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* Split overview chapter into a chapter on package management and a

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chapter on writing Nix expressions.
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Eelco Dolstra 2004-10-14 11:55:12 +00:00
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<chapter id='chap-package-management'><title>Package Management</title>
<para>Let's start from the perspective of an end user. Common
operations at this level are to install and remove packages, ask what
packages are installed or available for installation, and so on.
These are operations on the <emphasis>user environment</emphasis>: the
set of packages that a user <quote>sees</quote>. In a command line
Unix environment, this means the set of programs that are available
through the <envar>PATH</envar> environment variable. (In other
environments it might mean the set of programs available on the
desktop, through the start menu, and so on.)</para>
<para>The terms <quote>installation</quote> and
<quote>uninstallation</quote> are used in this context to denote the
act of adding or removing packages from the user environment. In Nix,
these operations are dissociated from the physical copying or deleting
of files. Installation requires that the files constituting the
package are present, but they may be present beforehand. Likewise,
uninstallation does not actually delete any files; this is done
automatically by running a garbage collector.</para>
<para>User environments are manipulated through the
<command>nix-env</command> command. The query operation can be used
to see what packages are currently installed.</para>
<screen>
$ nix-env -q
MozillaFirebird-0.7
sylpheed-0.9.7
pan-0.14.2</screen>
<para>(<option>-q</option> is actually short for <option>--query
--installed</option>.) The package names are symbolic: they don't
have any particular significance to Nix (as they shouldn't, since they
are not unique&mdash;there can be many derivations with the same
name). Note that these packages have many dependencies (e.g., Mozilla
uses the <literal>gtk+</literal> package) but these have not been
installed in the user environment, though they are present on the
system. Generally, there is no need to install such packages; only
packages containing programs should be installed.</para>
<para>To install packages, a <emphasis>Nix expression</emphasis> is
required that tells Nix how to build that package. There is a <ulink
url='https://svn.cs.uu.nl:12443/dist/trace/trace-nixpkgs-trunk.tar.bz2'>set
of standard of Nix expressions</ulink> for many common packages.
Assuming that you have downloaded and unpacked these, you can view the
set of available packages:</para>
<screen>
$ nix-env -qaf pkgs/system/i686-linux.nix
gettext-0.12.1
sylpheed-0.9.7
aterm-2.0
gtk+-1.2.10
apache-httpd-2.0.48
pan-0.14.2
...</screen>
<para>The Nix expression in the file
<filename>i686-linux.nix</filename> yields the set of packages for a
Linux system running on x86 hardware. For other platforms, copy and
modify this file for your platform as appropriate. [TODO: improve
this]</para>
<para>It is also possible to see the <emphasis>status</emphasis> of
available packages, i.e., whether they are installed into the user
environment and/or present in the system:</para>
<screen>
$ nix-env -qasf pkgs/system/i686-linux.nix
-P gettext-0.12.1
IP sylpheed-0.9.7
-- aterm-2.0
-P gtk+-1.2.10</screen>
<para>This reveals that the <literal>sylpheed</literal> package is
already installed, or more precisely, that exactly the same
instantiation of <literal>sylpheed</literal> is installed. This
guarantees that the available package is exactly the same as the
installed package with regard to sources, dependencies, build flags,
and so on. Similarly, we see that the <literal>gettext</literal> and
<literal>gtk+</literal> packages are present but not installed in the
user environment, while the <literal>aterm</literal> package is not
installed or present at all (so, if we were to install it, it would
have to be built or downloaded first).</para>
<para>The install operation is used install available packages from a
Nix environment. To install the <literal>pan</literal> package (a
newsreader), you would do:</para>
<screen>
$ nix-env -if pkgs/system/i686-linux.nix pan</screen>
<para>Since installation may take a long time, depending on whether
any packages need to be built or downloaded, it's a good idea to make
<command>nix-env</command> run verbosely by using the
<option>-v</option> (<option>--verbose</option>) option. This option
may be repeated to increase the level of verbosity. A good value is 3
(<option>-vvv</option>).</para>
<para>In fact, if you run this command verbosely you will observe that
Nix starts to build many packages, including large and fundamental
ones such as <literal>glibc</literal> and <literal>gcc</literal>.
I.e., you are performing a source installation. This is generally
undesirable, since installation from sources may require large amounts
of disk and CPU resources. Therefore a <quote>binary</quote>
installation is generally preferable.</para>
<para>Rather than provide different mechanisms to create and perform
the installation of binary packages, Nix supports binary deployment
<emphasis>transparently</emphasis> through a generic mechanism of
<emphasis>substitute expressions</emphasis>. If an request is made to
build some Nix expression, Nix will first try to build any substitutes
for that expression. These substitutes presumably perform an
identical build operation with respect to the result, but require less
resources. For instance, a substitute that downloads a pre-built
package from the network requires less CPU and disk resources, and
possibly less time.</para>
<para>Nix's use of cryptographic hashes makes this entirely safe. It
is not possible, for instance, to accidentally substitute a build of
some package for a Solaris or Windows system for a build on a SuSE/x86
system.</para>
<para>While the substitute mechanism is a generic mechanism, Nix
provides two standard tools called <command>nix-pull</command> and
<command>nix-push</command> that maintain and use a shared cache of
prebuilt derivations on some network site (reachable through HTTP).
If you attempt to install some package that someone else has
previously built and <quote>pushed</quote> into the cache, and you
have done a <quote>pull</quote> to register substitutes that download
these prebuilt packages, then the installation will automatically use
these.</para>
<para>For example, to pull from our <ulink
url='http://losser.st-lab.cs.uu.nl/~eelco/nix-dist/'>cache</ulink> of
prebuilt packages (at the time of writing, for SuSE Linux/x86), use
the following command:</para>
<screen>
$ nix-pull http://catamaran.labs.cs.uu.nl/dist/nix/nixpkgs-<replaceable>version</replaceable>/MANIFEST
obtaining list of Nix archives at http://catamaran.labs.cs.uu.nl/dist/nix/nixpkgs-<replaceable>version</replaceable>/MANIFEST...
...</screen>
<para>If <command>nix-pull</command> is run without any arguments, it
will pull from the URLs specified in the file
<filename><replaceable>prefix</replaceable>/etc/nix/prebuilts.conf</filename>.</para>
<para>Assuming that the <literal>pan</literal> installation produced
no errors, it can be used immediately, that is, it now appears in a
directory in the <envar>PATH</envar> environment variable.
Specifically, <envar>PATH</envar> includes the entry
<filename><replaceable>prefix</replaceable>/var/nix/profiles/default/bin</filename>,
where
<filename><replaceable>prefix</replaceable>/var/nix/profiles/default</filename>
is just a symlink to the current user environment:</para>
<screen>
$ ls -l /nix/var/nix/profiles/
...
lrwxrwxrwx 1 eelco ... default-15-link -> /nix/store/1871...12b0-user-environment
lrwxrwxrwx 1 eelco ... default-16-link -> /nix/store/59ba...df6b-user-environment
lrwxrwxrwx 1 eelco ... default -> default-16-link</screen>
<para>That is, <filename>default</filename> in this example is a link
to <filename>default-16-link</filename>, which is the current user
environment. Before the installation, it pointed to
<filename>default-15-link</filename>. Note that this means that you
can atomically roll-back to the previous user environment by pointing
the symlink <filename>default</filename> at
<filename>default-15-link</filename> again. This also shows that
operations such as installation are atomic in the Nix system: any
arbitrarily complex set of installation, uninstallation, or upgrade
actions eventually boil down to the single operation of pointing a
symlink somewhere else (which can be implemented atomically in Unix).</para>
<para>What's in a user environment? It's just a set of symlinks to the
files that constitute the installed packages. For instance:</para>
<screen>
$ ls -l /nix/var/nix/profiles/default-16-link/bin
lrwxrwxrwx 1 eelco ... MozillaFirebird -> /nix/store/35f8...4ae6-MozillaFirebird-0.7/bin/MozillaFirebird
lrwxrwxrwx 1 eelco ... svn -> /nix/store/3829...fb5d-subversion-0.32.1/bin/svn
...</screen>
<para>Note that, e.g., <filename>svn</filename> =
<filename>/nix/var/nix/profiles/default/bin/svn</filename> =
<filename>/nix/var/nix/profiles/default-16-link/bin/svn</filename> =
<filename>/nix/store/59ba...df6b-user-environment/bin/svn</filename> =
<filename>/nix/store/3829...fb5d-subversion-0.32.1/bin/svn</filename>.</para>
<para>Naturally, packages can also be uninstalled:</para>
<screen>
$ nix-env -e pan</screen>
<para>This means that the package is removed from the user
environment. It is <emphasis>not</emphasis> yet removed from the
system. When a package is uninstalled from a user environment, it may
still be used by other packages, or may still be present in other user
environments. Deleting it under such conditions would break those
other packages or user environments. To prevent this, packages are
only <quote>physically</quote> deleted by running the Nix garbage
collector, which searches for all packages in the Nix store that are
no longer <quote>reachable</quote> from outside the store. Thus,
uninstalling a package is always safe: it cannot break other
packages.</para>
<para>Upgrading packages is easy. Given a Nix expression that
contains newer versions of installed packages (that is, packages with
the same package name, but a higher version number), <command>nix-env
-u</command> will replace the installed package in the user
environment with the newer package. For example,
<screen>
$ nix-env -uf pkgs/system/i686-linux.nix pan</screen>
looks for a newer version of Pan, and installs it if found. Also
useful is the ability to upgrade <emphasis>all</emphasis> packages:
<screen>
$ nix-env -uf pkgs/system/i686-linux.nix '*'</screen>
The asterisk matches all installed packages<footnote><para>No, we
don't support arbitrary regular expressions</para></footnote>. Note
that <literal>*</literal> must be quoted to prevent shell
globbing.</para>
</chapter>