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Ansible Inventory Handling
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Inventory....

Inventory format (YAML, JSON, and INI formats) 
  https://docs.ansible.com/ansible/latest/user_guide/intro_inventory.html

Developing a dynamic inventory - covers both plugin and script
  https://docs.ansible.com/ansible/latest/dev_guide/developing_inventory.html

Test with
  ansible-inventory --list

Output is JSON, or with --yaml flag YAML.

You can set the $ANSIBLE_INVENTORY variable which points to the path for your
inventory plugin and/or script, e.g.
  export ANSIBLE_INVENTORY=your_inventory_script.py
or
  export ANSIBLE_INVENTORY=/root/your_inventory_plugin.py

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Ansible inventory must NOT be empty.

If the specified inventory, host pattern or "--limit" option leaves us with no
hosts to target, ansible will error, (not produce a warning).

However if you include a 'dummy' host, that is later rejected by the
host patterns within the playbook, then you will only get a warning....
   skipping: no hosts matched

A warning rather than a error is preferable in a gitlab CI pipeline.

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List all matching hosts...

  ansible -i inventory.yml {playbook} --list-hosts

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Get Facts in the inventory for a specific host (if there are any)

  ansible-inventory -i inventory.yml --host seet-hobbit.\*

NOTE: the host pattern must only match ONE host!
And only output the variables attached to the host,
and not the groups the host is in :-(

Basically it is VERY limited!

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Get the groups of a host..

get_groups.yml
=======8<--------CUT HERE----------
- hosts: *host pattern*
  tasks:

    # report the full inventory groupings (by group)
    - debug: var=groups
      run_once: true

    # report the groups each host belongs to (by host)
    - debug: var=group_names

    # List just the groups in inventory
    # This includes 'all', and 'ungrouped', whether it has hosts or not.
    - debug: msg='{{ groups.keys }}'
      run_once: true

=======8<--------CUT HERE----------

ansible -i inventory.yml get_groups.yml

NOTE: the above needs 'no gather facts' to make faster

=======8<--------CUT HERE----------
- hosts: localhost
  vars:
    my_groups: "{{ groups|difference(['all', 'ungrouped']) }}"
    my_hosts: "{{ my_groups|map('extract', groups)|list }}"
    my_inventory: "{{ dict(my_groups|zip(my_hosts)) }}"
  tasks:
    - debug:
        var: my_inventory
=======8<--------CUT HERE----------

===============================================================================
Local inventory plugins...

You can put your inventory plugin under
  ~/.ansible/plugins/inventory/
for example
  ~/.ansible/plugins/inventory/get_my_inv.py
then enable it...
  export ANSIBLE_INVENTORY_ENABLED=get_my_inv

Test with
  ansible-inventory --list

These python inventories require a python class with specific subroutines
that can be called. The subroutines places hosts and there groups using class
functions...

=======8<--------CUT HERE----------
from ansible.plugins.inventory import BaseInventoryPlugin, Cacheable

# Uncomment to see errors...
# logging.getLogger().setLevel(logging.DEBUG)

class InventoryModule(BaseInventoryPlugin, Cacheable):

  NAME = "get_inv"

  def verify_file(self, path):
      return true

  def parse(self, inventory, loader, path, cache=True):
      super(InventoryModule, self).parse(inventory, loader, path, cache)
      cache_key = self.get_cache_key(path)

      ...

          server = the FQDN of server
          group = group to add server to...

          self.inventory.add_group(group)
          self.inventory.add_host(server, group=group)

=======8<--------CUT HERE----------

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Using nmap...

Get a list of community pluging available.
  ansible-doc -t inventory -l

Get the list of hosts dynamically using the "nmap" command...
  nmap -v -n -sn --osscan-limit --max-os-tries 1 132.234.28.0/24

Save this yaml file as "nmap_inventory.yml" then run command
  ansible-inventory -i nmap_inventory.yml --list

=======8<--------CUT HERE----------
---
plugin: community.general.nmap
address: 132.234.28.0/24
strict: False
ipv4: True
ipv6: False
ports: False
groups:
  production: "'-prd-' in hostname"
  development: "'-prd-' not in hostname"
...
=======8<--------CUT HERE----------

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