nix run aliases

Sat, 1 January, 2019 (1300 Words)

I use NixOS each and every day, everywhere. One really cool feature of nix is nix-shell and more recently (with nix >= 2.0.0), nix run.

Usage: nix run <FLAGS>... <INSTALLABLES>...

Summary: run a shell in which the specified packages are available.

      --arg <NAME> <EXPR>         argument to be passed to Nix functions
      --argstr <NAME> <STRING>    string-valued argument to be passed to Nix functions
  -c, --command <COMMAND> <ARGS>  command and arguments to be executed; defaults to 'bash'
  -f, --file <FILE>               evaluate FILE rather than the default
  -i, --ignore-environment        clear the entire environment (except those specified with --keep)
  -I, --include <PATH>            add a path to the list of locations used to look up <...> file names
  -k, --keep <NAME>               keep specified environment variable
  -u, --unset <NAME>              unset specified environment variable


  To start a shell providing GNU Hello from NixOS 17.03:
  $ nix run -f channel:nixos-17.03 hello

  To start a shell providing youtube-dl from your 'nixpkgs' channel:
  $ nix run

  To run GNU Hello:
  $ nix run nixpkgs.hello -c hello --greeting 'Hi everybody!'

  To run GNU Hello in a chroot store:
  $ nix run --store ~/my-nix nixpkgs.hello -c hello

Note: this program is EXPERIMENTAL and subject to change.

As you can see from the -h summary, it makes it really easy to run a shell or a command with some packages that are not in your main configuration. It will download the package(s) if there are not available in the Nix store (/nix/store/).

A few month ago I decided it would be a perfect use-case for command I do not run often. My idea was, let’s define aliases (in the shell) that would make a simple command call, like ncdu, become nix run nixpkgs.ncdu -c ndcu. My shell of choice is fish, so I decided to dig into the language in order to implement that.

The use case is the following :

  • When I type foo, I want the command foo in package bar to be executed.
  • I want to be able to pin a channel for the package — I’m using Matthew Bauer Channel Changing with Nix setup for pin-pointing a given channel.

Fish aliases experimentation

I had a feeling the built-in alias would not work so I ended up trying to define a dynamic function that would be the name of the command. That’s the beauty of the shell, everything is a command, even function appears as commands. If you define a function foo(), you will be able to run foo in your shell, and it will take precedence over the foo executable file that would be in your PATH.

I ended up with two main helper function that would create those alias function.

function _nix_run_package
    set -l s $argv[1]
    set -l package (string split ":" $s)
    switch (count $package)
	case 1
	    _nix_run $s $s $argv[2] $argv[3]
	case 2
	    _nix_run $package[1] $package[2] $argv[2] $argv[3]

function _nix_run
    set -l c $argv[1]
    set -l p $argv[2]
    set -l channel $argv[3]
    set -l channelsfile $argv[4]
    function $c --inherit-variable c --inherit-variable p --inherit-variable channel --inherit-variable channelsfile
	set -l cmd nix run
	if test -n "$channelsfile"
	    set cmd $cmd -f $channelsfile
	eval $cmd $channel.$p -c $c $argv

In a nutshell, _nix_run is the function that create the alias function. There is so condition in there depending on whether we gave it a channel or not. So, a call like _nix_run foo bar unstable channels.nix would, in the end generate a function foo with the following call : nix run -f channels.nix -c foo.

The other function, _nix_run_package is there to make me write less when I define those aliases — aka if the command and the package share the same name, I don’t want to write it twice. So, a call like _nix_run_package foo nixpkgs would result in a _nix_run foo foo nixpkgs, whereas a call like _nix_run_package foo:bar unstable channels.nix would result in a _nix_run foo bar unstable channels.nix.

An example is gonna be better than the above paragraphs. This is what I used to have in my fish configuration.

function _def_nix_run_aliases
    set -l stable mr sshfs ncdu wakeonlan:python36Packages.wakeonlan lspci:pciutils lsusb:usbutils beet:beets gotop virt-manager:virtmanager pandoc nix-prefetch-git:nix-prefetch-scripts nix-prefetch-hg:nix-prefetch-scripts
    set -l unstable op:_1password update-desktop-database:desktop-file-utils lgogdownloader
    for s in $stable
	_nix_run_package $s nixpkgs
    for s in $unstable
	_nix_run_package $s unstable ~/.config/nixpkgs/channels.nix
# Call the function to create the aliases

This works like a charm, and for a while, I was happy. But I soon realized something : I’m not always on my shell — like, I tend to spend more and more time in eshell. This also doesn’t work with graphic tools like rofi. I needed actual command, so that external tools would benefit from that. I ended up writing a small tool, nr that integrates nicely with nix and home-manager.

A proper tool : nr

The gist for this tool is simple :

  • create an executable script that will call nix run ... instead of the command
  • as for the above fish script, support different channels
  • make sure we don’t have conflicts — if the command already exists, then don’t create the command

The nr tool would have to be able to manage multiple profile, which really stands for multiple file. The main reason is really about how I manage my configuration ; To make it simple, depending on the computer my configurations are setup, I may not have go, thus I don’t want any go-related aliases for a computer that doesn’t have go (using go here but you can replace with anything).

$ nr default
> nr generate default
> virtmanager already exists
$ nr git
> nr generate git

nr generates a bash script that does the nr run … and mark it as executable. nr needs to be able to clean files it has generated (in case we removed it from aliases). Thus, I went for a really naive comment in the script. When generating a new set of commands, nr will first remove previously generated script for this profile, and for that, it uses the comment. Let’s look at what a generated script looks like, for the default profile.

#!/usr/bin/env bash
# Generated by nr default
nix run nixpkgs.nix-prefetch-scripts -c nix-prefetch-git $@

The format used in nr is json. I’m not a huge fan of json but it really was the best format to use for this tool. The reason to use json are simple :

  • Go has encoding/json built-in, so it’s really easy to Marshall and Unmarshall structure.

    type alias struct {
        Command string `json:"cmd"`
        Package string `json:"pkg"`
        Channel string `json:"chan"`
  • Nix also has built-in support for json : builtins.toJSON will marshall a struct into a json file.

Finally, to avoid conflicts at build time (home-manager switch) I couldn’t use/define a nix package, but to execute command(s) at the end of the build. One way to achieve it is to use file.?.onChange script, which is executed after home-manager has updated the environment, if the file has changed. That means it’s possible to check for executable files in ~/.nix-profile/bin/ for defined aliases and create those that are not there, with nr. My configuration then looks like the following.

xdg.configFile."nr/default" = {
  text = builtins.toJSON [
    {cmd = "ncdu";} {cmd = "sshfs";} {cmd = "gotop";} {cmd = "pandoc";}
    {cmd = "wakeonlan"; pkg = "python36Packages.wakeonlan";}
    {cmd = "beet"; pkg = "beets";}
    {cmd = "virt-manager"; pkg = "virtmanager";}
    {cmd = "nix-prefetch-git"; pkg = "nix-prefetch-scripts";}
    {cmd = "nix-prefetch-hg"; pkg = "nix-prefetch-scripts";}
  onChange = "${}/bin/nr default";

And there you are, now, each time I update my environment (home-manager switch), nr will regenerate my nix run aliases.