glfs/introduction/important/unpacking.xml

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<sect1 id="intro-important-unpacking">
  <?dbhtml filename="unpacking.html"?>

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  <title>Notes on Building Software</title>

  <para>Those people who have built an LFS system will be aware
  of the general principles of downloading and unpacking software.  We will
  however repeat some of that information here for those new to building
  their own software.</para>

  <para>Each set of installation instructions contains a URL from which you
  can download the package.  We do however keep a selection of patches
  available via HTTP.  These are referenced as needed in the installation
  instructions.</para>

  <para>While you can keep the source files anywhere you like, we assume that
  you have unpacked the package and changed into the directory created by the
  unpacking process (the 'build' directory).  We also assume you have
  uncompressed any required patches and they are in the directory immediately
  above the 'build' directory.</para>

  <para>We can not emphasize strongly enough that you should start from a
  <emphasis>clean source tree</emphasis> each time. This means that if
  you have had an error, it's usually best to delete the source tree and
  re-unpack it <emphasis>before</emphasis> trying again. This obviously
  doesn't apply if you're an advanced user used to hacking
  <filename>Makefile</filename>s and C code, but if in doubt, start from a
  clean tree.</para>

  <sect2>
    <title>Building Software as an Unprivileged (non-root) User</title>

    <para>The golden rule of Unix System Administration is to use your
    superpowers only when necessary. Hence, BLFS recommends that you
    build software as an unprivileged user and only become the
    <systemitem class='username'>root</systemitem> user when installing the
    software. This philosophy is followed in all the packages in this book.
    Unless otherwise specified, all instructions should be executed as an
    unprivileged user. The book will advise you on instructions that need
    <systemitem class='username'>root</systemitem> privileges.</para>

  </sect2>

  <sect2>
    <title>Unpacking the Software</title>

    <para>If a file is in <filename class='extension'>.tar</filename> format
    and compressed, it is unpacked by running one of the following
    commands:</para>

<screen><userinput>tar -xvf filename.tar.gz
tar -xvf filename.tgz
tar -xvf filename.tar.Z
tar -xvf filename.tar.bz2</userinput></screen>

    <note>
      <para>You may omit using the <option>v</option> parameter in the commands
      shown above and below if you wish to suppress the verbose listing of all
      the files in the archive as they are extracted. This can help speed up the
      extraction as well as make any errors produced during the extraction
      more obvious to you.</para>
    </note>

    <para>You can also use a slightly different method:</para>

<screen><userinput>bzcat filename.tar.bz2 | tar -xv</userinput></screen>

    <para>Finally, you sometimes need to be able to unpack patches which are
    generally not in <filename class='extension'>.tar</filename> format. The
    best way to do this is to copy the patch file to parent of the 'build'
    directory and then run one of the following commands depending on whether
    the file is a <filename class='extension'>.gz</filename> or <filename
    class='extension'>.bz2</filename> file:</para>

<screen><userinput>gunzip -v patchname.gz
bunzip2 -v patchname.bz2</userinput></screen>

  </sect2>

  <sect2>
    <title>Verifying File Integrity Using 'md5sum'</title>

    <para>Generally, to verify that the downloaded file is genuine and complete,
    many package maintainers also distribute md5sums of the files. To verify the
    md5sum of the downloaded files, download both the file and the
    corresponding md5sum file to the same directory (preferably from different
    on-line locations), and (assuming <filename>file.md5sum</filename> is the
    md5sum file downloaded) run the following command:</para>

<screen><userinput>md5sum -c file.md5sum</userinput></screen>

    <para>If there are any errors, they will be reported. Note that the BLFS
    book includes md5sums for all the source files also. To use the BLFS
    supplied md5sums, you can create a <filename>file.md5sum</filename> (place
    the md5sum data and the exact name of the downloaded file on the same
    line of a file, separated by white space) and run the command shown above.
    Alternately, simply run the command shown below and compare the output
    to the md5sum data shown in the BLFS book.</para>

<screen><userinput>md5sum <replaceable>[name_of_downloaded_file]</replaceable></userinput></screen>

  </sect2>

  <sect2>
    <title>Creating Log Files During Installation</title>

    <para>For larger packages, it is convenient to create log files instead of
    staring at the screen hoping to catch a particular error or warning. Log
    files are also useful for debugging and keeping records. The following
    command allows you to create an installation log. Replace
    <replaceable>[command]</replaceable> with the command you intend to execute.</para>

<screen><userinput>( <replaceable>[command]</replaceable> 2&gt;&amp;1 | tee compile.log &amp;&amp; exit $PIPESTATUS )</userinput></screen>

    <para><option>2&gt;&amp;1</option> redirects error messages to the same
    location as standard output. The <command>tee</command> command allows
    viewing of the output while logging the results to a file. The parentheses
    around the command run the entire command in a subshell and finally the
    <command>exit $PIPESTATUS</command> command ensures the result of the
    <replaceable>[command]</replaceable> is returned as the result and not the
    result of the <command>tee</command> command.</para>

  </sect2>

</sect1>