159 lines
6.7 KiB
XML
159 lines
6.7 KiB
XML
<chapter xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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version="5.0"
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xml:id="sec-profiles">
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<title>Profiles</title>
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<para>Profiles and user environments are Nix’s mechanism for
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implementing the ability to allow different 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, packages 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 package might be stored in a directory
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<filename>/nix/store/dpmvp969yhdqs7lm2r1a3gng7pyq6vy4-subversion-1.1.3/</filename>,
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while another version might be stored in
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<filename>/nix/store/5mq2jcn36ldlmh93yj1n8s9c95pj7c5s-subversion-1.1.2</filename>.
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The long strings prefixed to the directory names are cryptographic
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hashes<footnote><para>160-bit truncations of SHA-256 hashes encoded in
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a base-32 notation, to be precise.</para></footnote> of
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<emphasis>all</emphasis> inputs involved in building the package —
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sources, dependencies, compiler flags, and so on. So if two
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packages differ in any way, they end up in different locations in
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the file system, so they don’t interfere with each other. <xref
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linkend='fig-user-environments' /> shows a part of a typical Nix
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store.</para>
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<figure xml: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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</imageobject>
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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/dpmvp969yhdq...-subversion-1.1.3/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 package 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> packages. These are called
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<emphasis>user environments</emphasis> and they are packages
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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/0c1p5z4kda11...-user-env</filename>
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contains a symlink to just Subversion 1.1.2 (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 1.1.2.</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/0c1p5z4kda11...-user-env/bin/svn</filename>
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either. That’s why 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>default-42-link</filename> and
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<filename>default-43-link</filename> in the example. These are called
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<emphasis>generations</emphasis> since every time you perform a
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<command>nix-env</command> operation, a new user environment is
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generated based on the current one. For instance, generation 43 was
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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/0c1p5z4kda11...-user-env
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lrwxrwxrwx 1 eelco ... default-43-link -> /nix/store/3aw2pdyx2jfc...-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 points
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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 packages doesn’t interfere in
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any way with old packages, 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 points 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. You can do that using the
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command <command>nix-env --switch-profile</command>:
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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 than the <filename>profiles</filename>
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directory, since otherwise it might not be used as a root of the
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garbage collector (see <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 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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</chapter> |