Ansible: Handling Different Operating Systems with Variables

A fundamental part of working with configuration management tools is the ability to handle different operating systems within the same logic stream, such as a module.

I’ve found a few challenges addressing this topic within Ansible, and hence I think it’s worth noting certain things that you can’t do, and I think is the most logical way (so far).

Things That do not Work

As with most things I learn in life, I figured these out via trial and error, via an approach something along the lines of “I should be able to do this, shouldn’t I?”

Variables and facts cannot be directly referenced in a handler

I tried this:

# handlers/main.yml
---
- name: (Ubuntu) restart ssh
  service: name=ssh state=restarted

- name: (CentOS) restart ssh
  service: name=sshd state=restarted

# tasks/main.yml
---
- name: test SSH restart
  lineinfile: dest=/etc/ssh/sshd_config line=#testing
  notify: ({{ ansible_distribution }}) restart ssh

This gives you this:

ERROR: change handler (({{ ansible_distribution }}) restart ssh) is not defined

Hmm.

Logic does not work in handler

You also can’t do something like this:

# handlers/main.yml
---
- name: restart ssh
  service: name=ssh state=restarted
  when: ansible_distribution == 'Ubuntu'

- name: restart ssh
  service: name=sshd state=restarted
  when: ansible_distribution == 'CentOS'

In this instance, only the first handler is looked at, and if the logic in when: does not evaluate to true, the handler will be skipped. Completely.

Defining variables within a task or handler

You cannot use a vars directive within a task or handler: you simply get this:

ERROR: vars is not a legal parameter in an Ansible task or handler

Note that facts can be set using the set_fact module but this is better used when there is a need to define host-specific variables for use later, in more of a host scope versus a role scope.

The Right Way

I mentioned that variables within a role should not be used, but that’s only half the story. Specifically, variables should not be used for parameters – if you are going to define those variables later within a playbook or via group_vars/ or host_vars/, it is better to make them defaults.

But on the other hand, they are better suited as variables if they are not intended for assignment outside of the scope of the role. This is perfect for what we are planning on using them for here.

Variables can be defined in a role pretty easy – see roles. Note that in here there is a vars/ directory, from which vars/main.yml is loaded with high priority.

This is not the file I am looking for though, as variable files are not very flexible: no additional files or logic can go into it, just values.

So how can variables be included logically then?

include_vars and with_first_found

include_vars is a module that allows you to include variables from within a task. Perfect for the kind of logic flow we are looking for, and allows you to specify a file based off either a jinja2 template handler or the special {{ item }} token.

That’s only part of it though. Logic is needed to handle different operating systems, and can get pretty messy if all that is being used is a bunch of when: clauses. Enter with_first_found:

# tasks/main.yml
---
- name: set distro-specific variables
  include_vars: '{{ item }}'
  with_first_found:
    - '{{ ansible_os_family }}.yml'
    - default.yml

Using this, files can now be placed like so:

# vars/Debian.yml
---
ssh_svc: ssh

# vars/RedHat.yml
---
ssh_svc: sshd

# vars/default.yml
---
ssh_svc: ssh

Now, if you have a handler such as:

# handlers/main.yml
- name: restart ssh
  service: name={{ ssh_svc }} state=restarted

Either ssh or sshd would be used, depending on distro.

Note that with_first_found can be used to set up logic for other things, not just include_vars, it can be especially useful within your task workflow, to avoid including logic that may artificially take up log space or time in a playbook. These includes could be used in the place of when: clauses, for example.

More detail on logic and templating below. Most with_ clauses can be found in the Loops page, but there are a couple of more within the other pages. I wish Ansible was better at documenting this!

http://docs.ansible.com/playbooks_loops.html
http://docs.ansible.com/playbooks_lookups.html
https://docs.ansible.com/playbooks_conditionals.html#selecting-files-and-templates-based-on-variables

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