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cached-nix-shell is a caching layer fornix-shell featuring instant startup time on subsequent runs.

It supports NixOS and Linux.

Install the release version from Nixpkgs:

nix-env -iA nixpkgs.cached-nix-shell

Or, install the latest development version from GitHub:

nix-env -if https://github.com/xzfc/cached-nix-shell/tarball/master

Just replacenix-shell withcached-nix-shell in the shebang line:

#! /usr/bin/env cached-nix-shell#! nix-shell -i python3 -p pythonprint("Hello, World!")

Alternatively, callcached-nix-shell directly:

$ cached-nix-shell ./hello.py$ cached-nix-shell -p python3 --run'python --version'

Or use the--wrap option for programs that callnix-shell internally.

$ cached-nix-shell --wrap stack build

$ time ./hello.py # first run; no cache usedcached-nix-shell: updating cacheHello, World!./hello.py  0.33s user 0.06s system 91% cpu 0.435 total$ time ./hello.pyHello, World!./hello.py  0.02s user 0.01s system 97% cpu 0.029 total

cached-nix-shell stores environment variables set up bynix-shell and reuses them on subsequent runs.Ittraces which files are read bynix during an evaluation, and performs a proper cache invalidation if any of the used files are changed.The cache is stored in~/.cache/cached-nix-shell/.

The following situations are covered:

• builtins.readFile is used
• builtins.readDir is used
• import ./file.nix is used
• updating/etc/nix/nix.conf or~/.config/nix/nix.conf
• updating nix channels
• updating$NIX_PATH environment variable

The following situations aren't handled bycached-nix-shell and may lead to staled cache:

• builtins.fetchurl or other network builtins are used (e.g. innixpkgs-mozilla)

• https://discourse.nixos.org/t/speeding-up-nix-shell-shebang/4048

• There are related projects focused on usingnix-shell for project developing:

  • direnv withuse_nix
  • Cached and Persistent Nix shell with direnv integration
  • lorri, anix-shell replacement for project development
    • lorri #167
  • nix-develop, universal build tool featuring cachednd shell command

• nix-community/home-manager#447