Ansible - Print message - debug: msg="line1 \n {{ var2 }} \n line3 with var3 = {{ var3 }}"

debug module support array, so you can do like this:

debug:
  msg:
    - "First line"
    - "Second line"

The output:

ok: [node1] => {
    "msg": [
        "First line",
        "Second line"
    ]
}

Or you can use the method from this answer:

In YAML, how do I break a string over multiple lines?

The most convenient way I found to print multi-line text with debug is:

- name: Print several lines of text
  vars:
    msg: |
         This is the first line.
         This is the second line with a variable like {{ inventory_hostname }}.
         And here could be more...
  debug:
    msg: "{{ msg.split('\n') }}"

It splits the message up into an array and debug prints each line as a string. The output is:

ok: [example.com] => {
    "msg": [
        "This is the first line.", 
        "This is the second line with a variable like example.com", 
        "And here could be more...", 
        ""
    ]
}

Thanks to jhutar.

Pause module:

The most convenient and simple way I found to display a message with formatting (ex: new lines, tabs ...) is to use the pause module instead of debug module:

    - pause:
        seconds: 1
        prompt: |
          ======================
            line_1
            line_2
          ======================

You can also include a variable that contains formatting (new lines, tabs...) inside the prompt and it will be displayed as expected:

- name: test
  hosts: all
  vars:
    line3: "\n  line_3"
  tasks:
    - pause:
        seconds: 1
        prompt: |
          /////////////////
            line_1
            line_2 {{ line3 }}
          /////////////////

Tip:

when you want to display an output from a command, and instead of running an extra task to run the command and register the output, you can directly use the pipe lookup inside the prompt and do the job in one shot:

    - pause:
        seconds: 1
        prompt: |
          =========================
            line_1
            {{ lookup('pipe', 'echo "line_2 with \t tab \n  line_3 "') }}
            line_4
          =========================

Extra notes regarding the pause module:

1. If you have multiple hosts, note that the pause task will run only once against the first host in the list of hosts.

   This means that if the variable you want to display exists only in part of the hosts and the first host does not contain that variable then you will get an error.

   To avoid such an issue, use {{ hostvars['my_host']['my_var'] }} instead of {{ my_var }}

2. Combining pause with when conditional might skip the task! Why? Because the task will only run once against the first host which might not conform to the stated when conditions.

   To avoid this, don't use conditions that constrain the number of hosts! As you don't need it either, because you know that the task will run only once anyway. Also use hostvars stated above to make sure you get the needed variable whatever the picked up host is.

Example:

Incorrect:

- name: test
  hosts: host1,host2
  vars:
    display_my_var: true
  tasks:
    - when: inventory_hostname == 'host2'
      set_fact:
        my_var: "hi there"
    - when:
      - display_my_var|bool
      - inventory_hostname == 'host2'
      pause:
        seconds: 1
        prompt: |
          {{ my_var }}

This example will skip the pause task, because it will choose only the first host host1 and then starts to evaluate conditions, when it finds that host1 is not conforming to the second condition it will skip the task.

Correct:

- name: test
  hosts: host1,host2
  vars:
    display_my_var: true
  tasks:
    - when: inventory_hostname == 'host2'
      set_fact:
        my_var: "hi there"
    - when: display_my_var|bool
      pause:
        seconds: 1
        prompt: |
          {{ hostvars['host2']['my_var'] }}

Another example to display messages where the content depends on the host:

    - set_fact:
        my_var: "hi from {{ inventory_hostname }}"
    - pause:
        seconds: 1
        prompt: |
          {% for host in ansible_play_hosts %}
            {{ hostvars[host]['my_var'] }}
          {% endfor %}