Variable Scoping

Understanding variable scope in Ansible is essential for effective configuration management. In this guide, we explore three primary scopes: Host Scope, Play Scope, and Global Variables. Mastering these concepts will help you write cleaner, more modular playbooks and troubleshoot variable-related issues more efficiently.

Host Scope

Host-specific variables are defined directly in the inventory file or host-specific configurations. These variables are available only when tasks are executed on that host.

Consider the following inventory file where the dns_server variable is specified only for host web2:

/etc/ansible/hosts
web1 ansible_host=172.20.1.100
web2 ansible_host=172.20.1.101 dns_server=10.5.5.4
web3 ansible_host=172.20.1.102

When executing a playbook that prints the dns_server variable for all hosts:

---
- name: Print DNS server
  hosts: all
  tasks:
    - debug:
        msg: '{{ dns_server }}'

The play output is as follows:

PLAY [Check /etc/ansible/hosts file] ***********************************************************************

TASK [debug] ****************************************************************************************
ok: [web1] => {
    "dns_server": "VARIABLE IS NOT DEFINED!"
}
ok: [web2] => {
    "dns_server": "10.5.5.4"
}
ok: [web3] => {
    "dns_server": "VARIABLE IS NOT DEFINED!"
}

Play Scope

Variables defined within a play are confined to that play's context. They do not persist into subsequent plays unless declared globally. This helps maintain clear boundaries for variable lifecycles within your playbook.

Consider the following playbook that uses the variable ntp_server in one play but not in another:

---
- name: Play1
  hosts: web1
  vars:
    ntp_server: 10.1.1.1
  tasks:
    - debug:
        var: ntp_server

- name: Play2
  hosts: web1
  tasks:
    - debug:
        var: ntp_server

The output for this playbook is:

PLAY [Play1] *********************************************************************

TASK [debug] **********************************************************************
ok: [web1] => {
    "ntp_server": "10.1.1.1"
}

PLAY [Play2] *********************************************************************

TASK [debug] **********************************************************************
ok: [web1] => {
    "ntp_server": "VARIABLE IS NOT DEFINED!"
}

Global Variables

Global variables are accessible across all plays and tasks within a playbook. A common method to define global variables is by passing them as extra variables using the command line.

Consider a playbook without explicit variable definition in any play:

---
- name: Play1
  hosts: web1
  tasks:
    - debug:
        var: ntp_server

- name: Play2
  hosts: web1
  tasks:
    - debug:
        var: ntp_server

When you run the playbook with the following command, the ntp_server variable is set globally:

$ ansible-playbook playbook.yml --extra-vars "ntp_server=10.1.1.1"

The output will show that ntp_server is available in both plays:

PLAY [Play1] ********************************************************************

TASK [debug] *********************************************************************
ok: [web1] => {
    "ntp_server": "10.1.1.1"
}

PLAY [Play2] ********************************************************************

TASK [debug] *********************************************************************
ok: [web1] => {
    "ntp_server": "10.1.1.1"
}

Summary of Variable Scopes

Having a solid grasp of these scopes will not only help in writing efficient and modular playbooks but also in troubleshooting and managing Ansible configurations effectively.

Thank you for reading this article. Happy automating, and see you in the next guide!