Jinja Template Renderer

Render an Ansible .j2 template against your variables — and see which ones are missing.

Why this exists

Every Ansible `template:` task goes through Jinja, and the usual loop for getting one right is ansible-playbook → wrong output → edit → rerun, with the whole inventory in the way. Paste the template and the group_vars beside it and the render is immediate. The list of undefined variables matters more than the output: that is the play that fails at 3 a.m. because one host had no override.

🔒 Runs in your browser — nothing is sent anywhere

template (.j2)

variables (YAML or JSON)

rendered

# managed by Ansible
upstream portfolio {
    server 10.0.0.51:8080;
    server 10.0.0.52:9000;
    server 10.0.0.53:8080 backup;
}

server {
    server_name opswithzaeem.com;

    listen 443 ssl http2;
    ssl_certificate     /etc/ssl/opswithzaeem.com.pem;

    location / {
        proxy_pass http://portfolio;
        proxy_set_header Host $host;
    }
}

Syntax reference — tags, filters, tests

Inside a for loop

loop.index: 1, 2, 3 …
loop.index0: 0, 1, 2 …
loop.first: true on the first pass
loop.last: true on the last — the usual way to skip a trailing comma
loop.length: how many items

Filters

default(v): Use v when the variable is undefined. default(v, true) also replaces empty/false.
join(', '): A list into a string.
upper / lower / capitalize / title: Case.
trim: Strip surrounding whitespace.
length: Items in a list, keys in a mapping, characters in a string.
replace(a, b): Substring replacement.
int / float / string / bool: Convert. int takes a fallback: | int(0).
first / last / sort / unique / reverse / list: List handling.
indent(4): Indent every line after the first — for nesting a block inside YAML.
tojson: Serialise, for embedding in JSON or a shell argument.

Tests and operators

x is defined: The Ansible idiom. Guard anything that might be absent.
x is not defined / is undefined: The negative.
x is none: Defined, but null. Not the same as undefined.
a in b: Membership in a list, mapping keys, or a substring.
and / or / not, == != > < >= <=: The usual operators.

The mistakes that actually cost time

An undefined variable fails the play. Ansible stops on it rather than rendering an empty string, so the warnings above are the real output of this tool. Guard anything optional with | default(...) or is defined.
default does not fire on an empty string. {{ name | default('anon') }} still renders nothing when name is "" — it is defined. Pass default('anon', true) to replace falsy values too.
Block tags leave blank lines. {% for %} on its own line emits that line's newline. Use {%- ... -%} to trim — it is why generated configs come out double-spaced.
A number in quotes is a string. port: "8080" compared with {% if port > 1024 %} behaves differently from an unquoted 8080. Use | int when you mean a number.
Whitespace matters in YAML output. When a template writes YAML, an interpolated multi-line value breaks the indentation. | indent(4) is the usual repair.

What this renderer does not do

No {% macro %}, {% include %}, {% raw %}, inline if expressions, or Ansible-only filters such as regex_replace and to_nice_yaml — those come from Ansible, not Jinja. Anything unsupported is reported in the warnings rather than dropped silently, so the output never looks more finished than it is.

Learn the theory

📚 The Ansible notes