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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><<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><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>
