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fx workflows

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 bash or zsh as a shell, source scripts/ to get some auto-completion.

Setting up fx

It is strongly recommended that you source scripts/ 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
$ echo "source \"$PWD/scripts/\"" >> "$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 $FUCHSIA_DIR/scripts/fx explicitly
  • Use a tool like dotenv to add $FUCHSIA_DIR/.jiri_root/bin to your $PATH while 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 with this:

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? (unsure: try workstation)
  • What board are you building for? (unsure: try x64)
  • What extra test targets do you want? (unsure: try //bundles:tools, and if you're working on features, you probably want //bundles:tests)

Armed with our above choices (if you didn't read above, do so now), you are ready to configure your build:

$ fx set workstation.x64 --with //bundles:tests

This command stores the configuration in an file in the build directory (which is out/default by default). You can edit this file using the fx args command to create more elaborate configurations.

What just happened?

  • You selected the product workstation (run fx list-products for a list of other product configurations).
  • You selected the board x64, which supports many typical boards based on the x64 architecture. (Note that arm64 boards are less interchangable, and you will most likely need to give fx set the specific board, when building to an arm64 architecture. Run fx list-boards for a list of known board configurations.)
  • You are ephemerally building tests as part of the universe package set, not as a part of the paving images.

Package deployment options

The --with option has three variants related to how packages are deployed to a Fuchsia device: --with-base, --with-cache, and --with (implies universe). (Note, fx set also has a --with-host option, for building host-only targets, such as host-based tools and libraries.)

So what are base, cache and universe?

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:

  • bringup is 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 fx commands such as fx serve and fx shell cannot be used with the bringup product.
  • core is 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 is 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

The --with flag for fx set takes in arbitrary build targets. 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 following bundles:

  • //bundles:tools contains 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:tools in the universe package set by default.
  • //bundles:tests causes all test programs to be built. Most test programs can be invoked using run-test-component on the device, or via fx run-test.
  • //bundles:kitchen_sink is 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

For most use cases, only fx build is needed. fx build builds both Zircon and the Fuchsia portions of the build. The build process is optimized for fast incremental rebuilds, as such, repeating this command does the minimal work required after code has been changed, and no work if the source files are unchanged.

Additionally to fx build, a few other build related commands provide more granular control:

  • fx clean clear out all build artifacts.
  • fx clean-build perform a clean, then a build.
  • fx gen repeat the gn gen process that fx set performed. Users making fine grained build argument changes (e.g. by editing directly) can run fx gen to reconfigure their build.

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 <output_dir>/host_x64/blah.

See the build system overview for a more detailed discussion of build targets.

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 convenient behind fx commands:

  • fx flash is used with most arm64 devices to perform a raw write of Zedboot to the device, preparing it for Paving.
  • fx mkzedboot is used with most x64 devices 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 system, execute 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. There are additional instructions for preparing a Pixelbook.

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 fx flash, or by booting a Zedboot USB key made by fx mkzedboot, then executing fx pave on the host system. In general most users actually will want to use fx serve instead of fx pave. fx serve is covered in the serve a build section.

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 using either 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".

The command fx serve performs two functions internally:

  • fx pave start a paving server, used for "fresh installs" of a Fuchsia device from a Zedboot state.
  • fx serve-updates start a package repository server, used for dynamic installation of software at runtime, as well as whole-system updates.

Internally the fx serve-updates command also searches for a device to configure, and upon discovery (which may be restricted/modulated with fx set-device or fx -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.

As the 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.

The 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 components, use basemgr_launcher shutdown.

Execute tests

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 runtests and /bin/run-test-component. 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 rolldice-tests

The above command will execute the rolldice tests every time a change is made to the source code in the tree. The -i flag to fx causes fx to repeat the rest of its command every time the source code in the tree is changed. As the 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 /hub and /dev. It is also useful for invoking programs such as /bin/run that provides facilities for launching Fuchsia components. If the tools bundle is available in the build configuration, many tools common to unix shell environments have been ported and are available, such as ps, ls, cat, curl, vim, fortune and so on.

Performing other common tasks

Getting logs

fx log captures all logs from low-level and high-level programs, including the kernel, drivers and other userspace programs. fx log depends upon a working high level network stack and SSH. As such, fx log 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 switch 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, fx klog is most useful when working on low-level software, such as the Zircon kernel, or drivers.

See Viewing Logs for more information.

Copying files

fx cp provides a basic wrapper around scp, similar to how fx shell is a wrapper around ssh.

# 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

Start Fuchsia in an Emulator

fx emu starts a Fuchsia build under the Fuchsia emulator, a general purpose virtual machine.

In order to run ephemerally delivered programs, users will need to setup TAP based networking, the full details of which are beyond the scope of this document. A quick overview is as follows:

On macOS: Install "" On Linux: Run sudo ip tuntap add dev qemu mode tap user $USER && sudo ip link set qemu up

Then to run the emulator using TAP networking, execute fx emu -N. You can attach a package server by running: fx serve as you would with a physical target device.

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.

The 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.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 command, the fx global flag -d allows overriding the target node name for a single command invocation.

Reboot a device

fx reboot

On some devices (most arm64 devices at present) there are also some useful flags:

  • fx reboot -r reboot into "recovery" (Zedboot)
  • fx reboot -b reboot 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
CL status: NEW

$ fx whereiscl I94c56fa4e59842d398bfa90a48c45b388f095184
CL status: MERGED
GI status: PASSED

$ fx whereiscl 6575aee
CL status: MERGED
GI status: PENDING

Debugging and developing fx commands

  • fx -x the -x flag turns on tracing for the fx scripts, printing out all expressions evaluated during the fx invocation.
  • fx exec executes an arbitrary program that follows inside of the current fx environment. As an example fx exec env prints all environment variables in that environment (fx exec env | grep FUCHSIA is likely of interest).

Getting help with fx

fx help <command> provides the best introductory documentation for that command. Some commands also support/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 -h or --help flags may not provide documentation for fx <command>, but instead for the program invoked downstream of fx.

Users should always start with fx help <command>.

fx help with no other arguments provides a list of all available commands in fx, as well as documentation for fx global flags.

Displaying pending commits

fx pending-commits displays the commits not yet rolled to global integration.

To view Fuchsia's integration dashboard, see Builders.