A web shell is a
program that combines the functionality of a WWW
browser, like Netscape or Internet Explorer, with
the features of a shell (e.g., a typical Unix shell
or a Telnet terminal). It acts as a client to web
servers, requesting files from them. It
deals with all different kinds of files that
can be transferred from a web server. It
however handles the contents of web documents
only in a limited way. For example, it
cannot parse arbitrary HTML tags or display an HTML
document in a way familiar to you from a
browser. Instead, it treats an HTML document as a
directory (aka folder) that may contain
documents. In other words, the web shell parses
documents for only HTML tags that reference
documents, such as the anchor (syntax:
<a
href="some.url">), a form's
action
parameter (syntax: <form
action="some.url">) or images (syntax:
<img
src="some.url">). Every
document that is linked from an HTML page is said to
be "contained in the HTML page's folder".
Documents
A web shell allows for traversal of the tree defined
by this relationship (back-links are ignored),
simple operations on the documents, and some
miscellaneous functions.
A directory is either a real
directory (http://www.cs.ucsb.edu/~cappello/) or
an HTML page. Its contents are defined as all
files that the web server returns on a request for
this URL (possibly empty).
Any other document is called plain.
The User Interface
Write a Java application that implements the
functions described below. The input
from the user has to be read from a text interface
("terminal") that strongly resembles that of a Unix
shell or Telnet terminal. A prompt (e.g., ">"
asks for the next command or sequence of
commands. After processing the command[s],
another prompt is displayed. The output
depends on the display. One display makes use of the
terminal that also works as the input
device. Terminal output, displayed before the next
prompt, is that part that is described before the
"/"; the other display is a graphical
representation of the document structure. That
output is described after the
"/". Hint: The Java Swing package makes
it easy to implement a visualization of both the
tree structure of interlinked documents and the
lists of documents - you have to display both. For
an example, look at the WindowsExplorer.
Web Shell Commands
These are the particular commands that can be used
in the web shell:
-
cd [name] (change directory)
The current document is set to name,
and the complete URL of the current document is
displayed. If name equals ".."
(two dots), the new current document is
set to the current document's (known)
parent. If there is more than one parent,
choose the logical parent or the first parent in
alphabetical order. The logical parent of
a document with the URL
http://www.cs.ucsb.edu/~cappello/172/ is
either
http://www.cs.ucsb.edu/~cappello/
or
http://www.cs.ucsb.edu/~cappello/index.htm(l),
whichever exists.
If no name is
provided, the current document stays unchanged.
OUTPUT: absolute path of new
current document / highlighting of new current
document.
-
ls [name]
The contents of the named document are
shown: all documents that are linked from
the current document. Special tags or other
means identify different types of documents
(directory, plain, or even more detailed). If
no name is given, the current document's
contents are listed.
The output list can
be sorted by any argument. This is specified in
the environment variable SORT.
OUTPUT: list of documents as text /
graphical representation of documents
-
info [name]
Display information about the document, such as
the absolute URL, its type and other attributes
if available.
OUTPUT: information
as text / popup window with information
represented appropriately.
-
open [name]
Spawn an external application to properly
interpret the contents of this document. The
helper application (aka viewer) could be a web
browser for HTML pages, GhostScript for
postscript documents, an MP3 player for sound
documents, etc. Provide the functionality
to open HTML pages and two other document types
of your choice.
OUTPUT: nothing -
return to prompt as soon as possible / nothing
-
set [variable [value]]
This command assigns an arbitrary value to an
environment variable. Those variables can be
either pre-defined (e.g., SORT, GUI) or
user-defined. A variable must not contain
whitespace. If no arguments are given, all
variables and their values are displayed. If the
value is omitted, the variable is removed from
the set of defined variables.
OUTPUT: nothing / if GUI is set to ALL
(this time or any time before this set),
then popup window with all variable settings;
otherwise nothing
-
alias [name value]
Create an alias for a command, a command
sequence, or any other linefeed-delimited string
sequence. The name alias can afterwards
be used instead of the value. It
hides other commands or aliases with the same
name. When no argument is given, all
pairs of name-values are displayed.
The
character sequence "!$" (exclamation-mark
dollar) is replaced at alias-invoke time with
everything behind the alias name. See example.
OUTPUT: nothing / if GUI is set to
ALL (this time or any time before this
set), then popup window with all alias
settings; otherwise nothing.
-
unalias [name]
Delete the latest (the currently used) entry for
name from the set of aliases.
OUTPUT: nothing / if GUI is set to ALL
(this time or any time before this set),
then popup window with all alias settings;
otherwise nothing
Not every command can be applied to every
document. Operations for directories are cd
and info; operations for plain documents are
info and open. Generally, one command
is entered at a time. Processing begins after a
linefeed character (return, enter). Commands also
can be concatenated by the semicolon (";"). After
the linefeed, commands are processed in sequence.
History, Attributes and Environment Variables
Attributes include the document's URL, type,
size, and date of last change. There may be
additional attributes for specific document types.
Pre-defined Environment Variables
SORT determines the sort order of directory
listings via the ls command. Its
values are NONE (equivalent to SORT not defined),
URL, NAME, and TYPE.
GUI determines
if a graphical output is displayed or not. Its
values are NONE (equivalent to GUI not defined),
DEFAULT (display documents in current document and
the tree structure of directories), and ALL (a lot
of additional popup windows for little messages).
History: Just like every other decent
shell, the web shell keeps a history of the most
recently entered commands (either a fixed number
of commands, say 50, or by "set HISTORY
number"). Given a prompt, the user can
"scroll" through the history by using the arrow-up
and arrow-down keys. On a new prompt, pressing
arrow-up once reprints the last command entered,
pressing it again reprints the second-to-last
command, etc. The commands are shown in the same
line; the cursor is placed behind the last
character, and a linefeed keystroke executes the
chosen command again (using the current directory,
environment variables, and so on).
Examples
Starting from the current document
http://www.cs.ucsb.edu/noframes.html, we can
change the current document to the URL
http://www.cs.ucsb.edu/search/iaquery:
> cd search/iaquery
http://www.cs.ucsb.edu/search/iaquery
>
Notice the relative path name we used, and notice
also the output of the absolute URL. This
"directory" contains:
> ls
http://www.cs.ucsb.edu/ui/icons/srchlogo.png
http://www.cs.ucsb.edu/ui/usa/help.html/
http://www.cs.ucsb.edu/search/iaquery?
NS-query-pat=/usa/NS-advquery.pat&
http://www.cs.ucsb.edu/search/iatoc/
>
Notice the slash / we used behind the second and
fourth document to indicate that these documents
are directories.
The next three examples demonstrate the usage of
concatenation and aliases. Output is not shown.
> set SORT NAME
> ls http://www.cs.ucsb.edu
> set SORT NONE
>
has the same effect as
> set SORT NAME; ls http://www.cs.ucsb.edu; set SORT NONE
>
and
> alias "lsname set SORT NAME; ls !$; set SORT NONE"
> lsname http://www.cs.ucsb.edu
>
Mathias Kölsch
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