Playbooks¶

Hosts and Users¶

For each play in a playbook, you get to choose which machines in your infrastructure to target and what remote user to complete the steps (called tasks) as.

The hosts line is a list of one or more groups or host patterns, separated by colons, as described in the Inventory & Patterns documentation. The user is just the name of the user account:

---
- hosts: webservers
  user: root

Support for running things from sudo is also available:

---
- hosts: webservers
  user: yourname
  sudo: True

You can also login as you, and then sudo to different users than root:

---
- hosts: webservers
  user: yourname
  sudo: True
  sudo_user: postgres

If you need to specify a password to sudo, run ansible-playbook with --ask-sudo-pass (-K). If you run a sudo playbook and the playbook seems to hang, it’s probably stuck at the sudo prompt. Just Control-C to kill it and run it again with -K.

Vars section¶

The vars section contains a list of variables and values that can be used in the plays, like this:

---
- hosts: webservers
  users: root
  vars:
     http_port: 80
     van_halen_port: 5150
     other: 'magic'

These variables can be used later in the playbook like this:

$varname or ${varname}

The later is useful in the event you need to do something like ${other}_some_string.

Inside templates, the full power of the Jinja2 templating language is also available, which looks like this:

{{ varname }}

The Jinja2 documentation provides information about how to construct loops and conditionals for those who which to use more advanced templating. This is optional and the $varname format still works in template files.

If there are discovered variables about the system, called ‘facts’, these variables bubble up back into the playbook, and can be used on each system just like explicitly set variables. Ansible provides several of these, prefixed with ‘ansible’, and are documented under setup in the module documentation. Additionally, facts can be gathered by ohai and facter if they are installed. Facter variables are prefixed with facter_ and Ohai variables are prefixed with ohai_.

So for instance, if I wanted to write the hostname into the /etc/motd file, I could say:

- name: write the motd
  action: template src=/srv/templates/motd.j2 dest=/etc/motd

And in /srv/templates/motd.j2:

You are logged into {{ facter_hostname }}

But we’re getting ahead of ourselves. Let’s talk about tasks.

Tasks list¶

Each play contains a list of tasks. Tasks are executed in order, one at a time, against all machines matched by the host pattern, before moving on to the next task. It is important to understand that, within a play, all hosts are going to get the same task directives. It is the purpose of a play to map a selection of hosts to tasks.

When running the playbook, which runs top to bottom, hosts with failed tasks are taken out of the rotation for the entire playbook. If things fail, simply correct the playbook file and rerun.

The goal of each task is to execute a module, with very specific arguments. Variables, as mentioned above, can be used in arguments to modules.

Modules are ‘idempotent’, meaning if you run them again, they will make the changes they are told to make to bring the system to the desired state. This makes it very safe to rerun the same playbook multiple times. They won’t change things unless they have to change things.

The command and shell modules will typically rerun the same command again, which is totally ok if the command is something like ‘chmod’ or ‘setsebool’, etc. Though there is a ‘creates’ flag available which can be used to make these modules also idempotent.

Every task should have a name, which is included in the output from running the playbook. This is output for humans, so it is nice to have reasonably good descriptions of each task step. If the name is not provided though, the string fed to ‘action’ will be used for output.

Here is what a basic task looks like, as with most modules, the service module takes key=value arguments:

tasks:
  - name: make sure apache is running
    action: service name=httpd state=running

The command and shell modules are the one modules that just takes a list of arguments, and don’t use the key=value form. This makes them work just like you would expect. Simple:

tasks:
  - name: disable selinux
    action: command /sbin/setenforce 0

The command and shell module care about return codes, so if you have a command who’s successful exit code is not zero, you may wish to do this:

tasks:

• name: run this command and ignore the result action: shell /usr/bin/somecommand & /bin/true

Variables can be used in action lines. Suppose you defined a variable called ‘vhost’ in the ‘vars’ section, you could do this:

tasks:
  - name: create a virtual host file for $vhost
    action: template src=somefile.j2 dest=/etc/httpd/conf.d/$vhost

Those same variables are usable in templates, which we’ll get to later.

Now in a very basic playbook all the tasks will be listed directly in that play, though it will usually make more sense to break up tasks using the ‘include:’ directive. We’ll show that a bit later.