ansible/docs/docsite/rst/user_guide/playbooks_loops.rst

.. _playbooks_loops:

*****
Loops
*****

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
repeating a polling step until a certain result is reached. Ansible offers two keywords for creating loops: ``loop`` and ``with_<lookup>``.

.. note::
   * 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.
   * We have not deprecated the use of ``with_<lookup>`` - that syntax will still be valid for the foreseeable future.
   * We are looking to improve ``loop`` syntax - watch this page and the `changelog <https://github.com/ansible/ansible/tree/devel/changelogs>`_ for updates.

.. contents::
   :local:

Comparing ``loop`` and ``with_*``
=================================

* The ``with_<lookup>`` keywords rely on :ref:`lookup_plugins` - even  ``items`` is a lookup.
* The ``loop`` keyword is equivalent to ``with_list``, and is the best choice for simple loops.
* The ``loop`` keyword will not accept a string as input, see :ref:`query_vs_lookup`.
* Generally speaking, any use of ``with_*`` covered in :ref:`migrating_to_loop` can be updated to use ``loop``.
* 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:

.. code-block:: yaml

  with_items:
    - 1
    - [2,3]
    - 4

you would need::

  loop: "{{ [1, [2,3] ,4] | flatten(1) }}"

* Any ``with_*`` statement that requires using ``lookup`` within a loop should not be converted to use the ``loop`` keyword. For example, instead of doing:

.. code-block:: yaml

  loop: "{{ lookup('fileglob', '*.txt', wantlist=True) }}"

it's cleaner to keep::

  with_fileglob: '*.txt'

.. _standard_loops:

Standard loops
==============

Iterating over a simple list
----------------------------

Repeated tasks can be written as standard loops over a simple list of strings. You can define the list directly in the task::

    - name: add several users
      user:
        name: "{{ item }}"
        state: present
        groups: "wheel"
      loop:
         - testuser1
         - testuser2

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::

    loop: "{{ somelist }}"

Either of these examples would be the equivalent of::

    - name: add user testuser1
      user:
        name: "testuser1"
        state: present
        groups: "wheel"

    - name: add user testuser2
      user:
        name: "testuser2"
        state: present
        groups: "wheel"

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::

   - name: optimal yum
     yum:
       name: "{{  list_of_packages  }}"
       state: present

   - name: non-optimal yum, slower and may cause issues with interdependencies
     yum:
       name: "{{  item  }}"
       state: present
     loop: "{{  list_of_packages  }}"

Check the :ref:`module documentation <modules_by_category>` to see if you can pass a list to any particular module's parameter(s).

Iterating over a list of hashes
-------------------------------

If you have a list of hashes, you can reference subkeys in a loop. For example::

    - name: add several users
      user:
        name: "{{ item.name }}"
        state: present
        groups: "{{ item.groups }}"
      loop:
        - { name: 'testuser1', groups: 'wheel' }
        - { name: 'testuser2', groups: 'root' }

When combining :ref:`playbooks_conditionals` with a loop, the ``when:`` statement is processed separately for each item.
See :ref:`the_when_statement` for examples.

Iterating over a dictionary
---------------------------

To loop over a dict, use the ``dict2items`` :ref:`dict_filter`::

    - name: create a tag dictionary of non-empty tags
      set_fact:
        tags_dict: "{{ (tags_dict|default({}))|combine({item.key: item.value}) }}"
      loop: "{{ tags|dict2items }}"
      vars:
        tags:
          Environment: dev
          Application: payment
          Another: "{{ doesnotexist|default() }}"
      when: item.value != ""

Here, we don't want to set empty tags, so we create a dictionary containing only non-empty tags.

Registering variables with a loop
=================================

You can register the output of a loop as a variable. For example::

   - shell: "echo {{ item }}"
     loop:
       - "one"
       - "two"
     register: echo

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::

    {
        "changed": true,
        "msg": "All items completed",
        "results": [
            {
                "changed": true,
                "cmd": "echo \"one\" ",
                "delta": "0:00:00.003110",
                "end": "2013-12-19 12:00:05.187153",
                "invocation": {
                    "module_args": "echo \"one\"",
                    "module_name": "shell"
                },
                "item": "one",
                "rc": 0,
                "start": "2013-12-19 12:00:05.184043",
                "stderr": "",
                "stdout": "one"
            },
            {
                "changed": true,
                "cmd": "echo \"two\" ",
                "delta": "0:00:00.002920",
                "end": "2013-12-19 12:00:05.245502",
                "invocation": {
                    "module_args": "echo \"two\"",
                    "module_name": "shell"
                },
                "item": "two",
                "rc": 0,
                "start": "2013-12-19 12:00:05.242582",
                "stderr": "",
                "stdout": "two"
            }
        ]
    }

Subsequent loops over the registered variable to inspect the results may look like::

    - name: Fail if return code is not 0
      fail:
        msg: "The command ({{ item.cmd }}) did not have a 0 return code"
      when: item.rc != 0
      loop: "{{ echo.results }}"

During iteration, the result of the current item will be placed in the variable::

    - shell: echo "{{ item }}"
      loop:
        - one
        - two
      register: echo
      changed_when: echo.stdout != "one"

.. _complex_loops:

Complex loops
=============

Iterating over nested lists
---------------------------

You can use Jinja2 expressions to iterate over complex lists. For example, a loop can combine nested lists::

    - name: give users access to multiple databases
      mysql_user:
        name: "{{ item[0] }}"
        priv: "{{ item[1] }}.*:ALL"
        append_privs: yes
        password: "foo"
      loop: "{{ ['alice', 'bob'] |product(['clientdb', 'employeedb', 'providerdb'])|list }}"


.. _do_until_loops:

Retrying a task until a condition is met
----------------------------------------

.. versionadded:: 1.4

You can use the ``until`` keyword to retry a task until a certain condition is met.  Here's an example::

    - shell: /usr/bin/foo
      register: result
      until: result.stdout.find("all systems go") != -1
      retries: 5
      delay: 10

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.

To see the results of individual retries, run the play with ``-vv``.

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.

.. 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.

Looping over inventory
----------------------

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::

    # show all the hosts in the inventory
    - debug:
        msg: "{{ item }}"
      loop: "{{ groups['all'] }}"

    # show all the hosts in the current play
    - debug:
        msg: "{{ item }}"
      loop: "{{ ansible_play_batch }}"

There is also a specific lookup plugin ``inventory_hostnames`` that can be used like this::

    # show all the hosts in the inventory
    - debug:
        msg: "{{ item }}"
      loop: "{{ query('inventory_hostnames', 'all') }}"

    # show all the hosts matching the pattern, ie all but the group www
    - debug:
        msg: "{{ item }}"
      loop: "{{ query('inventory_hostnames', 'all:!www') }}"

More information on the patterns can be found on :ref:`intro_patterns`

.. _query_vs_lookup:

Ensuring list input for ``loop``: ``query`` vs. ``lookup``
==========================================================

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.

You can force ``lookup`` to return a list to ``loop`` by using ``wantlist=True``, or you can use ``query`` instead.

These examples do the same thing::

    loop: "{{ query('inventory_hostnames', 'all') }}"

    loop: "{{ lookup('inventory_hostnames', 'all', wantlist=True) }}"


.. _loop_control:

Adding controls to loops
========================
.. versionadded:: 2.1

The ``loop_control`` keyword lets you manage your loops in useful ways.

Limiting loop output with ``label``
-----------------------------------
.. versionadded:: 2.2

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``::

    - name: create servers
      digital_ocean:
        name: "{{ item.name }}"
        state: present
      loop:
        - name: server1
          disks: 3gb
          ram: 15Gb
          network:
            nic01: 100Gb
            nic02: 10Gb
            ...
      loop_control:
        label: "{{ item.name }}"

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.

Pausing within a loop
---------------------
.. versionadded:: 2.2

To control the time (in seconds) between the execution of each item in a task loop, use the ``pause`` directive with ``loop_control``::

    # main.yml
    - name: create servers, pause 3s before creating next
      digital_ocean:
        name: "{{ item }}"
        state: present
      loop:
        - server1
        - server2
      loop_control:
        pause: 3

Tracking progress through a loop with ``index_var``
---------------------------------------------------
.. versionadded:: 2.5

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::

  - name: count our fruit
    debug:
      msg: "{{ item }} with index {{ my_idx }}"
    loop:
      - apple
      - banana
      - pear
    loop_control:
      index_var: my_idx

Defining inner and outer variable names with ``loop_var``
---------------------------------------------------------
.. versionadded:: 2.1

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.
You can specify the name of the variable for each loop using ``loop_var`` with ``loop_control``::

    # main.yml
    - include_tasks: inner.yml
      loop:
        - 1
        - 2
        - 3
      loop_control:
        loop_var: outer_item

    # inner.yml
    - debug:
        msg: "outer item={{ outer_item }} inner item={{ item }}"
      loop:
        - a
        - b
        - c

.. 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.

Extended loop variables
-----------------------
.. versionadded:: 2.8

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.

==========================  ===========
Variable                    Description
--------------------------  -----------
``ansible_loop.allitems``   The list of all items in the loop
``ansible_loop.index``      The current iteration of the loop. (1 indexed)
``ansible_loop.index0``     The current iteration of the loop. (0 indexed)
``ansible_loop.revindex``   The number of iterations from the end of the loop (1 indexed)
``ansible_loop.revindex0``  The number of iterations from the end of the loop (0 indexed)
``ansible_loop.first``      ``True`` if first iteration
``ansible_loop.last``       ``True`` if last iteration
``ansible_loop.length``     The number of items in the loop
``ansible_loop.previtem``   The item from the previous iteration of the loop. Undefined during the first iteration.
``ansible_loop.nextitem``   The item from the following iteration of the loop. Undefined during the last iteration.
==========================  ===========

::

      loop_control:
        extended: yes

Accessing the name of your loop_var
-----------------------------------
.. versionadded:: 2.8

As of Ansible 2.8 you can get the name of the value provided to ``loop_control.loop_var`` using the ``ansible_loop_var`` variable

For role authors, writing roles that allow loops, instead of dictating the required ``loop_var`` value, you can gather the value via::

    "{{ lookup('vars', ansible_loop_var) }}"

.. _migrating_to_loop:

Migrating from with_X to loop
=============================

.. include:: shared_snippets/with2loop.txt

.. seealso::

   :ref:`about_playbooks`
       An introduction to playbooks
   :ref:`playbooks_reuse_roles`
       Playbook organization by roles
   :ref:`playbooks_best_practices`
       Best practices in playbooks
   :ref:`playbooks_conditionals`
       Conditional statements in playbooks
   :ref:`playbooks_variables`
       All about variables
   `User Mailing List <https://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