How to prevent 'changed' flag when 'register'-ing a variable?

Question

I have a register task to test for the installation of a package:

tasks:
  - name: test for nginx
    command: dpkg -s nginx-common
    register         


        

Answer 1

It is the command module causing the changed state, not the register parameter.

You can set changed_when: to something that is only true when something changed (also look at failed_when). If your task don't change anything, you might want to set check_mode as well. (Especially if other steps depend on the value)

This gives:

tasks:
  - name: test for nginx
    command: dpkg -s nginx-common
    register: nginx_installed
    changed_when: False
    failed_when: False  # dpkg -s returns 1 when packages is not found
    check_mode: yes # this can safely run in check_mode

Answer 2

It’s described in official documentation here.

tasks:
  - name: test for nginx
    command: dpkg -s nginx-common
    register: nginx_installed
    changed_when: false