Appimage

AppImage is a monolithic packaging format for linux applications. It contains all dependencies in one file that is composed of an executable with a tacked on filesystem.

On most distros, all one has to do is download the .AppImage file, make it executable chmod +x $AppImage, and execute it. This doesn't work in NixOS out of the box though, as AppImage files usually (if not always) depend on certain libraries commonly found on other Linux distributions to exist on certain paths; such as /lib64/ld-linux-x86-64.so.2.

Running an AppImage file on NixOS

$ nix-shell -p appimage-run
$ appimage-run $AppImageFile

Register AppImage files as a binary type to binfmt_misc

You can tell the Linux kernel to use an interpreter (e.g. appimage-run) when executing certain binary files through the use of binfmt_misc, either by filename extension or magic number matching. Below NixOS configuration registers AppImage files (ELF files with magic number "AI" + 0x02) to be run with appimage-run as interpreter.

boot.binfmt.registrations.appimage = {
  wrapInterpreterInShell = false;
  interpreter = "${pkgs.appimage-run}/bin/appimage-run";
  recognitionType = "magic";
  offset = 0;
  mask = ''\xff\xff\xff\xff\x00\x00\x00\x00\xff\xff\xff'';
  magicOrExtension = ''\x7fELF....AI\x02'';
};

This way AppImage files can be invoked directly.

How AppImage files are packaged by NixOS

See the nixpkgs manual on wrapping AppImage packages. In short, the AppImage is extracted and any dependencies are added as nix build dependencies.