Here we use the post-build hook to upload to a binary cache. This is a simple and working example, but it is not suitable for all use cases. The post build hook program runs after each executed build, and blocks the build loop. The build loop exits if the hook program fails. Concretely, this implementation will make Nix slow or unusable when the internet is slow or unreliable. A more advanced implementation might pass the store paths to a user-supplied daemon or queue for processing the store paths outside of the build loop.

This tutorial assumes you have configured an S3-compatible binary cache according to the instructions at , and that the root user's default AWS profile can upload to the bucket.

Use nix-store --generate-binary-cache-key to create our public and private signing keys. We will sign paths with the private key, and distribute the public key for verifying the authenticity of the paths. # nix-store --generate-binary-cache-key example-nix-cache-1 /etc/nix/key.private /etc/nix/key.public # cat /etc/nix/key.public example-nix-cache-1:1/cKDz3QCCOmwcztD2eV6Coggp6rqc9DGjWv7C0G+rM= Then, add the public key and the cache URL to your nix.conf's and like: substituters = https://cache.nixos.org/ s3://example-nix-cache trusted-public-keys = cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= example-nix-cache-1:1/cKDz3QCCOmwcztD2eV6Coggp6rqc9DGjWv7C0G+rM= We will restart the Nix daemon in a later step.

Write the following script to /etc/nix/upload-to-cache.sh: #!/bin/sh set -eu set -f # disable globbing export IFS=' ' echo "Signing paths" $OUT_PATHS nix sign-paths --key-file /etc/nix/key.private $OUT_PATHS echo "Uploading paths" $OUT_PATHS exec nix copy --to 's3://example-nix-cache' $OUT_PATHS The $OUT_PATHS variable is a space-separated list of Nix store paths. In this case, we expect and want the shell to perform word splitting to make each output path its own argument to nix sign-paths. Nix guarantees the paths will not contain any spaces, however a store path might contain glob characters. The set -f disables globbing in the shell. Then make sure the hook program is executable by the root user: # chmod +x /etc/nix/upload-to-cache.sh

Edit /etc/nix/nix.conf to run our hook, by adding the following configuration snippet at the end: post-build-hook = /etc/nix/upload-to-cache.sh Then, restart the nix-daemon.

Build any derivation, for example: $ nix-build -E '(import <nixpkgs> {}).writeText "example" (builtins.toString builtins.currentTime)' this derivation will be built: /nix/store/s4pnfbkalzy5qz57qs6yybna8wylkig6-example.drv building '/nix/store/s4pnfbkalzy5qz57qs6yybna8wylkig6-example.drv'... running post-build-hook '/home/grahamc/projects/github.com/NixOS/nix/post-hook.sh'... post-build-hook: Signing paths /nix/store/ibcyipq5gf91838ldx40mjsp0b8w9n18-example post-build-hook: Uploading paths /nix/store/ibcyipq5gf91838ldx40mjsp0b8w9n18-example /nix/store/ibcyipq5gf91838ldx40mjsp0b8w9n18-example Then delete the path from the store, and try substituting it from the binary cache: $ rm ./result $ nix-store --delete /nix/store/ibcyipq5gf91838ldx40mjsp0b8w9n18-example Now, copy the path back from the cache: $ nix-store --realise /nix/store/ibcyipq5gf91838ldx40mjsp0b8w9n18-example copying path '/nix/store/m8bmqwrch6l3h8s0k3d673xpmipcdpsa-example from 's3://example-nix-cache'... warning: you did not specify '--add-root'; the result might be removed by the garbage collector /nix/store/m8bmqwrch6l3h8s0k3d673xpmipcdpsa-example

We now have a Nix installation configured to automatically sign and upload every local build to a remote binary cache. Before deploying this to production, be sure to consider the implementation caveats in .