# Building and running a session {#building-and-running-a-session}

A session is the first product-specific component started on boot. The session
component is responsible for building a product's user experience. For more
information on sessions, see
[session framework](/docs/concepts/session/introduction.md).

## Booting into a session {#booting-into-a-session}

To boot into a session, do the following:

1. For a session to run at boot you need to create a configuration file with the
session component URL.

   <pre><code>{
       "session_url": "fuchsia-pkg://fuchsia.com/<var>pkg-name</var>#meta/<var>your_session.cm</var>"
   }</code></pre>

   Replace the following:
   * <var>pkg-name</var>: the package name
   * <var>your_session.cm</var>: the name of your session, including the `.cm`
   extension.

   For more information, see
   [`fuchsia-pkg`](/docs/reference/components/url.md#fuchsia-pkg) and
   [Package name](/docs/concepts/packages/package_url.md#package-name).

1.  In the `BUILD.gn` file, include the configuration file for the session
component

   ```json
   import("//src/session/build/session_manager.gni")

   session_manager_package("your_session_manager_package") {
       config = "path/to/config.json"
   }
   ```

1. Run the following command to include the `session_manager` and `your_session`
   in your base image:

   <pre class="prettyprint"><code class="devsite-terminal">fx set <var>product</var>.<var>board</var> --with-base=<var>//path/to/your/session</var>,<var>//path/to/your/session:your_session_manager_package</var></code></pre>

   Note: Selecting a product that already has a session manager package will
   result in a build error because the packages will conflict. The `core`
   product would be a good choice as a starting point as it includes only the
   bare minimum needed to launch Fuchsia.

   `fx list-products` and `fx list-boards` will show lists of the products and
   boards available to be used in the `fx set` command. For more information on
   `fx` commands see the [fx documentation](/docs/development/build/fx.md).

1. Rebuild and re-pave the device.

   ```posix-terminal
   fx build
   fx ota
   ```

   This causes `session_manager` to start and launch your session.

For a full explanation of building a session component, see [Writing a Hello
World Session](/docs/development/sessions/writing-a-hello-world-session.md).

## Launch a session from the command line {#launch-a-session-from-the-command-line}

There are cases when you don't want your session to launch at boot but still
want to be able to launch it from the command line. `session_manager` needs
to be running to launch a session. The `session_manager_for_dev` target
ensures `session_manager` itself starts, but does not launch a session.

To launch a session from the command line, do the following:

1. Add the `session_manager_for_dev` target in the base dependency set, in
addition to the session target.

   <pre class="prettyprint"><code class="devsite-terminal">fx set <var>product</var>.<var>board</var> --with-base=//src/session/bin/session_manager:session_manager_for_dev --with=<var>//path/to/your/session</var></code></pre>

   `fx list-products` and `fx list-boards` will show lists of the products and
   boards available to be used in the `fx set` command. For more information on
   `fx` commands see the [fx documentation](/docs/development/build/fx.md).

1. Run the following command to rebuild and repave the device:

   ```posix-terminal
   fx build
   fx ota
   ```

   This causes `session_manager` to start without launching your session.

1. Your session can now be launched from the command line.

   Run the following command to launch your session:

   <pre class="prettyprint"><code class="devsite-terminal">ffx session launch fuchsia-pkg://fuchsia.com/<var>pkg-name</var>#meta/<var>your_session.cm</var></code></pre>

   For more information about the `ffx session` command, run
   `ffx session --help`. For more information about `ffx`, see the
   [`ffx documentation`](/docs/development/tools/ffx/overview.md).