Jinja2_Basics

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Table of Contents

• Jinja2 Basics
• Variables
• Loops
  • list
  • dict
    • Do something with a pair of key and value in a dict
    • Do somthing with keys in a dict
• Conditional
  • Simple if
  • if ... else ...
  • case ... when ...
• See if a key exists in a dict
• Merging two dict variables
• Creating filters and tests

Jinja2 Basics

Jinja2 is the template engine of ansible. Honestly, I am not a fun of it. The documentation is sometimes cryptic and does not clearly explain what you should know. Here is a list of what you should know about jinja2 to write ansible roles and plays.

Variables

In a template, you have access to all ansible variables. In rare cases, you need to create variables in a template. In those cases, remember that variables declared in a template have no scope, meaning the variables are always in global scope in the template. Use set to declare a variable in a template.

{% set foo = 'bar' %}
{% if something is defined %}
{% set foo = 'baz' %} # NOT local to the `if` scope
{% endif %}
{{ foo }}

The both of foo is same variable. If something is defined, the result is baz, which might not be what you expect.

Loops

list

{% set l = [ 'foo', 'bar' ] %}
{% for i in l %}
{{ i }}
{% endfor %}

dict

Do something with a pair of key and value in a dict

{% set d = { 'foo': 1, 'bar': 2 } %}
{% for k, v in d %}
{{ k }} = {{ v }}
{% endfor %}

But in almost all cases, you would need dictsort because a dict is not ordered, which breaks idempotentcy.

{% set d = { 'foo': 1, 'bar': 2 } %}
{% for k, v in d | dictsort %}
{{ k }} = {{ v }}
{% endfor %}

Do something with keys in a dict

If you are not interested in values, simply use for.

{% for key in d | dictsort %}
key == {{ key }}
{% endfor %}

Conditional

Simple if

{% if foo is defined %}
foo is defined
{% endif %}

if ... else ...

It's elif, not elseif or elsif..

{% if foo is defined %}
foo is defined
{% elif %}
foo is not defined
{% endif %}

case ... when ...

No, jinja2 does not have case. Use if ... elif ... elif ... endif .

See if a key exists in a dict

{% if 'the_key' in a_dict %}
... do something ...
{% endif %}

This is often used in tasks. The following example runs command when a dict, bar\_dict has foo in the keys.

- command: /path/to/command
  when: "{{ 'foo' in bar_dict }}"

Merging two dict variables

ansible 2.0 and later implements combine, which is useful to override keys when missing. See how it is used in a role, which overrides missing keys..

Creating filters and tests

See Extending_Ansible