how am I hosing this Process?

I'm trying to run a bash script from Java in order to run it under
JUnitPerf, but am failing miserably. Any suggestions about what I'm
missing? except chops :-(

I've called scripts from Java before, but not for awhile (i.e. back in
the Runtime.exec days). Process/Builder looked like the way to go, so
I coded up something like

  public static void execute(List<String> l) throws RunnerException {
    Process p = null;
    ProcessBuilder pb = new ProcessBuilder(l);
    String argsMsg = "\"" + Utils.list2String(l) + "\"";
    // start debugging
    Utils.logInfo("start output from " + argsMsg + ":");
    Utils.logEnvironment(pb);
    // end debugging
    try {
      p = pb.start();
    } catch (IOException e) {
      String errmsg = "Failed to run command=" + argsMsg;
      throw new RunnerException(errmsg, e);
    }
    InputStream is = p.getInputStream();
    InputStreamReader isr = new InputStreamReader(is);
    BufferedReader br = new BufferedReader(isr);
    String line;
    try {
      while ((line = br.readLine()) != null) {
        Utils.logInfo(line);
      }
    } catch (IOException e) {
      String errmsg = "Dropped output line from process";
      throw new RunnerException(errmsg, e);
    }
    // start debugging
    Utils.logInfo("end output from " + argsMsg);
    // end debugging
  }

but nothing's happening ... except the logging above. From that I can
tell that I'm calling the correct command line (i.e. argsMsg has the
expected value, with a fully-qualified executable and all the
arguments) and I've got a correct environment. But when I call the
command from an xterm (FWIW I'm running on linux) Stuff Actually
Happens (notably files get created in a directory, and a whole lot
more logging happens); when I call the java app, nothing actually
happens except the logging above. I.e. I get

- start output from "<correct commandline/>":
- start environment:
<correct environment/>
- end environment.
- end output from "<correct commandline/>"

when I should instead get

+ start output from "<correct commandline/>":
+ start environment:
<correct environment/>
+ end environment.
<LOTS MORE STUFF HERE/>
+ end output from "<correct commandline/>"

plus lots of files in the output directory.