Ansible Ternary Operator in Jinja2 Templates

Introduction

Writing efficient conditional logic in Ansible often means leveraging Jinja2 templating. One particularly powerful feature is the ternary operator, which helps you choose between two values based on a condition. However, many users encounter the dreaded error:

template error while templating string: unexpected character

This article walks through the correct way to use ternary expressions in Ansible using Jinja2 filters and shows how to avoid common pitfalls.

The Problem: Using JavaScript-style Ternary in Ansible

If you’ve ever written something like this in your Ansible playbook or template:

{{ env == 'dev' ? 'localhost' : 'db.prod.internal' }}

You’ve likely been met with a syntax error. That’s because Jinja2 does not support this JavaScript-style ternary operator. Instead, Ansible uses a different syntax via the ternary filter.

The Correct Way: Use the ternary Filter

To properly use conditional logic in Ansible, wrap your condition in parentheses and apply the ternary() filter:

{{ (env == 'dev') | ternary('localhost', 'db.prod.internal') }}

This line says: if env is 'dev', return 'localhost'; otherwise, return 'db.prod.internal'.

Example Use Case in a Template

Suppose you’re building an .env.j2 file and want to set the database host depending on the environment:

DB_HOST={{ (env | default('prod')) == 'dev' | ternary('localhost', 'db.prod.internal') }}

This ensures the condition works even if env isn’t defined.

Common Mistakes and Fixes

Mistake	Why it’s wrong	Correct Syntax
env == 'dev' ? 'a' : 'b'	Not valid Jinja2	`(env == ‘dev’)
Missing parentheses	Jinja2 parses it incorrectly	Always use parentheses around the condition
Forgetting `	beforeternary`	ternary is a filter, not a function

Debugging Tips

• Use debug tasks to check expressions:

  - debug:
      msg: "{{ (my_var is defined) | ternary(my_var, 'default') }}"
  

• Pipe | type_debug to see what kind of data you’re working with.

Why Use Ternary?

The ternary operator in Jinja2 helps you:

• Avoid verbose if-else blocks in templates.
• Keep playbooks and configuration files cleaner.
• Make conditional assignments more readable.

Conclusion

When used correctly, the ternary operator can drastically improve the clarity and maintainability of your Ansible playbooks and templates. Avoid syntax errors by remembering: wrap the condition in parentheses and use the | ternary(true_value, false_value) filter. Stick to this structure, and you’ll avoid the classic unexpected character template error for good.

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