Improve shell argument parsing

For example, this patch fixes the command "df | awk '{print $1}'"

commands.py

@@ -21,19 +21,18 @@ class RawCommand(BaseCommand):
     with the builtin subprocess module. Each output object is just a tree node
     whose data is a simple string."""
 
-    def __init__(self, cmd, input_generator=[], args=tuple()):
+    def __init__(self, cmd, input_generator=[]):
         self.input_generator = input_generator
         self.cmd = cmd
-        self.args = args
 
     def __call__(self, input_generator=[], *args, **kwargs):
         import subprocess
         try:
-            p = subprocess.Popen((self.cmd,) + self.args, stdin=subprocess.PIPE, stdout=subprocess.PIPE)
+            p = subprocess.Popen(self.cmd, stdin=subprocess.PIPE, stdout=subprocess.PIPE)
             def output_generator():
                 input_str = b""
                 for line in input_generator:
-                    input_str += line
+                    input_str += line + b'\n'
                 outs, errs = p.communicate(input_str)
                 if outs:
                     yield outs

main.py

@@ -4,6 +4,8 @@ from example_cmd import example_cmd, echo
 
 import os
 import os.path
+import shlex
+
 for path in os.environ['PATH'].split(':'):
     if os.path.exists(path):
         binaries = os.listdir(path)
@@ -25,10 +27,7 @@ def parse_cmd(potential_cmd):
     if args:
         args = args[1:]
     if cmd_name not in registered_cmds:
-        if args:
-            return "RawCommand('{}', args={})".format(cmd_name, str(tuple(args)))
-        else:
-            return "RawCommand('{}')".format(cmd_name)
+        return "RawCommand({})".format(shlex.split(potential_cmd))
     else:
         return "{}()".format(cmd_name)