<section 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="ssec-relnotes-2.0">

<title>Release 2.0 (2018-02-22)</title>

<para>The following incompatible changes have been made:</para>

<itemizedlist>

  <listitem>
    <para>The manifest-based substituter mechanism
    (<command>download-using-manifests</command>) has been <link
    xlink:href="https://github.com/NixOS/nix/commit/867967265b80946dfe1db72d40324b4f9af988ed">removed</link>. It
    has been superseded by the binary cache substituter mechanism
    since several years. As a result, the following programs have been
    removed:

    <itemizedlist>
      <listitem><para><command>nix-pull</command></para></listitem>
      <listitem><para><command>nix-generate-patches</command></para></listitem>
      <listitem><para><command>bsdiff</command></para></listitem>
      <listitem><para><command>bspatch</command></para></listitem>
    </itemizedlist>
    </para>
  </listitem>

  <listitem>
    <para>The “copy from other stores” substituter mechanism
    (<command>copy-from-other-stores</command> and the
    <envar>NIX_OTHER_STORES</envar> environment variable) has been
    removed. It was primarily used by the NixOS installer to copy
    available paths from the installation medium. The replacement is
    to use a chroot store as a substituter
    (e.g. <literal>--substituters /mnt</literal>), or to build into a
    chroot store (e.g. <literal>--store /mnt --substituters /</literal>).</para>
  </listitem>

  <listitem>
    <para>The command <command>nix-push</command> has been removed as
    part of the effort to eliminate Nix's dependency on Perl. You can
    use <command>nix copy</command> instead, e.g. <literal>nix copy
    --to file:///tmp/my-binary-cache <replaceable>paths…</replaceable></literal></para>
  </listitem>

  <listitem>
    <para>The “nested” log output feature (<option>--log-type
    pretty</option>) has been removed. As a result,
    <command>nix-log2xml</command> was also removed.</para>
  </listitem>

  <listitem>
    <para>OpenSSL-based signing has been <link
    xlink:href="https://github.com/NixOS/nix/commit/f435f8247553656774dd1b2c88e9de5d59cab203">removed</link>. This
    feature was never well-supported. A better alternative is provided
    by the <option>secret-key-files</option> and
    <option>trusted-public-keys</option> options.</para>
  </listitem>

  <listitem>
    <para>Failed build caching has been <link
    xlink:href="https://github.com/NixOS/nix/commit/8cffec84859cec8b610a2a22ab0c4d462a9351ff">removed</link>. This
    feature was introduced to support the Hydra continuous build
    system, but Hydra no longer uses it.</para>
  </listitem>

  <listitem>
    <para><filename>nix-mode.el</filename> has been removed from
    Nix. It is now <link
    xlink:href="https://github.com/NixOS/nix-mode">a separate
    repository</link> and can be installed through the MELPA package
    repository.</para>
  </listitem>

</itemizedlist>

<para>This release has the following new features:</para>

<itemizedlist>

  <listitem>
    <para>It introduces a new command named <command>nix</command>,
    which is intended to eventually replace all
    <command>nix-*</command> commands with a more consistent and
    better designed user interface. It currently provides replacements
    for some (but not all) of the functionality provided by
    <command>nix-store</command>, <command>nix-build</command>,
    <command>nix-shell -p</command>, <command>nix-env -qa</command>,
    <command>nix-instantiate --eval</command>,
    <command>nix-push</command> and
    <command>nix-copy-closure</command>. It has the following major
    features:</para>

    <itemizedlist>

      <listitem>
        <para>Unlike the legacy commands, it has a consistent way to
        refer to packages and package-like arguments (like store
        paths). For example, the following commands all copy the GNU
        Hello package to a remote machine:

        <screen>nix copy --to ssh://machine nixpkgs.hello</screen>
        <screen>nix copy --to ssh://machine /nix/store/0i2jd68mp5g6h2sa5k9c85rb80sn8hi9-hello-2.10</screen>
        <screen>nix copy --to ssh://machine '(with import &lt;nixpkgs> {}; hello)'</screen>

        By contrast, <command>nix-copy-closure</command> only accepted
        store paths as arguments.</para>
      </listitem>

      <listitem>
        <para>It is self-documenting: <option>--help</option> shows
        all available command-line arguments. If
        <option>--help</option> is given after a subcommand, it shows
        examples for that subcommand. <command>nix
        --help-config</command> shows all configuration
        options.</para>
      </listitem>

      <listitem>
        <para>It is much less verbose. By default, it displays a
        single-line progress indicator that shows how many packages
        are left to be built or downloaded, and (if there are running
        builds) the most recent line of builder output. If a build
        fails, it shows the last few lines of builder output. The full
        build log can be retrieved using <command>nix
        log</command>.</para>
      </listitem>

      <listitem>
        <para>It <link
        xlink:href="https://github.com/NixOS/nix/commit/b8283773bd64d7da6859ed520ee19867742a03ba">provides</link>
        all <filename>nix.conf</filename> configuration options as
        command line flags. For example, instead of <literal>--option
        http-connections 100</literal> you can write
        <literal>--http-connections 100</literal>. Boolean options can
        be written as
        <literal>--<replaceable>foo</replaceable></literal> or
        <literal>--no-<replaceable>foo</replaceable></literal>
        (e.g. <option>--no-auto-optimise-store</option>).</para>
      </listitem>

      <listitem>
        <para>Many subcommands have a <option>--json</option> flag to
        write results to stdout in JSON format.</para>
      </listitem>

    </itemizedlist>

    <warning><para>Please note that the <command>nix</command> command
    is a work in progress and the interface is subject to
    change.</para></warning>

    <para>It provides the following high-level (“porcelain”)
    subcommands:</para>

    <itemizedlist>

      <listitem>
        <para><command>nix build</command> is a replacement for
        <command>nix-build</command>.</para>
      </listitem>

      <listitem>
        <para><command>nix run</command> executes a command in an
        environment in which the specified packages are available. It
        is (roughly) a replacement for <command>nix-shell
        -p</command>. Unlike that command, it does not execute the
        command in a shell, and has a flag (<command>-c</command>)
        that specifies the unquoted command line to be
        executed.</para>

        <para>It is particularly useful in conjunction with chroot
        stores, allowing Linux users who do not have permission to
        install Nix in <command>/nix/store</command> to still use
        binary substitutes that assume
        <command>/nix/store</command>. For example,

        <screen>nix run --store ~/my-nix nixpkgs.hello -c hello --greeting 'Hi everybody!'</screen>

        downloads (or if not substitutes are available, builds) the
        GNU Hello package into
        <filename>~/my-nix/nix/store</filename>, then runs
        <command>hello</command> in a mount namespace where
        <filename>~/my-nix/nix/store</filename> is mounted onto
        <command>/nix/store</command>.</para>
      </listitem>

      <listitem>
        <para><command>nix search</command> replaces <command>nix-env
        -qa</command>. It searches the available packages for
        occurrences of a search string in the attribute name, package
        name or description. Unlike <command>nix-env -qa</command>, it
        has a cache to speed up subsequent searches.</para>
      </listitem>

      <listitem>
        <para><command>nix copy</command> copies paths between
        arbitrary Nix stores, generalising
        <command>nix-copy-closure</command> and
        <command>nix-push</command>.</para>
      </listitem>

      <listitem>
        <para><command>nix repl</command> replaces the external
        program <command>nix-repl</command>. It provides an
        interactive environment for evaluating and building Nix
        expressions. Note that it uses <literal>linenoise-ng</literal>
        instead of GNU Readline.</para>
      </listitem>

      <listitem>
        <para><command>nix upgrade-nix</command> upgrades Nix to the
        latest stable version. This requires that Nix is installed in
        a profile. (Thus it won’t work on NixOS, or if it’s installed
        outside of the Nix store.)</para>
      </listitem>

      <listitem>
        <para><command>nix verify</command> checks whether store paths
        are unmodified and/or “trusted” (see below). It replaces
        <command>nix-store --verify</command> and <command>nix-store
        --verify-path</command>.</para>
      </listitem>

      <listitem>
        <para><command>nix log</command> shows the build log of a
        package or path. If the build log is not available locally, it
        will try to obtain it from the configured substituters (such
        as <uri>cache.nixos.org</uri>, which now provides build
        logs).</para>
      </listitem>

      <listitem>
        <para><command>nix edit</command> opens the source code of a
        package in your editor.</para>
      </listitem>

      <listitem>
        <para><command>nix eval</command> replaces
        <command>nix-instantiate --eval</command>.</para>
      </listitem>

      <listitem>
        <para><command
        xlink:href="https://github.com/NixOS/nix/commit/d41c5eb13f4f3a37d80dbc6d3888644170c3b44a">nix
        why-depends</command> shows why one store path has another in
        its closure. This is primarily useful to finding the causes of
        closure bloat. For example,

        <screen>nix why-depends nixpkgs.vlc nixpkgs.libdrm.dev</screen>

        shows a chain of files and fragments of file contents that
        cause the VLC package to have the “dev” output of
        <literal>libdrm</literal> in its closure — an undesirable
        situation.</para>
      </listitem>

      <listitem>
        <para><command>nix path-info</command> shows information about
        store paths, replacing <command>nix-store -q</command>. A
        useful feature is the option <option>--closure-size</option>
        (<option>-S</option>). For example, the following command show
        the closure sizes of every path in the current NixOS system
        closure, sorted by size:

        <screen>nix path-info -rS /run/current-system | sort -nk2</screen>

        </para>
      </listitem>

      <listitem>
        <para><command>nix optimise-store</command> replaces
        <command>nix-store --optimise</command>. The main difference
        is that it has a progress indicator.</para>
      </listitem>

    </itemizedlist>

    <para>A number of low-level (“plumbing”) commands are also
    available:</para>

    <itemizedlist>

      <listitem>
        <para><command>nix ls-store</command> and <command>nix
        ls-nar</command> list the contents of a store path or NAR
        file. The former is primarily useful in conjunction with
        remote stores, e.g.

        <screen>nix ls-store --store https://cache.nixos.org/ -lR /nix/store/0i2jd68mp5g6h2sa5k9c85rb80sn8hi9-hello-2.10</screen>

        lists the contents of path in a binary cache.</para>
      </listitem>

      <listitem>
        <para><command>nix cat-store</command> and <command>nix
        cat-nar</command> allow extracting a file from a store path or
        NAR file.</para>
      </listitem>

      <listitem>
        <para><command>nix dump-path</command> writes the contents of
        a store path to stdout in NAR format. This replaces
        <command>nix-store --dump</command>.</para>
      </listitem>

      <listitem>
        <para><command
        xlink:href="https://github.com/NixOS/nix/commit/e8d6ee7c1b90a2fe6d824f1a875acc56799ae6e2">nix
        show-derivation</command> displays a store derivation in JSON
        format. This is an alternative to
        <command>pp-aterm</command>.</para>
      </listitem>

      <listitem>
        <para><command
        xlink:href="https://github.com/NixOS/nix/commit/970366266b8df712f5f9cedb45af183ef5a8357f">nix
        add-to-store</command> replaces <command>nix-store
        --add</command>.</para>
      </listitem>

      <listitem>
        <para><command>nix sign-paths</command> signs store
        paths.</para>
      </listitem>

      <listitem>
        <para><command>nix copy-sigs</command> copies signatures from
        one store to another.</para>
      </listitem>

      <listitem>
        <para><command>nix show-config</command> shows all
        configuration options and their current values.</para>
      </listitem>

    </itemizedlist>

  </listitem>

  <listitem>
    <para>The store abstraction that Nix has had for a long time to
    support store access via the Nix daemon has been extended
    significantly. In particular, substituters (which used to be
    external programs such as
    <command>download-from-binary-cache</command>) are now subclasses
    of the abstract <classname>Store</classname> class. This allows
    many Nix commands to operate on such store types. For example,
    <command>nix path-info</command> shows information about paths in
    your local Nix store, while <command>nix path-info --store
    https://cache.nixos.org/</command> shows information about paths
    in the specified binary cache. Similarly,
    <command>nix-copy-closure</command>, <command>nix-push</command>
    and substitution are all instances of the general notion of
    copying paths between different kinds of Nix stores.</para>

    <para>Stores are specified using an URI-like syntax,
    e.g. <uri>https://cache.nixos.org/</uri> or
    <uri>ssh://machine</uri>. The following store types are supported:

    <itemizedlist>

      <listitem>

        <para><classname>LocalStore</classname> (stori URI
        <literal>local</literal> or an absolute path) and the misnamed
        <classname>RemoteStore</classname> (<literal>daemon</literal>)
        provide access to a local Nix store, the latter via the Nix
        daemon. You can use <literal>auto</literal> or the empty
        string to auto-select a local or daemon store depending on
        whether you have write permission to the Nix store. It is no
        longer necessary to set the <envar>NIX_REMOTE</envar>
        environment variable to use the Nix daemon.</para>

        <para>As noted above, <classname>LocalStore</classname> now
        supports chroot builds, allowing the “physical” location of
        the Nix store
        (e.g. <filename>/home/alice/nix/store</filename>) to differ
        from its “logical” location (typically
        <filename>/nix/store</filename>). This allows non-root users
        to use Nix while still getting the benefits from prebuilt
        binaries from <uri>cache.nixos.org</uri>.</para>

      </listitem>

      <listitem>

        <para><classname>BinaryCacheStore</classname> is the abstract
        superclass of all binary cache stores. It supports writing
        build logs and NAR content listings in JSON format.</para>

      </listitem>

      <listitem>

        <para><classname>HttpBinaryCacheStore</classname>
        (<literal>http://</literal>, <literal>https://</literal>)
        supports binary caches via HTTP or HTTPS. If the server
        supports <literal>PUT</literal> requests, it supports
        uploading store paths via commands such as <command>nix
        copy</command>.</para>

      </listitem>

      <listitem>

        <para><classname>LocalBinaryCacheStore</classname>
        (<literal>file://</literal>) supports binary caches in the
        local filesystem.</para>

      </listitem>

      <listitem>

        <para><classname>S3BinaryCacheStore</classname>
        (<literal>s3://</literal>) supports binary caches stored in
        Amazon S3, if enabled at compile time.</para>

      </listitem>

      <listitem>

        <para><classname>LegacySSHStore</classname> (<literal>ssh://</literal>)
        is used to implement remote builds and
        <command>nix-copy-closure</command>.</para>

      </listitem>

      <listitem>

        <para><classname>SSHStore</classname>
        (<literal>ssh-ng://</literal>) supports arbitrary Nix
        operations on a remote machine via the same protocol used by
        <command>nix-daemon</command>.</para>

      </listitem>

    </itemizedlist>

    </para>

  </listitem>

  <listitem>

    <para>Security has been improved in various ways:

    <itemizedlist>

      <listitem>
        <para>Nix now stores signatures for local store
        paths. When paths are copied between stores (e.g., copied from
        a binary cache to a local store), signatures are
        propagated.</para>

        <para>Locally-built paths are signed automatically using the
        secret keys specified by the <option>secret-key-files</option>
        store option. Secret/public key pairs can be generated using
        <command>nix-store
        --generate-binary-cache-key</command>.</para>

        <para>In addition, locally-built store paths are marked as
        “ultimately trusted”, but this bit is not propagated when
        paths are copied between stores.</para>
      </listitem>

      <listitem>
        <para>Content-addressable store paths no longer require
        signatures — they can be imported into a store by unprivileged
        users even if they lack signatures.</para>
      </listitem>

      <listitem>
        <para>The command <command>nix verify</command> checks whether
        the specified paths are trusted, i.e., have a certain number
        of trusted signatures, are ultimately trusted, or are
        content-addressed.</para>
      </listitem>

      <listitem>
        <para>Substitutions from binary caches <link
        xlink:href="https://github.com/NixOS/nix/commit/ecbc3fedd3d5bdc5a0e1a0a51b29062f2874ac8b">now</link>
        require signatures by default. This was already the case on
        NixOS.</para>
      </listitem>

      <listitem>
        <para>In Linux sandbox builds, we <link
        xlink:href="https://github.com/NixOS/nix/commit/eba840c8a13b465ace90172ff76a0db2899ab11b">now</link>
        use <filename>/build</filename> instead of
        <filename>/tmp</filename> as the temporary build
        directory. This fixes potential security problems when a build
        accidentally stores its <envar>TMPDIR</envar> in some
        security-sensitive place, such as an RPATH.</para>
      </listitem>

    </itemizedlist>

    </para>

  </listitem>

  <listitem>
    <para><emphasis>Pure evaluation mode</emphasis>. This is a variant
    of the existing restricted evaluation mode. In pure mode, the Nix
    evaluator forbids access to anything that could cause different
    evaluations of the same command line arguments to produce a
    different result. This includes builtin functions such as
    <function>builtins.getEnv</function>, but more importantly,
    <emphasis>all</emphasis> filesystem or network access unless a
    content hash or commit hash is specified. For example, calls to
    <function>builtins.fetchGit</function> are only allowed if a
    <varname>rev</varname> attribute is specified.</para>

    <para>The goal of this feature is to enable true reproducibility
    and traceability of builds (including NixOS system configurations)
    at the evaluation level. For example, in the future,
    <command>nixos-rebuild</command> might build configurations from a
    Nix expression in a Git repository in pure mode. That expression
    might fetch other repositories such as Nixpkgs via
    <function>builtins.fetchGit</function>. The commit hash of the
    top-level repository then uniquely identifies a running system,
    and, in conjunction with that repository, allows it to be
    reproduced or modified.</para>

  </listitem>

  <listitem>
    <para>There are several new features to support binary
    reproducibility (i.e. to help ensure that multiple builds of the
    same derivation produce exactly the same output). When
    <option>enforce-determinism</option> is set to
    <literal>false</literal>, it’s <link
    xlink:href="https://github.com/NixOS/nix/commit/8bdf83f936adae6f2c907a6d2541e80d4120f051">no
    longer</link> a fatal error if build rounds produce different
    output. Also, a hook named <option>diff-hook</option> is <link
    xlink:href="https://github.com/NixOS/nix/commit/9a313469a4bdea2d1e8df24d16289dc2a172a169">provided</link>
    to allow you to run tools such as <command>diffoscope</command>
    when build rounds produce different output.</para>
  </listitem>

  <listitem>
    <para>Configuring remote builds is a lot easier now. Provided you
    are not using the Nix daemon, you can now just specify a remote
    build machine on the command line, e.g. <literal>--option builders
    'ssh://my-mac x86_64-darwin'</literal>. The environment variable
    <envar>NIX_BUILD_HOOK</envar> has been removed and is no longer
    needed. The environment variable <envar>NIX_REMOTE_SYSTEMS</envar>
    is still supported for compatibility, but it is also possible to
    specify builders in <command>nix.conf</command> by setting the
    option <literal>builders =
    @<replaceable>path</replaceable></literal>.</para>
  </listitem>

  <listitem>
    <para>If a fixed-output derivation produces a result with an
    incorrect hash, the output path is moved to the location
    corresponding to the actual hash and registered as valid. Thus, a
    subsequent build of the fixed-output derivation with the correct
    hash is unnecessary.</para>
  </listitem>

  <listitem>
    <para><command>nix-shell</command> <link
    xlink:href="https://github.com/NixOS/nix/commit/ea59f39326c8e9dc42dfed4bcbf597fbce58797c">now</link>
    sets the <varname>IN_NIX_SHELL</varname> environment variable
    during evaluation and in the shell itself. This can be used to
    perform different actions depending on whether you’re in a Nix
    shell or in a regular build. Nixpkgs provides
    <varname>lib.inNixShell</varname> to check this variable during
    evaluation.</para>
  </listitem>

  <listitem>
    <para><envar>NIX_PATH</envar> is now lazy, so URIs in the path are
    only downloaded if they are needed for evaluation.</para>
  </listitem>

  <listitem>
    <para>You can now use
    <uri>channel:<replaceable>channel-name</replaceable></uri> as a
    short-hand for
    <uri>https://nixos.org/channels/<replaceable>channel-name</replaceable>/nixexprs.tar.xz</uri>. For
    example, <literal>nix-build channel:nixos-15.09 -A hello</literal>
    will build the GNU Hello package from the
    <literal>nixos-15.09</literal> channel. In the future, this may
    use Git to fetch updates more efficiently.</para>
  </listitem>

  <listitem>
    <para>When <option>--no-build-output</option> is given, the last
    10 lines of the build log will be shown if a build
    fails.</para>
  </listitem>

  <listitem>
    <para>Networking has been improved:

    <itemizedlist>

      <listitem>
        <para>HTTP/2 is now supported. This makes binary cache lookups
        <link
        xlink:href="https://github.com/NixOS/nix/commit/90ad02bf626b885a5dd8967894e2eafc953bdf92">much
        more efficient</link>.</para>
      </listitem>

      <listitem>
        <para>We now retry downloads on many HTTP errors, making
        binary caches substituters more resilient to temporary
        failures.</para>
      </listitem>

      <listitem>
        <para>HTTP credentials can now be configured via the standard
        <filename>netrc</filename> mechanism.</para>
      </listitem>

      <listitem>
        <para>If S3 support is enabled at compile time,
        <uri>s3://</uri> URIs are <link
        xlink:href="https://github.com/NixOS/nix/commit/9ff9c3f2f80ba4108e9c945bbfda2c64735f987b">supported</link>
        in all places where Nix allows URIs.</para>
      </listitem>

      <listitem>
        <para>Brotli compression is now supported. In particular,
        <uri>cache.nixos.org</uri> build logs are now compressed using
        Brotli.</para>
      </listitem>

    </itemizedlist>

    </para>

  </listitem>

  <listitem>
    <para><command>nix-env</command> <link
    xlink:href="https://github.com/NixOS/nix/commit/b0cb11722626e906a73f10dd9a0c9eea29faf43a">now</link>
    ignores packages with bad derivation names (in particular those
    starting with a digit or containing a dot).</para>
  </listitem>

  <listitem>
    <para>Many configuration options have been renamed, either because
    they were unnecessarily verbose
    (e.g. <option>build-use-sandbox</option> is now just
    <option>sandbox</option>) or to reflect generalised behaviour
    (e.g. <option>binary-caches</option> is now
    <option>substituters</option> because it allows arbitrary store
    URIs). The old names are still supported for compatibility.</para>
  </listitem>

  <listitem>
    <para>The <option>max-jobs</option> option can <link
    xlink:href="https://github.com/NixOS/nix/commit/7251d048fa812d2551b7003bc9f13a8f5d4c95a5">now</link>
    be set to <literal>auto</literal> to use the number of CPUs in the
    system.</para>
  </listitem>

  <listitem>
    <para>Hashes can <link
    xlink:href="https://github.com/NixOS/nix/commit/c0015e87af70f539f24d2aa2bc224a9d8b84276b">now</link>
    be specified in base-64 format, in addition to base-16 and the
    non-standard base-32.</para>
  </listitem>

  <listitem>
    <para><command>nix-shell</command> now uses
    <varname>bashInteractive</varname> from Nixpkgs, rather than the
    <command>bash</command> command that happens to be in the caller’s
    <envar>PATH</envar>. This is especially important on macOS where
    the <command>bash</command> provided by the system is seriously
    outdated and cannot execute <literal>stdenv</literal>’s setup
    script.</para>
  </listitem>

  <listitem>
    <para>Nix can now automatically trigger a garbage collection if
    free disk space drops below a certain level during a build. This
    is configured using the <option>min-free</option> and
    <option>max-free</option> options.</para>
  </listitem>

  <listitem>
    <para><command>nix-store -q --roots</command> and
    <command>nix-store --gc --print-roots</command> now show temporary
    and in-memory roots.</para>
  </listitem>

  <listitem>
    <para>
      Nix can now be extended with plugins. See the documentation of
      the <option>plugin-files</option> option for more details.
    </para>
  </listitem>

</itemizedlist>

<para>The Nix language has the following new features:

<itemizedlist>

  <listitem>
    <para>It supports floating point numbers. They are based on the
    C++ <literal>float</literal> type and are supported by the
    existing numerical operators. Export and import to and from JSON
    and XML works, too.</para>
  </listitem>

  <listitem>
    <para>Derivation attributes can now reference the outputs of the
    derivation using the <function>placeholder</function> builtin
    function. For example, the attribute

<programlisting>
configureFlags = "--prefix=${placeholder "out"} --includedir=${placeholder "dev"}";
</programlisting>

    will cause the <envar>configureFlags</envar> environment variable
    to contain the actual store paths corresponding to the
    <literal>out</literal> and <literal>dev</literal> outputs.</para>
  </listitem>

</itemizedlist>

</para>

<para>The following builtin functions are new or extended:

<itemizedlist>

  <listitem>
    <para><function
    xlink:href="https://github.com/NixOS/nix/commit/38539b943a060d9cdfc24d6e5d997c0885b8aa2f">builtins.fetchGit</function>
    allows Git repositories to be fetched at evaluation time. Thus it
    differs from the <function>fetchgit</function> function in
    Nixpkgs, which fetches at build time and cannot be used to fetch
    Nix expressions during evaluation. A typical use case is to import
    external NixOS modules from your configuration, e.g.

    <programlisting>imports = [ (builtins.fetchGit https://github.com/edolstra/dwarffs + "/module.nix") ];</programlisting>

    </para>
  </listitem>

  <listitem>
    <para>Similarly, <function>builtins.fetchMercurial</function>
    allows you to fetch Mercurial repositories.</para>
  </listitem>

  <listitem>
    <para><function>builtins.path</function> generalises
    <function>builtins.filterSource</function> and path literals
    (e.g. <literal>./foo</literal>). It allows specifying a store path
    name that differs from the source path name
    (e.g. <literal>builtins.path { path = ./foo; name = "bar";
    }</literal>) and also supports filtering out unwanted
    files.</para>
  </listitem>

  <listitem>
    <para><function>builtins.fetchurl</function> and
    <function>builtins.fetchTarball</function> now support
    <varname>sha256</varname> and <varname>name</varname>
    attributes.</para>
  </listitem>

  <listitem>
    <para><function
    xlink:href="https://github.com/NixOS/nix/commit/b8867a0239b1930a16f9ef3f7f3e864b01416dff">builtins.split</function>
    splits a string using a POSIX extended regular expression as the
    separator.</para>
  </listitem>

  <listitem>
    <para><function
    xlink:href="https://github.com/NixOS/nix/commit/26d92017d3b36cff940dcb7d1611c42232edb81a">builtins.partition</function>
    partitions the elements of a list into two lists, depending on a
    Boolean predicate.</para>
  </listitem>

  <listitem>
    <para><literal>&lt;nix/fetchurl.nix&gt;</literal> now uses the
    content-addressable tarball cache at
    <uri>http://tarballs.nixos.org/</uri>, just like
    <function>fetchurl</function> in
    Nixpkgs. (f2682e6e18a76ecbfb8a12c17e3a0ca15c084197)</para>
  </listitem>

  <listitem>
    <para>In restricted and pure evaluation mode, builtin functions
    that download from the network (such as
    <function>fetchGit</function>) are permitted to fetch underneath a
    list of URI prefixes specified in the option
    <option>allowed-uris</option>.</para>
  </listitem>

</itemizedlist>

</para>

<para>The Nix build environment has the following changes:

<itemizedlist>

  <listitem>
    <para>Values such as Booleans, integers, (nested) lists and
    attribute sets can <link
    xlink:href="https://github.com/NixOS/nix/commit/6de33a9c675b187437a2e1abbcb290981a89ecb1">now</link>
    be passed to builders in a non-lossy way. If the special attribute
    <varname>__structuredAttrs</varname> is set to
    <literal>true</literal>, the other derivation attributes are
    serialised in JSON format and made available to the builder via
    the file <envar>.attrs.json</envar> in the builder’s temporary
    directory. This obviates the need for
    <varname>passAsFile</varname> since JSON files have no size
    restrictions, unlike process environments.</para>

    <para><link
    xlink:href="https://github.com/NixOS/nix/commit/2d5b1b24bf70a498e4c0b378704cfdb6471cc699">As
    a convenience to Bash builders</link>, Nix writes a script named
    <envar>.attrs.sh</envar> to the builder’s directory that
    initialises shell variables corresponding to all attributes that
    are representable in Bash. This includes non-nested (associative)
    arrays. For example, the attribute <literal>hardening.format =
    true</literal> ends up as the Bash associative array element
    <literal>${hardening[format]}</literal>.</para>
  </listitem>

  <listitem>
    <para>Builders can <link
    xlink:href="https://github.com/NixOS/nix/commit/88e6bb76de5564b3217be9688677d1c89101b2a3">now</link>
    communicate what build phase they are in by writing messages to
    the file descriptor specified in <envar>NIX_LOG_FD</envar>. The
    current phase is shown by the <command>nix</command> progress
    indicator.
    </para>
  </listitem>

  <listitem>
    <para>In Linux sandbox builds, we <link
    xlink:href="https://github.com/NixOS/nix/commit/a2d92bb20e82a0957067ede60e91fab256948b41">now</link>
    provide a default <filename>/bin/sh</filename> (namely
    <filename>ash</filename> from BusyBox).</para>
  </listitem>

  <listitem>
    <para>In structured attribute mode,
    <varname>exportReferencesGraph</varname> <link
    xlink:href="https://github.com/NixOS/nix/commit/c2b0d8749f7e77afc1c4b3e8dd36b7ee9720af4a">exports</link>
    extended information about closures in JSON format. In particular,
    it includes the sizes and hashes of paths. This is primarily
    useful for NixOS image builders.</para>
  </listitem>

  <listitem>
    <para>Builds are <link
    xlink:href="https://github.com/NixOS/nix/commit/21948deed99a3295e4d5666e027a6ca42dc00b40">now</link>
    killed as soon as Nix receives EOF on the builder’s stdout or
    stderr. This fixes a bug that allowed builds to hang Nix
    indefinitely, regardless of
    timeouts.</para>
  </listitem>

  <listitem>
    <para>The <option>sandbox-paths</option> configuration
    option can now specify optional paths by appending a
    <literal>?</literal>, e.g. <literal>/dev/nvidiactl?</literal> will
    bind-mount <varname>/dev/nvidiactl</varname> only if it
    exists.</para>
  </listitem>

  <listitem>
    <para>On Linux, builds are now executed in a user
    namespace with UID 1000 and GID 100.</para>
  </listitem>

</itemizedlist>

</para>

<para>A number of significant internal changes were made:

<itemizedlist>

  <listitem>
    <para>Nix no longer depends on Perl and all Perl components have
    been rewritten in C++ or removed. The Perl bindings that used to
    be part of Nix have been moved to a separate package,
    <literal>nix-perl</literal>.</para>
  </listitem>

  <listitem>
    <para>All <classname>Store</classname> classes are now
    thread-safe. <classname>RemoteStore</classname> supports multiple
    concurrent connections to the daemon. This is primarily useful in
    multi-threaded programs such as
    <command>hydra-queue-runner</command>.</para>
  </listitem>

</itemizedlist>

</para>

<para>This release has contributions from

Adrien Devresse,
Alexander Ried,
Alex Cruice,
Alexey Shmalko,
AmineChikhaoui,
Andy Wingo,
Aneesh Agrawal,
Anthony Cowley,
Armijn Hemel,
aszlig,
Ben Gamari,
Benjamin Hipple,
Benjamin Staffin,
Benno Fünfstück,
Bjørn Forsman,
Brian McKenna,
Charles Strahan,
Chase Adams,
Chris Martin,
Christian Theune,
Chris Warburton,
Daiderd Jordan,
Dan Connolly,
Daniel Peebles,
Dan Peebles,
davidak,
David McFarland,
Dmitry Kalinkin,
Domen Kožar,
Eelco Dolstra,
Emery Hemingway,
Eric Litak,
Eric Wolf,
Fabian Schmitthenner,
Frederik Rietdijk,
Gabriel Gonzalez,
Giorgio Gallo,
Graham Christensen,
Guillaume Maudoux,
Harmen,
Iavael,
James Broadhead,
James Earl Douglas,
Janus Troelsen,
Jeremy Shaw,
Joachim Schiele,
Joe Hermaszewski,
Joel Moberg,
Johannes 'fish' Ziemke,
Jörg Thalheim,
Jude Taylor,
kballou,
Keshav Kini,
Kjetil Orbekk,
Langston Barrett,
Linus Heckemann,
Ludovic Courtès,
Manav Rathi,
Marc Scholten,
Markus Hauck,
Matt Audesse,
Matthew Bauer,
Matthias Beyer,
Matthieu Coudron,
N1X,
Nathan Zadoks,
Neil Mayhew,
Nicolas B. Pierron,
Niklas Hambüchen,
Nikolay Amiantov,
Ole Jørgen Brønner,
Orivej Desh,
Peter Simons,
Peter Stuart,
Pyry Jahkola,
regnat,
Renzo Carbonara,
Rhys,
Robert Vollmert,
Scott Olson,
Scott R. Parish,
Sergei Trofimovich,
Shea Levy,
Sheena Artrip,
Spencer Baugh,
Stefan Junker,
Susan Potter,
Thomas Tuegel,
Timothy Allen,
Tristan Hume,
Tuomas Tynkkynen,
tv,
Tyson Whitehead,
Vladimír Čunát,
Will Dietz,
wmertens,
Wout Mertens,
zimbatm and
Zoran Plesivčak.
</para>

</section>