Q97841: IF EXISTS Command Exhibits Different Behavior on HPFS

Article: Q97841
Product(s): Microsoft LAN Manager
Version(s): 
Operating System(s): 
Keyword(s): 
Last Modified: 30-JUL-2001

SUMMARY
=======

Testing for the existence of a remote drive or directory via the IF EXISTS
drive:\nul command exhibits different behavior on an HPFS partition than it does
on a FAT partition. On an HPFS system, the IF EXISTS command always returns
FALSE when testing for the NUL device.

SYMPTOMS
========

        exist.bat

        if exist %1\NUL goto dir_ok
        echo %1 doesn't exist
        goto end
        :dir_ok
        echo %1 exists

     Usage:

        exist P:
        exist P:\dirname\dir

The code returns "P: doesn't exist," but if P is an HPFS partition linked to a
sharename on a FAT partition, then the code returns "P: exists." If you
substitute the local hard drive for P, then the command works as expected.

FAT and HPFS partitions respond differently to the IF EXISTS NUL from an MS-DOS
workstation, but not if you copy EXIST.BAT to EXIST.CMD and run it from both
types of partitions while at the server.

CAUSE
=====

This behavior exists because of design differences in the two file systems. In
the FAT file system, the NUL device is one associated with each partition, as
well as each directory on that partition. On an HPFS partition, however, the NUL
device does not live as part of the file system on the disk, and as such cannot
be tested for via the IF EXISTS from a remote client. Nevertheless, the device
does exist, and behaves identically to the implementation on FAT.

RESOLUTION
==========

The following batch file shows an alternative way to test for the existence of a
remote drive/directory:

        copy c:\config.sys %1\$$$$$.tst
        if exists %1\$$$$$.txt goto dir_there
        echo %1 does not exist!
        goto done
        :dir_there
        echo %1 does exist!
        :done

Also, using the LAN Manager PTK, it is possible to programatically test for the
existence of a remote drive/directory.

Additional query words: 2.00 2.10 2.10a 2.20

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Keywords          :  

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