fx is the entry-point for a set of subcommands that make many tasks
related to Fuchsia development easier. Run
fx help to see all the available
subcommands. If you use
zsh as a shell, source
scripts/fx-env.sh to get some auto-completion.
Setting up fx
It is strongly recommended that you
source scripts/fx-env.sh into your
shell. This is tested and regularly used with Bash and ZSH. It may work for
other compatible shells.
# In your fuchsia checkout: $ cd fuchsia # Add a configuration to your shell to include fx-env.sh $ echo "source \"$PWD/scripts/fx-env.sh\"" >> "$HOME/.$(basename "$SHELL")rc" # If you would like additional convenience tools from the Fuchsia scripts, also # optionally run the following: $ echo "fx-update-path" >> "$HOME/.$(basename "$SHELL")rc" # Restart your shell $ exec "$SHELL"
The above method provides the most well defined feature set, and should be generally non-invasive. If it causes bugs in your shell environment, please file project bugs.
If for some reason you need to work with multiple Fuchsia checkouts (recommended workflows below should obviate such a need), then you may want to do something other than the above. In this case, there are a few well supported methods:
- Always execute
- Use a tool like direnv or
dotenv to add
$PATHwhile working in a particular Fuchsia directory.
Common daily tools
The first thing you will want to do after checking out a Fuchsia tree is to
build Fuchsia, and then get it onto a device.
fx has some commands to help
fx setconfigure a build
fx buildexecute a build
fx flash ; fx mkzedbootflash a target; or prepare a zedboot USB key
fx serveserve a build
fx otaupdate a target
fx testexecute tests
fx shellconnect to a target shell
- and many other small tasks
Configure a build
First let's configure the build. To do this you need to make a few choices:
- What product configuration do you want?
- What board are you building for? (unsure: try
- What extra test targets do you want? (unsure: try
//bundles/tools, and if you're working on features, you probably want
Armed with our above choices (if you didn't read above, do so now), you are ready to configure your build:
fx set workstation_eng.x64 --with //bundles/tests
Once you ran
fx set in your checkout, there is no need to run it again unless
you need to modify the arguments that follow.
fx set stores its configuration in an
args.gn file in the output directory.
The output directly is
out/default. You can specify a different directory
fx --dir <out_dir> set <set_args>.
You can edit the generated
args.gn file using the
fx args command to create
more elaborate configurations.
fx args will open
args.gn in an editor, let
you make changes, and regenerate the build graph.
What just happened?
- You selected the product
fx list-productsfor a list of other product configurations).
- You selected the board
x64, which supports many typical boards based on the
x64architecture. (Note that
arm64boards are less interchangeable, and you will most likely need to give
fx setthe specific board, when building to an
fx list-boardsfor a list of known board configurations.)
- You are ephemerally building
testsas part of the
universepackage set, not as a part of the paving images.
Package deployment options
--with option has three variants related to how packages are deployed to a
fx set also has a
--with-host option, for building
host-only targets, such as host-based tools and libraries.)
So what are
Configurations ultimately specify dependencies (mostly packages) that contribute to output artifacts of the build (mostly images and package repositories). The build is parameterized to determine which dependencies (mostly packages) are added to which output artifacts (images or package repositories). The three axes are called "base", "cache", and "universe":
- Base: Packages that are added to base are included in paving images produced by the build. They are included in over-the-air updates, and are always updated as a single unit. Packages in base can not be evicted from a device at runtime - they encode the minimum possible size of a configuration.
- Cache: Packages in cache are included in paving images, but they are not included in over-the-air system updates, and are allowed to be evicted from the system in response to resource demands, such as disk-space pressure. Packages in cache can be updated at any time that updates are available, and each of these packages may be updated independently. This is software that is "optional", but is good to have available instantly "out of the box".
- Universe: Packages in universe are additional optional packages that can be fetched and run on-demand, but are not pre-baked into any paving images.
The "board" and "product" configurations pick a predefined set of members for each of these package sets. Most commonly the board configurations specify a set of boot-critical drivers to add to the base dependency set, and could for example include some optional but common peripheral drivers in the cache set. The board configuration may also include some board-specific development tools (more commonly host tools, rather than target packages) for interacting with the board in "universe". The product configurations make choices to add more or less software to the base, cache or universe package sets based on the definition and feature set of the product they represent. A speaker product, for example, adds many audio-media-related packages to the base. A workstation product adds a wide range of GUI, media and many other packages to the base.
Key product configurations
There are many more than below, but the following three particularly important configurations to be familiar with:
bringupis a minimal feature set product that is focused on being very simple and very lean. It exists to provide fast builds and small images (primarily used in a netboot rather than paved fashion), and is great for working on very low-level facilities, such as the Zircon kernel or board-specific drivers and configurations. It lacks most network capabilities, and therefore is not able to add new software at runtime or upgrade itself. This also means some
fxcommands such as
fx shellcannot be used with the
coreis a minimal feature set that can install additional software (such as items added to the "universe" dependency set). It is the starting point for all higher-level product configurations. It has common network capabilities and can update a system over-the-air.
workstation_engis a basis for a general purpose development environment, good for working on UI, media and many other high-level features. This is also the best environment for enthusiasts to play with and explore.
Key additional build targets
--with flag for
fx set takes in arbitrary
For convenience, a number of bundles are defined, which include a variety of
commonly used build targets. It is important to be familiarized with the
//bundles/toolscontains a broad array of the most common developer tools. This includes tools for spawning components from command-line shells, tools for reconfiguring and testing networks, making http requests, debugging programs, changing audio volume, and so on. The core product includes
//bundles/toolsin the universe package set by default.
//bundles/testscauses all test programs to be built. Most test programs can be invoked using
run-test-suiteon the device, or via
//bundles/kitchen_sinkis a target that causes all other build targets to be included. It is useful when testing the impact of core changes, or when making large scale changes in the code base. It also may be a fun configuration for enthusiasts to play with, as it includes all software available in the source tree. Note that kitchen sink will produce more than 20GB of build artifacts and requires at least 2GB of storage on the target device (size estimates from Q1/2019).
Execute a build
Once you configured your build with
fx set as shown above, you can run a build
fx build. This commands builds all required targets and their outputs.
Clean a build
You can continue to make changes to files in your checkout and run
fx build to
rebuild. The build system will attempt to do less work when there are results from
a previous build. A build that uses previous build results is called an incremental
build, and is usually much faster than a clean rebuild.
Changing configurations should not result in a broken incremental build, but this may still happen in rare cases due to limitations of the build system. If this happens, please file a detailed bug that captures any steps to reproduce the problem and any diagnostics such as build logs. Then use these commands to recover:
fx cleanwill clear out all build artifacts.
fx clean-buildis equivalent to
fx clean, then
If you find yourself changing configurations and cleaning your output directory
often then consider using
fx set --auto-dir instead. In this mode,
fx set will
choose different output directories for different configurations. Note that this
will increase your disk usage and you may need to delete old output directories
that are no longer needed.
Enabling incremental package rebuilds
fx build builds all packages for the specified product configuration.
This is necessary for some developer workflows, but it is excessive for many others.
If you are only iterating on your test, you shouldn't need to rebuild and republish
all ephemeral (universe) packages.
You can enable incremental rebuilds by adding
~/.bashrc or equivalent. This change results in the following:
pm(and by consequence
fx serve) watches for packages before they are created. When a package is created or modified,
pmauto-publishes that package, so you can keep
fx serverunning from an empty tree and it will publish incrementally as you go.
fx testonly builds the minimal targets required for running. For component tests, that's the package, its GN dependencies and
fx servedoes not pave or flash.
fx paveby default exits after paving a single time. Use
Note that the behavior of
fx build remains unchanged.
Building a specific target
fx build can be given the name of a specific target or file to build. For
example, a target with the label
//examples/hello_world:hello_world can be built with
fx build examples/hello_world:hello_world.
Note that this only works for targets declared in the default GN toolchain. For
targets in other toolchains, the path of an output file may be used instead. For
example, an executable target with the label
//foo/bar:blah(//build/toolchain:host_x64) can be built with
fx build host_x64/blah.
See the build system overview for a more detailed discussion of build targets.
Generating a build archive
In addition to executing a build, you can use the
fx build command
to generate a build archive file (that is,
.zip). This build
archive file comprises a specific blend of the build artifacts produced by
making your build output portable for various purposes. For instance, you can provide
this build archive file as input to the
ffx target flash command
for flashing the build to a Fuchsia device.
To generate a build archive file, run
fx build with the following special target:
fx build build-archive.FORMAT
zip, for example:
fx build build-archive.zip
Once the build is finished, this command creates the build archive file
build-archive.zip in the example above) in your Fuchsia build directory, which is
out/default by default. (To view the exact location of your build directory,
Flash a board and prepare Zedboot
The exact preparation required to put Fuchsia onto a target device varies by
specific device, but there are two general groups in common use today, made
fx flashis used with most
arm64devices to perform a raw write of Zedboot to the device, preparing it for Paving.
fx mkzedbootis used with most
x64devices to prepare a bootable USB key that boots into Zedboot, preparing the device for Paving.
What is Zedboot?
Zedboot is a special configuration of Zircon that contains a simple network stack, a simple device advertisement and discovery protocols, and a suite of protocols to write Fuchsia to a target disk and/or to network boot a target system. Zedboot is a term used for both the overall process, as well as a special build configuration. Many people come to know it as "the blue screen with ASCII art".
To enter Zedboot on an arm64 target, power on the device while triggering a
boot into fastboot flashing mode (often this involves holding a particular
button while rebooting or powering on that varies by particular hardware
target). Once in flashing mode, execute
fx flash on the host system.
To enter Zedboot on an x64 target, first produce a Zedboot USB key using
fx mkzedboot <path-to-usb-device> (to list suitable USB devices on your
fx list-usb-disks). Remove the USB key after completion,
insert it to the target device, and reboot the target device, selecting "Boot
from USB" from the boot options, or in the device BIOS.
What is Paving?
Paving is in many ways similar to "flashing" from other worlds, however, it has some differences. Specifically, paving refers to a group of processes and protocols in Fuchsia to transfer a set of artifacts to a target system that will be written into various partitions on a target system. By contrast, the process of "flashing" is more of a raw process of writing a raw data stream to a raw disk device, and not strictly partition-oriented.
Users can start a paving process by first flashing Zedboot using
or by booting a Zedboot USB key made by
fx mkzedboot, then executing
on the host system. In general most users actually will want to use
fx serve is covered in the serve a build
What is Netbooting?
In Fuchsia, "netboot" refers to sending a set of artifacts to a Zedboot
instance that instead of making changes to the disk, will just be booted from
RAM. Users can perform a "netboot" by first booting a device into Zedboot by
fx flash (arm64) or
fx mkzedboot (x64), and then executing
fx netboot on the host system.
Serve a build
A lot of build configurations for Fuchsia include software that is not immediately included in the base images that a build produces, that are written to devices during paving. Such software is instead made available to target devices on-demand, which is often colloquially referred to as "ephemeral software".
fx serve performs two functions internally:
fx pavestart a paving server, used for "fresh installs" of a Fuchsia device from a Zedboot state.
fx servestart a package repository server, used for dynamic installation of software at runtime, as well as whole-system updates.
fx serve command also searches for a device to configure, and
upon discovery (which may be restricted/modulated with
fx set-device or
-d) the target device is configured to use the repository server as a source of
dynamic packages and system updates.
Update a target device
As described in prior sections, there are different groups of software on a Fuchsia device:
- Software that is part of the core system "base", that is updated in a single transaction.
- Software that is part of Zedboot images other than base (cache) that can be updated ephemerally.
- Software that is always ephemeral (universe).
For new user development workflows, the most general command to assist with
updating a target device is
fx ota. The
fx ota command first
updates "base" and "cache" software, and then reboots the target device
when it is complete. The end result of this process should be
indistinguishable in terms of software versions from performing a fresh
pave of a device.
fx ota process causes a device reboot, it is sometimes not the
most efficient process for diagnosis, debugging or other non-testing based
workflows or needs. In these cases a user has some options for how to ensure
that software on a device is being regularly updated.
fx serve process configures a Fuchsia software repository with
automatic update features. The repository informs the target device of newly
updated software every time the underlying repository is updated (which
happens at the end of every successful
fx build). For many software
components, the easiest way to update them during development is to ensure
that they are not included in the base set, but instead included in
either "cache" or "universe". In that case, simply restarting the
software on the target (e.g. by closing it completely, or by invoking
killall) will result in the software being immediately updated when it is
started again. Specifically for shutting down Modular and all dependant
The Fuchsia codebase contains many tests. Most of these tests are themselves
components and can be launched on the target device in the same way as other
components. On the target device, some programs also assist with test-specific
concerns for component launching, such as
run-test-suite. The process can also
conveniently be controlled from the development host by way of
fx test. See
Run Fuchsia tests for more details.
Some users find that an effective high focus workflow is to have the system
build, push and execute tests whenever they save their source code. This can
be achieved with
fx very easily, for example:
fx -i test hello-world-rust-tests
The above command will execute the tests every time a change is made
to the source code in the tree. The
-i flag to
fx to repeat
the rest of its command every time the source code in the tree is changed.
fx test command first performs a build, then executes a test on
a target, this combination provides a convenient auto-test loop, great for
high focus workflows like test driven development.
Connect to a target shell
Most product configurations include an SSH
server with a Fuchsia specific configuration. The command
fx shell is a
convenient wrapper to connect to the target device over SSH and provides
access to a very simply POSIX-style shell. Users should note that while the
shell is a fork of a POSIX shell, it does not provide all features of a
common Unix shell. In particular users will find that CTRL+C has odd quirks,
and may often find quirks for sub-shell expressions and certain more advanced
IO redirections or environment variable propagations. These misfeatures are
side effects of Fuchsia not being a POSIX system.
Nonetheless the shell made available via
fx shell is extremely useful for
imperatively executing programs on the Fuchsia target, as well as exploring
some of the diagnostic / debug interfaces made available in a filesystem
tree, such as
/dev. It is also useful for invoking programs such
/bin/run that provides facilities for launching Fuchsia components. If
tools bundle is available in the build configuration, many tools common
to unix shell environments have been ported and are available, such as
fortune and so on.
Performing other common tasks
fx log captures all logs from low-level and high-level programs,
including the kernel, drivers and other userspace programs.
depends upon a working high level network stack and SSH. As such,
does not work with Zedboot or "bringup" product configurations. If a device
is in a state where
fx log ceases to function, it is often useful to
fx klog to capture more information about probable causes.
fx klog captures only a low-level log stream called "klog". The klog stream
includes logs from the Zircon kernel itself, as well as a subset of userspace
software (most notably drivers and low-level core software).
fx klog depends
on a lightweight network stack called
netsvc that has a tendency to remain
available even after problems in higher-level software. The netsvc suite is
also always available in "bringup" product configurations, as such,
is most useful when working on low-level software, such as the Zircon kernel,
See Viewing Logs for more information.
fx cp provides a basic wrapper around
scp, similar to how
fx shell is a
# copy ./book.txt from the host, to /tmp/book.txt on the target $ fx cp book.txt /tmp/book.txt # copy /tmp/poem.txt on the target to poem.txt on the host $ fx cp --to-host /tmp/poem.txt poem.txt
Using multiple Fuchsia devices
Some users will have more than one Fuchsia device on a network, and will want
to limit the effects of various commands to particular of those devices. The
fx set-device command exists to help with this use case.
fx set-device command binds a particular device node name to a
particular build directory. This is particularly useful when a user wishes to
keep several different devices in several build configurations, and could be
setup as follows:
$ fx --dir out/workstation set workstation_eng.x64 $ fx build $ fx set-device <workstation-node-name> $ fx --dir out/core set core.arm64 $ fx build $ fx set-device <core-node-name> # Start a server for the workstation: $ fx --dir=out/workstation serve # Set the default build-dir and target device to the arm64 core, and # connect to a shell on that device: $ fx use out/core $ fx shell
Additionally, for users who wish to execute a command against a single
Fuchsia device from the current default build directory, as a one-off
fx global flag
-d allows overriding the target node name for
a single command invocation.
Reboot a device
On some devices (most arm64 devices at present) there are also some useful flags:
fx reboot -rreboot into "recovery" (Zedboot)
fx reboot -breboot into "bootloader" (Flash)
Determine a CL's status
fx whereiscl <query>
This command tells whether the given change is merged, and if so whether it passed
Global Integration. The query can be either a Gerrit review URL, a change number, a
Change-Id, or a git revision.
$ fx whereiscl fxr/286748 CL status: MERGED GI status: PASSED $ fx whereiscl https://fuchsia-review.googlesource.com/c/fuchsia/+/287311/1/garnet/go/src/amber/source/source.go CL status: NEW $ fx whereiscl I94c56fa4e59842d398bfa90a48c45b388f095184 CL status: MERGED GI status: PASSED $ fx whereiscl 6575aee CL status: MERGED GI status: PENDING
Debugging and developing
-xflag turns on tracing for the
fxscripts, printing out all expressions evaluated during the
fx execexecutes an arbitrary program that follows inside of the current
fxenvironment. As an example
fx exec envprints all environment variables in that environment (
fx exec env | grep FUCHSIAis likely of interest).
Getting help with
fx help <command> provides the best introductory documentation for that
command. Some commands also support and provide
fx <command> -h or
fx <command> --help, however this help is not available for all commands.
This is unusual, but is a function of implementation details. Internally many
fx commands just run other programs, most often those produced by the
build, and flags are in many cases passed on unaltered to those programs. In
those cases, passing the usual
--help flags may not provide
fx <command>, but instead for the program invoked
Users should always start with
fx help <command>.
fx help with no other arguments provides a list of all available commands
fx, as well as documentation for
fx global flags.
Displaying pending commits
fx pending-commits displays the commits not yet rolled to global
To view Fuchsia's integration dashboard, see Builders.
Synchronizing the Fuchsia checkout to a release branch
To synchronize your local Fuchsia checkout to a specific release branch, run the following command:
fx sync-to RELEASE_BRANCH
RELEASE_BRANCH with the release branch that you want to switch to
refs/heads/releases/f6). To see the list of all available
release branches, see the Branches section on the
Fuchsia global integration repository page.
The example command below synchronizes the Fuchsia checkout to the F6 release branch:
fx sync-to refs/heads/releases/f6
To reset the Fuchsia checkout to the top of the tree, run the following command:
fx sync-to reset
For more options, see the
fx sync-to reference page.
Defining persistent local build arguments
If you want to define build arguments that will be included whenever running
fx set, add them to
$FUCHSIA_DIR/local/args.gn. They will be appended to
$FUCHSIA_BUILD_DIR/args.gn whenever regenerating the build arguments.
To suppress the inclusion of
fx set ... --skip-local-args.