diff --git a/core/__init__.py b/core/__init__.py
index d0eff64..1b97b3f 100644
--- a/core/__init__.py
+++ b/core/__init__.py
@@ -3,6 +3,7 @@
 
 from . import app
 from . import aux
+from . import cmd
 from . import file
 from . import misc
 from . import textui
diff --git a/core/cmd.py b/core/cmd.py
new file mode 100644
index 0000000..f3ef5e0
--- /dev/null
+++ b/core/cmd.py
@@ -0,0 +1,142 @@
+#! /bin/env python3
+# encoding: utf-8
+
+import queue
+import subprocess
+import threading
+
+# --------------------------------------------------
+
+def capture_output(stream, fifo):
+	while True:
+		chunk = stream.read1(128)
+		
+		if chunk:
+			fifo.put(chunk)
+		else:
+			fifo.put(None) # indicates a process has terminated
+			break
+	
+	stream.close()
+
+class Command:
+	'''
+	A shell command with argument substitution and output capture.
+	
+	>>> c = Command(['process.sh', '-x', 'ARGUMENT'])
+	>>> c.ARGUMENT = 'file.txt'
+	>>> c.dump()
+	['process.sh', '-x', 'file.txt']
+	>>> c.run(stderr=False) # capture stdout
+	>>> c.get_output()
+	b'some output'
+	>>> c.get_output()
+	b'' # there was no output since the last call
+	>>> c.get_output()
+	b'more of it\nand some more'
+	>>> c.get_output()
+	None # indicates the process ended and there is no more output
+	
+	'''
+	
+	def __init__(self, *sequence):
+		self.__dict__['sequence_original'] = list(sequence)
+		self.reset()
+	
+	def __setattr__(self, name, value):
+		found = False
+		
+		for i, word in enumerate(self.sequence):
+			if word == name:
+				self.sequence[i] = value
+				found = True
+		
+		#if not found:
+			#raise AttributeError('Command has no attribute \'%s\'' %(name))
+	
+	def reset(self):
+		self.__dict__['sequence'] = list(self.sequence_original)
+		self.__dict__['thread'] = None
+		self.__dict__['fifo'] = None
+		self.__dict__['terminated'] = False
+	
+	def dump(self, sequence=None, pretty=False):
+		out = []
+		
+		if not sequence:
+			sequence = self.sequence
+		
+		for item in sequence:
+			if type(item) is list:
+				out += self.dump(item)
+			elif type(item) is Command:
+				out += item.dump()
+			elif item is not None:
+				out.append(str(item))
+		
+		if pretty:
+			return [('"%s"' %(a) if ' ' in a else a) for a in out]
+		else:
+			return out
+	
+	def run(self, stderr=False):
+		'''
+		Executes the command in the current environment.
+		
+		stderr    capture stderr instead of stdout (can't do both yet)
+		'''
+		
+		self.__dict__['process'] = subprocess.Popen(self.dump(), stdout=subprocess.PIPE, stderr=subprocess.PIPE, bufsize=1)
+		self.__dict__['fifo'] = queue.Queue()
+		self.__dict__['thread'] = threading.Thread(
+			target=capture_output,
+			args=(self.process.stderr if stderr else self.process.stdout, self.fifo)
+		)
+		self.__dict__['terminated'] = False
+		self.thread.daemon = True # thread dies with the program
+		self.thread.start()
+	
+	def get_output(self, blocking=False):
+		'''
+		Returns the output of a currently running process and clears the buffer.
+		
+		Returns None if no process is running and no more output is available.
+		
+		blocking    block until some output is available or the process terminates
+		'''
+		
+		buffer = []
+		
+		if self.terminated:
+			return None
+		
+		if blocking:
+			buffer.append(self.fifo.get())
+		
+		while not self.fifo.empty():
+			buffer.append(self.fifo.get_nowait()) # FIXME nowait needed?
+		
+		if None in buffer:
+			self.__dict__['terminated'] = True
+			self.__dict__['returncode'] = self.process.poll()
+			
+			if len(buffer) == 1:
+				return None
+			else:
+				assert(buffer[-1] is None)
+				del buffer[-1]
+		
+		return b''.join(buffer)
+	
+	def get_all_output(self):
+		buffer = []
+		
+		while True:
+			chunk = self.get_output(blocking=True)
+			
+			if chunk is None:
+				break
+			else:
+				buffer.append(chunk)
+		
+		return b''.join(buffer) if buffer else None
\ No newline at end of file