- name: find

  lineinfile: 

    dest: /etc/passwd

    line: "user"

  check_mode: yes

  register: presence

  failed_when: presence.changed

- hosts: localhost

  tasks:

​

  - name: read the passwd file

    shell: cat /etc/passwd

    register: user_accts

​

  - name: a task that only happens if the user exists

    when: user_accts.stdout.find('hillsy') != -1

    debug: msg="user hillsy exists"

tasks:

  - shell: grep username /etc/passwd

    changed_when: false