There are two ways to determine if plays in a playbook will be successful, like a dry run, meaning that no changes will be made on the managed nodes (e.g. target systems)

• --check command line flag
• check_mode parameter (that is what is described in this article)

The --check flag will check every play in a playbook. The check_mode parameter will only check certain plays in a playbook.

Let's say you've a playbook named foo.yml that is using the lineinfile module will append "Hello World" to /tmp/foo.txt, like this. Notice the check_mode parameter is being used.

- name: append 'Hello World' to /tmp/foo.txt
  lineinfile:
    path: /tmp/foo.txt
    line: Hello World
  check_mode: yes

Something like this should be returned.

TASK [append 'Hello World' to /tmp/foo.txt]
changed: [server1.example.com]

PLAY RECAP
server1.example.com : ok=1    changed=1    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0

On the managed node, "Hello World" would not have been appended to /tmp/foo.txt. Now let's invoke the ansible-playbook command without check_mode parameter.

- name: append 'Hello World' to /tmp/foo.txt
  lineinfile:
    path: /tmp/foo.txt
    line: Hello World

This should produce the same exact output, which is something important to be aware of. The check_mode parameter will not change the output produced by the ansible-playbook command. However, now, "Hello World" should have been appended to /tmp/foo.txt on the managed node. Just like the check_mode parameter, the --check flag will not change the output produced by the ansible-playbook command.

TASK [append 'Hello World' to /tmp/foo.txt]
changed: [server1.example.com]

PLAY RECAP
server1.example.com : ok=1    changed=1    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0

It usually doesn't make sense to hard code in "check_mode: yes" in your playbooks, because then when you want to disable check mode, you would have to comment out or change the check_mode parameter to false. Instead, you could set the check_mode parameter to use a variable, like this.

- name: append 'Hello World' to /tmp/foo.txt
  lineinfile:
    path: /tmp/foo.txt
    line: Hello World
  check_mode: "{{ mode }}"

Then, when using the ansible-playbook command, the --extra-vars command line flag could be used to set the mode variable to either "yes" or "no".

ansible-playbook foo.yml --extra-vars "mode=yes"