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