Harnessing the power of Jinja2 with Ansible

2 min read

May 2, 2019 12:00:00 AM

Configuration files management is the core component of every configuration management tool. Templates are one of the best methods for creating configuration files as they allow dynamic variables. Ansible has a template module that uses Jinja2 in its core. Jinja2 is written in Python and allows Python-like expressions which allows us to have Python features and flexibility to be embedded into our configuration file management strategy.

Looping and Testing

The following is a standard template usage to create multiple files using the same template. Using the loop method with_items, multiple files can be created based on the declared variable log_path. [code] vars: set_category: true log_path: - name: "auth" path: "/var/log/auth.log" category: auth - name: "syslog" path: "/var/log/syslog" category: syslog tasks: - name: create config files template: src=./templates/sources.j2 dest=/etc/sumo/sumo.d/{{item.name}}.json with_items: "" [/code] With this, variables like log_path.path can be called as item.path from the template. Tests using “ if" expressions can be used as seen in the following example. If only set_category variable is set to TRUE, the category will be updated. [code] { "api.version": "v1", "source": { "name": "{{item.name}}", "sourceType": "LocalFile", "pathExpression": "{{item.path}}", } } [/code] The above code would create the following files: /etc/sumo/sumo.d/auth.json [code] { "api.version": "v1", "source": { "name": "auth", "sourceType": "LocalFile", "pathExpression": "/var/log/auth.log", "category": "auth" } } [/code] /etc/sumo/sumo.d/syslog.json [code] { "api.version": "v1", "source": { "name": "syslog", "sourceType": "LocalFile", "pathExpression": "/var/log/syslog", "category": "syslog" } } [/code] Repeated configuration becomes an easy task and if it is required in a file - “ for loop” can be used from a template. [code] [/code] This would create an entry like below: [code] auth : /var/log/auth.log syslog : /var/log/syslog [/code]

Filters

Jinja2 provides a wide range of filters that can transform the data inside the template. For example, join filter would take a list and concatenate it to a string with a separator. The following is an example: [code] [/code] The above join filter would take the following list: [code] allhosts: - localhost - 10.0.0.1 - 10.0.0.2 [/code] and create the following: [code] localhost,10.0.0.1,10.0.0.2 [/code] json_query is a filter which would allow searching inside a data structure. The below query would search inside the log_paths variable for a name key with value auth and would print the value of associated path: [code] /var/log/auth.log [/code] Another advantage of filters is that multiple filters can be lined up using pipe. For example, adding a basename filter to the above query would get the last name of the above file path. [code] auth.log [/code] map filter is another filter, which is very useful. It can either look up an attribute from a list or apply another filter on the objects. For example, the following would look for all values with attribute category: [code] auth, syslog [/code] In the following example, another filter called regex_replace is called by map filter to attach https:// to all the ip addresses which starts with 10: [code] localhost, https://10.0.0.1, https://10.0.0.2 [/code] As we can see, Ansible Jinja2 templates use the power of Python, allowing complex data manipulation while managing diverse environments. Custom filters can also be created using Python and added to filter_plugins/ under the ansible top directory.

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Harnessing the power of Jinja2 with Ansible

Looping and Testing

Filters

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