The ability to launch a shell or batch command periodically to determine if a
build is required. This feature could be configured to "poll" similar to the
"Poll SCM" build trigger, but instead it would invoke a shell or batch command,
using the exit code to determine if a build is needed. Alternatively, the output
of the command could be scanned for key words like "BUILD_NEEDED".

This feature would allow custom and complex build-when-necessary triggers and
could be used with SCM's not directly supported by Hudson, such as clearcase.

Example Scenario.

I wish to build a project that uses ClearCase for SCM. I only want to build if
source code has changed. However, since the machine is shared to perform lengthy
and cpu intensive data analysis. I do not want to invoke a build if the machine
is more than 80% busy.

Using a shell command build trigger, I could invoke a cleartool update and check
resulting log. I could also monitor the cpu for a small period of time to
determine if the machine is sufficiently idle.

By the way... fantastic product. So much easier than Continuum and
CruiseControl. Well done.