This assumes you have installed Ansible, are familiar with the ansible-playbook command and facts, and have created a playbook. If you are not familiar with variables, check out our article Getting Started with variables.

The set_fact module is used to create your own facts. In this example, a fact named "foo" is created with a value of "Hello World".

---
- hosts: all
  tasks:
    - set_fact: 
        foo: "Hello World"

The debug module can be used to print the variable.

- debug:
    msg: "{{ foo }}"

Which should produce the following.

TASK [debug]
ok: [server1.example.com] => {
    "msg": "Hello World"
}

Redefine variable

Let's say you used the vars module to give the "foo" key a value of "Hello", like this.

vars:
  foo: "Hello"

set_facts can be used to update the value of the "foo" key. In this example, the "foo" key will be updated to have a value of "World".

set_fact:
  foo: "World"

Array

Refer to Ansible - set_facts module (array of values) for the steps on how to create an array.

when

The when module can be used to set facts when a condition evaluates to true or false, like this.

---
- hosts: all
  tasks:
    - set_fact: 
        env: "dev"
      when: inventory_hostname is search ('dev')

    - set_fact: 
        env: "prod"
      when: inventory_hostname is search ('prod')