XXCOPY TECHNICAL BULLETIN #07

From:    Kan Yabumoto           tech@xxcopy.com
To:      XXCOPY user
Subject: XXCOPY's Handling the case of Cyclic Copy
Date:    2004-12-18 (revised)
===============================================================================

What is a Cyclic Copy?

    Try this with Microsoft's XCOPY (not XXCOPY).

        XCOPY  C:\  C:\temp\ /S

    You will be greeted by the following message:

        Cannot perform a cyclic copy.

    A cyclic copy refers to the situation where the source directory
    contains the destination directory.  Since the destination directory
    is part of the source, the copying process eventually starts copying
    the destination directory to a deeper portion of the destination.
    This process will continue until, of course, the finite disk space
    eventually runs out.

    To avoid this condition but to achieve the objective, one can
    specify a temporary destination which is on another volume (e.g.,
    D:\temp\) and later copy the temporary destination to the final
    destination, and delete the temporary directory at the end.


The safety net against a Cyclic Copy:

    Microsoft's XCOPY detects this disastrous situation with an error
    message.  With our XXCOPY, you may continue the normal file copy
    operation by excluding the directory that would be cyclic after
    a warning.


Cases where a Cyclic Copy is useful:

    A Cyclic Copy is certainly useful with a combination with other
    switches such as /TR0 (create corresponding zero-byte files) or
    /T (make a directory tree without files).

    Or, you may be tempted to make a collection of certain data files
    into one directory:

        XXCOPY  C:\*.doc   c:\my_word\  /S

    Of course, it is senseless to let the recursive meltdown to occur
    by allowing the freshly copied files in the destination to partake
    in the copy process.  By now, it is quite apparent that what we
    really need is a mechanism to cut off this vicious cycle in the
    process.


Enter XXCOPY's new switch for Cyclic Copy.

    XXCOPY comes with a few variations to handle the cyclic copy case.

    /CC     Warns you for a cyclic copy case and gives you a prompt
            where you may terminate the process immediately, or
            you may let XXCOPY continue by adding the destination
            directory in the exclusion list.

    /CCY    Continues the copy operation by automatically adding
            the destination directory in the exclusion list without
            prompting the user (as if you typed "Y" at the prompt).

    /CC0    Terminates the copy operation unconditionally without prompt.
            This is the old XCOPY behavior --- hardly useful but if
            you want to emulate XCOPY, you may do so with this switch.


A few interesting examples taking advantage of the /CCY feature:

    XXCOPY  C:\*.doc  C:\my_word\  /CCY /S

           This example collects directories which contains .DOC
           files in the destination with the .DOC files.

    XXCOPY  C:\*.doc  C:\my_word\  /CCY /SX

           This is a variation of the fist example.  It uses XXCOPY's
           handy /SX switch.  It gathers all .DOC files from the entire
           volume and saves them in a flattened directory.  If you have
           little idea what a "flattened directory" is, just try it
           and examine the destination.  You will see what it is.

    XXCOPY  C:\  C:\skeleton\      /CCY /T

           This gives the whole volume tree (except the \skeleton
           itself) inside the destination without files.

    XXCOPY  C:\  C:\summary\       /CCY /E /TR0

           This one includes files without copying the file contents
           (creates zero-length files corresponding to the source).


    These operations are not possible using Microsoft's XCOPY.
    Incidentally, the explicit /CCY suppresses the warning message:

     "Detected a cyclic copy (the source includes the destination)."

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