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Q77361: DOS "IF EXIST Z:\NUL" Fails on HPFS Servers

Article: Q77361
Product(s): Microsoft LAN Manager
Version(s): 
Operating System(s): 
Keyword(s): 
Last Modified: 30-JUL-2001

SYMPTOMS
========

Using the MS-DOS COMMAND.COM batch language IF EXIST <filespec> construct,
using a <filespec> of Z:\NUL (where Z: is a remote drive that is not using
the FAT file system), the IF EXIST will fail. Variations of this IF EXIST are
often used (incorrectly) as presence checks for remote drive letters or remote
subdirectories.

Using Z:\NUL will fail on LAN Manager HPFS servers. It will also probably fail on
any remote drive that neither is DOS-based nor emulates the MS-DOS-centric
behavior of making all character devices available at all portions in the file
system (that is, ignoring the drive letter and optional path when the base
filename is an MS-DOS device name).

Using Z:\*.* (that is, using "*.*" instead of "NUL") works on all tested servers,
as long as there is at least one file in that subdirectory (the special
directory files "." and ".." do not count in this case).

Using Z:\NUL on local MS-DOS drives or remote drives on MS-DOS MS-Net servers
(both of which are running MS-DOS, and thus, FAT file systems) will work.

Using Z:\NUL on LAN Manager servers that have FAT file system shares will also
work.

Using Z:\NUL on LAN Manager servers that have HPFS file system shares WILL work
if a FILE (not a device) called NUL exists that allows the check to work.

Using IF EXIST under OS/2's CMD.EXE command interpreter will work; however, this
is due to differences of behavior in CMD.EXE.

This problem occurs on all versions of MS-DOS.

CAUSE
=====

The use of Z:\NUL tells MS-DOS to look for a local character device on a remote
block device. Thus, DOS tells the LAN Manager redirector to ask the remote
server to find this file (using DosFindFirst() under OS/2 LAN Manager servers).
Because there is no device (or probably no file) on HPFS drives, this fails, and
the error is propagated back from the server to MS-DOS, via the redirector.

RESOLUTION
==========

To work around this problem, write a program that uses the MS-DOS Get Current
Drive (interrupt 21h ah=19h) and Set Current Drive (interrupt 21h ah=0Eh)
functions to check a drive letter for validity, and return an ERRORLEVEL, which
can be tested in DOS's batch language.

Additional query words: 1.x 2.00 2.10 dos51.1722 lanman21.1328 lanman21.3096

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Keywords          :  

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