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 and the local file system. 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. Every document that is
linked from an HTML page is said to be "contained in the HTML
page's folder".
Documents, Directories, Folders
A remote document is any file you can request
from a web server with a HTTP request.
A local document is any file in the local file
system, that is any file you can refer to via the java.io.File
class.
A directory is a document of type HTML. Its
contents are defined as all documents that the directory
references. A document is referenced by a directory, if the
directory has a link to this document. A link is any HTML tag
that has a URL as value (examples are: anchor (syntax:
<a href="some.url">), a form's
action parameter
(syntax: <form
action="some.url">) or images (syntax:
<img src="some.url">)). You may
disregard non-HTML tags such as JavaScript links, and HTML links
that are embedded in JavaScript.
Additionally, a local document can be a directory if it is a
directory in the sense of the local file system. It is called a
local directory, and you can easily test local
files for this property with the method
java.io.File.isDirectory().
Any other document is called plain.
The shell always has a current directory. On
startup, this is the local directory in which the WebShell
application was started. It can be changed using the commands
described below.
Parents, Tree Structure and Traversal
The shell allows for two ways to enter a directory.
-
A relative cd (change directory) is when you
change into one of the directories that are linked from the
current directory (example: cd AnyDocument.html). If the
new current directory was entered for the first time, the
directory you came from is called the parent
directory of the directory you changed to.
-
An absolute cd is the change to a directory that is not
linked from the current directory (example: cd http://www.cs.ucsb.edu/~matz).
In this case, if the new current directory was entered for the first time,
it will not have a parent, but it becomes a root directory.
Later traversals of the tree don't modify the parent of a document !
This relationship defines a tree structure which we will call the
"parent-tree", or simply "tree". There
will possibly be multiple parent-trees, but only one for each root
document. Parent-trees can be interlinked, but only "on the way
down": At most one parent exists for each document, but a document
can be linked from multiple directories. That's why we also define
a "Most-Recently-Visited-Queue", or
MRV-queue. It contains a history of the last
recently visited directories, that is every new current directory
is inserted at the beginning. The queue may have a fixed length,
such that directories at the end of queue drop out of it on
insertion of another directory. (If you ever need a longer
directory history than the queue contains, you may issue an error
message and continue without this information.) Two commands
(back and forward) allow to change the current
directory to an arbitrary entry in the queue without modifying
it. However, if a cd command is executed while we got to
the (old) current directory through a series of back
commands, the front end of the queue is discarded and the new
current directory is inserted right before the old directory (at
the new beginning of the queue).
A web shell allows for traversal of the tree defined by this
relationship, simple operations on the documents, and some
miscellaneous functions.
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. You can either use the terminal (window)
in which the Java interpreter for the WebShell was started or you
can pop up a new window. Your decision won't effect grading, but a
"text-only" application - one that does not use the AWT at all -
is much faster and usually easier to debug. 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 the part that is described
before the "/"; the other display is a graphical
representation of the document structure. This output is
described after the "/". Hint: The Java
Swing package makes it easy to implement a visualization of
both the tree structure of interlinked directory documents and the
content lists of directories - you have to display both. For an
example, look at the WindowsExplorer (not: Internet Explorer).
NOTE: The costumer is familiar with Unix shells and the
WindowsExplorer. She wants to use the WebShell as intuitively as
possible, so try to resemble this behaviour and interpretation -
but reconfirm it anyway !
GUI input: The customer made up her mind, she wants to be
able to input some commands from both the shell and the GUI. For
now, those commands are: double-click on a directory cds
to it, it is highlighted in the tree structure and its contents
are displayed. Double-click on a plain document opens it.
Web Shell Commands
These are the particular commands that can be used in the web
shell:
-
cd [name] (change current directory)
The current directory is set to name (which must be the
name of a directory), and the complete URL of the new current
directory is displayed.
If no name is provided,
the current directory stays unchanged.
OUTPUT:
absolute path of new current directory / highlighting of new
current directory.
-
up
The new current directory is set to the current directory's
parent. If it does not have a parent, the current directory
stays unchanged. Note: This replaces "cd ..".
OUTPUT: absolute path of new current directory /
highlighting of new current directory.
-
back
The new current directory is set to the previous current
directory, as determined by the MRV-queue.
OUTPUT:
absolute path of new current directory / highlighting of new
current directory.
-
forward
The new current directory is set to the following document of
the MRV-queue. This can be executed k times iff the k most
recent changes to the current directory were done using
back.
OUTPUT: absolute path of new current
directory / highlighting of new current directory.
-
ls [name ...]
If name is a directory, its contents are shown:
all documents that are linked from the it. Special tags or
other means identify different types of documents (directory,
plain, or even more detailed). If name is a plain
document, only this document is listed. 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 (see below).
OUTPUT: information as text / if GUI is set to ALL
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. Also, let the user
configure at startup-time of the WebShell which helpers
are used for what type of documents.
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. The value can be enclosed in double-quotes
("), they are not part of the actual value. If no arguments
are given, all variables and their values are displayed. If
the value is omitted, the variable is set to the empty
string. This can be interpreted as a default value in
applications/commands that use this variable.
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. The value can be enclosed in
double-quotes ("), they are not part of the actual value.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 (every string
and character, including whitespaces) behind the alias name,
but before a new line or semicolon. This functions as
an argument to the alias. 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, remote directories can
be opened also; 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.
[name ...] means that more than one name can be supplied, then
the operation is executed for each of the documents. An asterix
(*) can be used for the name argument of commands for
which it is meaningful. It has the same functionality as in a Unix
shell: "*.jpg" is expanded to all documents in the respective
directory that end with ".jpg".
Command 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, but you have to support only the standard
attributes.
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, DEFAULT (initial setting, equivalent to GUI not
defined or defined as the empty string) displays the tree
structure of directories and the documents in the current
directory in one window, and ALL (a lot of additional popup
windows for little messages).
Command History
Just like every other decent shell, the web shell keeps a
command 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
>
Be aware of the difference between the two following commands. The
first is a single "alias" command, the second is composed of three
commands. Note the output of the "ls" command issued in the
second line. The name-value pairs for the aliases are shown also.
> alias lsname1 "set SORT NAME; ls; set SORT NONE"
> alias lsname2 set SORT NAME; ls; set SORT NONE
..
SomeDocument.html
AnotherDocument.html
> alias
lsname1 set SORT NAME; ls; set SORT NONE
lsname2 set SORT NAME
Mathias Kölsch
Last modified: Tue Jan 12 00:28:29 PST 1999
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