1337 fake Screensaver

It’s 2022. Screens don’t really need saving anymore - in fact, when there’s talk of a “screensaver”, it’s usually a lock-screen and/or just some animation that would be cool if “screensavers” were still a thing. What I’m about to present is no different.

The vision

One Vision

one flesh, one bone, one true religion
one voice, one hope, one real decision
gimme gimme gimme gimme
...fried chicken

I want each of my monitors to display a fullscreen Matrix-like text crawl, and then when I quit one of them it quits all of them. That’s what I want. Should be pretty simple, right?

The obvious solution

We should be able to just open one big Cmatrix window that covers all three screens (yes I have three screens. Only two monitors though - the other one is a laptop). That way there’s only one to quit. Elementary, Watson-kun.

Problem 1: resolution

Here’s how my monitors look physically:

+---------------+---------------+    |   | |  +----------+
|               |               |    pillar|  |          |
|   1920x1080   |    1366x760   |    |   | |  | 1920x1080|
|               |               | <--cable--> +----------+
+---------------+---------------+    |___|/   \___________\

But here’s how they look to the computer:

|               |          |               |
|   1920x1080   | 1366x760 |   1920x1080   |
|               |----------|               |
+---------------+          +---------------+

So a window that looks fine on the left monitor will be zoomed in on the middle monitor, and small on the right monitor​laptop.

This means that having one big window stretching over all three screens will look all wonky. We need separate windows with different zoom levels.

Problem 2: Separate windows

Opening multiple terminal windows is fine - just call them with & and call it a day.

Closing them is another matter.

Thankfully, things like kill and killall exist. If we have the PIDs of each terminal process, it’s killable. There are various shell-scripting tricks for getting the PID straight off the process invocation, but I decided not to bother with anything fancy. Instead, I’ll just use a terminal I don’t normally – Xterm – and kill all of those when one exits.

Problem 2.5: Window Placement

I’m using ubuntu on this computer because I don’t care. Snaps aren’t that bad, Gnome isn’t that bad, and all the server things I want to run, run. However, Gnome is also a Wayland compositor, making ordinary Xorg tools insufficient for automation - specifically stuff like xdotool for sending keystrokes and mouse events. In addition, XWayland+Gnome doesn’t seem to honor a window’s request to be placed on a specific monitor.

So behold! Ydotool​! An Xdotool replacement for Wayland!

… which doesn’t work on gnome either. At least, I couldn’t get it to work. No idea why, it’s been a week since this part.

Okay, for real this time. Behold dotool. A display-server-agnostic desktop automator and event simulator. Works sort of like xdotool or ydotool but with pipe syntax for reasons.

Once it’s installed and working, we can run basic commands like

echo "type twelve" | dotool

resulting in


getting typed to the current window.

On my gnome setup which is hacky and probably non-standard, the keybinding to move a window to the right by one monitor is super + shift + right. Let’s define this as a function in bash:

moveright () {
    # move a window right one workspace
    echo "key super+shift+right" | dotool

now running moveright in the terminal will move the currently selected window to the right. It leaves the mouse in the same place because that’s what the keybinding does.

But I want to be able to move it twice. If we leave the mouse, another window might get selected (this is a setting that I enable because it’s very convenient on tiling WMs). So another function

mouseright () {
    # move mouse one monitor right (ish)
    echo "mousemove 1300 0" | dotool

1300 isn’t quite right, but it’s close enough for now.

We should also have one for moving it back to the left:

mouseleft () {
    # move mouse one monitor left (ish)
    echo "mousemove -1300 0" | dotool

Problem 2.5.1: Actually throwing windows onto the screen an moving them around

Assuming we start with the mouse on the left monitor, here’s the basic control flow:

  • Spawn the window that will be on the right monitor
    • Move it right (to middle monitor)
    • Move the mouse to follow it
    • Move it right (to right monitor)
  • Move mouse left (back to left monitor)
  • Spawn the window that will be on the middle monitor
    • Move it right (to middle monitor)
  • Spawn the window that will be on the left monitor

Also, the font sizes. The rightmost screen is small but with a high resolution, so the text should be larger. The middle screen is large with low resolution, so small text. Then somewhere in between for the left monitor.

As previously mentioned, we’re using Xterm. Xterm’s (relevant) command-line options include:

-fullscreen                 start in fullscreen
-bg COLOR                   background color, very intuitive
-fa FONT                    font, semi-intuitive
-e COMMAND                  command to run, in quotes

So to asynchronously launch a fullscreen Xterm with a black background and (not my) Meslo font in size 12 running Cmatrix in screensaver mode, we can do:

xterm -fullscreen -bg black -fa "MesloLGS NF:size=12" -e "cmatrix" &

Sprucing that up into a function taking an argument for the font size:

FONT=MeseloLGS\ NF:size=
matrixterm () {
    # spawn terminal with fun matrix in it
    # $1 = the size of font
    xterm -fullscreen -bg black -fa "${FONT}$1" -e "cmatrix -abC$COLOR" &

Notice the expanded variable $FONT, and $COLOR. Cmatrix takes different colors and at the moment I’m partial to blue.

So the control flow including fonts can go something like

openonall () {
    # Open $1 on all monitors
    $1 14
    $1 9
    $1 12
openall matrixterm

Problem 3: on opening and closing

As previously mentioned, processes are okay. If we know the PIDs we can just kill them. Here’s an article on finding and killing processes. I personally don’t believe in Xargs, so we’re going to do this in multiple steps.

XTERMS=$(ps -ef | grep xterm | grep -v 'grep' | awk '{print $2}')

So here’s an array of all the PIDs of running xterm​s. I want to streamline checking if each of these are still running and signal an error if any aren’t:

isrunning () {
    # $1 = process to check
    ps -p $1 >/dev/null
checkallprocs () {
    for proc in $*
      isrunning $proc || return 1

So any run of checkallprocs $XTERMS will succeed as long as all of the original xterms are running, but will fail once one is manually terminated. This is perfect for a While loop:

while checkallprocs $XTERMS
    sleep $INTERVAL

The interpreter doesn’t move on until this loop is broken, meaning we can simply put the final Kill right after it.

for proc in $XTERMS
    kill -9 $proc 2>/dev/null

One of those iterations will fail because that xterm is already closed. But oh well.

Wrapping it up in a script with a hotkey

And I bound it to super+z because nothing else appeared to be bound to that key and it’s easy to hit when leaving the computer.

There’s definitely improvements that can be made to this, but I found it to be a positive learning experience and wanted to share it with thou, the void.