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OLPC France has deployed Samsung Galaxy Tab A 2018 tablets with Sugarizer OS at the Mendela school. For more details, see https://wiki.sugarlabs.org/go/Sugarizer_Saint-Ouen_deployment

Learners at Tooraweenah Public School in outback Australia being taught how to use OLPC XO-4 with Turtle Blocks. Their task was to use the turtle to draw shapes, like squares, and hexagons.

Part of a weekly visit by a volunteer technology teacher and OLPC’s CTO.

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This guide will build on the previous three guides, and look at creating a wider variety of useful nix packages.

Nix is built around the concept of derivations. A derivation is simply defined as "a build action". It produces 1 (or maybe more) output paths in the nix store.

Basically, a derivation is a pure function that takes some inputs (dependencies, source code, etc.) and makes some output (binaries, assets, etc.). These outputs are referenceable by their unique nix-store path.

Derivation Examples

It's important to note that literally everything in NixOS is built around derivations:

• Applications? Of course they are derivations.
• Configuration files? In NixOS, they are a derivation that takes the nix configuration and outputs an appropriate config file for the application.
• The system configuration as a whole (/run/current-system)?

sam@vcs ~> ls -lsah /run/current-system
0 lrwxrwxrwx 1 root root 83 Jan 25 13:22 /run/current-system -> /nix/store/wb9fj59cgnjmkndkkngbwxwzj3msqk9c-nixos-system-vcs-17.09.2683.360089b3521

It's a symbolic link to a derivation!

It's derivations all the way down.

If you've followed this series from the beginning, you would have noticed that we've already made some derivations. Our nix-shell scripts are based off having a derivation. When packaging a shell script, we also made a derivation.

I think it is easiest to learn how to make a derivation through examples. Most packaging tasks are vaguely similar to packaging tasks done in the past by other people. So this will be going through example of using mkDerivation.

mkDerivation

Making a derivation manually requires fussing with things like processor architecture and having zero standard build-inputs. This is often not necessary. So instead, NixPkgs provides a function function stdenv.mkDerivation; which handles the common patterns.

The only real requirement to use mkDerivation is that you have some folder of source material. This can be a reference to a local folder, or something fetched from the internet by another nix function. If you have no source, or just 1 file; consider the "trivial builders" covered in part three of this series

mkDerivation does most a lot of work automatically. It divides the build up into "phases", all of which include a little bit of default behaviour - although it is usually unintrusive or can be can be overridden. The most important phases are:

1. unpack: unzips, untarz, or copies your source folder to the nix store
2. patch: applies any patches provided in the patches variable
3. configure: runs ./configure if it exists
4. build: runs make if it exists
5. check: skipped by default
6. install: runs make install
7. fixup: automagically fixes up things that don't jell with the nix store; such as using incorrect interpreter paths
8. installCheck: runs make installcheck if it exists and is enabled

You can see all the phases in the docs. But with a bit of practice from the examples below you'll likely get the feel for how this works quickly.

Example #1: A static site

Nix makes writing packages really easy; and with NixOps (which we'll learn later) Nix derivations are automagiaclly built and deployed.

First we need to answer the question of how we would build the static site ourself. This is a jekyll site, so you'd run the jekyll command

with import <nixpkgs> {};

stdenv.mkDerivation {
  name = "example-website-content";

  # fetchFromGitHub is a build support function that fetches a GitHub
  # repository and extracts into a directory; so we can use it
  # fetchFromGithub is actually a derivation itself :)
  src = fetchFromGitHub {
    owner = "jekyll";
    repo = "example";
    rev = "5eb1b902ca3bda6f4b50d4cfcdc7bc0097bac4b7";
    sha256 = "1jw35hmgx2gsaj2ad5f9d9ks4yh601wsxwnb17pmb9j02hl3vgdm";
  };
  # the src can also be a local folder, like:
  # src = /home/sam/my-site;

  # This overrides the shell code that is run during the installPhase.
  # By default; this runs `make install`.
  # The install phase will fail if there is no makefile; so it is the
  # best choice to replace with our custom code.
  installPhase = ''
    # Build the site to the $out directory
    export JEKYLL_ENV=production
    ${pkgs.jekyll}/bin/jekyll build --destination $out
  '';
}

Now we can see that this derivation builds the site. If you save it to test.nix, you can trigger a build by running:

> nix-build test.nix
/nix/store/b8wxbwrvxk8dfpyk8mqg8iqhp7j2c9bs-example-website-content

The path printed by nix-build is where $out was in the Nix store. Your path might be a little different; if you are running a different version of NixPkgs, then the build inputs are different.

We can see the site has built successfully by entering that directory:

> ls /nix/store/b8wxbwrvxk8dfpyk8mqg8iqhp7j2c9bs-example-website-content
2014  about  css  feed.xml  index.html  LICENSE  README.md

Using the content

We can then use that derivation as a webroot in a nginx virtualHost. If you have a server, you could add the following to your NixOS configuration:

let
  content = stdenv.mkDerivation {
  name = "example-website-content";

    ... # code from above snipped
  }
in
  services.nginx.virtualHosts."example.com" = {
    locations = {
      "/" = {
        root = "${content}";
      }
    }
  };

So how does this work? Ultimately, the "root" attribute needs to be set to the output directory of the content derivation.

Using the "${content}" expression, we force the derivation to be converted to a string (remembering ${...} is string interpolation syntax). When a derivation is converted to a string in Nix, it becomes the output path in the Nix store.

If you don't have a server handy, we can use the content in this a simple http server script:

# server.nix
with import <nixpkgs> {};

let
  content = stdenv.mkDerivation {
    name = "example-website-content";

    src = fetchFromGitHub {
      owner = "jekyll";
      repo = "example";
      rev = "5eb1b902ca3bda6f4b50d4cfcdc7bc0097bac4b7";
      sha256 = "1jw35hmgx2gsaj2ad5f9d9ks4yh601wsxwnb17pmb9j02hl3vgdm";
    };

    installPhase = ''
      export JEKYLL_ENV=production
      # The site expects to be served as http://hostname/example/...
      ${pkgs.jekyll}/bin/jekyll build --destination $out/example
    '';
  };
in
let
  serveSite = pkgs.writeShellScriptBin "serveSite" ''
    # -F = do not fork
    # -p = port
    # -r = content root
    echo "Running server: visit http://localhost:8000/example/index.html"
    # See how we reference the content derivation by `${content}`
    ${webfs}/bin/webfsd -F -p 8000 -r ${content}
  '';
in
stdenv.mkDerivation {
  name = "server-environment";
  # Kind of evil shellHook - you don't get a shell you just get my site
  shellHook = ''
    ${serveSite}/bin/serveSite
  '';
}

Then run nix-shell server.nix, you'll then start the server and can view the site!

Example #2: A more complex shell app

We've already talked a lot about shell scripts. But sometimes whole apps get built in shell scripts. One such example is emojify, a CLI tool for replacing words with emojis.

We can make a derivation for that. All we need to do is copy the shell script into the PATH, and mark it as executable.

If we were writing the script ourself, we'd need to pay special attention to fixing up dependencies (such as changing /bin/bash to a Nix store path). But mkDerivation has the fixup phase, which does this automatically. The defaults are smart, and in this case it works perfectly.

It is quite simple to write a derivation for a shell script.

with import <nixpkgs> {};

let
  emojify = let
    version = "2.0.0";
  in
    stdenv.mkDerivation {
      name = "emojify-${version}";

      # Using this build support function to fetch it from github
      src = fetchFromGitHub {
        owner = "mrowa44";
        repo = "emojify";
        # The git tag to fetch
        rev = "${version}";
        # Hashes must be specified so that the build is purely functional
        sha256 = "0zhbfxabgllpq3sy0pj5mm79l24vj1z10kyajc4n39yq8ibhq66j";
      };

      # We override the install phase, as the emojify project doesn't use make
      installPhase = ''
        # Make the output directory
        mkdir -p $out/bin

        # Copy the script there and make it executable
        cp emojify $out/bin/
        chmod +x $out/bin/emojify
      '';
    };
in
stdenv.mkDerivation {
  name = "emojify-environment";
  buildInputs = [ emojify ];
}

And see it in action:

> nix-shell test.nix

[nix-shell:~]$ emojify "Hello world :smile:"
Hello world 😄

Example #3: The infamous GNU Hello example

If you've ever read anything about Nix, you might have seen an example of making a derivation for GNU Hello. Something like this:

with import <nixpkgs> {};

let
  # Let's separate the version number so we can update it easily in the future
  version = "2.10";

  # Now define the derivation for the app
  helloApp = stdenv.mkDerivation {
    # String interpolation to include the version number in the name
    # Including a version in the name is idiomatic
    name = "hello-${version}";

    # fetchurl is a build support again; and does some funky stuff to support
    # selecting from a predefined set of mirrors
    src = fetchurl {
      url = "mirror://gnu/hello/hello-${version}.tar";
      sha256 = "0ssi1wpaf7plaswqqjwigppsg5fyh99vdlb9kzl7c9lng89ndq1i";
    };

    # Will run `make check`
    doCheck = true;
  };
in
# Make an environment for nix-shell
stdenv.mkDerivation {
  name = "hello-environment";
  buildInputs = [ helloApp ];
}

You can build and run this:

> nix-shell test.nix

[nix-shell:~]$ hello
Hello, world!

Ultimately this is a terrible and indirect example. This doesn't explicitly specify anything that the builder will actually run! It really confused me when I was learning Nix.

To understand it, we need to remember the default build phases from stdenv.mkDerivtion. From above, we had a list of the most important phases. If we annotate the defaults with what happens in the case of GNU Hello, things start to make sense:

Since GNU Hello uses Make & ./configure, the defaults work perfectly for us in this case. That is why this GNU Hello example is so short!

Your Packing Future

While it's amazing to use mkDerivation (so much easier than an RPM spec), there are many cases when you should not use mkDerivation. NixPkgs contains many useful build support functions. These are functions that return derivations, but do a bit of the hard work and boilerplate for you. These make it easy to build packages that meet specified criteria.

We've seen a few build support today; such as fetchFromGitHub or fetchurl. These just functions that return derivations. In these cases, they return derivations to download and extract the source files.

For example, there is pkgs.python36Packages.buildPythonPackage, which is a super easy way to build a python package.

When making packages, there are helpful resources to check:

• Chapter 9. Support for specific programming languages and frameworks of the NixPkgs manual. This documents language specific build support functions.
• The NixPkgs repository. Many packages you make could be similar to packages that already exist. (Note that packages inside of NixPkgs are written in a bit of a different way to ours; since they can't reference NixPkgs directly. Instead, they are structured as functions. If a package depends on another, it takes the other packages as an argument. For more on this subject; see Nix Pill 13: Callpackage Design Patten)

Up Next

In part 5, we'll learn about functions in the Nix programming language. With the knowledge of functions, we can write go on and write our own build support function!

Follow the series on GitHub

Hero image from nix-artwork by Luca Bruno

• <<

Getting radical realtime transparency in a project can be slow and frustrating, especially in the beginning. Most folks don’t know this, but in order to have public conversations, leaders need to send out a ridiculous number of private messages to get things rolling. In fact, looking at my own inbox history for the past half-decade, I’ve sent anywhere between 2-20 private messages – on average  (not maximum, average) – to get a single public message during the early stages of a project’s “open” life.

You really need to keep poking people in private asking them to put their messages public. It’s thankless and invisible work. It takes a while to build a new cultural habit, and for a while it’s going to seem like you’ll be doing this forever… but trust me, it will come. It’s going to take longer than you want it to, it’s going to take an unexpected route, but keep the faith – it will come.

There are three strategies it’s useful to have up your sleeve for times like this.

Start the conversation in private, then say something like “hey, this is really good, could you resend it to the public list and I’ll reply there?” This is good for starters if folks are new to the “default to open” concept and are reacting with great nervousness. This nervousness stems from wariness that they may not want to go public with some hypothetical future thing – in effect, worrying about a problem that hasn’t happened yet. Going this route allows beginners in radical transparency to look at something they’ve already written and assess the risk for only that specific situation – no unknowns here, no future commitments. After a few times of going “oh, I guess that retroactive transparency was okay!” it’s much easier to ask people to give “open by default” a chance.

Publicly announce that you’ll only respond to things sent to the public list. Reply to private emails with a reminder of this. This only works only if the people you’re trying to persuade are unable to route around you. It’s also a bit of a strongarm tactic, not appropriate for all situations and best used in moderation if at all. But if you’re a project manager, or an instructor, or a senior engineer, or something of the sort, you might be able to get away with it – and boy, folks learn fast this way.

Get others to help you with the nudges-to-public. Those 20 private emails to get a single public email? No reason why you’ve got to be the only one doing it. Train others to become Agents of Transparency as soon as you can, especially if they were once on the other side of the conversation. To begin with, ask them to work specific mailing lists, specific people, or specific conversation threads into the public eye – coach them from behind if needed. After a little while, they’ll be able to do it on their own – then just ask them to keep an eye out in general, and hey presto!

The key thing to keep in mind is that this is an investment. You’re putting resources into something that may not see returns for a little while. But the returns will come, and they’ll be worth it – when a project tips over into living, breathing, and practicing true realtime transparency, the results of the culture shift can be stunningly refreshing.

Keep going.

This guide will build on the previous two guides, and look at creating your first useful derivation (or "package").

This will teach you how to package a shell script.

Packaging a shell script (with no dependencies)

We can use the function pkgs.writeShellScriptBin from NixPkgs, which handles generating a derivation for us.

This function takes 2 arguments; what name you want the script to have in your PATH, and a string being the contents of the script.

So we could have:

pkgs.writeShellScriptBin "helloWorld" "echo Hello World"

That would create a shell script named "helloWorld", that printed "Hello World".

Let's put that in an environment; so we can use it in nix-shell. Write this to test.nix:

with import <nixpkgs> {};

let
  # Use the let-in clause to assign the derivation to a variable
  myScript = pkgs.writeShellScriptBin "helloWorld" "echo Hello World";
in
stdenv.mkDerivation rec {
  name = "test-environment";

  # Add the derivation to the PATH
  buildInputs = [ myScript ];
}

We can then enter the nix-shell and run it:

sam@vcs ~> nix-shell test.nix

[nix-shell:~]$ helloWorld
Hello World

Great! You've successfully made your first package. If you use NixOS, you can modify your system configuration and include it in your environment.systemPackages list. Or you can use it in a nix-shell (like we just did). Or whatever you want! Despite being one line of code, this is a real Nix derivation that we can use.

Referencing other commands in your script

For this example/section; we are going to look at something more complex. Say you want to write a script to find your public IP address. We're basically going to run this command:

curl http://httpbin.org/get | jq --raw-output .origin

But running this requires dependencies; you need curl and jq installed. How do we specify dependencies in Nix?

Well, we could just add them to the build input for the shell:

# DO NOT USE THIS; this is a BAD example
with import <nixpkgs> {};

let
  # This is the WORST way to do dependencies
  # We just specify the derivation the same way as before
  simplePackage = pkgs.writeShellScriptBin "whatIsMyIp" ''
    curl http://httpbin.org/get | jq --raw-output .origin
  '';
in
stdenv.mkDerivation rec {
  name = "test-environment";

  # Then we add curl & jq to the list of buildInputs for the shell
  # So curl and jq will be added to the PATH inside the shell
  buildInputs = [ simplePackage pkgs.jq pkgs.curl ];
}

This would work OK; you could go nix-shell then run whatIsMyIp and get your IP.

But it has a problem. The script would work unpredictably. If you took this package, and used it outside of the nix-shell, it wouldn't work - because you didn't have the dependencies. It also pollutes the environment of the end user; as they need to have a compatible version jq and curl in their path.

The more eloquent way to do this is to reference the exact packages in the shell script:

with import <nixpkgs> {};

let
  # The ${...} is for string interpolation
  # The '' quotes are used for multi-line strings
  simplePackage = pkgs.writeShellScriptBin "whatIsMyIp" ''
    ${pkgs.curl}/bin/curl http://httpbin.org/get \
      | ${pkgs.jq}/bin/jq --raw-output .origin
  '';
in
stdenv.mkDerivation rec {
  name = "test-environment";

  buildInputs = [ simplePackage ];
}

Here we reference the dependency package inside the derivation. To understand what this is doing, we need to see what the script is written to disk as. You can do that by running:

sam@vcs ~> nix-shell test.nix

[nix-shell:~]$ cat $(which whatIsMyIp)

Which gives us:

#!/nix/store/hqi64wjn83nw4mnf9a5z9r4vmpl72j3r-bash-4.4-p12/bin/bash
/nix/store/pkc7g36m95jymw3ga2i7pwrykcfs78il-curl-7.57.0-bin/bin/curl http://httpbin.org/get \
  | /nix/store/znqn0z505i0bm1aiz2jaj1ki7z4ck1sv-jq-1.5/bin/jq --raw-output .origin

As we can see, all the binaries referenced in this script are absolute paths, something like /nix/store/...../bin/name. The /nix/store/... is the path of the derivation's (package's) build output.

Due to the pure and functional of Nix, that path will be the same on every machine that ever runs Nix. Replacing fuzzy references (eg. jq) with definitive and unambiguous ones (/nix/store/...) is a core tenant of Nix; as it means packages come will all their dependencies and don't pollute your environment.

Since it is an absolute path, that script doesn't rely on the PATH environment variable; so the script can be used anywhere.

When you reference the path (like ${pkgs.curl} from above), Nix automatically knows to download the package into the machine whenever your package is downloaded.

Why do we do it like this? Ultimately, the goal of package management is to make consuming software easier. Creating less dependencies on the environment that runs the package makes it easier to use the script.

So the TL;DR is:

# BAD; not very explicit
# - we need to remember to add curl to the environment again later
badPackage = pkgs.writeShellScriptBin "something" ''
  curl ...
'';

# GOOD: Nix will do the magic for us
goodPackage = pkgs.writeShellScriptBin "something" ''
  ${pkgs.curl}/bin/curl ...
'';

Functions make creating packages easier

One of the main lessons from this process is that when you use functions (like pkgs.writeShellScriptBin) to create packages, it is pretty simple. Compare this to a traditional RPM or DEB workflow; where you would have needed to write a long spec file, put the script in a separate file, and fight your way through too much boilerplate.

Luckily; NixPkgs (the standard library of packages) includes a whole raft of functions that make packaging easier for specific needs. Most of these are in the build support folder of the NixPkgs repository. These are defined in the Nix expression language; the same language you are learning to write. For example, the pkgs.writeShellScriptBin function is defined as a ~10 line function.

Some of the more complex build support functions are documented in the NixPkgs manual. There is currently documentation for packaging Python, Go, Haskell, Qt, Rust, Perl, Node and many other types of applications.

Some of the more simple build support functions (like pkgs.writeShellScriptBin) are not documented (when I write this). Most of them are self explanatory, and can be found by reading their names in the so called trivial builders file.

Up Next

Derivations 102 - Learning Nix pt 4

Follow the series on GitHub

Hero image from nix-artwork by Eric Sagnes

This is just an idea I had last night for improving the welcome screen UX, if it’s too much work or Dave and Yash don’t like it I understand. However, I may try to code it myself for fun if Yash doesn’t have time. :-)

My fear is that when users start the Edit Fonts activity for the first time they will be be lost and not understand what to do. Some users might not even have a basic understand of what vector drawing is or how a font is made. This welcome screen will at least give the users a basic idea about how to use the activity. Most importantly, this makes the first screen visualy interesting, interactive and fun. Many users may not continue with the activity if the first page is dull and boring.

I’m proposing that the welcome screen have 4 options, represented by icons and text, plus an editable .glyph that reads “Edit Fonts” in the Geo typeface. The Edit-Fonts logotype will be one .glyph file that is only loaded and never saved. see below:

I have added a Geo-Regular.ufo file to the gh-pages repo with a special “editfonts.glyph” logotype:

https://github.com/sugarlabs/edit-fonts-activity/tree/gh-pages/files/fonts/Geo-Regular.ufo

There are two neat things about this approach. First, it uses components we already have, the only work will be laying out the page, which Dave or I can attempt if Yash is too busy. Second, if the user never realizes that the edit fonts logotype is editable, it still functions as a logotype. A similar UX design pattern was used for the start screen of the game Super Mario 64, see below:

Mario 64 easter-egg

Last Saturday (July 9th) Eli and I met up to review the codebase, and the main issue I identified was that Travis was not set up with flake8 to test the codebase was conforming to the pep8 guidelines.

I’d filed Issue #17 for this, back at the start of the project on May 19. Yash had started to develop the [.travis.yml](https://github.com/sugarlabs/edit-fonts-activity/blob/gh-pages/.travis.yml) file to build a .xo bundle but hadn’t complete this just yet, so I commended out most of the code and what remained is very simple:

# this makes travis run a fast Docker container system
sudo: false
# we use python 2.7
language: python
python:
  - "2.7"
# we need to install flake8 to use it
before_install:
  - "pip install flake8"
# we check the codebase
script: 
  - "flake8 --statistics --ignore=E402 --exclude=defcon,extractor,fontTools,fontmake,robofab,ufo2ft,ufoLib,snippets ."

You can see there’s a few arguments passed that are pretty simple.

Stastics prints the number of occurences of each error, so you can fix the most common issues across the codebase first.

E402 is about the order of imports, but since we need to import gi to version later imports, we can’t adhere to that rule, so we ignore it.

We also exclude all the third party libraries, and our snippets.

Eli and I worked together on this and I finished it up on Sunday in Pull Request #65

Yash had already set up Travis configuration, at https://travis-ci.org/sugarlabs/edit-fonts-activity, so once this was merged, our button went green:

Finally I added a CONTRIBUTING.md file that explains how to use it.

I’ll get a similar travis set up for the gh-pages branch too.

Perhaps we could also set up a git hook that runs the flake8 command on each commit…