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diff --git a/docs/docsite/rst/reference_appendices/general_precedence.rst b/docs/docsite/rst/reference_appendices/general_precedence.rst new file mode 100644 index 00000000..90494b69 --- /dev/null +++ b/docs/docsite/rst/reference_appendices/general_precedence.rst @@ -0,0 +1,140 @@ +.. _general_precedence_rules: + +Controlling how Ansible behaves: precedence rules +================================================= + +To give you maximum flexibility in managing your environments, Ansible offers many ways to control how Ansible behaves: how it connects to managed nodes, how it works once it has connected. +If you use Ansible to manage a large number of servers, network devices, and cloud resources, you may define Ansible behavior in several different places and pass that information to Ansible in several different ways. +This flexibility is convenient, but it can backfire if you do not understand the precedence rules. + +These precedence rules apply to any setting that can be defined in multiple ways (by configuration settings, command-line options, playbook keywords, variables). + +.. contents:: + :local: + +Precedence categories +--------------------- + +Ansible offers four sources for controlling its behavior. In order of precedence from lowest (most easily overridden) to highest (overrides all others), the categories are: + + * Configuration settings + * Command-line options + * Playbook keywords + * Variables + +Each category overrides any information from all lower-precedence categories. For example, a playbook keyword will override any configuration setting. + +Within each precedence category, specific rules apply. However, generally speaking, 'last defined' wins and overrides any previous definitions. + +Configuration settings +^^^^^^^^^^^^^^^^^^^^^^ + +:ref:`Configuration settings<ansible_configuration_settings>` include both values from the ``ansible.cfg`` file and environment variables. Within this category, values set in configuration files have lower precedence. Ansible uses the first ``ansible.cfg`` file it finds, ignoring all others. Ansible searches for ``ansible.cfg`` in these locations in order: + + * ``ANSIBLE_CONFIG`` (environment variable if set) + * ``ansible.cfg`` (in the current directory) + * ``~/.ansible.cfg`` (in the home directory) + * ``/etc/ansible/ansible.cfg`` + +Environment variables have a higher precedence than entries in ``ansible.cfg``. If you have environment variables set on your control node, they override the settings in whichever ``ansible.cfg`` file Ansible loads. The value of any given environment variable follows normal shell precedence: the last value defined overwrites previous values. + +Command-line options +^^^^^^^^^^^^^^^^^^^^ + +Any command-line option will override any configuration setting. + +When you type something directly at the command line, you may feel that your hand-crafted values should override all others, but Ansible does not work that way. Command-line options have low precedence - they override configuration only. They do not override playbook keywords, variables from inventory or variables from playbooks. + +You can override all other settings from all other sources in all other precedence categories at the command line by :ref:`general_precedence_extra_vars`, but that is not a command-line option, it is a way of passing a :ref:`variable<general_precedence_variables>`. + +At the command line, if you pass multiple values for a parameter that accepts only a single value, the last defined value wins. For example, this :ref:`ad-hoc task<intro_adhoc>` will connect as ``carol``, not as ``mike``:: + + ansible -u mike -m ping myhost -u carol + +Some parameters allow multiple values. In this case, Ansible will append all values from the hosts listed in inventory files inventory1 and inventory2:: + + ansible -i /path/inventory1 -i /path/inventory2 -m ping all + +The help for each :ref:`command-line tool<command_line_tools>` lists available options for that tool. + +Playbook keywords +^^^^^^^^^^^^^^^^^ + +Any :ref:`playbook keyword<playbook_keywords>` will override any command-line option and any configuration setting. + +Within playbook keywords, precedence flows with the playbook itself; the more specific wins against the more general: + +- play (most general) +- blocks/includes/imports/roles (optional and can contain tasks and each other) +- tasks (most specific) + +A simple example:: + + - hosts: all + connection: ssh + tasks: + - name: This task uses ssh. + ping: + + - name: This task uses paramiko. + connection: paramiko + ping: + +In this example, the ``connection`` keyword is set to ``ssh`` at the play level. The first task inherits that value, and connects using ``ssh``. The second task inherits that value, overrides it, and connects using ``paramiko``. +The same logic applies to blocks and roles as well. All tasks, blocks, and roles within a play inherit play-level keywords; any task, block, or role can override any keyword by defining a different value for that keyword within the task, block, or role. + +Remember that these are KEYWORDS, not variables. Both playbooks and variable files are defined in YAML but they have different significance. +Playbooks are the command or 'state description' structure for Ansible, variables are data we use to help make playbooks more dynamic. + +.. _general_precedence_variables: + +Variables +^^^^^^^^^ + +Any variable will override any playbook keyword, any command-line option, and any configuration setting. + +Variables that have equivalent playbook keywords, command-line options, and configuration settings are known as :ref:`connection_variables`. Originally designed for connection parameters, this category has expanded to include other core variables like the temporary directory and the python interpreter. + +Connection variables, like all variables, can be set in multiple ways and places. You can define variables for hosts and groups in :ref:`inventory<intro_inventory>`. You can define variables for tasks and plays in ``vars:`` blocks in :ref:`playbooks<about_playbooks>`. However, they are still variables - they are data, not keywords or configuration settings. Variables that override playbook keywords, command-line options, and configuration settings follow the same rules of :ref:`variable precedence <ansible_variable_precedence>` as any other variables. + +When set in a playbook, variables follow the same inheritance rules as playbook keywords. You can set a value for the play, then override it in a task, block, or role:: + + - hosts: cloud + gather_facts: false + become: yes + vars: + ansible_become_user: admin + tasks: + - name: This task uses admin as the become user. + dnf: + name: some-service + state: latest + - block: + - name: This task uses service-admin as the become user. + # a task to configure the new service + - name: This task also uses service-admin as the become user, defined in the block. + # second task to configure the service + vars: + ansible_become_user: service-admin + - name: This task (outside of the block) uses admin as the become user again. + service: + name: some-service + state: restarted + +Variable scope: how long is a value available? +"""""""""""""""""""""""""""""""""""""""""""""" + +Variable values set in a playbook exist only within the playbook object that defines them. These 'playbook object scope' variables are not available to subsequent objects, including other plays. + +Variable values associated directly with a host or group, including variables defined in inventory, by vars plugins, or using modules like :ref:`set_fact<set_fact_module>` and :ref:`include_vars<include_vars_module>`, are available to all plays. These 'host scope' variables are also available via the ``hostvars[]`` dictionary. + +.. _general_precedence_extra_vars: + +Using ``-e`` extra variables at the command line +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +To override all other settings in all other categories, you can use extra variables: ``--extra-vars`` or ``-e`` at the command line. Values passed with ``-e`` are variables, not command-line options, and they will override configuration settings, command-line options, and playbook keywords as well as variables set elsewhere. For example, this task will connect as ``brian`` not as ``carol``:: + + ansible -u carol -e 'ansible_user=brian' -a whoami all + +You must specify both the variable name and the value with ``--extra-vars``. |