Kestra Expressions Operators, Tags, and Tests

Use this page when you need the control-flow side of Pebble rather than data transformation helpers.

Operators

Comparisons

Supported comparison operators:

• ==
• !=
• <
• >
• <=
• >=

Logic and boolean checks

Use:

• and
• or
• not
• is
• contains

Use parentheses to group expressions and make precedence explicit:

{% if 2 is even and 3 is odd %}
  ...
{% endif %}

{% if (3 is not even) and (2 is odd or 3 is even) %}
  ...
{% endif %}

contains

Checks whether an item exists within a list, string, map, or array:

{% if ["apple", "pear", "banana"] contains "apple" %}
  ...
{% endif %}

For maps, contains checks for a matching key:

{% if {"apple": "red", "banana": "yellow"} contains "banana" %}
  ...
{% endif %}

To check for multiple items at once, pass a list on the right-hand side:

{% if ["apple", "pear", "banana", "peach"] contains ["apple", "peach"] %}
  ...
{% endif %}

contains also works inline in output expressions:

{{ inputs.mainString contains inputs.subString }}

Math and concatenation

Use:

• +, -, *, /, %
• ~ for string concatenation

Example:

{{ "apple" ~ "pear" ~ "banana" }}
{{ 2 + 2 / (10 % 3) * (8 - 1) }}

Fallbacks and conditionals

Use:

• ?? for null-coalescing: returns the first non-null value
• ??? for undefined-coalescing: returns the right-hand side only when the left is undefined (not just null)
• ? : for ternary expressions

Examples:

{{ foo ?? bar ?? "default" }}     {# first non-null value #}
{{ foo ??? "default" }}           {# only if foo is undefined #}
{{ foo == null ? bar : baz }}
{{ foo ?? bar ?? raise }}         {# raises an exception if all are undefined #}

For detailed null vs undefined behavior, see the Handling null and undefined values guide.

Operator precedence

Pebble operators are evaluated in this order:

1. .
2. |
3. %, /, *
4. -, +
5. ==, !=, >, <, >=, <=
6. is, is not
7. and
8. or

Tags

Pebble tags are enclosed in {% %} and control template flow.

set

Defines a variable in the template context:

{% set header = "Welcome Page" %}
{{ header }}
{# output: Welcome Page #}

if

Evaluates conditional logic. Use elseif and else for multiple branches:

{% if users is empty %}
  No users available.
{% elseif users.length == 1 %}
  One user found.
{% else %}
  Multiple users found.
{% endif %}

for

Iterates over arrays, maps, or any java.lang.Iterable.

Iterating over a list:

{% for user in users %}
  {{ user.name }} lives in {{ user.city }}.
{% else %}
  No users found.
{% endfor %}

The else block runs when the collection is empty.

Iterating over a map:

{% for entry in map %}
  {{ entry.key }}: {{ entry.value }}
{% endfor %}

Loop special variables:

Inside any for loop, Pebble provides a loop object with these properties:

Example:

{% for user in users %}
  {{ loop.index }}: {{ user.name }}{% if loop.last %} (last){% endif %}
{% endfor %}

filter

Applies a filter to a block of content. Filters can be chained:

{% filter upper %}
  hello
{% endfilter %}
{# output: HELLO #}

{% filter lower | title %}
  hello world
{% endfilter %}
{# output: Hello World #}

raw

Prevents Pebble from parsing its content — useful when you need to output literal {{ }} syntax:

{% raw %}{{ user.name }}{% endraw %}
{# output: {{ user.name }} #}

macro

Defines a reusable template snippet. Macros only have access to their own arguments by default:

{% macro input(type="text", name, value="") %}
  type: "{{ type }}", name: "{{ name }}", value: "{{ value }}"
{% endmacro %}

{{ input(name="country") }}
{# output: type: "text", name: "country", value: "" #}

To access variables from the outer template context, pass _context explicitly:

{% set foo = "bar" %}

{% macro display(_context) %}
  {{ _context.foo }}
{% endmacro %}

{{ display(_context) }}
{# output: bar #}

block

Defines a named, reusable template block. Use the block() function to render the block elsewhere:

{% block "header" %}
  Introduction
{% endblock %}

{{ block("header") }}

Tests

Tests are used with is and is not to perform type and value checks.

defined

Checks whether a variable exists in the context (regardless of its value):

{% if missing is not defined %}
  Variable is not defined.
{% endif %}

empty

Returns true when a variable is null, an empty string, an empty collection, or an empty map:

{% if user.email is empty %}
  No email on record.
{% endif %}

null

Checks whether a variable is null:

{% if user.email is null %}
  ...
{% endif %}

{% if name is not null %}
  ...
{% endif %}

even and odd

Check whether an integer is even or odd:

{% if 2 is even %}
  ...
{% endif %}

{% if 3 is odd %}
  ...
{% endif %}

iterable

Returns true when a variable implements java.lang.Iterable. Use this to guard a for loop when the collection may not always be present:

{% if users is iterable %}
  {% for user in users %}
    {{ user.name }}
  {% endfor %}
{% endif %}

json

Returns true when a variable is a valid JSON string:

{% if '{"test": 1}' is json %}
  ...
{% endif %}

map

Returns true when a variable is a map:

{% if {"apple": "red", "banana": "yellow"} is map %}
  ...
{% endif %}

When to use this page

• Need expression-writing basics: Pebble Syntax
• Need data transformations: Filter Reference
• Need runtime helpers: Function Reference