Understand the nix derivation

This page records some advanced concepts and commands for nix

In Nix, packages are defined in a domain specific language called Nix Expression Language (details)

Note

  • Nix derivation: A derivation in Nix is a definition of a build, which takes some inputs and produces an output (in other words, the derivation is a function that describes a build process). Using Derivation is an alternative of using a channel, and is much more reproducible as the tarball can be fixed to a specific version as it is done here.

Nix Derivation

The core for a derivation is the derivation script, an example can be accessed here

There are at least two major components in a nix build script:

  • Input (e.g., channel for getting building tools)

  • Build process

    • name of the package

    • where to get the package

    • building tools needed

    • build phases defination

The description for the major components in a .nix build script is described below

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The file can be obtained here

Build the package with nix-build

After download the above nix build script, we can build the package as:

nix-build chord_example.nix

The output should sit in ./result/bin, we may be able to test the build as chord --help

Build the package with nix-shell (manually)

You can manually build the package by diving into the `nix-shell, for example

nix-shell --pure chord_example.nix

The shell will create a environment for us to manually build/debug the package.

Note

  • with --pure: the environment is clean with only tools defined in .nix (e.g., simgrid, boost and cmake).

  • without --pure: the environment starts from the current environment variables

Now let’s do the building manually,

git config --global http.sslVerify false
git clone https://gitlab.inria.fr/nix-tutorial/chord-tuto-nix-2019.git /tmp/chord-tuto-nix-2019
cmake .
make
mkdir -p $out/bin
mv chord $out/bin

Note that the reason we may have to run git config --global http.sslVerify false is that we may get SSL certificate problem from git if we don’t …

Create a generic environment with nix-shell

Soemtimes we may want to have an completely isolated environment for developing or debugging a package, nix-shell can do this for us as well

Thw following shows the difference between a nix build script and a nix env script

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You can download a sample env shell script from here

The generaic environment (or shell) can be triggered by

nix-shell shell.nix