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15 February 2006

Nerd Factor X Provocation



xcopy brain damage

And now, a software development war story, if you’ll indulge me.

The Problem

Once upon a time I worked on a project where one of the deliverables was a Windows executable. We had a build.bat batch file with commands to copy executables and libraries from the build directories into a bin directory so they can be run.

The initial version of this used the vanilla Windows copy command. However this always copies every single file to the bin directory, resulting in a small, but noticable, turnaround time between building the executable and running it.

I thought: surely it would be easy to only copy the files if they had actually changed? Most of the files were third-party DLLs and they never change.

Attempt #1: nmake

I initially looked at using a makefile for this, more specifically the nmake tool that comes with Visual C++. After wrangling this tool for a bit, and eventually realising that it implements maybe 10% of the functionality of a real make tool (eg GNU make), I gave up. Not only was the makefile a monster (the equivalent in GNU make would be about 2 lines), but also it didn’t quite do what I wanted. The existing build script ignores executables which haven’t been built yet (e.g. test harnesses for modules you’re not working on), and it was too hard to get nmake to do this.

Why not just use GNU make? Well as a general principle I try not to introduce additional dependencies on the build system unless absolutely necessary. I had one other alternative that was available for free…

Attempt #2: xcopy

So the next step was to modify build.bat to use xcopy. This tool has a \d option which (according to the help) “Copies files changed on or after the specified date. If no date is given, copies only those files whose source time is newer than the destination time.” Sounds just the ticket right?

Unfortunately this option is so broken that I wonder if anyone has ever managed to put it to any worthwhile use, ever.

Note that it says it compares the source time with the destination time. Unfortunately if you specify the destination as a directory it will use the timestamp of the directory as the comparison, even if the source is a file. Duh.

Attempt #3: xcopy, again

OK, so we just specify the destination as a full filename, such as xcopy \d build\blah\foo.exe bin\foo.exe, right? Nope, Microsoft strikes again! xcopy gets confused and prompts you as to whether the destination is a directory or a file (!) As Dave Barry says, I am not making this up:

Does c:\path\to\bin\foo.exe specify a file name or directory name
on the target (F = file, D = directory)?

And of course there is no option to tell it which, beforehand.

After being struck with stupidity of such awesome magnitude, I simply gave up and went back to just blindly copying everything every time. And I can’t recall ever having used xcopy since then.


Posted by
Alan Green
2006-02-14 23:54:23 -0600

Your story illustrates why Windows programmers are paid more than Unix programmers: danger money.

As an alternative to introducing GNU make, has a Windows version of the Unix "cp". "cp -u srcfile destfile" will do what you want.

Posted by
2006-02-14 23:54:23 -0600

If you're going to install stuff then I'd install cygwin and rsync which will do what you want.

Posted by
2006-02-14 23:54:23 -0600

As I recall, we did eventually introduce a dependency on Cygwin. It just seems like such a simple task that you'd think Windows would be able to do it out of the box. I guess not?

Posted by
2006-02-14 23:54:23 -0600


echo F | xcopy /d build\blah\foo.exe bin\foo.exe

Posted by
2006-02-14 23:54:23 -0600

I tried:

system("echo xcopy \"$returnstr\" \"$mezteststr\" /f /s /e /i");

nothing happens, but it is asking me

Does C:\build\6.2.3\noncompiled\Release6.2\InStorecard\Web\Assets\buysafe\shopper\ttlHd_crdts.gif specify a file name or directory name on the target (F = file, D = directory)?

i tired:

system("echo F|xcopy \"$returnstr\" \"$mezteststr\" /f /s /e /i");

and I get a parse error on the F|.

Not sure what to do now. using XPerl

Does anyone know what to do. This is under cygwin as well.

Posted by
2006-02-14 23:54:23 -0600

Try not using "\" at the end of the source path and adding one at the end of the destination. Something like this:

C:\temp>xcopy d:\h-drive\caching-cdn-proxy-streaming\ h:\caching-cdn-proxy-streaming\ /t /e

or maybe like xcopy drive:\path*.exe \drive:\path\ /various parameters.

also, if you copy xcopy.exe to mcopy.exe, it changes how it works. The difference I found is that mcopy requires that the destination path exist, where xcopy will create it if you answer the file or directory question interactively.

Good Luck. Let me know if it works.

Posted by
2006-02-14 23:54:23 -0600

I’ve used and I am still using XCOPY a lot of times, and I never had any issue with it, whatsoever. I only use it to refresh directory structures (/s), never without this option, never to copy 1 file. I’ve checked it multiple times, and it works perfectly, I am 100% sure that it works, using this method :

xcopy C:\something*.* D:\backup /s /d /y

The issue as described in Attempt#3 is indeed annoying, but I guess that the /d option makes this go away, as this way he knows what he is copying, and doesn’t come and asks this silly question.

The issue as you described in Attempt2, I have never seen. I assume it appears when not using “.”, or when not using “/s”, but I am guessing. The timestamp comparision system works all right, I have seen it do time and time again, it is reliable as nothing else. I use it for incremental backups.

Problem is, but that is not xcopy’s fault, is that if you use it for incremental backup purposes (for which this /d option is designed anyway), is that removed files/directories on the source, still appear on the target. Depending on the circumstances, this may be a problem.


Posted by
2006-02-14 23:54:23 -0600


xcopy \d build\blah\foo.exe bin\

Posted by
2006-02-14 23:54:23 -0600

So here’s the test case for those of you that want to have a try:

C:\temp>mkdir bin

C:\temp>mkdir build

C:\temp>mkdir build\blah

C:\temp>echo hello > build\blah\foo.txt

Now your task is to find an xcopy command that will copy build\blah\foo.txt to bin\foo.txt but only when the former changes. In other words it should copy the first time, and subsequently not copy the file. Then, if you change build\blah\foo.txtit should copy the file the first time and not after that. Make sense?


The \d option is the whole point of the exercise. Without it I go back to copying everything all the time, and I can just use copy for that.


C:\temp>xcopy \d build\blah\foo.txt bin\
Invalid number of parameters

Thanks for playing.

Posted by
2006-02-14 23:54:23 -0600

Try using the slash the right way around. DOS options use leading /’s, not 's or -‘s.

C:\temp>xcopy /d build\blah\foo.txt bin\
1 File(s) copied

Posted by
2006-02-14 23:54:23 -0600

I am refering to the problem mentioned at the begining of the thread for “Attempt #3: xcopy, again”, this might be a solution to just copy only the file without being prompted to choose either file or directory

xcopy /f /s /y build\blah\foo.exe bin\


Posted by
Matt Winchester
2006-02-14 23:54:23 -0600

I am experiencing the same problem. The answer to your test case is to use the /m option instead of the /d option. That way it will copy the file the first time, then reset the archive flag for that file. it will then not copy that file again until you modify it, causing the archive flag to be set again.

That is no good in my situation though as I am trying to back up to a removable tape drive, and I need the file to be copied to each tape, not just the first tape that is inserted after the file was modified.

Does anyone know why the /d option doesn’t do what it is supposed to?


Posted by
2006-02-14 23:54:23 -0600

Can this be the OS/version issue. I just tried on 2k3 (cmd/xcopy.exe ver: 5.02.3790.1830) with dest folder having the timestamp lower but the file having higher then the source file and no problems at all:

02/19/2007 18:29

bin 02/19/2007 18:33 build

Directory of C:\E\temp\bin 02/19/2007 18:34 8 foo.txt

Directory of C:\E\temp\build\blah 02/19/2007 18:30 8 foo.txt

xcopy /I/Y/R/d build\blah\foo.txt bin
0 File(s) copied

Tried several times, making bigger time diff (just in case) and never ever got a hiccup.

If we are still talking about building and on Windows, then you may want to take a look at the Rotor source distro ( Has the full build setup from DDK and a few extras (yes MS builds with Perl :-). Then look for (pcopy” and makefile.plt (PUBLISH_CMD=). The mention of “pcopy” in is the place to add extra tests.