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	xmlns="https://trac.frantovo.cz/xml-web-generator/wiki/xmlns/strana"
	xmlns:m="https://trac.frantovo.cz/xml-web-generator/wiki/xmlns/makro">
	
	<nadpis>Formatting fstab</nadpis>
	<perex>implementing a simple relpipe-out-fstab filter using -in-fstab, -out-nullbyte, xargs and Perl</perex>
	<m:pořadí-příkladu>00300</m:pořadí-příkladu>

	<text xmlns="http://www.w3.org/1999/xhtml">
	
		<p>
			As we have seen before, we can convert <code>/etc/fstab</code> (or <code>mtab</code>)
			to e.g. an XML or a nice and colorful table using <m:name/>.
			But we can also convert these data back to the <code>fstab</code> format. And do it with proper indentation/padding.
			Fstab has a simple format where values are separated by one or more whitespace characters.
			But without proper indentation, these files look a bit obfuscated and hard to read (however, they are valid).
		</p>
		
		<m:pre jazyk="text" src="examples/relpipe-out-fstab.txt"/>
		
		<p>
			So let's build a pipeline that reformats the <code>fstab</code> and makes it more readable.
		</p>
			
		<m:pre jazyk="bash">relpipe-in-fstab | relpipe-out-fstab &gt; reformatted-fstab.txt</m:pre>
			
		<p>
			We can hack together a script called <code>relpipe-out-fstab</code> that accepts relational data and produces <code>fstab</code> data.
			Later this will be probably implemented as a regular tool, but for now, it is just an example of a ad-hoc shell script:
		</p>
		
		<m:pre jazyk="bash" src="examples/relpipe-out-fstab.sh" odkaz="ano"/>
		
		<p>
			In the first part, we prepend a single record (<code>relpipe-in-cli</code>) before the data coming from STDIN (<code>cat</code>).
			Then, we use <code>relpipe-out-nullbyte</code> to convert relational data to values separated by a null-byte.
			This command processes only attribute values (skips relation and attribute names).
			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>).
			Perl does the actual formatting: adds padding and does some little tunning (merges two attributes and replaces empty values with <em>none</em>).
		</p>
		
		<p>This is formatted version of the <code>fstab</code> above:</p>
		
		<m:pre jazyk="text" src="examples/relpipe-out-fstab.formatted.txt"/>
		
		<p>
			And using following command we can verify, that the files differ only in comments and whitespace:
		</p>
		
		<pre>relpipe-in-fstab | relpipe-out-fstab | diff -w /etc/fstab -</pre>

		<p>
			Another check (should print same hashes):
		</p>
		
		<pre><![CDATA[relpipe-in-fstab | sha512sum 
relpipe-in-fstab | relpipe-out-fstab | relpipe-in-fstab | sha512sum]]></pre>
		
		<p>
			Regular implementation of <code>relpipe-out-fstab</code> will probably keep the comments
			(it needs also one more attribute and small change in <code>relpipe-in-fstab</code>).
		</p>
		
		<p>
			For just mere <code>fstab</code> reformatting, this approach is a bit overengineering.
			We could skip the whole relational thing and do just something like this:
		</p>
		
		<m:pre jazyk="bash">cat /etc/fstab | grep -v '^#' | sed -E 's/\s+/\n/g' | tr \\n \\0 | xargs -0 -n7 ...</m:pre>
		
		<p>
			plus prepend the comment (or do everything in Perl).
			But this example is intended as a demostration, how we can
			1) prepend some additional data before the data from STDIN
			2) use <m:name/> and traditional tools like <code>xargs</code> or <code>perl</code> together.
			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 :-)
		</p>
		
		<blockquote>
			<p>
				There is more Unix-nature in one line of shell script than there is in ten thousand lines of C.
				<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>
			</p>
		</blockquote>
				
	</text>

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