How can I get a list of child groups in Ansible?

Question

I have an inventory file that looks like this:

[master]
host01

[nl]
host02

[us]
host03

[satellites:children]
nl
us

How can I get a list of groups that have satellites as their parent?

I am looking for a solution that works similar to this:

- debug: msg="{{ item }}"
  with_items: "{{ groups['satellites:children'] }}"

Update:

The only solution I was able to come with is this:

- debug: {{ item }}
  with_items: "{{ groups }}"
  when: item != "master" and item != "satellites" and item != "all" and item != "ungrouped"

But that is not very flexible.

Answer 1

You can try the following approaches:

  vars:
    parent: 'satellites'
  tasks:
      # functional style
    - debug: msg="{{hostvars[item].group_names | select('ne', parent) | first}}"
      with_items: "{{groups[parent]}}"
      # set style
    - debug: msg="{{hostvars[item].group_names | difference(parent) | first}}"
      with_items: "{{groups[parent]}}"

Also select('ne', parent) is interchangeable with reject('equalto', parent) depending on what is more readable to you.

Links:
set theory operator
select and reject filters

---

Updated answer based on comments. inspiration from this thread.

vars:
    parent: 'satellites'
tasks:
    - name: build a subgroups list
      set_fact: subgroups="{{ ((subgroups | default([])) + hostvars[item].group_names) | difference(parent) }}"
      with_items: "{{groups[parent]}}"

    - debug: var=subgroups

output:

 "subgroups": [
        "nl",
        "us"
    ]