OneSpace 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 OneSpace 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:

• Batch programming using DOS command syntax. Highly underrated, probably because of its heritage and its peculiar syntax, but actually 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 Perl or Python
• UNIX-like shell scripting languages, such as ksh or bash
• Last, but not least, scripting languages supported by the Windows Scripting Host.

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

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

In VBScript, it is possible to instantiate COM components such as the 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:

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

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:

  (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))))

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 loop statement's result accumulation capabilities (collecting).

-- ClausBrod - 13 Feb 2005

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