258 lines
9.8 KiB
ReStructuredText
258 lines
9.8 KiB
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Conditionals
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============
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.. contents:: Topics
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Often the result of a play may depend on the value of a variable, fact (something learned about the remote system),
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or previous task result. In some cases, the values of variables may depend on other variables.
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Further, additional groups can be created to manage hosts based on
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whether the hosts match other criteria. There are many options to control execution flow in Ansible.
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Let's dig into what they are.
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The When Statement
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``````````````````
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Sometimes you will want to skip a particular step on a particular host. This could be something
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as simple as not installing a certain package if the operating system is a particular version,
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or it could be something like performing some cleanup steps if a filesystem is getting full.
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This is easy to do in Ansible, with the `when` clause, which contains a Jinja2 expression (see :doc:`playbooks_variables`).
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It's actually pretty simple::
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tasks:
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- name: "shutdown Debian flavored systems"
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command: /sbin/shutdown -t now
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when: ansible_os_family == "Debian"
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A number of Jinja2 "filters" can also be used in when statements, some of which are unique
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and provided by Ansible. Suppose we want to ignore the error of one statement and then
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decide to do something conditionally based on success or failure::
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tasks:
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- command: /bin/false
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register: result
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ignore_errors: True
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- command: /bin/something
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when: result|failed
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- command: /bin/something_else
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when: result|success
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- command: /bin/still/something_else
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when: result|skipped
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Note that was a little bit of foreshadowing on the 'register' statement. We'll get to it a bit later in this chapter.
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As a reminder, to see what facts are available on a particular system, you can do::
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ansible hostname.example.com -m setup
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Tip: Sometimes you'll get back a variable that's a string and you'll want to do a math operation comparison on it. You can do this like so::
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tasks:
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- shell: echo "only on Red Hat 6, derivatives, and later"
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when: ansible_os_family == "RedHat" and ansible_lsb.major_release|int >= 6
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.. note:: the above example requires the lsb_release package on the target host in order to return the ansible_lsb.major_release fact.
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Variables defined in the playbooks or inventory can also be used. An example may be the execution of a task based on a variable's boolean value::
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vars:
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epic: true
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Then a conditional execution might look like::
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tasks:
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- shell: echo "This certainly is epic!"
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when: epic
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or::
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tasks:
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- shell: echo "This certainly isn't epic!"
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when: not epic
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If a required variable has not been set, you can skip or fail using Jinja2's
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`defined` test. For example::
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tasks:
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- shell: echo "I've got '{{ foo }}' and am not afraid to use it!"
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when: foo is defined
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- fail: msg="Bailing out. this play requires 'bar'"
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when: bar is not defined
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This is especially useful in combination with the conditional import of vars
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files (see below).
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Note that when combining `when` with `with_items` (see :doc:`playbooks_loops`), be aware that the `when` statement is processed separately for each item. This is by design::
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tasks:
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- command: echo {{ item }}
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with_items: [ 0, 2, 4, 6, 8, 10 ]
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when: item > 5
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Loading in Custom Facts
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```````````````````````
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It's also easy to provide your own facts if you want, which is covered in :doc:`developing_modules`. To run them, just
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make a call to your own custom fact gathering module at the top of your list of tasks, and variables returned
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there will be accessible to future tasks::
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tasks:
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- name: gather site specific fact data
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action: site_facts
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- command: /usr/bin/thingy
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when: my_custom_fact_just_retrieved_from_the_remote_system == '1234'
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Applying 'when' to roles and includes
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`````````````````````````````````````
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Note that if you have several tasks that all share the same conditional statement, you can affix the conditional
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to a task include statement as below. Note this does not work with playbook includes, just task includes. All the tasks
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get evaluated, but the conditional is applied to each and every task::
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- include: tasks/sometasks.yml
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when: "'reticulating splines' in output"
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Or with a role::
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- hosts: webservers
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roles:
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- { role: debian_stock_config, when: ansible_os_family == 'Debian' }
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You will note a lot of 'skipped' output by default in Ansible when using this approach on systems that don't match the criteria.
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Read up on the 'group_by' module in the :doc:`modules` docs for a more streamlined way to accomplish the same thing.
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Conditional Imports
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```````````````````
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.. note:: This is an advanced topic that is infrequently used. You can probably skip this section.
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Sometimes you will want to do certain things differently in a playbook based on certain criteria.
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Having one playbook that works on multiple platforms and OS versions is a good example.
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As an example, the name of the Apache package may be different between CentOS and Debian,
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but it is easily handled with a minimum of syntax in an Ansible Playbook::
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---
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- hosts: all
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remote_user: root
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vars_files:
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- "vars/common.yml"
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- [ "vars/{{ ansible_os_family }}.yml", "vars/os_defaults.yml" ]
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tasks:
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- name: make sure apache is running
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service: name={{ apache }} state=running
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.. note::
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The variable 'ansible_os_family' is being interpolated into
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the list of filenames being defined for vars_files.
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As a reminder, the various YAML files contain just keys and values::
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---
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# for vars/CentOS.yml
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apache: httpd
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somethingelse: 42
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How does this work? If the operating system was 'CentOS', the first file Ansible would try to import
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would be 'vars/CentOS.yml', followed by '/vars/os_defaults.yml' if that file
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did not exist. If no files in the list were found, an error would be raised.
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On Debian, it would instead first look towards 'vars/Debian.yml' instead of 'vars/CentOS.yml', before
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falling back on 'vars/os_defaults.yml'. Pretty simple.
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To use this conditional import feature, you'll need facter or ohai installed prior to running the playbook, but
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you can of course push this out with Ansible if you like::
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# for facter
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ansible -m yum -a "pkg=facter state=present"
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ansible -m yum -a "pkg=ruby-json state=present"
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# for ohai
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ansible -m yum -a "pkg=ohai state=present"
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Ansible's approach to configuration -- separating variables from tasks, keeps your playbooks
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from turning into arbitrary code with ugly nested ifs, conditionals, and so on - and results
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in more streamlined & auditable configuration rules -- especially because there are a
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minimum of decision points to track.
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Selecting Files And Templates Based On Variables
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````````````````````````````````````````````````
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.. note:: This is an advanced topic that is infrequently used. You can probably skip this section.
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Sometimes a configuration file you want to copy, or a template you will use may depend on a variable.
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The following construct selects the first available file appropriate for the variables of a given host, which is often much cleaner than putting a lot of if conditionals in a template.
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The following example shows how to template out a configuration file that was very different between, say, CentOS and Debian::
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- name: template a file
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template: src={{ item }} dest=/etc/myapp/foo.conf
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with_first_found:
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- files:
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- {{ ansible_distribution }}.conf
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- default.conf
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paths:
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- search_location_one/somedir/
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- /opt/other_location/somedir/
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Register Variables
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``````````````````
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Often in a playbook it may be useful to store the result of a given command in a variable and access
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it later. Use of the command module in this way can in many ways eliminate the need to write site specific facts, for
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instance, you could test for the existence of a particular program.
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The 'register' keyword decides what variable to save a result in. The resulting variables can be used in templates, action lines, or *when* statements. It looks like this (in an obviously trivial example)::
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- name: test play
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hosts: all
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tasks:
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- shell: cat /etc/motd
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register: motd_contents
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- shell: echo "motd contains the word hi"
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when: motd_contents.stdout.find('hi') != -1
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As shown previously, the registered variable's string contents are accessible with the 'stdout' value.
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The registered result can be used in the "with_items" of a task if it is converted into
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a list (or already is a list) as shown below. "stdout_lines" is already available on the object as
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well though you could also call "home_dirs.stdout.split()" if you wanted, and could split by other
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fields::
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- name: registered variable usage as a with_items list
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hosts: all
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tasks:
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- name: retrieve the list of home directories
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command: ls /home
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register: home_dirs
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- name: add home dirs to the backup spooler
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file: path=/mnt/bkspool/{{ item }} src=/home/{{ item }} state=link
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with_items: home_dirs.stdout_lines
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# same as with_items: home_dirs.stdout.split()
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.. seealso::
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:doc:`playbooks`
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An introduction to playbooks
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:doc:`playbooks_roles`
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Playbook organization by roles
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:doc:`playbooks_best_practices`
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Best practices in playbooks
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:doc:`playbooks_conditionals`
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Conditional statements in playbooks
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:doc:`playbooks_variables`
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All about variables
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`User Mailing List <http://groups.google.com/group/ansible-devel>`_
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Have a question? Stop by the google group!
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`irc.freenode.net <http://irc.freenode.net>`_
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#ansible IRC chat channel
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