• Why? Doesn't feh --bg-max --randomize do a good job?
• It does, for slideshows, but if you use that to set a random background, then you will run into the birthday problem meaning that you will start to see repeats more often than you would expect, and it will take you a long time to see all the pictures. This instead uses a state file BGSTATE (shell environment variable) to keep track of what has not been seen yet. It initially populates it with files from BGDIR (if it is not an absolute path, be sure to start this script in the same directory for all runs).

BGDIR (defaults to "$HOME/backgrounds") the directory in which to look for image files. BGSTATE (defaults to "$HOME/.feh-random-background") the state file which holds the not-yet-seen images. BGEXTENSIONRE (defaults to "png|jpg|jpeg|gif|tiff") regular expression matching the extensions of the files to be used as wallpapers.

The file home-manager-service.nix is usable with home-manager, e.g. with imports = [ path/to/this/repo/home-manager-service.nix ]. This probably wouldn't currently work in a NixOS configuration as that uses a slightly different syntax for the systemd service files (probably something like <nixpkgs/nixos/modules/services/x11/urxvtd.nix> should be a good place to start looking for the NixOS syntax).

To use this in a flake, add this input:

feh-random-background = {
  url = github:KoviRobi/feh-random-background;
  flake = false;
};

then you can import the home manager service with:

imports = [
  (inputs.feh-random-background + /home-manager-service.nix)
];