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This Ansible playbook example helps you execute actions only if you are on a certain distribution. You might have a mixed environment with CentOS and Debian and when using Ansible to execute actions on nodes you don't need to run Yum on Debian, or Apt on CentOS. Some package names are different and such, so this helps you with an only if statement to select a specific distribution. As a bonus, you also get an only_if for specific distribution versions, like Ubuntu precise (12.04 LTS) or Ubuntu Trusty (14.04 LTS).

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Package module (2015 short update)

As my former colleague Stein pointed me to, Ansible 2.0 has been released and it features the package module. This is a generic module that installs, upgrade and removes packages using the underlying OS package manager. This module actually calls the pertinent package modules for each system (apt, yum, etc).

This means that if you use this article because you want a package install on Debian and CentOS, you can now just do the following:

- name: install (or upgrade to) the latest version of htop
  package: name=htop state=latest

# This uses a variable as this changes per distribution.
# Ubuntu: apache2
# CentOS: httpd
- name: remove the apache package
  package : name={{apache}} state=absent

Read more about the package module on the ansible Docs website

Specific Distribution

On a specific action, add the following when statement:

when: ansible_distribution == 'CentOS' or ansible_distribution == 'Red Hat Enterprise Linux'

This is for RHEL and Centos, the following is for Debian/Ubuntu:

when: ansible_distribution == 'Debian' or ansible_distribution == 'Ubuntu'

This example playbook installs ntpd on both Debian/Ubuntu and CentOS.

---
- hosts: example
  sudo: True
  user: remy
  connection: ssh # or paramiko

  tasks:
  - apt: name=$item state=latest
    with_items:
     - ntp
    when: ansible_distribution == 'Debian' or ansible_distribution == 'Ubuntu'

  - yum: name=$item state=latest
    with_items:
     - ntp
    when: ansible_distribution == 'CentOS' or ansible_distribution == 'Red Hat Enterprise Linux'

  - service: name=ntpd state=started enabled=yes

Specific Distribution Version

You might also need to do different actions based on distribution version, because some things are available on CentOS 6 but not on 5, or on Ubuntu Lucid you need to install some backported packages and not on Ubuntu Precise.

For those situations, you can use either the {{ ansible_distribution_version } or {{ ansible_distribution_release }} variable. See some example output from ansible all -m setup -a "filter=ansible_distribution*":

    "ansible_distribution": "CentOS",
    "ansible_distribution_release": "Final",
    "ansible_distribution_version": "5.9"

    "ansible_distribution": "CentOS",
    "ansible_distribution_release": "Final",
    "ansible_distribution_version": "6.4"

    "ansible_distribution": "Ubuntu",
    "ansible_distribution_release": "lucid",
    "ansible_distribution_version": "10.04"

    "ansible_distribution": "Ubuntu",
    "ansible_distribution_release": "precise",
    "ansible_distribution_version": "12.04"

    "ansible_distribution": "Debian",
    "ansible_distribution_release": "wheezy",
    "ansible_distribution_version": "7"

Using these, you can filter the output by changing the when statement in your ansible playbook:

when: ansible_distribution == 'CentOS' and ansible_distribution_version == '6.4'

when: ansible_distribution == 'Ubuntu' and ansible_distribution_release == 'precise'

when: ansible_distribution == 'Debian' and ansible_distribution_version == '7'

when: ansible_os_family == "RedHat" and ansible_lsb.major_release|int >= 5