How to remotely send a keypress to an X11 application

You are asking five questions here, and might be better asking five questions ☺ But I'll jump in:

1. X is a server and can be compared to a "web server" in that it is a process that listens for incoming connections that speak a particular protocol (the X protocol) and it issues answers. The connections come from X clients, which may be on the local host or on a remote host accessed over a network.

2. X11 is a "major version" of the X protocol, which has evolved since inception. X11 is the most recent protocol and the most common. (Xorg is an implementation of an X server, X libraries and a collection of clients, all talking X11. There are other implementations: the largely defunct XFree86; commercial ones such as Hummingbird Exceed; forks such as XQuartz, which runs on Mac OS X)

3. A "window manager" is an X client which manages windows. It typically has the responsibility to decorate windows with bevels/borders, drop shadows, a menu bar containing buttons, etc. -- and handle the logic of window placement; support windows being dragged, resized or re-arranged, etc.

4. X requires input and it has a shared responsibility to manage that with the Kernel. Historically, X did a lot of hardware management itself. In modern times, on the Linux platform, X is gradually becoming "smaller" and delegating this responsibility to the Linux kernel. The advantages of this are: smaller X codebase; fewer "crossed-wires" with the kernel and X both trying to manage the same stuff. (an example of this is the Linux event interface, where events from mice etc. are interpreted and translated by the kernel and its drivers into the standard interface at /dev/input/event*, which is consumed by the X server). Note that on other platforms (BSD, Solaris) X is still quite monolithic.

5. There are many, many ways for processes to communicate with each other. I believe X clients can interact by way of the X protocol (for example, traditional X window managers need to know when other clients draw windows, in order to decorate them; pagers need to know when a window's size or position changes, in order to reflect that in the pager). D-Bus is a modern inter-process communication (IPC) technology developed to address the shortcomings of other methods. It is not X specific.

At least in my case, the problem was that the address needs to be in lower case! So, in your case, change ATTRS{address}=="00:1F:20:76:41:30" to the following:

ATTRS{address}=="00:1f:20:76:41:30"

In case that doesn't do it, I'd double check the permissions.

Also, udev should set a DEVNAME variable (among others) which you can use, so you don't really need the grep the logs (another possible candidate for a permission issue). To troubleshoot further, you could just create a log file every time the script is run (from the script) - that way, you will know if the script was run at all - i.e. if the udev rule triggered at all, or if the error is somewhere later on.

So, the authors script solution (on the page you have already linked) is better IMO. I've adapted it as such:

Permissions:

# ls -l /etc/udev/rules.d/50-k810.rules /opt/bin/k810*
-rw-r--r-- 1 root root   106 2014-07-16 19:21 /etc/udev/rules.d/50-k810.rules
-rwxr-xr-x 1 root root   304 2014-07-16 19:39 /opt/bin/k810.sh
-rwxr-xr-x 1 root root 13102 2014-06-07 22:05 /opt/bin/k810_conf

50-k810.rules:

KERNEL=="hidraw*", SUBSYSTEM=="hidraw", ATTRS{address}=="my:k8:10:ad:re:ss" \
 RUN+="/opt/bin/k810.sh %p"

k810.sh:

#!/bin/sh
LOGFILE=/tmp/logfilek810sh.log
echo "RUN: at `date` by `whoami` act $ACTION \$1 $1 DEVPATH $DEVPATH DEVNAME $DEVNAME" >> ${LOGFILE}
echo "Setting F-keys on for your K810!"

if [ "$ACTION" == "add" ];
then
    # configure k810 ($DEVPATH) at $DEVNAME.
    /opt/bin/k810_conf -d $DEVNAME -f on
fi

Also, one little thing: You can use udevadm info -a -n /dev/hidraw1 to get the right address instead of hcitool (replace with the right hidraw). It should match, but just to double check (that's how I figured udev is seeing a lower case address).