Create pull request for adding --no-parallel as coded by @BernieSumption

 in issue gorakhargosh#227

src/watchdog/tricks/__init__.py

@@ -78,14 +78,20 @@ class ShellCommandTrick(Trick):
     """Execeutes shell commands in response to matched events."""
 
     def __init__(self, shell_command=None, patterns=None, ignore_patterns=None,
-                 ignore_directories=False, wait_for_process=False):
+                 ignore_directories=False, wait_for_process=False,
+                 no_parallel_processes=False):
         super(ShellCommandTrick, self).__init__(patterns, ignore_patterns,
                                                 ignore_directories)
         self.shell_command = shell_command
         self.wait_for_process = wait_for_process
+        self.no_parallel_processes = no_parallel_processes
+        self.process = None
 
     def on_any_event(self, event):
         from string import Template
+
+        if self.no_parallel_processes and self.process and self.process.poll() is None:
+            return
 
         if event.is_directory:
             object_type = 'directory'
@@ -111,9 +117,9 @@ def on_any_event(self, event):
             command = self.shell_command
 
         command = Template(command).safe_substitute(**context)
-        process = subprocess.Popen(command, shell=True)
+        self.process = subprocess.Popen(command, shell=True)
         if self.wait_for_process:
-            process.wait()
+            self.process.wait()
 
 
 class AutoRestartTrick(Trick):

src/watchdog/watchmedo.py

@@ -416,6 +416,11 @@ def log(args):
      action='store_true',
      default=False,
      help="wait for process to finish to avoid multiple simultaneous instances")
+@arg('-n', '--no-parallel',
+     dest='no_parallel_processes',
+     action='store_true',
+     default=False,
+     help="Ignore events that happen while the previous process is still running")
 def shell_command(args):
     """
     Subcommand to execute shell commands in response to file system events.
@@ -435,7 +440,8 @@ def shell_command(args):
                                 patterns=patterns,
                                 ignore_patterns=ignore_patterns,
                                 ignore_directories=args.ignore_directories,
-                                wait_for_process=args.wait_for_process)
+                                wait_for_process=args.wait_for_process,
+                                no_parallel_processes=args.no_parallel_processes)
     observer = Observer(timeout=args.timeout)
     observe_with(observer, handler, args.directories, args.recursive)