nix/doc/manual/nix-env.xml

<refentry xmlns="http://docbook.org/ns/docbook"
          xmlns:xlink="http://www.w3.org/1999/xlink">
  
<refnamediv>
  <refname>nix-env</refname>
  <refpurpose>manipulate or query Nix user environments</refpurpose>
</refnamediv>

<refsynopsisdiv>
  <cmdsynopsis>
    <command>nix-env</command>
    <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="opt-common-syn.xml#xpointer(/nop/*)" />
    <arg><option>--arg</option> <replaceable>name</replaceable> <replaceable>value</replaceable></arg>
    <arg>
      <group choice='req'>
        <arg choice='plain'><option>--attr</option></arg>
        <arg choice='plain'><option>-A</option></arg>
      </group>
      <replaceable>attrPath</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><option>--from-expression</option></arg>
    <arg><option>-E</option></arg>
    <arg><option>--from-profile</option> <replaceable>path</replaceable></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 components
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.  See also <xref
linkend="sec-common-options" />.</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 sequence of
    user environments called <emphasis>generations</emphasis>, one of
    which is the <emphasis>current generation</emphasis>.  The default
    profile is the target of the symbolic link
    <filename>~/.nix-profile</filename> (see below).</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> 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></listitem>
    
  </varlistentry>
      
  <varlistentry><term><option>--system-filter</option> <replaceable>system</replaceable></term>
  
    <listitem><para>By default, operations such as <option>--query
    --available</option> only include derivations matching the current
    platform.  This option allows you to use derivations for the
    specified platform <replaceable>system</replaceable>.  The special
    value <literal>*</literal> causes derivations for any platform to
    be included.</para></listitem>
    
  </varlistentry>

</variablelist>

</refsection>

  

<!--######################################################################-->

<refsection><title>Files</title>

<variablelist>

  <varlistentry><term><filename>~/.nix-defexpr</filename></term>
  
    <listitem><para>The default Nix expression used by the
    <option>--install</option>, <option>--upgrade</option>, and
    <option>--query --available</option> operations to obtain
    derivations.  It is generally a symbolic link to some other
    location set using the <option>--import</option> operation.  The
    <option>--file</option> option may be used to override this
    default.</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>
  <group choice='opt'>
    <arg choice='plain'><option>--preserve-installed</option></arg>
    <arg choice='plain'><option>-P</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></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 a 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>--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>

</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 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
attribute set returned by calling the function defined in
<filename>./foo.nix</filename>.</para>

</refsection>
    
</refsection>


  
<!--######################################################################-->

<refsection><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>
  <group choice='opt'>
    <arg choice='plain'><option>--lt</option></arg>
    <arg choice='plain'><option>--leq</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>--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>

</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><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><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>
  <group choice='req'>
    <arg choice='plain'><option>--status</option></arg>
    <arg choice='plain'><option>-s</option></arg>
    <arg choice='plain'><option>--no-name</option></arg>
    <arg choice='plain'><option>--compare-versions</option></arg>
    <arg choice='plain'><option>-c</option></arg>
    <arg choice='plain'><option>--system</option></arg>
    <arg choice='plain'><option>--drv-path</option></arg>
    <arg choice='plain'><option>--out-path</option></arg>
  </group>
</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>).</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>--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>--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
    components 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 component is available
        or installed.</para></listitem>

      </varlistentry>
      
      <varlistentry><term><literal>=</literal> <replaceable>version</replaceable></term>

        <listitem><para>At most the same version of the component is
        available or installed.</para></listitem>

      </varlistentry>
      
      <varlistentry><term><literal>></literal> <replaceable>version</replaceable></term>

        <listitem><para>Only older versions of the component are
        available or installed.</para></listitem>

      </varlistentry>
      
      <varlistentry><term><literal>- ?</literal></term>

        <listitem><para>No version of the component 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>

</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>
...

$ nix-env -f ./foo.nix -qa <lineannotation>(show available derivations in the Nix expression <!-- !!! <filename>-->foo.nix<!-- </filename> -->)</lineannotation>
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>
</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, or the
special value <literal>old</literal> to delete all non-current
generations.  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 -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 --rolback
error: no generation older than the current (91) exists</screen>

</refsection>
    
</refsection>


  
<!--######################################################################-->

<refsection><title>Operation <option>--import</option></title>

<refsection><title>Synopsis</title>

<cmdsynopsis>
  <command>nix-env</command>
  <group choice='req'>
    <arg choice='plain'><option>--import</option></arg>
    <arg choice='plain'><option>-I</option></arg>
  </group>
  <arg choice='req'><replaceable>path</replaceable></arg>
</cmdsynopsis>

</refsection>


<refsection><title>Description</title>
            
<para>This operation makes <replaceable>path</replaceable> the default
active Nix expression for the user.  That is, the symlink
<filename>~/.nix-userenv</filename> is made to point to
<replaceable>path</replaceable>.</para>

</refsection>


<refsection><title>Examples</title>

<screen>
$ nix-env -I ~/nixpkgs-0.5/</screen>

</refsection>
    
</refsection>


  
</refentry>