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 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:

  • 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

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

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)))
        (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

Much nicer and shorter version of listdrives:

  (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))))

-- ClausBrod - 18 Feb 2009

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r1.12 - 24 Jul 2009 - 19:54 - ClausBrod to top

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