Frequently Asked Questions



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


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?

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 nix decides which parts of the environment affect a derivation and its sha256 hash

How to pin nixpkgs to a specific commit/branch?

Ways to provide/pin nixpkgs:

  • nix-channel --add URL nixpkgs && nix-channel --update sets it globally for the user, but it doesn’t allow precision (pinning to specific version)
  • As environment variable: $NIX_PATH=URL
  • -I command line parameter to most of commands like nix-build, nix-shell, etc
  • Using builtins.fetchTarball function that fetches the channel at evaluation time

Possible URL values:

  • Local file path. Using just . means that nixpkgs is located in current folder.
  • Pinned to a specific commit:
  • Using latest channel, meaning all tests have passed:
  • Using latest channel, but hosted by github:
  • Using latest commit for release branch, but not tested yet:


  • nix-build -I ~/dev
  • nix-build -I ~/dev
  • nix-build -I nixpkgs=
  • NIX_PATH=nixpkgs= nix-build ...
  • Using just Nix:
with import (fetchTarball {};

stdenv.mkDerivation { … }

How to build reverse dependencies of a package?

nox-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?

Subquestion: how stable is unstable?

How do I mirror tarballs?

We have a content-addressed tarball mirror at for this purpose. “fetchurl” will automatically use this mirror to obtain files by hash. However:

How can I manage dotfiles in $HOME with Nix?

See following solutions:

Are there some known impurities in builds?


  • 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)


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

Apply following patch:

diff --git a/nixos/lib/test-driver/ b/nixos/lib/test-driver/
index 8ad0d67..838fbdd 100644
--- a/nixos/lib/test-driver/
+++ b/nixos/lib/test-driver/
@@ -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:


What to do if cache/hydra is down or unreachable?

Pass following to Nix commands:

  • --option connect-timeout 5 to wait only 5 second on binary package
  • --fallback to build from source if binary package fetching fails

How do I add a new binary cache?

Using NixOS:

trustedBinaryCaches = [ "" "" ];
binaryCaches = trustedBinaryCaches;
binaryCachePublicKeys = [ "" ];

Using Nix:

$ echo "trusted-binary-caches =" >> /etc/nix/nix.conf
$ nix-build helpers/bench.nix --option extra-binary-caches`