toplevel

$ nix-build -A toplevel
/nix/store/9xcrrpznh480n2d6axp4h3z7z5a96nj6-toplevel

The toplevel contains two scripts and a directory (actually a symlink):

$ ls /nix/store/q40j6y70nwhgazvhzrlzh79cjlfik7jv-not-os/
activate  init  sw

• activate content is the value of config.system.activationScripts.script. In particular it calls the setup-etc.pl script.
• init content is the value of stage-2, which is stage-2-init.sh, with systemConfig replaced by toplevel’s path, and sw/bin/ set as its $PATH. The init script mounts a few special filesystems, calls the activate script, then execute `runit.
• sw is a symlink to path, which contains only a bin/ directory with symlinks to base executables (e.g. [, nix-build, yes, …).

The toplevel is packaged as a squashfs image.

The toplevel’s path is sometimes called systemConfig or sysconfig.