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Ansible variables let one playbook deploy different hosts, environments, and releases without changing task logic. Define stable values in inventory or playbooks, load environment data with vars_files, calculate values with set_fact, capture module output with register, and supply a one-time override with --extra-vars (or -e). The difficult part is not the syntax; it is knowing which definition wins when the same name appears in several places.
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This guide shows where variables come from, how to reference them safely, how to override them deliberately, and how to diagnose a value that is not the one you expected. Precedence details vary by Ansible version, so use the precedence page for the exact ansible-core release installed on your controller when an ordering decision matters.
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A variable is a named value that Ansible resolves while it parses a play, renders a template, evaluates a condition, or runs a task. Values may be strings, numbers, booleans, lists, dictionaries, or structured data returned by a module. A task can use the same variable name on every host while each host receives a different value from inventory.
Variable resolution is separate from task execution. Ansible collects values from several sources, merges the available definitions into the host context, and then applies precedence rules when names collide. The winning value is what a Jinja expression such as {{ app_port }} sees at that point in the run.
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Where variables can be defined
Choose the narrowest location that matches the value’s purpose. Shared inventory data belongs with the inventory; play-specific settings belong in the play; a value calculated during execution belongs in a fact or registered result.
| Location | Best use | Example path or syntax | Important behavior |
|---|---|---|---|
Play vars |
Constants used only by one play | vars: followed by key/value pairs |
Readable and close to the tasks. If a name is defined twice in the same vars section, the second definition wins. |
vars_files |
Environment or deployment data kept outside the play file | vars_files: with one or more YAML files |
The selected file is loaded for the play; keep separate files for development, staging, and production when their values differ. |
group_vars |
Defaults shared by all hosts in an inventory group | group_vars/web.yml or a directory matching the group name |
Useful for service ports, package names, and regional defaults. |
host_vars |
Exceptions for one inventory host | host_vars/web-01.yml |
Host-specific values rank above group values in the documented precedence model, so an exception can override a group default. |
set_fact |
Values computed during a run | ansible.builtin.set_fact: |
The value is attached to the host for later tasks. A stale fact can continue to hide a corrected inventory value. |
register |
Output from a task or module | register: package_result |
The registered variable contains a result structure, such as rc, stdout, and changed, rather than just the module’s primary value. |
| Prompted variables | A value entered when a play starts | vars_prompt: |
Useful for an operator-supplied value, but unsuitable for unattended automation unless a non-interactive strategy is provided. |
| Extra variables | A deliberate run-time override | ansible-playbook site.yml -e 'release=2026.10' |
Extra vars sit at the top of the precedence stack and are the normal way to force a specific value for one invocation. |
Define variables in a play
Put a play-scoped value under vars, then reference it in task arguments, templates, or conditions. YAML types matter: an unquoted number is numeric, while a quoted value is a string.
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hosts: web
become: true
vars:
app_name: storefront
app_port: 8080
app_enabled: true
app_packages:
- nginx
- python3
tasks:
- name: Install application packages
ansible.builtin.package:
name: "{{ app_packages }}"
state: present
- name: Write the service port
ansible.builtin.template:
src: app.conf.j2
dest: /etc/storefront/app.conf
mode: "0644"
In app.conf.j2, the value can be rendered as port={{ app_port }}. Keep the variable name descriptive and avoid reusing a generic name such as port across unrelated roles.
Use group_vars and host_vars for inventory data
Shared group defaults
Suppose the inventory contains a web group. Create group_vars/web.yml:
app_port: 8080
app_environment: production
log_retention_days: 14
Every host in the web group receives these values. This keeps host lists focused on identity and connection details instead of repeating configuration.
Per-host exceptions
For a single machine that needs a different port, create a file named for the inventory host, such as host_vars/web-01.yml:
app_port: 8181
When web-01 belongs to web, its host variable overrides the group’s app_port. Other web hosts continue to use 8080.
Conflicting groups
A host can belong to multiple groups. If those groups define the same variable, the last group loaded wins according to the latest official “Using variables” guidance. Do not rely on an accidental load order for an important setting: give the value one authoritative home or use a host-specific exception.
Load separate files with vars_files
vars_files is useful when the playbook should stay identical while a file supplies environment-specific values.
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- name: Deploy the selected environment
hosts: web
vars_files:
- "vars/{{ deploy_environment }}.yml"
tasks:
- name: Show the selected release
ansible.builtin.debug:
msg: "Deploying {{ release_name }} to {{ deploy_environment }}"
The file selected by deploy_environment must exist before Ansible can load it. Keep secrets out of ordinary repository files; use an approved secret-management process and protect any variable file that contains credentials.
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Reference variables in tasks and templates
Basic interpolation
Use Jinja expressions inside a quoted YAML value:
- name: Create the application directory
ansible.builtin.file:
path: "{{ app_root }}/{{ app_name }}"
state: directory
mode: "0755"
Quoting the complete YAML value avoids parsing surprises when the expression produces a string beginning with a special character.
Nested dictionaries and lists
Dictionary members can be accessed with dot notation when the key is a valid identifier, or with bracket notation for keys that contain punctuation:
database:
host: db.internal
port: 5432
- name: Configure the database endpoint
ansible.builtin.debug:
msg: "{{ database.host }}:{{ database['port'] }}"
List items are zero-indexed. For example, {{ app_packages[0] }} selects the first package. In most tasks, passing the complete list is clearer than indexing it.
Defaults for optional values
Use the default filter when a variable is optional:
worker_count: "{{ configured_workers | default(2) }}"
If an empty string should also trigger the fallback, use the filter’s boolean form, default(2, true). Apply defaults intentionally: silently replacing a required value can hide a configuration error.
Conditions and loops
Conditions use the expression without wrapping it in {{ }}:
- name: Install the monitoring agent
ansible.builtin.package:
name: monitoring-agent
state: present
when: monitoring_enabled | bool
- name: Create application directories
ansible.builtin.file:
path: "{{ item }}"
state: directory
loop: "{{ app_directories }}"
Convert user-supplied strings explicitly with filters such as bool when a condition must behave consistently.
Compute values with set_fact
Use set_fact when the value depends on information discovered during the run or on a calculation that would be awkward to repeat.
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- name: Choose a configuration directory
ansible.builtin.set_fact:
app_config_dir: "/etc/{{ app_name }}"
- name: Record the active release path
ansible.builtin.set_fact:
release_path: "{{ app_root }}/releases/{{ release_name }}"
Later tasks on that host can use app_config_dir and release_path. Because a set fact participates high in variable precedence, it can override a value that was corrected later in group_vars. If a play appears to ignore an inventory edit, search for an earlier set_fact, a cached fact, or a fact-setting role. Rename the computed value or remove the stale assignment rather than repeatedly changing the lower-precedence file.
Fact caching can make a value survive beyond one playbook process, depending on the configured cache plugin and options. Treat cached facts as run state and clear or refresh them when changing the logic that creates them.
Capture module output with register
register stores the complete result of a task. Inspect the fields you need instead of treating the variable as a scalar.
- name: Check whether the marker file exists
ansible.builtin.stat:
path: /etc/storefront/installed
register: marker
- name: Install the application once
ansible.builtin.command: /usr/local/bin/install-storefront
when: not marker.stat.exists
register: install_result
- name: Report the installer output
ansible.builtin.debug:
msg: "{{ install_result.stdout | default('installer was not run') }}"
Registered results commonly expose status fields such as changed, failed, and rc, plus module-specific data. A result from a skipped task may contain a skipped flag and not contain the fields produced by a task that actually ran, so guard optional fields with default or test the status before reading them.
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When the same name is defined in multiple places, Ansible loads all definitions and chooses one according to precedence. The broad pattern is that low-priority defaults are easy to override, while values supplied specifically for a run are harder to override.
| Practical layer | Typical sources | How to use it |
|---|---|---|
| Low-priority defaults | Role defaults and other baseline settings | Provide safe behavior that callers can replace. |
| Inventory data | group_vars, host_vars, inventory variables, and facts |
Describe what a host or group normally needs; host values outrank group values. |
| Play and loaded-file values | vars, vars_files, and prompted values |
Keep settings specific to one play or deployment selection. |
| Task-time values | Task or block variables, include_vars, set_fact, and registered results |
Use for local overrides, computed state, and module output. Exact ordering among these sources is version-dependent. |
| Role and include parameters | Arguments passed to roles or includes | Make a reusable component explicit at its call site. |
| Highest priority | Extra vars supplied with -e or --extra-vars |
Force a value for a particular invocation, especially in controlled automation. |
The official ansible-core 2.15 documentation presents a 13-step order. The Ansible 7 documentation presents a longer 22-step order that separately lists items such as vars_files, role vars, block and task vars, include_vars, set facts, registered vars, and role or include parameters. Both place extra vars at the top, but the exact placement of set_fact, register, and include parameters differs between versions and third-party summaries. Check the precedence page matching your installed version before depending on a precise rank.
Override a value deliberately at the command line
Extra vars are the clearest way to change one run without editing inventory:
ansible-playbook site.yml -e "deploy_environment=staging release_name=2026.10.14"
For structured data, pass JSON:
ansible-playbook site.yml -e '{"database":{"host":"staging-db.internal","port":5432}}'
Use shell quoting appropriate to your operating system. Do not put passwords or tokens directly in shell history. Prefer a protected secret mechanism, an encrypted variable file, or an automation system’s secret store. Because extra vars override lower layers, a typo in a command-line name can create a new variable while the intended variable remains unchanged; validate the final value before making destructive changes.
A repeatable workflow for building variable-driven playbooks
- Define the contract. List required inputs, optional inputs, types, safe defaults, and which values are allowed to vary by host or environment.
- Choose one authoritative source. Put group-wide values in
group_vars, host exceptions inhost_vars, play-only constants invars, and environment files invars_files. - Give names a scope-specific prefix. A role prefix such as
storefront_app_portreduces collisions with inventory and other roles. - Set values that are discovered, not configured. Use
registerfor module output andset_factfor derived values; do not use either as a hidden replacement for documented configuration. - Expose intentional overrides. Document the variables that operators may pass with
-e, including accepted values and whether the change is safe in production. - Validate early. Add an assertion before changing the host:
- name: Validate the application port
ansible.builtin.assert:
that:
- app_port | int >= 1
- app_port | int <= 65535
fail_msg: "app_port must be between 1 and 65535"
- Render or print a safe summary. Use
debugfor non-secret values and avoid dumping complete variable dictionaries that may contain credentials. - Run with an explicit inventory and limit. During development, target one host or a small group before applying a new precedence arrangement broadly.
Debug a variable that has the wrong value
Confirm the inventory value
Ask Ansible what inventory data it associates with a host:
ansible-inventory -i inventory.yml --host web-01
This helps distinguish an inventory parsing problem from a playbook override. Verify that the host name in host_vars exactly matches the inventory name and that the group name in group_vars matches the actual inventory group.
Print the value at the point of use
- name: Inspect the selected configuration
ansible.builtin.debug:
msg:
app_port: "{{ app_port | default('UNDEFINED') }}"
environment: "{{ app_environment | default('UNDEFINED') }}"
For host-specific investigation, hostvars[inventory_hostname] exposes the current host's variable context. Print only the keys you need; unrestricted output can disclose secrets and produce unreadable logs.
Trace every assignment
- Search the repository for the exact variable name, including role defaults, role vars, task files,
group_vars,host_vars, andvars_files. - Check whether an earlier task sets the name with
set_factor creates a registered variable with the same name. - Check the command invocation for
-eor--extra-vars. - Check whether multiple inventory groups assign the name and whether load order is being relied upon.
- Confirm the value's type. The string
"false"is not the same as the booleanfalsewhen a condition is evaluated without an explicit conversion.
Common mistakes and their fixes
Using an undefined variable
An expression fails when no source defines the name and no default is provided. Define a required value in the appropriate inventory or play scope, or use default only when omission is genuinely valid. An early assert produces a more useful message than a later template error.
Putting a host exception in group_vars
A value intended for one machine affects every member of the group. Move it to the matching host_vars file, or create a narrower inventory group with an explicit purpose.
Expecting register to contain a simple value
A registered name is a result object. Read the relevant member, such as result.stdout, result.rc, or a module-specific dictionary, and account for skipped tasks.
Changing group_vars but seeing no change
Look for a higher-precedence set fact, cached fact, task variable, role parameter, or extra var. Remove the stale assignment or change the variable name used for the derived value.
Embedding secrets in ordinary variables
Variables are not automatically encrypted. Keep credentials in a protected secret workflow, restrict file permissions, and prevent debug output from printing them.
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Version-aware rules for maintainers
Precedence is part of Ansible's behavior, but the published lists have changed as variable sources were documented and separated. The ansible-core 2.15 page describes 13 levels; Ansible 7 documentation describes 22. Do not copy a numeric list from a blog written for another release. Pin the Ansible version used by your automation, read that release's official precedence page, and test any change that depends on the relative order of set_fact, registered values, includes, role parameters, or prompted values.
The stable design rule is version-independent: keep one source authoritative, reserve high-precedence mechanisms for intentional overrides, validate inputs, and make the final value observable without exposing secrets. With that discipline, variables let the same playbook adapt cleanly to hosts and environments instead of turning precedence surprises into production outages.
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