nix.dev/source/recipes/faq.md

Frequently Asked Questions

Nix

Should I enable flakes?

You have to judge for yourself based on your needs.

Flakes and the nix command suite bring multiple improvements that are relevant for both software users and package authors:

• The new command line interface, together with flakes, makes dealing with existing packages significantly more convenient.
• The constraints imposed on flakes strengthen reproducibility by default, and enable various performance improvements when interacting with a large Nix package repository like {term}Nixpkgs.
• Flake references allow for easier handling of version upgrades for existing packages or project dependencies.
• The flake schema helps with composing Nix projects from multiple sources in an ordered fashion.

Other than that, and below the surface of the flake schema, Nix and the Nix language work exactly the same in both cases. In principle, the same level of reproducibility can be achieved with or without flakes. In particular, the process of adding software to {term}Nixpkgs or maintaining {term}NixOS modules and configurations is not affected by flakes at all.

Both paradigms have their own set of unique concepts and support tooling that have to be learned, with varying ease of use, implementation quality, and support status. At the moment, neither the stable nor the experimental interface is clearly superior to the other in all aspects. While flakes reduce complexity in some regards, they introduce additional concepts and you will have to learn more about the system to fully understand how it works.

There are downsides to relying on experimental features in general:

• Interfaces and behavior of experimental features could still be changed by Nix developers. This may require you to adapt your code at some point in the future, which will be more effort when it has grown in complexity. Currently there is no agreed-upon plan or timeline for stabilising flakes.
• The Nix maintainer team focuses on fixing bugs and regressions in stable interfaces, supporting well-understood use cases, as well as improving the internal design and overall contributor experience in order to ease future development. Improvements to experimental features have low priority.
• The Nix documentation team focuses on improving documentation and learning materials for stable features and common principles. Using flakes, you will have to rely more heavily on user-to-user support, third-party documentation, and the source code.

What to do if a binary cache is down or unreachable?

Pass --option substitute false to Nix commands.

How do I add a new binary cache?

Using NixOS (≥ 22.05):

nix.settings = {
  trusted-substituters = [ "https://cache.nixos.org" ];
  substituters = [ "https://cache.nixos.org" ];
};

Using NixOS (≤ 21.11):

nix = {
  trustedBinaryCaches = [ "https://cache.nixos.org" ];
  binaryCaches = [ "https://cache.nixos.org" ];
};

Using Nix:

$ echo "trusted-binary-caches = https://cache.nixos.org" >> /etc/nix/nix.conf
$ nix-build helpers/bench.nix --option extra-binary-caches https://cache.nixos.org

How do I force nix to re-check whether something exists at a binary cache?

Nix caches the contents of binary caches so that it doesn't have to query them on every command. This includes negative answers (cache doesn't have something). The default timeout for that is 1 hour as of writing.

To wipe all cache-lookup-caches:

$ rm $HOME/.cache/nix/binary-cache-v*.sqlite*

Alternatively, use the narinfo-cache-negative-ttl option to reduce the cache timeout.

How do I fix: error: querying path in database: database disk image is malformed

Try:

$ sqlite3 /nix/var/nix/db/db.sqlite "pragma integrity_check"

Which will print the errors in the database. If the errors are due to missing references, the following may work:

$ mv /nix/var/nix/db/db.sqlite /nix/var/nix/db/db.sqlite-bkp
$ sqlite3 /nix/var/nix/db/db.sqlite-bkp ".dump" | sqlite3 /nix/var/nix/db/db.sqlite

How to operate between Nix paths and strings?

http://stackoverflow.com/a/43850372

How do I fix: error: current Nix store schema is version 10, but I only support 7

This means you have upgraded Nix sqlite schema to a newer version, but then tried to use older Nix.

The solution is to dump the db and use old Nix version to initialize it:

$ /path/to/nix/unstable/bin/nix-store --dump-db > /tmp/db.dump
$ mv /nix/var/nix/db /nix/var/nix/db.toonew
$ mkdir /nix/var/nix/db
$ nix-store --init # this is the old nix-store
$ nix-store --load-db < /tmp/db.dump

How to build reverse dependencies of a package?

$ nix-shell -p nixpkgs-review --run "nixpkgs-review wip"

I'm getting: writing to file: Connection reset by peer

Too big files in src, out of resources (HDD space, memory)

What are channels and different branches on github?

See https://nixos.wiki/wiki/Nix_channels

How can I manage dotfiles in $HOME with Nix?

https://github.com/nix-community/home-manager

Are there some known impurities in builds?

Yes.

• CPU (we try hard to avoid compiling native instructions, but rather hardcode supported ones)
• current time/date
• FileSystem (ext4 has a known bug creating empty files on power loss)
• Kernel
• Timing behaviour of the build system (parallel Make not getting correct inputs in some cases)

What's the recommended process for building custom packages?

E.g. if I git clone nixpkgs how do I use the cloned repo to define new / updated packages?

NixOS

How to build my own ISO?

http://nixos.org/nixos/manual/index.html#sec-building-image

How do I connect to any of the machines in NixOS tests?

Apply following patch:

diff --git a/nixos/lib/test-driver/test-driver.pl b/nixos/lib/test-driver/test-driver.pl
index 8ad0d67..838fbdd 100644
--- a/nixos/lib/test-driver/test-driver.pl
+++ b/nixos/lib/test-driver/test-driver.pl
@@ -34,7 +34,7 @@ foreach my $vlan (split / /, $ENV{VLANS} || "") {
     if ($pid == 0) {
         dup2(fileno($pty->slave), 0);
         dup2(fileno($stdoutW), 1);
-        exec "vde_switch -s $socket" or _exit(1);
+        exec "vde_switch -tap tap0 -s $socket" or _exit(1);
     }
     close $stdoutW;
     print $pty "version\n";

And then the vde_switch network should be accessible locally.

How to bootstrap NixOS inside an existing Linux installation?

There are a couple of tools:

• https://github.com/jeaye/nixos-in-place
• https://github.com/elitak/nixos-infect
• https://github.com/cleverca22/nix-tests/tree/master/kexec