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229 lines
No EOL
10 KiB
XML
<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>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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<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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...</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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<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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<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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...
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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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...</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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<para>Naturally, packages can also be uninstalled:</para>
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<screen>
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$ nix-env -e pan</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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<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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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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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> |