lfs-book-zh_CN/chapter06/pkgmgt.xml

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<sect1 id="ch-system-pkgmgt">
  <?dbhtml filename="pkgmgt.html"?>

  <title>Package Management</title>

  <para>Package Management is an often requested addition to the LFS Book. A
  Package Manager allows tracking the installation of files making it easy to
  remove and upgrade packages. Before you begin to wonder, NO&mdash;this section
  will not talk about nor recommend any particular package manager. What it
  provides is a roundup of the more popular techniques and how they work. The
  perfect package manager for you may be among these techniques or may be a
  combination of two or more of these techniques. This section briefly mentions
  issues that may arise when upgrading packages.</para>

  <para>Some reasons why no package manager is mentioned in LFS or BLFS
  include:</para>

  <itemizedlist>
    <listitem>
      <para>Dealing with package management takes the focus away from the goals
      of these books&mdash;teaching how a Linux system is built.</para>
    </listitem>

    <listitem>
      <para>There are multiple solutions for package management, each having
      its strengths and drawbacks.  Including one that satisfies all audiences
      is difficult.</para>
    </listitem>
  </itemizedlist>

  <para>There are some hints written on the topic of package management. Visit
  the <ulink url="&hints-root;">Hints subproject</ulink> and see if one of them
  fits your need.</para>

  <sect2>
    <title>Upgrade Issues</title>

    <para>A Package Manager makes it easy to upgrade to newer versions when they
    are released. Generally the instructions in the LFS and BLFS Book can be
    used to upgrade to the newer versions. Here are some points that you should
    be aware of when upgrading packages, especially on a running system.</para>

    <itemizedlist>
      <listitem>
        <para>If one of the toolchain packages (Glibc, GCC or Binutils) needs
        to be upgraded to a newer minor version, it is safer to rebuild LFS.
        Though you <emphasis>may</emphasis> be able to get by rebuilding all
        the packages in their dependency order, we do not recommend it. For
        example, if glibc-2.2.x needs to be updated to glibc-2.3.x, it is safer
        to rebuild. For micro version updates, a simple reinstallation usually
        works, but is not guaranteed. For example, upgrading from glibc-2.3.4
        to glibc-2.3.5 will not usually cause any problems.</para>
      </listitem>

      <listitem>
        <para>If a package containing a shared library is updated, and if the
        name of the library changes, then all the packages dynamically linked
        to the library need to be recompiled to link against the newer library.
        (Note that there is no correlation between the package version and the
        name of the library.) For example, consider a package foo-1.2.3 that
        installs a shared library with name
        <filename class='libraryfile'>libfoo.so.1</filename>. Say you upgrade
        the package to a newer version foo-1.2.4 that installs a shared library
        with name <filename class='libraryfile'>libfoo.so.2</filename>. In this
        case, all packages that are dynamically linked to
        <filename class='libraryfile'>libfoo.so.1</filename> need to be
        recompiled to link against
        <filename class='libraryfile'>libfoo.so.2</filename>. Note that you
        should not remove the previous libraries until the dependent packages
        are recompiled.</para>
      </listitem>

      <listitem>
        <para>If you are upgrading a running system, be on the lookout for
        packages that use <command>cp</command> instead of
        <command>install</command> to install files. The latter command is
        usually safer if the executable or library is already loaded in memory.
        </para>
      </listitem>
    </itemizedlist>

  </sect2>

  <sect2>
    <title>Package Management Techniques</title>

    <para>The following are some common package management techniques. Before
    making a decision on a package manager, do some research on the various
    techniques, particularly the drawbacks of the particular scheme.</para>

    <sect3>
      <title>It is All in My Head!</title>

      <para>Yes, this is a package management technique. Some folks do not find
      the need for a package manager because they know the packages intimately
      and know what files are installed by each package. Some users also do not
      need any package management because they plan on rebuilding the entire
      system when a package is changed.</para>

    </sect3>

    <sect3>
      <title>Install in Separate Directories</title>

      <para>This is a simplistic package management that does not need any extra
      package to manage the installations. Each package is installed in a
      separate directory. For example, package foo-1.1 is installed in
      <filename class='directory'>/usr/pkg/foo-1.1</filename>
      and a symlink is made from <filename>/usr/pkg/foo</filename> to
      <filename class='directory'>/usr/pkg/foo-1.1</filename>. When installing
      a new version foo-1.2, it is installed in
      <filename class='directory'>/usr/pkg/foo-1.2</filename> and the previous
      symlink is replaced by a symlink to the new version.</para>

      <para>Environment variables such as <envar>PATH</envar>,
      <envar>LD_LIBRARY_PATH</envar>, <envar>MANPATH</envar>,
      <envar>INFOPATH</envar> and <envar>CPPFLAGS</envar> need to be expanded to
      include <filename>/usr/pkg/foo</filename>. For more than a few packages,
      this scheme becomes unmanageable.</para>

    </sect3>

    <sect3>
      <title>Symlink Style Package Management</title>

      <para>This is a variation of the previous package management technique.
      Each package is installed similar to the previous scheme. But instead of
      making the symlink, each file is symlinked into the
      <filename class='directory'>/usr</filename> hierarchy. This removes the
      need to expand the environment variables. Though the symlinks can be
      created by the user to automate the creation, many package managers have
      been written using this approach. A few of the popular ones include Stow,
      Epkg, Graft, and Depot.</para>

      <para>The installation needs to be faked, so that the package thinks that
      it is installed in <filename class="directory">/usr</filename> though in
      reality it is installed in the
      <filename class="directory">/usr/pkg</filename> hierarchy. Installing in
      this manner is not usually a trivial task. For example, consider that you
      are installing a package libfoo-1.1. The following instructions may
      not install the package properly:</para>

<screen role="nodump"><userinput>./configure --prefix=/usr/pkg/libfoo/1.1
make
make install</userinput></screen>

      <para>The installation will work, but the dependent packages may not link
      to libfoo as you would expect. If you compile a package that links against
      libfoo, you may notice that it is linked to
      <filename class='libraryfile'>/usr/pkg/libfoo/1.1/lib/libfoo.so.1</filename>
      instead of <filename class='libraryfile'>/usr/lib/libfoo.so.1</filename>
      as you would expect. The correct approach is to use the
      <envar>DESTDIR</envar> strategy to fake installation of the package. This
      approach works as follows:</para>

<screen role="nodump"><userinput>./configure --prefix=/usr
make
make DESTDIR=/usr/pkg/libfoo/1.1 install</userinput></screen>

      <para>Most packages support this approach, but there are some which do not.
      For the non-compliant packages, you may either need to manually install the
      package, or you may find that it is easier to install some problematic
      packages into <filename class='directory'>/opt</filename>.</para>

    </sect3>

    <sect3>
      <title>Timestamp Based</title>

      <para>In this technique, a file is timestamped before the installation of
      the package. After the installation, a simple use of the
      <command>find</command> command with the appropriate options can generate
      a log of all the files installed after the timestamp file was created. A
      package manager written with this approach is install-log.</para>

      <para>Though this scheme has the advantage of being simple, it has two
      drawbacks. If, during installation, the files are installed with any
      timestamp other than the current time, those files will not be tracked by
      the package manager. Also, this scheme can only be used when one package
      is installed at a time. The logs are not reliable if two packages are
      being installed on two different consoles.</para>

    </sect3>

    <sect3>
      <title>LD_PRELOAD Based</title>

      <para>In this approach, a library is preloaded before installation. During
      installation, this library tracks the packages that are being installed by
      attaching itself to various executables such as <command>cp</command>,
      <command>install</command>, <command>mv</command> and tracking the system
      calls that modify the filesystem. For this approach to work, all the
      executables need to be dynamically linked without the suid or sgid bit.
      Preloading the library may cause some unwanted side-effects during
      installation. Therefore, it is advised that one performs some tests to
      ensure that the package manager does not break anything and logs all the
      appropriate files.</para>

    </sect3>

    <sect3>
      <title>Creating Package Archives</title>

      <para>In this scheme, the package installation is faked into a separate
      tree as described in the Symlink style package management. After the
      installation, a package archive is created using the installed files.
      This archive is then used to install the package either on the local
      machine or can even be used to install the package on other machines.</para>

      <para>This approach is used by most of the package managers found in the
      commercial distributions. Examples of package managers that follow this
      approach are RPM (which, incidentally, is required by the <ulink
      url="http://lsbbook.gforge.freestandards.org/package.html#RPM">Linux
      Standard Base Specification</ulink>), pkg-utils, Debian's apt, and
      Gentoo's Portage system.  A hint describing how to adopt this style of
      package management for LFS systems is located at <ulink
      url="&hints-root;/fakeroot.txt"/>.</para>

    </sect3>

    <sect3>
      <title>User Based Management</title>

      <para>This scheme, unique to LFS, was devised by Matthias Benkmann, and is
      available from the <ulink url="&hints-root;">Hints Project</ulink>. In
      this scheme, each package is installed as a separate user into the
      standard locations. Files belonging to a package are easily identified by
      checking the user ID. The features and shortcomings of this approach are
      too complex to describe in this section. For the details please see the
      hint at <ulink url="&hints-root;/more_control_and_pkg_man.txt"/>.</para>

    </sect3>

  </sect2>

</sect1>