tvl-depot/doc/manual/nix-store.xml

<refentry xmlns="http://docbook.org/ns/docbook"
          xmlns:xlink="http://www.w3.org/1999/xlink"
          xmlns:xi="http://www.w3.org/2001/XInclude"
          xml:id="sec-nix-store">

<refmeta>
  <refentrytitle>nix-store</refentrytitle>
  <manvolnum>1</manvolnum>
  <refmiscinfo class="source">Nix</refmiscinfo>
  <refmiscinfo class="version"><xi:include href="version.txt" parse="text"/></refmiscinfo>
</refmeta>

<refnamediv>
  <refname>nix-store</refname>
  <refpurpose>manipulate or query the Nix store</refpurpose>
</refnamediv>

<refsynopsisdiv>
  <cmdsynopsis>
    <command>nix-store</command>
    <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="opt-common-syn.xml#xmlns(db=http://docbook.org/ns/docbook)xpointer(/db:nop/*)" />
    <arg><option>--add-root</option> <replaceable>path</replaceable></arg>
    <arg><option>--indirect</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-store</command> performs primitive
operations on the Nix store.  You generally do not need to run this
command manually.</para>

<para><command>nix-store</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" /> for a list of common options.</para>

<variablelist>

  <varlistentry xml:id="opt-add-root"><term><option>--add-root</option> <replaceable>path</replaceable></term>

    <listitem><para>Causes the result of a realisation
    (<option>--realise</option> and <option>--force-realise</option>)
    to be registered as a root of the garbage collector (see <xref
    linkend="ssec-gc-roots" />).  The root is stored in
    <replaceable>path</replaceable>, which must be inside a directory
    that is scanned for roots by the garbage collector (i.e.,
    typically in a subdirectory of
    <filename>/nix/var/nix/gcroots/</filename>)
    <emphasis>unless</emphasis> the <option>--indirect</option> flag
    is used.</para>

    <para>If there are multiple results, then multiple symlinks will
    be created by sequentially numbering symlinks beyond the first one
    (e.g., <filename>foo</filename>, <filename>foo-2</filename>,
    <filename>foo-3</filename>, and so on).</para></listitem>

  </varlistentry>

  <varlistentry><term><option>--indirect</option></term>

    <listitem>

    <para>In conjunction with <option>--add-root</option>, this option
    allows roots to be stored <emphasis>outside</emphasis> of the GC
    roots directory.  This is useful for commands such as
    <command>nix-build</command> that place a symlink to the build
    result in the current directory; such a build result should not be
    garbage-collected unless the symlink is removed.</para>

    <para>The <option>--indirect</option> flag causes a uniquely named
    symlink to <replaceable>path</replaceable> to be stored in
    <filename>/nix/var/nix/gcroots/auto/</filename>.  For instance,

    <screen>
$ nix-store --add-root /home/eelco/bla/result --indirect -r <replaceable>...</replaceable>

$ ls -l /nix/var/nix/gcroots/auto
lrwxrwxrwx    1 ... 2005-03-13 21:10 dn54lcypm8f8... -> /home/eelco/bla/result

$ ls -l /home/eelco/bla/result
lrwxrwxrwx    1 ... 2005-03-13 21:10 /home/eelco/bla/result -> /nix/store/1r11343n6qd4...-f-spot-0.0.10</screen>

    Thus, when <filename>/home/eelco/bla/result</filename> is removed,
    the GC root in the <filename>auto</filename> directory becomes a
    dangling symlink and will be ignored by the collector.</para>

    <warning><para>Note that it is not possible to move or rename
    indirect GC roots, since the symlink in the
    <filename>auto</filename> directory will still point to the old
    location.</para></warning>

    </listitem>

  </varlistentry>
    
</variablelist>

</refsection>

  

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

<refsection xml:id='rsec-nix-store-realise'><title>Operation <option>--realise</option></title>

<refsection><title>Synopsis</title>

<cmdsynopsis>
  <command>nix-store</command>
  <group choice='req'>
    <arg choice='plain'><option>--realise</option></arg>
    <arg choice='plain'><option>-r</option></arg>
  </group>
  <arg choice='plain' rep='repeat'><replaceable>paths</replaceable></arg>
  <arg><option>--dry-run</option></arg>
</cmdsynopsis>

</refsection>

<refsection><title>Description</title>
            
<para>The operation <option>--realise</option> essentially “builds”
the specified store paths.  Realisation is a somewhat overloaded term:

<itemizedlist>

  <listitem><para>If the store path is a
  <emphasis>derivation</emphasis>, realisation ensures that the output
  paths of the derivation are <link
  linkend="gloss-validity">valid</link> (i.e., the output path and its
  closure exist in the file system).  This can be done in several
  ways.  First, it is possible that the outputs are already valid, in
  which case we are done immediately.  Otherwise, there may be <link
  linkend="gloss-substitute">substitutes</link> that produce the
  outputs (e.g., by downloading them).  Finally, the outputs can be
  produced by performing the build action described by the
  derivation.</para></listitem>

  <listitem><para>If the store path is not a derivation, realisation
  ensures that the specified path is valid (i.e., it and its closure
  exist in the file system).  If the path is already valid, we are
  done immediately.  Otherwise, the path and any missing paths in its
  closure may be produced through substitutes.  If there are no
  (succesful) subsitutes, realisation fails.</para></listitem>

</itemizedlist>

</para>

<para>The output path of each derivation is printed on standard
output.  (For non-derivations argument, the argument itself is
printed.)</para>

<para>If the <option>--dry-run</option> option is used, then
<command>nix-store</command> will print on standard error a
description of what packages would be built or downloaded, and then
quit.</para>

</refsection>
            

<refsection><title>Examples</title>

<para>This operation is typically used to build store derivations
produced by <link
linkend="sec-nix-instantiate"><command>nix-instantiate</command></link>:
    
<screen>
$ nix-store -r $(nix-instantiate ./test.nix)
/nix/store/31axcgrlbfsxzmfff1gyj1bf62hvkby2-aterm-2.3.1</screen>

This is essentially what <link
linkend="sec-nix-build"><command>nix-build</command></link> does.</para>

</refsection>


</refsection>

  

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

<refsection xml:id='rsec-nix-store-gc'><title>Operation <option>--gc</option></title>

<refsection><title>Synopsis</title>

<cmdsynopsis>
  <command>nix-store</command>
  <arg choice='plain'><option>--gc</option></arg>
  <group>
    <arg choice='plain'><option>--print-roots</option></arg>
    <arg choice='plain'><option>--print-live</option></arg>
    <arg choice='plain'><option>--print-dead</option></arg>
    <arg choice='plain'><option>--delete</option></arg>
  </group>
  <arg><option>--max-freed</option> <replaceable>bytes</replaceable></arg>
  <arg><option>--max-links</option> <replaceable>nrlinks</replaceable></arg>
  <arg><option>--max-atime</option> <replaceable>atime</replaceable></arg>
  <arg><option>--use-atime</option></arg>
</cmdsynopsis>

</refsection>

<refsection><title>Description</title>
            
<para>Without additional flags, the operation <option>--gc</option>
performs a garbage collection on the Nix store.  That is, all paths in
the Nix store not reachable via file system references from a set of
“roots”, are deleted.</para>

<para>The following suboperations may be specified:</para>

<variablelist>

  <varlistentry><term><option>--print-roots</option></term>
  
    <listitem><para>This operation prints on standard output the set
    of roots used by the garbage collector.  What constitutes a root
    is described in <xref linkend="ssec-gc-roots"
    />.</para></listitem>
    
  </varlistentry>

  <varlistentry><term><option>--print-live</option></term>
  
    <listitem><para>This operation prints on standard output the set
    of “live” store paths, which are all the store paths reachable
    from the roots.  Live paths should never be deleted, since that
    would break consistency — it would become possible that
    applications are installed that reference things that are no
    longer present in the store.</para></listitem>
    
  </varlistentry>

  <varlistentry><term><option>--print-dead</option></term>
  
    <listitem><para>This operation prints out on standard output the
    set of “dead” store paths, which is just the opposite of the set
    of live paths: any path in the store that is not live (with
    respect to the roots) is dead.</para></listitem>
    
  </varlistentry>

  <varlistentry><term><option>--delete</option></term>
  
    <listitem><para>This operation performs an actual garbage
    collection.  All dead paths are removed from the
    store.  This is the default.</para></listitem>
    
  </varlistentry>

</variablelist>

<para>By default, all unreachable paths are deleted.  The following
options control what gets deleted and in what order:

<variablelist>

  <varlistentry><term><option>--max-freed</option> <replaceable>bytes</replaceable></term>
  
    <listitem><para>Keep deleting paths until at least
    <replaceable>bytes</replaceable> bytes have been
    deleted, then stop.</para></listitem>
    
  </varlistentry>

  <varlistentry><term><option>--max-links</option> <replaceable>nrlinks</replaceable></term>
  
    <listitem><para>Keep deleting paths until the hard link count on
    <filename>/nix/store</filename> is less than
    <replaceable>nrlinks</replaceable>, then stop.  This is useful for
    very large Nix stores on filesystems with a 32000 subdirectories
    limit (like <literal>ext3</literal>).</para></listitem>
    
  </varlistentry>

  <varlistentry><term><option>--max-atime</option> <replaceable>atime</replaceable></term>
  
    <listitem><para>Only delete a store path if its last-accessed time
    is less than <replaceable>atime</replaceable>.  This allows you to
    garbage-collect only packages that haven’t been used recently.
    The time is expressed as the number of seconds in the Unix epoch,
    i.e., since 1970-01-01 00:00:00 UTC.  An easy way to convert to
    this format is <literal>date +%s -d "<replaceable>date
    specification</replaceable>"</literal>.</para>

    <para>For directories, the last-accessed time is the highest
    last-accessed time of any regular file in the directory (or in any
    of its subdirectories).  That is, the <literal>atime</literal>
    field maintained by the filesystem is ignored for directories.
    This is because operations such as rebuilding the
    <command>locate</command> database tend to update the
    <literal>atime</literal> values of all directories, so they’re not
    a useful indicator of whether a package was recently used.</para>

    <para>Note that <command>nix-store --optimise</command> reads all
    regular files in the Nix store, and so causes all last-accessed
    times to be set to the present time.  This makes
    <option>--max-atime</option> ineffective (for a while at
    least).</para></listitem>
    
  </varlistentry>

  <varlistentry><term><option>--use-atime</option></term>

    <listitem><para>Delete store paths in order of ascending
    last-accessed time.  This is useful in conjunction with the other
    options to delete only the least recently used
    packages.</para></listitem>

  </varlistentry>
  
</variablelist>

</para>

<para>The behaviour of the collector is also influenced by the <link
linkend="conf-gc-keep-outputs"><literal>gc-keep-outputs</literal></link>
and <link
linkend="conf-gc-keep-derivations"><literal>gc-keep-derivations</literal></link>
variables in the Nix configuration file.</para>

<para>With <option>--delete</option>, the collector prints the total
number of freed bytes when it finishes (or when it is interrupted).
With <option>--print-dead</option>, it prints the number of bytes that
would be freed.</para>

</refsection>


<refsection><title>Examples</title>

<para>To delete all unreachable paths, just do:
    
<screen>
$ nix-store --gc
deleting `/nix/store/kq82idx6g0nyzsp2s14gfsc38npai7lf-cairo-1.0.4.tar.gz.drv'
<replaceable>...</replaceable>
8825586 bytes freed (8.42 MiB)</screen>

</para>

<para>To delete unreachable paths not accessed in the last two months:

<screen>
$ nix-store --gc -v --max-atime $(date +%s -d "2 months ago")</screen>

</para>

<para>To delete at least 100 MiBs of unreachable paths:

<screen>
$ nix-store --gc --max-freed $((100 * 1024 * 1024))</screen>

</para>

</refsection>


</refsection>


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

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

<refsection><title>Synopsis</title>

<cmdsynopsis>
  <command>nix-store</command>
  <arg choice='plain'><option>--gc</option></arg>
  <arg><option>--ignore-liveness</option></arg>
  <arg choice='plain' rep='repeat'><replaceable>paths</replaceable></arg>
</cmdsynopsis>

</refsection>

<refsection><title>Description</title>

<para>The operation <option>--delete</option> deletes the store paths
<replaceable>paths</replaceable> from the Nix store, but only if it is
safe to do so; that is, when the path is not reachable from a root of
the garbage collector.  This means that you can only delete paths that
would also be deleted by <literal>nix-store --gc</literal>.  Thus,
<literal>--delete</literal> is a more targeted version of
<literal>--gc</literal>.</para>

<para>With the option <option>--ignore-liveness</option>, reachability
from the roots is ignored.  However, the path still won’t be deleted
if there are other paths in the store that refer to it (i.e., depend
on it).</para>

</refsection>

<refsection><title>Example</title>

<screen>
$ nix-store --delete /nix/store/zq0h41l75vlb4z45kzgjjmsjxvcv1qk7-mesa-6.4
0 bytes freed (0.00 MiB)
error: cannot delete path `/nix/store/zq0h41l75vlb4z45kzgjjmsjxvcv1qk7-mesa-6.4' since it is still alive</screen>

</refsection>

</refsection>



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

<refsection xml:id='refsec-nix-store-query'><title>Operation <option>--query</option></title>

<refsection><title>Synopsis</title>

<cmdsynopsis>
  <command>nix-store</command>
  <group choice='req'>
    <arg choice='plain'><option>--query</option></arg>
    <arg choice='plain'><option>-q</option></arg>
  </group>
  <group choice='req'>
    <arg choice='plain'><option>--outputs</option></arg>
    <arg choice='plain'><option>--requisites</option></arg>
    <arg choice='plain'><option>-R</option></arg>
    <arg choice='plain'><option>--references</option></arg>
    <arg choice='plain'><option>--referrers</option></arg>
    <arg choice='plain'><option>--referrers-closure</option></arg>
    <arg choice='plain'><option>--deriver</option></arg>
    <arg choice='plain'><option>--deriver</option></arg>
    <arg choice='plain'><option>--graph</option></arg>
    <arg choice='plain'><option>--tree</option></arg>
    <arg choice='plain'><option>--binding</option> <replaceable>name</replaceable></arg>
    <arg choice='plain'><option>--hash</option></arg>
  </group>
  <arg><option>--use-output</option></arg>
  <arg><option>-u</option></arg>
  <arg><option>--force-realise</option></arg>
  <arg><option>-f</option></arg>
  <arg choice='plain' rep='repeat'><replaceable>paths</replaceable></arg>
</cmdsynopsis>

</refsection>


<refsection><title>Description</title>
            
<para>The operation <option>--query</option> displays various bits of
information about the store paths .  The queries are described below.  At
most one query can be specified.  The default query is
<option>--outputs</option>.</para>

<para>The paths <replaceable>paths</replaceable> may also be symlinks
from outside of the Nix store, to the Nix store.  In that case, the
query is applied to the target of the symlink.</para>


</refsection>


<refsection><title>Common query options</title>

<variablelist>

  <varlistentry><term><option>--use-output</option></term>
    <term><option>-u</option></term>
  
    <listitem><para>For each argument to the query that is a store
    derivation, apply the query to the output path of the derivation
    instead.</para></listitem>
    
  </varlistentry>

  <varlistentry><term><option>--force-realise</option></term>
    <term><option>-f</option></term>
  
    <listitem><para>Realise each argument to the query first (see
    <link linkend="rsec-nix-store-realise"><command>nix-store
    --realise</command></link>).</para></listitem>

  </varlistentry>

</variablelist>
        
</refsection>
    

<refsection xml:id='nixref-queries'><title>Queries</title>
            
<variablelist>

  <varlistentry><term><option>--outputs</option></term>

    <listitem><para>Prints out the <link
    linkend="gloss-output-path">output paths</link> of the store
    derivations <replaceable>paths</replaceable>.  These are the paths
    that will be produced when the derivation is
    built.</para></listitem>
    
  </varlistentry>

  <varlistentry><term><option>--requisites</option></term>
    <term><option>-R</option></term>

    <listitem><para>Prints out the <link
    linkend="gloss-closure">closure</link> of the store path
    <replaceable>paths</replaceable>.</para>

    <para>This query has one option:</para>

    <variablelist>

      <varlistentry><term><option>--include-outputs</option></term>
      
        <listitem><para>Also include the output path of store
        derivations, and their closures.</para></listitem>
        
      </varlistentry>

    </variablelist>

    <para>This query can be used to implement various kinds of
    deployment.  A <emphasis>source deployment</emphasis> is obtained
    by distributing the closure of a store derivation.  A
    <emphasis>binary deployment</emphasis> is obtained by distributing
    the closure of an output path.  A <emphasis>cache
    deployment</emphasis> (combined source/binary deployment,
    including binaries of build-time-only dependencies) is obtained by
    distributing the closure of a store derivation and specifying the
    option <option>--include-outputs</option>.</para>
    
    </listitem>

  </varlistentry>

  <varlistentry><term><option>--references</option></term>
  
    <listitem><para>Prints the set of <link
    linkend="gloss-reference">references</link> of the store paths
    <replaceable>paths</replaceable>, that is, their immediate
    dependencies.  (For <emphasis>all</emphasis> dependencies, use
    <option>--requisites</option>.)</para></listitem>

  </varlistentry>
  
  <varlistentry><term><option>--referrers</option></term>
  
    <listitem><para>Prints the set of <emphasis>referrers</emphasis> of
    the store paths <replaceable>paths</replaceable>, that is, the
    store paths currently existing in the Nix store that refer to one
    of <replaceable>paths</replaceable>.  Note that contrary to the
    references, the set of referrers is not constant; it can change as
    store paths are added or removed.</para></listitem>

  </varlistentry>
  
  <varlistentry><term><option>--referrers-closure</option></term>
  
    <listitem><para>Prints the closure of the set of store paths
    <replaceable>paths</replaceable> under the referrers relation; that
    is, all store paths that directly or indirectly refer to one of
    <replaceable>paths</replaceable>.  These are all the path currently
    in the Nix store that are dependent on
    <replaceable>paths</replaceable>.</para></listitem>

  </varlistentry>

  <varlistentry><term><option>--deriver</option></term>
  
    <listitem><para>Prints the <link
    linkend="gloss-deriver">deriver</link> of the store paths
    <replaceable>paths</replaceable>.  If the path has no deriver
    (e.g., if it is a source file), or if the deriver is not known
    (e.g., in the case of a binary-only deployment), the string
    <literal>unknown-deriver</literal> is printed.</para></listitem>

  </varlistentry>

  <varlistentry><term><option>--graph</option></term>
  
    <listitem><para>Prints the references graph of the store paths
    <replaceable>paths</replaceable> in the format of the
    <command>dot</command> tool of AT&amp;T's <link
    xlink:href="http://www.graphviz.org/">Graphviz package</link>.
    This can be used to visualise dependency graphs.  To obtain a
    build-time dependency graph, apply this to a store derivation.  To
    obtain a runtime dependency graph, apply it to an output
    path.</para></listitem>

  </varlistentry>

  <varlistentry><term><option>--tree</option></term>
  
    <listitem><para>Prints the references graph of the store paths
    <replaceable>paths</replaceable> as a nested ASCII tree.
    References are ordered by descending closure size; this tends to
    flatten the tree, making it more readable.  The query only
    recurses into a store path when it is first encountered; this
    prevents a blowup of the tree representation of the
    graph.</para></listitem>

  </varlistentry>

  <varlistentry><term><option>--binding</option> <replaceable>name</replaceable></term>
  
    <listitem><para>Prints the value of the attribute
    <replaceable>name</replaceable> (i.e., environment variable) of
    the store derivations <replaceable>paths</replaceable>.  It is an
    error for a derivation to not have the specified
    attribute.</para></listitem>

  </varlistentry>

  <varlistentry><term><option>--hash</option></term>
  
    <listitem><para>Prints the SHA-256 hash of the contents of the
    store path <replaceable>paths</replaceable>.  Since the hash is
    stored in the Nix database, this is a fast
    operation.</para></listitem>

  </varlistentry>

</variablelist>

</refsection>


<refsection><title>Examples</title>

<para>Print the closure (runtime dependencies) of the
<command>svn</command> program in the current user environment:
    
<screen>
$ nix-store -qR $(which svn)
/nix/store/5mbglq5ldqld8sj57273aljwkfvj22mc-subversion-1.1.4
/nix/store/9lz9yc6zgmc0vlqmn2ipcpkjlmbi51vv-glibc-2.3.4
<replaceable>...</replaceable></screen>

</para>

<para>Print the build-time dependencies of <command>svn</command>:

<screen>
$ nix-store -qR $(nix-store -qd $(which svn))
/nix/store/02iizgn86m42q905rddvg4ja975bk2i4-grep-2.5.1.tar.bz2.drv
/nix/store/07a2bzxmzwz5hp58nf03pahrv2ygwgs3-gcc-wrapper.sh
/nix/store/0ma7c9wsbaxahwwl04gbw3fcd806ski4-glibc-2.3.4.drv
<replaceable>... lots of other paths ...</replaceable></screen>

The difference with the previous example is that we ask the closure of
the derivation (<option>-qd</option>), not the closure of the output
path that contains <command>svn</command>.</para>

<para>Show the build-time dependencies as a tree:

<screen>
$ nix-store -q --tree $(nix-store -qd $(which svn))
/nix/store/7i5082kfb6yjbqdbiwdhhza0am2xvh6c-subversion-1.1.4.drv
+---/nix/store/d8afh10z72n8l1cr5w42366abiblgn54-builder.sh
+---/nix/store/fmzxmpjx2lh849ph0l36snfj9zdibw67-bash-3.0.drv
|   +---/nix/store/570hmhmx3v57605cqg9yfvvyh0nnb8k8-bash
|   +---/nix/store/p3srsbd8dx44v2pg6nbnszab5mcwx03v-builder.sh
<replaceable>...</replaceable></screen>

</para>

<para>Show all paths that depend on the same OpenSSL library as
<command>svn</command>:

<screen>
$ nix-store -q --referrers $(nix-store -q --binding openssl $(nix-store -qd $(which svn)))
/nix/store/23ny9l9wixx21632y2wi4p585qhva1q8-sylpheed-1.0.0
/nix/store/5mbglq5ldqld8sj57273aljwkfvj22mc-subversion-1.1.4
/nix/store/dpmvp969yhdqs7lm2r1a3gng7pyq6vy4-subversion-1.1.3
/nix/store/l51240xqsgg8a7yrbqdx1rfzyv6l26fx-lynx-2.8.5</screen>

</para>

<para>Show all paths that directly or indirectly depend on the Glibc
(C library) used by <command>svn</command>:

<screen>
$ nix-store -q --referrers-closure $(ldd $(which svn) | grep /libc.so | awk '{print $3}')
/nix/store/034a6h4vpz9kds5r6kzb9lhh81mscw43-libgnomeprintui-2.8.2
/nix/store/15l3yi0d45prm7a82pcrknxdh6nzmxza-gawk-3.1.4
<replaceable>...</replaceable></screen>

Note that <command>ldd</command> is a command that prints out the
dynamic libraries used by an ELF executable.</para>

<para>Make a picture of the runtime dependency graph of the current
user environment:

<screen>
$ nix-store -q --graph ~/.nix-profile | dot -Tps > graph.ps
$ gv graph.ps</screen>

</para>

</refsection>


</refsection>

  

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

<!--
<refsection xml:id="rsec-nix-store-reg-val"><title>Operation <option>-XXX-register-validity</option></title>

<refsection><title>Synopsis</title>

<cmdsynopsis>
  <command>nix-store</command>
  <arg choice='plain'><option>-XXX-register-validity</option></arg>
</cmdsynopsis>
</refsection>

<refsection><title>Description</title>
            
<para>TODO</para>

</refsection>
            
</refsection>
-->



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

<refsection xml:id='refsec-nix-store-verify'><title>Operation <option>--verify</option></title>

<refsection>
  <title>Synopsis</title>
  <cmdsynopsis>
    <command>nix-store</command>
    <arg choice='plain'><option>--verify</option></arg>
    <arg><option>--check-contents</option></arg>
  </cmdsynopsis>
</refsection>

<refsection><title>Description</title>
            
<para>The operation <option>--verify</option> verifies the internal
consistency of the Nix database, and the consistency between the Nix
database and the Nix store.  Any inconsistencies encountered are
automatically repaired.  Inconsistencies are generally the result of
the Nix store or database being modified by non-Nix tools, or of bugs
in Nix itself.</para>

<para>There is one option:

<variablelist>

  <varlistentry><term><option>--check-contents</option></term>
  
    <listitem><para>Checks that the contents of every valid store path
    has not been altered by computing a SHA-256 hash of the contents
    and comparing it with the hash stored in the Nix database at build
    time.  Paths that have been modified are printed out.  For large
    stores, <option>--check-contents</option> is obviously quite
    slow.</para></listitem>
    
  </varlistentry>
  
</variablelist>

</para>

</refsection>
            

</refsection>


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

<refsection xml:id='refsec-nix-store-dump'><title>Operation <option>--dump</option></title>

<refsection>
  <title>Synopsis</title>
  <cmdsynopsis>
    <command>nix-store</command>
    <arg choice='plain'><option>--dump</option></arg>
    <arg choice='plain'><replaceable>path</replaceable></arg>
  </cmdsynopsis>
</refsection>

<refsection><title>Description</title>
            
<para>The operation <option>--dump</option> produces a NAR (Nix
ARchive) file containing the contents of the file system tree rooted
at <replaceable>path</replaceable>.  The archive is written to
standard output.</para>

<para>A NAR archive is like a TAR or Zip archive, but it contains only
the information that Nix considers important.  For instance,
timestamps are elided because all files in the Nix store have their
timestamp set to 0 anyway.  Likewise, all permissions are left out
except for the execute bit, because all files in the Nix store have
644 or 755 permission.</para>

<para>Also, a NAR archive is <emphasis>canonical</emphasis>, meaning
that “equal” paths always produce the same NAR archive.  For instance,
directory entries are always sorted so that the actual on-disk order
doesn’t influence the result.  This means that the cryptographic hash
of a NAR dump of a path is usable as a fingerprint of the contents of
the path.  Indeed, the hashes of store paths stored in Nix’s database
(see <link linkend="refsec-nix-store-query"><literal>nix-store -q
--hash</literal></link>) are SHA-256 hashes of the NAR dump of each
store path.</para>

<para>NAR archives support filenames of unlimited length and 64-bit
file sizes.  They can contain regular files, directories, and symbolic
links, but not other types of files (such as device nodes).</para>

<para>A Nix archive can be unpacked using <literal>nix-store
--restore</literal>.</para>

</refsection>
            

</refsection>


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

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

<refsection>
  <title>Synopsis</title>
  <cmdsynopsis>
    <command>nix-store</command>
    <arg choice='plain'><option>--restore</option></arg>
    <arg choice='plain'><replaceable>path</replaceable></arg>
  </cmdsynopsis>
</refsection>

<refsection><title>Description</title>
            
<para>The operation <option>--restore</option> unpacks a NAR archive
to <replaceable>path</replaceable>, which must not already exist.  The
archive is read from standard input.</para>

</refsection>
            

</refsection>


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

<refsection xml:id='refsec-nix-store-export'><title>Operation <option>--export</option></title>

<refsection>
  <title>Synopsis</title>
  <cmdsynopsis>
    <command>nix-store</command>
    <arg choice='plain'><option>--export</option></arg>
    <arg choice='plain' rep='repeat'><replaceable>paths</replaceable></arg>
  </cmdsynopsis>
</refsection>

<refsection><title>Description</title>
            
<para>The operation <option>--export</option> writes a serialisation
of the specified store paths to standard output in a format that can
be imported into another Nix store with <command
linkend="refsec-nix-store-import">nix-store --import</command>.  This
is like <command linkend="refsec-nix-store-dump">nix-store
--dump</command>, except that the NAR archive produced by that command
doesn’t contain the necessary meta-information to allow it to be
imported into another Nix store (namely, the set of references of the
path).</para>

<para>This command does not produce a <emphasis>closure</emphasis> of
the specified paths, so if a store path references other store paths
that are missing in the target Nix store, the import will fail.  To
copy a whole closure, do something like

<screen>
$ nix-store --export $(nix-store -qR <replaceable>paths</replaceable>) > out</screen>

</para>

<para>For an example of how <option>--export</option> and
<option>--import</option> can be used, see the source of the <command
linkend="sec-nix-copy-closure">nix-copy-closure</command>
command.</para>

</refsection>
            

</refsection>


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

<refsection xml:id='refsec-nix-store-import'><title>Operation <option>--import</option></title>

<refsection>
  <title>Synopsis</title>
  <cmdsynopsis>
    <command>nix-store</command>
    <arg choice='plain'><option>--import</option></arg>
  </cmdsynopsis>
</refsection>

<refsection><title>Description</title>
            
<para>The operation <option>--export</option> reads a serialisation of
a set of store paths produced by <command
linkend="refsec-nix-store-export">nix-store --import</command> from
standard input and adds those store paths to the Nix store.  Paths
that already exist in the Nix store are ignored.  If a path refers to
another path that doesn’t exist in the Nix store, the import
fails.</para>

</refsection>
            

</refsection>


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

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

<refsection>
  <title>Synopsis</title>
  <cmdsynopsis>
    <command>nix-store</command>
    <arg choice='plain'><option>--optimise</option></arg>
  </cmdsynopsis>
</refsection>

<refsection><title>Description</title>
            
<para>The operation <option>--optimise</option> reduces Nix store disk
space usage by finding identical files in the store and hard-linking
them to each other.  It typically reduces the size of the store by
something like 25-35%.  Only regular files and symlinks are
hard-linked in this manner.  Files are considered identical when they
have the same NAR archive serialisation: that is, regular files must
have the same contents and permission (executable or non-executable),
and symlinks must have the same contents.</para>

<para>After completion, or when the command is interrupted, a report
on the achieved savings is printed on standard error.</para>

<para>Use <option>-vv</option> or <option>-vvv</option> to get some
progress indication.</para>

</refsection>
            
<refsection><title>Example</title>

<screen>
$ nix-store --optimise
hashing files in `/nix/store/qhqx7l2f1kmwihc9bnxs7rc159hsxnf3-gcc-4.1.1'
<replaceable>...</replaceable>
541838819 bytes (516.74 MiB) freed by hard-linking 54143 files;
there are 114486 files with equal contents out of 215894 files in total
</screen>

</refsection>


</refsection>


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

<refsection><title>Operation <option>--read-log</option></title>

<refsection>
  <title>Synopsis</title>
  <cmdsynopsis>
    <command>nix-store</command>
    <group choice='req'>
      <arg choice='plain'><option>--read-log</option></arg>
      <arg choice='plain'><option>-l</option></arg>
    </group>
    <arg choice='plain' rep='repeat'><replaceable>paths</replaceable></arg>
  </cmdsynopsis>
</refsection>

<refsection><title>Description</title>
            
<para>The operation <option>--read-log</option> prints the build log
of the specified store paths on standard output.  The build log is
whatever the builder of a derivation wrote to standard output and
standard error.  If a store path is not a derivation, the deriver of
the store path is used.</para>

<para>Build logs are kept in
<filename>/nix/var/log/nix/drvs</filename>.  However, there is no
guarantee that a build log is available for any particular store
path.  For instance, if the path was downloaded as a pre-built binary
through a substitute, then the log is unavailable.</para>

</refsection>
            
<refsection><title>Example</title>

<screen>
$ nix-store -l $(which ktorrent)
building /nix/store/dhc73pvzpnzxhdgpimsd9sw39di66ph1-ktorrent-2.2.1
unpacking sources
unpacking source archive /nix/store/p8n1jpqs27mgkjw07pb5269717nzf5f8-ktorrent-2.2.1.tar.gz
ktorrent-2.2.1/
ktorrent-2.2.1/NEWS
<replaceable>...</replaceable>
</screen>

</refsection>


</refsection>


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

<refsection><title>Operation <option>--dump-db</option></title>

<refsection>
  <title>Synopsis</title>
  <cmdsynopsis>
    <command>nix-store</command>
    <arg choice='plain'><option>--dump-db</option></arg>
  </cmdsynopsis>
</refsection>

<refsection><title>Description</title>
            
<para>The operation <option>--dump-db</option> writes a dump of the
Nix database to standard output.  It can be loaded into an empty Nix
store using <option>--load-db</option>.  This is useful for making
backups and when migrating to different database schemas.</para>

</refsection>
            
</refsection>


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

<refsection><title>Operation <option>--dump-db</option></title>

<refsection>
  <title>Synopsis</title>
  <cmdsynopsis>
    <command>nix-store</command>
    <arg choice='plain'><option>--load-db</option></arg>
  </cmdsynopsis>
</refsection>

<refsection><title>Description</title>
            
<para>The operation <option>--load-db</option> reads a dump of the Nix
database created by <option>--dump-db</option> from standard input and
loads it into the Nix database.</para>

</refsection>
            
</refsection>


</refentry>