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+
+:mod:`subprocess` --- Subprocess management
+===========================================
+
+.. module:: subprocess
+   :synopsis: Subprocess management.
+.. moduleauthor:: Peter Åstrand <astrand@lysator.liu.se>
+.. sectionauthor:: Peter Åstrand <astrand@lysator.liu.se>
+
+
+.. versionadded:: 2.4
+
+The :mod:`subprocess` module allows you to spawn new processes, connect to their
+input/output/error pipes, and obtain their return codes.  This module intends to
+replace several other, older modules and functions, such as::
+
+   os.system
+   os.spawn*
+   os.popen*
+   popen2.*
+   commands.*
+
+Information about how the :mod:`subprocess` module can be used to replace these
+modules and functions can be found in the following sections.
+
+
+Using the subprocess Module
+---------------------------
+
+This module defines one class called :class:`Popen`:
+
+
+.. class:: Popen(args, bufsize=0, executable=None, stdin=None, stdout=None, stderr=None, preexec_fn=None, close_fds=False, shell=False, cwd=None, env=None, universal_newlines=False, startupinfo=None, creationflags=0)
+
+   Arguments are:
+
+   *args* should be a string, or a sequence of program arguments.  The program to
+   execute is normally the first item in the args sequence or string, but can be
+   explicitly set by using the executable argument.
+
+   On Unix, with *shell=False* (default): In this case, the Popen class uses
+   :meth:`os.execvp` to execute the child program. *args* should normally be a
+   sequence.  A string will be treated as a sequence with the string as the only
+   item (the program to execute).
+
+   On Unix, with *shell=True*: If args is a string, it specifies the command string
+   to execute through the shell.  If *args* is a sequence, the first item specifies
+   the command string, and any additional items will be treated as additional shell
+   arguments.
+
+   On Windows: the :class:`Popen` class uses CreateProcess() to execute the child
+   program, which operates on strings.  If *args* is a sequence, it will be
+   converted to a string using the :meth:`list2cmdline` method.  Please note that
+   not all MS Windows applications interpret the command line the same way:
+   :meth:`list2cmdline` is designed for applications using the same rules as the MS
+   C runtime.
+
+   *bufsize*, if given, has the same meaning as the corresponding argument to the
+   built-in open() function: :const:`0` means unbuffered, :const:`1` means line
+   buffered, any other positive value means use a buffer of (approximately) that
+   size.  A negative *bufsize* means to use the system default, which usually means
+   fully buffered.  The default value for *bufsize* is :const:`0` (unbuffered).
+
+   The *executable* argument specifies the program to execute. It is very seldom
+   needed: Usually, the program to execute is defined by the *args* argument. If
+   ``shell=True``, the *executable* argument specifies which shell to use. On Unix,
+   the default shell is :file:`/bin/sh`.  On Windows, the default shell is
+   specified by the :envvar:`COMSPEC` environment variable.
+
+   *stdin*, *stdout* and *stderr* specify the executed programs' standard input,
+   standard output and standard error file handles, respectively.  Valid values are
+   ``PIPE``, an existing file descriptor (a positive integer), an existing file
+   object, and ``None``.  ``PIPE`` indicates that a new pipe to the child should be
+   created.  With ``None``, no redirection will occur; the child's file handles
+   will be inherited from the parent.  Additionally, *stderr* can be ``STDOUT``,
+   which indicates that the stderr data from the applications should be captured
+   into the same file handle as for stdout.
+
+   If *preexec_fn* is set to a callable object, this object will be called in the
+   child process just before the child is executed. (Unix only)
+
+   If *close_fds* is true, all file descriptors except :const:`0`, :const:`1` and
+   :const:`2` will be closed before the child process is executed. (Unix only).
+   Or, on Windows, if *close_fds* is true then no handles will be inherited by the
+   child process.  Note that on Windows, you cannot set *close_fds* to true and
+   also redirect the standard handles by setting *stdin*, *stdout* or *stderr*.
+
+   If *shell* is :const:`True`, the specified command will be executed through the
+   shell.
+
+   If *cwd* is not ``None``, the child's current directory will be changed to *cwd*
+   before it is executed.  Note that this directory is not considered when
+   searching the executable, so you can't specify the program's path relative to
+   *cwd*.
+
+   If *env* is not ``None``, it defines the environment variables for the new
+   process.
+
+   If *universal_newlines* is :const:`True`, the file objects stdout and stderr are
+   opened as text files, but lines may be terminated by any of ``'\n'``, the Unix
+   end-of-line convention, ``'\r'``, the Macintosh convention or ``'\r\n'``, the
+   Windows convention. All of these external representations are seen as ``'\n'``
+   by the Python program.
+
+   .. note::
+
+      This feature is only available if Python is built with universal newline support
+      (the default).  Also, the newlines attribute of the file objects :attr:`stdout`,
+      :attr:`stdin` and :attr:`stderr` are not updated by the communicate() method.
+
+   The *startupinfo* and *creationflags*, if given, will be passed to the
+   underlying CreateProcess() function.  They can specify things such as appearance
+   of the main window and priority for the new process.  (Windows only)
+
+
+Convenience Functions
+^^^^^^^^^^^^^^^^^^^^^
+
+This module also defines two shortcut functions:
+
+
+.. function:: call(*popenargs, **kwargs)
+
+   Run command with arguments.  Wait for command to complete, then return the
+   :attr:`returncode` attribute.
+
+   The arguments are the same as for the Popen constructor.  Example::
+
+      retcode = call(["ls", "-l"])
+
+
+.. function:: check_call(*popenargs, **kwargs)
+
+   Run command with arguments.  Wait for command to complete. If the exit code was
+   zero then return, otherwise raise :exc:`CalledProcessError.` The
+   :exc:`CalledProcessError` object will have the return code in the
+   :attr:`returncode` attribute.
+
+   The arguments are the same as for the Popen constructor.  Example::
+
+      check_call(["ls", "-l"])
+
+   .. versionadded:: 2.5
+
+
+Exceptions
+^^^^^^^^^^
+
+Exceptions raised in the child process, before the new program has started to
+execute, will be re-raised in the parent.  Additionally, the exception object
+will have one extra attribute called :attr:`child_traceback`, which is a string
+containing traceback information from the childs point of view.
+
+The most common exception raised is :exc:`OSError`.  This occurs, for example,
+when trying to execute a non-existent file.  Applications should prepare for
+:exc:`OSError` exceptions.
+
+A :exc:`ValueError` will be raised if :class:`Popen` is called with invalid
+arguments.
+
+check_call() will raise :exc:`CalledProcessError`, if the called process returns
+a non-zero return code.
+
+
+Security
+^^^^^^^^
+
+Unlike some other popen functions, this implementation will never call /bin/sh
+implicitly.  This means that all characters, including shell metacharacters, can
+safely be passed to child processes.
+
+
+Popen Objects
+-------------
+
+Instances of the :class:`Popen` class have the following methods:
+
+
+.. method:: Popen.poll()
+
+   Check if child process has terminated.  Returns returncode attribute.
+
+
+.. method:: Popen.wait()
+
+   Wait for child process to terminate.  Returns returncode attribute.
+
+
+.. method:: Popen.communicate(input=None)
+
+   Interact with process: Send data to stdin.  Read data from stdout and stderr,
+   until end-of-file is reached.  Wait for process to terminate. The optional
+   *input* argument should be a string to be sent to the child process, or
+   ``None``, if no data should be sent to the child.
+
+   communicate() returns a tuple (stdout, stderr).
+
+   .. note::
+
+      The data read is buffered in memory, so do not use this method if the data size
+      is large or unlimited.
+
+The following attributes are also available:
+
+
+.. attribute:: Popen.stdin
+
+   If the *stdin* argument is ``PIPE``, this attribute is a file object that
+   provides input to the child process.  Otherwise, it is ``None``.
+
+
+.. attribute:: Popen.stdout
+
+   If the *stdout* argument is ``PIPE``, this attribute is a file object that
+   provides output from the child process.  Otherwise, it is ``None``.
+
+
+.. attribute:: Popen.stderr
+
+   If the *stderr* argument is ``PIPE``, this attribute is file object that
+   provides error output from the child process.  Otherwise, it is ``None``.
+
+
+.. attribute:: Popen.pid
+
+   The process ID of the child process.
+
+
+.. attribute:: Popen.returncode
+
+   The child return code.  A ``None`` value indicates that the process hasn't
+   terminated yet.  A negative value -N indicates that the child was terminated by
+   signal N (Unix only).
+
+
+Replacing Older Functions with the subprocess Module
+----------------------------------------------------
+
+In this section, "a ==> b" means that b can be used as a replacement for a.
+
+.. note::
+
+   All functions in this section fail (more or less) silently if the executed
+   program cannot be found; this module raises an :exc:`OSError` exception.
+
+In the following examples, we assume that the subprocess module is imported with
+"from subprocess import \*".
+
+
+Replacing /bin/sh shell backquote
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+::
+
+   output=`mycmd myarg`
+   ==>
+   output = Popen(["mycmd", "myarg"], stdout=PIPE).communicate()[0]
+
+
+Replacing shell pipe line
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+::
+
+   output=`dmesg | grep hda`
+   ==>
+   p1 = Popen(["dmesg"], stdout=PIPE)
+   p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
+   output = p2.communicate()[0]
+
+
+Replacing os.system()
+^^^^^^^^^^^^^^^^^^^^^
+
+::
+
+   sts = os.system("mycmd" + " myarg")
+   ==>
+   p = Popen("mycmd" + " myarg", shell=True)
+   sts = os.waitpid(p.pid, 0)
+
+Notes:
+
+* Calling the program through the shell is usually not required.
+
+* It's easier to look at the :attr:`returncode` attribute than the exit status.
+
+A more realistic example would look like this::
+
+   try:
+       retcode = call("mycmd" + " myarg", shell=True)
+       if retcode < 0:
+           print >>sys.stderr, "Child was terminated by signal", -retcode
+       else:
+           print >>sys.stderr, "Child returned", retcode
+   except OSError, e:
+       print >>sys.stderr, "Execution failed:", e
+
+
+Replacing os.spawn\*
+^^^^^^^^^^^^^^^^^^^^
+
+P_NOWAIT example::
+
+   pid = os.spawnlp(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg")
+   ==>
+   pid = Popen(["/bin/mycmd", "myarg"]).pid
+
+P_WAIT example::
+
+   retcode = os.spawnlp(os.P_WAIT, "/bin/mycmd", "mycmd", "myarg")
+   ==>
+   retcode = call(["/bin/mycmd", "myarg"])
+
+Vector example::
+
+   os.spawnvp(os.P_NOWAIT, path, args)
+   ==>
+   Popen([path] + args[1:])
+
+Environment example::
+
+   os.spawnlpe(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg", env)
+   ==>
+   Popen(["/bin/mycmd", "myarg"], env={"PATH": "/usr/bin"})
+
+
+Replacing os.popen\*
+^^^^^^^^^^^^^^^^^^^^
+
+::
+
+   pipe = os.popen(cmd, mode='r', bufsize)
+   ==>
+   pipe = Popen(cmd, shell=True, bufsize=bufsize, stdout=PIPE).stdout
+
+::
+
+   pipe = os.popen(cmd, mode='w', bufsize)
+   ==>
+   pipe = Popen(cmd, shell=True, bufsize=bufsize, stdin=PIPE).stdin
+
+::
+
+   (child_stdin, child_stdout) = os.popen2(cmd, mode, bufsize)
+   ==>
+   p = Popen(cmd, shell=True, bufsize=bufsize,
+             stdin=PIPE, stdout=PIPE, close_fds=True)
+   (child_stdin, child_stdout) = (p.stdin, p.stdout)
+
+::
+
+   (child_stdin,
+    child_stdout,
+    child_stderr) = os.popen3(cmd, mode, bufsize)
+   ==>
+   p = Popen(cmd, shell=True, bufsize=bufsize,
+             stdin=PIPE, stdout=PIPE, stderr=PIPE, close_fds=True)
+   (child_stdin,
+    child_stdout,
+    child_stderr) = (p.stdin, p.stdout, p.stderr)
+
+::
+
+   (child_stdin, child_stdout_and_stderr) = os.popen4(cmd, mode, bufsize)
+   ==>
+   p = Popen(cmd, shell=True, bufsize=bufsize,
+             stdin=PIPE, stdout=PIPE, stderr=STDOUT, close_fds=True)
+   (child_stdin, child_stdout_and_stderr) = (p.stdin, p.stdout)
+
+
+Replacing popen2.\*
+^^^^^^^^^^^^^^^^^^^
+
+.. note::
+
+   If the cmd argument to popen2 functions is a string, the command is executed
+   through /bin/sh.  If it is a list, the command is directly executed.
+
+::
+
+   (child_stdout, child_stdin) = popen2.popen2("somestring", bufsize, mode)
+   ==>
+   p = Popen(["somestring"], shell=True, bufsize=bufsize,
+             stdin=PIPE, stdout=PIPE, close_fds=True)
+   (child_stdout, child_stdin) = (p.stdout, p.stdin)
+
+::
+
+   (child_stdout, child_stdin) = popen2.popen2(["mycmd", "myarg"], bufsize, mode)
+   ==>
+   p = Popen(["mycmd", "myarg"], bufsize=bufsize,
+             stdin=PIPE, stdout=PIPE, close_fds=True)
+   (child_stdout, child_stdin) = (p.stdout, p.stdin)
+
+The popen2.Popen3 and popen2.Popen4 basically works as subprocess.Popen, except
+that:
+
+* subprocess.Popen raises an exception if the execution fails
+
+* the *capturestderr* argument is replaced with the *stderr* argument.
+
+* stdin=PIPE and stdout=PIPE must be specified.
+
+* popen2 closes all file descriptors by default, but you have to specify
+  close_fds=True with subprocess.Popen.
+