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<chapter xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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version="5.0"
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xml:id="ch-basic-package-mgmt">
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<title>Basic Package Management</title>
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<para>The main command for package management is <link
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linkend="sec-nix-env"><command>nix-env</command></link>. You can use
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it to install, upgrade, and erase packages, and to query what
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packages are installed or are available for installation.</para>
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<para>In Nix, different users can have different “views”
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on the set of installed applications. That is, there might be lots of
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applications present on the system (possibly in many different
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versions), but users can have a specific selection of those active —
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where “active” just means that it appears in a directory
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in the user’s <envar>PATH</envar>. Such a view on the set of
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installed applications is called a <emphasis>user
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environment</emphasis>, which is just a directory tree consisting of
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symlinks to the files of the active applications. </para>
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<para>Components are installed from a set of <emphasis>Nix
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expressions</emphasis> that tell Nix how to build those packages,
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including, if necessary, their dependencies. There is a collection of
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Nix expressions called the Nixpkgs package collection that contains
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packages ranging from basic development stuff such as GCC and Glibc,
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to end-user applications like Mozilla Firefox. (Nix is however not
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tied to the Nixpkgs package collection; you could write your own Nix
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expressions based on Nixpkgs, or completely new ones.)</para>
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<para>You can manually download the latest version of Nixpkgs from
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<link xlink:href='http://nixos.org/nixpkgs/download.html'/>. However,
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it’s much more convenient to use the Nixpkgs
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<emphasis>channel</emphasis>, since it makes it easy to stay up to
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date with new versions of Nixpkgs. (Channels are described in more
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detail in <xref linkend="sec-channels"/>.) Nixpkgs is automatically
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added to your list of “subscribed” channels when you install
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Nix. If this is not the case for some reason, you can add it as
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follows:
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<screen>
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$ nix-channel --add https://nixos.org/channels/nixpkgs-unstable
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$ nix-channel --update
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</screen>
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</para>
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<note><para>On NixOS, you’re automatically subscribed to a NixOS
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channel corresponding to your NixOS major release
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(e.g. <uri>http://nixos.org/channels/nixos-14.12</uri>). A NixOS
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channel is identical to the Nixpkgs channel, except that it contains
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only Linux binaries and is updated only if a set of regression tests
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succeed.</para></note>
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<para>You can view the set of available packages in Nixpkgs:
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<screen>
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$ nix-env -qa
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aterm-2.2
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bash-3.0
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binutils-2.15
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bison-1.875d
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blackdown-1.4.2
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bzip2-1.0.2
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…</screen>
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The flag <option>-q</option> specifies a query operation, and
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<option>-a</option> means that you want to show the “available” (i.e.,
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installable) packages, as opposed to the installed packages. If you
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downloaded Nixpkgs yourself, or if you checked it out from GitHub,
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then you need to pass the path to your Nixpkgs tree using the
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<option>-f</option> flag:
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<screen>
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$ nix-env -qaf <replaceable>/path/to/nixpkgs</replaceable>
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</screen>
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where <replaceable>/path/to/nixpkgs</replaceable> is where you’ve
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unpacked or checked out Nixpkgs.</para>
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<para>You can select specific packages by name:
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<screen>
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$ nix-env -qa firefox
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firefox-34.0.5
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firefox-with-plugins-34.0.5
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</screen>
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and using regular expressions:
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<screen>
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$ nix-env -qa 'firefox.*'
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</screen>
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</para>
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<para>It is also possible to see the <emphasis>status</emphasis> of
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available packages, i.e., whether they are installed into the user
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environment and/or present in the system:
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<screen>
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$ nix-env -qas
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…
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-PS bash-3.0
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--S binutils-2.15
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IPS bison-1.875d
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…</screen>
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The first character (<literal>I</literal>) indicates whether the
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package is installed in your current user environment. The second
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(<literal>P</literal>) indicates whether it is present on your system
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(in which case installing it into your user environment would be a
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very quick operation). The last one (<literal>S</literal>) indicates
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whether there is a so-called <emphasis>substitute</emphasis> for the
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package, which is Nix’s mechanism for doing binary deployment. It
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just means that Nix knows that it can fetch a pre-built package from
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somewhere (typically a network server) instead of building it
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locally.</para>
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<para>You can install a package using <literal>nix-env -i</literal>.
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For instance,
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<screen>
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$ nix-env -i subversion</screen>
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will install the package called <literal>subversion</literal> (which
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is, of course, the <link
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xlink:href='http://subversion.tigris.org/'>Subversion version
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management system</link>).</para>
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<note><para>When you ask Nix to install a package, it will first try
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to get it in pre-compiled form from a <emphasis>binary
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cache</emphasis>. By default, Nix will use the binary cache
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<uri>https://cache.nixos.org</uri>; it contains binaries for most
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packages in Nixpkgs. Only if no binary is available in the binary
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cache, Nix will build the package from source. So if <literal>nix-env
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-i subversion</literal> results in Nix building stuff from source,
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then either the package is not built for your platform by the Nixpkgs
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build servers, or your version of Nixpkgs is too old or too new. For
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instance, if you have a very recent checkout of Nixpkgs, then the
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Nixpkgs build servers may not have had a chance to build everything
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and upload the resulting binaries to
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<uri>https://cache.nixos.org</uri>. The Nixpkgs channel is only
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updated after all binaries have been uploaded to the cache, so if you
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stick to the Nixpkgs channel (rather than using a Git checkout of the
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Nixpkgs tree), you will get binaries for most packages.</para></note>
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<para>Naturally, packages can also be uninstalled:
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<screen>
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$ nix-env -e subversion</screen>
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</para>
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<para>Upgrading to a new version is just as easy. If you have a new
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release of Nix Packages, you can do:
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<screen>
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$ nix-env -u subversion</screen>
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This will <emphasis>only</emphasis> upgrade Subversion if there is a
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“newer” version in the new set of Nix expressions, as
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defined by some pretty arbitrary rules regarding ordering of version
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numbers (which generally do what you’d expect of them). To just
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unconditionally replace Subversion with whatever version is in the Nix
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expressions, use <parameter>-i</parameter> instead of
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<parameter>-u</parameter>; <parameter>-i</parameter> will remove
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whatever version is already installed.</para>
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<para>You can also upgrade all packages for which there are newer
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versions:
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<screen>
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$ nix-env -u</screen>
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</para>
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<para>Sometimes it’s useful to be able to ask what
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<command>nix-env</command> would do, without actually doing it. For
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instance, to find out what packages would be upgraded by
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<literal>nix-env -u</literal>, you can do
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<screen>
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$ nix-env -u --dry-run
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(dry run; not doing anything)
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upgrading `libxslt-1.1.0' to `libxslt-1.1.10'
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upgrading `graphviz-1.10' to `graphviz-1.12'
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upgrading `coreutils-5.0' to `coreutils-5.2.1'</screen>
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</para>
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</chapter>
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