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diff --git a/doc/manual/introduction/about-nix.xml b/doc/manual/introduction/about-nix.xml new file mode 100644 index 000000000000..e6dfb7a5a565 --- /dev/null +++ b/doc/manual/introduction/about-nix.xml @@ -0,0 +1,267 @@ +<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.</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 Mac OS X.</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> |