Ansible: Check if my user exists on remote host, else use root user to connect with ssh

Question

I'm currently writting an Ansible script which should update openssl on every host running Debian or CentOS. On the hosts our SSH-Keys are deposited for my own user or root. I want to check if my user is existing on the host, if not I want to authenticate with the root user. Is there a possibility to do this? I tried it with a bash command but I want to check if my user exists before I'm running the tasks. Maybe there are other solutions to my problem but I don't know them. Running this playbook throws a syntax error. My Script looks like this right now:

---
- hosts: "{{ host_group }}" 
  remote_user: "{{ username }}"
  tasks:

# Check whether there's a existinig user or whether you have to use root
    - name: Check whether there's your user on the machine
      action: shell /usr/bin/getent passwd $username | /usr/bin/wc -l | tr -d ''
      register: user_exist
      remote_user: root
      when: user_exist.stdout == 0
      tags:
         - users

# Install openssl on Ubuntu or Debian
    - name: Install openssl on Ubuntu or Debian
      become: True
      become_user: root
      apt: name=openssl state=latest
      when: ansible_distribution == 'Debian' or ansible_distribution == 'Ubuntu'

# Install openssl on CentOS or RHEL
    - name: Install openssl on CentOS or RHEL
      become: True
      become_user: root
      yum: name=openssl state=latest
      when: ansible_distribution == 'CentOS' or ansible_distribution == 'Red Hat Enterprise Linux'

Answer 1

A more native and elegant way to test your SSH connection is with the Ansible ping module (which verifies the end-to-end SSH connection, not ICMP), and to use the playbook keyword ignore_unreachable, which was added in Ansible 2.7.

The technique below puts SSH testing into its own play where facts are not gathered; subsequent plays will gather facts as normal:

---
###
## First play: Dynamically configure SSH user
##
- hosts: "{{ host_group }}"
  gather_facts: false  # don't try to ssh yet!!
  vars:
    ansible_ssh_user: "{{ username }}"

  tasks:
    - name: "Test SSH connection"
      ping:  # <-- no args needed
      ignore_unreachable: true
      ignore_errors: true
      changed_when: false
      register: ssh_test

    - name: "Fall back to root user?"
      when: ssh_test.unreachable is defined
      connection: local
      set_fact:
        ansible_ssh_user: root


###
## Next play: Install Stuff
###
- hosts: "{{ host_group }}"
  tasks:
    - name: Install openssl on Ubuntu or Debian
      # ...