Fetch a file from a nix binary cache via HTTP, given its store path

fetch-from-store.sh

#!/usr/bin/env bash
# TODO: run in nix shell gitlab:abstract-binary/nix-nar-rs, nix-shell does not support
# flakes, nix shell no shebangs atm.

STORE_PATH="/nix/store/4q7ij8ivy09vbhini8j0gzd7f77z0bwn-disk-image"
FILE_NAME="nixos.root.qcow2"
CACHE_URL="https://nix-dabei.cachix.org"

HASH="$(echo "$STORE_PATH" | sed 's|^/nix/store/\([^-]*\)-.*$|\1|')"
NAR_INFO_URL="$CACHE_URL/$HASH.narinfo"
NAR_URL="$CACHE_URL/$(curl -s "$NAR_INFO_URL" | awk '$1 == "URL:" {print $2}')"

curl "$NAR_URL" \
    | xz -d \
    > "$HASH.nar"

nix-nar cat "$HASH.nar" "/$FILE_NAME" \
    > "$FILE_NAME"