I haven't tested, but is there a reason not to use ansible.builtin.command instead of shell for this?

Something like this might work

- name: Set bootstrap password
  ansible.builtin.command: /usr/share/elasticsearch/bin/elasticsearch-keystore add -x 'bootstrap.password'
  args:
    stdin: {{ elasticsearch_bootstrap_pw }}
  when: "'bootstrap.password' not in elasticsearch_keystore.stdout_lines"

I haven't tested, but is there a reason not to use ansible.builtin.command instead of shell for this?

Something like this might work

- name: Set bootstrap password
  ansible.builtin.command: /usr/share/elasticsearch/bin/elasticsearch-keystore add -x 'bootstrap.password'
  args:
    stdin: {{ elasticsearch_bootstrap_pw }}
  when: "'bootstrap.password' not in elasticsearch_keystore.stdout_lines"

To be honest, I don't see a reason. Maybe the contributer back then had an idea I do not recognize. I guess, in the current implementation, command should be fine and make things even faster.

In the long term, it might be a good idea to replace these tasks with a custom module, though.

@ivareri I guess it was that the original contributor (like me) didn't think of the possibility to implement it with stdin.

I still think this would fit into another PR so not to change too many things at once. Maybe a module would be even better.