linux

i3, udev, & xrandr: Hotplugging & Output Switching

bsilvereagle

• 4 min read

One of the benefits of using a Desktop Environment (DE) like Gnome or KDE is that a lot of "user friendly" things are automagically taken care of. With i3, a minimalist Window Manager, that is not the case. One of the more convenient things missing from a default i3 installation is the ability to hotplug monitors and to quickly change whether monitors are extending/mirroring/off.

Handling hotplugs with udev

udev is a device manager and handles hardware events. It allows scripts to be tied to specific hardware events, plugging in a USB device or a monitor, for example.

Detecting udev events is straightforward with the udevadm command:

$ sudo udevadm monitor

monitor will print the received events for:
UDEV - the event which udev sends out after rule processing
KERNEL - the kernel uevent


KERNEL[3959.208769] change   /devices/pci0000:00/0000:00:03.0/0000:01:00.0/drm/card0 (drm)
UDEV  [3960.654344] change   /devices/pci0000:00/0000:00:03.0/0000:01:00.0/drm/card0 (drm)

Plugging/unplugging a HDMI/VGA/DisplayPort cable will cause events to fire and be printed to the terminal.

Writing a rule to respond to a particular event is accomplished by creating a file in /etc/udev/rules.d/. For example, the following rule corresponds to the change event on kernel card0 on subsystem drm and runs /bin/bash /path/to/script.sh

$ cat /etc/udev/rules.d/98-monitor-hotplug.rules 
KERNEL=="card0", SUBSYSTEM=="drm", ACTION=="change", ENV{DISPLAY}=":0", ENV{XAUTHORITY}="/home/ben/.Xauthority", RUN+="/bin/bash /path/to/script.sh"

The udev event does not specify which graphics output was plugged. For a card with multiple outputs, VGA or HDMI could have been plugged in. The information about which outputs are plugged in (or not) is available in /sys/. Notice that the path matches the KERNEL and SUBSYSTEM from the udev event.

$ echo $(</sys/class/drm/card0/card0-HDMI-A-1/status )
disconnected
$ echo $(</sys/class/drm/card0/card0-VGA-1/status )
connected

Once the output is known, it's very easy to use xrandr to setup the outputs properly. If a notification daemon is running (such as dunst) notify-send will cause a pop-up.

#!/bin/sh

# Get out of town if something errors
set -e

HDMI_STATUS=$(</sys/class/drm/card0/card0-HDMI-A-1/status )
VGA_STATUS=$(</sys/class/drm/card0/card0-VGA-1/status )

if [ "connected" == "$HDMI_STATUS" ]; then
	/usr/bin/xrandr --output HDMI-1 --left-of LVDS-1 --auto
	/usr/bin/xrandr --output VGA-1 --off
    /usr/bin/notify-send --urgency=low -t 5000 "Graphics Update" "HDMI plugged in"
elif [ "connected" == "$VGA_STAUTS" ]; then
	/usr/bin/xrandr --output HDMI-1 --off
	/usr/bin/xrandr --output VGA-1 --left-of LVDS-1 --auto
    /usr/bin/notify-send --urgency=low -t 5000 "Graphics Update" "VGA plugged in"
else 
	/usr/bin/xrandr --output HDMI-1 --off
	/usr/bin/xrandr --output VGA-1 --off
	/usr/bin/notify-send --urgency=low -t 5000 "Graphics Update" "External monitor disconnected"	
	exit
fi

Convenient Output Switching

With monitors being detected on hotplug, the next phase of user friendliness is quickly being able to change whether the display is mirroring, extending, or off.

arandr is a convenient GUI to position windows and change output behavior. However, hotkeys can be far superior. The goal is to have a hotkey that toggles between a few states:

  1. LVDS-1 (laptop screen) is on and external display is off
  2. LVDS-1 is off and external display is off
  3. LVDS-1 and external display are mirroring
  4. LVDS-1 and external display are extending

To maintain state through multiple executions of the script, a temporary file to store the next state will be created.

#!/bin/sh

# Get out of town if something errors
# set -e

# Get info on the monitors
LVDS_STATUS=$(</sys/class/drm/card0/card0-LVDS-1/status )
HDMI_STATUS=$(</sys/class/drm/card0/card0-HDMI-A-1/status )
VGA_STATUS=$(</sys/class/drm/card0/card0-VGA-1/status )

LVDS_ENABLED=$(</sys/class/drm/card0/card0-LVDS-1/enabled)
HDMI_ENABLED=$(</sys/class/drm/card0/card0-HDMI-A-1/enabled)
VGA_ENABLED=$(</sys/class/drm/card0/card0-VGA-1/enabled)

# Check to see if our state log exists
if [ ! -f /tmp/monitor ]; then
	touch /tmp/monitor
	STATE=5
else
	STATE=$(</tmp/monitor)
fi

# The state log has the NEXT state to go to in it

# If monitors are disconnected, stay in state 1
if [ "disconnected" == "$HDMI_STATUS" -a "disconnected" == "$VGA_STATUS" ]; then
	STATE=1
fi

case $STATE in
	1)
	# LVDS is on, projectors not connected
	/usr/bin/xrandr --output LVDS-1 --auto
	STATE=2
	;;
	2)
	# LVDS is on, projectors are connected but inactive
	/usr/bin/xrandr --output LVDS-1 --auto --output HDMI-1 --off --output VGA-1 --off
	STATE=3	
	;;
	3)
	# LVDS is off, projectors are on
	if [ "connected" == "$HDMI_STATUS" ]; then
		/usr/bin/xrandr --output LVDS-1 --off --output HDMI-1 --auto
		TYPE="HDMI"
	elif [ "connected" == "$VGA_STATUS" ]; then
		/usr/bin/xrandr --output VGA-1 --off --output VGA-1 --auto
		TYPE="VGA"
	fi
	/usr/bin/notify-send -t 5000 --urgency=low "Graphics Update" "Switched to $TYPE"
	STATE=4
	;;
	4)
	# LVDS is on, projectors are mirroring
	if [ "connected" == "$HDMI_STATUS" ]; then
		/usr/bin/xrandr --output LVDS-1 --auto --output HDMI-1 --auto
		TYPE="HDMI"
	elif [ "connected" == "$VGA_STATUS" ]; then
		/usr/bin/xrandr --output VGA-1 --auto --output VGA-1 --auto
		TYPE="VGA"
	fi
	/usr/bin/notify-send -t 5000 --urgency=low "Graphics Update" "Switched to $TYPE mirroring"
	STATE=5
	;;
	5) 
	# LVDS is on, projectors are extending
	if [ "connected" == "$HDMI_STATUS" ]; then
		/usr/bin/xrandr --output LVDS-1 --auto --output HDMI-1 --auto --left-of LVDS-1
		TYPE="HDMI"
	elif [ "connected" == "$VGA_STATUS" ]; then
		/usr/bin/xrandr --output VGA-1 --auto --output VGA-1 --auto --left-of LVDS-1
		TYPE="VGA"
	fi
	/usr/bin/notify-send -t 5000 --urgency=low "Graphics Update" "Switched to $TYPE extending"
	STATE=2
	;;
	*)
	# Unknown state, assume we're in 1
	STATE=1	
esac	

echo $STATE > /tmp/monitor

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