A C program by @ibawt that can be used to execute commands using a watch performantly on OSX.

fswait.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <CoreServices/CoreServices.h> 

/* fswait.c
 * 
 * usage: ./fswait /some/directory[:/some/otherdirectory:...]
 *
 * compile me with something like: gcc fswait.c -framework CoreServices -o fswait
 *
 * adapted from the FSEvents api PDF
*/

extern char **environ;

//fork a process when there's any change in watch file
void callback( 
    ConstFSEventStreamRef streamRef, 
    void *clientCallBackInfo, 
    size_t numEvents, 
    void *eventPaths, 
    const FSEventStreamEventFlags eventFlags[], 
    const FSEventStreamEventId eventIds[]) 
{ 
 exit(0);
} 
 
//set up fsevents and callback
int main(int argc, char **argv) {

  if(argc != 2) {
    fprintf(stderr, "You must specify a directory to watch\n");
    exit(1);
  }

  CFStringRef mypath = CFStringCreateWithCString(NULL, argv[1], kCFStringEncodingUTF8); 
  CFArrayRef pathsToWatch = CFStringCreateArrayBySeparatingStrings (NULL, mypath, CFSTR(":"));

  void *callbackInfo = NULL; 
  FSEventStreamRef stream; 
  CFAbsoluteTime latency = 1.0;

  stream = FSEventStreamCreate(NULL,
    &callback,
    callbackInfo,
    pathsToWatch,
    kFSEventStreamEventIdSinceNow,
    latency,
    kFSEventStreamCreateFlagNone
  ); 

  FSEventStreamScheduleWithRunLoop(stream, CFRunLoopGetCurrent(), kCFRunLoopDefaultMode); 
  FSEventStreamStart(stream);
  CFRunLoopRun();
}