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-<chapter xmlns="http://docbook.org/ns/docbook"
-      xmlns:xlink="http://www.w3.org/1999/xlink"
-      xmlns:xi="http://www.w3.org/2001/XInclude"
-      version="5.0"
-      xml:id="sec-profiles">
-
-<title>Profiles</title>
-
-<para>Profiles and user environments are Nix’s mechanism for
-implementing the ability to allow different users to have different
-configurations, and to do atomic upgrades and rollbacks.  To
-understand how they work, it’s useful to know a bit about how Nix
-works.  In Nix, packages are stored in unique locations in the
-<emphasis>Nix store</emphasis> (typically,
-<filename>/nix/store</filename>).  For instance, a particular version
-of the Subversion package might be stored in a directory
-<filename>/nix/store/dpmvp969yhdqs7lm2r1a3gng7pyq6vy4-subversion-1.1.3/</filename>,
-while another version might be stored in
-<filename>/nix/store/5mq2jcn36ldlmh93yj1n8s9c95pj7c5s-subversion-1.1.2</filename>.
-The long strings prefixed to the directory names are cryptographic
-hashes<footnote><para>160-bit truncations of SHA-256 hashes encoded in
-a base-32 notation, to be precise.</para></footnote> of
-<emphasis>all</emphasis> inputs involved in building the package —
-sources, dependencies, compiler flags, and so on.  So if two
-packages differ in any way, they end up in different locations in
-the file system, so they don’t interfere with each other.  <xref
-linkend='fig-user-environments' /> shows a part of a typical Nix
-store.</para>
-
-<figure xml:id='fig-user-environments'><title>User environments</title>
-  <mediaobject>
-    <imageobject>
-      <imagedata fileref='../figures/user-environments.png' format='PNG' />
-    </imageobject>
-  </mediaobject>
-</figure>
-
-<para>Of course, you wouldn’t want to type
-
-<screen>
-$ /nix/store/dpmvp969yhdq...-subversion-1.1.3/bin/svn</screen>
-
-every time you want to run Subversion.  Of course we could set up the
-<envar>PATH</envar> environment variable to include the
-<filename>bin</filename> directory of every package we want to use,
-but this is not very convenient since changing <envar>PATH</envar>
-doesn’t take effect for already existing processes.  The solution Nix
-uses is to create directory trees of symlinks to
-<emphasis>activated</emphasis> packages.  These are called
-<emphasis>user environments</emphasis> and they are packages
-themselves (though automatically generated by
-<command>nix-env</command>), so they too reside in the Nix store.  For
-instance, in <xref linkend='fig-user-environments' /> the user
-environment <filename>/nix/store/0c1p5z4kda11...-user-env</filename>
-contains a symlink to just Subversion 1.1.2 (arrows in the figure
-indicate symlinks).  This would be what we would obtain if we had done
-
-<screen>
-$ nix-env -i subversion</screen>
-
-on a set of Nix expressions that contained Subversion 1.1.2.</para>
-
-<para>This doesn’t in itself solve the problem, of course; you
-wouldn’t want to type
-<filename>/nix/store/0c1p5z4kda11...-user-env/bin/svn</filename>
-either.  That’s why there are symlinks outside of the store that point
-to the user environments in the store; for instance, the symlinks
-<filename>default-42-link</filename> and
-<filename>default-43-link</filename> in the example.  These are called
-<emphasis>generations</emphasis> since every time you perform a
-<command>nix-env</command> operation, a new user environment is
-generated based on the current one.  For instance, generation 43 was
-created from generation 42 when we did
-
-<screen>
-$ nix-env -i subversion firefox</screen>
-
-on a set of Nix expressions that contained Firefox and a new version
-of Subversion.</para>
-
-<para>Generations are grouped together into
-<emphasis>profiles</emphasis> so that different users don’t interfere
-with each other if they don’t want to.  For example:
-
-<screen>
-$ ls -l /nix/var/nix/profiles/
-...
-lrwxrwxrwx  1 eelco ... default-42-link -> /nix/store/0c1p5z4kda11...-user-env
-lrwxrwxrwx  1 eelco ... default-43-link -> /nix/store/3aw2pdyx2jfc...-user-env
-lrwxrwxrwx  1 eelco ... default -> default-43-link</screen>
-
-This shows a profile called <filename>default</filename>.  The file
-<filename>default</filename> itself is actually a symlink that points
-to the current generation.  When we do a <command>nix-env</command>
-operation, a new user environment and generation link are created
-based on the current one, and finally the <filename>default</filename>
-symlink is made to point at the new generation.  This last step is
-atomic on Unix, which explains how we can do atomic upgrades.  (Note
-that the building/installing of new packages doesn’t interfere in
-any way with old packages, since they are stored in different
-locations in the Nix store.)</para>
-
-<para>If you find that you want to undo a <command>nix-env</command>
-operation, you can just do
-
-<screen>
-$ nix-env --rollback</screen>
-
-which will just make the current generation link point at the previous
-link.  E.g., <filename>default</filename> would be made to point at
-<filename>default-42-link</filename>.  You can also switch to a
-specific generation:
-
-<screen>
-$ nix-env --switch-generation 43</screen>
-
-which in this example would roll forward to generation 43 again.  You
-can also see all available generations:
-
-<screen>
-$ nix-env --list-generations</screen></para>
-
-<para>You generally wouldn’t have
-<filename>/nix/var/nix/profiles/<replaceable>some-profile</replaceable>/bin</filename>
-in your <envar>PATH</envar>.  Rather, there is a symlink
-<filename>~/.nix-profile</filename> that points to your current
-profile.  This means that you should put
-<filename>~/.nix-profile/bin</filename> in your <envar>PATH</envar>
-(and indeed, that’s what the initialisation script
-<filename>/nix/etc/profile.d/nix.sh</filename> does).  This makes it
-easier to switch to a different profile.  You can do that using the
-command <command>nix-env --switch-profile</command>:
-
-<screen>
-$ nix-env --switch-profile /nix/var/nix/profiles/my-profile
-
-$ nix-env --switch-profile /nix/var/nix/profiles/default</screen>
-
-These commands switch to the <filename>my-profile</filename> and
-default profile, respectively.  If the profile doesn’t exist, it will
-be created automatically.  You should be careful about storing a
-profile in another location than the <filename>profiles</filename>
-directory, since otherwise it might not be used as a root of the
-garbage collector (see <xref linkend='sec-garbage-collection'
-/>).</para>
-
-<para>All <command>nix-env</command> operations work on the profile
-pointed to by <command>~/.nix-profile</command>, but you can override
-this using the <option>--profile</option> option (abbreviation
-<option>-p</option>):
-
-<screen>
-$ nix-env -p /nix/var/nix/profiles/other-profile -i subversion</screen>
-
-This will <emphasis>not</emphasis> change the
-<command>~/.nix-profile</command> symlink.</para>
-
-</chapter>