#template #yaml #hcl #yaml-config #json #cli #render-template

bin+lib template-cli

CLI for templating based on JSON, YAML or HCL configuration

16 releases

0.4.1 Apr 27, 2024
0.3.7 Nov 4, 2023
0.3.3 Jul 4, 2023
0.2.0 Mar 3, 2023
0.1.0 Dec 11, 2022

#28 in Template engine

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MIT license

962 lines

Template crates.io Github status

CLI for templating based on JSON, YAML or HCL configuration.

Inspired by Consul Template, Sprig, Handlebars, Mustache, and the Kotlin, Rust Go and Python standard libraries.



Install with

cargo install template-cli

Precompiled binaries

You can download a pre-compiled binaries for multiple architectures and operating systems from the Latest Release page.


The binaries are shipped as Docker images.

In the usage instructions below, replace all usages of template with docker run --rm ghcr.io/hiddewie/template.


template --template ./path/to/template.template --configuration ./path/to/configuration.json

Show the usage information with the --help option:

template --help

which outputs:

CLI for templating based on JSON, YAML or HCL configuration

Usage: template [OPTIONS] --template <TEMPLATE> --configuration <CONFIGURATION>

  -t, --template <TEMPLATE>            Absolute or relative path to the template file
  -c, --configuration <CONFIGURATION>  Absolute or relative path to the configuration file. Provide `-` as path to read the configuration input from the standard input stream
  -f, --format <FORMAT>                Specify the format of the configuration input. Useful when the configuration file has a non-standard extension, or when the input is given in the standard input stream [possible values: json, hcl, yaml]
  -h, --help                           Print help information
  -V, --version                        Print version information

The output is rendered to the standard output stream. Log messages are output to the standard error stream. The log level can be controlled using the RUST_LOG environment variable.

This tool combines well with other command-line utilities, following the UNIX philosophy, for example:

curl -s 'https://dummyjson.com/products' \
  | jq '{ items: .products }' \
  | template --configuration - --template products.template \
  | head -n 5 \
  > product-list.txt

Exit codes

  • 0: Success. The standard output contains the rendered template.
  • 1: Template file cannot be read.
  • 2: One of the required CLI options is not given.
  • 3: Configuration file cannot be read.
  • 4: Configuration file cannot be parsed.
  • 5: Template file cannot be parsed.
  • 6: Template cannot be rendered.
  • 101: Panic. An unexpected error has occurred, and was not handled correctly. Please create an issue to report the configuration, the template and the error output.


This software:

  • reads the configuration file or standard input
  • reads the template file
  • reads the environment to set the log level, and if the environment function is used
  • does not write any files
  • does not make any network connections


  • JSON, extension .json, default parsing format
  • YAML, extension .yml and .yaml
  • HCL, extension .hcl

If the configuration file has a different extension, or when the configuration is given in the standard input stream, you can configure the configuration format with the --format option.


See Pest grammar for formal template grammar.

It is recommended to use the extension .template for template files.

Render configuration data


  "value": "Hello",
  "nested": {
    "value": "world"

with template

{% value %} {% nested.value %}{% does_not_exist %}!

renders output

Hello world!

The YAML configuration file

value: Hello
  value: world

or the HCL configuration file

value = "Hello"
nested {
  value = "world"

will give equivalent output.


Comments are ignored and the content will not be rendered as output

This is rendered {# comment #} 
{# {% for item in array %}
This is not a loop
{% end %} #}
This is rendered


Null: {% null %}
Boolean: {% true %} and {% false %}
Integer: {% 0 %}, {% -0 %}, {% -100 %} and {% 100 %}
Floating point: {% .0 %}, {% 0. %}, {% -1.0 %}, {% 10.47e1 %} and {% -1.47e-10 %}
String: {% "" %} and {% "value" %}
Array: {% [] %}, {% [1] %} and {% ["", null, expression, [], {}, ] %}
Dictionary: {% {} %}, {% {a:1} %} and {% {item: expression, " space ": "spacy", "array": [], } %}

Flow control

Test expressions with if / unless, elif and else:

{% if some_expression %}
  Rendered when the expression is truthy
{% elif else_if_expression %}
  Rendered when the above is not true and the expression is truthy
{% else %}
  Rendered when the above are not truthy
{% end %}

{% unless expression %}
  Rendered when the expression is falsy
{% end %}

Loop over arrays with for and else:

{% for item in array_value %}
  Rendered content for each item
  reference a property in iteration array: {% item.property %}
  0-based index: {% loop.index0 %}
  1-based index: {% loop.index1 %}
  true if this is the first iteration: {% loop.first %}
  true if this is the last iteration: {% loop.last %}
  the number of iterations in the loop: {% loop.size %}
  alternate items in an array, treating it as circular: {% loop.index0 | alternate(["one", "two", "three"]) %}
{% else %}
  Rendered when the array did not contain any values
{% end %}

Notice that within the for loop, the loop variable provides information about the loop iteration.

Context variables

Set a variable within a block using with:

{% with a = 1 %}
  Value is {% a %}
{% end %}


Apply a function in a template by using the pipe | operator:

{% value | function1 | function2 | function3 %}

Some functions take one or more arguments, which can be passed by using parentheses:

{% value | function(argument1, argument2) %}

General functions

  • default(value): default value if the argument is falsy.
  • coalesce(value): default value if the argument is null.
  • toString: transform the value to a string.
  • empty: whether the value is not "truthy", i.e. null, 0, 0.0, -0.0, "", " ", [] or {}.
  • toJson, toPrettyJson: format a value to JSON, either compact or multi-line indented.

String functions

  • length: length of the string.
  • upperCase: transform a string into upper case.
  • lowerCase: transform a string into lower case.
  • kebabCase: transform a string into kebab case: lowercase-words-joined-with-dashes.
  • snakeCase: transform a string into snake case: lowercase_words_joined_with_underscores.
  • camelCase: transform a string into camel case: capitalizedWordsWithoutSpaces.
  • pascalCase: transform a string into pascal case: CapitalizedWordsWithoutSpaces.
  • capitalize: make the first character uppercase.
  • capitalizeWords: make the first character of every word uppercase.
  • environment: read an environment variable.
  • reverse: the string in reverse order.
  • split(splitter): split the string for each occurrence of splitter.
  • lines: split the string into an array of lines. Leading and trailing whitespace will be dropped.
  • parseBoolean: parses a boolean.
  • parseInteger: parses an integer.
  • parseDecimal: parses a decimal number.
  • parseNumber: parses an integer or decimal number.
  • substring(from), substring(from, to): creates a substring from the string. from is inclusive, to is exclusive.
  • take(n): takes the first n characters from the string.
  • drop(n): drops the first n characters from the string.
  • fromJson: parse a value from JSON.
  • abbreviate: ensure the value is not longer than n characters. If it is longer, the value will be shortened until n-1 characters, and suffixed with .
  • trimLeft, trimRight, trim: trim the left, right or both sides of the string from whitespace.
  • matches(regex): checks if the string matches a regular expression.
  • replace(search, replacement): replace the search string with the replacement.
  • regexReplace(search, replacement): replace matches of the regular expression with the replacement. The replacement may contain $0 (entire match), $1, $2, etc. for matched groups, and $name for matched named groups.
  • contains(substring): whether the string contains the substring.
  • startsWith(start), endsWith(end): whether the string starts or ends with the given value.

Array functions

  • length: length of the array.
  • reverse: the array in reverse order.
  • take(n): takes the first n items from the array.
  • drop(n): drops the first n items from the array.
  • first: the first item from the array, if it exists.
  • last: the last item from the array, if it exists.
  • index(n): the nth item from the array, if it exists.
  • contains(value): whether the array contains the value.
  • unique: remove all duplicates from the array.
  • all: true if all arguments are truthy, ∧.
  • any: true if any arguments are truthy, ∨.
  • none: true if all of the arguments are falsy.
  • some: true if any of the arguments are falsy.
  • chunked(items, overlap): chunks the array into an array of arrays, each subarray at most items items, with overlap overlapping items with the previous chunk.

Dictionary functions

  • length: size of the dictionary
  • containsKey(key): whether the array contains the key.
  • containsValue(value): whether the array contains the value.
  • keys: the keys of the dictionary.
  • values: the values of the dictionary.
  • invert: the values become the keys and the keys become the values. Values can only be strings. Duplicate values are not preserved.

Boolean functions

  • negate: negation of the boolean, ¬.

Date/Time functions

  • parseFormatDateTime(parse, format): parse the date/time, and format it to the given format. If the input is the string now, it will be parsed to the current instant. Otherwise, the parsing and formatting follows the strftime specifiers.


The debug statement can be used to log an expression and it's evaluated result without outputting the content into the templated output:

Template content...
{% debug some_value %}
... more template content

Logged output:

[2023-03-05T11:18:56Z INFO  template_cli::evaluate] Debug expression: some_value = {"b":"c"}

The assert function can be used to assert the expected value of an expression. If the assertion fails, the template rendering will fail with an error indicated by the given assertion message. The template:

{% true | assert(true, "This is fine") %}
{% false | assert(true, "This is not OK") %}

will exit with an error code, and log the output

[2023-03-19T16:02:48Z ERROR template] ERROR: Could not render template: Assertion failed: Expected value 'true' but found 'false': This is not OK



cargo build


cargo test


Go to the Release workflow.

Click the Run workflow button, and give the following inputs:

  • Branch: master
  • Version: The next version to release, e.g. 1.2.3


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