register

FAQ

Does set_fact persist across plays?

Can I unset a fact?

set_fact vs vars?

Basic set_fact

Conditional set_fact

From Registered Results

Complex Data Structures

Cacheable Facts

Append to List

Merge Dictionaries

set_fact vs vars

Cross-Host Access

FAQ

set_fact persists between plays?

How to unset a fact?

set_fact in a loop?

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