<section xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xml:id="sec-common-env">
<title>Common environment variables</title>
<para>Most Nix commands interpret the following environment variables:</para>
<variablelist>
<varlistentry><term><envar>NIX_ROOT</envar></term>
<listitem><para>If <envar>NIX_ROOT</envar> is set, the Nix command
will on startup perform a <function>chroot()</function> to the
specified directory. This is useful in certain bootstrapping
situations (e.g., when installing a Nix installation onto a hard
disk from CD-ROM).</para></listitem>
</varlistentry>
<varlistentry><term><envar>NIX_IGNORE_SYMLINK_STORE</envar></term>
<listitem>
<para>Normally, the Nix store directory (typically
<filename>/nix/store</filename>) is not allowed to contain any
symlink components. This is to prevent “impure” builds. Builders
sometimes “canonicalise” paths by resolving all symlink components.
Thus, builds on different machines (with
<filename>/nix/store</filename> resolving to different locations)
could yield different results. This is generally not a problem,
except when builds are deployed to machines where
<filename>/nix/store</filename> resolves differently. If you are
sure that you’re not going to do that, you can set
<envar>NIX_IGNORE_SYMLINK_STORE</envar> to <envar>1</envar>.</para>
<para>Note that if you’re symlinking the Nix store so that you can
put it on another file system than the root file system, on Linux
you’re better off using <literal>bind</literal> mount points, e.g.,
<screen>
$ mkdir /nix
$ mount -o bind /mnt/otherdisk/nix /nix</screen>
Consult the <citerefentry><refentrytitle>mount</refentrytitle>
<manvolnum>8</manvolnum></citerefentry> manual page for details.</para>
</listitem>
</varlistentry>
<varlistentry><term><envar>NIX_STORE_DIR</envar></term>
<listitem><para>Overrides the location of the Nix store (default
<filename><replaceable>prefix</replaceable>/store</filename>).</para></listitem>
</varlistentry>
<varlistentry><term><envar>NIX_DATA_DIR</envar></term>
<listitem><para>Overrides the location of the Nix static data
directory (default
<filename><replaceable>prefix</replaceable>/share</filename>).</para></listitem>
</varlistentry>
<varlistentry><term><envar>NIX_LOG_DIR</envar></term>
<listitem><para>Overrides the location of the Nix log directory
(default <filename><replaceable>prefix</replaceable>/log/nix</filename>).</para></listitem>
</varlistentry>
<varlistentry><term><envar>NIX_STATE_DIR</envar></term>
<listitem><para>Overrides the location of the Nix state directory
(default <filename><replaceable>prefix</replaceable>/var/nix</filename>).</para></listitem>
</varlistentry>
<varlistentry><term><envar>NIX_DB_DIR</envar></term>
<listitem><para>Overrides the location of the Nix database (default
<filename><replaceable>$NIX_STATE_DIR</replaceable>/db</filename>, i.e.,
<filename><replaceable>prefix</replaceable>/var/nix/db</filename>).</para></listitem>
</varlistentry>
<varlistentry><term><envar>NIX_CONF_DIR</envar></term>
<listitem><para>Overrides the location of the Nix configuration
directory (default
<filename><replaceable>prefix</replaceable>/etc/nix</filename>).</para></listitem>
</varlistentry>
<varlistentry><term><envar>NIX_LOG_TYPE</envar></term>
<listitem><para>Equivalent to the <link
linkend="opt-log-type"><option>--log-type</option>
option</link>.</para></listitem>
</varlistentry>
<varlistentry><term><envar>TMPDIR</envar></term>
<listitem><para>Use the specified directory to store temporary
files. In particular, this includes temporary build directories;
these can take up substantial amounts of disk space. The default is
<filename>/tmp</filename>.</para></listitem>
</varlistentry>
<varlistentry xml:id="envar-build-hook"><term><envar>NIX_BUILD_HOOK</envar></term>
<listitem>
<para>Specifies the location of the <emphasis>build hook</emphasis>,
which is a program (typically some script) that Nix will call
whenever it wants to build a derivation. This is used to implement
distributed builds (see <xref linkend="sec-distributed-builds"
/>). The protocol by which the calling Nix process and the build
hook communicate is as follows.</para>
<para>The build hook is called with the following command-line
arguments:
<orderedlist>
<listitem><para>A boolean value <literal>0</literal> or
<literal>1</literal> specifying whether Nix can locally execute
more builds, as per the <link
linkend="opt-max-jobs"><option>--max-jobs</option> option</link>.
The purpose of this argument is to allow the hook to not have to
maintain bookkeeping for the local machine.</para></listitem>
<listitem><para>The Nix platform identifier for the local machine
(e.g., <literal>i686-linux</literal>).</para></listitem>
<listitem><para>The Nix platform identifier for the derivation,
i.e., its <link linkend="attr-system"><varname>system</varname>
attribute</link>.</para></listitem>
<listitem><para>The store path of the derivation.</para></listitem>
</orderedlist>
</para>
<para>On the basis of this information, and whatever persistent
state the build hook keeps about other machines and their current
load, it has to decide what to do with the build. It should print
out on file descriptor 3 one of the following responses (terminated
by a newline, <literal>"\n"</literal>):
<variablelist>
<varlistentry><term><literal>decline</literal></term>
<listitem><para>The build hook is not willing or able to perform
the build; the calling Nix process should do the build itself,
if possible.</para></listitem>
</varlistentry>
<varlistentry><term><literal>postpone</literal></term>
<listitem><para>The build hook cannot perform the build now, but
can do so in the future (e.g., because all available build slots
on remote machines are in use). The calling Nix process should
postpone this build until at least one currently running build
has terminated.</para></listitem>
</varlistentry>
<varlistentry><term><literal>accept</literal></term>
<listitem><para>The build hook has accepted the
build.</para></listitem>
</varlistentry>
</variablelist>
</para>
<para>If the build hook accepts the build, it is possible that it is
no longer necessary to do the build because some other process has
performed the build in the meantime. To prevent races, the hook
must read from file descriptor 4 a single line that tells it whether
to continue:
<variablelist>
<varlistentry><term><literal>cancel</literal></term>
<listitem><para>The build has already been done, so the hook
should exit.</para></listitem>
</varlistentry>
<varlistentry><term><literal>okay</literal></term>
<listitem><para>The hook should proceed with the build. At this
point, the calling Nix process has acquired locks on the output
path, so no other Nix process will perform the
build.</para></listitem>
</varlistentry>
</variablelist>
</para>
<para>If the hook has been told to proceed, Nix will store in the
hook’s current directory a number of text files that contain
information about the derivation:
<variablelist>
<varlistentry><term><filename>inputs</filename></term>
<listitem><para>The set of store paths that are inputs to the
build process (one per line). These have to be copied
<emphasis>to</emphasis> the remote machine (in addition to the
store derivation itself).</para></listitem>
</varlistentry>
<varlistentry><term><filename>outputs</filename></term>
<listitem><para>The set of store paths that are outputs of the
derivation (one per line). These have to be copied
<emphasis>from</emphasis> the remote machine if the build
succeeds.</para></listitem>
</varlistentry>
<varlistentry><term><filename>references</filename></term>
<listitem><para>The reference graph of the inputs, in the format
accepted by the command <command>nix-store
--register-validity</command>. It is necessary to run this
command on the remote machine after copying the inputs to inform
Nix on the remote machine that the inputs are valid
paths.</para></listitem>
</varlistentry>
</variablelist>
</para>
<para>The hook should copy the inputs to the remote machine,
register the validity of the inputs, perform the remote build, and
copy the outputs back to the local machine. An exit code other than
<literal>0</literal> indicates that the hook has failed.</para>
</listitem>
</varlistentry>
</variablelist>
</section>
