Ansible Hosts Pattern Checking

A simple Ansible playbook to verify host pattern matching

The following is a playbook to verify that the host patten you want to use matches the desired hosts without actually connecting to the devices. This is a good way to make sure that a complicated host matching pattern is only acting on the desired hosts for a play/playbook. Simply change the pattern for hosts in the playbook:

---
- name: Print Inventory Hostname For hosts that this playbook will act on 
  hosts: RTR:!2921
  vars:
    ansible_connection: local
  tasks:
  - debug:
      msg: 'This will play will act on '

Other ways to test Ansible hosts pattern matches

For an Ad Hoc command that will parse the ansible inventory directly, do the following:

ansible -i test_inventory.yml --list-hosts 'asw:!catalyst'

*NOTE - you cannot pass the ‘hosts’ variable from the cli with -e (long form: –extra-vars) but if you set the hosts variable to a a new variable name you can reference that name from the cli with –extra-vars:

Another example playbook that will take CLI arguments for the host key pattern to match:

---
- name: Print Inventory Hostname For hosts that this playbook will run on
  hosts: ''
  vars:
    ansible_connection: local
  tasks:
  - debug:
      msg: 'This will play will act on '

Here you would call that playbook from the cli (watch out for quotes, they need to be single quotes otherwise ! can be evaluated by bash)

ansible-playbook -i test_inventory test_playbook.yml -e 'test_hosts=host_pattern'

Official Ansible Docs on the subject:

https://docs.ansible.com/ansible/latest/user_guide/intro_patterns.html