Ansible set_fact Module: Create Dynamic Variables at Runtime (Guide)

Exploring Ansible's set_fact Module with Example

Ansible is an open-source automation tool widely used for configuration management, application deployment, and orchestration. It simplifies the process of managing IT infrastructure by allowing you to define desired state configurations in a declarative manner. One powerful feature of Ansible is the set_fact module, which enables you to create or modify variables dynamically during playbook execution. It is part of the ansible.builtin collection.

See also: ansible_date_time: Access Date, Time & Timestamp Facts in Ansible

Links

Demo

In this article, we will explore the set_fact module and its usage through a practical example. We will Playbooknstrate how to retrieve the latest kernel package from a CentOS repository using Ansible's uri module and store it in a variable called kernel. Let's dive into the code example:

---
- name: Regex Playbook
  hosts: all
  vars:
    centos_repo: http://mirror.centos.org/centos/7/os/x86_64/Packages/
  tasks:
    - name: Get Latest Kernel
      ansible.builtin.uri:
        url: "{{ centos_repo }}"
        method: GET
        return_content: true
        body_format: json
      register: available_packages
- name: Save
      ansible.builtin.set_fact:
        kernel: "{{ available_packages.content | ansible.builtin.regex_replace('<.*?>') | regex_findall('kernel-[0-9].*rpm') }}"
- name: Print
      ansible.builtin.debug:
        var: kernel

Let's break down this playbook step by step to understand its functionality: The name directive gives our playbook a descriptive title, "regex Playbook." The hosts directive specifies the target hosts on which the playbook will be executed. In this case, it's set to all, meaning it will apply to all hosts in the inventory. The vars section allows us to define variables used within the playbook. Here, we set the centos_repo variable to the URL of the CentOS 7 package repository. The tasks section contains the actual work to be performed. We have three tasks defined. The first task, named "Get Latest Kernel," uses Ansible's uri module to send an HTTP GET request to the centos_repo URL. It retrieves the content of the repository in JSON format and stores it in the available_packages variable using the register directive. The second task, named "Save," utilizes the set_fact module. It takes the content stored in available_packages and applies two filters consecutively. First, it uses the regex_replace filter from the ansible.builtin plugin to remove any HTML tags from the content. Then, it applies the regex_findall filter from the same plugin to extract the kernel package names matching the pattern "kernel-[0-9].\rpm." The resulting list of kernel package names is stored in the kernel variable using the ansible.builtin.set_fact module. The third task, named "Print," uses Ansible's debug module to display the value of the kernel variable.

    - name: Define a few facts
      ansible.builtin.set_fact:
        app_name: myapp
        app_port: 8080
        app_url: "http://{{ ansible_default_ipv4.address }}:{{ app_port }}"

set_fact and register both create variables at runtime, so they are easy to confuse: • register captures the result object of a task — rc, stdout, changed, and so on — and is scoped to the host that ran the task. • set_fact defines a variable with a value you choose, often derived* from a registered result after you have filtered it down to what you need.

A common pattern is to register a command's output and then set_fact to pull out just the piece you care about:

    - name: Read the current kernel
      ansible.builtin.command: uname -r
      register: uname_out
      changed_when: false
- name: Keep just the version string
      ansible.builtin.set_fact:
        kernel_version: "{{ uname_out.stdout | trim }}"

By default facts set this way are not cached between runs; add cacheable: true if you want them written to the fact cache like gathered facts.

See also: Ansible set_fact: Create & Set Runtime Variables (Complete Guide)

Conclusion

In summary, the set_fact module is a powerful tool in Ansible for creating or modifying variables dynamically during playbook execution. It enables us to extract, transform, and store data from various sources, making it available for subsequent tasks or playbooks. In our example, we used set_fact to extract the latest kernel package name from a CentOS.

Related Articles