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+<refentry 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-nix-env">
+
+<refmeta>
+  <refentrytitle>nix-env</refentrytitle>
+  <manvolnum>1</manvolnum>
+  <refmiscinfo class="source">Nix</refmiscinfo>
+  <refmiscinfo class="version"><xi:include href="version.txt" parse="text"/></refmiscinfo>
+</refmeta>
+
+<refnamediv>
+  <refname>nix-env</refname>
+  <refpurpose>manipulate or query Nix user environments</refpurpose>
+</refnamediv>
+
+<refsynopsisdiv>
+  <cmdsynopsis>
+    <command>nix-env</command>
+    <xi:include href="opt-common-syn.xml#xmlns(db=http://docbook.org/ns/docbook)xpointer(/db:nop/*)" />
+    <arg><option>--arg</option> <replaceable>name</replaceable> <replaceable>value</replaceable></arg>
+    <arg><option>--argstr</option> <replaceable>name</replaceable> <replaceable>value</replaceable></arg>
+    <arg>
+      <group choice='req'>
+        <arg choice='plain'><option>--file</option></arg>
+        <arg choice='plain'><option>-f</option></arg>
+      </group>
+      <replaceable>path</replaceable>
+    </arg>
+    <arg>
+      <group choice='req'>
+        <arg choice='plain'><option>--profile</option></arg>
+        <arg choice='plain'><option>-p</option></arg>
+      </group>
+      <replaceable>path</replaceable>
+    </arg>
+    <arg>
+      <arg choice='plain'><option>--system-filter</option></arg>
+      <replaceable>system</replaceable>
+    </arg>
+    <arg><option>--dry-run</option></arg>
+    <arg choice='plain'><replaceable>operation</replaceable></arg>
+    <arg rep='repeat'><replaceable>options</replaceable></arg>
+    <arg rep='repeat'><replaceable>arguments</replaceable></arg>
+  </cmdsynopsis>
+</refsynopsisdiv>
+
+
+<refsection><title>Description</title>
+
+<para>The command <command>nix-env</command> is used to manipulate Nix
+user environments.  User environments are sets of software packages
+available to a user at some point in time.  In other words, they are a
+synthesised view of the programs available in the Nix store.  There
+may be many user environments: different users can have different
+environments, and individual users can switch between different
+environments.</para>
+
+<para><command>nix-env</command> takes exactly one
+<emphasis>operation</emphasis> flag which indicates the subcommand to
+be performed.  These are documented below.</para>
+
+</refsection>
+
+
+
+<!--######################################################################-->
+
+<refsection><title>Common options</title>
+
+<para>This section lists the options that are common to all
+operations.  These options are allowed for every subcommand, though
+they may not always have an effect.  <phrase condition="manual">See
+also <xref linkend="sec-common-options" />.</phrase></para>
+
+<variablelist>
+
+  <varlistentry><term><option>--file</option></term>
+    <term><option>-f</option></term>
+
+    <listitem><para>Specifies the Nix expression (designated below as
+    the <emphasis>active Nix expression</emphasis>) used by the
+    <option>--install</option>, <option>--upgrade</option>, and
+    <option>--query --available</option> operations to obtain
+    derivations.  The default is
+    <filename>~/.nix-defexpr</filename>.</para></listitem>
+
+  </varlistentry>
+
+  <varlistentry><term><option>--profile</option></term>
+    <term><option>-p</option></term>
+
+    <listitem><para>Specifies the profile to be used by those
+    operations that operate on a profile (designated below as the
+    <emphasis>active profile</emphasis>).  A profile is a sequence of
+    user environments called <emphasis>generations</emphasis>, one of
+    which is the <emphasis>current
+    generation</emphasis>.</para></listitem>
+
+  </varlistentry>
+
+  <varlistentry><term><option>--dry-run</option></term>
+
+    <listitem><para>For the <option>--install</option>,
+    <option>--upgrade</option>, <option>--uninstall</option>,
+    <option>--switch-generation</option>,
+    <option>--delete-generations</option> and
+    <option>--rollback</option> operations, this flag will cause
+    <command>nix-env</command> to print what
+    <emphasis>would</emphasis> be done if this flag had not been
+    specified, without actually doing it.</para>
+
+    <para><option>--dry-run</option> also prints out which paths will
+    be <link linkend="gloss-substitute">substituted</link> (i.e.,
+    downloaded) and which paths will be built from source (because no
+    substitute is available).</para></listitem>
+
+  </varlistentry>
+
+  <varlistentry><term><option>--system-filter</option> <replaceable>system</replaceable></term>
+
+    <listitem><para>By default, operations such as <option>--query
+    --available</option> show derivations matching any platform.  This
+    option allows you to use derivations for the specified platform
+    <replaceable>system</replaceable>.</para></listitem>
+
+  </varlistentry>
+
+</variablelist>
+
+<variablelist condition="manpage">
+  <xi:include href="opt-common.xml#xmlns(db=http://docbook.org/ns/docbook)xpointer(//db:variablelist[@xml:id='opt-common']/*)" />
+</variablelist>
+
+</refsection>
+
+
+
+<!--######################################################################-->
+
+<refsection><title>Files</title>
+
+<variablelist>
+
+  <varlistentry><term><filename>~/.nix-defexpr</filename></term>
+
+    <listitem><para>A directory that contains the default Nix
+    expressions used by the <option>--install</option>,
+    <option>--upgrade</option>, and <option>--query
+    --available</option> operations to obtain derivations.  The
+    <option>--file</option> option may be used to override this
+    default.</para>
+
+    <para>The Nix expressions in this directory are combined into a
+    single set, with each file as an attribute that has the name of
+    the file.  Thus, if <filename>~/.nix-defexpr</filename> contains
+    two files, <filename>foo</filename> and <filename>bar</filename>,
+    then the default Nix expression will essentially be
+
+<programlisting>
+{
+  foo = import ~/.nix-defexpr/foo;
+  bar = import ~/.nix-defexpr/bar;
+}</programlisting>
+
+    </para>
+
+    <para>The command <command>nix-channel</command> places symlinks
+    to the downloaded Nix expressions from each subscribed channel in
+    this directory.</para>
+
+    </listitem>
+
+  </varlistentry>
+
+  <varlistentry><term><filename>~/.nix-profile</filename></term>
+
+    <listitem><para>A symbolic link to the user's current profile.  By
+    default, this symlink points to
+    <filename><replaceable>prefix</replaceable>/var/nix/profiles/default</filename>.
+    The <envar>PATH</envar> environment variable should include
+    <filename>~/.nix-profile/bin</filename> for the user environment
+    to be visible to the user.</para></listitem>
+
+  </varlistentry>
+
+</variablelist>
+
+</refsection>
+
+
+
+<!--######################################################################-->
+
+<refsection xml:id="rsec-nix-env-install"><title>Operation <option>--install</option></title>
+
+<refsection><title>Synopsis</title>
+
+<cmdsynopsis>
+  <command>nix-env</command>
+  <group choice='req'>
+    <arg choice='plain'><option>--install</option></arg>
+    <arg choice='plain'><option>-i</option></arg>
+  </group>
+  <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="opt-inst-syn.xml#xmlns(db=http://docbook.org/ns/docbook)xpointer(/db:nop/*)" />
+  <group choice='opt'>
+    <arg choice='plain'><option>--preserve-installed</option></arg>
+    <arg choice='plain'><option>-P</option></arg>
+  </group>
+  <group choice='opt'>
+    <arg choice='plain'><option>--remove-all</option></arg>
+    <arg choice='plain'><option>-r</option></arg>
+  </group>
+  <arg choice='plain' rep='repeat'><replaceable>args</replaceable></arg>
+</cmdsynopsis>
+
+</refsection>
+
+
+<refsection><title>Description</title>
+
+<para>The install operation creates a new user environment, based on
+the current generation of the active profile, to which a set of store
+paths described by <replaceable>args</replaceable> is added.  The
+arguments <replaceable>args</replaceable> map to store paths in a
+number of possible ways:
+
+<itemizedlist>
+
+  <listitem><para>By default, <replaceable>args</replaceable> is a set
+  of derivation names denoting derivations in the active Nix
+  expression.  These are realised, and the resulting output paths are
+  installed.  Currently installed derivations with a name equal to the
+  name of a derivation being added are removed unless the option
+  <option>--preserve-installed</option> is
+  specified.</para>
+
+  <para>If there are multiple derivations matching a name in
+  <replaceable>args</replaceable> that have the same name (e.g.,
+  <literal>gcc-3.3.6</literal> and <literal>gcc-4.1.1</literal>), then
+  the derivation with the highest <emphasis>priority</emphasis> is
+  used.  A derivation can define a priority by declaring the
+  <varname>meta.priority</varname> attribute.  This attribute should
+  be a number, with a higher value denoting a lower priority.  The
+  default priority is <literal>0</literal>.</para>
+
+  <para>If there are multiple matching derivations with the same
+  priority, then the derivation with the highest version will be
+  installed.</para>
+
+  <para>You can force the installation of multiple derivations with
+  the same name by being specific about the versions.  For instance,
+  <literal>nix-env -i gcc-3.3.6 gcc-4.1.1</literal> will install both
+  version of GCC (and will probably cause a user environment
+  conflict!).</para></listitem>
+
+  <listitem><para>If <link
+  linkend='opt-attr'><option>--attr</option></link>
+  (<option>-A</option>) is specified, the arguments are
+  <emphasis>attribute paths</emphasis> that select attributes from the
+  top-level Nix expression.  This is faster than using derivation
+  names and unambiguous.  To find out the attribute paths of available
+  packages, use <literal>nix-env -qaP '*'</literal>.</para></listitem>
+
+  <listitem><para>If <option>--from-profile</option>
+  <replaceable>path</replaceable> is given,
+  <replaceable>args</replaceable> is a set of names denoting installed
+  store paths in the profile <replaceable>path</replaceable>.  This is
+  an easy way to copy user environment elements from one profile to
+  another.</para></listitem>
+
+  <listitem><para>If <option>--from-expression</option> is given,
+  <replaceable>args</replaceable> are Nix <link
+  linkend="ss-functions">functions</link> that are called with the
+  active Nix expression as their single argument.  The derivations
+  returned by those function calls are installed.  This allows
+  derivations to be specified in an unambiguous way, which is necessary
+  if there are multiple derivations with the same
+  name.</para></listitem>
+
+  <listitem><para>If <replaceable>args</replaceable> are store
+  derivations, then these are <link
+  linkend="rsec-nix-store-realise">realised</link>, and the resulting
+  output paths are installed.</para></listitem>
+
+  <listitem><para>If <replaceable>args</replaceable> are store paths
+  that are not store derivations, then these are <link
+  linkend="rsec-nix-store-realise">realised</link> and
+  installed.</para></listitem>
+
+</itemizedlist>
+
+</para>
+
+</refsection>
+
+
+<refsection><title>Flags</title>
+
+<variablelist>
+
+  <varlistentry><term><option>--prebuild-only</option> / <option>-b</option></term>
+
+    <listitem><para>Use only derivations for which a substitute is
+    registered, i.e., there is a pre-built binary available that can
+    be downloaded in lieu of building the derivation.  Thus, no
+    packages will be built from source.</para></listitem>
+
+  </varlistentry>
+
+  <varlistentry><term><option>--preserve-installed</option></term>
+    <term><option>-P</option></term>
+
+    <listitem><para>Do not remove derivations with a name matching one
+    of the derivations being installed.  Usually, trying to have two
+    versions of the same package installed in the same generation of a
+    profile will lead to an error in building the generation, due to
+    file name clashes between the two versions.  However, this is not
+    the case for all packages.</para></listitem>
+
+  </varlistentry>
+
+  <varlistentry><term><option>--remove-all</option></term>
+    <term><option>-r</option></term>
+
+    <listitem><para>Remove all previously installed packages first.
+    This is equivalent to running <literal>nix-env -e '*'</literal>
+    first, except that everything happens in a single
+    transaction.</para></listitem>
+
+  </varlistentry>
+
+</variablelist>
+
+</refsection>
+
+
+<refsection xml:id='refsec-nix-env-install-examples'><title>Examples</title>
+
+<para>To install a specific version of <command>gcc</command> from the
+active Nix expression:
+
+<screen>
+$ nix-env --install gcc-3.3.2
+installing `gcc-3.3.2'
+uninstalling `gcc-3.1'</screen>
+
+Note the the previously installed version is removed, since
+<option>--preserve-installed</option> was not specified.</para>
+
+<para>To install an arbitrary version:
+
+<screen>
+$ nix-env --install gcc
+installing `gcc-3.3.2'</screen>
+
+</para>
+
+<para>To install using a specific attribute:
+
+<screen>
+$ nix-env -i -A gcc40mips
+$ nix-env -i -A xorg.xorgserver</screen>
+
+</para>
+
+<para>To install all derivations in the Nix expression <filename>foo.nix</filename>:
+
+<screen>
+$ nix-env -f ~/foo.nix -i '*'</screen>
+
+</para>
+
+<para>To copy the store path with symbolic name <literal>gcc</literal>
+from another profile:
+
+<screen>
+$ nix-env -i --from-profile /nix/var/nix/profiles/foo -i gcc</screen>
+
+</para>
+
+<para>To install a specific store derivation (typically created by
+<command>nix-instantiate</command>):
+
+<screen>
+$ nix-env -i /nix/store/fibjb1bfbpm5mrsxc4mh2d8n37sxh91i-gcc-3.4.3.drv</screen>
+
+</para>
+
+<para>To install a specific output path:
+
+<screen>
+$ nix-env -i /nix/store/y3cgx0xj1p4iv9x0pnnmdhr8iyg741vk-gcc-3.4.3</screen>
+
+</para>
+
+<para>To install from a Nix expression specified on the command-line:
+
+<screen>
+$ nix-env -f ./foo.nix -i -E \
+    'f: (f {system = "i686-linux";}).subversionWithJava'</screen>
+
+I.e., this evaluates to <literal>(f: (f {system =
+"i686-linux";}).subversionWithJava) (import ./foo.nix)</literal>, thus
+selecting the <literal>subversionWithJava</literal> attribute from the
+set returned by calling the function defined in
+<filename>./foo.nix</filename>.</para>
+
+<para>A dry-run tells you which paths will be downloaded or built from
+source:
+
+<screen>
+$ nix-env -f pkgs/top-level/all-packages.nix -i f-spot --dry-run
+(dry run; not doing anything)
+installing `f-spot-0.0.10'
+the following derivations will be built:
+  /nix/store/0g63jv9aagwbgci4nnzs2dkxqz84kdja-libgnomeprintui-2.12.1.tar.bz2.drv
+  /nix/store/0gfarvxq6sannsdw8a1ir40j1ys2mqb4-ORBit2-2.14.2.tar.bz2.drv
+  /nix/store/0i9gs5zc04668qiy60ga2rc16abkj7g8-sqlite-2.8.17.drv
+  <replaceable>...</replaceable>
+the following paths will be substituted:
+  /nix/store/8zbipvm4gp9jfqh9nnk1n3bary1a37gs-perl-XML-Parser-2.34
+  /nix/store/b8a2bg7gnyvvvjjibp4axg9x1hzkw36c-mono-1.1.4
+  <replaceable>...</replaceable></screen>
+
+</para>
+
+</refsection>
+
+</refsection>
+
+
+
+<!--######################################################################-->
+
+<refsection xml:id="rsec-nix-env-upgrade"><title>Operation <option>--upgrade</option></title>
+
+<refsection><title>Synopsis</title>
+
+<cmdsynopsis>
+  <command>nix-env</command>
+  <group choice='req'>
+    <arg choice='plain'><option>--upgrade</option></arg>
+    <arg choice='plain'><option>-u</option></arg>
+  </group>
+  <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="opt-inst-syn.xml#xmlns(db=http://docbook.org/ns/docbook)xpointer(/db:nop/*)" />
+  <group choice='opt'>
+    <arg choice='plain'><option>--lt</option></arg>
+    <arg choice='plain'><option>--leq</option></arg>
+    <arg choice='plain'><option>--eq</option></arg>
+    <arg choice='plain'><option>--always</option></arg>
+  </group>
+  <arg choice='plain' rep='repeat'><replaceable>args</replaceable></arg>
+</cmdsynopsis>
+
+</refsection>
+
+<refsection><title>Description</title>
+
+<para>The upgrade operation creates a new user environment, based on
+the current generation of the active profile, in which all store paths
+are replaced for which there are newer versions in the set of paths
+described by <replaceable>args</replaceable>.  Paths for which there
+are no newer versions are left untouched; this is not an error.  It is
+also not an error if an element of <replaceable>args</replaceable>
+matches no installed derivations.</para>
+
+<para>For a description of how <replaceable>args</replaceable> is
+mapped to a set of store paths, see <link
+linkend="rsec-nix-env-install"><option>--install</option></link>.  If
+<replaceable>args</replaceable> describes multiple store paths with
+the same symbolic name, only the one with the highest version is
+installed.</para>
+
+</refsection>
+
+<refsection><title>Flags</title>
+
+<variablelist>
+
+  <varlistentry><term><option>--lt</option></term>
+
+    <listitem><para>Only upgrade a derivation to newer versions.  This
+    is the default.</para></listitem>
+
+  </varlistentry>
+
+  <varlistentry><term><option>--leq</option></term>
+
+    <listitem><para>In addition to upgrading to newer versions, also
+    “upgrade” to derivations that have the same version.  Version are
+    not a unique identification of a derivation, so there may be many
+    derivations that have the same version.  This flag may be useful
+    to force “synchronisation” between the installed and available
+    derivations.</para></listitem>
+
+  </varlistentry>
+
+  <varlistentry><term><option>--eq</option></term>
+
+    <listitem><para><emphasis>Only</emphasis> “upgrade” to derivations
+    that have the same version.  This may not seem very useful, but it
+    actually is, e.g., when there is a new release of Nixpkgs and you
+    want to replace installed applications with the same versions
+    built against newer dependencies (to reduce the number of
+    dependencies floating around on your system).</para></listitem>
+
+  </varlistentry>
+
+  <varlistentry><term><option>--always</option></term>
+
+    <listitem><para>In addition to upgrading to newer versions, also
+    “upgrade” to derivations that have the same or a lower version.
+    I.e., derivations may actually be downgraded depending on what is
+    available in the active Nix expression.</para></listitem>
+
+  </varlistentry>
+
+</variablelist>
+
+<para>For the other flags, see <option
+linkend="rsec-nix-env-install">--install</option>.</para>
+
+</refsection>
+
+<refsection><title>Examples</title>
+
+<screen>
+$ nix-env --upgrade gcc
+upgrading `gcc-3.3.1' to `gcc-3.4'
+
+$ nix-env -u gcc-3.3.2 --always <lineannotation>(switch to a specific version)</lineannotation>
+upgrading `gcc-3.4' to `gcc-3.3.2'
+
+$ nix-env --upgrade pan
+<lineannotation>(no upgrades available, so nothing happens)</lineannotation>
+
+$ nix-env -u '*' <lineannotation>(try to upgrade everything)</lineannotation>
+upgrading `hello-2.1.2' to `hello-2.1.3'
+upgrading `mozilla-1.2' to `mozilla-1.4'</screen>
+
+</refsection>
+
+<refsection xml:id="ssec-version-comparisons"><title>Versions</title>
+
+<para>The upgrade operation determines whether a derivation
+<varname>y</varname> is an upgrade of a derivation
+<varname>x</varname> by looking at their respective
+<literal>name</literal> attributes.  The names (e.g.,
+<literal>gcc-3.3.1</literal> are split into two parts: the package
+name (<literal>gcc</literal>), and the version
+(<literal>3.3.1</literal>).  The version part starts after the first
+dash not following by a letter.  <varname>x</varname> is considered an
+upgrade of <varname>y</varname> if their package names match, and the
+version of <varname>y</varname> is higher that that of
+<varname>x</varname>.</para>
+
+<para>The versions are compared by splitting them into contiguous
+components of numbers and letters.  E.g., <literal>3.3.1pre5</literal>
+is split into <literal>[3, 3, 1, "pre", 5]</literal>.  These lists are
+then compared lexicographically (from left to right).  Corresponding
+components <varname>a</varname> and <varname>b</varname> are compared
+as follows.  If they are both numbers, integer comparison is used.  If
+<varname>a</varname> is an empty string and <varname>b</varname> is a
+number, <varname>a</varname> is considered less than
+<varname>b</varname>.  The special string component
+<literal>pre</literal> (for <emphasis>pre-release</emphasis>) is
+considered to be less than other components.  String components are
+considered less than number components.  Otherwise, they are compared
+lexicographically (i.e., using case-sensitive string comparison).</para>
+
+<para>This is illustrated by the following examples:
+
+<screen>
+1.0 &lt; 2.3
+2.1 &lt; 2.3
+2.3 = 2.3
+2.5 > 2.3
+3.1 > 2.3
+2.3.1 > 2.3
+2.3.1 > 2.3a
+2.3pre1 &lt; 2.3
+2.3pre3 &lt; 2.3pre12
+2.3a &lt; 2.3c
+2.3pre1 &lt; 2.3c
+2.3pre1 &lt; 2.3q</screen>
+
+</para>
+
+</refsection>
+
+</refsection>
+
+
+
+<!--######################################################################-->
+
+<refsection><title>Operation <option>--uninstall</option></title>
+
+<refsection><title>Synopsis</title>
+
+<cmdsynopsis>
+  <command>nix-env</command>
+  <group choice='req'>
+    <arg choice='plain'><option>--uninstall</option></arg>
+    <arg choice='plain'><option>-e</option></arg>
+  </group>
+  <arg choice='plain' rep='repeat'><replaceable>drvnames</replaceable></arg>
+</cmdsynopsis>
+</refsection>
+
+<refsection><title>Description</title>
+
+<para>The uninstall operation creates a new user environment, based on
+the current generation of the active profile, from which the store
+paths designated by the symbolic names
+<replaceable>names</replaceable> are removed.</para>
+
+</refsection>
+
+<refsection><title>Examples</title>
+
+<screen>
+$ nix-env --uninstall gcc
+$ nix-env -e '*' <lineannotation>(remove everything)</lineannotation></screen>
+
+</refsection>
+
+</refsection>
+
+
+
+<!--######################################################################-->
+
+<refsection xml:id="rsec-nix-env-set-flag"><title>Operation <option>--set-flag</option></title>
+
+<refsection><title>Synopsis</title>
+
+<cmdsynopsis>
+  <command>nix-env</command>
+  <arg choice='plain'><option>--set-flag</option></arg>
+  <arg choice='plain'><replaceable>name</replaceable></arg>
+  <arg choice='plain'><replaceable>value</replaceable></arg>
+  <arg choice='plain' rep='repeat'><replaceable>drvnames</replaceable></arg>
+</cmdsynopsis>
+</refsection>
+
+<refsection><title>Description</title>
+
+<para>The <option>--set-flag</option> operation allows meta attributes
+of installed packages to be modified.  There are several attributes
+that can be usefully modified, because they affect the behaviour of
+<command>nix-env</command> or the user environment build
+script:
+
+<itemizedlist>
+
+  <listitem><para><varname>priority</varname> can be changed to
+  resolve filename clashes.  The user environment build script uses
+  the <varname>meta.priority</varname> attribute of derivations to
+  resolve filename collisions between packages.  Lower priority values
+  denote a higher priority.  For instance, the GCC wrapper package and
+  the Binutils package in Nixpkgs both have a file
+  <filename>bin/ld</filename>, so previously if you tried to install
+  both you would get a collision.  Now, on the other hand, the GCC
+  wrapper declares a higher priority than Binutils, so the former’s
+  <filename>bin/ld</filename> is symlinked in the user
+  environment.</para></listitem>
+
+  <listitem><para><varname>keep</varname> can be set to
+  <literal>true</literal> to prevent the package from being upgraded
+  or replaced.  This is useful if you want to hang on to an older
+  version of a package.</para></listitem>
+
+  <listitem><para><varname>active</varname> can be set to
+  <literal>false</literal> to “disable” the package.  That is, no
+  symlinks will be generated to the files of the package, but it
+  remains part of the profile (so it won’t be garbage-collected).  It
+  can be set back to <literal>true</literal> to re-enable the
+  package.</para></listitem>
+
+</itemizedlist>
+
+</para>
+
+</refsection>
+
+<refsection><title>Examples</title>
+
+<para>To prevent the currently installed Firefox from being upgraded:
+
+<screen>
+$ nix-env --set-flag keep true firefox</screen>
+
+After this, <command>nix-env -u</command> will ignore Firefox.</para>
+
+<para>To disable the currently installed Firefox, then install a new
+Firefox while the old remains part of the profile:
+
+<screen>
+$ nix-env -q \*
+firefox-2.0.0.9 <lineannotation>(the current one)</lineannotation>
+
+$ nix-env --preserve-installed -i firefox-2.0.0.11
+installing `firefox-2.0.0.11'
+building path(s) `/nix/store/myy0y59q3ig70dgq37jqwg1j0rsapzsl-user-environment'
+collision between `/nix/store/<replaceable>...</replaceable>-firefox-2.0.0.11/bin/firefox'
+  and `/nix/store/<replaceable>...</replaceable>-firefox-2.0.0.9/bin/firefox'.
+<lineannotation>(i.e., can’t have two active at the same time)</lineannotation>
+
+$ nix-env --set-flag active false firefox
+setting flag on `firefox-2.0.0.9'
+
+$ nix-env --preserve-installed -i firefox-2.0.0.11
+installing `firefox-2.0.0.11'
+
+$ nix-env -q \*
+firefox-2.0.0.11 <lineannotation>(the enabled one)</lineannotation>
+firefox-2.0.0.9 <lineannotation>(the disabled one)</lineannotation></screen>
+
+</para>
+
+<para>To make files from <literal>binutils</literal> take precedence
+over files from <literal>gcc</literal>:
+
+<screen>
+$ nix-env --set-flag priority 5 binutils
+$ nix-env --set-flag priority 10 gcc</screen>
+
+</para>
+
+</refsection>
+
+</refsection>
+
+
+
+<!--######################################################################-->
+
+<refsection><title>Operation <option>--query</option></title>
+
+<refsection><title>Synopsis</title>
+
+<cmdsynopsis>
+  <command>nix-env</command>
+  <group choice='req'>
+    <arg choice='plain'><option>--query</option></arg>
+    <arg choice='plain'><option>-q</option></arg>
+  </group>
+  <group choice='opt'>
+    <arg choice='plain'><option>--installed</option></arg>
+    <arg choice='plain'><option>--available</option></arg>
+    <arg choice='plain'><option>-a</option></arg>
+  </group>
+
+  <sbr />
+
+  <arg>
+    <group choice='req'>
+      <arg choice='plain'><option>--status</option></arg>
+      <arg choice='plain'><option>-s</option></arg>
+    </group>
+  </arg>
+  <arg>
+    <group choice='req'>
+      <arg choice='plain'><option>--attr-path</option></arg>
+      <arg choice='plain'><option>-P</option></arg>
+    </group>
+  </arg>
+  <arg><option>--no-name</option></arg>
+  <arg>
+    <group choice='req'>
+      <arg choice='plain'><option>--compare-versions</option></arg>
+      <arg choice='plain'><option>-c</option></arg>
+    </group>
+  </arg>
+  <arg><option>--system</option></arg>
+  <arg><option>--drv-path</option></arg>
+  <arg><option>--out-path</option></arg>
+  <arg><option>--description</option></arg>
+  <arg><option>--meta</option></arg>
+
+  <sbr />
+
+  <arg><option>--xml</option></arg>
+  <arg><option>--json</option></arg>
+  <arg>
+    <group choice='req'>
+      <arg choice='plain'><option>--prebuilt-only</option></arg>
+      <arg choice='plain'><option>-b</option></arg>
+    </group>
+  </arg>
+
+  <arg>
+    <group choice='req'>
+      <arg choice='plain'><option>--attr</option></arg>
+      <arg choice='plain'><option>-A</option></arg>
+    </group>
+    <replaceable>attribute-path</replaceable>
+  </arg>
+
+  <sbr />
+
+  <arg choice='plain' rep='repeat'><replaceable>names</replaceable></arg>
+</cmdsynopsis>
+
+</refsection>
+
+
+<refsection><title>Description</title>
+
+<para>The query operation displays information about either the store
+paths that are installed in the current generation of the active
+profile (<option>--installed</option>), or the derivations that are
+available for installation in the active Nix expression
+(<option>--available</option>).  It only prints information about
+derivations whose symbolic name matches one of
+<replaceable>names</replaceable>.  The wildcard <literal>*</literal>
+shows all derivations.</para>
+
+<para>The derivations are sorted by their <literal>name</literal>
+attributes.</para>
+
+</refsection>
+
+
+<refsection><title>Source selection</title>
+
+<para>The following flags specify the set of things on which the query
+operates.</para>
+
+<variablelist>
+
+  <varlistentry><term><option>--installed</option></term>
+
+    <listitem><para>The query operates on the store paths that are
+    installed in the current generation of the active profile.  This
+    is the default.</para></listitem>
+
+  </varlistentry>
+
+  <varlistentry><term><option>--available</option></term>
+    <term><option>-a</option></term>
+
+    <listitem><para>The query operates on the derivations that are
+    available in the active Nix expression.</para></listitem>
+
+  </varlistentry>
+
+</variablelist>
+
+</refsection>
+
+
+<refsection><title>Queries</title>
+
+<para>The following flags specify what information to display about
+the selected derivations.  Multiple flags may be specified, in which
+case the information is shown in the order given here.  Note that the
+name of the derivation is shown unless <option>--no-name</option> is
+specified.</para>
+
+<!-- TODO: fix the terminology here; i.e., derivations, store paths,
+user environment elements, etc. -->
+
+<variablelist>
+
+  <varlistentry><term><option>--xml</option></term>
+
+    <listitem><para>Print the result in an XML representation suitable
+    for automatic processing by other tools.  The root element is
+    called <literal>items</literal>, which contains a
+    <literal>item</literal> element for each available or installed
+    derivation.  The fields discussed below are all stored in
+    attributes of the <literal>item</literal>
+    elements.</para></listitem>
+
+  </varlistentry>
+
+  <varlistentry><term><option>--json</option></term>
+
+    <listitem><para>Print the result in a JSON representation suitable
+    for automatic processing by other tools.</para></listitem>
+
+  </varlistentry>
+
+  <varlistentry><term><option>--prebuild-only</option> / <option>-b</option></term>
+
+    <listitem><para>Show only derivations for which a substitute is
+    registered, i.e., there is a pre-built binary available that can
+    be downloaded in lieu of building the derivation.  Thus, this
+    shows all packages that probably can be installed
+    quickly.</para></listitem>
+
+  </varlistentry>
+
+  <varlistentry><term><option>--status</option></term>
+    <term><option>-s</option></term>
+
+    <listitem><para>Print the <emphasis>status</emphasis> of the
+    derivation.  The status consists of three characters.  The first
+    is <literal>I</literal> or <literal>-</literal>, indicating
+    whether the derivation is currently installed in the current
+    generation of the active profile.  This is by definition the case
+    for <option>--installed</option>, but not for
+    <option>--available</option>.  The second is <literal>P</literal>
+    or <literal>-</literal>, indicating whether the derivation is
+    present on the system.  This indicates whether installation of an
+    available derivation will require the derivation to be built.  The
+    third is <literal>S</literal> or <literal>-</literal>, indicating
+    whether a substitute is available for the
+    derivation.</para></listitem>
+
+  </varlistentry>
+
+  <varlistentry><term><option>--attr-path</option></term>
+    <term><option>-P</option></term>
+
+    <listitem><para>Print the <emphasis>attribute path</emphasis> of
+    the derivation, which can be used to unambiguously select it using
+    the <link linkend="opt-attr"><option>--attr</option> option</link>
+    available in commands that install derivations like
+    <literal>nix-env --install</literal>.</para></listitem>
+
+  </varlistentry>
+
+  <varlistentry><term><option>--no-name</option></term>
+
+    <listitem><para>Suppress printing of the <literal>name</literal>
+    attribute of each derivation.</para></listitem>
+
+  </varlistentry>
+
+  <varlistentry><term><option>--compare-versions</option> /
+  <option>-c</option></term>
+
+    <listitem><para>Compare installed versions to available versions,
+    or vice versa (if <option>--available</option> is given).  This is
+    useful for quickly seeing whether upgrades for installed
+    packages are available in a Nix expression.  A column is added
+    with the following meaning:
+
+    <variablelist>
+
+      <varlistentry><term><literal>&lt;</literal> <replaceable>version</replaceable></term>
+
+        <listitem><para>A newer version of the package is available
+        or installed.</para></listitem>
+
+      </varlistentry>
+
+      <varlistentry><term><literal>=</literal> <replaceable>version</replaceable></term>
+
+        <listitem><para>At most the same version of the package is
+        available or installed.</para></listitem>
+
+      </varlistentry>
+
+      <varlistentry><term><literal>></literal> <replaceable>version</replaceable></term>
+
+        <listitem><para>Only older versions of the package are
+        available or installed.</para></listitem>
+
+      </varlistentry>
+
+      <varlistentry><term><literal>- ?</literal></term>
+
+        <listitem><para>No version of the package is available or
+        installed.</para></listitem>
+
+      </varlistentry>
+
+    </variablelist>
+
+    </para></listitem>
+
+  </varlistentry>
+
+  <varlistentry><term><option>--system</option></term>
+
+    <listitem><para>Print the <literal>system</literal> attribute of
+    the derivation.</para></listitem>
+
+  </varlistentry>
+
+  <varlistentry><term><option>--drv-path</option></term>
+
+    <listitem><para>Print the path of the store
+    derivation.</para></listitem>
+
+  </varlistentry>
+
+  <varlistentry><term><option>--out-path</option></term>
+
+    <listitem><para>Print the output path of the
+    derivation.</para></listitem>
+
+  </varlistentry>
+
+  <varlistentry><term><option>--description</option></term>
+
+    <listitem><para>Print a short (one-line) description of the
+    derivation, if available.  The description is taken from the
+    <literal>meta.description</literal> attribute of the
+    derivation.</para></listitem>
+
+  </varlistentry>
+
+  <varlistentry><term><option>--meta</option></term>
+
+    <listitem><para>Print all of the meta-attributes of the
+    derivation.  This option is only available with
+    <option>--xml</option>.</para></listitem>
+
+  </varlistentry>
+
+</variablelist>
+
+</refsection>
+
+
+<refsection><title>Examples</title>
+
+<screen>
+$ nix-env -q '*' <lineannotation>(show installed derivations)</lineannotation>
+bison-1.875c
+docbook-xml-4.2
+firefox-1.0.4
+MPlayer-1.0pre7
+ORBit2-2.8.3
+...
+
+$ nix-env -qa '*' <lineannotation>(show available derivations)</lineannotation>
+firefox-1.0.7
+GConf-2.4.0.1
+MPlayer-1.0pre7
+ORBit2-2.8.3
+...
+
+$ nix-env -qas '*' <lineannotation>(show status of available derivations)</lineannotation>
+-P- firefox-1.0.7   <lineannotation>(not installed but present)</lineannotation>
+--S GConf-2.4.0.1   <lineannotation>(not present, but there is a substitute for fast installation)</lineannotation>
+--S MPlayer-1.0pre3 <lineannotation>(i.e., this is not the installed MPlayer, even though the version is the same!)</lineannotation>
+IP- ORBit2-2.8.3    <lineannotation>(installed and by definition present)</lineannotation>
+...
+
+<lineannotation>(show available derivations in the Nix expression <!-- !!! <filename>-->foo.nix<!-- </filename> -->)</lineannotation>
+$ nix-env -f ./foo.nix -qa '*'
+foo-1.2.3
+
+$ nix-env -qc '*' <lineannotation>(compare installed versions to what’s available)</lineannotation>
+<replaceable>...</replaceable>
+acrobat-reader-7.0 - ?      <lineannotation>(package is not available at all)</lineannotation>
+autoconf-2.59      = 2.59   <lineannotation>(same version)</lineannotation>
+firefox-1.0.4      &lt; 1.0.7  <lineannotation>(a more recent version is available)</lineannotation>
+<replaceable>...</replaceable>
+
+<lineannotation>(show info about a specific package, in XML)</lineannotation>
+$ nix-env -qa --xml --description firefox
+<![CDATA[<?xml version='1.0' encoding='utf-8'?>
+<items>
+  <item attrPath="0.0.firefoxWrapper"
+    description="Mozilla Firefox - the browser, reloaded (with various plugins)"
+    name="firefox-1.5.0.7" system="i686-linux" />
+</items>]]></screen>
+
+</refsection>
+
+</refsection>
+
+
+
+<!--######################################################################-->
+
+<refsection><title>Operation <option>--switch-profile</option></title>
+
+<refsection><title>Synopsis</title>
+
+<cmdsynopsis>
+  <command>nix-env</command>
+  <group choice='req'>
+    <arg choice='plain'><option>--switch-profile</option></arg>
+    <arg choice='plain'><option>-S</option></arg>
+  </group>
+  <arg choice='req'><replaceable>path</replaceable></arg>
+</cmdsynopsis>
+
+</refsection>
+
+
+<refsection><title>Description</title>
+
+<para>This operation makes <replaceable>path</replaceable> the current
+profile for the user.  That is, the symlink
+<filename>~/.nix-profile</filename> is made to point to
+<replaceable>path</replaceable>.</para>
+
+</refsection>
+
+<refsection><title>Examples</title>
+
+<screen>
+$ nix-env -S ~/my-profile</screen>
+
+</refsection>
+
+</refsection>
+
+
+
+<!--######################################################################-->
+
+<refsection><title>Operation <option>--list-generations</option></title>
+
+<refsection><title>Synopsis</title>
+
+<cmdsynopsis>
+  <command>nix-env</command>
+  <arg choice='plain'><option>--list-generations</option></arg>
+</cmdsynopsis>
+
+</refsection>
+
+
+<refsection><title>Description</title>
+
+<para>This operation print a list of all the currently existing
+generations for the active profile.  These may be switched to using
+the <option>--switch-generation</option> operation.  It also prints
+the creation date of the generation, and indicates the current
+generation.</para>
+
+</refsection>
+
+
+<refsection><title>Examples</title>
+
+<screen>
+$ nix-env --list-generations
+  95   2004-02-06 11:48:24
+  96   2004-02-06 11:49:01
+  97   2004-02-06 16:22:45
+  98   2004-02-06 16:24:33   (current)</screen>
+
+</refsection>
+
+</refsection>
+
+
+
+<!--######################################################################-->
+
+<refsection><title>Operation <option>--delete-generations</option></title>
+
+<refsection><title>Synopsis</title>
+
+<cmdsynopsis>
+  <command>nix-env</command>
+  <arg choice='plain'><option>--delete-generations</option></arg>
+  <arg choice='plain' rep='repeat'><replaceable>generations</replaceable></arg>
+</cmdsynopsis>
+
+</refsection>
+
+
+<refsection><title>Description</title>
+
+<para>This operation deletes the specified generations of the current
+profile.  The generations can be a list of generation numbers, the
+special value <literal>old</literal> to delete all non-current
+generations, or a value such as <literal>30d</literal> to delete all
+generations older than the specified number of days (except for the
+generation that was active at that point in time).
+Periodically deleting old generations is important to make garbage
+collection effective.</para>
+
+</refsection>
+
+<refsection><title>Examples</title>
+
+<screen>
+$ nix-env --delete-generations 3 4 8
+
+$ nix-env --delete-generations 30d
+
+$ nix-env -p other_profile --delete-generations old</screen>
+
+</refsection>
+
+</refsection>
+
+
+
+<!--######################################################################-->
+
+<refsection><title>Operation <option>--switch-generation</option></title>
+
+<refsection><title>Synopsis</title>
+
+<cmdsynopsis>
+  <command>nix-env</command>
+  <group choice='req'>
+    <arg choice='plain'><option>--switch-generation</option></arg>
+    <arg choice='plain'><option>-G</option></arg>
+  </group>
+  <arg choice='req'><replaceable>generation</replaceable></arg>
+</cmdsynopsis>
+
+</refsection>
+
+
+<refsection><title>Description</title>
+
+<para>This operation makes generation number
+<replaceable>generation</replaceable> the current generation of the
+active profile.  That is, if the
+<filename><replaceable>profile</replaceable></filename> is the path to
+the active profile, then the symlink
+<filename><replaceable>profile</replaceable></filename> is made to
+point to
+<filename><replaceable>profile</replaceable>-<replaceable>generation</replaceable>-link</filename>,
+which is in turn a symlink to the actual user environment in the Nix
+store.</para>
+
+<para>Switching will fail if the specified generation does not exist.</para>
+
+</refsection>
+
+
+<refsection><title>Examples</title>
+
+<screen>
+$ nix-env -G 42
+switching from generation 50 to 42</screen>
+
+</refsection>
+
+</refsection>
+
+
+
+<!--######################################################################-->
+
+<refsection><title>Operation <option>--rollback</option></title>
+
+<refsection><title>Synopsis</title>
+
+<cmdsynopsis>
+  <command>nix-env</command>
+  <arg choice='plain'><option>--rollback</option></arg>
+</cmdsynopsis>
+
+</refsection>
+
+<refsection><title>Description</title>
+
+<para>This operation switches to the “previous” generation of the
+active profile, that is, the highest numbered generation lower than
+the current generation, if it exists.  It is just a convenience
+wrapper around <option>--list-generations</option> and
+<option>--switch-generation</option>.</para>
+
+</refsection>
+
+
+<refsection><title>Examples</title>
+
+<screen>
+$ nix-env --rollback
+switching from generation 92 to 91
+
+$ nix-env --rollback
+error: no generation older than the current (91) exists</screen>
+
+</refsection>
+
+</refsection>
+
+
+<refsection condition="manpage"><title>Environment variables</title>
+
+<variablelist>
+  
+  <varlistentry><term><envar>NIX_PROFILE</envar></term>
+
+    <listitem><para>Location of the Nix profile.  Defaults to the
+    target of the symlink <filename>~/.nix-profile</filename>, if it
+    exists, or <filename>/nix/var/nix/profiles/default</filename>
+    otherwise.</para></listitem>
+
+  </varlistentry>
+
+  <xi:include href="env-common.xml#xmlns(db=http://docbook.org/ns/docbook)xpointer(//db:variablelist[@xml:id='env-common']/*)" />
+</variablelist>
+
+</refsection>
+  
+
+</refentry>