Why doesn't xsetwacom work from udev?

It works if you create two files, one wrapper script being called by udev, which in turns calls the actual configuration script in the background. The configuration script needs to sleep for a short while, so that X11 has time to do its job. Here's the setup I use:

Wrapper script called by udev (/usr/local/bin/setupwacom.sh):

#!/usr/bin/env bash
/usr/local/bin/setupwacom-post-X11.sh &

Configuration script called by the wrapper script (/usr/local/bin/setupwacom-post-X11.sh):

#!/usr/bin/env bash
sleep 2
export XAUTHORITY=/home/adrian/.Xauthority
export DISPLAY=:0
# Put your xsetwacom commands here, for example: 
xsetwacom --set "Wacom Intuos S Pad pad" Button 1 "key +ctrl +shift e"

There is a fairly easy workaround, you can add something like this to your xorg.conf (or a file in xorg.conf.d, as I've done):

anthony@Watt:/etc/X11/xorg.conf.d$ cat 55-local-wacom.conf 
Section "InputClass"
       Identifier "Wacom Left Handed"
       MatchDriver "wacom"
       Option "Rotate" "half"
EndSection

Check the wacom(4) manpage for details of all the options you can set.

(In theory, you can use MatchProduct to individually configure the touchpad, pen, eraser, etc., but when I tried that a while back, it caused Xorg to segfault. Same if I tried to float them. But you're not doing any of that... and maybe the bug is fixed now.)

None of the answers here worked for me, and the options I wanted to set could not be specified in xorg.conf:

$ xsetwacom -x get 'Wacom Intuos PT S Pad pad' button 1 
Button: Actions are not supported by xorg.conf. Try shell format (-s) instead.

I ended up having to start the script with a systemd service triggered by a udev rule:

$ cat /etc/udev/rules.d/99-wacom.rules
SUBSYSTEM=="usb", ENV{ID_VENDOR_ID}=="056a", ENV{ID_MODEL_ID}=="0302", TAG+="systemd"

Vendor and model id can be found running lsusb with the device plugged in.

To reload udev rules:

$ udevadm control --reload-rules
$ udevadm trigger

The TAG+="systemd" enables other systemd services (system or user) to depend on the device (registers it as a device unit, see man systemd.device). To find the name of the device unit run udevadm monitor and plug in the tablet. I get

UDEV  [2918.098423] add      /devices/pci0000:00/0000:00:1d.0/usb2/2-1/2-1.3 (usb)
...

To check that systemd is picking it up do

$ systemctl status /sys/devices/pci0000:00/0000:00:1d.0/usb2/2-1/2-1.3/
● sys-devices-pci0000:00-0000:00:1d.0-usb2-2\x2d1-2\x2d1.3.device - CTH-480 [Intuos Pen & Touch (S)]
   Loaded: loaded
   Active: active (plugged) since Mon 2016-06-20 11:14:20 UYT; 29min ago
   Device: /sys/devices/pci0000:00/0000:00:1d.0/usb2/2-1/2-1.3

So the device unit is sys-devices-pci0000:00-0000:00:1d.0-usb2-2\x2d1-2\x2d1.3.device, and it can be used in the systemd service unit

 $ cat .config/systemd/user/wacom.service    
[Service]
Type=forking
Restart=no
ExecStart=/path/to/wacom-pad-button-setup

[Install]
WantedBy=default.target
WantedBy=sys-devices-pci0000:00-0000:00:1d.0-usb2-2\x2d1-2\x2d1.1.device
WantedBy=sys-devices-pci0000:00-0000:00:1d.0-usb2-2\x2d1-2\x2d1.2.device
WantedBy=sys-devices-pci0000:00-0000:00:1d.0-usb2-2\x2d1-2\x2d1.3.device

There is one device unit per usb port.

Then enable and reload the unit with systemctl --user enable wacom.service and systemctl --user daemon-reload.

The script still needs to sleep a bit for xsetwacom to find the device, and to set $DISPLAY and $XAUTHORITY. Type=oneshot works fine when plugging it in, but it doesn't run it if the device was already plugged in when booting the computer. That is why I needed to use a user service instead of a system one, and why the unit also has WantedBy=default.target. The problem with oneshot is that it was blocking startx. Type=forking and Restart=no tells systemd not to wait for the script's forked process to exit, so the script can then sleep in the background waiting for Xorg to start.

$ cat bin/wacom-pad-button-setup
#!/bin/rc
{
    sleep 2

    if (~ $DISPLAY ()) {
        DISPLAY=:0
        XAUTHORITY=/home/spelufo/.Xauthority
    }

    xsetwacom set 'Wacom Intuos PT S Pad pad' button 9 'button +3 -3'
    xsetwacom set 'Wacom Intuos PT S Pad pad' button 8 'button +4 -4'
    xsetwacom set 'Wacom Intuos PT S Pad pad' button 3 'button +1 -1'
    xsetwacom set 'Wacom Intuos PT S Pad pad' button 1 'button +2 -2'
} &