mirror of
https://github.com/privatevoid-net/nix-super.git
synced 2024-11-25 07:16:17 +02:00
268 lines
9.4 KiB
XML
268 lines
9.4 KiB
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="ch-about-nix">
|
||
|
||
<title>About Nix</title>
|
||
|
||
<para>Nix is a <emphasis>purely functional package manager</emphasis>.
|
||
This means that it treats packages like values in purely functional
|
||
programming languages such as Haskell — they are built by functions
|
||
that don’t have side-effects, and they never change after they have
|
||
been built. Nix stores packages in the <emphasis>Nix
|
||
store</emphasis>, usually the directory
|
||
<filename>/nix/store</filename>, where each package has its own unique
|
||
subdirectory such as
|
||
|
||
<programlisting>
|
||
/nix/store/b6gvzjyb2pg0kjfwrjmg1vfhh54ad73z-firefox-33.1/
|
||
</programlisting>
|
||
|
||
where <literal>b6gvzjyb2pg0…</literal> is a unique identifier for the
|
||
package that captures all its dependencies (it’s a cryptographic hash
|
||
of the package’s build dependency graph). This enables many powerful
|
||
features.</para>
|
||
|
||
|
||
<simplesect><title>Multiple versions</title>
|
||
|
||
<para>You can have multiple versions or variants of a package
|
||
installed at the same time. This is especially important when
|
||
different applications have dependencies on different versions of the
|
||
same package — it prevents the “DLL hell”. Because of the hashing
|
||
scheme, different versions of a package end up in different paths in
|
||
the Nix store, so they don’t interfere with each other.</para>
|
||
|
||
<para>An important consequence is that operations like upgrading or
|
||
uninstalling an application cannot break other applications, since
|
||
these operations never “destructively” update or delete files that are
|
||
used by other packages.</para>
|
||
|
||
</simplesect>
|
||
|
||
|
||
<simplesect><title>Complete dependencies</title>
|
||
|
||
<para>Nix helps you make sure that package dependency specifications
|
||
are complete. In general, when you’re making a package for a package
|
||
management system like RPM, you have to specify for each package what
|
||
its dependencies are, but there are no guarantees that this
|
||
specification is complete. If you forget a dependency, then the
|
||
package will build and work correctly on <emphasis>your</emphasis>
|
||
machine if you have the dependency installed, but not on the end
|
||
user's machine if it's not there.</para>
|
||
|
||
<para>Since Nix on the other hand doesn’t install packages in “global”
|
||
locations like <filename>/usr/bin</filename> but in package-specific
|
||
directories, the risk of incomplete dependencies is greatly reduced.
|
||
This is because tools such as compilers don’t search in per-packages
|
||
directories such as
|
||
<filename>/nix/store/5lbfaxb722zp…-openssl-0.9.8d/include</filename>,
|
||
so if a package builds correctly on your system, this is because you
|
||
specified the dependency explicitly. This takes care of the build-time
|
||
dependencies.</para>
|
||
|
||
<para>Once a package is built, runtime dependencies are found by
|
||
scanning binaries for the hash parts of Nix store paths (such as
|
||
<literal>r8vvq9kq…</literal>). This sounds risky, but it works
|
||
extremely well.</para>
|
||
|
||
</simplesect>
|
||
|
||
|
||
<simplesect><title>Multi-user support</title>
|
||
|
||
<para>Nix has multi-user support. This means that non-privileged
|
||
users can securely install software. Each user can have a different
|
||
<emphasis>profile</emphasis>, a set of packages in the Nix store that
|
||
appear in the user’s <envar>PATH</envar>. If a user installs a
|
||
package that another user has already installed previously, the
|
||
package won’t be built or downloaded a second time. At the same time,
|
||
it is not possible for one user to inject a Trojan horse into a
|
||
package that might be used by another user.</para>
|
||
|
||
</simplesect>
|
||
|
||
|
||
<simplesect><title>Atomic upgrades and rollbacks</title>
|
||
|
||
<para>Since package management operations never overwrite packages in
|
||
the Nix store but just add new versions in different paths, they are
|
||
<emphasis>atomic</emphasis>. So during a package upgrade, there is no
|
||
time window in which the package has some files from the old version
|
||
and some files from the new version — which would be bad because a
|
||
program might well crash if it’s started during that period.</para>
|
||
|
||
<para>And since packages aren’t overwritten, the old versions are still
|
||
there after an upgrade. This means that you can <emphasis>roll
|
||
back</emphasis> to the old version:</para>
|
||
|
||
<screen>
|
||
$ nix-env --upgrade <replaceable>some-packages</replaceable>
|
||
$ nix-env --rollback
|
||
</screen>
|
||
|
||
</simplesect>
|
||
|
||
|
||
<simplesect><title>Garbage collection</title>
|
||
|
||
<para>When you uninstall a package like this…
|
||
|
||
<screen>
|
||
$ nix-env --uninstall firefox
|
||
</screen>
|
||
|
||
the package isn’t deleted from the system right away (after all, you
|
||
might want to do a rollback, or it might be in the profiles of other
|
||
users). Instead, unused packages can be deleted safely by running the
|
||
<emphasis>garbage collector</emphasis>:
|
||
|
||
<screen>
|
||
$ nix-collect-garbage
|
||
</screen>
|
||
|
||
This deletes all packages that aren’t in use by any user profile or by
|
||
a currently running program.</para>
|
||
|
||
</simplesect>
|
||
|
||
|
||
<simplesect><title>Functional package language</title>
|
||
|
||
<para>Packages are built from <emphasis>Nix expressions</emphasis>,
|
||
which is a simple functional language. A Nix expression describes
|
||
everything that goes into a package build action (a “derivation”):
|
||
other packages, sources, the build script, environment variables for
|
||
the build script, etc. Nix tries very hard to ensure that Nix
|
||
expressions are <emphasis>deterministic</emphasis>: building a Nix
|
||
expression twice should yield the same result.</para>
|
||
|
||
<para>Because it’s a functional language, it’s easy to support
|
||
building variants of a package: turn the Nix expression into a
|
||
function and call it any number of times with the appropriate
|
||
arguments. Due to the hashing scheme, variants don’t conflict with
|
||
each other in the Nix store.</para>
|
||
|
||
</simplesect>
|
||
|
||
|
||
<simplesect><title>Transparent source/binary deployment</title>
|
||
|
||
<para>Nix expressions generally describe how to build a package from
|
||
source, so an installation action like
|
||
|
||
<screen>
|
||
$ nix-env --install firefox
|
||
</screen>
|
||
|
||
<emphasis>could</emphasis> cause quite a bit of build activity, as not
|
||
only Firefox but also all its dependencies (all the way up to the C
|
||
library and the compiler) would have to built, at least if they are
|
||
not already in the Nix store. This is a <emphasis>source deployment
|
||
model</emphasis>. For most users, building from source is not very
|
||
pleasant as it takes far too long. However, Nix can automatically
|
||
skip building from source and instead use a <emphasis>binary
|
||
cache</emphasis>, a web server that provides pre-built binaries. For
|
||
instance, when asked to build
|
||
<literal>/nix/store/b6gvzjyb2pg0…-firefox-33.1</literal> from source,
|
||
Nix would first check if the file
|
||
<uri>https://cache.nixos.org/b6gvzjyb2pg0….narinfo</uri> exists, and
|
||
if so, fetch the pre-built binary referenced from there; otherwise, it
|
||
would fall back to building from source.</para>
|
||
|
||
</simplesect>
|
||
|
||
|
||
<!--
|
||
<simplesect><title>Binary patching</title>
|
||
|
||
<para>In addition to downloading binaries automatically if they’re
|
||
available, Nix can download binary deltas that patch an existing
|
||
package in the Nix store into a new version. This speeds up
|
||
upgrades.</para>
|
||
|
||
</simplesect>
|
||
-->
|
||
|
||
|
||
<simplesect><title>Nix Packages collection</title>
|
||
|
||
<para>We provide a large set of Nix expressions containing hundreds of
|
||
existing Unix packages, the <emphasis>Nix Packages
|
||
collection</emphasis> (Nixpkgs).</para>
|
||
|
||
</simplesect>
|
||
|
||
|
||
<simplesect><title>Managing build environments</title>
|
||
|
||
<para>Nix is extremely useful for developers as it makes it easy to
|
||
automatically set up the build environment for a package. Given a
|
||
Nix expression that describes the dependencies of your package, the
|
||
command <command>nix-shell</command> will build or download those
|
||
dependencies if they’re not already in your Nix store, and then start
|
||
a Bash shell in which all necessary environment variables (such as
|
||
compiler search paths) are set.</para>
|
||
|
||
<para>For example, the following command gets all dependencies of the
|
||
Pan newsreader, as described by <link
|
||
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/networking/newsreaders/pan/default.nix">its
|
||
Nix expression</link>:</para>
|
||
|
||
<screen>
|
||
$ nix-shell '<nixpkgs>' -A pan
|
||
</screen>
|
||
|
||
<para>You’re then dropped into a shell where you can edit, build and test
|
||
the package:</para>
|
||
|
||
<screen>
|
||
[nix-shell]$ tar xf $src
|
||
[nix-shell]$ cd pan-*
|
||
[nix-shell]$ ./configure
|
||
[nix-shell]$ make
|
||
[nix-shell]$ ./pan/gui/pan
|
||
</screen>
|
||
|
||
<!--
|
||
<para>Since Nix packages are reproducible and have complete dependency
|
||
specifications, Nix makes an excellent basis for <a
|
||
href="[%root%]hydra">a continuous build system</a>.</para>
|
||
-->
|
||
|
||
</simplesect>
|
||
|
||
|
||
<simplesect><title>Portability</title>
|
||
|
||
<para>Nix runs on Linux and macOS.</para>
|
||
|
||
</simplesect>
|
||
|
||
|
||
<simplesect><title>NixOS</title>
|
||
|
||
<para>NixOS is a Linux distribution based on Nix. It uses Nix not
|
||
just for package management but also to manage the system
|
||
configuration (e.g., to build configuration files in
|
||
<filename>/etc</filename>). This means, among other things, that it
|
||
is easy to roll back the entire configuration of the system to an
|
||
earlier state. Also, users can install software without root
|
||
privileges. For more information and downloads, see the <link
|
||
xlink:href="http://nixos.org/">NixOS homepage</link>.</para>
|
||
|
||
</simplesect>
|
||
|
||
|
||
<simplesect><title>License</title>
|
||
|
||
<para>Nix is released under the terms of the <link
|
||
xlink:href="http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html">GNU
|
||
LGPLv2.1 or (at your option) any later version</link>.</para>
|
||
|
||
</simplesect>
|
||
|
||
|
||
</chapter>
|