ansible/docsite/latest/rst/playbooks_variables.rst

Variables
=========

While automation exists to make it easier to make things repeatable, all of your systems are likely not exactly alike.

All of your systems are likely not the same.  On some systems you may want to set some behavior
or configuration that is slightly different from others. 

Also, some of the observed behavior or state 
of remote systems might need to influence how you configure those systems.  (Such as you might need to find out the IP
address of a system and even use it as a configuration value on another system).

You might have some templates for configuration files that are mostly the same, but slightly different
based on those variables.  

Variables in Ansible are how we deal with differences between systems.  

Once understanding variables you'll also want to dig into `playbooks_conditionals` and `playbooks_loops`.  
Useful things like the "group_by" module
and the "when" conditional can also be used with variables, and to help manage differences between systems.

It's highly recommended that you consult the ansible-examples github repository to see a lot of examples of variables put to use.

.. contents::
   :depth: 2

.. _valid_variable_names:

What Makes A Valid Variable Name
````````````````````````````````

Before we start using variables it's important to know what are valid variable names.

Variable names should be letters, numbers, and underscores.  Variables should always start with a letter.

"foo_port" is a great variable.  "foo5" is fine too.  

"foo-port", "foo port", "foo.port" and "12" are not valid variable names.

Easy enough, let's move on.

.. _variables_in_inventory:

Variables Defined in Inventory
``````````````````````````````

We've actually already covered a lot about variables in another section, so so far this shouldn't be terribly new, but
a bit of a refresher.

Often you'll want to set variables based on what groups a machine is in.  For instance, maybe machines in Boston
want to use 'boston.ntp.example.com' as an NTP server.

See the `intro_inventory` document for multiple ways on how to define variables in inventory.  

.. _playbook_variables:

Variables Defined in a Playbook
```````````````````````````````

In a playbook, it's possible to define variables directly inline like so::

   - hosts: webservers
     vars:
       http_port: 80

This can be nice as it's right there when you are reading the playbook.

.. _included_variables:

Variables defined from included files and roles
-----------------------------------------------

It turns out we've already talked about variables in another place too.

As described in `intro_roles`, variables can also be included in the playbook via include files, which may or may
not be part of an "Ansible Role".  Usage of roles is preferred as it provides a nice organizational system.

.. _about_jinja2:

Using Variables: About Jinja2
`````````````````````````````

It's nice enough to know about how to define variables, but how do you use them?

Ansible allows you to
reference variables in your playbooks using the Jinja2 templating system.  While you can do a lot of complex
things in Jinja, only the basics are things you really need to learn at first.

For instance, in a simple template, you can do something like

    My amp goes to {{ max_amp_value }}

And that will provide the most basic form of variable substitution.

This is also valid directly in playbooks, and you'll occasionally want to do things like:

    template: src=foo.cfg.j2 dest={{ remote_install_path}}/foo.cfg

In the above example, we used a variable to help decide where to place a file.

Inside a template you automatically have access to all of the variables that are in scope for a host.  Actually
it's more than that -- you can also read variables about other hosts.  We'll show how to do that in a bit.

.. note:: ansible allows Jinja2 loops and conditionals in templates, but in playbooks, we do not use them.  Ansible
   templates are pure machine-parseable YAML.  This is an rather important feature as it means it is possible to code-generate
   pieces of files, or to have other ecosystem tools read Ansible files.  Not everyone will need this but it can unlock
   possibilities.

.. _jinja2_filters:

Jinja2 Filters
``````````````

.. note: These are infrequently utilized features.  Use them if they fit a use case you have, but this is optional knowledge.

Filters in Jinja2 are a way of transforming template expressions from one kind of data into another.  Jinja2
ships with many of these as documented on the official Jinja2 template documentation.

In addition to these, Ansible supplies many more.  

.. _filters_for_formatting_data:

Filters For Formatting Data
---------------------------

The following filters will take a data structure in a template and render it in a slightly different format.  These
are occasionally useful for debugging::

    {{ some_variable | to_nice_json }}
    {{ some_variable | to_nice_yaml }}

.. _filters_used_with_conditionals:

Filters Often Used With Conditionals
------------------------------------

The following tasks are illustrative of how filters can be used with conditionals::

    tasks:
      - shell: /usr/bin/foo
        register: result
        ignore_errors: True

      - debug: msg="it failed"
        when: result|failed

      # in most cases you'll want a handler, but if you want to do something right now, this is nice
      - debug: msg="it changed"
        when: result|changed

      - debug: msg="it succeeded"
        when: result|success

      - debug: msg="it was skipped"
        when: result|skipped

.. _forcing_variables_to_be_defined:

Forcing Variables To Be Defined
-------------------------------

The default behavior from ansible and ansible.cfg is to fail if variables are undefined, but you can turn this off.

This allows an explicit check with this feature off::

    {{ variable | mandatory }}

The variable value will be used as is, but the template evaluation will raise an error if it is undefined.

.. _other_useful_filters:

Other Useful Filters
--------------------

To get the last name of a file path, like 'foo.txt' out of '/etc/asdf/foo.txt'::

    {{ path | basename }} 

To get the directory from a path::

    {{ path | dirname }}

To work with Base64 encoded strings::

    {{ encoded | b64decode }}
    {{ decoded | b64encode }}

To take an md5sum of a filename::

    {{ filename | md5 }}

To cast values as certain types, such as when you input a string as "True" from a vars_prompt and the system
doesn't know it is a boolean value::

   - debug: msg=test
     when: some_string_value | bool

A few useful filters are typically added with each new Ansible release.  The development documentation shows
how to extend Ansible filters by writing your own as plugins, though in general, we encourage new ones
to be added to core so everyone can make use of them.

.. _yaml_gotchas:

Hey Wait, A YAML Gotcha
```````````````````````

YAML syntax requires that if you start a value with {{ foo }} you quote the whole line, since it wants to be
sure you aren't trying to start a YAML dictionary.  This is covered on the `YAMLSyntax` page.

This won't work::

    - hosts: app_servers
      vars:
          app_path: {{ base_path }}/22

Do it like this and you'll be fine::

    - hosts: app_servers
      vars:
           app_path: "{{ base_path }}/22"

.. _vars_and_facts:

Information discovered from systems: Facts
``````````````````````````````````````````

There are other places where variables can come from, but these are a type of variable that are discovered, not set by the user.

Facts are information derived from speaking with your remote systems.

An example of this might be the ip address of the remote host, or what the operating system is. 

To see what information is available, try the following::

    ansible hostname -m setup

This will return a ginormous amount of variable data, which may look like this, as taken from Ansible 1.4 on a Ubuntu 12.04 system::

        "ansible_all_ipv4_addresses": [
            "REDACTED IP ADDRESS"
        ], 
        "ansible_all_ipv6_addresses": [
            "REDACTED IPV6 ADDRESS"
        ], 
        "ansible_architecture": "x86_64", 
        "ansible_bios_date": "09/20/2012", 
        "ansible_bios_version": "6.00", 
        "ansible_cmdline": {
            "BOOT_IMAGE": "/boot/vmlinuz-3.5.0-23-generic", 
            "quiet": true, 
            "ro": true, 
            "root": "UUID=4195bff4-e157-4e41-8701-e93f0aec9e22", 
            "splash": true
        }, 
        "ansible_date_time": {
            "date": "2013-10-02", 
            "day": "02", 
            "epoch": "1380756810", 
            "hour": "19", 
            "iso8601": "2013-10-02T23:33:30Z", 
            "iso8601_micro": "2013-10-02T23:33:30.036070Z", 
            "minute": "33", 
            "month": "10", 
            "second": "30", 
            "time": "19:33:30", 
            "tz": "EDT", 
            "year": "2013"
        }, 
        "ansible_default_ipv4": {
            "address": "REDACTED", 
            "alias": "eth0", 
            "gateway": "REDACTED", 
            "interface": "eth0", 
            "macaddress": "REDACTED", 
            "mtu": 1500, 
            "netmask": "255.255.255.0", 
            "network": "REDACTED", 
            "type": "ether"
        }, 
        "ansible_default_ipv6": {}, 
        "ansible_devices": {
            "fd0": {
                "holders": [], 
                "host": "", 
                "model": null, 
                "partitions": {}, 
                "removable": "1", 
                "rotational": "1", 
                "scheduler_mode": "deadline", 
                "sectors": "0", 
                "sectorsize": "512", 
                "size": "0.00 Bytes", 
                "support_discard": "0", 
                "vendor": null
            }, 
            "sda": {
                "holders": [], 
                "host": "SCSI storage controller: LSI Logic / Symbios Logic 53c1030 PCI-X Fusion-MPT Dual Ultra320 SCSI (rev 01)", 
                "model": "VMware Virtual S", 
                "partitions": {
                    "sda1": {
                        "sectors": "39843840", 
                        "sectorsize": 512, 
                        "size": "19.00 GB", 
                        "start": "2048"
                    }, 
                    "sda2": {
                        "sectors": "2", 
                        "sectorsize": 512, 
                        "size": "1.00 KB", 
                        "start": "39847934"
                    }, 
                    "sda5": {
                        "sectors": "2093056", 
                        "sectorsize": 512, 
                        "size": "1022.00 MB", 
                        "start": "39847936"
                    }
                }, 
                "removable": "0", 
                "rotational": "1", 
                "scheduler_mode": "deadline", 
                "sectors": "41943040", 
                "sectorsize": "512", 
                "size": "20.00 GB", 
                "support_discard": "0", 
                "vendor": "VMware,"
            }, 
            "sr0": {
                "holders": [], 
                "host": "IDE interface: Intel Corporation 82371AB/EB/MB PIIX4 IDE (rev 01)", 
                "model": "VMware IDE CDR10", 
                "partitions": {}, 
                "removable": "1", 
                "rotational": "1", 
                "scheduler_mode": "deadline", 
                "sectors": "2097151", 
                "sectorsize": "512", 
                "size": "1024.00 MB", 
                "support_discard": "0", 
                "vendor": "NECVMWar"
            }
        }, 
        "ansible_distribution": "Ubuntu", 
        "ansible_distribution_release": "precise", 
        "ansible_distribution_version": "12.04", 
        "ansible_domain": "", 
        "ansible_env": {
            "COLORTERM": "gnome-terminal", 
            "DISPLAY": ":0", 
            "HOME": "/home/mdehaan", 
            "LANG": "C", 
            "LESSCLOSE": "/usr/bin/lesspipe %s %s", 
            "LESSOPEN": "| /usr/bin/lesspipe %s", 
            "LOGNAME": "root", 
            "LS_COLORS": "rs=0:di=01;34:ln=01;36:mh=00:pi=40;33:so=01;35:do=01;35:bd=40;33;01:cd=40;33;01:or=40;31;01:su=37;41:sg=30;43:ca=30;41:tw=30;42:ow=34;42:st=37;44:ex=01;32:*.tar=01;31:*.tgz=01;31:*.arj=01;31:*.taz=01;31:*.lzh=01;31:*.lzma=01;31:*.tlz=01;31:*.txz=01;31:*.zip=01;31:*.z=01;31:*.Z=01;31:*.dz=01;31:*.gz=01;31:*.lz=01;31:*.xz=01;31:*.bz2=01;31:*.bz=01;31:*.tbz=01;31:*.tbz2=01;31:*.tz=01;31:*.deb=01;31:*.rpm=01;31:*.jar=01;31:*.war=01;31:*.ear=01;31:*.sar=01;31:*.rar=01;31:*.ace=01;31:*.zoo=01;31:*.cpio=01;31:*.7z=01;31:*.rz=01;31:*.jpg=01;35:*.jpeg=01;35:*.gif=01;35:*.bmp=01;35:*.pbm=01;35:*.pgm=01;35:*.ppm=01;35:*.tga=01;35:*.xbm=01;35:*.xpm=01;35:*.tif=01;35:*.tiff=01;35:*.png=01;35:*.svg=01;35:*.svgz=01;35:*.mng=01;35:*.pcx=01;35:*.mov=01;35:*.mpg=01;35:*.mpeg=01;35:*.m2v=01;35:*.mkv=01;35:*.webm=01;35:*.ogm=01;35:*.mp4=01;35:*.m4v=01;35:*.mp4v=01;35:*.vob=01;35:*.qt=01;35:*.nuv=01;35:*.wmv=01;35:*.asf=01;35:*.rm=01;35:*.rmvb=01;35:*.flc=01;35:*.avi=01;35:*.fli=01;35:*.flv=01;35:*.gl=01;35:*.dl=01;35:*.xcf=01;35:*.xwd=01;35:*.yuv=01;35:*.cgm=01;35:*.emf=01;35:*.axv=01;35:*.anx=01;35:*.ogv=01;35:*.ogx=01;35:*.aac=00;36:*.au=00;36:*.flac=00;36:*.mid=00;36:*.midi=00;36:*.mka=00;36:*.mp3=00;36:*.mpc=00;36:*.ogg=00;36:*.ra=00;36:*.wav=00;36:*.axa=00;36:*.oga=00;36:*.spx=00;36:*.xspf=00;36:", 
            "MAIL": "/var/mail/root", 
            "OLDPWD": "/root/ansible/docsite", 
            "PATH": "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin", 
            "PWD": "/root/ansible", 
            "SHELL": "/bin/bash", 
            "SHLVL": "1", 
            "SUDO_COMMAND": "/bin/bash", 
            "SUDO_GID": "1000", 
            "SUDO_UID": "1000", 
            "SUDO_USER": "mdehaan", 
            "TERM": "xterm", 
            "USER": "root", 
            "USERNAME": "root", 
            "XAUTHORITY": "/home/mdehaan/.Xauthority", 
            "_": "/usr/local/bin/ansible"
        }, 
        "ansible_eth0": {
            "active": true, 
            "device": "eth0", 
            "ipv4": {
                "address": "REDACTED", 
                "netmask": "255.255.255.0", 
                "network": "REDACTED"
            }, 
            "ipv6": [
                {
                    "address": "REDACTED", 
                    "prefix": "64", 
                    "scope": "link"
                }
            ], 
            "macaddress": "REDACTED", 
            "module": "e1000", 
            "mtu": 1500, 
            "type": "ether"
        }, 
        "ansible_form_factor": "Other", 
        "ansible_fqdn": "ubuntu2", 
        "ansible_hostname": "ubuntu2", 
        "ansible_interfaces": [
            "lo", 
            "eth0"
        ], 
        "ansible_kernel": "3.5.0-23-generic", 
        "ansible_lo": {
            "active": true, 
            "device": "lo", 
            "ipv4": {
                "address": "127.0.0.1", 
                "netmask": "255.0.0.0", 
                "network": "127.0.0.0"
            }, 
            "ipv6": [
                {
                    "address": "::1", 
                    "prefix": "128", 
                    "scope": "host"
                }
            ], 
            "mtu": 16436, 
            "type": "loopback"
        }, 
        "ansible_lsb": {
            "codename": "precise", 
            "description": "Ubuntu 12.04.2 LTS", 
            "id": "Ubuntu", 
            "major_release": "12", 
            "release": "12.04"
        }, 
        "ansible_machine": "x86_64", 
        "ansible_memfree_mb": 74, 
        "ansible_memtotal_mb": 991, 
        "ansible_mounts": [
            {
                "device": "/dev/sda1", 
                "fstype": "ext4", 
                "mount": "/", 
                "options": "rw,errors=remount-ro", 
                "size_available": 15032406016, 
                "size_total": 20079898624
            }
        ], 
        "ansible_os_family": "Debian", 
        "ansible_pkg_mgr": "apt", 
        "ansible_processor": [
            "Intel(R) Core(TM) i7 CPU         860  @ 2.80GHz"
        ], 
        "ansible_processor_cores": 1, 
        "ansible_processor_count": 1, 
        "ansible_processor_threads_per_core": 1, 
        "ansible_processor_vcpus": 1, 
        "ansible_product_name": "VMware Virtual Platform", 
        "ansible_product_serial": "REDACTED", 
        "ansible_product_uuid": "REDACTED", 
        "ansible_product_version": "None", 
        "ansible_python_version": "2.7.3", 
        "ansible_selinux": false, 
        "ansible_ssh_host_key_dsa_public": "REDACTED KEY VALUE"
        "ansible_ssh_host_key_ecdsa_public": "REDACTED KEY VALUE"
        "ansible_ssh_host_key_rsa_public": "REDACTED KEY VALUE"
        "ansible_swapfree_mb": 665, 
        "ansible_swaptotal_mb": 1021, 
        "ansible_system": "Linux", 
        "ansible_system_vendor": "VMware, Inc.", 
        "ansible_user_id": "root", 
        "ansible_userspace_architecture": "x86_64", 
        "ansible_userspace_bits": "64", 
        "ansible_virtualization_role": "guest", 
        "ansible_virtualization_type": "VMware"

In the above the model of the first harddrive may be referenced in a template or playbook as::

    {{ ansible_devices.sda.model }}

Similarly, the hostname as the system reports it is::

    {{ ansible_hostname }}


Facts are frequently used in conditionals (see `playbook_conditionals`) and also in templates.

Facts can be also used to create dynamic groups of hosts that match particular criteria, see the :doc:`modules` documentation on 'group_by' for details, as well as in generalized conditional statements as discussed in the `playbook_conditionals` chapter.

.. _disabling_facts:

Turning Off Facts
`````````````````

If you know you don't need any fact data about your hosts, and know everything about your systems centrally, you
can turn off fact gathering.  This has advantages in scaling Ansible in push mode with very large numbers of
systems, mainly, or if you are using Ansible on experimental platforms.   In any play, just do this::

    - hosts: whatever
      gather_facts: no

.. _local_facts:

Local Facts (Facts.d)
`````````````````````

.. versionadded:: 1.3

As discussed in the playbooks chapter, Ansible facts are a way of getting data about remote systems for use in playbook variables.
Usually these are discovered automatically by the 'setup' module in Ansible. Users can also write custom facts modules, as described
in the API guide.  However, what if you want to have a simple way to provide system or user
provided data for use in Ansible variables, without writing a fact module?  

For instance, what if you want users to be able to control some aspect about how their systems are managed? "Facts.d" is one such mechanism.

.. note:: Perhaps "local facts" is a bit of a misnomer, it means "locally supplied user values" as opposed to "centrally supplied user values", or what facts are -- "locally dynamically determined values".

If a remotely managed system has an "/etc/ansible/facts.d" directory, any files in this directory
ending in ".fact", can be JSON, INI, or executable files returning JSON, and these can supply local facts in Ansible.

For instance assume a /etc/ansible/facts.d/preferences.fact::

    [general]
    asdf=1
    bar=2

This will produce a hash variable fact named "general" with 'asdf' and 'bar' as members.
To validate this, run the following::

    ansible <hostname> -m setup -a "filter=ansible_local"

And you will see the following fact added::

    "ansible_local": {
            "preferences": {
                "general": {
                    "asdf" : "1", 
                    "bar"  : "2"
                }
            }
     }

And this data can be accessed in a template/playbook as::

     {{ ansible_local.preferences.general.asdf }}

The local namespace prevents any user supplied fact from overriding system facts
or variables defined elsewhere in the playbook.

.. _registered_variables:

Registered Variables
````````````````````

Another major use of variables is running a command and using the result of that command to save the result into a variable.

The value of a task being executed in ansible can be saved in a variable and used later.  See some examples of this in the
`playbooks_conditionals` chapter.

While it's mentioned elsewhere in that document too, here's a quick syntax example::

   - hosts: web_servers

     tasks:

        - shell: /usr/bin/foo
          register: foo_result
          ignore_errors: True

        - shell: /usr/bin/bar
          when: foo_result.rc == 5

Registered variables are valid on the host the remainder of the playbook run, which is the same as the lifetime of "facts"
in Ansible.  Effectively registered variables are just like facts.

.. _accessing_complex_variable_data:

Accessing Complex Variable Data
```````````````````````````````

We already talked about facts a little higher up in the documentation.

Some provided facts, like networking information, are made available as nested data structures.  To access
them a simple {{ foo }} is not sufficient, but it is still easy to do.   Here's how we get an IP address::

    {{ ansible_eth0["ipv4"]["address"] }}

OR alternatively::

    {{ ansible_eth0.ipv4.address }}

Similarly, this is how we access the first element of an array::

    {{ foo[0] }}

.. _magic_variables_and_hostvars:

Magic Variables, and How To Access Information About Other Hosts
````````````````````````````````````````````````````````````````

Even if you didn't define them yourself, Ansible provides a few variables for you automatically.
The most important of these are 'hostvars', 'group_names', and 'groups'.  Users should not use
these names themselves as they are reserved.  'environment' is also reserved.

Hostvars lets you ask about the variables of another host, including facts that have been gathered
about that host.  If, at this point, you haven't talked to that host yet in any play in the playbook
or set of playbooks, you can get at the variables, but you will not be able to see the facts.

If your database server wants to use the value of a 'fact' from another node, or an inventory variable
assigned to another node, it's easy to do so within a template or even an action line::

    {{ hostvars['test.example.com']['ansible_distribution'] }}

Additionally, *group_names* is a list (array) of all the groups the current host is in.  This can be used in templates using Jinja2 syntax to make template source files that vary based on the group membership (or role) of the host::

   {% if 'webserver' in group_names %}
      # some part of a configuration file that only applies to webservers
   {% endif %}

*groups* is a list of all the groups (and hosts) in the inventory.  This can be used to enumerate all hosts within a group.
For example::

   {% for host in groups['app_servers'] %}
      # something that applies to all app servers.
   {% endfor %}

A frequently used idiom is walking a group to find all IP addresses in that group::

   {% for host in groups['app_servers'] %}
      {{ hostvars[host]['ansible_eth0']['ipv4']['address'] }}
   {% endfor %}

An example of this could include pointing a frontend proxy server to all of the app servers, setting up the correct firewall rules between servers, etc.

Just a few other 'magic' variables are available...  There aren't many.

Additionally, *inventory_hostname* is the name of the hostname as configured in Ansible's inventory host file.  This can
be useful for when you don't want to rely on the discovered hostname `ansible_hostname` or for other mysterious
reasons.  If you have a long FQDN, *inventory_hostname_short* also contains the part up to the first
period, without the rest of the domain.

Don't worry about any of this unless you think you need it.  You'll know when you do.

Also available, *inventory_dir* is the pathname of the directory holding Ansible's inventory host file, *inventory_file* is the pathname and the filename pointing to the Ansible's inventory host file.

.. _variable_file_seperation_details:

Variable File Separation
````````````````````````

It's a great idea to keep your playbooks under source control, but
you may wish to make the playbook source public while keeping certain
important variables private.  Similarly, sometimes you may just
want to keep certain information in different files, away from
the main playbook.

You can do this by using an external variables file, or files, just like this::

    ---
    - hosts: all
      remote_user: root
      vars:
        favcolor: blue
      vars_files:
        - /vars/external_vars.yml
      tasks:
      - name: this is just a placeholder
        command: /bin/echo foo

This removes the risk of sharing sensitive data with others when
sharing your playbook source with them.

The contents of each variables file is a simple YAML dictionary, like this::

    ---
    # in the above example, this would be vars/external_vars.yml
    somevar: somevalue
    password: magic

.. note::
   It's also possible to keep per-host and per-group variables in very
   similar files, this is covered in :doc:`intro_patterns`.

.. _passing_variables_on_the_command_line:

Passing Variables On The Command Line
`````````````````````````````````````

In addition to `vars_prompt` and `vars_files`, it is possible to send variables over
the Ansible command line.  This is particularly useful when writing a generic release playbook
where you may want to pass in the version of the application to deploy::

    ansible-playbook release.yml --extra-vars "version=1.23.45 other_variable=foo"

This is useful, for, among other things, setting the hosts group or the user for the playbook.

Example::

    ---
    - remote_user: '{{ user }}'
      hosts: '{{ hosts }}'
      tasks:
         - ...

    ansible-playbook release.yml --extra-vars "hosts=vipers user=starbuck"

As of Ansible 1.2, you can also pass in extra vars as quoted JSON, like so::

    --extra-vars '{"pacman":"mrs","ghosts":["inky","pinky","clyde","sue"]}'

The key=value form is obviously simpler, but it's there if you need it!

As of Ansible 1.3, extra vars can be loaded from a JSON file with the "@" syntax::

    --extra-vars "@some_file.json"

Also as of Ansible 1.3, extra vars can be formatted as YAML, either on the command line
or in a file as above.

.. _conditional_imports:

Conditional Imports
```````````````````

.. note: this behavior is infrequently used in Ansible.  You may wish to skip this section.  The 'group_by' module as described in the module documentation is a better way to achieve this behavior in most cases.

Sometimes you will want to do certain things differently in a playbook based on certain criteria.
Having one playbook that works on multiple platforms and OS versions is a good example.

As an example, the name of the Apache package may be different between CentOS and Debian,
but it is easily handled with a minimum of syntax in an Ansible Playbook::

    ---
    - hosts: all
      remote_user: root
      vars_files:
        - "vars/common.yml"
        - [ "vars/{{ ansible_os_family }}.yml", "vars/os_defaults.yml" ]
      tasks:
      - name: make sure apache is running
        service: name={{ apache }} state=running

.. note::
   The variable 'ansible_os_family' is being interpolated into
   the list of filenames being defined for vars_files.

As a reminder, the various YAML files contain just keys and values::

    ---
    # for vars/CentOS.yml
    apache: httpd
    somethingelse: 42

How does this work?  If the operating system was 'CentOS', the first file Ansible would try to import
would be 'vars/CentOS.yml', followed by '/vars/os_defaults.yml' if that file
did not exist.   If no files in the list were found, an error would be raised.
On Debian, it would instead first look towards 'vars/Debian.yml' instead of 'vars/CentOS.yml', before
falling back on 'vars/os_defaults.yml'. Pretty simple.

To use this conditional import feature, you'll need facter or ohai installed prior to running the playbook, but
you can of course push this out with Ansible if you like::

    # for facter
    ansible -m yum -a "pkg=facter ensure=installed"
    ansible -m yum -a "pkg=ruby-json ensure=installed"

    # for ohai
    ansible -m yum -a "pkg=ohai ensure=installed"

Ansible's approach to configuration -- separating variables from tasks, keeps your playbooks
from turning into arbitrary code with ugly nested ifs, conditionals, and so on - and results
in more streamlined & auditable configuration rules -- especially because there are a
minimum of decision points to track.

.. _variable_precedence:

Variable Precedence: Where Should I Put A Variable?
```````````````````````````````````````````````````

A lot of folks may ask about how variables override another.  Ultimately it's Ansible's philosophy that it's better
you know where to put a variable, and then you have to think about it a lot less.  

Avoid defining the variable "x" in 47 places and then ask the question "which x gets used".  
Why?  Because that's not Ansible's Zen philosophy of doing things.

There is only one Empire State Building. One Mona Lisa, etc.  Figure out where to define a variable, and don't make
it complicated.

However, let's go ahead and get precedence out of the way!  It exists.  It's a real thing, and you might have
a use for it.

If multiple variables of the same name are defined in different places, they win in a certain order, which is::

    * -e variables always win
    * then comes "most everything else"
    * then comes variables defined in inventory
    * then "role defaults", which are the most "defaulty" and lose in priority to everything.

That seems a little theoretical.  Let's show some examples and where you would choose to put what based on the kind of 
control you might want over values.

First off, group variables are super powerful.

Site wide defaults should be defined as a 'group_vars/all' setting.  Group variables are generally placed alongside
your inventory file.  They can also be returned by a dynamic inventory script (see `intro_dynamic_inventory`) or defined
in things like AnsibleWorks AWX from the UI or API::

    ---
    # file: /etc/ansible/group_vars/all
    # this is the site wide default
    ntp_server: default-time.example.com

Regional information might be defined in a 'group_vars/region' variable.  If this group is a child of the 'all' group (which it is, because all groups are), it will override the group that is higher up and more general::

    ---
    # file: /etc/ansible/group_vars/boston
    ntp_server: boston-time.example.com 

If for some crazy reason we wanted to tell just a specific host to use a specific NTP server, it would then override the group variable!::

    ---
    # file: /etc/ansible/host_vars/xyz.boston.example.com
    ntp-server: override.example.com

So that covers inventory and what you would normally set there.  It's a great place for things that deal with geography or behavior.  Since groups are frequently the entity that maps roles onto hosts, it is sometimes a shortcut to set variables on the group instead of defining them on a role.  You could go either way.

Remember:  Child groups override parent groups, and hosts always override their groups.

Next up: learning about role variable precedence.

We'll pretty much assume you are using roles at this point.  You should be using roles for sure.  Roles are great.  You are using
roles aren't you?  Hint hint.  

Ok, so if you are writing a redistributable role with reasonable defaults, put those in the 'roles/x/defaults/main.yml' file.  This means
the role will bring along a default value but ANYTHING in Ansible will override it.  It's just a default.  That's why it says "defaults" :)
See `intro_roles` for more info about this::

    ----
    # file: roles/x/defaults/main.yml
    # if not overriden in inventory or as a parameter, this is the value that will be used
    http_port: 80

if you are writing a role and want to ensure the value in the role is absolutely used in that role, and is not going to be overridden
by inventory, you should but it in roles/x/vars/main.yml like so, and inventory values cannot override it.  -e however, still will::

    ----
    # file: roles/x/vars/main.yml
    # this will absolutely be used in this role
    http_port: 80

So the above is a great way to plug in constants about the role that are always true.  If you are not sharing your role with others,
app specific behaviors like ports is fine to put in here.  But if you are sharing roles with others, putting variables in here might
be bad. Nobody will be able to override them with inventory, but they still can by passing a parameter to the role.

Parameterized roles are useful.

If you are using a role and want to override a default, pass it as a parameter to the role like so::

    roles:
       - { name: apache, http_port: 8080 }

This makes it clear to the playbook reader that you've made a conscious choice to override some default in the role, or pass in some
configuration that the role can't assume by itself.  It also allows you to pass something site-specific that isn't really part of the
role you are sharing with others.

This can often be used for things that might apply to some hosts multiple times,
like so::

    roles:
       - { role: app_user, name: Ian    }
       - { role: app_user, name: Terry  }
       - { role: app_user, name: Graham }
       - { role: app_user, name: John   }

That's a bit arbitrary, but you can see how the same role was invoked multiple Times.  In that example it's quite likely there was
no default for 'name' supplied at all.  Ansible can yell at you when variables aren't defined -- it's the default behavior in fact.

So that's a bit about roles.

There are a few bonus things that go on with roles.

Generally speaking, variables set in one role are available to others.  This means if you have a "roles/common/vars/main.yml" you
can set variables in there and make use of them in other roles and elsewhere in your playbook::

     roles:
        - { role: common_settings }
        - { role: something, foo: 12 }
        - { role: something_else }

.. note:: There are some protections in place to avoid the need to namespace variables.  
          In the above, variables defined in common_settings are most definitely available to 'app_user' and 'something_else' tasks, but if
          "something's" guaranteed to have foo set at 12, even if somewhere deep in common settings it set foo to 20.

So, that's precedence, explained in a more direct way.  Don't worry about precedence, just think about if your role is defining a
variable that is a default, or a "live" variable you definitely want to use.  Inventory lies in precedence right in the middle, and
if you want to forcibly override something, use -e.

If you found that a little hard to understand, take a look at the "ansible-examples" repo on our github for a bit more about
how all of these things can work together.

.. seealso::

   :doc:`playbooks`
       An introduction to playbooks
   :doc:`playbooks_conditionals`
       Conditional statements in playbooks
   :doc:`playbooks_loops`
       Looping in playbooks
   :doc:`playbooks_roles`
       Playbook organization by roles
   :doc:`playbooks_best_practices`
       Best practices in playbooks
   `User Mailing List <http://groups.google.com/group/ansible-devel>`_
       Have a question?  Stop by the google group!
   `irc.freenode.net <http://irc.freenode.net>`_
       #ansible IRC chat channel