<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>Writing an output filter in Bash</nadpis>
	<perex>processing relational data in GNU Bash or some other shell</perex>
	<m:pořadí-příkladu>00600</m:pořadí-příkladu>

	<text xmlns="http://www.w3.org/1999/xhtml">
		
		<p>
			In previous example we created an output filter in Perl. 
			We converted a relation to values separated by <code>\0</code> and then passed it through <code>xargs</code> to a perl <em>one-liner</em> (or a <em>multi-liner</em> in this case).
			But we can write such output filter in pure Bash without <code>xargs</code> and <code>perl</code>.
			Of course, it is still limited to a single relation (or it can process multiple relations of same type and do something like implicit <code>UNION ALL</code>).
		</p>
		
		<p>
			We will define a function that will help us with reading the <code>\0</code>-separated values and putting them into shell variables:
		</p>
		
		<m:pre jazyk="bash"><![CDATA[read_nullbyte() { local IFS=; for v in "$@"; do export "$v"; read -r -d '' "$v"; done }]]></m:pre>
		
		<!--
			This version will not require the last \0:
				read_zero() { for v in "$@"; do export "$v"; read -r -d '' "$v" || [ ! -z "${!v}" ]; done }
			at least in case when the last value is not missing.
			Other values might be null/missing: \0\0 is OK.
		-->
		
		<p>
			Currently, there is no known way how to do this without a custom function (just with <code>read</code> built-in command of Bash and its parameters).
			But it is just a single line function, so not a big deal.
		</p>
		
		<p>
			And then we just read the values, put them in shell variables and process them in a cycle in a shell block of code:
		</p>
		
		<m:pre jazyk="bash"><![CDATA[relpipe-in-fstab \
	| relpipe-out-nullbyte \
	| while read_nullbyte scheme device mount_point fs_type options dump pass; do
		echo "Device ${scheme:+$scheme=}$device is mounted" \
		     "at $mount_point and contains $fs_type.";
	done]]></m:pre>
	
		<p>
			Which will print:
		</p>
		
		<pre><![CDATA[Device UUID=29758270-fd25-4a6c-a7bb-9a18302816af is mounted at / and contains ext4.
Device /dev/sr0 is mounted at /media/cdrom0 and contains udf,iso9660.
Device /dev/sde is mounted at /mnt/data and contains ext4.
Device UUID=a2b5f230-a795-4f6f-a39b-9b57686c86d5 is mounted at /home and contains btrfs.
Device /dev/mapper/sdf_crypt is mounted at /mnt/private and contains xfs.]]></pre>

		<p>
			Using this method, we can convert any single relation to any format (preferably some text one, but <code>printf</code> can produce also binary data).
			This is good for ad-hoc conversions and single-relation data.
			More powerful tools can be written in C++ and other languages like Java, Python, Guile etc. (when particular libraries are available).
		</p>
		
	</text>

</stránka>