\begin{chapter}[id=ch-about-nix]{About Nix}

Nix is a \emph{purely functional package manager}.  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 \emph{Nix store}, usually the directory
\filename{/nix/store}, where each package has its own unique
subdirectory such as

  \listing{/nix/store/b6gvzjyb2pg0kjfwrjmg1vfhh54ad73z-firefox-33.1/}

where \code{b6gvzjyb2pg0…} 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.


\simplesect{Multiple versions}{

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.

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.

}

\simplesect{Complete dependencies}{

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 \emph{your} machine if you
have the dependency installed, but not on the end user's machine if
it's not there.

Since Nix on the other hand doesn’t install packages in “global”
locations like \filename{/usr/bin} 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}, so if a
package builds correctly on your system, this is because you specified
the dependency explicitly.

Once a package is built, runtime dependencies are found by scanning
binaries for the hash parts of Nix store paths (such as
\code{r8vvq9kq…}).  This sounds risky, but it works extremely well.

}

\simplesect{Multi-user support}{

Nix has multi-user support.  This means that non-privileged users can
securely install software.  Each user can have a different
\emph{profile}, a set of packages in the Nix store that appear in the
user’s \envar{PATH}.  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.

}

\simplesect{Atomic upgrades and rollbacks}{

Since package management operations never overwrite packages in the
Nix store but just add new versions in different paths, they are
\emph{atomic}.  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.

And since packages aren’t overwritten, the old versions are still
there after an upgrade.  This means that you can \emph{roll
back} to the old version:

  \screen{
  $ nix-env --upgrade <replaceable>some-packages</replaceable>
  $ nix-env --rollback
  }

}

\simplesect{Garbage collection}{

When you uninstall a package like this…

  \screen{$ nix-env --uninstall firefox}

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
\emph{garbage collector}:

  \screen{$ nix-collect-garbage}

This deletes all packages that aren’t in use by any user profile or by
a currently running program.

}

\simplesect{Functional package language}{

Packages are built from \emph{Nix expressions},
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 \emph{deterministic}: building a Nix
expression twice should yield the same result.

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.

}

\simplesect{Transparent source/binary deployment}{

Nix expressions generally describe how to build a package from
source, so an installation action like

  \screen{$ nix-env --install firefox}

\emph{could} 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 \emph{source deployment model}.  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 \emph{binary cache}, a web server that provides pre-built
binaries. For instance, when asked to build
\code{/nix/store/b6gvzjyb2pg0…-firefox-33.1} from source, Nix would
first check if the file
\uri{https://cache.nixos.org/b6gvzjyb2pg0….narinfo} exists, and if so,
fetch the pre-built binary referenced from there; otherwise, it would
fall back to building from source.

}

\#{\simplesect{Binary patching}{

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.

}}

\simplesect{Nix Packages collection}{

We provide a large set of Nix expressions containing hundreds of
existing Unix packages, the \emph{Nix Packages
collection} (Nixpkgs).

}

\simplesect{Managing build environments}{

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} 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.

For example, the following command gets all dependencies of the Pan
newsreader, as described by
\link{https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/networking/newsreaders/pan/default.nix}{its
  Nix expression}:

  \screen{$ nix-shell '<nixpkgs>' -A pan}

You’re then dropped into a shell where you can edit, build and test
the package:

  \screen{{{
  [nix-shell]$ tar xf $src
  [nix-shell]$ cd pan-*
  [nix-shell]$ ./configure
  [nix-shell]$ make
  [nix-shell]$ ./pan/gui/pan
  }}}

\#{
Since Nix packages are reproducible and have complete dependency
specifications, Nix makes an excellent basis for
\link{https://nixos.org/hydra}{a continuous build system}.
}

}

\simplesect{Portability}{

Nix runs on Linux and macOS.

}

\simplesect{NixOS}{

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}).  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{http://nixos.org/}{NixOS homepage}.

}

\simplesect{License}{

Nix is released under the terms of the
\link{http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html}{GNU
  LGPLv2.1 or (at your option) any later version}.

}

\end{chapter}