<stránka
	xmlns="https://trac.frantovo.cz/xml-web-generator/wiki/xmlns/strana"
	xmlns:m="https://trac.frantovo.cz/xml-web-generator/wiki/xmlns/makro">
	
	<nadpis>Renaming VG in /etc/fstab using relpipe-tr-sed</nadpis>
	<perex>sed-like substitutions in the relational stream</perex>
	<m:pořadí-příkladu>00700</m:pořadí-příkladu>

	<text xmlns="http://www.w3.org/1999/xhtml">
		
		<p>
			Assume that we have an <code>/etc/fstab</code> with many lines defining the mount-points (directories) of particular devices (disks) and we are using LVM.
			If we rename a volume group (VG), we have to change all of them. The lines look like this one:
		</p>
		
		<pre>/dev/alpha/photos    /mnt/photos/    btrfs    noauto,noatime,nodiratime    0  0</pre>
		
		<p>
			We want to change all lines from <code>alpha</code> to <code>beta</code> (the new VG name).
			This can be done by the power of regular expressions<m:podČarou>see <a href="https://en.wikibooks.org/wiki/Regular_Expressions/Simple_Regular_Expressions">Regular Expressions</a> at Wikibooks</m:podČarou> and this pipeline:
		</p>
		
		<m:pre jazyk="bash"><![CDATA[relpipe-in-fstab \
	| relpipe-tr-sed \
		--relation 'fstab' \
			--attribute 'device' \
			--value '^/dev/alpha/' \
			--replacement '/dev/beta/' \
	| relpipe-out-fstab]]></m:pre>
	
		<p>
			The <code>relpipe-tr-sed</code> tool works only with given relation (<code>fstab</code>) and given attribute (<code>device</code>)
			and it would leave untouched other relations and attributes in the stream.
			So it would not replace the strings on unwanted places (if there are any random matches).
		</p>
		
		<p>
			Even the relation names and attribute names are specified as a regular expression, so we can (purposefully) modify multiple relations or attributes.
			For example we can put zeroes in both <code>dump</code> and <code>pass</code> attributes:
		</p>
		
		<m:pre jazyk="bash"><![CDATA[relpipe-in-fstab \
	| relpipe-tr-sed \
		--relation 'fstab' \
			--attribute 'dump|pass' \
			--value '.+' \
			--replacement '0' \
	| relpipe-out-fstab]]></m:pre>
		
		<p>
			n.b. the data types must be respected, we can not e.g. put <code>abc</code> in the <code>pass</code> attribute because it is declared as <code>integer</code>.
		</p>
		
	</text>

</stránka>