By default, before executing the set of tasks defined in a playbook, Ansible will take a few moments to gather information about the systems that are being provisioned. This information, referred to as facts, contain details such as network interfaces and addresses, the operating system running on remote nodes, and available memory, among other things.

Ansible stores facts in JSON format, with items grouped in nodes. To check what kind of information is available for the systems you’re provisioning, you can run the setup module with an ad hoc command:

ansible all -i inventory -m setup -u sammy

This command will output an extensive JSON containing information about your server. To obtain a subset of that data, you can use the filter parameter and provide a pattern. For instance, if you’d like to obtain information about all IPv4 addresses in the remote nodes, you can use the following command:

ansible all -i inventory -m setup -a "filter=*ipv4*" -u sammy 

You’ll see output like this:

Output
203.0.113.10 | SUCCESS => {
    "ansible_facts": {
        "ansible_all_ipv4_addresses": [
            "203.0.113.10", 
            "198.51.100.23"
        ], 
        "ansible_default_ipv4": {
            "address": "203.0.113.10", 
            "alias": "eth0", 
            "broadcast": "203.0.113.255", 
            "gateway": "203.0.113.1", 
            "interface": "eth0", 
            "macaddress": "06:c7:91:16:2e:b7", 
            "mtu": 1500, 
            "netmask": "203.0.113.0", 
            "network": "203.0.113.0", 
            "type": "ether"
        }
    }, 
    "changed": false
}

Once you have found the facts that will be useful for your play, you can update your playbook accordingly. As an example, the following playbook will print out the IPv4 address of the default network interface. From the previous command output, we can see that this value is available through ansible_default_ipv4.address in the JSON provided by Ansible.

Create a new file called playbook-03.yml in your ansible-practice directory:

nano ~/ansible-practice/playbook-03.yml

Then add the following lines to the new playbook file:

---
- hosts: all
  tasks:
    - name: print facts
      debug:
        msg: "IPv4 address: {{ ansible_default_ipv4.address }}"

Save and close the file when you’re done.

To try this playbook on servers from your inventory file, run ansible-playbook with the same connection arguments you’ve used before when running our first example. Again, we’ll be using an inventory file named inventory and the sammy user to connect to the remote servers:

ansible-playbook -i inventory playbook-03.yml -u sammy

When you run the playbook, you’ll see your remote server’s IPv4 address in the output as expected:

Output
...

TASK [print facts] ***************************************************************************************************************************************************************************
ok: [server1] => {
    "msg": "IPv4 address: 203.0.113.10"
}

...

Facts encapsulate important data that you can leverage to better customize your playbooks. To learn more about all the information you can obtain through facts, please refer to the official Ansible documentation.