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Divide the "Service Management" chapter into two sections. The 1st (the original) explaining General, not NixOS specific ways to interact with Systemd. The 2nd section, explaining NixOS specific things worth knowing. Explain in the 2nd section a bit NixOS modules and services of Nixpkgs, and mention `systemd.user.services` option. Give an example demonstrating how to enable imperatively an upstream provided unit file for a user. Explain why `systemctl --user enable` doesn't work for the long term on NixOS.
To build the manual, you need Nix installed on your system (no need
for NixOS). To install Nix, follow the instructions at
https://nixos.org/nix/download.html
When you have Nix on your system, in the root directory of the project
(i.e., `nixpkgs`), run:
nix-build nixos/release.nix -A manual.x86_64-linux
When this command successfully finishes, it will tell you where the
manual got generated.