source: introduction/important/unpacking.xml@ d5f2a3f

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<sect1 id="intro-important-unpacking">
<sect1info>
  <othername>$LastChangedBy$</othername>
  <date>$Date$</date>
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<?dbhtml filename="unpacking.html"?>
<title>Notes on building software</title>

<para>Those people who have built an <acronym>LFS</acronym> 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 <acronym>URL</acronym> 
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 them and unzipped any required patches
into <filename>/usr/src</filename>.</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 Makefiles and C
code, but if in doubt, start from a clean tree.</para>

<sect2>
<title>Unpacking the software</title>

<para>If a file is tar'ed and compressed, it is unpacked by running one of
the following commands:</para>

<screen><command>tar -xf filename.tar.gz
tar -xf filename.tgz
tar -xf filename.tar.Z
tar -xf filename.tar.bz2</command></screen>

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

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

<para>Finally, you sometimes need to be able to unpack patches which are
generally not tar'ed.  The best way to do this is to copy the patch file to
<filename>/usr/src</filename> and then to run one of the following commands
depending on whether the file is .gz or .bz2:</para>

<screen><command>gunzip patchname.gz
bunzip2 patchname.bz2</command></screen>

</sect2>

<sect2>
<title>Verifying file integrity using md5sum</title>

<para>Generally, to verify that the downloaded file is genuine and complete,
most 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 file.md5sum is the md5sum file downloaded)
run the following command:</para>

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

<para>If there are any errors, they will be reported.</para>

</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 &lt;command&gt; with the
command you intend to execute.</para>

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

<para><parameter>2&gt;&amp;1</parameter> 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> ensures the result of the &lt;command&gt;
is returned as the result and not the result of the <command>tee</command> command.</para>

</sect2>

</sect1>