Ansible Fix 'VARIABLE IS NOT DEFINED' Error: Undefined Variables — Video Tutorial

Introduction Today we’re going to talk about Ansible troubleshooting, specifically about VARIABLE IS NOT DEFINED! Message. Most of the time the root cause is a misspelled variable or a variable really not defined. This use case is special about the `ansible_hostname` internal variable. I’m Luca Berton and welcome to today’s episode of Ansible Pilot. Playbook The best way of talking about Ansible troubleshooting is to jump in a live Playbook to show you practically the VARIABLE IS NOT DEFINED! and how to solve it! error code ```yaml --- - name: hostname Playbook hosts: all gather_facts: false tasks: - name: print hostname ansible.builtin.debug: var: ansible_hostname ``` error execution ```bash ansible-pilot $ ansible-playbook -i virtualmachines/demo/inventory troubleshooting/variablenotdefined_error.yml PLAY [hostname Playbook] ****************************************************************************** TASK [print hostname] ***************************************************************************** ok: [demo.example.com] => { "ansible_hostname": "VARIABLE IS NOT DEFINED!" } PLAY RECAP **************************************************************************************** demo.example.com : ok=1 changed=0 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0 ansible-pilot $ ``` fix code ```yaml --- - name: hostname Playbook hosts: all gather_facts: true tasks: - name: print hostname ansible.builtin.debug: var: ansible_hostname ``` fix execution ```bash ansible-pilot $ ansible-playbook -i virtualmachines/demo/inventory troubleshooting/variablenotdefined_fix.yml PLAY [hostname Playbook] ****************************************************************************** TASK [Gathering Facts] **************************************************************************** ok: [demo.example.com] TASK [print hostname] ***************************************************************************** ok: [demo.example.com] => { "ansible_hostname": "Playbook" } PLAY RECAP **************************************************************************************** demo.example.com : ok=2 changed=0 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0 ansible-pilot $ ``` [code with ❤️ in GitHub](https://github.com/lucab85/ansible-pilot/tree/master/troubleshooting) Conclusion Now you know better how to troubleshoot the Ansible VARIABLE IS NOT DEFINED! message. Why Does This Error Happen? Ansible has two types of variables: 1. **User-defined variables** — you create them with `vars:`, `set_fact`, `register`, etc. 2. **Facts (built-in variables)** — Ansible collects them automatically from managed hosts Facts like `ansible_hostname`, `ansible_os_family`, `ansible_distribution`, and `ansible_default_ipv4` are **only available if `gather_facts: true`** (which is the default). If you set `gather_facts: false`, these variables don't exist. Common Vari