How to make Ansible playbook run on first host in the group?

Question

How can I run a playbook only on first host in the group?

I am expecting something like this:

---
- name: playbook that only run on first host in the group
  hosts: "{{ groups[group_name] | first }}"

  tasks:
   - debug:
       msg: "on {{ inventory_hostname }}"

But this doesn't work, gives error:

'groups' is undefined

How can I make it work?

Pimiento

Answer 1

You can use:

hosts: group_name[0]

Inventory hosts values (specified in the hosts directive) are processed with a custom parser, which does not allow Jinja2 expressions like the regular template engine does.

Read about Patterns.

Italianize

When I use this, I get a warning saying "[WARNING]: Could not match supplied host pattern, ignoring: group_name" – Formalize

… because I'm stupid. I thought you meant the literal string group_name[0] but you clearly mean my_group_name[0] like in the docs. – Formalize