Restructuring the Nix manual

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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="ch-basic-package-mgmt">
+
+<title>Basic Package Management</title>
+
+<para>The main command for package management is <link
+linkend="sec-nix-env"><command>nix-env</command></link>.  You can use
+it to install, upgrade, and erase packages, and to query what
+packages are installed or are available for installation.</para>
+
+<para>In Nix, different users can have different “views”
+on the set of installed applications.  That is, there might be lots of
+applications present on the system (possibly in many different
+versions), but users can have a specific selection of those active —
+where “active” just means that it appears in a directory
+in the user’s <envar>PATH</envar>.  Such a view on the set of
+installed applications is called a <emphasis>user
+environment</emphasis>, which is just a directory tree consisting of
+symlinks to the files of the active applications.  </para>
+
+<para>Components are installed from a set of <emphasis>Nix
+expressions</emphasis> that tell Nix how to build those packages,
+including, if necessary, their dependencies.  There is a collection of
+Nix expressions called the Nix Package collection that contains
+packages ranging from basic development stuff such as GCC and Glibc,
+to end-user applications like Mozilla Firefox.  (Nix is however not
+tied to the Nix Package collection; you could write your own Nix
+expressions based on it, or completely new ones.)  You can download
+the latest version from <link
+xlink:href='http://nixos.org/nixpkgs/download.html' />.</para>
+
+<para>Assuming that you have downloaded and unpacked a release of Nix
+Packages, you can view the set of available packages in the release:
+
+<screen>
+$ nix-env -qaf nixpkgs-<replaceable>version</replaceable> '*'
+ant-blackdown-1.4.2
+aterm-2.2
+bash-3.0
+binutils-2.15
+bison-1.875d
+blackdown-1.4.2
+bzip2-1.0.2
+...</screen>
+
+where <literal>nixpkgs-<replaceable>version</replaceable></literal> is
+where you’ve unpacked the release.  The flag <option>-q</option>
+specifies a query operation; <option>-a</option> means that you want
+to show the “available” (i.e., installable) packages, as opposed to
+the installed packages; and <option>-f</option>
+<filename>nixpkgs-<replaceable>version</replaceable></filename>
+specifies the source of the packages.  The argument
+<literal>'*'</literal> shows all installable packages. (The quotes are
+necessary to prevent shell expansion.)  You can also select specific
+packages by name:
+
+<screen>
+$ nix-env -qaf nixpkgs-<replaceable>version</replaceable> gcc
+gcc-3.4.6
+gcc-4.0.3
+gcc-4.1.1</screen>
+
+</para>
+
+<para>It is also possible to see the <emphasis>status</emphasis> of
+available packages, i.e., whether they are installed into the user
+environment and/or present in the system:
+
+<screen>
+$ nix-env -qasf nixpkgs-<replaceable>version</replaceable> '*'
+...
+-PS bash-3.0
+--S binutils-2.15
+IPS bison-1.875d
+...</screen>
+
+The first character (<literal>I</literal>) indicates whether the
+package is installed in your current user environment.  The second
+(<literal>P</literal>) indicates whether it is present on your system
+(in which case installing it into your user environment would be a
+very quick operation).  The last one (<literal>S</literal>) indicates
+whether there is a so-called <emphasis>substitute</emphasis> for the
+package, which is Nix’s mechanism for doing binary deployment.  It
+just means that Nix knows that it can fetch a pre-built package from
+somewhere (typically a network server) instead of building it
+locally.</para>
+
+<para>So now that we have a set of Nix expressions we can build the
+packages contained in them.  This is done using <literal>nix-env
+-i</literal>.  For instance,
+
+<screen>
+$ nix-env -f nixpkgs-<replaceable>version</replaceable> -i subversion</screen>
+
+will install the package called <literal>subversion</literal> (which
+is, of course, the <link
+xlink:href='http://subversion.tigris.org/'>Subversion version
+management system</link>).</para>
+
+<para>When you do this for the first time, Nix will start building
+Subversion and all its dependencies.  This will take quite a while —
+typically an hour or two on modern machines.  Fortunately, there is a
+faster way (so do a Ctrl-C on that install operation!): you just need
+to tell Nix that pre-built binaries of all those packages are
+available somewhere.  This is done using the
+<command>nix-pull</command> command, which must be supplied with a URL
+containing a <emphasis>manifest</emphasis> describing what binaries
+are available.  This URL should correspond to the Nix Packages release
+that you’re using.  For instance, if you obtained a release from <link
+xlink:href='http://nixos.org/releases/nixpkgs/nixpkgs-0.12pre11712-4lrp7j8x'
+/>, then you should do:
+
+<screen>
+$ nix-pull http://nixos.org/releases/nixpkgs/nixpkgs-0.12pre11712-4lrp7j8x/MANIFEST</screen>
+
+If you then issue the installation command, it should start
+downloading binaries from <systemitem
+class='fqdomainname'>nixos.org</systemitem>, instead of building
+them from source.  This might still take a while since all
+dependencies must be downloaded, but on a reasonably fast connection
+such as a DSL line it’s on the order of a few minutes.</para>
+
+<para>Naturally, packages can also be uninstalled:
+
+<screen>
+$ nix-env -e subversion</screen>
+
+</para>
+
+<para>Upgrading to a new version is just as easy.  If you have a new
+release of Nix Packages, you can do:
+
+<screen>
+$ nix-env -f nixpkgs-<replaceable>version</replaceable> -u subversion</screen>
+
+This will <emphasis>only</emphasis> upgrade Subversion if there is a
+“newer” version in the new set of Nix expressions, as
+defined by some pretty arbitrary rules regarding ordering of version
+numbers (which generally do what you’d expect of them).  To just
+unconditionally replace Subversion with whatever version is in the Nix
+expressions, use <parameter>-i</parameter> instead of
+<parameter>-u</parameter>; <parameter>-i</parameter> will remove
+whatever version is already installed.</para>
+
+<para>You can also upgrade all packages for which there are newer
+versions:
+
+<screen>
+$ nix-env -f nixpkgs-<replaceable>version</replaceable> -u '*'</screen>
+
+</para>
+
+<para>Sometimes it’s useful to be able to ask what
+<command>nix-env</command> would do, without actually doing it.  For
+instance, to find out what packages would be upgraded by
+<literal>nix-env -u '*'</literal>, you can do
+
+<screen>
+$ nix-env ... -u '*' --dry-run
+(dry run; not doing anything)
+upgrading `libxslt-1.1.0' to `libxslt-1.1.10'
+upgrading `graphviz-1.10' to `graphviz-1.12'
+upgrading `coreutils-5.0' to `coreutils-5.2.1'</screen>
+
+</para>
+
+</chapter>
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