[ansible] Override hosts variable of Ansible playbook from the command line

This is a fragment of a playbook that I'm using (server.yml):

- name: Determine Remote User
  hosts: web
  gather_facts: false
  roles:
    - { role: remote-user, tags: [remote-user, always] }

My hosts file has different groups of servers, e.g.

[web]
x.x.x.x

[droplets]
x.x.x.x

Now I want to execute ansible-playbook -i hosts/<env> server.yml and override hosts: web from server.yml to run this playbook for [droplets].

Can I just override as a one time off thing, without editing server.yml directly?

Thanks.

If you want to run a task that's associated with a host, but on different host, you should try delegate_to.

In your case, you should delegate to your localhost (ansible master) and calling ansible-playbook command

Here's a cool solution I came up to safely specify hosts via the --limit option. In this example, the play will end if the playbook was executed without any hosts specified via the --limit option.

This was tested on Ansible version 2.7.10

---
- name: Playbook will fail if hosts not specified via --limit option.
  # Hosts must be set via limit. 
  hosts: "{{ play_hosts }}"
  connection: local
  gather_facts: false
  tasks:
  - set_fact:
      inventory_hosts: []
  - set_fact:
      inventory_hosts: "{{inventory_hosts + [item]}}"
    with_items: "{{hostvars.keys()|list}}"

  - meta: end_play
    when: "(play_hosts|length) == (inventory_hosts|length)"

  - debug:
      msg: "About to execute tasks/roles for {{inventory_hostname}}"

I am using ansible 2.5 (2.5.3 exactly), and it seems that the vars file is loaded before the hosts param is executed. So you can set the host in a vars.yml file and just write hosts: {{ host_var }} in your playbook

For example, in my playbook.yml:

---
- hosts: "{{ host_name }}"
  become: yes
  vars_files:
    - vars/project.yml
  tasks:
    ... 

And inside vars/project.yml:

---

# general
host_name: your-fancy-host-name

We use a simple fail task to force the user to specify the Ansible limit option, so that we don't execute on all hosts by default/accident.

The easiest way I found is this:

---
- name: Force limit
  # 'all' is okay here, because the fail task will force the user to specify a limit on the command line, using -l or --limit
  hosts: 'all'

  tasks:
  - name: checking limit arg
    fail:
      msg: "you must use -l or --limit - when you really want to use all hosts, use -l 'all'"
    when: ansible_limit is not defined
    run_once: true

Now we must use the -l (= --limit option) when we run the playbook, e.g.

ansible-playbook playbook.yml -l www.example.com

Limit option docs:

Limit to one or more hosts This is required when one wants to run a playbook against a host group, but only against one or more members of that group.

Limit to one host

ansible-playbook playbooks/PLAYBOOK_NAME.yml --limit "host1"

Limit to multiple hosts

ansible-playbook playbooks/PLAYBOOK_NAME.yml --limit "host1,host2"

Negated limit.
NOTE: Single quotes MUST be used to prevent bash interpolation.

ansible-playbook playbooks/PLAYBOOK_NAME.yml --limit 'all:!host1'

Limit to host group

ansible-playbook playbooks/PLAYBOOK_NAME.yml --limit 'group1'

For anyone who might come looking for the solution.
Play Book

- hosts: '{{ host }}'
  tasks:
  - debug: msg="Host is {{ ansible_fqdn }}"

Inventory

[web]
x.x.x.x

[droplets]
x.x.x.x

Command: ansible-playbook deplyment.yml -i hosts --extra-vars "host=droplets" So you can specify the group name in the extra-vars

An other solution is to use the special variable ansible_limit which is the contents of the --limit CLI option for the current execution of Ansible.

- hosts: "{{ ansible_limit | default(omit) }}"

If the --limit option is omitted, then Ansible issues a warning, but does nothing since no host matched.

[WARNING]: Could not match supplied host pattern, ignoring: None

PLAY ****************************************************************
skipping: no hosts matched

I'm using another approach that doesn't need any inventory and works with this simple command:

ansible-playbook site.yml -e working_host=myhost

To perform that, you need a playbook with two plays:

• first play runs on localhost and add a host (from given variable) in a known group in inmemory inventory
• second play runs on this known group

A working example (copy it and runs it with previous command):

- hosts: localhost
  connection: local
  tasks:
  - add_host:
      name: "{{ working_host }}"
      groups: working_group
    changed_when: false

- hosts: working_group
  gather_facts: false
  tasks:
  - debug:
      msg: "I'm on {{ ansible_host }}"

I'm using ansible 2.4.3 and 2.3.3

I changed mine to default to no host and have a check to catch it. That way the user or cron is forced to provide a single host or group etc. I like the logic from the comment from @wallydrag. The empty_group contains no hosts in the inventory.

- hosts: "{{ variable_host | default('empty_group') }}"

Then add the check in tasks:

   tasks:
   - name: Fail script if required variable_host parameter is missing
     fail:
       msg: "You have to add the --extra-vars='variable_host='"
     when: (variable_host is not defined) or (variable_host == "")

This is a bit late, but I think you could use the --limit or -l command to limit the pattern to more specific hosts. (version 2.3.2.0)

You could have - hosts: all (or group) tasks: - some_task

and then ansible-playbook playbook.yml -l some_more_strict_host_or_pattern and use the --list-hosts flag to see on which hosts this configuration would be applied.

Just came across this googling for a solution. Actually, there is one in Ansible 2.5. You can specify your inventory file with --inventory, like this: ansible --inventory configs/hosts --list-hosts all