Remote Control Slideshow

Controlling the default tty / framebuffer on a raspberry pi zero w.

The Login Screen

When you power on a raspberry pi, after a lot of status messages etc scroll through the pi presents a login prompt:

This screen is tty1. You can find it in the /dev/tty1 device on the filesystem.

Printing to tty1 from another `tty`

A super user can write directly to this tty1 device, being that tty devices are text-mode terminals in a simple sense.

So, it is possible to write text to the device.

Now the login screen has new characters printed on it...

Clearing the screen and printing something using ansi codes

Using terminal escape codes it is possible to send commands to tty1 much as you would on the tty you are currently using.

The escape code \033c will clear the screen. So we can execute a simple command to clear the screen before it says hello:

Now the entire terminal `tty1` has been cleared.

This unlocks some powerful abilities.

It is possible to log in through ssh and drive the monitor connected to a raspberry pi remotely. This is excellent for presentations - for example we could use catimg to send ascii art to the monitor connected to the raspberry pi (tty1) - sometimes it looks cool, sometimes not so much - YMMV ;)

sudo -- sh -c 'echo "\033c" > /dev/tty1 ; catimg Pictures/yoda.jpg > /dev/tty1'

It looks like Yoda to me!

Ok so now it's pretty straight forward to write a little script that pushes a bunch of ascii to tty1 from a file in the (say,) Pictures folder.

That way, we can make a slideshow type script:

clear the screen
blast some ascii
wait x seconds

The example script proves that we can do that.

#!/bin/sh

echo "\033c" > /dev/tty1

catimg /home/user/Pictures/yoda.jpg > /dev/tty1

sleep 5

echo "\033c" > /dev/tty1

catimg /home/user/Pictures/darth-vader.jpg > /dev/tty1

Making it a continuous thing

The Raspberry Pi OS Lite linux comes with the watch command which is highly useful for running something over and over again.

Given the script above, the "slideshow" we have created will last for ~5 seconds. This means we can tell watch to run once every, say, 10 seconds and have a functional continuous slideshow controlled by basic tools.

Things to watch out for.

I have a theory that if the interval given to watch is less than the time it takes to get through the script it will create a second running copy of the script - over time, the number of processes and scripts and images being decoded into ascii and whatnot will likely consume all the RAM on the pi and cause problems.

Remember to make sure your script is finished before watch runs it again.

I am sure there is wizardry that could be applied here, to avoid this but for the sake of keeping it as basic as possible I just recommend this:

add up the number of sleep seconds (5 in the script above)
give watch an interval a few seconds more, just in case

A Continuously Running Slideshow

watch --interval 10 sudo ./show-ascii-image-on-tty1.sh

Monitoring using the `htop` utility

htop is a good way to keep an eye on your pi's resource use. You can use it to check (by eye) whether or not your continuously looping screenshow script is eating memory or whatnot.

Above, htop is showing there is a call to sudo to execute the script, and below the script itself running.

whatbirdisthat/super-simple-slideshow