Run an example component

This guide shows you how to build Fuchsia to include an example package from Fuchsia source's //examples directory and run its component on your Fuchsia device.

Examine a Fuchsia package

Open the examples/hello_world/BUILD.gn file.

This example, written in both C++ and Rust, prints Hello, world!. Each language-dependent directory has the following:

A BUILD.gn file that defines its Fuchsia package.
A meta subdirectory with component manifests (.cmx) files.

BUILD.gn

In short, GN is a meta build system. Output files from GN serve as inputs to Ninja{:.external}, the actual build system. If you aren't familiar with GN, see Introduction to GN.

In the example package's top level BUILD.gn file, the hello_world target is a group containing other dependencies, notably cpp and rust. Therefore, this target builds both of them:

group("hello_world") {
  testonly = true
  deps = [
    ":tests",
    "cpp",
    "rust",
  ]
}

To learn more about how GN defines Fuchsia packages, see the build/package.gni file.

Component manifest

A .cmx file, known as a component manifest, describes how to run an application on Fuchsia as a component. In other words, a component manifest creates a Fuchsia package.

Fuchsia package

To include a package in your Fuchsia image, you have the following options:

Base: Packages that are produced by build and included in paving images. These packages are included in over-the-air updates and are always updated as a single unit.
Cache: Packages that are included in paving images, but are not included in over-the-air system updates. These packages can be updated at any time when updates are available.
Universe: Packages that are not included in paving image. These optional packages are fetched and run on-demand,

Include the example package in your Fuchsia image

To include the example package in Universe (so that it can be fetched on-demand), use the --with flag when setting your product and board environment:

fx set <PRODUCT>.<ARCH> --with //examples/hello_world
fx build

For more information on setting up fx, see fx workflows.

You now have a build that includes the example package in Universe.

Run the example component

To run a Fuchsia component, use its Fuchsia package URL as an argument to the fx shell run command:

Open a terminal and run fx serve:
```
fx serve
```

Open another terminal and run the example component:

fx shell run fuchsia-pkg://fuchsia.com/hello_world_cpp#meta/hello_world_cpp.cmx

This command prints the following output:

Hello, World!

If fx serve is not running, the command prints an error message from the device:

fuchsia-pkg://fuchsia.com/hello_world_cpp#meta/hello_world_cpp.cmx: not found.

If fx serve is running, but the package is not found, then try rebuilding your Fuchsia image to include this package and repaving it to the device. See Include the example package in your Fuchsia image for details.

Run the example component using a simple string

The fx shell run command can match a string to a package URL if the string is only mapped to one component in your product configuration. For example:

$ fx shell run hello_world_cpp.cmx

If multiple matches exist, the command prints the list of matches:

$ fx shell run hello_world
fuchsia-pkg://fuchsia.com/hello_world_cpp_tests#meta/hello_world_cpp_unittests.cmx
fuchsia-pkg://fuchsia.com/hello_world_rust_tests#meta/hello_world_rust_bin_test.cm
fuchsia-pkg://fuchsia.com/hello_world_rust_tests#meta/hello_world_rust_bin_test.cmx
fuchsia-pkg://fuchsia.com/hello_world_cpp#meta/hello_world_cpp.cmx
fuchsia-pkg://fuchsia.com/hello_world_rust#meta/hello_world_rust.cmx
fuchsia-pkg://fuchsia.com/hello_world_rust#meta/hello_world_rust.cm
Error: "hello_world" matched multiple components

You can explore what components are in your product configuration using the locate command.

Find your favorite component:
```
fx shell locate hello_world_cpp
```
Find all runnable components:
```
fx shell locate --list cmx
```
Find multiple test components:
```
fx shell locate --list test
```