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<h1 class="SECT1"><a id="ARCHIVE-FILES-TAR" name="ARCHIVE-FILES-TAR">15.3 <tt
class="COMMAND">tar</tt></a></h1>
<p><tt class="COMMAND">tar</tt>(1) is the GNU tape archiver. It takes several files or
directories and creates one large file. This allows you to compress an entire directory
tree, which is impossible by just using <tt class="COMMAND">gzip</tt> or <tt
class="COMMAND">bzip2</tt>. <tt class="COMMAND">tar</tt> has many command line options,
which are explained in its man page. This section will just cover the most common uses of
<tt class="COMMAND">tar</tt>.</p>
<p>The most common use for <tt class="COMMAND">tar</tt> is to decompress and unarchive a
package that you've downloaded from a web site or ftp site. Most files will come with a
<tt class="FILENAME">.tar.gz</tt> extension. This is commonly known as a
“tarball”. It means that several files were archived using <tt
class="COMMAND">tar</tt> and then compressed using <tt class="COMMAND">gzip</tt>. You
might also see this listed as a <tt class="FILENAME">.tar.Z</tt> file. It means the same
thing, but this is usually encountered on older Unix systems.</p>
<p>Alternatively, you might find a <tt class="FILENAME">.tar.bz2</tt> file somewhere.
Kernel source is distributed as such because it is a smaller download. As you might have
guessed, this is several files archived with <tt class="COMMAND">tar</tt> and then
bzipped.</p>
<p>You can get to all the files in this archive by making use of <tt
class="COMMAND">tar</tt> and some command line arguments. Unarchiving a tarball makes use
of the <var class="OPTION">-z</var> flag, which means to first run the file through <tt
class="COMMAND">gunzip</tt> and decompress it. The most common way to decompress a
tarball is like so:</p>
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<pre class="SCREEN">
<samp class="PROMPT">%</samp> <kbd class="USERINPUT">tar -xvzf filename.tar.gz</kbd>
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<p>That's quite a few options. So what do they all mean? The <var class="OPTION">-x</var>
means to extract. This is important, as it tells <tt class="COMMAND">tar</tt> exactly
what to do with the input file. In this case, we'll be splitting it back up into all the
files that it came from. <var class="OPTION">-v</var> means to be verbose. This will list
all the files that are being unarchived. It is perfectly acceptable to leave this option
off, if somewhat boring. Alternatively, you could use <var class="OPTION">-vv</var> to be
very verbose and list even more information about each file being unarchived. The <var
class="OPTION">-z</var> option tells <tt class="COMMAND">tar</tt> to run <tt
class="FILENAME">filename.tar.gz</tt> through <tt class="COMMAND">gunzip</tt> first. And
finally, the <var class="OPTION">-f</var> option tells <tt class="COMMAND">tar</tt> that
the next string on the command line is the file to operate on.</p>
<p>There are a few other ways to write this same command. On older systems lacking a
decent copy of GNU <tt class="COMMAND">tar</tt>, you might see it written like so:</p>
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<samp class="PROMPT">%</samp> <kbd
class="USERINPUT">gunzip filename.tar.gz | tar -xvf -</kbd>
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<p>This command line will uncompress the file and send the output to <tt
class="COMMAND">tar</tt>. Since <tt class="COMMAND">gzip</tt> will write its output to
standard out if told to do so, this command will write the decompressed file to standard
out. The pipe then sends it to <tt class="COMMAND">tar</tt> for unarchiving. The
“-” means to operate on standard input. It will unarchive the stream of data
that it gets from <tt class="COMMAND">gzip</tt> and write that to the disk.</p>
<p>Another way to write the first command line is to leave off the dash before the
options, like so:</p>
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<samp class="PROMPT">%</samp> <kbd class="USERINPUT">tar xvzf filename.tar.gz</kbd>
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<p>You might also encounter a bzipped archive. The version of <tt
class="COMMAND">tar</tt> that comes with Slackware Linux can handle these the same as
gzipped archives. Instead of the <var class="OPTION">-z</var> command line option, you'd
use <var class="OPTION">-j</var>:</p>
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<samp class="PROMPT">%</samp> <kbd class="USERINPUT">tar -xvjf filename.tar.bz2</kbd>
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<p>It is important to note that <tt class="COMMAND">tar</tt> will place the unarchived
files in the current directory. So, if you had an archive in <tt
class="FILENAME">/tmp</tt> that you wanted to decompress into your home directory, there
are a few options. First, the archive could be moved into your home directory and then
run through <tt class="COMMAND">tar</tt>. Second, you could specify the path to the
archive file on the command line. Third, you can use the <var class="OPTION">-C</var>
option to “explode” the tarball in a specified directory.</p>
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<samp class="PROMPT">%</samp> <kbd class="USERINPUT">cd $HOME</kbd>
<samp class="PROMPT">%</samp> <kbd class="USERINPUT">cp /tmp/filename.tar.gz .</kbd>
<samp class="PROMPT">%</samp> <kbd class="USERINPUT">tar -xvzf filename.tar.gz</kbd>
<samp class="PROMPT">%</samp> <kbd class="USERINPUT">cd $HOME</kbd>
<samp class="PROMPT">%</samp> <kbd class="USERINPUT">tar -xvzf /tmp/filename.tar.gz</kbd>
<samp class="PROMPT">%</samp> <kbd class="USERINPUT">cd /</kbd>
<samp class="PROMPT">%</samp> <kbd
class="USERINPUT">tar -xvzf /tmp/filename.tar.gz -C $HOME</kbd>
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<p>All the above statements are equivalent. In each case, the archive is unpacked inside
your home directory and the original uncompressed archive is left in place.</p>
<p>So what good is being able to uncompress these archives if you can't make them? Well,
<tt class="COMMAND">tar</tt> handles that too. In most cases it's as easy as removing the
“<var class="OPTION">-x</var>” option and replacing it with the “<var
class="OPTION">-c</var>” option.</p>
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<samp class="PROMPT">%</samp> <kbd class="USERINPUT">tar -cvzf filename.tar.gz .</kbd>
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<p>In this command line, the <var class="OPTION">-c</var> option tells <tt
class="COMMAND">tar</tt> to create an archive, while the <var class="OPTION">-z</var>
option runs the resulting archive file through <tt class="COMMAND">gzip</tt> to compress
it. <tt class="FILENAME">filename.tar.gz</tt> is the file that you want to create.</p>
<p>Specifying the “<var class="OPTION">-f</var>” option isn't always
necessary, but is typically good practice anyway. Without it, <tt
class="COMMAND">tar</tt> writes to standard output, which is usually desired for piping
<tt class="COMMAND">tar</tt>'s output to another program, like so.</p>
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<samp class="PROMPT">%</samp> <kbd
class="USERINPUT">tar -cv filename.tar . | gpg --encrypt</kbd>
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<p>That command creates an non-compressed tar archive of the current directory, pipes the
tarball through <tt class="COMMAND">gpg</tt> which encrypts and compresses the tarball,
making it realistically impossible to read by anyone other than the person knowing the
secret key.</p>
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