41 		

    42 		<h3>/etc/fstab formatting using -in-fstab, -out-nullbyte, xargs and Perl</h3>

    43 		

    44 		<p>

    45 			As we have seen before, we can convert <code>/etc/fstab</code> (or <code>mtab</code>)

    46 			to e.g. an XML or a nice and colorful table using <m:name/>.

    47 			But we can also convert these data back to the <code>fstab</code> format. And do it with proper indentation/padding.

    48 			Fstab has a simple format where values are separated by one or more whitespace characters.

    49 			But without proper indentation, these files look a bit obfuscated and hard to read (however, they are valid).

    50 		</p>

    51 		

    52 		<m:pre jazyk="text" src="examples/relpipe-out-fstab.txt"/>

    53 		

    54 		<p>

    55 			So let's build a pipeline that reformats the <code>fstab</code> and makes it more readable.

    56 		</p>

    57 			

    58 		<m:pre jazyk="bash">relpipe-in-fstab | relpipe-out-fstab &gt; reformatted-fstab.txt</m:pre>

    59 			

    60 		<p>

    61 			We can hack together a script called <code>relpipe-out-fstab</code> that accepts relational data and produces <code>fstab</code> data.

    62 			Later this will be probably implemented as a regular tool, but for now, it is just an example of a ad-hoc shell script:

    63 		</p>

    64 		

    65 		<m:pre jazyk="bash" src="examples/relpipe-out-fstab.sh" odkaz="ano"/>

    66 		

    67 		<p>

    68 			In the first part, we prepend a single record (<code>relpipe-in-cli</code>) before the data coming from STDIN (<code>cat</code>).

    69 			Then, we use <code>relpipe-out-nullbyte</code> to convert relational data to values separated by a null-byte.

    70 			This command processes only attribute values (relation and attribute names are skipped).

    71 			Then we used <code>xargs</code> to read the null-separated values and execute a Perl command for each record (pass to it a same number of arguments, as we have attributes: <code>--max-args=7</code>).

    72 			Perl does the actual formatting: adds padding and does some little tunning (merges two attributes and replaces empty values with <em>none</em>).

    73 		</p>

    74 		

    75 		<p>This is formatted version of the <code>fstab</code> above:</p>

    76 		

    77 		<m:pre jazyk="text" src="examples/relpipe-out-fstab.formatted.txt"/>

    78 		

    79 		<p>

    80 			And using following command we can verify, that the files differ only in comments and whitespace:

    81 		</p>

    82 		

    83 		<pre>relpipe-in-fstab | relpipe-out-fstab | diff -w /etc/fstab -</pre>

    84 		

    85 		<p>

    86 			Regular implementation of <code>relpipe-out-fstab</code> will probably keep the comments

    87 			(it needs also one more attribute and small change in <code>relpipe-in-fstab</code>).

    88 		</p>

    89 		

    90 		<p>

    91 			For just mere <code>fstab</code> reformatting, this approach is a bit overengineering.

    92 			Wo could skip the whole relational thing and do just something like this:

    93 		</p>

    94 		

    95 		<m:pre jazyk="bash">cat /etc/fstab | grep -v '^#' | sed -E 's/\s+/\n/g' | tr \\n \\0 | xargs -0 -n7 ...</m:pre>

    96 		

    97 		<p>

    98 			plus prepend the comment (or do everything in Perl).

    99 			But this example is intended as a demostration, how we can

   100 			1) prepend some additional data before the data from STDIN

   101 			2) use <m:name/> and traditional tools like <code>xargs</code> or <code>perl</code> together.

   102 			And BTW we have implemented a (simple but working) <em>relpipe output filter</em> – and did it without any serious programming, just put some existing commands together :-)

   103 		</p>

   104 		

   105 		<blockquote>

   106 			<p>

   107 				There is more Unix-nature in one line of shell script than there is in ten thousand lines of C.

   108 				<m:podČarou>see <a href="http://www.catb.org/~esr/writings/unix-koans/ten-thousand.html">Master Foo and the Ten Thousand Lines</a></m:podČarou>

   109 			</p>

   110 		</blockquote>

   111