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275 lines
8.6 KiB
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275 lines
8.6 KiB
XML
<sect1 id="sec-common-env"><title>Common environment variables</title>
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<para>Most Nix commands interpret the following environment variables:</para>
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<variablelist>
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<varlistentry><term><envar>NIX_ROOT</envar></term>
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<listitem><para>If <envar>NIX_ROOT</envar> is set, the Nix command
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will on startup perform a <function>chroot()</function> to the
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specified directory. This is useful in certain bootstrapping
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situations (e.g., when installing a Nix installation onto a hard
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disk from CD-ROM).</para></listitem>
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</varlistentry>
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<varlistentry><term><envar>NIX_IGNORE_SYMLINK_STORE</envar></term>
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<listitem>
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<para>Normally, the Nix store directory (typically
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<filename>/nix/store</filename>) is not allowed to contain any
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symlink components. This is to prevent “impure” builds. Builders
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sometimes “canonicalise” paths by resolving all symlink components.
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Thus, builds on different machines (with
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<filename>/nix/store</filename> resolving to different locations)
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could yield different results. This is generally not a problem,
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except when builds are deployed to machines where
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<filename>/nix/store</filename> resolves differently. If you are
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sure that you’re not going to do that, you can set
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<envar>NIX_IGNORE_SYMLINK_STORE</envar> to <envar>1</envar>.</para>
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<para>Note that if you’re symlinking the Nix store so that you can
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put it on another file system than the root file system, on Linux
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you’re better off using <literal>bind</literal> mount points, e.g.,
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<screen>
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$ mkdir /nix
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$ mount -o bind /mnt/otherdisk/nix /nix</screen>
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Consult the <citerefentry><refentrytitle>mount</refentrytitle>
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<manvolnum>8</manvolnum></citerefentry> manual page for details.</para>
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</listitem>
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</varlistentry>
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<varlistentry><term><envar>NIX_STORE_DIR</envar></term>
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<listitem><para>Overrides the location of the Nix store (default
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<filename><replaceable>prefix</replaceable>/store</filename>).</para></listitem>
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</varlistentry>
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<varlistentry><term><envar>NIX_DATA_DIR</envar></term>
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<listitem><para>Overrides the location of the Nix static data
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directory (default
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<filename><replaceable>prefix</replaceable>/share</filename>).</para></listitem>
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</varlistentry>
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<varlistentry><term><envar>NIX_LOG_DIR</envar></term>
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<listitem><para>Overrides the location of the Nix log directory
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(default <filename><replaceable>prefix</replaceable>/log/nix</filename>).</para></listitem>
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</varlistentry>
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<varlistentry><term><envar>NIX_STATE_DIR</envar></term>
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<listitem><para>Overrides the location of the Nix state directory
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(default <filename><replaceable>prefix</replaceable>/var/nix</filename>).</para></listitem>
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</varlistentry>
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<varlistentry><term><envar>NIX_DB_DIR</envar></term>
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<listitem><para>Overrides the location of the Nix database (default
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<filename><replaceable>$NIX_STATE_DIR</replaceable>/db</filename>, i.e.,
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<filename><replaceable>prefix</replaceable>/var/nix/db</filename>).</para></listitem>
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</varlistentry>
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<varlistentry><term><envar>NIX_CONF_DIR</envar></term>
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<listitem><para>Overrides the location of the Nix configuration
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directory (default
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<filename><replaceable>prefix</replaceable>/etc/nix</filename>).</para></listitem>
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</varlistentry>
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<varlistentry><term><envar>NIX_LOG_TYPE</envar></term>
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<listitem><para>Equivalent to the <link
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linkend="opt-log-type"><option>--log-type</option>
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option</link>.</para></listitem>
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</varlistentry>
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<varlistentry><term><envar>TMPDIR</envar></term>
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<listitem><para>Use the specified directory to store temporary
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files. In particular, this includes temporary build directories;
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these can take up substantial amounts of disk space. The default is
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<filename>/tmp</filename>.</para></listitem>
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</varlistentry>
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<varlistentry id="envar-build-hook"><term><envar>NIX_BUILD_HOOK</envar></term>
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<listitem>
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<para>Specifies the location of the <emphasis>build hook</emphasis>,
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which is a program (typically some script) that Nix will call
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whenever it wants to build a derivation. This is used to implement
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distributed builds (see <xref linkend="sec-distributed-builds"
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/>). The protocol by which the calling Nix process and the build
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hook communicate is as follows.</para>
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<para>The build hook is called with the following command-line
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arguments:
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<orderedlist>
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<listitem><para>A boolean value <literal>0</literal> or
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<literal>1</literal> specifying whether Nix can locally execute
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more builds, as per the <link
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linkend="opt-max-jobs"><option>--max-jobs</option> option</link>.
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The purpose of this argument is to allow the hook to not have to
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maintain bookkeeping for the local machine.</para></listitem>
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<listitem><para>The Nix platform identifier for the local machine
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(e.g., <literal>i686-linux</literal>).</para></listitem>
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<listitem><para>The Nix platform identifier for the derivation,
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i.e., its <link linkend="attr-system"><varname>system</varname>
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attribute</link>.</para></listitem>
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<listitem><para>The store path of the derivation.</para></listitem>
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</orderedlist>
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</para>
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<para>On the basis of this information, and whatever persistent
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state the build hook keeps about other machines and their current
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load, it has to decide what to do with the build. It should print
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out on file descriptor 3 one of the following responses (terminated
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by a newline, <literal>"\n"</literal>):
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<variablelist>
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<varlistentry><term><literal>decline</literal></term>
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<listitem><para>The build hook is not willing or able to perform
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the build; the calling Nix process should do the build itself,
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if possible.</para></listitem>
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</varlistentry>
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<varlistentry><term><literal>postpone</literal></term>
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<listitem><para>The build hook cannot perform the build now, but
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can do so in the future (e.g., because all available build slots
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on remote machines are in use). The calling Nix process should
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postpone this build until at least one currently running build
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has terminated.</para></listitem>
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</varlistentry>
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<varlistentry><term><literal>accept</literal></term>
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<listitem><para>The build hook has accepted the
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build.</para></listitem>
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</varlistentry>
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</variablelist>
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</para>
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<para>If the build hook accepts the build, it is possible that it is
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no longer necessary to do the build because some other process has
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performed the build in the meantime. To prevent races, the hook
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must read from file descriptor 4 a single line that tells it whether
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to continue:
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<variablelist>
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<varlistentry><term><literal>cancel</literal></term>
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<listitem><para>The build has already been done, so the hook
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should exit.</para></listitem>
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</varlistentry>
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<varlistentry><term><literal>okay</literal></term>
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<listitem><para>The hook should proceed with the build. At this
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point, the calling Nix process has acquired locks on the output
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path, so no other Nix process will perform the
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build.</para></listitem>
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</varlistentry>
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</variablelist>
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</para>
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<para>If the hook has been told to proceed, Nix will store in the
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hook’s current directory a number of text files that contain
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information about the derivation:
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<variablelist>
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<varlistentry><term><filename>inputs</filename></term>
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<listitem><para>The set of store paths that are inputs to the
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build process (one per line). These have to be copied
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<emphasis>to</emphasis> the remote machine (in addition to the
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store derivation itself).</para></listitem>
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</varlistentry>
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<varlistentry><term><filename>outputs</filename></term>
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<listitem><para>The set of store paths that are outputs of the
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derivation (one per line). These have to be copied
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<emphasis>from</emphasis> the remote machine if the build
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succeeds.</para></listitem>
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</varlistentry>
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<varlistentry><term><filename>references</filename></term>
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<listitem><para>The reference graph of the inputs, in the format
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accepted by the command <link
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linkend="rsec-nix-store-reg-val"><command>nix-store
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--register-validity</command></link>. It is necessary to run
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this command on the remote machine after copying the inputs to
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inform Nix on the remote machine that the inputs are valid
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paths.</para></listitem>
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</varlistentry>
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</variablelist>
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</para>
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<para>The hook should copy the inputs to the remote machine,
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register the validity of the inputs, perform the remote build, and
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copy the outputs back to the local machine. An exit code other than
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<literal>0</literal> indicates that the hook has failed.</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect1>
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