266 lines
9.3 KiB
XML
266 lines
9.3 KiB
XML
<chapter xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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version="5.0"
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xml:id="ch-about-nix">
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<title>About Nix</title>
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<para>Nix is a <emphasis>purely functional package manager</emphasis>.
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This means that it treats packages like values in purely functional
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programming languages such as Haskell — they are built by functions
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that don’t have side-effects, and they never change after they have
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been built. Nix stores packages in the <emphasis>Nix
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store</emphasis>, usually the directory
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<filename>/nix/store</filename>, where each package has its own unique
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subdirectory such as
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<programlisting>
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/nix/store/b6gvzjyb2pg0kjfwrjmg1vfhh54ad73z-firefox-33.1/
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</programlisting>
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where <literal>b6gvzjyb2pg0…</literal> is a unique identifier for the
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package that captures all its dependencies (it’s a cryptographic hash
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of the package’s build dependency graph). This enables many powerful
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features.</para>
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<simplesect><title>Multiple versions</title>
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<para>You can have multiple versions or variants of a package
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installed at the same time. This is especially important when
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different applications have dependencies on different versions of the
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same package — it prevents the “DLL hell”. Because of the hashing
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scheme, different versions of a package end up in different paths in
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the Nix store, so they don’t interfere with each other.</para>
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<para>An important consequence is that operations like upgrading or
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uninstalling an application cannot break other applications, since
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these operations never “destructively” update or delete files that are
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used by other packages.</para>
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</simplesect>
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<simplesect><title>Complete dependencies</title>
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<para>Nix helps you make sure that package dependency specifications
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are complete. In general, when you’re making a package for a package
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management system like RPM, you have to specify for each package what
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its dependencies are, but there are no guarantees that this
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specification is complete. If you forget a dependency, then the
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package will build and work correctly on <emphasis>your</emphasis>
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machine if you have the dependency installed, but not on the end
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user's machine if it's not there.</para>
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<para>Since Nix on the other hand doesn’t install packages in “global”
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locations like <filename>/usr/bin</filename> but in package-specific
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directories, the risk of incomplete dependencies is greatly reduced.
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This is because tools such as compilers don’t search in per-packages
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directories such as
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<filename>/nix/store/5lbfaxb722zp…-openssl-0.9.8d/include</filename>,
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so if a package builds correctly on your system, this is because you
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specified the dependency explicitly.</para>
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<para>Runtime dependencies are found by scanning binaries for the hash
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parts of Nix store paths (such as <literal>r8vvq9kq…</literal>). This
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sounds risky, but it works extremely well.</para>
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</simplesect>
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<simplesect><title>Multi-user support</title>
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<para>Nix has multi-user support. This means that non-privileged
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users can securely install software. Each user can have a different
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<emphasis>profile</emphasis>, a set of packages in the Nix store that
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appear in the user’s <envar>PATH</envar>. If a user installs a
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package that another user has already installed previously, the
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package won’t be built or downloaded a second time. At the same time,
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it is not possible for one user to inject a Trojan horse into a
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package that might be used by another user.</para>
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</simplesect>
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<simplesect><title>Atomic upgrades and rollbacks</title>
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<para>Since package management operations never overwrite packages in
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the Nix store but just add new versions in different paths, they are
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<emphasis>atomic</emphasis>. So during a package upgrade, there is no
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time window in which the package has some files from the old version
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and some files from the new version — which would be bad because a
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program might well crash if it’s started during that period.</para>
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<para>And since package aren’t overwritten, the old versions are still
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there after an upgrade. This means that you can <emphasis>roll
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back</emphasis> to the old version:</para>
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<screen>
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$ nix-env --upgrade <replaceable>some-packages</replaceable>
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$ nix-env --rollback
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</screen>
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</simplesect>
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<simplesect><title>Garbage collection</title>
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<para>When you uninstall a package like this…
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<screen>
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$ nix-env --uninstall firefox
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</screen>
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the package isn’t deleted from the system right away (after all, you
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might want to do a rollback, or it might be in the profiles of other
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users). Instead, unused packages can be deleted safely by running the
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<emphasis>garbage collector</emphasis>:
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<screen>
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$ nix-collect-garbage
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</screen>
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This deletes all packages that aren’t in use by any user profile or by
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a currently running program.</para>
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</simplesect>
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<simplesect><title>Functional package language</title>
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<para>Packages are built from <emphasis>Nix expressions</emphasis>,
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which is a simple functional language. A Nix expression describes
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everything that goes into a package build action (a “derivation”):
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other packages, sources, the build script, environment variables for
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the build script, etc. Nix tries very hard to ensure that Nix
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expressions are <emphasis>deterministic</emphasis>: building a Nix
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expression twice should yield the same result.</para>
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<para>Because it’s a functional language, it’s easy to support
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building variants of a package: turn the Nix expression into a
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function and call it any number of times with the appropriate
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arguments. Due to the hashing scheme, variants don’t conflict with
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each other in the Nix store.</para>
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</simplesect>
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<simplesect><title>Transparent source/binary deployment</title>
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<para>Nix expressions generally describe how to build a package from
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source, so an installation action like
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<screen>
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$ nix-env --install firefox
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</screen>
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<emphasis>could</emphasis> cause quite a bit of build activity, as not
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only Firefox but also all its dependencies (all the way up to the C
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library and the compiler) would have to built, at least if they are
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not already in the Nix store. This is a <emphasis>source deployment
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model</emphasis>. For most users, building from source is not very
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pleasant as it takes far too long. However, Nix can automatically
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skip building from source and instead use a <emphasis>binary
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cache</emphasis>, a web server that provides pre-built binaries. For
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instance, when asked to build
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<literal>/nix/store/b6gvzjyb2pg0…-firefox-33.1</literal> from source,
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Nix would first check if the file
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<uri>https://cache.nixos.org/b6gvzjyb2pg0….narinfo</uri> exists, and
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if so, fetch the pre-built binary referenced from there; otherwise, it
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would fall back to building from source.</para>
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</simplesect>
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<!--
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<simplesect><title>Binary patching</title>
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<para>In addition to downloading binaries automatically if they’re
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available, Nix can download binary deltas that patch an existing
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package in the Nix store into a new version. This speeds up
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upgrades.</para>
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</simplesect>
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-->
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<simplesect><title>Nix Packages collection</title>
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<para>We provide a large set of Nix expressions containing hundreds of
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existing Unix packages, the <emphasis>Nix Packages
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collection</emphasis> (Nixpkgs).</para>
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</simplesect>
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<simplesect><title>Managing build environments</title>
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<para>Nix is extremely useful for developers as it makes it easy to
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automatically set up the the build environment for a package. Given a
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Nix expression that describes the dependencies of your package, the
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command <command>nix-shell</command> will build or download those
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dependencies if they’re not already in your Nix store, and then start
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a Bash shell in which all necessary environment variables (such as
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compiler search paths) are set.</para>
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<para>For example, the following command gets all dependencies of the
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Pan newsreader, as described by <link
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xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/networking/newsreaders/pan/default.nix">its
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Nix expression</link>:</para>
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<screen>
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$ nix-shell '<nixpkgs>' -A pan
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</screen>
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<para>You’re then dropped into a shell where you can edit, build and test
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the package:</para>
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<screen>
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[nix-shell]$ tar xf $src
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[nix-shell]$ cd pan-*
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[nix-shell]$ ./configure
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[nix-shell]$ make
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[nix-shell]$ ./pan/gui/pan
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</screen>
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<!--
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<para>Since Nix packages are reproducible and have complete dependency
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specifications, Nix makes an excellent basis for <a
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href="[%root%]hydra">a continuous build system</a>.</para>
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-->
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</simplesect>
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<simplesect><title>Portability</title>
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<para>Nix runs on Linux and Mac OS X.</para>
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</simplesect>
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<simplesect><title>NixOS</title>
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<para>NixOS is a Linux distribution based on Nix. It uses Nix not
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just for package management but also to manage the system
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configuration (e.g., to build configuration files in
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<filename>/etc</filename>). This means, among other things, that it
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is easy to roll back the entire configuration of the system to an
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earlier state. Also, users can install software without root
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privileges. For more information and downloads, see the <link
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xlink:href="http://nixos.org/">NixOS homepage</link>.</para>
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</simplesect>
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<simplesect><title>License</title>
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<para>Nix is released under the terms of the <link
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xlink:href="http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html">GNU
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LGPLv2.1 or (at your option) any later version</link>.</para>
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</simplesect>
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</chapter>
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