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<refentry>
<refnamediv>
<refname>nix-prefetch-url</refname>
<refpurpose>copy a file from a URL into the store and print its MD5 hash</refpurpose>
</refnamediv>
<refsynopsisdiv>
<cmdsynopsis>
<command>nix-prefetch-url</command>
<arg choice='plain'><replaceable>url</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 MD5 cryptographic hash code, 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 to Nix expression writers.
Often a Nix expressions fetch some source distribution from the
network using the <literal>fetchurl</literal> expression
contained in <literal>nixpkgs</literal>. However,
<literal>fetchurl</literal> requires an MD5 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>
</refsection>
<refsection>
<title>Examples</title>
<screen>
$ nix-prefetch-url ftp://ftp.nluug.nl/pub/gnu/make/make-3.80.tar.bz2
...
file has hash 0bbd1df101bc0294d440471e50feca71
...</screen>
</refsection>
</refentry>
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