Ansible: variables and templates

Ansible can define variables in playbooks and various places. These variables can be used in tasks and templates using Jinja2 templating to define dynamic expressions. Syntax is {{ variable_name }}.

See Ansible docs on variables.

Use variable in playbook

Add variables to playbook.yml:

apache_data_owner: www-data
apache_data_group: www-data

And update tasks so that index.html owner and group are defined using variables and not hardcoded. Run the updated playbook.

Use variables with templates

Templates are a way to define file content using variables. template module is like the copy module, but the content of our copied file can contain variables which will be replaced by Ansible.

Replace the copy module by the template module to copy index.html:

• Add a Playbook variable apache_hello_message: Hello from Playbook!
• Update index.html to use apache_hello_message variable

Run the updated playbook using your newly defined template.

Note: by default, template and copy modules will look for src files under:

• playbook root directory
• templates/ directory

See Search paths in Ansible for details about how Ansible looks up files and templates.

Our index.html is present in playbook root directory, but we could also have created a template directory and put our template file here. See Search paths in Ansible for details about how Ansible looks up files and templates.

Variable precedence

Variables can be defined in multiple places with Ansible with an order of precedence

• Create a file in your inventory named inventories/dev/group_vars/all/main.yml
• Add the variable apache_hello_message: Hello from inventory!

inventories/dev/group_vars/all/main.yml will set variables for all targets defined in inventories/dev/hosts. It is then possible to define a set of nodes for different environments: dev, prod, etc. each using a different inventory. This is the main purpose of Ansible inventories.

Run the playbook and check the content of index.html on your managed node. Adapt your playbook if needed to ensure index_content from inventory is used.

Using module return value

Most Ansible modules return values after execution with details about execution - like calling a function with a return value!

It is possible to store a module’s result in a variable with Ansible, this mechanism is called registering variables.

Ansible copy and template modules return values about the managed file such as whether a change was performed, file owner/group, checksum, etc.

Exercise: register the output of the task used to copy index.html and find a way to output the checksum of the file after module execution.

• the debug module can be used to output variables

Setting variable during playbook run

Ansible can define variables during a playbook run with set_fact tasks. Use set_fact to declare a variable index_checksum_b64 with value as the checksum returned by copy or template task encoded in base64.

This is mostly used when some module return values are used as input for other modules or to transform data, such as retrieving a server IP created/updated with ec2_instance module to set sub-sequent firewall rules or passing formatted data from a plain variable.