Ansible setting of hostname from inventory, but ignoring IP addresses

hosts.yml

- name: Ensure hostname set
  hostname:
    name: {{ inventory_hostname }}
  when: not inventory_hostname|trim is match('(\d{1,3}\.){3}\d{1,3}')

- name: Ensure hostname is in /etc/hosts
  lineinfile:
    dest: /etc/hosts
    regexp: "^{{ ansible_default_ipv4.address }}.+$"
    line: "{{ ansible_default_ipv4.address }} {{ ansible_fqdn }} {{ ansible_hostname }}"

ziamler, i hope this help you

- name: Configure slave's hostname
  hosts: slaves
  become: True
  become_user: root
  gather_facts: True
  tasks:
    - hostname:
        name: "slave{{ play_hosts.index(inventory_hostname)|int+1 }}"
    - debug: msg="{{ play_hosts.index(inventory_hostname) }}"
    - debug: msg="{{ inventory_hostname }}"

With this: play_hosts.index(inventory_hostname) you obtain the current index

- name: Add Ansible inventory mappings to /etc/hosts
  become: true
  blockinfile:
    path: /etc/hosts
    block: |
      {% for host in groups['all'] %}
      {{ hostvars[host].ansible_host }} {{ host }}
      {% endfor %}

In case this has stopped working for anyone else recently, with errors like

fatal: [mydomain.net]: FAILED! => {"msg": "The conditional check 'not inventory_hostname | match ('(\d{1,3}\.){3}\d{1,3}')' failed. The error was: template error while templating string: no filter named 'match'. String: {% if not inventory_hostname | match ('(\d{1,3}\.){3}\d{1,3}') %} True {% else %} False {% endif %}\n\nThe error appears to be in '/Users/config-hostname.yaml': line 5, column 5, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n tasks:\n - name: Ensure hostname set\n ^ here\n"}

changing line 3 to:

when: not inventory_hostname|trim is match('(\d{1,3}\.){3}\d{1,3}')

Should fix the issue.

---
- name: inventory name to hostname
  hostname:
    name: "{{ inventory_hostname }}"

---

• name: inventory name to hostname
  hostname:
  name: "{{ inventory_hostname }}"

Indeed @patsevanton — the original gist is in a really old format. You've correctly put the way things should be now 👍