Urbit / Docs

Web (Eyre)

Your urbit runs its own webserver, Eyre, that handles all things HTTP. Here we'll cover the features of Eyre that most resemble a traditional web server.

By default Eyre uses Tree. Tree is a simple ui for browsing the contents of the filesystem, statically publishing a blog or any kind of content and for running single-page apps.

Tree was built primarily to display both our documentation and the contents of urbit.org. If you're interested in doing anything complex, it's worth checking out the docs source and the urbit.org source.

We ship tree as compiled JavaScript on your urbit, but the source lives in its own repo here.


Your urbit runs a web server and can be found locally at http://localhost:8080/ This is the fastest and easiest way to find your urbit's web server. Note that the port that a given urbit uses might be 8081, 8082, or so on if you're running other processes that might be occupying 8080, such as another urbit. Upon the starting of a ship, it will print a message telling you which port it's using.

Your urbit's web server can be accessed remotely at http://your-urbit.urbit.org/. You'll need to enter a code to access it in this way, one that is generated by running the +code command in the Dojo.

(Make sure you have a desk mounted to unix, as covered in the setup section).


The /web directory of the %home desk is served by default. You can change which desk you're serving by using the command |serve in the Dojo.

Let's create a desk for development called %sandbox.

Use |merge to create the new desk containing the contents of %home:

~your-urbit:dojo> |merge %home our %sandbox

And use |sync to continuously update based on changes to %home:

~your-urbit:dojo> |sync %home our %sandbox

Now let's mount it to unix so that we can save files to it:

~your-urbit:dojo> |mount /=sandbox=

And now, to switch the desk you're serving from:

~your-planet:dojo> |serve %sandbox
Hosting files

To host a file on the web, save the following in /sandbox/web/test.udon in your ship's pier through unix.

# Hello

This is a simple Udon file.

Our Tree system allows for pages to be designated as children of other pages. To do this for any given page, simply make a new folder in the directory that contains the page that will be the parent, and give that directory the same name. So, the children of a page /home/web/foo.udon will live in a directory called /home/web/foo/.

Let's continue our with our test.udon example. Create /sandbox/web/test/ and add two more Udon files in /sandbox/web/test/daughter.udon /sandbox/web/test/son.udon, respectively. Make sure that they contain valid Udon. Udon is our markup language, similar to Markdown; check out the guide here.

NOTE: Due to a bug in Urbit, you may have to create the directory and the file in it in rapid succession using something like mkdir sandbox/web/test && touch sandbox/web/test/first.md.

Now modify /sandbox/web/test.udon to list the children using ;list;:

# Hello

This is a simple Udon file.


To view your file rendered using tree:


To view your file as raw Udon:


To view your file as raw html:


Now navigate to the “daughter” file that you created:


In Urbit, there is no distinction between a directory and a file. So, if you're adding files directly from unix, simply mkdir test and Urbit will treat the file and directory as the same entity.

You can use this feature to change your homepage at your-urbit.urbit.org with the page /home/web.udon, where home is the current mounted desk. By default, web.udon will display some info about your urbit, but you can customize it to your heart's content.



This is the equivalent of a public_html directory. Your urbit looks in this directory to serve content to the world. You can change what desk you're serving from using |serve.

Switch to the %sandbox desk:

|serve %sandbox

Let's quickly step through the default contents of web/:

  • 404.hoon - The default 404 page.

  • dojo.hoon - Page body that loads the dojo React module.

  • lib/css/bootstrap.css - The Urbit-flavored bootstrap

  • lib/css/codemirror.css - Syntax highlighting

  • lib/css/fonts.css - The Urbit custom fonts

  • lib/js/hoon.js - Hoon syntax highlighting

  • lib/js/sole.js - dojo operational transformation JS

  • lib/js/urb.js - Base Urbit JS library. Handles AJAX and polling.

  • static.udon - Sample static file

  • talk/ - Talk compiled JS / CSS

  • talk.hoon - Page body that loads the talk React module.

  • testing.udon - Guide to writing unit tests in Urbit

  • tree/ - Tree compiled JS / CSS

  • tree.hoon - Page body that loads the tree React module.

  • unmark/ - Used for testing .udon.

  • womb/ - Womb compiled by JS/CSS

  • womb.hoon - Used for issuing planets on the test network.


Tree is a simple publishing tool for files that live on your urbit. You're probably reading this file using Tree right now. The main driving use-case for Tree has been for us to host our own documentation and the surrounding discussion. Tree remains a pretty simple and flexible tool for getting content online. It resembles a cross between Jekyll, Kirby and macOS's Finder. We think it's pretty great.

Tree is also the basic UI for browsing the filesystem. We take it one step further and make it possible to write modules (tiny little apps), like Talk and Dojo, that can be run directly inside of Tree.

In the browser Tree is a React / Flux app that uses a slightly modifed version of Bootstrap. On your Urbit Tree is a renderer that translates data in %clay, our filesystem, into JSON / JSX that can be read by the client.

Your urbit comes with a pre-built copy of the tree (as a single .js file). If you're interested in helping to develop tree or learning about how it works, it has its own GitHub repo here. We have our own fork of Bootstrap, which lives here.


To render content without the Tree put it in /web/pages. Files in this directory aren't wrapped by Tree.

Publishing files

Publishing files in Tree is simple. Try dropping a file in /web (as covered in the Quickstart) and navigating to it.

The best example of more complicated usage of Tree is covered by the Urbit docs themselves. See anything while perusing the docs that you'd like to use yourself? The docs are all on GitHub here.


Most .udon files begin with some frontmatter, which determines the title and other settings for rendering the file correctly. E.g.:

:-  :~  navhome/'/home/'
        title/'Example Frontmatter Title'

Here are the fields we support:


String. Example: title/'Tree manual'.

This sets the title in the nav or in lists as distinct from the filename.


Boolean. Example: next/'true'

Adds a 'next' link at the bottom of the page that links to the next item as decided by the sort order.


Integer. Example: sort/'7'

User-defined sort. Indicates the sort order as distinct from alphabetical sort. These numbers should not be overlapping.


Boolean. Example: hide/'true'

Whether or not to hide the item from the tree.


Urbit date. Example: date/'~2016.2.22'

Used for sorting posts by date.

Arbitrary Frontmatter

Any arbitrary category can be used as long as it's given data of valid types. Arbitrary categories can be used with the ones mentioned above. Below is valid front matter:

:-  :~  date/~2018.9.12

Using components

Tree is built using React with a small library of components that you can add to your pages using JSX tags.

If you don't know anything about JSX that's okay: basically we just give you a few custom HTML tags that you can use to create components on the page.

For those familiar with React / Flux each JSX tag corresponds to a component in the source.


Example: <list src="/posts" titlesOnly="true" />

Creates a list of children at a path. Defaults to the current path.

Optional properties:

  • src: Path to load kids from.

  • titlesOnly: Only show the 'title' which is either the filename or the first h1.

  • sortBy: Can be date if items have a date YAML.

  • className: Class to be assigned to the container.


Example: <kids src="/posts" />

Includes all the children at the specified path separated by <hr> tags.

Optional properties:

  • src: Path to load kids from.

  • className: Class to be assigned to the container.


Example: <toc noHeader="true" />

Creates a table of contents from the h1 - h6 tags on the page.

Optional properties:

  • match: Tag name to match. If you wanted to only match h2 tags it could be match="h2"

  • noHeader: By default we add a 'Table of Contents' header to the top. Remove it with noHeader="true".


On the frontend Tree is mostly just a standard Flux app: components generate actions that get dispatched to stores which are picked up by components. If you're not familiar with Flux, this overview is a good place to start.

As a file browser Tree does a simple job: it fetches data and keeps a cache of what it has seen. To do this simple job we do three fancy things: manx to JSON encoding, the Async component and Modules.

manx to JSON format

Since we want to use React to manage our DOM lifecycle we actually transfer the DOM structure of each page as JSON over the wire, and 'reactify' it on the client side.

On the Urbit side we have a type for html: ++manx. Our wire format for sending HTML as JSON is really similar to a ++manx. If you're not interested here's what the json looks like:

"string" || {gn:"string", ga:{key:"string", [...]}, c:[...]}

To make this clearer let's look at some HTML and its JSON representation:

<div class="blue">
{gn:"div", ga:{className:"blue"}, [
    {gn:"h1", ga:null, "Title"}

To see the mechanics of how this is handled on the frontend, checkout components/Reactify.coffee in the urbit/tree repo.

Async component

Each of our components (even those we fetched from the server) can depend on data that may not exist in our stores, so we wrap our components in a general-purpose Async component. You can find the component in components/Async.coffee in the urbit/tree repo.

When we say that we just fetch HTML as JSON it's actually slightly more complicated. When we fetch a node in the filesystem we might not want to fetch its entire contents. Sometimes we just get the title as a part of a list of children, sometimes we might get a snippet (in a list with previews), and sometimes we get the whole thing. On the Urbit side these results are provided by ren/tree/json.

From the perspective of the frontend we just send queries on a path that obey a simple API. To see this in action, here's our BodyComponent:

query {

Here we're asking for the body, name, path, metadata and parent of the current path. When it loads that data gets passed to our component as props.

Module basics

A module is a special component that can kind of 'take over' the page. It can change properties of the nav bar as well as load custom scripts / css.

Talk, for example, looks like this:

<module nav_title="Talk" nav_no-dpad="" nav_no-sibs="" nav_subnav="talk-station">
    <script src="/~~/~/at/lib/js/urb.js" />
    <script src="/talk/main.js" />
    <link href="/talk/main.css" rel="stylesheet" />
    <talk />

A module will first download all the scripts and styles that it specifies before trying to load any contained components. This means that you can define React components in external scripts and a module can be entirely self contained. The <talk> component isn't a part of Tree — it's loaded first before it's executed.

If you're interested in seeing how a module works you can check out the source for Talk here. Dojo also ships on your urbit, its source is here.


The most basic interface between your browser and your urbit is urb.js. This small library provides the basic utilities for sending and receiving messages from an urbit.


Each JSON message sent to your urbit needs an app endpoint. Can be set as a global window.urb.appl or set on a per-request basis in the payload to send().


CSRF token. Written to the page by Eyre, our webserver, when logged in. Required part of any authenticated request and included by default.


Our basic request wrapper. You can see here how a request needs to be packaged to be properly understood by your urbit.


Subscribe to an endpoint. Subscription data is returned through the continuous long poll.


Close subscription on specified path.


Mechanics of the continuous long poll. Used mostly internally.


Library of utility and helper functions.