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Running tests as components

Tests on Fuchsia can either be run as standalone executables or as components. Standalone executables are invoked in whatever environment the test runner happens to be in, whereas components executed in a test runner are run in a hermetic environment.

These hermetic environments are fully separated from the host services, and the test manifests can stipulate that new instances of services should be started in this environment, or services from the host should be plumbed in to the test environment.

This document aims to outline the idiomatic way for a developer to configure their test artifacts to be run as components. This document is targeted towards developers working inside of fuchsia.git, and the workflow described is unlikely to work for SDK consumers.

An example setup of a test component is available at //examples/hello_world/rust.

Building the test

The exact GN invocations that should be used to produce a test vary between different classes of tests and different languages. The rest of this document assumes that test logic is being built somewhere, and that the test output is something that can be run as a component. For C++ and Rust, this would be the executable file the build produces.

Further documentation for building tests is available for Rust.

Packaging and component-ifying the tests

Once the build rule for building a test executable exists, a component manifest referencing the executable and a package build rule containing the executable and manifest must be created.

Component manifests

The component manifest exists to inform the component framework how to run something. In this case, it's explaining how to run the test binary. This file typically lives in a meta directory next to the file, and will be included in the package under a top level directory also called meta.

The simplest possible component manifest for running a test would look like this:

    "program": {
        "binary": "test/hello_world_rust_bin_test"

This component, when run, would invoke the test/hello_world_rust_bin_test binary in the package.

This example manifest may be insufficient for many use cases as the program will have a rather limited set of capabilities, for example there will be no mutable storage available and no services it can access. The sandbox portion of the manifest can be used to expand on this. As an alternative to the prior example, this example will give the component access to storage at /cache and will allow it to talk to the service located at /svc/fuchsia.logger.LogSink.

    "program": {
        "binary": "test/hello_world_rust_bin_test"
    "sandbox": {
        "features": [ "isolated-cache-storage" ],
        "services": [ "fuchsia.logger.LogSink" ]

Test components can also have new instances of services created inside their test environment, thus isolating the impact of the test from the host. In the following example, the service available at /svc/fuchsia.example.Service will be handled by a brand new instance of the service referenced by the URL.

    "program": {
        "binary": "test/hello_world_rust_bin_test"
    "facets": {
        "fuchsia.test": {
            "injected-services": {
                "fuchsia.example.Service": "fuchsia-pkg://"
    "sandbox": {
        "services": [

For a more thorough description of what is valid in a component manifest, please see the documentation on component manifests.

Component and package build rules

With a component manifest written the GN build rule can now be added to create a package that holds the test component.


test_package("hello_world_rust_tests") {
  deps = [
  tests = [
      name = "hello_world_rust_bin_test"

This example will produce a new package named hello_world_rust_tests that contains the artifacts necessary to run a test component. This example requires that the :bin target produce a test binary named hello_world_rust_bin_test.

The test_package template requires that meta/${TEST_NAME}.cmx exist and that the destination of the test binary match the target name. In this example, this means that meta/hello_world_rust_bin_test.cmx must exist. This template produces a package in the same way that the package template does, but it has extra checks in place to ensure that the test is set up properly. For more information, please see the documentation on test_package.

Running tests

To test the package, use the fx test command with the name of the package:

fx test TEST_NAME

If the package you specified is a test component, the command makes your Fuchsia device load and run said component. However, if the package you specified is a host test, the command directly invokes that test binary. Note that this can lead to the execution of multiple components.

Customize fx test invocations

In most cases, you should run the entire subset of test that verify the code that you are editing. You can run fx test with arguments to run specific tests or test suites, and flags to filter down to just host or device tests. To customize fx test:

fx test [FLAGS] [TEST_NAME [TEST_NAME [...]]]

Multiple ways to specify a test

fx test supports multiple ways to reference a specific test.

  • Full or partial paths:

    Provide a partial path to match against all test binaries in children directories.

    fx test //host_x64/gen/sdk

    Provide a full path to match against that exact binary.

    fx test //host_x64/pm_cmd_pm_genkey_test
  • Full or partial Fuchsia Package URLs:

    Provide a partial URL to match against all test components whose Package URLs start with the supplied value.

    fx test fuchsia-pkg://

    Provide a full URL to match against that exact test component.

    fx test fuchsia-pkg://
  • Package name:

    Provide a package name to run all test components in that package:

    fx test my_test_pkg
  • Component name:

    Provide a resource-path to test a single component in a package:

    fx test my_test

Running multiple tests

If you want to run multiple sets of Fuchsia tests, configure your Fuchsia build to include several of the primary testing bundles, build Fuchsia, and then run all tests in the build:

fx set core.x64 --with //bundles:tools,//bundles:tests,//garnet/packages/tests:all
fx build
fx test

You can also provide multiple targets in a single invocation:

fx test package_1 package_2 component_1 component_2

Converting from run-test or run-host-tests


For run-test, you should always be able to change fx run-test to fx test, for example:


Now becomes:



For run-host-tests, you should always be able to change fx run-host-tests to fx test, for example:

fx run-host-tests PATH_TO_HOST_TEST

Now becomes:


the -t flag

Unlike fx run-test (which operated on packages), fx test matches against tests in many different ways. This means that you can easily target tests either by their package name or directly by a component's name. One common workflow with run-test was to use the -t flag to specify a single component:


Now, with fx test, that simply becomes:


Passing arguments to individual tests

Use the -- flag to provide additional arguments which are ignored by fx test and passed to test components.

For example, to pass a timeout flag to a test that accepts it, execute:

fx test TEST_NAME -- --timeout=5

Internally, this command runs the following:

fx shell run-test-component TEST_NAME --timeout=5

If your test is a Components v2 test, then you can specify a filter which controls which test cases actually run:

fx test TEST_NAME -- --test-filter FooTest*

Note that there must be a space between --test-filter and the filter; you cannot use =. The filter is a glob style pattern.


When fx test runs your test component, fx test calls run-test-component on the target device with the test url to run the test.

Running tests (Legacy)

Tests can be exercised with the fx run-test command by providing the name of the package containing the tests.


This command will rebuild any modified files, push the named package to the device, and run it.

Tests can also be run directly from the shell on a Fuchsia device with the run-test-component command, which can take either a fuchsia-pkg URL or a prefix to search pkgfs for.

If using a fuchsia-pkg URL the test will be automatically updated on the device, but not rebuilt like if fx run-test was used. The test will be neither rebuilt nor updated if a prefix is provided.

In light of the above facts, the recommended way to run tests from a Fuchsia shell is:

fx shell run-test-component `locate TEST_PACKAGE_NAME`

The locate tool will search for and return fuchsia-pkg URLs based on a given search query. If there are multiple matches for the query the above command will fail, so locate should be invoked directly to discover the URL that should be provided to run-test-component.

run-test-component will create transient isolated storage for the test. See isolated-storage for more info.