<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>.</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-14.12.tar.gz</screen> tells Nix to download the latest revision in the Nixpkgs/NixOS 14.12 channel.</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>/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>