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Tips And Tricks for ChromiumOS Developers

This page contains tips and tricks for developing on ChromiumOS.

The free-form content on this page is contributed by developers. Before adding content to this page, consider whether the content might fit better in a different document, such as the ChromiumOS Developer Guide or the ChromiumOS Developer FAQ:

*** note Note: The tips on this page generally assume that you've followed the instructions in the ChromiumOS Developer Guide to build your image.

Device Tips

How to get more commands available on the release image

If you're using the shell in a ChromiumOS device (requires developer mode, or a system where you've set a custom chronos password), you may find that you're missing commands you wish you had. Try putting busybox on a USB key or an SD Card. I formatted by USB key with ext3 and named it utils (and that is assumed in these instructions) using the Disk Utility that comes with Ubuntu (go to the System menu at the top of the screen, then Administration). Since busybox is built as part of a normal build (it's used in the factory image), you can copy it to your USB key like this (run from outside the chroot).

sudo chown root:root /media/utils
sudo chmod o+rx /media/utils
sudo cp ~/chromiumos/chroot/build/${BOARD}/bin/busybox /media/utils/
sudo chmod o+rx /media/utils/busybox

Then, you can go add symlinks for whatever your favorite busybox commands are:

for cmd in less zip; do
  sudo ln -s busybox /media/utils/$cmd
done

Unmount your USB key:

sudo umount /media/utils

Plug your USB key into your ChromiumOS device. Make sure that the browser has started (i.e., you're not still on the login screen) so that ChromiumOS will mount your USB key to /media/utils. You can get access to your commands with:

sudo mount -o remount,exec /media/utils
export PATH="$PATH:/media/utils"

If you want a list of all of the commands that busybox supports, you can just run busybox to get a list. If you want to run a busybox command without making a symlink, just type busybox first. Like: busybox netstat.

*** note SIDE NOTE: If you didn't build ChromiumOS, it's possible that you can run a busybox binary obtained from some other source (e.g. official binaries. You'll have the best luck if the busybox that you get was statically linked.

How to wipe the stateful partition

You can use:

crossystem clear_tpm_owner_request=1 && reboot

You can also reflash the DUT with with the --clobber-stateful flag enabled:

cros flash myDUTHostName --clobber-stateful ${BOARD}/latest

How to enable a local user account

This is not to be confused with how to enable the chronos account. This set of instructions allows you to login to the browser as something other than guest without having any network connectivity. Most people don't need to do this.

The local user account allows login with no password even if you can not connect to the Internet. If you are customizing ChromiumOS and having trouble logging in due to your customizations, it may be handy to be able to bypass authentication and log yourself in as a test user. This is disabled by default for security reasons, but if you want to enable it for a backdoor user USERNAME, enter the following from inside the chroot:

***note Note: You will be prompted to enter a password. The -d flag preserves existing user accounts on the device.

ssh myDUTHostName "/usr/local/autotest/bin/autologin.py -d -u '${USERNAME}'"

How to SSH to DUT without a password

The cros shell command can be used to quickly connect to a DUT and use the default set of testing ssh keys. There's no need to setup configs or keys yourself with it. It can be used in place of ssh for interactive shells & running arbitrary remote commands. It works inside & outside the SDK.

# Interactive session.
$ cros shell localhost:9222
Warning: Permanently added '[localhost.corp.google.com]:9222' (ED25519) to the list of known hosts.
localhost ~ #

# Run a remote command.
$ cros shell localhost:9222 uname -r
5.15.94-16357-g93a4635b23be

How to get a fully functional vim on a test device

By default there is a very minimalist build of vim included on test images. To replace it with a fully functional copy of vim do the following:

export BOARD=eve # Replace with the board name of your test device
export TESTHOST=127.0.0.1 # Replace with the hostname or ip address of the test device
export JOBS=$(nproc)

emerge-${BOARD} --noreplace --usepkg -j${JOBS} app-editors/vim-core app-vim/gentoo-syntax
USE=-minimal emerge-${BOARD} -j${JOBS} app-editors/vim

cros deploy "${TESTHOST}" app-vim/gentoo-syntax app-editors/vim-core app-editors/vim

How to get a larger serial console on a test device

By default, the AP serial console might act as though it is 80 columns and 24 lines, even if your terminal window is much larger. It might scroll and wrap prematurely while listing long directories or running programs like vim or more. To increase the effective size, do this:

stty rows 50 cols 132

The actual numbers can vary depending on window size.

CrOS Chroot Tips

How to modify my prompt to tell me which git branch I'm in

Add the following to your ~/.bash_profile:

export PS1='\h:\W$(__git_ps1 "(%s)") \u\$ '

How to search the code quickly

When you installed depot_tools, you got a tool called git-gs that's useful for searching your code. According to the depo_tools info page, git-gs is a "Wrapper for git grep with relevant source types". That means it should be the fastest way to search through your code.

You can use it to search the git project associated with the current directory by doing:

git gs ${SEARCH_STRING}

If you want to search all git projects in the repo project associated with the current directory:

repo forall -c git gs ${SEARCH_STRING}

On Linux, you can install Silversearcher, which is a faster and smarter version of ack/grep. Ag is very fast and works independent of git repositories, so you can do queries from your top-level folder.

sudo apt-get install silversearcher-ag
# For keyword based search
ag "keyword" .
# To ignore certain paths
ag --ignore chroot/ "keyword" .
# ag has the same syntax as grep. For example, excluding files that match a pattern
ag -L "keyword" .

How to free up space after lots of builds

Use cros clean outside the chroot. This should take care of purging your .cache, builds downloaded from devserver, etc.

How to share files for inside and outside the chroot

The cros_sdk command supports mounting additional directories into your chroot environment. This can be used to share editor configurations, a directory full of recovery images, etc.

You can create a src/scripts/.local_mounts file listing all the directories (outside of the chroot) that you'd like to access inside the chroot. For example:

# source(path outside chroot) destination(path inside chroot)
/usr/share/vim/google
/home/${USER}/Downloads /Downloads

Each line of .local_mounts refers to a directory you'd like to mount, and where you'd like it mounted. If there is only one path name, it will be used as both the source and the destination directory. If there are two paths listed on one line, the first is considered to be the path OUTSIDE the chroot and the second will be the path INSIDE the chroot. The source directory must exist; otherwise, cros_sdk will give off an ugly python error and fail to enter the chroot.

***note Note: if sharing your .bashrc file for development purposes, be very wary of how its contents might interact with the chroot. A known issue is a bashrc that exports PYTHONPYCACHEPREFIX (or any python variables), which will conflict with what emerge expects and may lead to permissions errors.

How to symbolize a crash dump from a ChromiumOS device

There is a nifty little script in your chroot, under chromiumos/src/scripts called cros_show_stacks. The script takes in the parameters: board name, IP of the remote device you want to fetch and symbolize crashes from, and a path to the debug symbols. If the path is not specified, it picks it up from your local build correctly (/build/$BOARD/usr/lib/debug/breakpad).

Typical usage. Note that /Downloads below refers to the Downloads folder outside the chroot, the shared folder mapping is specified in ~/chromiumos/src/scripts/.local_mount.

./cros_show_stacks --board=eve --remote $DUT_IP --breakpad_root /Downloads/eve_debug_syms/debug/breakpad/

How to make sudo a little more permissive

***note If you are at Google, follow the instructions on the internal workstation notes page instead.

Follow these instructions to make sudo request your password less frequently:

cd /tmp
cat > ./sudo_editor <<EOF
#!/bin/sh
[ "\$1" == "--" ] && shift                 # visudo might pass '--' arg
echo Defaults \!tty_tickets > \$1          # Entering your password in one shell affects all shells
echo Defaults timestamp_timeout=180 >> \$1 # Time between re-requesting your password, in minutes
EOF
chmod +x ./sudo_editor
sudo EDITOR=./sudo_editor visudo -f /etc/sudoers.d/relax_requirements

***note Note: See the sudoers man page for full detail on the options available in sudoers.

***note Note: depending on your Linux distribution's configuration, sudoers.d may not be the correct directory, you may check /etc/sudoers for an #includedir directive to see what the actual directory is.

How to use the chroot's ClangD Language Server

You can leverage IDE-like functionality in your editor by following this guide to use the clangd already installed in the chroot.

How to use the chroot's rust-analyzer Language Server

rust-analyzer provides IDE-like functionality for the Rust language, similar to what clangd provides for C++. In order to use the rust-analyzer in the chroot, you'll need to

For details and more thorough instructions see chromiumos/src/third_party/toolchain-utils/rust-analyzer-chromiumos-wrapper/README.md.

Repo Tips

How to set up repo bash completion

Most distributions (Ubuntu included) come with repo available for installation. See the upstream repo install instructions for more details.

Installing repo will set up bash completion automatically.

How to configure repo to sync private repositories

Create a file called .repo/local_manifests/my_projects.xml and add your private project into it.

<?xml version="1.0" encoding="UTF-8"?>
<manifest>
  <remote  name   = "private"
           fetch  = "https://chromium.googlesource.com"
           review = "https://chromium-review.googlesource.com/" />
  <project path   = "src/thirdparty/location"
           name   = "nameofgitrepo"
           remote = "private" />
</manifest>

If you want to pull in from a different git server, you will need to add a different remote for the project. Please type repo manifest -o tmp.xml to dump your current manifest to see an example. More documentation on the manifest file format is available on the repo Manifest format docs.

How to make repo sync less disruptive by running the stages separately

Just running repo sync does both the network part (fetching updates for each repository) and the local part (attempting to merge those updates in to your tree) in one go. This has a couple of drawbacks. Firstly, because both stages are mixed up, you need to avoid making changes to your tree for the whole time, including during the slow network part. Secondly, errors telling you which repositories weren't updated because of your local changes get lost among the many messages about the update fetching.

To alleviate these problems, you can run the two stages separately using the -n (network) and -l (local) switches. First, run repo sync -n. This performs the network stage, fetching updates for each of the repositories in your tree, but not applying them. While it's running, you can continue working as usual. Once it's finished and you've come to a convenient point in your work, run repo sync -l. This will perform the local stage, actually checking out the updated code in each repository where it doesn't conflict with your changes. It generally only prints output when such a conflict occurs, which makes it much easier to see which repositories you need to manually update. You can run it multiple times until all the conflicts are fixed.

How to find a list of branches that I've created with repo

See the repo branches command (i.e. repo help branches).

Gerrit and Crbug Tips

Be sure to also take a look at the git and gerrit intro.

How to refer to bugs concisely

If you have a bug number and want to send it to someone else in an email, you can use the crbug shortcut. So, to refer to bug 1234, you'd refer to:

https://crbug.com/1234

How to refer to change-lists (CLs) concisely

When you want to send a short link for a chromium CL you can make short-links such as:

https://crrev.com/c/1234

See https://crrev.com for instructions on linking to Chrome (internal) CLs or making links based on the git hash.

How to fetch patches/changes from a Gerrit CL with the Command Line

Very often, you may want to fetch changes from someone else's or your own CL uploaded for code-review on chromium-review.googlesource.com. If you only want to download and test a single change use the following repo command:

repo download -c 344778

If you want to download a specific patch set you can add a / followed by the patch set number (e.g. 344778/2). You can also list multiple CLs separated by spaces.

***note Note: this will clear other changes you may have already cherry-picked. If you need to test multiple commits you either need to list them all or use the git command below.

If you are more familiar with working with git, you can use the following. Make sure to be cd-ed into the correct directory for that git repository. Replace cros with whatever remote name is listed in your .git/config (e.g. origin, aosp, etc.).

COMMIT_HASH=2a22b9dc94c9c13dbff0a1c397f49cb6456f4f2c
git fetch cros "${COMMIT_HASH}" && git cherry-pick FETCH_HEAD

This works because the CLs are stored on the same remote git server as where all the branches and refs reside. You can switch to a branch that has its HEAD pointing to a CL, based on just the CL id as follows.

Continuing the first example, to fetch https://crrev.com/c/344778/ in a local branch called test-branch:

repo download -b test-branch 344778