ansible_variable_precedence's Introduction

Ansible Variable Precedence

This playbook demonstrates the many places that it is possible to assign variables in Ansible and the precedence rules that apply.

Precedence Rules

In order from highest to lowest:

Register Variables
Ansible assigned fact vars
Role Dependency Parameters
Vars file vars
Command line extra var
Playbook vars
Playbook Role parameter
Role var
Inventory Host variable
Inventory Group variable
Role default variable

Usage

To run the playbook, use the predefined script called bin/run_playbook. This defaults to using the -v flag so that the stdout of executed tasks is visible. Look for the stdout of the "THE MAIN EVENT echo echo echo" task to see which variable is currently active with the highest precedence. To demonstrate the rules in action, find the variable defined in the stdout of your most recent run of the playbook and go remove or comment out that specific assignment of the echo_var. Once you're done, if you rerun the playbook, you should see the next variable in the precedence order as the stdout for the task name mentioned above.

Prerequisites

Ansible v1.3
Ability to SSH into localhost without a password
ansible-playbook script available in Bash shell environment

v1.2 and earlier

To test the precedence rules against 1.2, simply change the required role in the playbook from dummy to precedence since v1.2 does not support role dependencies.

The predecedence list using this setup against Ansible v1.2 in order from highest to lowest:

Register Variables
Ansible assigned fact vars
Playbook Role parameter
Vars file vars
Role var
Command line extra var
Playbook vars
Inventory Host variable
Inventory Group variable

Versions prior to v1.2 were not tested.

Official Documentation

http://www.ansibleworks.com/docs/playbooks_variables.html

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