In addition to selecting the right monitor after docking my ThinkPad, I wanted to set the correct sound output since I have headphones connected to my Ultra Dock. This can be done fairly easily using Pulseaudio.

Switching to a different pulseaudio output

To find the device name and the output name I need to provide to pacmd, I ran pacmd list-sinks:

2 sink(s) available.
...
  * index: 1
    name: <alsa_output.pci-0000_00_1b.0.analog-stereo>
    driver: <module-alsa-card.c>
...
    ports:
        analog-output: Analog Output (priority 9900, latency offset 0 usec, available: unknown)
            properties:

        analog-output-speaker: Speakers (priority 10000, latency offset 0 usec, available: unknown)
            properties:
                device.icon_name = "audio-speakers"

From there, I extracted the soundcard name (alsa_output.pci-0000_00_1b.0.analog-stereo) and the names of the two output ports (analog-output and analog-output-speaker).

To switch between the headphones and the speakers, I can therefore run the following commands:

pacmd set-sink-port alsa_output.pci-0000_00_1b.0.analog-stereo analog-output
pacmd set-sink-port alsa_output.pci-0000_00_1b.0.analog-stereo analog-output-speaker

Listening for headphone events

Then I looked for the ACPI event triggered when my headphones are detected by the laptop after docking.

After looking at the output of acpi_listen, I found jack/headphone HEADPHONE plug.

Combining this with the above pulseaudio names, I put the following in /etc/acpi/events/thinkpad-dock-headphones:

event=jack/headphone HEADPHONE plug
action=su francois -c "pacmd set-sink-port alsa_output.pci-0000_00_1b.0.analog-stereo analog-output"

to automatically switch to the headphones when I dock my laptop.

Finding out whether or not the laptop is docked

While it is possible to hook into the docking and undocking ACPI events and run scripts, there doesn't seem to be an easy way from a shell script to tell whether or not the laptop is docked.

In the end, I settled on detecting the presence of USB devices.

I ran lsusb twice (once docked and once undocked) and then compared the output:

lsusb  > docked 
lsusb  > undocked 
colordiff -u docked undocked 

This gave me a number of differences since I have a bunch of peripherals attached to the dock:

--- docked  2017-07-07 19:10:51.875405241 -0700
+++ undocked    2017-07-07 19:11:00.511336071 -0700
@@ -1,15 +1,6 @@
 Bus 001 Device 002: ID 8087:8000 Intel Corp. 
 Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
-Bus 003 Device 081: ID 0424:5534 Standard Microsystems Corp. Hub
-Bus 003 Device 080: ID 17ef:1010 Lenovo 
 Bus 003 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
-Bus 002 Device 041: ID xxxx:xxxx ...
-Bus 002 Device 040: ID xxxx:xxxx ...
-Bus 002 Device 039: ID xxxx:xxxx ...
-Bus 002 Device 038: ID 17ef:100f Lenovo 
-Bus 002 Device 037: ID xxxx:xxxx ...
-Bus 002 Device 042: ID 0424:2134 Standard Microsystems Corp. Hub
-Bus 002 Device 036: ID 17ef:1010 Lenovo 
 Bus 002 Device 002: ID xxxx:xxxx ...
 Bus 002 Device 004: ID xxxx:xxxx ...
 Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

I picked 17ef:1010 as it appeared to be some internal bus on the Ultra Dock (none of my USB devices were connected to Bus 003) and then ended up with the following port toggling script:

#!/bin/bash

if /usr/bin/lsusb | grep 17ef:1010 > /dev/null ; then
    # docked
    pacmd set-sink-port alsa_output.pci-0000_00_1b.0.analog-stereo analog-output
else
    # undocked
    pacmd set-sink-port alsa_output.pci-0000_00_1b.0.analog-stereo analog-output-speaker
fi