264 lines
11 KiB
XML
264 lines
11 KiB
XML
<section xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xml:id="sec-conf-file">
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<title>Nix configuration file</title>
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<para>A number of persistent settings of Nix are stored in the file
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<filename><replaceable>prefix</replaceable>/etc/nix/nix.conf</filename>.
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This file is a list of <literal><replaceable>name</replaceable> =
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<replaceable>value</replaceable></literal> pairs, one per line.
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Comments start with a <literal>#</literal> character. An example
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configuration file is shown in <xref linkend="ex-nix-conf" />.</para>
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<example xml:id='ex-nix-conf'><title>Nix configuration file</title>
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<programlisting>
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gc-keep-outputs = true # Nice for developers
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gc-keep-derivations = true # Idem
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env-keep-derivations = false
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</programlisting>
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</example>
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<para>The following variables are currently available:
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<variablelist>
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<varlistentry xml:id="conf-gc-keep-outputs"><term><literal>gc-keep-outputs</literal></term>
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<listitem><para>If <literal>true</literal>, the garbage collector
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will keep the outputs of non-garbage derivations. If
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<literal>false</literal> (default), outputs will be deleted unless
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they are GC roots themselves (or reachable from other roots).</para>
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<para>In general, outputs must be registered as roots separately.
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However, even if the output of a derivation is registered as a
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root, the collector will still delete store paths that are used
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only at build time (e.g., the C compiler, or source tarballs
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downloaded from the network). To prevent it from doing so, set
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this option to <literal>true</literal>.</para></listitem>
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</varlistentry>
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<varlistentry xml:id="conf-gc-keep-derivations"><term><literal>gc-keep-derivations</literal></term>
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<listitem><para>If <literal>true</literal> (default), the garbage
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collector will keep the derivations from which non-garbage store
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paths were built. If <literal>false</literal>, they will be
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deleted unless explicitly registered as a root (or reachable from
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other roots).</para>
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<para>Keeping derivation around is useful for querying and
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traceability (e.g., it allows you to ask with what dependencies or
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options a store path was built), so by default this option is on.
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Turn it off to safe a bit of disk space (or a lot if
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<literal>gc-keep-outputs</literal> is also turned on).</para></listitem>
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</varlistentry>
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<varlistentry xml:id="conf-gc-reserved-space"><term><literal>gc-reserved-space</literal></term>
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<listitem><para>This option specifies how much space should be
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reserved in normal use so that the garbage collector can run
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succesfully. Since the garbage collector must perform Berkeley DB
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transactions, it needs some disk space for itself. However, when
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the disk is full, this space is not available, so the collector
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would not be able to run precisely when it is most needed.</para>
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<para>For this reason, when Nix is run, it allocates a file
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<filename>/nix/var/nix/db/reserved</filename> of the size
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specified by this option. When the garbage collector is run, this
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file is deleted before the Berkeley DB environment is opened.
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This should give it enough room to proceed.</para>
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<para>The default is <literal>1048576</literal> (1
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MiB).</para></listitem>
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</varlistentry>
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<varlistentry><term><literal>env-keep-derivations</literal></term>
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<listitem><para>If <literal>false</literal> (default), derivations
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are not stored in Nix user environments. That is, the derivation
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any build-time-only dependencies may be garbage-collected.</para>
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<para>If <literal>true</literal>, when you add a Nix derivation to
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a user environment, the path of the derivation is stored in the
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user environment. Thus, the derivation will not be
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garbage-collected until the user environment generation is deleted
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(<command>nix-env --delete-generations</command>). To prevent
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build-time-only dependencies from being collected, you should also
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turn on <literal>gc-keep-outputs</literal>.</para>
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<para>The difference between this option and
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<literal>gc-keep-derivations</literal> is that this one is
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“sticky”: it applies to any user environment created while this
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option was enabled, while <literal>gc-keep-derivations</literal>
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only applies at the moment the garbage collector is
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run.</para></listitem>
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</varlistentry>
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<varlistentry xml:id="conf-build-max-jobs"><term><literal>build-max-jobs</literal></term>
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<listitem><para>This option defines the maximum number of jobs
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that Nix will try to build in parallel. The default is
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<literal>1</literal>. You should generally set it to the number
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of CPUs in your system (e.g., <literal>2</literal> on a Athlon 64
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X2). It can be overriden using the <option
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linkend='opt-max-jobs'>--max-jobs</option> (<option>-j</option>)
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command line switch.</para></listitem>
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</varlistentry>
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<varlistentry xml:id="conf-build-max-silent-time"><term><literal>build-max-silent-time</literal></term>
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<listitem>
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<para>This option defines the maximum number of seconds that a
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builder can go without producing any data on standard output or
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standard error. This is useful (for instance in a automated
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build system) to catch builds that are stuck in an infinite
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loop, or to catch remote builds that are hanging due to network
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problems. It can be overriden using the <option
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linkend="opt-max-silent-time">--max-silent-time</option> command
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line switch.</para>
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<para>The value <literal>0</literal> means that there is no
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timeout. This is also the default.</para>
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</listitem>
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</varlistentry>
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<varlistentry xml:id="conf-build-users-group"><term><literal>build-users-group</literal></term>
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<listitem><para>This options specifies the Unix group containing
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the Nix build user accounts. In multi-user Nix installations,
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builds should not be performed by the Nix account since that would
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allow users to arbitrarily modify the Nix store and database by
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supplying specially crafted builders; and they cannot be performed
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by the calling user since that would allow him/her to influence
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the build result.</para>
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<para>Therefore, if this option is non-empty and specifies a valid
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group, builds will be performed under the user accounts that are a
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member of the group specified here (as listed in
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<filename>/etc/group</filename>). Those user accounts should not
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be used for any other purpose!</para>
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<para>Nix will never run two builds under the same user account at
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the same time. This is to prevent an obvious security hole: a
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malicious user writing a Nix expression that modifies the build
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result of a legitimate Nix expression being built by another user.
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Therefore it is good to have as many Nix build user accounts as
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you can spare. (Remember: uids are cheap.)</para>
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<para>The build users should have permission to create files in
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the Nix store, but not delete them. Therefore,
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<filename>/nix/store</filename> should be owned by the Nix
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account, its group should be the group specified here, and its
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mode should be <literal>1775</literal>.</para>
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<para>If the build users group is empty, builds will be performed
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under the uid of the Nix process (that is, the uid of the caller
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if <envar>NIX_REMOTE</envar> is empty, the uid under which the Nix
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daemon runs if <envar>NIX_REMOTE</envar> is
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<literal>daemon</literal>, or the uid that owns the setuid
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<command>nix-worker</command> program if <envar>NIX_REMOTE</envar>
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is <literal>slave</literal>). Obviously, this should not be used
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in multi-user settings with untrusted users.</para>
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</listitem>
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</varlistentry>
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<varlistentry><term><literal>build-use-chroot</literal></term>
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<listitem><para>If set to <literal>true</literal>, builds will be
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performed in a <emphasis>chroot environment</emphasis>, i.e., the
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build will be isolated from the normal file system hierarchy and
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will only see the Nix store, the temporary build directory, and
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the directories configured with the <link
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linkend='conf-build-chroot-dirs'><literal>build-chroot-dirs</literal>
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option</link> (such as <filename>/proc</filename> and
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<filename>/dev</filename>). This is useful to prevent undeclared
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dependencies on files in directories such as
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<filename>/usr/bin</filename>.</para>
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<para>The use of a chroot requires that Nix is run as root (but
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you can still use the <link
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linkend='conf-build-users-group'>“build users” feature</link> to
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perform builds under different users than root). Currently,
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chroot builds only work on Linux because Nix uses “bind mounts” to
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make the Nix store and other directories available inside the
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chroot.</para>
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</listitem>
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</varlistentry>
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<varlistentry xml:id="conf-build-chroot-dirs"><term><literal>build-chroot-dirs</literal></term>
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<listitem><para>When builds are performed in a chroot environment,
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Nix will mount (using <command>mount --bind</command> on Linux)
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some directories from the normal file system hierarchy inside the
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chroot. These are the Nix store, the temporary build directory
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(usually
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<filename>/tmp/nix-<replaceable>pid</replaceable>-<replaceable>number</replaceable></filename>)
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and the directories listed here. The default is <literal>dev
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/proc</literal>. Files in <filename>/dev</filename> (such as
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<filename>/dev/null</filename>) are needed by many builds, and
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some files in <filename>/proc</filename> may also be needed
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occasionally.</para>
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<para>The value used on NixOS is
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<programlisting>
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build-use-chroot = /dev /proc /bin</programlisting>
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to make the <filename>/bin/sh</filename> symlink available (which
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is still needed by many builders).</para>
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</listitem>
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</varlistentry>
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<varlistentry><term><literal>system</literal></term>
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<listitem><para>This option specifies the canonical Nix system
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name of the current installation, such as
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<literal>i686-linux</literal> or
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<literal>powerpc-darwin</literal>. Nix can only build derivations
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whose <literal>system</literal> attribute equals the value
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specified here. In general, it never makes sense to modify this
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value from its default, since you can use it to ‘lie’ about the
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platform you are building on (e.g., perform a Mac OS build on a
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Linux machine; the result would obviously be wrong). It only
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makes sense if the Nix binaries can run on multiple platforms,
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e.g., ‘universal binaries’ that run on <literal>powerpc-darwin</literal> and
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<literal>i686-darwin</literal>.</para>
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<para>It defaults to the canonical Nix system name detected by
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<filename>configure</filename> at build time.</para></listitem>
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</varlistentry>
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</variablelist>
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</para>
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</section>
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