Lock on unplug

   🏷 android 🏷 hack

Here’s a hack for Gnome (vaguely relevant to Android too) that I’ve been using for a little while. When I get up from a computer, I generally want to lock the screensaver. I also pick up my phone from a USB dock (not the Nexus One Desktop Dock; that doesn’t connect to a computer). So why not have the screensaver lock automatically when the phone disappears from USB?

This is helpful when you’re hurriedly picking up the phone to answer a call, and might otherwise forget to lock the screen. Of course, if you get a lot of calls that you want to answer while looking at your computer, this is not for you. :-)

This is pretty easy to achieve using udev. First we ask udev to run a script when this device disappears. The following goes in /etc/udev/rules.d/51-android.rules (or a similar name; the number is used for ordering). While I’m at it, I’ll set sensible permissions and group ownership, using the first line.

# Set mode and group for (some) devices made by HTC
SUBSYSTEM=="usb", ATTRS{idVendor}=="18d1", MODE="0660", GROUP="mygroup"
# Nexus One without debugging enabled
SUBSYSTEM=="usb", ATTRS{idVendor}=="18d1", ATTRS{idProduct}=="4e11", \
  ATTRS{serial}=="HT123P456789", ENV{IS_MY_PHONE}="yes"
# Nexus One with debugging enabled
SUBSYSTEM=="usb", ATTRS{idVendor}=="18d1", ATTRS{idProduct}=="4e12", \
  ATTRS{serial}=="HT123P456789", ENV{IS_MY_PHONE}="yes"
# You could unlock on connect if you want:
#ENV{IS_MY_PHONE}=="yes", ACTION=="add", RUN+="/somewhere/unlock"
# Lock when my phone goes away
ENV{IS_MY_PHONE}=="yes", ACTION=="remove", RUN+="/somewhere/lock"
  • The device doesn’t have to be a Nexus One or a phone at all, run lsusb to find youridVendor/idProduct values (for Android, this may differ when you turn on/off USB debugging).
  • To find your device’s serial number, run sudo lsusb -v and look for iSerial (or run adb devices if you already set that up).
  • Pick somewhere to put the lock script referenced in the last line.
  • The contents of that script should be: 
    #!/bin/sh
exec su user -c ". /home/user/.dbus/session-bus/*-0; \
export DBUS_SESSION_BUS_ADDRESS; \
gnome-screensaver-command --lock"

The unlock script would be the same with --deactivate. Replace user with your username, and if your display isn’t :0 then you might need to do something smarter to get the right session bus. Oh, and I’m on Ubuntu Karmic (9.10); adjustments may be required for other environments. When you’ve done all that, run sudo reload udev to pick up the changes, unplug the phone, and watch the screen fade to black.

Question: how easy is it to fake a serial number of a USB device? Equivalently, how silly is it to have the unlock line uncommented? I’m guessing fairly easy.