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+<chapter xmlns="http://docbook.org/ns/docbook"
+      xmlns:xlink="http://www.w3.org/1999/xlink"
+      xmlns:xi="http://www.w3.org/2001/XInclude"
+      version="5.0"
+      xml:id="sec-common-env">
+
+<title>Common Environment Variables</title>
+
+
+<para>Most Nix commands interpret the following environment variables:</para>
+
+<variablelist xml:id="env-common">
+
+<varlistentry><term><envar>IN_NIX_SHELL</envar></term>
+
+  <listitem><para>Indicator that tells if the current environment was set up by
+  <command>nix-shell</command>.  Since Nix 2.0 the values are
+  <literal>"pure"</literal> and <literal>"impure"</literal></para></listitem>
+
+</varlistentry>
+
+<varlistentry xml:id="env-NIX_PATH"><term><envar>NIX_PATH</envar></term>
+
+  <listitem>
+
+    <para>A colon-separated list of directories used to look up Nix
+    expressions enclosed in angle brackets (i.e.,
+    <literal>&lt;<replaceable>path</replaceable>></literal>).  For
+    instance, the value
+
+    <screen>
+/home/eelco/Dev:/etc/nixos</screen>
+
+    will cause Nix to look for paths relative to
+    <filename>/home/eelco/Dev</filename> and
+    <filename>/etc/nixos</filename>, in that order.  It is also
+    possible to match paths against a prefix.  For example, the value
+
+    <screen>
+nixpkgs=/home/eelco/Dev/nixpkgs-branch:/etc/nixos</screen>
+
+    will cause Nix to search for
+    <literal>&lt;nixpkgs/<replaceable>path</replaceable>></literal> in
+    <filename>/home/eelco/Dev/nixpkgs-branch/<replaceable>path</replaceable></filename>
+    and
+    <filename>/etc/nixos/nixpkgs/<replaceable>path</replaceable></filename>.</para>
+
+    <para>If a path in the Nix search path starts with
+    <literal>http://</literal> or <literal>https://</literal>, it is
+    interpreted as the URL of a tarball that will be downloaded and
+    unpacked to a temporary location. The tarball must consist of a
+    single top-level directory. For example, setting
+    <envar>NIX_PATH</envar> to
+
+    <screen>
+nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/nixos-15.09.tar.gz</screen>
+
+    tells Nix to download the latest revision in the Nixpkgs/NixOS
+    15.09 channel.</para>
+
+    <para>A following shorthand can be used to refer to the official channels:
+    
+    <screen>nixpkgs=channel:nixos-15.09</screen>
+    </para>
+
+    <para>The search path can be extended using the <option
+    linkend="opt-I">-I</option> option, which takes precedence over
+    <envar>NIX_PATH</envar>.</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>/var/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_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>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-remote"><term><envar>NIX_REMOTE</envar></term>
+
+  <listitem><para>This variable should be set to
+  <literal>daemon</literal> if you want to use the Nix daemon to
+  execute Nix operations. This is necessary in <link
+  linkend="ssec-multi-user">multi-user Nix installations</link>.
+  If the Nix daemon's Unix socket is at some non-standard path,
+  this variable should be set to <literal>unix://path/to/socket</literal>.
+  Otherwise, it should be left unset.</para></listitem>
+
+</varlistentry>
+
+
+<varlistentry><term><envar>NIX_SHOW_STATS</envar></term>
+
+  <listitem><para>If set to <literal>1</literal>, Nix will print some
+  evaluation statistics, such as the number of values
+  allocated.</para></listitem>
+
+</varlistentry>
+
+
+<varlistentry><term><envar>NIX_COUNT_CALLS</envar></term>
+
+  <listitem><para>If set to <literal>1</literal>, Nix will print how
+  often functions were called during Nix expression evaluation.  This
+  is useful for profiling your Nix expressions.</para></listitem>
+
+</varlistentry>
+
+
+<varlistentry><term><envar>GC_INITIAL_HEAP_SIZE</envar></term>
+
+  <listitem><para>If Nix has been configured to use the Boehm garbage
+  collector, this variable sets the initial size of the heap in bytes.
+  It defaults to 384 MiB.  Setting it to a low value reduces memory
+  consumption, but will increase runtime due to the overhead of
+  garbage collection.</para></listitem>
+
+</varlistentry>
+
+
+</variablelist>
+
+
+</chapter>