* Rewrite of package management stuff.
This commit is contained in:
Eelco Dolstra
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<chapter id='chap-package-management'><title>Package Management</title>
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<para>Let's start from the perspective of an end user. Common
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operations at this level are to install and remove packages, ask what
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packages are installed or available for installation, and so on.
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These are operations on the <emphasis>user environment</emphasis>: the
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set of packages that a user <quote>sees</quote>. In a command line
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Unix environment, this means the set of programs that are available
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through the <envar>PATH</envar> environment variable. (In other
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environments it might mean the set of programs available on the
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desktop, through the start menu, and so on.)</para>
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<para>This chapter discusses how to do package management with Nix,
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i.e., how to obtain, install, upgrade, and erase components. This is
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the <quote>user's</quote> perspective of the Nix system — people
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who want to <emphasis>create</emphasis> components should consult
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<xref linkend='chap-writing-nix-expressions' />.</para>
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<para>The terms <quote>installation</quote> and
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<quote>uninstallation</quote> are used in this context to denote the
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act of adding or removing packages from the user environment. In Nix,
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these operations are dissociated from the physical copying or deleting
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of files. Installation requires that the files constituting the
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package are present, but they may be present beforehand. Likewise,
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uninstallation does not actually delete any files; this is done
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automatically by running a garbage collector.</para>
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<para>User environments are manipulated through the
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<command>nix-env</command> command. The query operation can be used
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to see what packages are currently installed.</para>
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<sect1><title>Basic package management</title>
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<para>The main command for package management is
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<command>nix-env</command>. You can use it to install, upgrade, and
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erase components, and to query what components are installed or are
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available for installation.</para>
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<para>In Nix, different users can have different <quote>views</quote>
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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
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active — where <quote>active</quote> just means that it appears
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in a directory in the user's <envar>PATH</envar>.</para>
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<para>Such a view on the set of installed applications is called a
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<emphasis>user environment</emphasis>, which is just a directory tree
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consisting of symlinks to the files of the active applications. In
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Nix, operations such as upgrading or removing components never
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overwrite or remove the files of those components, and they don't even
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touch the user environments that point to them. Rather, they cause a
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<emphasis>new</emphasis> user environment to be constructed based on
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the old one.</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 components,
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including, if necessary, their dependencies. There is a collection of
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Nix expressions called the Nix Package collection that contains
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components 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 Nix Package collection; you could write your own Nix
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expression based on that, or completely new.) You can download the
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latest version from <ulink
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url='http://catamaran.labs.cs.uu.nl/dist/nix' />. You probably want
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the latest unstable release; currently the stable releases tend to lag
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behind quite a bit.</para>
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<para>Assuming that you have downloaded and unpacked a release of Nix
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Packages, you can view the set of available components in the release:
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<screen>
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$ nix-env -q
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MozillaFirebird-0.7
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sylpheed-0.9.7
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pan-0.14.2</screen>
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<para>(<option>-q</option> is actually short for <option>--query
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--installed</option>.) The package names are symbolic: they don't
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have any particular significance to Nix (as they shouldn't, since they
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are not unique—there can be many derivations with the same
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name). Note that these packages have many dependencies (e.g., Mozilla
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uses the <literal>gtk+</literal> package) but these have not been
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installed in the user environment, though they are present on the
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system. Generally, there is no need to install such packages; only
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packages containing programs should be installed.</para>
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<para>To install packages, a <emphasis>Nix expression</emphasis> is
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required that tells Nix how to build that package. There is a <ulink
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url='https://svn.cs.uu.nl:12443/dist/trace/trace-nixpkgs-trunk.tar.bz2'>set
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of standard of Nix expressions</ulink> for many common packages.
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Assuming that you have downloaded and unpacked these, you can view the
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set of available packages:</para>
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<screen>
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$ nix-env -qaf pkgs/system/i686-linux.nix
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gettext-0.12.1
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sylpheed-0.9.7
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aterm-2.0
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gtk+-1.2.10
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apache-httpd-2.0.48
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pan-0.14.2
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$ nix-env -qaf nixpkgs-<replaceable>version</replaceable>
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ant-blackdown-1.4.2
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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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<para>The Nix expression in the file
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<filename>i686-linux.nix</filename> yields the set of packages for a
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Linux system running on x86 hardware. For other platforms, copy and
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modify this file for your platform as appropriate. [TODO: improve
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this]</para>
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where <literal>nixpkgs-<replaceable>version</replaceable></literal> is
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where you've unpacked the release.</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:</para>
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available component, 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 -qasf pkgs/system/i686-linux.nix
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-P gettext-0.12.1
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IP sylpheed-0.9.7
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-- aterm-2.0
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-P gtk+-1.2.10</screen>
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<para>This reveals that the <literal>sylpheed</literal> package is
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already installed, or more precisely, that exactly the same
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instantiation of <literal>sylpheed</literal> is installed. This
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guarantees that the available package is exactly the same as the
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installed package with regard to sources, dependencies, build flags,
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and so on. Similarly, we see that the <literal>gettext</literal> and
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<literal>gtk+</literal> packages are present but not installed in the
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user environment, while the <literal>aterm</literal> package is not
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installed or present at all (so, if we were to install it, it would
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have to be built or downloaded first).</para>
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<para>The install operation is used install available packages from a
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Nix environment. To install the <literal>pan</literal> package (a
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newsreader), you would do:</para>
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<screen>
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$ nix-env -if pkgs/system/i686-linux.nix pan</screen>
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<para>Since installation may take a long time, depending on whether
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any packages need to be built or downloaded, it's a good idea to make
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<command>nix-env</command> run verbosely by using the
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<option>-v</option> (<option>--verbose</option>) option. This option
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may be repeated to increase the level of verbosity. A good value is 3
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(<option>-vvv</option>).</para>
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<para>In fact, if you run this command verbosely you will observe that
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Nix starts to build many packages, including large and fundamental
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ones such as <literal>glibc</literal> and <literal>gcc</literal>.
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I.e., you are performing a source installation. This is generally
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undesirable, since installation from sources may require large amounts
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of disk and CPU resources. Therefore a <quote>binary</quote>
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installation is generally preferable.</para>
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<para>Rather than provide different mechanisms to create and perform
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the installation of binary packages, Nix supports binary deployment
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<emphasis>transparently</emphasis> through a generic mechanism of
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<emphasis>substitute expressions</emphasis>. If an request is made to
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build some Nix expression, Nix will first try to build any substitutes
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for that expression. These substitutes presumably perform an
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identical build operation with respect to the result, but require less
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resources. For instance, a substitute that downloads a pre-built
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package from the network requires less CPU and disk resources, and
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possibly less time.</para>
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<para>Nix's use of cryptographic hashes makes this entirely safe. It
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is not possible, for instance, to accidentally substitute a build of
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some package for a Solaris or Windows system for a build on a SuSE/x86
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system.</para>
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<para>While the substitute mechanism is a generic mechanism, Nix
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provides two standard tools called <command>nix-pull</command> and
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<command>nix-push</command> that maintain and use a shared cache of
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prebuilt derivations on some network site (reachable through HTTP).
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If you attempt to install some package that someone else has
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previously built and <quote>pushed</quote> into the cache, and you
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have done a <quote>pull</quote> to register substitutes that download
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these prebuilt packages, then the installation will automatically use
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these.</para>
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<para>For example, to pull from our <ulink
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url='http://losser.st-lab.cs.uu.nl/~eelco/nix-dist/'>cache</ulink> of
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prebuilt packages (at the time of writing, for SuSE Linux/x86), use
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the following command:</para>
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<screen>
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$ nix-pull http://catamaran.labs.cs.uu.nl/dist/nix/nixpkgs-<replaceable>version</replaceable>/MANIFEST
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obtaining list of Nix archives at http://catamaran.labs.cs.uu.nl/dist/nix/nixpkgs-<replaceable>version</replaceable>/MANIFEST...
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...</screen>
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<para>If <command>nix-pull</command> is run without any arguments, it
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will pull from the URLs specified in the file
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<filename><replaceable>prefix</replaceable>/etc/nix/prebuilts.conf</filename>.</para>
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<para>Assuming that the <literal>pan</literal> installation produced
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no errors, it can be used immediately, that is, it now appears in a
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directory in the <envar>PATH</envar> environment variable.
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Specifically, <envar>PATH</envar> includes the entry
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<filename><replaceable>prefix</replaceable>/var/nix/profiles/default/bin</filename>,
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where
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<filename><replaceable>prefix</replaceable>/var/nix/profiles/default</filename>
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is just a symlink to the current user environment:</para>
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<screen>
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$ ls -l /nix/var/nix/profiles/
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$ nix-env -qasf nixpkgs-<replaceable>version</replaceable>
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...
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lrwxrwxrwx 1 eelco ... default-15-link -> /nix/store/1871...12b0-user-environment
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lrwxrwxrwx 1 eelco ... default-16-link -> /nix/store/59ba...df6b-user-environment
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lrwxrwxrwx 1 eelco ... default -> default-16-link</screen>
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<para>That is, <filename>default</filename> in this example is a link
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to <filename>default-16-link</filename>, which is the current user
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environment. Before the installation, it pointed to
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<filename>default-15-link</filename>. Note that this means that you
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can atomically roll-back to the previous user environment by pointing
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the symlink <filename>default</filename> at
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<filename>default-15-link</filename> again. This also shows that
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operations such as installation are atomic in the Nix system: any
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arbitrarily complex set of installation, uninstallation, or upgrade
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actions eventually boil down to the single operation of pointing a
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symlink somewhere else (which can be implemented atomically in Unix).</para>
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<para>What's in a user environment? It's just a set of symlinks to the
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files that constitute the installed packages. For instance:</para>
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<screen>
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$ ls -l /nix/var/nix/profiles/default-16-link/bin
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lrwxrwxrwx 1 eelco ... MozillaFirebird -> /nix/store/35f8...4ae6-MozillaFirebird-0.7/bin/MozillaFirebird
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lrwxrwxrwx 1 eelco ... svn -> /nix/store/3829...fb5d-subversion-0.32.1/bin/svn
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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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<para>Note that, e.g., <filename>svn</filename> =
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<filename>/nix/var/nix/profiles/default/bin/svn</filename> =
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<filename>/nix/var/nix/profiles/default-16-link/bin/svn</filename> =
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<filename>/nix/store/59ba...df6b-user-environment/bin/svn</filename> =
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<filename>/nix/store/3829...fb5d-subversion-0.32.1/bin/svn</filename>.</para>
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The first character (<literal>I</literal>) indicates whether the
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component 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 very
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quick). The last one (<literal>S</literal>) indicates whether there
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is a so-called <emphasis>substitute</emphasis> for the component,
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which is Nix's mechanism for doing binary deployment. It just means
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that Nix know that it can fetch a pre-built component from somewhere
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(typically a network server) instead of building it locally.</para>
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<para>Naturally, packages can also be uninstalled:</para>
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<para>So now that we have a set of Nix expressions we can build the
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components contained in them. This is done using <literal>nix-env
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-i</literal>. For instance,
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<screen>
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$ nix-env -e pan</screen>
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$ nix-env -f nixpkgs-<replaceable>version</replaceable> -i subversion</screen>
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<para>This means that the package is removed from the user
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environment. It is <emphasis>not</emphasis> yet removed from the
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system. When a package is uninstalled from a user environment, it may
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still be used by other packages, or may still be present in other user
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environments. Deleting it under such conditions would break those
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other packages or user environments. To prevent this, packages are
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only <quote>physically</quote> deleted by running the Nix garbage
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collector, which searches for all packages in the Nix store that are
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no longer <quote>reachable</quote> from outside the store. Thus,
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uninstalling a package is always safe: it cannot break other
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packages.</para>
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will install the component called <literal>subversion</literal> (which
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is, of course, the <ulink
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url='http://subversion.tigris.org/'>Subversion version management
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system</ulink>).</para>
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<para>When you do this for the first time, Nix will start building
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Subversion and all its dependencies. This will take quite a while
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— typically an hour or two on modern machines. Fortunately,
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there is a faster way (so just do a Ctrl-C on that install
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operation!): you just need to tell Nix that pre-built binaries of all
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those components are available somewhere. This is done using the
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<command>nix-pull</command> command, which must be supplied with a URL
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containing a <emphasis>manifest</emphasis> describing what binaries
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are available. This URL should correspond to the Nix Packages release
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that you're using. For instance, if you obtained a release from
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<ulink
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url='http://catamaran.labs.cs.uu.nl/dist/nix/nixpkgs-0.6pre1554/' />,
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then you should do:
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<para>Upgrading packages is easy. Given a Nix expression that
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contains newer versions of installed packages (that is, packages with
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the same package name, but a higher version number), <command>nix-env
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-u</command> will replace the installed package in the user
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environment with the newer package. For example,
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<screen>
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$ nix-env -uf pkgs/system/i686-linux.nix pan</screen>
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$ nix-pull http://catamaran.labs.cs.uu.nl/dist/nix/nixpkgs-0.6pre1554/MANIFEST</screen>
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If you then issue the installation command, it should start
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downloading binaries from <systemitem
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class='fqdomainname'>catamaran.labs.cs.uu.nl</systemitem>, instead of
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building them from source. This might still take a while since all
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dependencies must be downloaded, but on a reasonably fast connection
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such as an ADSL line it's on the order of a few minutes.</para>
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<para>Naturally, packages can also be uninstalled:
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looks for a newer version of Pan, and installs it if found. Also
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useful is the ability to upgrade <emphasis>all</emphasis> packages:
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<screen>
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$ nix-env -uf pkgs/system/i686-linux.nix '*'</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 -f nixpkgs-<replaceable>version</replaceable> -u subversion</screen>
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This will <emphasis>only</emphasis> upgrade Subversion if there is a
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<quote>newer</quote> version in the new set of Nix expressions, as
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defined by some pretty much arbitrary rules regarding ordering of
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version numbers (which generally do what you'd expect of them). To
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just unconditionally replace Subversion with whatever version is in
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the Nix expressions, use <parameter>-i</parameter> instead of
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<parameter>-u</parameter> — <parameter>-i</parameter> will
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remove whatever version is already installed.</para>
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<para>You can also upgrade all components for which there are newer
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versions:
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<screen>
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$ nix-env -f nixpkgs-<replaceable>version</replaceable> -u '*'</screen>
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</para>
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<para>If you grow tired of specifying the Nix expressions using
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<parameter>-f</parameter> all the time, you can set a default
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location:
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<screen>
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$ nix-env -I nixpkgs-<replaceable>version</replaceable></screen>
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After this you can just say, for instance, <literal>nix-env -u
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'*'</literal>.<footnote><para>Setting a default using
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<parameter>-I</parameter> currently clashes with using Nix channels,
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since <literal>nix-channel --update</literal> calls <literal>nix-env
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-I</literal> to set the default to the Nix expressions it downloaded
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from the channel, replacing whatever default you had
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set.</para></footnote></para>
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</sect1>
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<sect1><title>Profiles</title>
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<para>Bla</para>
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</sect1>
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<sect1><title>Garbage collection</title>
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<para>Bla</para>
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</sect1>
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<sect1><title>Channels</title>
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<para>Bla</para>
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</sect1>
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The asterisk matches all installed packages<footnote><para>No, we
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don't support arbitrary regular expressions</para></footnote>. Note
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that <literal>*</literal> must be quoted to prevent shell
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globbing.</para>
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</chapter>
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