Evaluate dynamic variable name in ansible

Evaluate dynamic variable name in ansible

In Ansible, you can evaluate dynamic variable names using the vars lookup plugin or the hostvars dictionary. This is particularly useful when you need to construct variable names dynamically and access their values. Here are a few methods to achieve this:

1. Using vars Lookup Plugin

The vars lookup plugin allows you to access the value of a variable whose name is stored in another variable.

Example:

Suppose you have a variable name stored in another variable and want to access its value.

--- - name: Example playbook hosts: localhost vars: var_name: "dynamic_var" dynamic_var: "This is a dynamically named variable" tasks: - name: Evaluate dynamic variable name debug: msg: "{{ lookup('vars', var_name) }}"

2. Using hostvars Dictionary

The hostvars dictionary allows you to access variables of a specific host, including dynamically constructed variable names.

Example:

Suppose you have host-specific variables and want to dynamically construct and access them.

--- - name: Example playbook hosts: localhost vars: var_prefix: "my_var" my_var_localhost: "Value for localhost" tasks: - name: Evaluate dynamic variable name using hostvars debug: msg: "{{ hostvars[inventory_hostname][var_prefix + '_' + inventory_hostname] }}"

3. Using Jinja2 Templating

You can use Jinja2 templating to dynamically construct variable names within your tasks.

Example:

--- - name: Example playbook hosts: localhost vars: var_name: "dynamic_var" dynamic_var: "This is a dynamically named variable" tasks: - name: Evaluate dynamic variable name using Jinja2 templating debug: msg: "{{ vars[var_name] }}"

4. Combining with set_fact

Sometimes, you might need to set a fact based on dynamic variable names.

Example:

--- - name: Example playbook hosts: localhost vars: var_prefix: "dynamic" dynamic_var1: "Value 1" dynamic_var2: "Value 2" tasks: - name: Set a fact with dynamic variable name set_fact: dynamic_variable: "{{ lookup('vars', var_prefix + '_var1') }}" - name: Display the dynamically set fact debug: msg: "{{ dynamic_variable }}"

Summary

These methods show how to handle dynamic variable names in Ansible. Using the vars lookup plugin, hostvars dictionary, and Jinja2 templating, you can construct and evaluate dynamic variable names as needed. This flexibility can be particularly useful when dealing with complex configurations or dynamically generated data.

Examples

  1. How to access a dynamic variable name using a variable in Ansible?

    • Description: This query shows how to use a variable to dynamically access another variable's value in Ansible.
    • Code:
      --- - hosts: localhost vars: var_name: "dynamic_var" dynamic_var: "Hello, World!" tasks: - name: Display dynamic variable value debug: msg: "{{ vars[var_name] }}"
      • vars[var_name] accesses the value of the variable named by var_name.

  2. How to use set_fact to create dynamic variables in Ansible?

    • Description: This query explains how to use set_fact to define variables dynamically in Ansible.
    • Code:
      --- - hosts: localhost tasks: - name: Set a dynamic variable name set_fact: "{{ 'dynamic_' + item }}": "{{ item }}" with_items: - var1 - var2 - name: Display dynamic variables debug: msg: "Dynamic var1 value: {{ dynamic_var1 }}, Dynamic var2 value: {{ dynamic_var2 }}"
      • Uses set_fact with dynamic variable names generated by concatenating strings.

  3. How to loop over a set of dynamic variables in Ansible?

    • Description: This query demonstrates how to loop through a list of dynamic variable names and their values.
    • Code:
      --- - hosts: localhost vars: var1: "Value 1" var2: "Value 2" var_list: - var1 - var2 tasks: - name: Display values of dynamic variables debug: msg: "{{ item }}: {{ vars[item] }}" loop: "{{ var_list }}"
      • Loops through var_list to access each dynamic variable��s value.

  4. How to use Jinja2 templating to evaluate dynamic variable names in Ansible?

    • Description: This query illustrates how to use Jinja2 templating to dynamically evaluate variable names.
    • Code:
      --- - hosts: localhost vars: var_name: "dynamic_var" dynamic_var: "Dynamic Value" tasks: - name: Display dynamic variable using Jinja2 debug: msg: "{{ lookup('vars', var_name) }}"
      • Utilizes Jinja2��s lookup function to dynamically evaluate variable names.

  5. How to access nested dynamic variables in Ansible?

    • Description: This query shows how to access nested dynamic variables.
    • Code:
      --- - hosts: localhost vars: base_var: "nested" nested: dynamic_nested_var: "Nested Value" tasks: - name: Display nested dynamic variable debug: msg: "{{ lookup('vars', base_var + '_nested_var') }}"
      • Accesses a nested dynamic variable using a combination of variables.

  6. How to dynamically create variable names based on conditions in Ansible?

    • Description: This query demonstrates creating variable names dynamically based on conditional logic.
    • Code:
      --- - hosts: localhost vars: condition: "foo" tasks: - name: Set a dynamic variable based on a condition set_fact: "dynamic_{{ condition }}": "Condition met" - name: Display the dynamic variable debug: msg: "{{ vars['dynamic_' + condition] }}"
      • Creates a variable dynamically based on the condition variable.

  7. How to combine multiple dynamic variables into a dictionary in Ansible?

    • Description: This query explains how to aggregate multiple dynamic variables into a dictionary.
    • Code:
      --- - hosts: localhost vars: var1: "Value 1" var2: "Value 2" dynamic_vars: var1: "{{ var1 }}" var2: "{{ var2 }}" tasks: - name: Display dynamic variables dictionary debug: msg: "{{ dynamic_vars }}"
      • Combines multiple dynamic variables into a dictionary for easier management.

  8. How to use dynamic variable names with Ansible roles?

    • Description: This query shows how to pass and access dynamic variable names within Ansible roles.
    • Code:
      # roles/example/tasks/main.yml - name: Access dynamic variable in role debug: msg: "{{ vars[role_var_name] }}" # playbook.yml - hosts: localhost vars: role_var_name: "dynamic_role_var" dynamic_role_var: "Role Variable Value" roles: - example
      • Uses a dynamic variable within a role by passing it through the playbook.

  9. How to evaluate dynamic variables in Ansible playbooks using loops?

    • Description: This query demonstrates evaluating and using dynamic variables within loops in Ansible.
    • Code:
      --- - hosts: localhost vars: items: - { name: "item1", value: "Value 1" } - { name: "item2", value: "Value 2" } tasks: - name: Set and display dynamic variables in loop set_fact: "{{ item.name }}": "{{ item.value }}" loop: "{{ items }}" - name: Display dynamic variables debug: msg: "{{ item.name }}: {{ vars[item.name] }}" loop: "{{ items }}"
      • Loops through a list of items to dynamically create and access variables.

  10. How to troubleshoot dynamic variable access issues in Ansible?

    • Description: This query provides tips on troubleshooting problems with accessing dynamic variables in Ansible.
    • Code:
      --- - hosts: localhost vars: var_name: "undefined_var" tasks: - name: Attempt to display an undefined dynamic variable debug: msg: "{{ vars[var_name] | default('Variable not found') }}"
      • Uses default to handle cases where the dynamic variable might not exist.

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