How to make Ansible run one certain task only on one host? - ansible

Question

The playbook looks like:
- hosts: all
tasks:
- name: "run on all hosts,1"
shell: something1
- name: "run on all hosts,2"
shell: something2
- name: "run on one host, any host would do"
shell: this_command_should_run_on_one_host
- name: "run on all hosts,3"
shell: something3
I know with command line option --limit, I can limit to one host, is it possible to do it in playbook?

Answer 1

For any host (with defaults it will match the first on the list):
- name: "run on first found host"
shell: this_command_should_run_on_one_host
run_once: true
For a specific host:
- name: "run on that_one_host host"
shell: this_command_should_run_on_one_host
when: ansible_hostname == 'that_one_host'
Or inventory_hostname (hostname as defined in the Ansible inventory) instead of ansible_hostname (hostname as defined on the target machine), depending on which name you want to use.

Answer 2

Techraf's first answer is the exact one for the OP's question.
I just wanted to show a better way to run a task on a specific host:
- name: "run on that_one_host host"
shell: this_command_should_run_on_one_host
run_once: true
delegate_to: that_one_host
If the group the playbook is run on contains many hosts, using when: ansible_hostname == 'that_one_host' or when: ansible_hostname == play_hosts[0] will evaluate the when-clause on all the hosts (which could be long if the when-clause had other more complicated conditions) and result in a long list of skipped hosts in the playbook's output.
Combining run_once and delegate_to, the playbook's output will be cleaner, only showing the task being executed on the chosen host.
- hosts: all
gather_facts: no
tasks:
- name: Run on one specific host | when-clause
debug:
msg: "Hello world"
when: inventory_hostname == play_hosts[0]
- name: Run on one specific host | run_once + delegate_to
debug:
msg: "Hello world"
run_once: true
delegate_to: play_hosts[0]
TASK [Run on one specific host | when-clause] **********************************************************************************
skipping: [host2]
skipping: [host3]
ok: [host1] => {
"msg": "Hello world"
}
skipping: [host4]
TASK [Run on one specific host | run_once + delegate_to] ***********************************************************************
ok: [host2 -> play_hosts[0]] => {
"msg": "Hello world"
}
Whichever solution you choose, never combine these, if you want a task to be run once on a precise host:
run_once: true and
when: inventory_hostname == 'that_one_host'
If you do, it will be impossible to know in advance if the task will be executed or not (I have learnt it the hard way). The reason is that run_once: true will select a random host in the play's group and only later apply the when-clause:
if run_once selected to run the task on 'that_one_host', the task will be executed
if run_once selected 'another_host', the task will be skipped.

Answer 3

True way
- include_tasks: custom-tasks.yml
when: inventory_hostname == item
with_items: "{{ ansible_play_hosts }}"