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The World Is a libferris Filesystem

Issue 146

From Issue #146
June 2006

Apr 28, 2006  By Ben Martin
 in
With libferris, the boundary of your filesystem extends to include PostgreSQL, XML, db4, RDF, the X Window System, Evolution and much more.

Listing 1. A Long Listing of a Directory with Explicit Metadata

$ fls -l \
--show-ea=size-human-readable,width,height,name
4.5k    48      46      emacs.png
1.9k    48      48      gnome-warning.png
3.2k    48      48      gnome-xterm.png
2.5k    48      48      gtkvim.png

Listing 2. Asking libferris itself to determine which EAs are of interest for the current directory and producing an XML document as output. 


$ fls -0 --xml
<ferrisls>
<ferrisls url="file:///tmp/lj"  name="lj"  >
 <context  size-human-readable="4.5k"
  protection-ls="-rw-r-----"
  mtime-display="05 Dec  4 23:39"
  name="emacs.png"  width="48"  height="46"  />
 ...
</ferrisls>
</ferrisls>

As mentioned previously, if you are sorting a directory on an EA that does not provide a complete ordering, you can chain together sorting predicates. For example, in Listing 3, I have sorted the output based on the numeric EA height and then used a version string sort on the name EA. A version sort is similar to the ls(1) -v option, which in Listing 3 has placed foo20.png after foo3.png. Such sorting is very useful when sorting by file type or MIME major type followed by name.

Listing 3. Sorting Your Output 

$ fls --show-ea=width,height,size,name \
  --ferris-sort='(:#:height)(:V:name)'
48      48      1968    gnome-warning.png
48      48      3253    gnome-xterm.png
48      48      2550    gtkvim.png
48      46      4589    emacs.png
48      46      4589    foo3.png
48      46      4589    foo20.png

The two concepts of files forming a tree and files having key-value pairs attached to them are similar to the structure of XML. With libferris, you can poke inside XML documents as though they were just another filesystem. For example, see Listing 4.

Listing 4. Initial Exploration of XML as a Filesystem 


$ cat example.xml
<root>
  <file1 size="200" />
  <file2 interesting="yes" />
  <file3>filesystems rock
</file3>
</root>

$ fls -0 ./example.xml/root
file1
file2
file3

$ fls -d --show-ea=name,interesting \
   ./example.xml/root/file2
file2   yes

$ fcat example.xml/root/file3
filesystems rock

By interacting with your filesystem, you can cause updates on the underlying XML document as well. The ferris-redirect client exists to allow shell-like redirection into libferris files. The -T or --trunc option truncates an existing file before writing stdin into it. This is much like the >| shell option. As you can see from the interaction in Listing 5, we have changed the structure of the example.xml document significantly through filesystem interaction.

Listing 5. Changing an XML File through Its Filesystem 


$ echo "VIRTUAL filesystems rock more" | \
  ferris-redirect -T ./example.xml/root/file3

$ echo "a new way" | \
  ferris-redirect ./example.xml/root/file4

$ ferrisrm ./example.xml/root/file2

$ ftouch ./example.xml/root/touched

$ cat example.xml
<?xml version="1.0" encoding="UTF-8"
   standalone="no" ?>
<root>
  <file1 size="200"/>

  <file3>VIRTUAL filesystems rock more
</file3>
  <file4>a new way
</file4>

  <touched/>

</root>

As many modern word-processing documents are XML inside a compressed container, libferris allows you to drill down into the office document as though it were a filesystem. In Listing 6, I am listing a simple OpenOffice.org Writer document as a filesystem.

Listing 6. OpenOffice.org Documents Are Filesystems Too 

$ fls -lh show-ea=size,name,content \
~/sample-oo-writer.odt/content.xml/ \
office:document-content/office:body/office:text
 0       office:forms
 18      text:p Paragraph number 1
 0       text:p-1
 116     text:p-2 This is the second paragraph ...
 0       text:p-3
 39      text:p-4 And in summary, this is really...
 0       text:p=5
 0       text:sequence-decls

A Xerces-C Document Object Model (DOM) can be obtained for any libferris filesystem, just as a Xerces-C DOM can be mounted as a libferris filesystem. Creation of a DOM for a filesystem is evaluated lazily, so you can get a DOM for file:// and only the parts of the DOM that are required are ever created.

The ability to convert any libferris filesystem into a DOM allows you to apply XSLT to your filesystems easily. The example C++ code in Listing 7 applies a stylesheet to a mounted OpenOffice.org document.

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