* Profiles section.
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Eelco Dolstra
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@ -106,7 +106,7 @@ these files (and <filename>fetchurl.nix</filename> checks
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them).</para></listitem>
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<listitem><para>We should switch away from MD5, since it has been
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cracked. We don't currently depend very much on the
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more-or-less cracked. We don't currently depend very much on the
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collision-resistance of MD5, but we will once we start sharing build
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results between users.</para></listitem>
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@ -204,7 +204,7 @@ variables is to include the file
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in your <filename>~/.bashrc</filename> (or similar), like this:</para>
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<screen>
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. <replaceable>prefix</replaceable>/etc/profile.d/nix.sh</screen>
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source <replaceable>prefix</replaceable>/etc/profile.d/nix.sh</screen>
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</sect1>
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@ -131,8 +131,8 @@ defined by some pretty much arbitrary rules regarding ordering of
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version numbers (which generally do what you'd expect of them). To
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just unconditionally replace Subversion with whatever version is in
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the Nix expressions, use <parameter>-i</parameter> instead of
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<parameter>-u</parameter> — <parameter>-i</parameter> will
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remove whatever version is already installed.</para>
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<parameter>-u</parameter>; <parameter>-i</parameter> will remove
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whatever version is already installed.</para>
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<para>You can also upgrade all components for which there are newer
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versions:
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@ -142,7 +142,21 @@ $ nix-env -f nixpkgs-<replaceable>version</replaceable> -u '*'</screen>
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</para>
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<para>If you grow tired of specifying the Nix expressions using
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<para>Sometimes it's useful to be able to ask what
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<command>nix-env</command> would do, without actually doing it. For
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instance, to find out what packages would be upgraded by
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<literal>nix-env -u '*'</literal>, you can do
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<screen>
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$ nix-env ... -u '*' --dry-run
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(dry run; not doing anything)
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upgrading `libxslt-1.1.0' to `libxslt-1.1.10'
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upgrading `graphviz-1.10' to `graphviz-1.12'
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upgrading `coreutils-5.0' to `coreutils-5.2.1'</screen>
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</para>
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<para>If you grow bored of specifying the Nix expressions using
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<parameter>-f</parameter> all the time, you can set a default
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location:
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@ -162,14 +176,30 @@ set.</para></footnote></para>
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<sect1><title>Profiles</title>
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<para>In Nix, operations such as upgrading or removing components
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never overwrite or remove the files of those components, and they
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don't even touch the user environments that point to them. Rather,
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they cause a <emphasis>new</emphasis> user environment to be
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constructed based on the old one. This is illustrated in Figure
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bla.</para>
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<para>Profiles and user environments are Nix's mechanism for
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implementing the ability to allow differens users to have different
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configurations, and to do atomic upgrades and rollbacks. To
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understand how they work, it's useful to know a bit about how Nix
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works. In Nix, components are stored in unique locations in the
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<emphasis>Nix store</emphasis> (typically,
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<filename>/nix/store</filename>). For instance, a particular version
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of the Subversion component might be stored in a directory
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<filename>/nix/store/eeeeaf42e56b...-subversion-0.32.1/</filename>,
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while another version might be stored in
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<filename>/nix/store/58823d558a6a...-subversion-0.34/</filename>. The
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long hexadecimal numbers prefixed to the directory names are
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cryptographic hashes<footnote><para>128 bit MD5 hashes, to be
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precise.</para></footnote> of <emphasis>all</emphasis> inputs involved
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in building the component — sources, dependencies, compiler flags, and
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so on. So if two components differ in any way, they end up in
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different locations in the file system, so they don't interfere with
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each other. <xref linkend='fig-user-environments'
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/><footnote><para>TODO: the figure isn't entirely up to date. It
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should show multiple profiles and
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<filename>~/.nix-profile</filename>.</para></footnote> shows a part of
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a typical Nix store.</para>
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<figure><title>User environments</title>
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<figure id='fig-user-environments'><title>User environments</title>
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<mediaobject>
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<imageobject>
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<imagedata fileref='figures/user-environments.png' format='PNG' />
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@ -177,20 +207,139 @@ bla.</para>
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</mediaobject>
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</figure>
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<para>Of course, you wouldn't want to type
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<screen>
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$ /nix/store/eeeeaf42e56b...-subversion-0.32.1/bin/svn</screen>
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every time you want to run Subversion. Of course we could set up the
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<envar>PATH</envar> environment variable to include the
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<filename>bin</filename> directory of every component we want to use,
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but this is not very convenient since changing <envar>PATH</envar>
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doesn't take effect for already existing processes. The solution Nix
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uses is to create directory trees of symlinks to
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<emphasis>activated</emphasis> components. These are called
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<emphasis>user environments</emphasis> and they are components
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themselves (though automatically generated by
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<command>nix-env</command>), so they too reside in the Nix store. For
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instance, in <xref linkend='fig-user-environments' /> the user
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environment <filename>/nix/store/068150f63831...-user-env</filename>
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contains a symlink to just Subversion 0.32.1 (arrows in the figure
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indicate symlinks). This would be what we would obtain if we had done
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<screen>
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$ nix-env -i subversion</screen>
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on a set of Nix expressions that contained Subversion 0.32.1.</para>
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<para>This doesn't in itself solve the problem, of course; you
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wouldn't want to type
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<filename>/nix/store/068150f63831...-user-env/bin/svn</filename>
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either. Therefore there are symlinks outside of the store that point
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to the user environments in the store; for instance, the symlinks
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<filename>42</filename> and <filename>43</filename> in the example.
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These are called <emphasis>generations</emphasis> since every time you
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perform a <command>nix-env</command> operation, a new user environment
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is generated based on the current one. For instance, generation 43
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was created from generation 42 when we did
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<screen>
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$ nix-env -i subversion mozilla</screen>
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on a set of Nix expressions that contained Mozilla and a new version
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of Subversion.</para>
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<para>Generations are grouped together into
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<emphasis>profiles</emphasis> so that different users don't interfere
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with each other if they don't want to. For example:
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<screen>
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$ ls -l /nix/var/nix/profiles/
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...
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lrwxrwxrwx 1 eelco ... default-42-link -> /nix/store/068150f63831...-user-env
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lrwxrwxrwx 1 eelco ... default-43-link -> /nix/store/84c85f89ddbf...-user-env
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lrwxrwxrwx 1 eelco ... default -> default-43-link</screen>
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This shows a profile called <filename>default</filename>. The file
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<filename>default</filename> itself is actually a symlink that point
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to the current generation. When we do a <command>nix-env</command>
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operation, a new user environment and generation link are created
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based on the current one, and finally the <filename>default</filename>
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symlink is made to point at the new generation. This last step is
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atomic on Unix, which explains how we can do atomic upgrades. (Note
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that the building/installing of new components doesn't interfere in
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any way with old components, since they are stored in different
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locations in the Nix store.)</para>
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<para>If you find that you want to undo a <command>nix-env</command>
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operation, you can just do
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<screen>
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$ nix-env --rollback</screen>
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which will just make the current generation link point at the previous
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link. E.g., <filename>default</filename> would be made to point at
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<filename>default-42-link</filename>. You can also switch to a
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specific generation:
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<screen>
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$ nix-env --switch-generation 43</screen>
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which in this example would roll forward to generation 43 again. You
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can also see all available generations:
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<screen>
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$ nix-env --list-generations</screen></para>
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<para>Actually, there is another level of indirection not shown in the
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figure above. You generally wouldn't have
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<filename>/nix/var/nix/profiles/<replaceable>some-profile</replaceable>/bin</filename>
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in your <envar>PATH</envar>. Rather, there is a symlink
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<filename>~/.nix-profile</filename> that point to your current
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profile. This means that you should put
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<filename>~/.nix-profile/bin</filename> in your <envar>PATH</envar>
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(and indeed, that's what the initialisation script
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<filename>/nix/etc/profile.d/nix.sh</filename> does). This makes it
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easier to switch to a different profile, which is exactly what the
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command <command>nix-env --switch-profile</command> does:
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<screen>
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$ nix-env --switch-profile /nix/var/nix/profiles/my-profile
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$ nix-env --switch-profile /nix/var/nix/profiles/default</screen>
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These commands switch to the <filename>my-profile</filename> and
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default profile, respectively. If the profile doesn't exist, it will
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be created automatically. You should be careful about storing a
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profile in another location that the <filename>profiles</filename>
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directory, since otherwise it might not be used as a root to the
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garbage collection (see section <xref linkend='sec-garbage-collection'
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/>).</para>
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<para>All <command>nix-env</command> operations work on the profile
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pointed to by <command>~/.nix-profile</command>, but you can override
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this on using the <option>--profile</option> option (abbreviation
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<option>-p</option>):
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<screen>
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$ nix-env -p /nix/var/nix/profiles/other-profile -i subversion</screen>
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This will <emphasis>not</emphasis> change the
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<command>~/.nix-profile</command> symlink.</para>
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</sect1>
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<sect1><title>Garbage collection</title>
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<sect1 id='sec-garbage-collection'><title>Garbage collection</title>
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<para>Bla</para>
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<para>TODO</para>
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</sect1>
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<sect1><title>Channels</title>
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<para>Bla</para>
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<para>TODO</para>
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</sect1>
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