TranslateProject/sources/tech/20230228.4 ⭐️⭐️ Some notes on using nix.md

[#]: subject: "Some notes on using nix"
[#]: via: "https://jvns.ca/blog/2023/02/28/some-notes-on-using-nix/"
[#]: author: "Julia Evans https://jvns.ca/"
[#]: collector: "lkxed"
[#]: translator: " "
[#]: reviewer: " "
[#]: publisher: " "
[#]: url: " "

Some notes on using nix
======

Recently I started using a Mac for the first time. The biggest downside I’ve
noticed so far is that the package management is much worse than on Linux.
At some point I got frustrated with homebrew because I felt like it was
spending too much time upgrading when I installed new packages, and so I
thought – maybe I’ll try the [nix][1] package manager!

nix has a reputation for being confusing (it has its whole
own programming language!), so I’ve been trying to figure out how to use nix in
a way that’s as simple as possible and does not involve managing any
configuration files or learning a new programming language. Here’s what I’ve
figured out so far! We’ll talk about how to:

- install packages with nix
- build a custom nix package for a C++ program called [paperjam][2]
- install a 5-year-old version of [hugo][3] with nix

As usual I’ve probably gotten some stuff wrong in this post since I’m still
pretty new to nix. I’m also still not sure how much I like nix – it’s very
confusing! But it’s helped me compile some software that I was struggling to
compile otherwise, and in general it seems to install things faster than
homebrew.

#### what’s interesting about nix?

People often describe nix as “declarative package management”. I don’t
care that much about declarative package management, so here are two things
that I appreciate about nix:

- It provides binary packages (hosted at [https://cache.nixos.org/][4]) that you can quickly download and install
- For packages which don’t have binary packages, it makes it easier to compile them

I think that the reason nix is good at compiling software is that:

- you can have multiple versions of the same library or program installed at a time (you could have 2 different versions of libc for instance). For example I have two versions of node on my computer right now, one at `/nix/store/4ykq0lpvmskdlhrvz1j3kwslgc6c7pnv-nodejs-16.17.1` and one at `/nix/store/5y4bd2r99zhdbir95w5pf51bwfg37bwa-nodejs-18.9.1`.
- when nix builds a package, it builds it in isolation, using only the
specific versions of its dependencies that you explicitly declared. So
there’s no risk that the package secretly depends on another package on your
system that you don’t know about. No more fighting with `LD_LIBRARY_PATH`!
- a lot of people have put a lot of work into writing down all of the
dependencies of packages

I’ll give a couple of examples later in this post of two times nix made it easier for me to compile software.

#### how I got started with nix

here’s how I got started with nix:

- Install nix. I forget exactly how I did this, but it looks like there’s an [official installer][5] and an [unofficial installer from zero-to-nix.com][6]. The [instructions for uninstalling nix on MacOS with the standard multi-user install][7] are a bit complicated, so it might be worth choosing an installation method with simpler uninstall instructions.
- Put `~/.nix-profile/bin` on my PATH
- Install packages with `nix-env -iA nixpkgs.NAME`
- That’s it.

Basically the idea is to treat `nix-env -iA` like `brew install` or `apt-get install`.

For example, if I want to install `fish`, I can do that like this:

```
nix-env -iA nixpkgs.fish
```

This seems to just download some binaries from [https://cache.nixos.org][8] – pretty simple.

Some people use nix to install their Node and Python and Ruby packages, but I haven’t
been doing that – I just use `npm install` and `pip install` the same way I
always have.

#### some nix features I’m not using

There are a bunch of nix features/tools that I’m not using, but that I’ll
mention. I originally thought that you _had_ to use these features to use nix,
because most of the nix tutorials I’ve read talk about them. But you don’t have to use them.

- NixOS (a Linux distribution)
- [nix-shell][9]
- [nix flakes][10]
- [home-manager][11]
- [devenv.sh][12]

I won’t go into these because I haven’t really used them and there are lots of
explanations out there.

#### where are nix packages defined?

I think packages in the main nix package repository are defined in [https://github.com/NixOS/nixpkgs/][13]

It looks like you can search for packages at [https://search.nixos.org/packages][14]. The two official ways to search packages seem to be:

- `nix-env -qaP NAME`, which is very extremely slow and which I haven’t been able to get to actually work
- `nix --extra-experimental-features 'nix-command flakes' search nixpkgs NAME`, which does seem to work but is kind of a mouthful. Also all of the packages it prints out start with `legacyPackages` for some reason

I found a way to search nix packages from the command line that I liked better:

- Run `nix-env -qa '*' > nix-packages.txt` to get a list of every package in the Nix repository
- Write a short `nix-search` script that just greps `packages.txt` (`cat ~/bin/nix-packages.txt | awk '{print $1}' | rg "$1"`)

#### everything is installed with symlinks

One of nix’s major design choices is that there isn’t one single `bin` with all
your packages, instead you use symlinks.  There are a lot of layers of symlinks. A few examples of symlinks:

- `~/.nix-profile` on my machine is (indirectly) a symlink to `/nix/var/nix/profiles/per-user/bork/profile-111-link/`
- `~/.nix-profile/bin/fish` is a symlink to `/nix/store/afkwn6k8p8g97jiqgx9nd26503s35mgi-fish-3.5.1/bin/fish`

When I install something, it creates a new `profile-112-link` directory with new symlinks and updates my `~/.nix-profile` to point to that directory.

I think this means that if I install a new version of `fish` and I don’t like it, I can
easily go back just by running `nix-env --rollback` – it’ll move me to my previous profile directory.

#### uninstalling packages doesn’t delete them

If I uninstall a nix package like this, it doesn’t actually free any hard drive space, it just removes the symlinks.

```
$ nix-env --uninstall oil
```

I’m still not sure how to actually delete the package – I ran a garbage collection like this, which seemed to delete some things:

```
$ nix-collect-garbage
...
85 store paths deleted, 74.90 MiB freed
```

But I still have `oil` on my system at `/nix/store/8pjnk6jr54z77jiq5g2dbx8887dnxbda-oil-0.14.0`.

There’s a more aggressive version of `nix-collect-garbage` that also deletes old versions of your profiles (so that you can’t rollback)

```
$ nix-collect-garbage -d --delete-old
```

That doesn’t delete `/nix/store/8pjnk6jr54z77jiq5g2dbx8887dnxbda-oil-0.14.0` either though and I’m not sure why.

#### upgrading

It looks like you can upgrade nix packages like this:

```
nix-channel --update
nix-env --upgrade
```

(similar to `apt-get update && apt-get upgrade`)

I haven’t really upgraded anything yet. I think that if something goes wrong with an upgrade, you can roll back (because everything is immutable in nix!) with

```
nix-env --rollback
```

Someone linked me to [this post from Ian Henry][15] that
talks about some confusing problems with `nix-env --upgrade` – maybe it
doesn’t work the way you’d expect? I guess I’ll be wary around upgrades.

#### next goal: make a custom package of paperjam

After a few months of installing existing packages, I wanted to make a custom package with nix for a program called [paperjam][2] that wasn’t already packaged.

I was actually struggling to compile `paperjam` at all even without nix  because the version I had
of `libiconv` I has on my system was wrong. I thought it might be easier to
compile it with nix even though I didn’t know how to make nix packages yet. And
it actually was!

But figuring out how to get there was VERY confusing, so here are some notes about how I did it.

#### how to build an example package

Before I started working on my `paperjam` package, I wanted to build an example existing package just to
make sure I understood the process for building a package. I was really
struggling to figure out how to do this, but I asked in Discord and someone
explained to me how I could get a working package from [https://github.com/NixOS/nixpkgs/][13] and build it. So here
are those instructions:

**step 1:** Download some arbitrary package from [nixpkgs][13] on github, for example the `dash` package:

```
wget https://raw.githubusercontent.com/NixOS/nixpkgs/47993510dcb7713a29591517cb6ce682cc40f0ca/pkgs/shells/dash/default.nix -O dash.nix
```

**step 2**: Replace the first statement (`{ lib , stdenv , buildPackages , autoreconfHook , pkg-config , fetchurl , fetchpatch , libedit , runCommand , dash }:` with `with import <nixpkgs> {};` I don’t know why you have to do this,
but it works.

**step 3**: Run `nix-build dash.nix`

This compiles the package

**step 4**: Run `nix-env -i -f dash.nix`

This installs the package into my `~/.nix-profile`

That’s all! Once I’d done that, I felt like I could modify the `dash` package and make my own package.

#### how I made my own package

`paperjam` has one dependency (`libpaper`) that also isn’t packaged yet, so I needed to build `libpaper` first.

Here’s `libpaper.nix`. I basically just wrote this by copying and pasting from
other packages in the [nixpkgs][13] repository.
My guess is what’s happening here is that nix has some default rules for
compiling C packages (like “run `make install`”), so the `make install` happens
default and I don’t need to configure it explicitly.

```
with import <nixpkgs> {};

stdenv.mkDerivation rec {
  pname = "libpaper";
  version = "0.1";

  src = fetchFromGitHub {
    owner = "naota";
    repo = "libpaper";
    rev = "51ca11ec543f2828672d15e4e77b92619b497ccd";
    hash = "sha256-S1pzVQ/ceNsx0vGmzdDWw2TjPVLiRgzR4edFblWsekY=";
  };

  buildInputs = [ ];

  meta = with lib; {
    homepage = "https://github.com/naota/libpaper";
    description = "libpaper";
    platforms = platforms.unix;
    license = with licenses; [ bsd3 gpl2 ];
  };
}
```

Basically this just tells nix how to download the source from GitHub.

I built this by running `nix-build libpaper.nix`

Next, I needed to compile `paperjam`. Here’s a link to the [nix package I wrote][16]. The main things I needed to do other than telling it where to download the source were:

- add some extra build dependencies (like `asciidoc`)
- set some environment variables for the install (`installFlags = [ "PREFIX=$(out)" ];`) so that it installed in the correct directory instead of `/usr/local/bin`.

I set the hashes by first leaving the hash empty, then running `nix-build` to get an error message complaining about a mismatched hash. Then I copied the correct hash out of the error message.

I figured out how to set `installFlags` just by running `rg PREFIX`
in the nixpkgs repository – I figured that needing to set a `PREFIX` was
pretty common and someone had probably done it before, and I was right. So I
just copied and pasted that line from another package.

Then I ran:

```
nix-build paperjam.nix
nix-env -i -f paperjam.nix
```

and then everything worked and I had `paperjam` installed! Hooray!

#### next goal: install a 5-year-old version of hugo

Right now I build this blog using Hugo 0.40, from 2018. I don’t need any new
features so I haven’t felt a need to upgrade. On Linux this is easy: Hugo’s
releases are a static binary, so I can just download the 5-year-old binary from
the [releases page][17] and
run it. Easy!

But on this Mac I ran into some complications. Mac hardware has changed in the
last 5 years, so the Mac Hugo binary I downloaded crashed. And when I tried to
build it from source with `go build`, that didn’t work either because Go build
norms have changed in the last 5 years as well.

I was working around this by running Hugo in a Linux docker container, but I
didn’t love that: it was kind of slow and it felt silly. It shouldn’t be that
hard to compile one Go program!

Nix to the rescue! Here’s what I did to install the old version of Hugo with
nix.

#### installing Hugo 0.40 with nix

I wanted to install Hugo 0.40 and put it in my PATH as `hugo-0.40`. Here’s how
I did it. I did this in a kind of weird way, but it worked ([Searching and installing old versions of Nix packages][18]
describes a probably more normal method).

**step 1**: Search through the nixpkgs repo to find Hugo 0.40

I found the `.nix` file here [https://github.com/NixOS/nixpkgs/blob/17b2ef2/pkgs/applications/misc/hugo/default.nix][19]

**step 2**: Download that file and build it

I downloaded that file (and another file called `deps.nix` in the same directory), replaced the first line with `with import <nixpkgs> {};`, and built it with `nix-build hugo.nix`.

That almost worked without any changes, but I had to make two changes:

- replace `with stdenv.lib` to `with lib` for some reason.
- rename the package to `hugo040` so that it wouldn’t conflict with the other version of `hugo` that I had installed

**step 3**: Rename `hugo` to `hugo-0.40`

I write a little post install script to rename the Hugo binary.

```
postInstall = ''
    mv $out/bin/hugo $out/bin/hugo-0.40
  '';
```

I figured out how to run this by running `rg 'mv '` in the nixpkgs repository and just copying and modifying something that seemed related.

**step 4**: Install it

I installed into my `~/.nix-profile/bin` by running `nix-env -i -f hugo.nix`.

And it all works! I put the final `.nix` file into my own personal [nixpkgs repo][20] so that I can use it again later if I
want.

#### reproducible builds aren’t magic, they’re really hard

I think it’s worth noting here that this `hugo.nix` file isn’t magic – the
reason I can easily compile Hugo 0.40 today is that many people worked for a long time to make it possible to
package that version of Hugo in a reproducible way.

#### that’s all!

Installing `paperjam` and this 5-year-old version of Hugo were both
surprisingly painless and actually much easier than compiling it without nix,
because nix made it much easier for me to compile the `paperjam` package with
the right version of `libiconv`, and because someone 5 years ago had already
gone to the trouble of listing out the exact dependencies for Hugo.

I don’t have any plans to get much more complicated with nix (and it’s still
very possible I’ll get frustrated with it and go back to homebrew!), but we’ll
see what happens! I’ve found it much easier to start in a simple way and then
start using more features if I feel the need instead of adopting a whole bunch
of complicated stuff all at once.

I probably won’t use nix on Linux – I’ve always been happy enough with `apt`
(on Debian-based distros) and `pacman` (on Arch-based distros), and they’re
much less confusing. But on a Mac it seems like it might be worth it. We’ll
see! It’s very possible in 3 months I’ll get frustrated with nix and just go back to homebrew.

--------------------------------------------------------------------------------

via: https://jvns.ca/blog/2023/02/28/some-notes-on-using-nix/

作者：[Julia Evans][a]
选题：[lkxed][b]
译者：[译者ID](https://github.com/译者ID)
校对：[校对者ID](https://github.com/校对者ID)

本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出

[a]: https://jvns.ca/
[b]: https://github.com/lkxed/
[1]: https://nixos.org/
[2]: https://mj.ucw.cz/sw/paperjam/
[3]: https://github.com/gohugoio/hugo/
[4]: https://cache.nixos.org/
[5]: https://nixos.org/download
[6]: https://zero-to-nix.com/concepts/nix-installer
[7]: https://nixos.org/manual/nix/stable/installation/installing-binary.html#macos
[8]: https://cache.nixos.org
[9]: https://nixos.org/guides/nix-pills/developing-with-nix-shell.html
[10]: https://nixos.wiki/wiki/Flakes
[11]: https://github.com/nix-community/home-manager
[12]: https://devenv.sh/
[13]: https://github.com/NixOS/nixpkgs/
[14]: https://search.nixos.org/packages
[15]: https://ianthehenry.com/posts/how-to-learn-nix/my-first-package-upgrade/
[16]: https://github.com/jvns/nixpkgs/blob/22b70a48a797538c76b04261b3043165896d8f69/paperjam.nix
[17]: https://github.com/gohugoio/hugo/releases/tag/v0.40
[18]: https://lazamar.github.io/download-specific-package-version-with-nix/
[19]: https://github.com/NixOS/nixpkgs/blob/17b2ef2/pkgs/applications/misc/hugo/default.nix
[20]: https://github.com/jvns/nixpkgs/