A universal root: u-root creates a root file system (initramfs) containing a busybox-like set of tools in Go.

go get



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u-root is a "universal root". It's a root file system with mostly Go source with the exception of 5 binaries.

u-root contains simple Go versions of many standard Linux tools, similar to busybox. u-root can create an initramfs in two different modes:

  • source mode: Go toolchain binaries + simple shell + Go source for tools to be compiled on the fly by the shell.

    When you run a command that is not built, you fall through to the command that does a go build of the command, and then execs the command once it is built. From that point on, when you run the command, you get the one in tmpfs. This is fast.

  • bb mode: One busybox-like binary comprised of all the Go tools you ask to include.

    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.

That's the interesting part. This set of utilities is all Go, and mostly source.


Make sure your Go version is the latest (>=1.9). Make sure your GOPATH is set up correctly.

Download and install u-root:

go get

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

# Build a bb-mode cpio initramfs of all the Go cmds in ./cmds/...
u-root -build=bb

# Generate a cpio archive named initramfs.cpio.
u-root -format=cpio -build=source -o initramfs.cpio

# Generate a bb-mode archive with only these given commands.
u-root -format=cpio -build=bb ./cmds/{ls,ip,dhclient,wget,tcz,cat}

-format=cpio and -build=source are the default flag values. 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:

# Both are required for the elvish shell.
go get
go get
u-root -build=bb ./cmds/*

Side note: elvish is a nicer shell than our default shell rush; and also written in Go.

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.

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

qemu-system-x86_64 -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.

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:

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

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.


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



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.