<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>Examples</nadpis>
<perex>Usage examples of Relational pipes tools</perex>
<pořadí>40</pořadí>
<text xmlns="http://www.w3.org/1999/xhtml">
<h3>relpipe-in-cli: Hello Wordl!</h3>
<p>
Let's start with and obligatory Hello World example.
</p>
<m:pre jazyk="bash"><![CDATA[relpipe-in-cli generate "relation_from_cli" 3 \
"a" "integer" \
"b" "string" \
"c" "boolean" \
"1" "Hello" "true" \
"2" "World!" "false"]]></m:pre>
<p>
This command generates relational data.
In order to see them, we need to convert them to some other format.
For now, we will use the "tabular" format and pipe relational data to the <code>relpipe-out-tabular</code>.
</p>
<m:pre jazyk="bash"><![CDATA[relpipe-in-cli generate "relation_from_cli" 3 \
"a" "integer" \
"b" "string" \
"c" "boolean" \
"1" "Hello" "true" \
"2" "World!" "false" \
| relpipe-out-tabular]]></m:pre>
<p>Output:</p>
<pre><![CDATA[relation_from_cli:
╭─────────────┬────────────┬─────────────╮
│ a (integer) │ b (string) │ c (boolean) │
├─────────────┼────────────┼─────────────┤
│ 1 │ Hello │ true │
│ 2 │ World! │ false │
╰─────────────┴────────────┴─────────────╯
Record count: 2
]]></pre>
<p>
The syntax is simple as we see above. We specify the name of the relation, number of attributes,
and then their definitions (names and types),
followed by the data.
</p>
<h3>relpipe-in-cli: STDIN</h3>
<p>
The number of CLI arguments is limited and their are passed at once to the process.
So there is option to pass the values from STDIN instead of CLI arguments.
Values on STDIN are expected to be separated by the null-byte.
We can generate such data e.g. using <code>echo</code> and <code>tr</code> (or using <code>printf</code> or other commands):
</p>
<m:pre jazyk="bash"><![CDATA[echo -e "1\nHello\ntrue\n2\nWorld\nfalse" \
| tr \\n \\0 \
| relpipe-in-cli generate-from-stdin relation_from_stdin 3 \
a integer \
b string \
c boolean \
| relpipe-out-tabular]]></m:pre>
<p>
The output is same as above.
We can use this approach to convert various formats to relational data.
There are lot of data already in the form of null-separated values e.g. the process arguments:
</p>
<m:pre jazyk="bash"><![CDATA[cat /proc/$(pidof mc)/cmdline \
| relpipe-in-cli generate-from-stdin mc_args 1 a string \
| relpipe-out-tabular
]]></m:pre>
<p>If we have <code>mc /etc/ /tmp/</code> running in some other terminal, the output will be:</p>
<pre><![CDATA[mc_args:
╭────────────╮
│ a (string) │
├────────────┤
│ mc │
│ /etc/ │
│ /tmp/ │
╰────────────╯
Record count: 3]]></pre>
<p>
Also the <code>find</code> command can produce data separated by the null-byte:
</p>
<m:pre jazyk="bash"><![CDATA[find /etc/ -name '*ssh*_*' -print0 \
| relpipe-in-cli generate-from-stdin files 1 file_name string \
| relpipe-out-tabular]]></m:pre>
<p>Will display something like this:</p>
<pre><![CDATA[files:
╭───────────────────────────────────╮
│ file_name (string) │
├───────────────────────────────────┤
│ /etc/ssh/ssh_host_ecdsa_key │
│ /etc/ssh/sshd_config │
│ /etc/ssh/ssh_host_ed25519_key.pub │
│ /etc/ssh/ssh_host_ecdsa_key.pub │
│ /etc/ssh/ssh_host_rsa_key │
│ /etc/ssh/ssh_config │
│ /etc/ssh/ssh_host_ed25519_key │
│ /etc/ssh/ssh_import_id │
│ /etc/ssh/ssh_host_rsa_key.pub │
╰───────────────────────────────────╯
Record count: 9]]></pre>
<h3>relpipe-in-fstab</h3>
<p>
Using command <code>relpipe-in-fstab</code> we can convert the <code>/etc/fstab</code> or <code>/etc/mtab</code> to relational data
</p>
<m:pre jazyk="bash"><![CDATA[relpipe-in-fstab | relpipe-out-tabular]]></m:pre>
<p>
and see them as a nice table:
</p>
<pre><![CDATA[fstab:
╭─────────────────┬──────────────────────────────────────┬──────────────────────┬───────────────┬───────────────────────────────────────┬────────────────┬────────────────╮
│ scheme (string) │ device (string) │ mount_point (string) │ type (string) │ options (string) │ dump (integer) │ pass (integer) │
├─────────────────┼──────────────────────────────────────┼──────────────────────┼───────────────┼───────────────────────────────────────┼────────────────┼────────────────┤
│ UUID │ 29758270-fd25-4a6c-a7bb-9a18302816af │ / │ ext4 │ relatime,user_xattr,errors=remount-ro │ 0 │ 1 │
│ │ /dev/sr0 │ /media/cdrom0 │ udf,iso9660 │ user,noauto │ 0 │ 0 │
│ │ /dev/sde │ /mnt/data │ ext4 │ relatime,user_xattr,errors=remount-ro │ 0 │ 2 │
│ UUID │ a2b5f230-a795-4f6f-a39b-9b57686c86d5 │ /home │ btrfs │ relatime │ 0 │ 2 │
│ │ /dev/mapper/sdf_crypt │ /mnt/private │ xfs │ relatime │ 0 │ 2 │
╰─────────────────┴──────────────────────────────────────┴──────────────────────┴───────────────┴───────────────────────────────────────┴────────────────┴────────────────╯
Record count: 5]]></pre>
<p>And we can do the same also with a remote <code>fstab</code> or <code>mtab</code>; just by adding <code>ssh</code> to the pipeline:</p>
<m:pre jazyk="bash"><![CDATA[ssh example.com cat /etc/mtab | relpipe-in-fstab | relpipe-out-tabular]]></m:pre>
<p>
The <code>cat</code> runs remotely. The <code>relpipe-in-fstab</code> and <code>relpipe-out-tabular</code> run on our machine.
</p>
<p>
n.b. the <code>relpipe-in-fstab</code> reads the <code>/etc/fstab</code> if executed on TTY. Otherwise, it reads the STDIN.
</p>
<h3>relpipe-out-xml</h3>
<p>
Relational data can be converted to various formats and one of them is the XML.
This is a good option for further processing e.g. using XSLT transformation or passing the XML data to some other tool.
Just use <code>relpipe-out-xml</code> instead of <code>relpipe-out-tabular</code> and the rest of the pipeline remains unchanged:
</p>
<m:pre jazyk="bash"><![CDATA[ssh example.com cat /etc/mtab | relpipe-in-fstab | relpipe-out-xml]]></m:pre>
<p>
Will produce XML like this:
</p>
<m:pre jazyk="xml"><![CDATA[<?xml version="1.0" encoding="UTF-8"?>
<pipe>
<relation>
<name>fstab</name>
<record>
<attribute>UUID</attribute>
<attribute>29758270-fd25-4a6c-a7bb-9a18302816af</attribute>
<attribute>/</attribute>
<attribute>ext4</attribute>
<attribute>relatime,user_xattr,errors=remount-ro</attribute>
<attribute>0</attribute>
<attribute>1</attribute>
</record>
<record>
<attribute></attribute>
<attribute>/dev/sr0</attribute>
<attribute>/media/cdrom0</attribute>
<attribute>udf,iso9660</attribute>
<attribute>user,noauto</attribute>
<attribute>0</attribute>
<attribute>0</attribute>
</record>
<record>
<attribute></attribute>
<attribute>/dev/sde</attribute>
<attribute>/mnt/data</attribute>
<attribute>ext4</attribute>
<attribute>relatime,user_xattr,errors=remount-ro</attribute>
<attribute>0</attribute>
<attribute>2</attribute>
</record>
<record>
<attribute>UUID</attribute>
<attribute>a2b5f230-a795-4f6f-a39b-9b57686c86d5</attribute>
<attribute>/home</attribute>
<attribute>btrfs</attribute>
<attribute>relatime</attribute>
<attribute>0</attribute>
<attribute>2</attribute>
</record>
<record>
<attribute></attribute>
<attribute>/dev/mapper/sdf_crypt</attribute>
<attribute>/mnt/private</attribute>
<attribute>xfs</attribute>
<attribute>relatime</attribute>
<attribute>0</attribute>
<attribute>2</attribute>
</record>
</relation>
</pipe>]]></m:pre>
<p>
Thanks to XSLT, this XML can be easily converted e.g. to an XHTML table (<code>table|tr|td</code>) or other format.
Someone can convert such data to a (La)TeX table.
</p>
<p>
n.b. the format is not final and will change i future versions (XML namespace, more metadata etc.).
</p>
<h3>relpipe-tr-validator</h3>
<p>
Just a passthrough command, so these pipelines should produce the same hash:
</p>
<m:pre jazyk="bash"><![CDATA[
relpipe-in-fstab | relpipe-tr-validator | sha512sum
relpipe-in-fstab | sha512sum]]></m:pre>
<p>
This tool can be used for testing whether a file contains valid relational data:
</p>
<m:pre jazyk="bash"><![CDATA[
if relpipe-tr-validator < "some-file.rp" &> /dev/null; then
echo "valid relational data";
else
echo "garbage";
fi]]></m:pre>
<p>or as a one-liner:</p>
<m:pre jazyk="bash"><![CDATA[relpipe-tr-validator < "some-file.rp" &> /dev/null && echo "ok" || echo "error"]]></m:pre>
<p>
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 (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>
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>
</stránka>