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+# Using the `post-build-hook`
+
+# Implementation Caveats
+
+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.
+
+# Prerequisites
+
+This tutorial assumes you have configured an S3-compatible binary cache
+according to the instructions at
+[???](#ssec-s3-substituter-authenticated-writes), and that the `root`
+user's default AWS profile can upload to the bucket.
+
+# Set up a Signing Key
+
+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
+[???](#conf-trusted-public-keys) and [???](#conf-substituters) 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.
+
+# Implementing the build hook
+
+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' $OUT_PATHS
+
+> **Note**
+> 
+> The `$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
+
+# Updating Nix Configuration
+
+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`.
+
+# Testing
+
+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
+
+# Conclusion
+
+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 [Implementation
+Caveats](#chap-post-build-hook-caveats).