I would like to show a custom message during boot, or even better the program cmatrix.I boot with the quiet parameter so there are no startup messages, instead I would like to show a custom-message or the output of a program like cmatrix.Is this even possible?How could I achieve this with systemd?
Show Custom Boot-Message
arch linux;systemd
You can't achieve the boot animation with the systemd but there is something called plymouth you can look at this package.Plymouth is an application runs very early in the boot process[Before root file system is mounted].It is designed to work on system with DRM modesetting drivers.For the system that doesn't have DRM mode,plymouth falls back to text mode.So, the plymouth will be having the two binariesplymouthd and the plymouth.plymouthd is responsible for showing the splash screen and the session logging.plymouth is responsible for controlling the interface to plymouthd.In internet you can find a lot of information regarding the plymouth.Edit: (Added some more info)Plymouth is shipped with some of the default themes or you can customize your own theme by giving a series of images(.gif doesn't work)to a theme script written by you.There are  2 ways to specify themes to plymouth1.Passing a parameter to the kernel command line.2.Specifying the theme in plymouth configuration file.So,Yeah,It's a huge topic,You can search in interent for more information on this.