Logging

Enable verbose logging

You can view verbose logs (VLOGs and LOG(VERBOSE)) by passing runtime arguments to enable logging. You must first enable passing runtime arguments; see the Getting Started.

Once you’ve done that, specific flags need to be added to the /etc/chrome_dev.conf file.

Note: vim as-is on ChromeOS devices behaves like classic vi. If you haven't done so, you can create a .vimrc file with the command below:

$ echo "set nocompatible backspace=2" >> ~/.vimrc

You can now use vim as expected. Add the following lines to the /etc/chrome_dev.conf file:

--enable-logging
--v=1

You can enable verbose logging (printed by the VLOG() macros) of a given log level and for particular modules (in this case, I chose bluetooth, dbus, bluez) like so:

--vmodule=*bluetooth*=1,*dbus*=1,*bluez*=2

The flags in the /etc/chrome_dev.conf file are added as arguments when the chrome binary is run during startup. In the same way, you can enable logging when running tests by adding args like so:

$ ./out/Default/extensions_browsertests --gtest_filter=*BluetoothSocket* --vmodule=*bluetooth*=1,*dbus*=1,*bluez*=2

Viewing logs

There are 4 separate log files kept by ChromeOS. Those log files are:

1. /var/log/messages

   1. Logged to by the actual operating system or kernel.
   2. Past files are kept as /var/log/messages.1, /var/log/messages.2, etc.

2. /var/log/chrome/chrome

   1. Logged to by chrome, but only before the user logs in. However, in dev-mode, it’s also logged to during a user session. Therefore, this will be the primary log file you use when developing.
   2. In non-dev-mode devices, you will only need to inspect this file if you are curious about operations that occur before user login.
   3. /var/log/chrome/chrome is a symbolic link to the current log file. The current log file and all past files are formatted like /var/log/chrome/chrome_YYYYMMDD-HHDDMM. There is also a symbolic link to the log file for the previous session, called /var/log/chrome/chrome.PREVIOUS.

3. /var/log/ui/ui.LATEST

   1. You will generally never use this file in dev-mode devices.
   2. Logged to by chrome after user login, but before it creates the /home/chronos/user/log/chrome log file.
   3. /var/log/ui/ui.LATEST is a symbolic link the current log file. The current log file and all past files are formatted like /var/log/ui/ui.20170331-101012.

4. /home/chronos/user/log/chrome

   1. Only exists in non dev-mode devices. You will generally never use this file.
   2. Logged to by chrome after user login.
   3. You will primarily be inspecting logs from this file.
   4. /home/chronos/user/log/chrome is a symbolic link the current log file. The current log file and all past files are formatted like /home/chronos/user/log/chrome_20170330-182904.
   5. (For context, chronos is the name of the user which launches the chrome executable)

You can simply read through the logs with vim or cat. The files can be fairly spammy, so I recommend using cat or tail with grep like so:

$ cat /var/log/chrome/chrome | grep -e “keyword 1” -e keyword2

Files in chrome can also be accessed directly with file:// urls. For example, to view /var/log/chrome/chrome, you can open up the browser and enter:

file:///var/log/chrome/chrome

If you wish to move log files from your test device onto your workstation and your workstation is a ChromeOS device, you can use Nearby Share to transfer files over. In order to use Nearby Share, the log files must be accessible from the Files app. To move a file, such as /var/log/chrome/chrome, into the Downloads folder, ssh into your test device, then run:

mv /var/log/chrome/chrome /home/chronos/user/MyFiles/Downloads