nix-freeze-tree: Convert a tree of files into fixed-output derivations

[ agpl, application, library, nix, program ] [ Propose Tags ]

nix-freeze-tree walks a directory tree and generates a tree of Nix expressions that recreate the tree from fixed-output derivations, one derivation per file in the source tree. See to get started.

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Dependencies base (>=4.9 && <4.14), binary (>= && <0.9), bytestring (>= && <0.11), data-fix (>=0.2.0 && <0.3), directory (>= && <1.4), directory-tree (>=0.12.1 && <0.13), hnix (>=0.6.1 && <0.7), hnix-store-core (>= && <0.2), optparse-applicative (>= && <0.15), path (>=0.6.1 && <0.7), prettyprinter (>= && <1.3), raw-strings-qq (==1.1.*), text (>= && <1.3), transformers (>= && <0.6) [details]
License AGPL-3.0-or-later
Copyright (c) 2020 Jack Kelly
Author Jack Kelly
Category Application, Nix
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Uploaded by jack at 2020-01-25T04:37:42Z
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Executables nix-freeze-tree
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Maintainer's Corner

For package maintainers and hackage trustees

Readme for nix-freeze-tree-

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nix-freeze-tree status AGPLv3

nix-freeze-tree is a utility that walks a directory tree, and writes out Nix expressions which rebuild that tree. The generated expressions have two important properties:

  1. Each file in the tree is built by a separate fixed-output derivation, and

  2. Directories are built by derivations that symlink their contents recursively.

If you are using nix copy to ship a derivation between nix stores, copying the derivation built by evaluating the output of nix-freeze-tree can reuse existing files in the destination store, as fixed-output derivations can be checked against a hash before copying.

I use this to speed up deploys to my personal website. Running nix-freeze-tree on the output from my site generator and importing the result means that nix copy only uploads new files, instead of copying a single giant derivation containing every image on the site.



If you use Nix, you can install the command into your local environment by running nix-env -f -iA command.


If you have GHC installed, you can also build and install the binary by running cabal v2-install nix-freeze-tree. You can also run the binary from the build directory with cabal v2-run nix-freeze-tree -- ARGS.


Usage: nix-freeze-tree [--version] [-f|--force] [-v|--verbose]
                       [-o|--out-root OUT_ROOT] [IN_DIR]
  Write a tree of nix expressions to OUT_ROOT that build a derivation,
  symlinking every file in IN_DIR as a separate fixed-output derivation.

Available options:
  --version                Display version information and exit.
  -h,--help                Show this help text
  -f,--force               If any default.nix exist in the output directory,
                           remove them and generate anyway
  -v,--verbose             Display messages while working.
  -o,--out-root OUT_ROOT   Where to write the nix files (default: IN_DIR)
  IN_DIR                   Directory to freeze (default: .)

Calling from Nix

This repository's default.nix gives access to a function freeze, which can be used to freeze the output of an existing derivation.

You can use it with Nix code that looks a bit like:

  # Some nixpkgs from somewhere
  nixpkgs = ...;

  # Whether you get it from a channel, a local checkout,
  # builtins.fetchTarball or builtins.fetchgit is up to you
  nix-freeze-tree-repo = builtins.fetchTarball { ... };

  # Derivation to freeze
  drv = mkDerivation { ... };

  # Evaluate default.nix from nix-freeze-tree-repo and hold onto the
  # freeze function
  freeze = (import nix-freeze-tree-repo { inherit nixpkgs; }).freeze;
  freeze drv


Q: Why didn't you use symlinkJoin to tie all the subtrees together?

A: I had the program mostly working before I remembered that symlinkJoin existed. I thought about making each file's derivation contain the directories leading up to it and then symlinkJoining them all together, but decided against it for three reasons:

  1. You can easily build subtrees using the Nix expressions generated by nix-freeze-tree;
  2. Building the output of nix-freeze-tree will correctly create empty directories present in the input; and
  3. Files are hashed based on their content alone, not their position in the tree-to-be-frozen (reducing duplication).

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