How to install systemd service on nixos

Question

If I do this:

#!/usr/bin/env bash

set -e;

cd "$(dirname "$BASH_SOURCE")"

ln -sf "$(pwd)/interos-es-mdb.service" '/etc/systemd/system/interos-es-mdb.service'

systemctl enable interos-es-mdb.service
systemctl start interos-es-mdb.service

then I get this error:

ln: failed to create symbolic link '/etc/systemd/system/interos-es-mdb.service': Read-only file system

anyone know the right way to install a service on nixos machine? (I am the root user)...here is the service for reference:

[Unit]
Description=Interos MongoDB+ES log capture
After=network.target

[Service]
Environment=interos_emit_only_json=yes
EnvironmentFile=/root/interos/env/es-service.env
StartLimitIntervalSec=0
Type=simple
Restart=always
RestartSec=1
ExecStart=/root/interos/repos/elastic-search-app/syslog-exec.sh

[Install]
WantedBy=multi-user.target

update: perhaps what I am looking for is "per-user" service, not something run as root etcetera.

Answer 1

An appropriate entry in your /etc/nixos/configuration.nix might look like:

let
  # assumes you build a derivation for your software and put it in
  # /etc/nixos/pkgs/interosEsMdb/default.nix
  interosEsMdb = import ./pkgs/interosEsMdb {};
in config.systemd.services.interosEsMdb = {
  description = "Interos MongoDB+ES log capture";
  after = ["network.target"];
  wantedBy = ["multi-user.target"];

  serviceConfig = {
    # change this to refer to your actual derivation
    ExecStart = "${interosEsMdb}/bin/syslog-exec.sh";
    EnvironmentFile = "${interosEsMdb}/lib/es-service.env";
    Restart = "always";
    RestartSec = 1;
  }
}

...assuming you actually build a derivation for interosEsMdb (which is the only sane and proper way to package software on NixOS).