<refentry xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xi="http://www.w3.org/2001/XInclude" version="5.0" xml:id="sec-nix-prefetch-url"> <refmeta> <refentrytitle>nix-prefetch-url</refentrytitle> <manvolnum>1</manvolnum> <refmiscinfo class="source">Nix</refmiscinfo> <refmiscinfo class="version"><xi:include href="../version.txt" parse="text"/></refmiscinfo> </refmeta> <refnamediv> <refname>nix-prefetch-url</refname> <refpurpose>copy a file from a URL into the store and print its hash</refpurpose> </refnamediv> <refsynopsisdiv> <cmdsynopsis> <command>nix-prefetch-url</command> <arg><option>--type</option> <replaceable>hashAlgo</replaceable></arg> <arg choice='plain'><replaceable>url</replaceable></arg> <arg><replaceable>hash</replaceable></arg> </cmdsynopsis> </refsynopsisdiv> <refsection><title>Description</title> <para>The command <command>nix-prefetch-url</command> downloads the file referenced by the URL <replaceable>url</replaceable>, prints its cryptographic hash, and copies it into the Nix store. The file name in the store is <filename><replaceable>hash</replaceable>-<replaceable>baseName</replaceable></filename>, where <replaceable>baseName</replaceable> is everything following the final slash in <replaceable>url</replaceable>.</para> <para>This command is just a convenience for Nix expression writers. Often a Nix expression fetches some source distribution from the network using the <literal>fetchurl</literal> expression contained in Nixpkgs. However, <literal>fetchurl</literal> requires a cryptographic hash. If you don't know the hash, you would have to download the file first, and then <literal>fetchurl</literal> would download it again when you build your Nix expression. Since <literal>fetchurl</literal> uses the same name for the downloaded file as <command>nix-prefetch-url</command>, the redundant download can be avoided.</para> <para>If <replaceable>hash</replaceable> is specified, then a download is not performed if the Nix store already contains a file with the same hash and base name. Otherwise, the file is downloaded, and an error if signaled if the actual hash of the file does not match the specified hash.</para> <para>This command prints the hash on standard output. Additionally, if the environment variable <envar>PRINT_PATH</envar> is set, the path of the downloaded file in the Nix store is also printed.</para> </refsection> <refsection><title>Options</title> <variablelist> <varlistentry><term><option>--type</option> <replaceable>hashAlgo</replaceable></term> <listitem><para>Use the specified cryptographic hash algorithm, which can be one of <literal>md5</literal>, <literal>sha1</literal>, and <literal>sha256</literal>.</para></listitem> </varlistentry> </variablelist> </refsection> <refsection><title>Examples</title> <screen> $ nix-prefetch-url ftp://ftp.nluug.nl/pub/gnu/make/make-3.80.tar.bz2 0bbd1df101bc0294d440471e50feca71 $ PRINT_PATH=1 nix-prefetch-url ftp://ftp.nluug.nl/pub/gnu/make/make-3.80.tar.bz2 0bbd1df101bc0294d440471e50feca71 /nix/store/wvyz8ifdn7wyz1p3pqyn0ra45ka2l492-make-3.80.tar.bz2</screen> </refsection> </refentry>