1 <stránka

     2 	xmlns="https://trac.frantovo.cz/xml-web-generator/wiki/xmlns/strana"

     3 	xmlns:m="https://trac.frantovo.cz/xml-web-generator/wiki/xmlns/makro">

     4 	

     5 	<nadpis>Renaming VG in /etc/fstab using relpipe-tr-sed</nadpis>

     6 	<perex>sed-like substitutions in the relational stream</perex>

     7 	<m:pořadí-příkladu>00700</m:pořadí-příkladu>

     8 

     9 	<text xmlns="http://www.w3.org/1999/xhtml">

    10 		

    11 		<p>

    12 			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.

    13 			If we rename a volume group (VG), we have to change all of them. The lines look like this one:

    14 		</p>

    15 		

    16 		<pre>/dev/alpha/photos    /mnt/photos/    btrfs    noauto,noatime,nodiratime    0  0</pre>

    17 		

    18 		<p>

    19 			We want to change all lines from <code>alpha</code> to <code>beta</code> (the new VG name).

    20 			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:

    21 		</p>

    22 		

    23 		<m:pre jazyk="bash"><![CDATA[relpipe-in-fstab \

    24 	| relpipe-tr-sed 'fstab' 'device' '^/dev/alpha/' '/dev/beta/' \

    25 	| relpipe-out-fstab]]></m:pre>

    26 	

    27 		<p>

    28 			The <code>relpipe-tr-sed</code> tool works only with given relation (<code>fstab</code>) and given attribute (<code>device</code>)

    29 			and it would leave untouched other relations and attributes in the stream.

    30 			So it would not replace the strings on unwanted places (if there are any random matches).

    31 		</p>

    32 		

    33 		<p>

    34 			Even the relation names and attribute names are specified as a regular expression, so we can (purposefully) modify multiple relations or attributes.

    35 			For example we can put zeroes in both <code>dump</code> and <code>pass</code> attributes:

    36 		</p>

    37 		

    38 		<m:pre jazyk="bash"><![CDATA[relpipe-in-fstab | relpipe-tr-sed 'fstab' 'dump|pass' '.+' '0' | relpipe-out-fstab]]></m:pre>

    39 		

    40 		<p>

    41 			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>.

    42 		</p>

    43 		

    44 	</text>

    45 

    46 </stránka>