nix.dev/source/faq.rst

Frequently Asked Questions
==========================

Nix
***

Secrets?
--------

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 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 <https://nixos.org/nix/manual/#ssec-builtins>`_ 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: ``https://github.com/NixOS/nixpkgs/archive/addcb0dddf2b7db505dae5c38fceb691c7ed85f9.tar.gz``

- Using latest channel, meaning all tests have passed: ``http://nixos.org/channels/nixos-17.03/nixexprs.tar.xz``

- Using latest channel, but hosted by github: ``https://github.com/NixOS/nixpkgs-channels/archive/nixos-17.03.tar.gz``

- Using latest commit for release branch, but not tested yet: ``https://github.com/NixOS/nixpkgs/archive/release-17.03.tar.gz``

Examples:


- ``nix-build -I ~/dev``
- ``nix-build -I ~/dev``
- ``nix-build -I nixpkgs=http://nixos.org/channels/nixos-17.03/nixexprs.tar.xz``
- ``NIX_PATH=nixpkgs=http://nixos.org/channels/nixos-17.03/nixexprs.tar.xz nix-build ...``
- Using just Nix:

::

    with import (fetchTarball https://github.com/NixOS/nixpkgs-channels/archive/nixos-14.12.tar.gz) {};

    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 tarballs.nixos.org for this
purpose. "fetchurl" will automatically use this mirror to obtain files by hash.
However:

* The mirroring script was not running lately. I've revived it so 16.03 tarballs
  are mirrored now
  (https://github.com/NixOS/nixos-org-configurations/commit/a17ccf87deae4fb86639c8d34ab5938edd68d8c4).

* The mirroring script only copies tarballs of packages in the Nixpkgs Hydra
  jobset. Since moreutils is not part of the jobset, it's not mirrored. This can
  be fixed by adding a meta.platforms attribute to moreutils.

How can I manage dotfiles in $HOME with Nix?
--------------------------------------------

See following solutions:

- https://github.com/sheenobu/nix-home

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 <https://github.com/NixOS/nixpkgs/issues/15581>`_)
- 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-cd

Hydra
*****

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

Pass following to Nix commands:

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


How do I add a new binary cache?
--------------------------------

Using `NixOS`:

.. code-block:: nix

    trustedBinaryCaches = [ "https://cache.nixos.org" "https://hydra.snabb.co" ];
    binaryCaches = trustedBinaryCaches;
    binaryCachePublicKeys = [ "hydra.snabb.co-1:zPzKSJ1mynGtYEVbUR0QVZf9TLcaygz/OyzHlWo5AMM=" ];

Using `Nix`:

.. code-block:: bash

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