#dataset #generator #json #template

bin+lib jen

A simple JSON generator CLI tool for creating large datasets

7 stable releases

1.3.1 Nov 29, 2021
1.2.1 Nov 23, 2021
1.1.0 Mar 9, 2021
1.0.1 Jul 11, 2019
1.0.0 Jun 13, 2019

#31 in Template engine

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363 downloads per month

MIT license

375 lines


Build Status Crates.io

A simple (but extensible) tool for generating large random datasets.

Jen is a combination of a core library and a CLI, used to generate random datasets based on a template. There are existing tools for this purpose, but most of them live in a browser and they're naturally insufficient when it comes to generating large amounts of data. Jen was created to fill the niche of creating larger amounts of data for things like unit tests and database state.

Jen's underlying template syntax is drive by Tera to aid in familiarity and to avoid re-inventing a templating language. On top of this layer, Jen offers many helpers based around randomizing data. Several of these helpers are based on fake, with additional helpers provided where there are gaps. You can naturally attach your own helpers when using Jen programmatically.


Jen will be available via Crates.io, so it can be installed from there directly. You can use Jen either as a command line utility, or directly via the programmatic API.

If you wish to install Jen as a command line utility, you can install it via an easy one-liner in your terminal:

$ cargo install jen

If you wish to use it as a library, you can add it to your Cargo.toml as a dependency of your application:

jen = { version = "1.1", default-features = false }

You should disable the default features as it includes several dependencies which are required for the CLI use case. These dependencies are not included in your application when these features are disables.


The first step is to construct a template file which Jen will then use when generating data. There are many template helpers provided by default, via the internal jen::helpers module. You can check the documentation for the latest list of helpers, although a fairly up to date table of helpers can be found below. Once you have this template, you can either use Jen via the CLI, or programmatically.

Command Line

The CLI is fairly simple, with a basic structure of:

$ jen <template>

Using this syntax will generate a random document based on the provided template (which must be a valid Tera template). There is a basic document you can test with inside the example directory.

There are various switches you can provide to adjust the output, including the following (at the time of writing, there may be more):

    -h, --help       Prints help information
    -t, --textual    Treat the input as textual, rather than JSON
    -V, --version    Prints version information

    -l, --limit <limit>        An upper limit of documents to generate [default: none]
    -w, --workers <workers>    Number of threads used to generate data [default: 4]

The Jen CLI was written under the assumption that you're dealing with JSON documents, and most of the options and features revolve around this being the case. If you're not using JSON, you can provide the -r flag to treat the incoming data as "raw". In this case many options will have no effect (such as pretty printing), but templating can still be carried out properly.

During development, Jen had options to combined and prettify JSON inputs. This was removed to keep things simple, and to allow Jen to support more input types. Instead it is recommended to use jq for these purposes.

For a complete and up to date list of options, please use jen -h in your terminal.

Programmatic API

The programmatic API is pretty simple. Everything is handled through the use of the Generator struct, which implements the Iterator trait to provide continuous (lazy) documents.

Generators are constructed using a path to a template on disk, and you then generate documents using the Iterator methods, as shown below.

let mut generator = Generator::load("./example/example.tera")
    .expect("provided a value template");

for document in generator.take(5) {
    println!("{}", document);

This will generate five documents from the provided template and print them to the terminal.

Template Helpers

Below is a list of current helpers (at the time of writing). Please see the documentation uploaded to Crates.io for an up-to-date listing.

Helper Description
bool() Generates a random boolean value
city() Generates a random city name
company() Generates a random company name
domain() Generates a random domain name
email() Generates a random email address
firstName() Generates a random first name
float(start=f64::MIN, end=f64::MAX) Generates a random float value between two bounds
index() Retrieves the current index of the generated document
industry() Generates a random industry type
integer(start=i64::MIN, end=i64::MAX) Generates a random integer value between two bounds
lastName() Generates a random last name
latitude() Generates a random latitude location value
longitude() Generates a random longitude location value
name() Generates a random full name
objectId() Generates a random object identifier
paragraph() Generates a random paragraph of Lorem Ipsum
phone() Generates a random phone number
postcode() Generates a random postcode value
profession() Generates a random job profession
random(values=["red","blue","yellow"]) Retrieves a random value from the provided values
sentence() Generates a random sentence of Lorem Ipsum
state() Retrieves a random US state name
stateCode() Retrieves a random US state code
street() Generates a random street name
timestamp() Generates a random timestamp value in seconds
title() Generates a random job title
userAgent() Generates a random browser user agent
username() Generates a random account username
uuid() Generates a v4 UUID
word() Retrieves a random word of Lorem Ipsum
zip() Generates a random US zip code


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