Nix Cookbook

Environment Tasks

Creating Shell Scripts

Arbitrary system shell scripts can be created with pkgs.writeScriptBin. It creates a derivation which you add to environment.systemPackages.

{ pkgs, ... }:

let
  helloWorld = pkgs.writeScriptBin "helloWorld" ''
    #!${pkgs.stdenv.shell}
    echo Hello World
  '';

in {
  environment.systemPackages = [ helloWorld ];
}

Creating Periodic Services

Using the systemd support periodic services can be defined. In this case a service named simple-timer writes out the current time to /tmp/simple-timer.log every minute.

{ pkgs, ... }:

{
  systemd = {
    timers.simple-timer = {
      wantedBy = [ "timers.target" ];
      partOf = [ "simple-timer.service" ];
      timerConfig.OnCalendar = "minutely";
    };
    services.simple-timer = {
      serviceConfig.Type = "oneshot";
      script = ''
        echo "Time: $(date)." >> /tmp/simple-timer.log
        chmod +r /tmp/simple-timer.log
      '';
    };
  };
}

Debugging

Common Errors

Bad configuration option: gssapikexalgorithms

Found when using an SSH binary from Nix on typically RPM-based distros like CentOS, Fedora, Scientific Linux, Redhat, etc. The quick fix: Just comment out the configuration option in the ssh config file, you probably don't need it.

Desktop Environment does not find .desktop files

IF your DE does not look in $HOME/.nix-profile/share for .desktop files. You need to add that path to the XDG_DATA_DIRS, the position reflects precedence so files in earlier directories shadow files in later directories. This can be accomplished in various ways depending on your login manager, see Arch wiki: Xprofile for more information. For example using ~/.xprofile as follows:

$ export XDG_DATA_DIRS=$HOME/.nix-profile/share:/usr/local/share:/usr/share

Notice that you have to include the default locations on your system, otherwise they will be overwritten. Find out the proper paths using echo $XDG_DATA_DIRS. (Note: export XDG_DATA_DIRS=$HOME/.nix-profile/share:$XDG_DATA_DIRS did not work, XDG_DATA_DIRS ended up containing only $HOME/.nix-profile/share: which isn't even a valid path.)