nixpkgs/pkgs/build-support/fetchurl/builder.sh

#! /bin/sh

. $stdenv/setup

echo "downloading $url into $out..."

wget --passive-ftp "$url" -O "$out" || exit 1

actual=$(md5sum -b $out | cut -c1-32)
if test "$actual" != "$md5"; then
    echo "hash is $actual, expected $md5"
    exit 1
fi