--- a/relpipe-data/examples.xml Tue Dec 04 18:43:01 2018 +0100
+++ b/relpipe-data/examples.xml Tue Dec 04 22:33:36 2018 +0100
@@ -38,6 +38,77 @@
If an error is found, it is reported on STDERR. So just omit the <code>&</code> in order to see the error message.
</p>
+
+ <h3>/etc/fstab formatting using -in-fstab, -out-nullbyte, xargs and Perl</h3>
+
+ <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 > 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 (relation and attribute names are skipped).
+ 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>
+ 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.
+ Wo 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>
+
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