In Ansible, how do I assign a hostvar to a playbook's host?

Question

The directory of my Ansible project has the following structure:

project/
  playbook.yml
  host_vars/
    localhsot.yml
  roles/
    ...
    terraform-provision/
      ...
  terraform/
      ...

Here's project/playbook.yml:

- hosts: localhost
  tasks:
  - name: trigger role
    include_role:
      name: terraform-provision
- hosts: hostvars['localhost'].aws_instance_host_group
  tasks:
  - name: debugging
    debug:
      var: hostvars['localhost'].aws_instance_host_group

Here's the relevant part of project/host_vars/localhost.yml:

hostvars['localhost'].aws_instance_host_group
aws_instance_host_group: "aws_instance"

Here's the relevant part of project/roles/terraform-provision/tasks/main.yml:

- name: Add the provisioned AWS instance to the inventory
  add_host:
    name: "{{ aws_instance_public_ip.value }}" # set using set_fact elsewhere in file
    ansible_user: "{{ aws_instance_user_name }}"
    groups: "{{ aws_instance_host_group }}"

When I execute project/playbook.yml, it throws the following error when trying to connect to the host created by the terraform-provision role:

[WARNING]: Could not match supplied host pattern, ignoring: hostvars['localhost'].aws_instance_host_group

However, the debugging task of project/playbook.yml prints the group's name (aws_instance) successfully.

What am I missing? Is there a better approach to achieve the same thing?

Answer 1

To be evaluated, a variable must be closed in quoted double curly braces, e.g.

- hosts: "{{ hostvars.localhost.aws_instance_host_group }}"

The dot notation simplifies the code.

---

Minimal reproducible example

shell> cat pb.yml
---
- hosts: localhost
  gather_facts: false
  tasks:
    - set_fact:
        aws_instance_host_group: aws_instance
    - add_host:
        name: 10.1.0.61
        groups: "{{ aws_instance_host_group }}"

- hosts: "{{ hostvars.localhost.aws_instance_host_group }}"
  gather_facts: false
  tasks:
    - debug:
        var: groups
    - debug:
        var: inventory_hostname

shell> cat hosts
localhost

shell> ansible-playbook -i hosts pb.yml 

PLAY [localhost] *****************************************************************************

TASK [set_fact] ******************************************************************************
ok: [localhost]

TASK [add_host] ******************************************************************************
changed: [localhost]

PLAY [aws_instance] **************************************************************************

TASK [debug] *********************************************************************************
ok: [10.1.0.61] => 
  groups:
    all:
    - 10.1.0.61
    - localhost
    aws_instance:
    - 10.1.0.61
    ungrouped:
    - localhost

TASK [debug] *********************************************************************************
ok: [10.1.0.61] => 
  inventory_hostname: 10.1.0.61

PLAY RECAP ***********************************************************************************
10.1.0.61: ok=2    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0   
localhost: ok=2    changed=1    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0