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u-root embodies four different projects.

  • Go versions of many standard Linux tools, such as ls, cp, or shutdown. See cmds/core for most of these.

  • A way to compile many Go programs into a single binary with busybox mode.

  • A way to create initramfs (an archive of files) to use with Linux kernels.

  • Go bootloaders that use kexec to boot Linux or multiboot kernels such as ESXi, Xen, or tboot. They are meant to be used with LinuxBoot. With that, parsers for GRUB config files or syslinux config files are to make transition to LinuxBoot easier.


Make sure your Go version is 1.13. Make sure your GOPATH is set up correctly. While u-root uses Go modules, it still vendors dependencies and builds with GO111MODULE=off.

Download and install u-root:

go get

You can now use the u-root command to build an initramfs. Here are some examples:

# Build an initramfs of all the Go cmds in ./cmds/core/... (default)

# Generate an archive with bootloaders
# core and boot are templates that expand to sets of commands
u-root core boot

# Generate an archive with only these given commands
u-root cmds/core/{init,ls,ip,dhclient,wget,cat,elvish}

# Generate an archive with all of the core tools with some exceptions
u-root core -cmds/core/{installcommand,losetup}

# Generate an archive with a tool outside of u-root
u-root cmds/core/{init,ls,elvish}

The default set of packages included is all packages in

In addition to using paths to specify Go source packages to include, you may also use Go package import paths (e.g. to include commands. Only the main package and its dependencies in those source directories will be included. For example:

You can build the initramfs built by u-root into the kernel via the CONFIG_INITRAMFS_SOURCE config variable or you can load it separately via an option in for example Grub or the QEMU command line or coreboot config variable.

Extra Files

You may also include additional files in the initramfs using the -files flag. If you add binaries with -files are listed, their ldd dependencies will be included as well. As example for Debian, you want to add two kernel modules for testing, executing your currently booted kernel:

NOTE: these files will be placed in the $HOME dir in the initramfs.

u-root -files "$HOME/hello.ko $HOME/hello2.ko"
qemu-system-x86_64 -kernel /boot/vmlinuz-$(uname -r) -initrd /tmp/initramfs.linux_amd64.cpio

To specify the location in the initramfs, use <sourcefile>:<destinationfile>. For example:

u-root -files "root-fs/usr/bin/runc:usr/bin/run"

Init and Uinit

u-root has a very simple (exchangable) init system controlled by the -initcmd and -uinitcmd command-line flags.

  • -initcmd determines what /init is symlinked to. -initcmd may be a u-root command name or a symlink target.
  • -uinitcmd is run by the default u-root init after some basic file system setup. There is no default, users should optionally supply their own. -uinitcmd may be a u-root command name with arguments or a symlink target with arguments.
  • After running a uinit (if there is one), init will start a shell determined by the -defaultsh argument.

We expect most users to keep their -initcmd as init, but to supply their own uinit for additional initialization or to immediately load another operating system.

All three command-line args accept both a u-root command name or a target symlink path. Only -uinitcmd accepts command-line arguments, however. For example,

u-root -uinitcmd="echo Go Gopher" ./cmds/core/{init,echo,elvish}

cpio -ivt < /tmp/initramfs.linux_amd64.cpio
# ...
# lrwxrwxrwx   0 root     root           12 Dec 31  1969 bin/uinit -> ../bbin/echo
# lrwxrwxrwx   0 root     root            9 Dec 31  1969 init -> bbin/init

qemu-system-x86_64 -kernel $KERNEL -initrd /tmp/initramfs.linux_amd64.cpio -nographic -append "console=ttyS0"
# ...
# [    0.848021] Freeing unused kernel memory: 896K
# 2020/05/01 04:04:39 Welcome to u-root!
#                              _
#   _   _      _ __ ___   ___ | |_
#  | | | |____| '__/ _ \ / _ \| __|
#  | |_| |____| | | (_) | (_) | |_
#   \__,_|    |_|  \___/ \___/ \__|
# Go Gopher
# ~/>

The command you name must be present in the command set. The following will not work:

u-root -uinitcmd="echo Go Gopher" ./cmds/core/{init,elvish}
# 2020/04/30 21:05:57 could not create symlink from "bin/uinit" to "echo": command or path "echo" not included in u-root build: specify -uinitcmd="" to ignore this error and build without a uinit

You can also refer to non-u-root-commands; they will be added as symlinks. We don't presume to know whether your symlink target is correct or not.

This will build, but not work unless you add a /bin/foobar to the initramfs.

u-root -uinitcmd="/bin/foobar Go Gopher" ./cmds/core/{init,elvish}

This will boot the same as the above.

u-root -uinitcmd="/bin/foobar Go Gopher" -files /bin/echo:bin/foobar ./cmds/core/{init,elvish}

This will bypass the regular u-root init and just launch a shell:

u-root -initcmd=elvish ./cmds/core/{elvish,ls}

cpio -ivt < /tmp/initramfs.linux_amd64.cpio
# ...
# lrwxrwxrwx   0 root     root            9 Dec 31  1969 init -> bbin/elvish

qemu-system-x86_64 -kernel $KERNEL -initrd /tmp/initramfs.linux_amd64.cpio -nographic -append "console=ttyS0"
# ...
# [    0.848021] Freeing unused kernel memory: 896K
# failed to put myself in foreground: ioctl: inappropriate ioctl for device
# ~/>

(It fails to do that because some initialization is missing when the shell is started without a proper init.)

Cross Compilation (targeting different architectures and OSes)

Cross-OS and -architecture compilation comes for free with Go. In fact, every PR to the u-root repo is built against the following architectures: amd64, x86 (i.e. 32bit), mipsle, armv7, arm64, and ppc64le.

Further, we run integration tests on linux/amd64, freebsd/amd64 and linux/arm64, using several CI systems. If you need to add another CI system, processor or OS, please let us know.

To cross compile for an ARM, on Linux:

GOARCH=arm u-root

If you are on OSX, and wish to build for Linux on AMD64:

GOOS=linux GOARCH=amd64 u-root

Testing in QEMU

A good way to test the initramfs generated by u-root is with qemu:

qemu-system-x86_64 -nographic -kernel path/to/kernel -initrd /tmp/initramfs.linux_amd64.cpio

Note that you do not have to build a special kernel on your own, it is sufficient to use an existing one. Usually you can find one in /boot.

If you quickly need to obtain a kernel, for example, when you are on a non-Linux system, you can assemble a URL to download one through Arch Linux's iPXE menu file. It would download from ${mirrorurl}iso/${release}/arch/boot/x86_64/vmlinuz, so just search for a mirror URL you prefer and a release version, for example,


For framebuffer support, append a VESA mode via the vga kernel parameter:

qemu-system-x86_64 \
  -kernel path/to/kernel \
  -initrd /tmp/initramfs.linux_amd64.cpio \
  -append "vga=786"

For a list of modes, refer to the Linux kernel documentation.

Entropy / Random Number Generator

Some utilities, e.g., dhclient, require entropy to be present. For a speedy virtualized random number generator, the kernel should have the following:


Then you can run your kernel in QEMU with a virtio-rng-pci device:

qemu-system-x86_64 \
    -device virtio-rng-pci \
    -kernel vmlinuz \
    -initrd /tmp/initramfs.linux_amd64.cpio

In addition, you can pass your host's RNG:

qemu-system-x86_64 \
    -object rng-random,filename=/dev/urandom,id=rng0 \
    -device virtio-rng-pci,rng=rng0 \
    -kernel vmlinuz \
    -initrd /tmp/initramfs.linux_amd64.cpio


SystemBoot is a set of bootloaders written in Go. It is meant to be a distribution for LinuxBoot to create a system firmware + bootloader. All of these use kexec to boot. The commands are in cmds/boot.

  • pxeboot: a network boot client that uses DHCP and HTTP or TFTP to get a boot configuration which can be parsed as PXELinux or iPXE configuration files to get a boot program.

  • boot: finds all bootable kernels on local disk, shows a menu, and boots them. Supports (basic) GRUB, (basic) syslinux, (non-EFI) BootLoaderSpec, and ESXi configurations.

  • fbnetboot: a network boot client that uses DHCP and HTTP to get a boot program based on Linux, and boots it. To be merged with pxeboot.

  • localboot: a tool that finds bootable kernel configurations on the local disks and boots them.

  • systemboot: a wrapper around fbnetboot and localboot that just mimicks a BIOS/UEFI BDS behaviour, by looping between network booting and local booting. Use -uinitcmd argument to the u-root build tool to make it the boot program.

This project started as a loose collection of programs in u-root by various LinuxBoot contributors, as well as a personal experiment by Andrea Barberio that has since been merged in. It is now an effort of a broader community and graduated to a real project for system firmwares.

More detailed information about the build process for a full LinuxBoot firmware image using u-root/systemboot and coreboot can be found in the LinuxBoot book chapter about LinuxBoot using coreboot, u-root and systemboot.

You can build systemboot like this:

u-root -build=bb -uinitcmd=systemboot core{systemboot,localboot,fbnetboot}


You can compress the initramfs. However, for xz compression, the kernel has some restrictions on the compression options and it is suggested to align the file to 512 byte boundaries:

xz --check=crc32 -9 --lzma2=dict=1MiB \
   --stdout /tmp/initramfs.linux_amd64.cpio \
   | dd conv=sync bs=512 \

Getting Packages of TinyCore

Using the tcz command included in u-root, you can install tinycore linux packages for things you want.

You can use QEMU NAT to allow you to fetch packages. Let's suppose, for example, you want bash. Once u-root is running, you can do this:

% tcz bash

The tcz command computes and fetches all dependencies. If you can't get to, or you want package fetching to be faster, you can run your own server for tinycore packages.

You can do this to get a local server using the u-root srvfiles command:

% srvfiles -p 80 -d path-to-local-tinycore-packages

Of course you have to fetch all those packages first somehow :-)

Build an Embeddable U-root

You can build this environment into a kernel as an initramfs, and further embed that into firmware as a coreboot payload.

In the kernel and coreboot case, you need to configure ethernet. We have a dhclient command that works for both ipv4 and ipv6. Since v6 does not yet work that well for most people, a typical invocation looks like this:

% dhclient -ipv4 -ipv6=false

Or, on newer linux kernels (> 4.x) boot with ip=dhcp in the command line, assuming your kernel is configured to work that way.

Build Modes

u-root can create an initramfs in two different modes:

  • source mode includes Go toolchain binaries + simple shell + Go source files in the initramfs archive. Tools are compiled from source on the fly by the shell.

    When you try to run a command that is not built, it is compiled first and stored in tmpfs. From that point on, when you run the command, you get the one in tmpfs. Don't worry: the Go compiler is pretty fast.

  • bb mode: One busybox-like binary comprising all the Go tools you ask to include. See here for how it works.

    In this mode, u-root copies and rewrites the source of the tools you asked to include to be able to compile everything into one busybox-like binary.

Updating Dependencies

go get -u
go mod tidy
go mod vendor


If you want to see u-root on real hardware, this board is a good start.


For information about contributing, including how we sign off commits, please see

Improving existing commands (e.g., additional currently unsupported flags) is very welcome. In this case it is not even required to build an initramfs, just enter the cmds/ directory and start coding. A list of commands that are on the roadmap can be found here.

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