nix-super/doc/manual/packages/profiles.xml

<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 mozilla</screen>

on a set of Nix expressions that contained Mozilla 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>Actually, there is another level of indirection not shown in the
figure above.  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>