Create an Ansible Playbook file which is just a YAML file:

examples/ansible/debug_playbook.yml

---
- name: Demo
  hosts: localhost
  gather_facts: False
  become: False

  tasks:
    - debug:
        msg: Just echo something

Run it as

$  ansible-playbook examples/ansible/debug_playbook.yml

The result will look like this:

 [WARNING]: Unable to parse /etc/ansible/hosts as an inventory source

 [WARNING]: No inventory was parsed, only implicit localhost is available

 [WARNING]: provided hosts list is empty, only localhost is available. Note that the implicit
localhost does not match 'all'


PLAY [Demo] *****************************************************************************************

TASK [debug] ****************************************************************************************
ok: [localhost] => {
    "msg": "Just echo something"
}

PLAY RECAP ******************************************************************************************
localhost                  : ok=1    changed=0    unreachable=0    failed=0

We run this on the local machine called localhost. It generates a few warnings, but we don't need an extra machine to play with. If you'd like to eliminate these warnings you can create an inventory file that onlys lists localhost:

examples/ansible/localhost.cfg

[localhost]
127.0.0.1

And supply it on the command line:

$  ansible-playbook -i examples/ansible/localhost.cfg  examples/ansible/debug_playbook.yml

Incorrect indentation: Ignoring invalid attribute: msg

One thing you need to be aware is that YAML derives its structure from the indentations and thus you need to make sure the msg tag is indented properly. If the indentation is incorrect as in the next example:

examples/ansible/debug_playbook_bad.yml

---
- name: Demo
  hosts: localhost
  gather_facts: False
  become: False

  tasks:
    - debug:
      msg: Just echo something

Then running it:

$  ansible-playbook -i examples/ansible/localhost.cfg  examples/ansible/debug_playbook_bad.yml

will yield the following output including the warning:

 [WARNING]: Ignoring invalid attribute: msg


PLAY [Demo] *****************************************************************************************

TASK [debug] ****************************************************************************************
ok: [127.0.0.1] => {
    "msg": "Hello world!"
}

PLAY RECAP ******************************************************************************************
127.0.0.1                  : ok=1    changed=0    unreachable=0    failed=0

No name

Finally, just to make anothe small change, one does not need to provide the name field at all:

examples/ansible/debug_playbook_noname.yml

---
- hosts: localhost
  gather_facts: False
  become: False

  tasks:
    - debug:
        msg: Just echo something

Result:

$  ansible-playbook -i examples/ansible/localhost.cfg  examples/ansible/debug_playbook_noname.yml

PLAY [localhost] ************************************************************************************

TASK [debug] ****************************************************************************************
ok: [127.0.0.1] => {
    "msg": "Just echo something"
}

PLAY RECAP ******************************************************************************************
127.0.0.1                  : ok=1    changed=0    unreachable=0    failed=0

Print results of shell commands

examples/ansible/debug_playbook_shell.yml

---
- name: Everything
  hosts: localhost
  tasks:
      #- name: run shell command
      #- shell: hostname
    - debug: msg="hello world"

    - debug:
        msg: "one two"

    - shell: hostname
      register: hostname
    - debug:
        var: hostname


    - debug:
        msg: "This is hostname: {{ hostname.stdout_lines[0] }}"

    - shell: |
        hostname
        whoami
      register: res
    - debug:
        var: res