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a361140680
* first review of loops page * round two * round three * puts 'not deprecating' message in a note * fixes note syntax * clarifies loop_control sections * loops require list input * incorporates bcoca and sivel feedback * fixes links * more bcoca feedback * adds examples to loop update guidelines * adds explicit code-blocks * remove stray copypasta
425 lines
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ReStructuredText
425 lines
14 KiB
ReStructuredText
.. _playbooks_loops:
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*****
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Loops
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*****
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Sometimes you want to repeat a task multiple times. In computer programming, this is called a loop. Common Ansible loops include changing ownership on several files and/or directories with the :ref:`file module <file_module>`, creating multiple users with the :ref:`user module <user_module>`, and
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repeating a polling step until a certain result is reached. Ansible offers two keywords for creating loops: ``loop`` and ``with_<lookup>``.
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.. note::
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* We added ``loop`` in Ansible 2.5. It is not yet a full replacement for ``with_<lookup>``, but we recommend it for most use cases.
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* We have not deprecated the use of ``with_<lookup>`` - that syntax will still be valid for the foreseeable future.
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* We are looking to improve ``loop`` syntax - watch this page and the `changelog <https://github.com/ansible/ansible/tree/devel/changelogs>`_ for updates.
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.. contents::
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:local:
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Comparing ``loop`` and ``with_*``
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=================================
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* The ``with_<lookup>`` keywords rely on :ref:`lookup_plugins` - even ``items`` is a lookup.
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* The ``loop`` keyword is equivalent to ``with_list``, and is the best choice for simple loops.
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* The ``loop`` keyword will not accept a string as input, see :ref:`query_vs_lookup`.
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* Generally speaking, any use of ``with_*`` covered in :ref:`migrating_to_loop` can be updated to use ``loop``.
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* Be careful when changing ``with_items`` to ``loop``, as ``with_items`` performed implicit single-level flattening. You may need to use ``flatten(1)`` with ``loop`` to match the exact outcome. For example, to get the same output as:
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.. code-block:: yaml
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with_items:
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- 1
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- [2,3]
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- 4
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you would need::
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loop: [1, [2,3] ,4] | flatten(1)
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* Any ``with_*`` statement that requires using ``lookup`` within a loop should not be converted to use the ``loop`` keyword. For example, instead of doing:
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.. code-block:: yaml
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loop: "{{ lookup('fileglob', '*.txt', wantlist=True) }}"
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it's cleaner to keep::
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with_fileglob: '*.txt'
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.. _standard_loops:
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Standard loops
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==============
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Iterating over a simple list
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----------------------------
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Repeated tasks can be written as standard loops over a simple list of strings. You can define the list directly in the task::
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- name: add several users
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user:
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name: "{{ item }}"
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state: present
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groups: "wheel"
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loop:
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- testuser1
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- testuser2
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You can define the list in a variables file, or in the 'vars' section of your play, then refer to the name of the list in the task::
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loop: "{{ somelist }}"
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Either of these examples would be the equivalent of::
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- name: add user testuser1
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user:
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name: "testuser1"
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state: present
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groups: "wheel"
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- name: add user testuser2
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user:
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name: "testuser2"
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state: present
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groups: "wheel"
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You can pass a list directly to a parameter for some plugins. Most of the packaging modules, like :ref:`yum_module` and :ref:`apt_module`, have this capability. When available, passing the list to a parameter is better than looping over the task. For example::
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- name: optimal yum
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yum:
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name: "{{ list_of_packages }}"
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state: present
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- name: non-optimal yum, slower and may cause issues with interdependencies
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yum:
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name: "{{ item }}"
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state: present
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loop: "{{ list_of_packages }}"
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Check the :ref:`module documentation <modules_by_category>` to see if you can pass a list to any particular module's parameter(s).
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Iterating over a list of hashes
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-------------------------------
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If you have a list of hashes, you can reference subkeys in a loop. For example::
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- name: add several users
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user:
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name: "{{ item.name }}"
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state: present
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groups: "{{ item.groups }}"
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loop:
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- { name: 'testuser1', groups: 'wheel' }
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- { name: 'testuser2', groups: 'root' }
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When combining :ref:`playbooks_conditionals` with a loop, the ``when:`` statement is processed separately for each item.
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See :ref:`the_when_statement` for examples.
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Iterating over a dictionary
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---------------------------
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To loop over a dict, use the ``dict2items`` :ref:`dict_filter`::
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- name: create a tag dictionary of non-empty tags
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set_fact:
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tags_dict: "{{ (tags_dict|default({}))|combine({item.key: item.value}) }}"
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loop: "{{ tags|dict2items }}"
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vars:
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tags:
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Environment: dev
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Application: payment
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Another: "{{ doesnotexist|default() }}"
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when: item.value != ""
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Here, we don't want to set empty tags, so we create a dictionary containing only non-empty tags.
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Registering variables with a loop
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=================================
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You can register the output of a loop as a variable. For example::
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- shell: "echo {{ item }}"
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loop:
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- "one"
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- "two"
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register: echo
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When you use ``register`` with a loop, the data structure placed in the variable will contain a ``results`` attribute that is a list of all responses from the module. This differs from the data structure returned when using ``register`` without a loop::
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{
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"changed": true,
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"msg": "All items completed",
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"results": [
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{
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"changed": true,
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"cmd": "echo \"one\" ",
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"delta": "0:00:00.003110",
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"end": "2013-12-19 12:00:05.187153",
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"invocation": {
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"module_args": "echo \"one\"",
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"module_name": "shell"
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},
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"item": "one",
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"rc": 0,
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"start": "2013-12-19 12:00:05.184043",
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"stderr": "",
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"stdout": "one"
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},
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{
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"changed": true,
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"cmd": "echo \"two\" ",
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"delta": "0:00:00.002920",
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"end": "2013-12-19 12:00:05.245502",
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"invocation": {
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"module_args": "echo \"two\"",
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"module_name": "shell"
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},
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"item": "two",
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"rc": 0,
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"start": "2013-12-19 12:00:05.242582",
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"stderr": "",
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"stdout": "two"
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}
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]
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}
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Subsequent loops over the registered variable to inspect the results may look like::
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- name: Fail if return code is not 0
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fail:
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msg: "The command ({{ item.cmd }}) did not have a 0 return code"
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when: item.rc != 0
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loop: "{{ echo.results }}"
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During iteration, the result of the current item will be placed in the variable::
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- shell: echo "{{ item }}"
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loop:
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- one
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- two
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register: echo
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changed_when: echo.stdout != "one"
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.. _complex_loops:
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Complex loops
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=============
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Iterating over nested lists
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---------------------------
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You can use Jinja2 expressions to iterate over complex lists. For example, a loop can combine nested lists::
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- name: give users access to multiple databases
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mysql_user:
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name: "{{ item[0] }}"
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priv: "{{ item[1] }}.*:ALL"
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append_privs: yes
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password: "foo"
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loop: "{{ ['alice', 'bob'] |product(['clientdb', 'employeedb', 'providerdb'])|list }}"
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.. _do_until_loops:
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Retrying a task until a condition is met
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----------------------------------------
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.. versionadded:: 1.4
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You can use the ``until`` keyword to retry a task until a certain condition is met. Here's an example::
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- shell: /usr/bin/foo
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register: result
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until: result.stdout.find("all systems go") != -1
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retries: 5
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delay: 10
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This task runs up to 5 times with a delay of 10 seconds between each attempt. If the result of any attempt has "all systems go" in its stdout, the task succeeds. The default value for "retries" is 3 and "delay" is 5.
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To see the results of individual retries, run the play with ``-vv``.
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When you run a task with ``until`` and register the result as a variable, the registered variable will include a key called "attempts", which records the number of the retries for the task.
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.. note:: You must set the ``until`` parameter if you want a task to retry. If ``until`` is not defined, the value for the ``retries`` parameter is forced to 1.
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Looping over inventory
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----------------------
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To loop over your inventory, or just a subset of it, you can use a regular ``loop`` with the ``ansible_play_batch`` or ``groups`` variables::
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# show all the hosts in the inventory
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- debug:
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msg: "{{ item }}"
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loop: "{{ groups['all'] }}"
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# show all the hosts in the current play
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- debug:
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msg: "{{ item }}"
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loop: "{{ ansible_play_batch }}"
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There is also a specific lookup plugin ``inventory_hostnames`` that can be used like this::
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# show all the hosts in the inventory
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- debug:
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msg: "{{ item }}"
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loop: "{{ query('inventory_hostnames', 'all') }}"
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# show all the hosts matching the pattern, ie all but the group www
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- debug:
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msg: "{{ item }}"
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loop: "{{ query('inventory_hostnames', 'all:!www') }}"
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More information on the patterns can be found on :ref:`intro_patterns`
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.. _query_vs_lookup:
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Ensuring list input for ``loop``: ``query`` vs. ``lookup``
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==========================================================
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The ``loop`` keyword requires a list as input, but the ``lookup`` keyword returns a string of comma-separated values by default. Ansible 2.5 introduced a new Jinja2 function named :ref:`query` that always returns a list, offering a simpler interface and more predictable output from lookup plugins when using the ``loop`` keyword.
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You can force ``lookup`` to return a list to ``loop`` by using ``wantlist=True``, or you can use ``query`` instead.
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These examples do the same thing::
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loop: "{{ query('inventory_hostnames', 'all') }}"
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loop: "{{ lookup('inventory_hostnames', 'all', wantlist=True) }}"
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.. _loop_control:
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Adding controls to loops
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========================
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.. versionadded:: 2.1
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The ``loop_control`` keyword lets you manage your loops in useful ways.
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Limiting loop output with ``label``
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-----------------------------------
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.. versionadded:: 2.2
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When looping over complex data structures, the console output of your task can be enormous. To limit the displayed output, use the ``label`` directive with ``loop_control``::
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- name: create servers
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digital_ocean:
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name: "{{ item.name }}"
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state: present
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loop:
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- name: server1
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disks: 3gb
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ram: 15Gb
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network:
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nic01: 100Gb
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nic02: 10Gb
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...
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loop_control:
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label: "{{ item.name }}"
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The output of this task will display just the ``name`` field for each ``item`` instead of the entire contents of the multi-line ``{{ item }}`` variable.
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Pausing within a loop
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---------------------
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.. versionadded:: 2.2
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To control the time (in seconds) between the execution of each item in a task loop, use the ``pause`` directive with ``loop_control``::
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# main.yml
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- name: create servers, pause 3s before creating next
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digital_ocean:
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name: "{{ item }}"
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state: present
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loop:
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- server1
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- server2
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loop_control:
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pause: 3
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Tracking progress through a loop with ``index_var``
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---------------------------------------------------
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.. versionadded:: 2.5
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To keep track of where you are in a loop, use the ``index_var`` directive with ``loop_control``. This directive specifies a variable name to contain the current loop index::
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- name: count our fruit
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debug:
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msg: "{{ item }} with index {{ my_idx }}"
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loop:
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- apple
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- banana
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- pear
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loop_control:
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index_var: my_idx
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Defining inner and outer variable names with ``loop_var``
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---------------------------------------------------------
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.. versionadded:: 2.1
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You can nest two looping tasks using ``include_tasks``. However, by default Ansible sets the loop variable ``item`` for each loop. This means the inner, nested loop will overwrite the value of ``item`` from the outer loop.
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You can specify the name of the variable for each loop using ``loop_var`` with ``loop_control``::
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# main.yml
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- include_tasks: inner.yml
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loop:
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- 1
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- 2
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- 3
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loop_control:
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loop_var: outer_item
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# inner.yml
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- debug:
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msg: "outer item={{ outer_item }} inner item={{ item }}"
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loop:
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- a
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- b
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- c
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.. note:: If Ansible detects that the current loop is using a variable which has already been defined, it will raise an error to fail the task.
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.. versionadded:: 2.8
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As of Ansible 2.8 you can get extended loop information using the ``extended`` option to loop control. This option will expose the following information.
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========================== ===========
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Variable Description
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-------------------------- -----------
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``ansible_loop.allitems`` The list of all items in the loop
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``ansible_loop.index`` The current iteration of the loop. (1 indexed)
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``ansible_loop.index0`` The current iteration of the loop. (0 indexed)
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``ansible_loop.revindex`` The number of iterations from the end of the loop (1 indexed)
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``ansible_loop.revindex0`` The number of iterations from the end of the loop (0 indexed)
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``ansible_loop.first`` ``True`` if first iteration
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``ansible_loop.last`` ``True`` if last iteration
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``ansible_loop.length`` The number of items in the loop
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``ansible_loop.previtem`` The item from the previous iteration of the loop. Undefined during the first iteration.
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``ansible_loop.nextitem`` The item from the following iteration of the loop. Undefined during the last iteration.
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========================== ===========
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::
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loop_control:
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extended: yes
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.. _migrating_to_loop:
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Migrating from with_X to loop
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=============================
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.. include:: shared_snippets/with2loop.txt
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.. seealso::
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:ref:`about_playbooks`
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An introduction to playbooks
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:ref:`playbooks_reuse_roles`
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Playbook organization by roles
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:ref:`playbooks_best_practices`
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Best practices in playbooks
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:ref:`playbooks_conditionals`
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Conditional statements in playbooks
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:ref:`playbooks_variables`
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All about variables
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`User Mailing List <https://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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