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What the generated script does

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Karim Vergnes edited this page Mar 25, 2023 · 3 revisions

The script generated by Nixie is the end product and main focus of the Nixie project. It is a simple bash script, which can be found here, with the -----BEGIN ARCHIVE SECTION----- suffix and a gzipped tar archive appended to it.

The bootstrap process

The features file

The first task accomplished by the script is to read the features file at the root of its appended archive, into its corresponding environment variables. The file is generated by builder/nixie/output/script.py and follows this simple format:

EXTRA_FEATURES="(string to pass to the --extra-experimental-features nix option)"
EXTRA_SUBSTITUTERS="(string to pass to the --extra-substituters nix option)"
EXTRA_TRUSTED_PUBLIC_KEYS="(string to pass to the --extra-trusted-public-keys nix option)"
SOURCE_CACHE="nix-wrap.cachix.org"
SOURCE_DERIVATION="(nix hash of sources derivation)"
BINARIES_DERIVATION="(nix hash of binaries derivation)"
NIXIE_VERSION="(optional, version of the nixie builder which created the script)"

If one of the EXTRA_* variables is empty, the corresponding option won't be passed to Nix.

Figuring out call context

In order to remain functionally compatible with the official nix command, the script performs a few checks around here:

  • Are we being run as nix-shell or another Nix subcommand? If so, unpack our channels into .nixie/channels before running
  • Are we being run as a shebang (#!)? If so, transform the nix-shell command to emulate official behavior.
  • Are we being run as an arbitrary command? If so, run that command in the default develop shell.

Retrieving the nix binary

Next up on the list is figuring out if the user has Nix installed (nix exists in $PATH and the /nix directory exists at the system root)

The sources and binaries derivations

In order to balance script size, launch speed and portability, the Nixie script relies on key derivations built by automated tasks in this repository:

  • github:nixie-dev/nixie#static-bins, the set of prebuilt static Nix binaries and the prebuilt libfakedir.dylib
  • github:nixie-dev/nixie#sources, the minimal set of configured source code required to build Nix on a target machine

All resource files are at the root of the produced path, and are listed in a filelist file, which is used by the Nixie builder to retrieve and embed these resources offline if applicable.
If those resources aren't included, the script will attempt to retrieve them using the /serve/(hash)/(file) HTTPS endpoint provided by Cachix. If a custom server and/or custom derivations are specified, the script expects these derivations to be available at that endpoint.

Example: https://nix-wrap.cachix.org/serve/bh38pf33bgyywhpakhn1b1rc4zagmsy4/nix.Linux.x86_64

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