Ansible has an installation guide ready which will guide you through any missing dependencies as well: https://docs.ansible.com/ansible/latest/installation_guide/intro_installation.html

There is also the development version on GitHub: https://github.com/ansible/ansible

Here are the basic guidelines, taken from the guide above:

1. Locate where Python is installed using the following command:

   which python3
   

2. Ensure that pip (part of Python) is installed by running this command:

   python3 -m pip -V
   

   If all is well, you should see something like the following:

   pip 21.0.1 from /usr/lib/python3.9/site-packages/pip (python 3.9)
   

   If so, pip is available, and you can move on to the next step.

   If you see an error like No module named pip, you’ll need to install pip under your chosen Python interpreter before proceeding. This may mean installing an additional OS package (for example, python3-pip), or installing the latest pip directly from the Python Packaging Authority by running the following:

   curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py
   python3 get-pip.py --user
   

3. Install Ansible for the current user using pip in your selected Python environment:

   python3 -m pip install --user ansible
   

You haven't mentioned your Ubuntu release installation, so I assume you can have something above 18.04, for which the above should work out fine.

For anything extra, refer to the guide and Ansible's official website, mentioned in the start of this answer.

Good luck.

I wouldn't recommend using shell for this, as Ansible has the apt module designed for just this purpose. I've detailed using apt below.

In a playbook, you can update and upgrade like so:

- name: Update and upgrade apt packages
  become: true
  apt:
    upgrade: yes
    update_cache: yes
    cache_valid_time: 86400 #One day

The cache_valid_time value can be omitted. Its purpose from the docs:

Update the apt cache if its older than the cache_valid_time. This option is set in seconds.

So it's good to include if you don't want to update the cache when it has only recently been updated.

To do this as an ad-hoc command you can run:

$ ansible all -m apt -a "upgrade=yes update_cache=yes cache_valid_time=86400" --become

ad-hoc commands are described in detail here

Note that I am using --become and become: true. This is an example of typical privilege escalation through Ansible. You use -u user and -K (ask for privilege escalation password). Use whichever works for you, this is just to show you the most common form.

 In other words, why not both be done with an apt module?

We can't say for certain, but likely it's just a mistake coming from ignorance. The latter command can be fully implemented using the Ansible apt module, and you should always prefer that over command; command exists as an escape for when there isn't a module for what you want to do.