Better way of incrementing build number?

I have been using a shell script as part of my Xcode build process to increment the build number within the plist file, however it's making Xcode 4.2.1 crash frequently (with an error about the target not belonging to a project; I'm guessing the changing of the plist file is confusing Xcode in some way).

The shell script did this so that the build number is only incremented by agvtool when a file is newer than the plist file (so just building didn't increment the value):

if [ -n \"`find ProjDir -newer ProjDir/Project-Info.plist`\" ]; then agvtool -noscm next-version -all; else echo \"Version not incremented\"; fi

Is there a way to increment the build number (in the plist file, or anywhere else) that doesn't break Xcode?

FINAL EDIT: I now do this kind of stuff using a python script which I have just made public on github. It's not well documented but shouldn't be difficult to work out. As a bonus this repo also contains a useful script to automatically bundle 3rd party library into an app bundle.