Safely limiting Ansible playbooks to a single machine?

I'm using Ansible for some simple user management tasks with a small group of computers. Currently, I have my playbooks set to hosts: all and my hosts file is just a single group with all machines listed:

# file: hosts
[office]
imac-1.local
imac-2.local
imac-3.local

I've found myself frequently having to target a single machine. The ansible-playbook command can limit plays like this:

ansible-playbook --limit imac-2.local user.yml

But that seems kind of fragile, especially for a potentially destructive playbook. Leaving out the limit flag means the playbook would be run everywhere. Since these tools only get used occasionally, it seems worth taking steps to foolproof playback so we don't accidentally nuke something months from now.

Is there a best practice for limiting playbook runs to a single machine? Ideally the playbooks should be harmless if some important detail was left out.

Turns out it is possible to enter a host name directly into the playbook, so running the playbook with hosts: imac-2.local will work fine. But it's kind of clunky.

A better solution might be defining the playbook's hosts using a variable, then passing in a specific host address via --extra-vars:

# file: user.yml  (playbook)
---
- hosts: '{{ target }}'
  user: ...

Running the playbook:

ansible-playbook user.yml --extra-vars "target=imac-2.local"

If {{ target }} isn't defined, the playbook does nothing. A group from the hosts file can also be passed through if need be. Overall, this seems like a much safer way to construct a potentially destructive playbook.

Playbook targeting a single host:

$ ansible-playbook user.yml --extra-vars "target=imac-2.local" --list-hosts

playbook: user.yml

  play #1 (imac-2.local): host count=1
    imac-2.local

Playbook with a group of hosts:

$ ansible-playbook user.yml --extra-vars "target=office" --list-hosts

playbook: user.yml

  play #1 (office): host count=3
    imac-1.local
    imac-2.local
    imac-3.local

Forgetting to define hosts is safe!

$ ansible-playbook user.yml --list-hosts

playbook: user.yml

  play #1 ({{target}}): host count=0

There's also a cute little trick that lets you specify a single host on the command line (or multiple hosts, I guess), without an intermediary inventory:

ansible-playbook -i "imac1-local," user.yml

Note the comma (,) at the end; this signals that it's a list, not a file.

Now, this won't protect you if you accidentally pass a real inventory file in, so it may not be a good solution to this specific problem. But it's a handy trick to know!

This approach will exit if more than a single host is provided by checking the play_hosts variable. The fail module is used to exit if the single host condition is not met. The examples below use a hosts file with two hosts alice and bob.

user.yml (playbook)

---
- hosts: all
  tasks:
    - name: Check for single host
      fail: msg="Single host check failed."
      when: "{{ play_hosts|length }} != 1"
    - debug: msg='I got executed!'

Run playbook with no host filters

$ ansible-playbook user.yml
PLAY [all] ****************************************************************
TASK: [Check for single host] *********************************************
failed: [alice] => {"failed": true}
msg: Single host check failed.
failed: [bob] => {"failed": true}
msg: Single host check failed.
FATAL: all hosts have already failed -- aborting

Run playbook on single host

$ ansible-playbook user.yml --limit=alice

PLAY [all] ****************************************************************

TASK: [Check for single host] *********************************************
skipping: [alice]

TASK: [debug msg='I got executed!'] ***************************************
ok: [alice] => {
    "msg": "I got executed!"
}

There's IMHO a more convenient way. You can indeed interactively prompt the user for the machine(s) he wants to apply the playbook to thanks to vars_prompt:

---

- hosts: "{{ setupHosts }}"
  vars_prompt:
    - name: "setupHosts"
      prompt: "Which hosts would you like to setup?"
      private: no
  tasks:
    […]