CoCreate Modeling code examples: Enumerating all drives on a Windows system

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---++ <nop>CoCreate Modeling: Enumerating all drives on a Windows system

Even though Common Lisp has very broad functionality, sometimes you need to get a little
help from your friends to accomplish a task.
Let's say we want to enumerate all available drives on a Windows system.
Neither Common Lisp nor <nop>CoCreate Modeling's Developer's Kit have any built-in inquiries
for this purpose - but a solution can be built easily by glueing together
an external Windows shell script and a short Lisp function! The following
example will demonstrate important techniques in getting this done.

An abundance of scripting solutions is available for the Windows platform:
   * [[http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/findstr.mspx][Batch programming]]
     using DOS command syntax. Highly underrated, probably because of its 
     heritage and its peculiar syntax, but actually 
     [[http://home7.inet.tele.dk/batfiles/][very powerful]],
     at least in Windows NT/2000/XP! Best of all: The command shell 
     is available everywhere, so no special installation
     steps are necessary.
   * Platform-independent scripting languages such as [[http://www.perl.org][Perl]]
     or [[http://www.python.org/][Python]]
   * UNIX-like shell scripting languages, such as ksh or [[http://www.cygwin.com][bash]]
   * Last, but not least, scripting languages supported by the
     [[http://www.microsoft.com/scripting][Windows Scripting Host]].

In our example, we'll use the Windows Scripting Host to execute the following short script
in <nop>VBScript syntax:

<pre>
  set fs=CreateObject("Scripting.FileSystemObject")
  for each drive in fs.drives
    WScript.Echo(drive.DriveLetter)
  next
</pre>

In <nop>VBScript, it is possible to instantiate COM components such as the
[[http://msdn.microsoft.com/library/en-us/script56/html/jsobjFileSystem.asp][<nop>FileSystemObject]],
which provides functionality to enumerate drives, inquire
drive properties and drill into a filesystem. Exactly what we need!

Save this script into a file and then try running it using =cscript=:

<pre>
  c:\home\clausb\>cscript /nologo drives.vbs
  A
  C
  D
  E
</pre>

Looks good! Now we have the drive information and only need to get it across
to LISP. The second part of the solution is the following LISP code:

<pre>
  (defun listdrives()
    (let (l (s (open "| cscript /nologo drives.vbs" :direction :input)))
      (prog1 
        (loop while (setf l (read-line s nil)) collecting (remove #\Return l))  
        (close s))))
</pre>

The file name parameter syntax for the =open= command probably caught your eye:
The "file name" starts with a pipe (=|=) character. This means that what follows
is actually a command and not a file. With this syntax, the specified
command is executed, and its output is then piped into the LISP stream =s= from
where it can be read line by line using =read-line=.

The return value of =listdrives= is a list containing the drive letters of 
all available drives. To build that list, the code above uses the
[[http://www.lisp.org/HyperSpec/Body/mac_loop.html][loop]] statement's
result accumulation capabilities ([[http://www-2.cs.cmu.edu/Groups/AI/html/cltl/clm/node246.html][collecting]]).

-- Main.ClausBrod - 13 Feb 2005

Much nicer and shorter version of =listdrives=:

<pre>
  (defun listdrives()
    (with-open-file (s "| cscript /nologo drives.vbs" :direction :input)
      (loop for l = (read-line s nil) while l collecting (remove #\Return l))))
</pre>

-- Main.ClausBrod - 18 Feb 2009

%COMMENT{type="below" nonotify="on"}%
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