See NixOS manual "Definition of systemd service units" page 
, systemd.service page , and "Execution environment configuration" (Paths) page for details.
wiki.nix:
{config, ...}: { systemd.services.wiki = { wantedBy = [ "multi-user.target" ]; after = [ "network.target" ]; description = "Federated Wiki (Node.js server version)"; serviceConfig = { Type = "simple"; User = "rgb"; # /nix/store/c7k8mr6d7fmqf1ghmngqjyhky42k8mba-node_wiki-0.28.1 # /nix/store/c3cr1n5pgmbxblsxcac8ac4yki04hp32-wiki-0.30.0/ # /nix/store/3s5lzzrsy18ypas2jzflhfjlyrvj3wnd-wiki-0.31.0/ # result -> /nix/store/wwgk5brb5zz273kmlndjj4rs08900n6w-wiki-0.33.0/ wwgk5brb5zz273kmlndjj4rs08900n6w-wiki-0.33.0 ExecStart = "/nix/store/wwgk5brb5zz273kmlndjj4rs08900n6w-wiki-0.33.0/bin/wiki --config /home/rgb/.wiki/config.json"; Restart = "on-failure"; }; } systemd.services.elm-pages = { wantedBy = [ "multi-user.target" ]; after = [ "network.target" ]; description = "blog.dreyeck.ch/elm-pages-starter-express"; serviceConfig = { Type = "simple"; User = "rgb"; WorkingDirectory = "/home/rgb/.wiki/blog.dreyeck.ch/elm-pages-starter-express"; ExecStart = "/run/current-system/sw/bin/node dist-server/server.mjs"; Restart = "on-failure"; }; }; }