
``subprocess`` --- Subprocess management
****************************************

The ``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*

Information about how the ``subprocess`` module can be used to replace
these modules and functions can be found in the following sections.

See also:

   **PEP 324** -- PEP proposing the subprocess module


Using the subprocess Module
===========================

This module defines one class called ``Popen``:

class subprocess.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 the string if a string is given, but can be explicitly set by
   using the *executable* argument.

   On Unix, with *shell=False* (default): In this case, the Popen
   class uses ``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 ``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 ``list2cmdline()``
   method.  Please note that not all MS Windows applications interpret
   the command line the same way: ``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: ``0`` means unbuffered,
   ``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 ``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
   ``/bin/sh``.  On Windows, the default shell is specified by the
   **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 ``0``, ``1``
   and ``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 ``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 must be a mapping that defines the
   environment variables for the new process; these are used instead
   of inheriting the current process' environment, which is the
   default behavior.

   If *universal_newlines* is ``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 old
   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 ``stdout``, ``stdin`` and ``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)

subprocess.PIPE

   Special value that can be used as the *stdin*, *stdout* or *stderr*
   argument to ``Popen`` and indicates that a pipe to the standard
   stream should be opened.

subprocess.STDOUT

   Special value that can be used as the *stderr* argument to
   ``Popen`` and indicates that standard error should go into the same
   handle as standard output.


Convenience Functions
---------------------

This module also defines four shortcut functions:

subprocess.call(*popenargs, **kwargs)

   Run command with arguments.  Wait for command to complete, then
   return the ``returncode`` attribute.

   The arguments are the same as for the Popen constructor.  Example:

      retcode = call(["ls", "-l"])

subprocess.check_call(*popenargs, **kwargs)

   Run command with arguments.  Wait for command to complete. If the
   exit code was zero then return, otherwise raise
   ``CalledProcessError``. The ``CalledProcessError`` object will have
   the return code in the ``returncode`` attribute.

   The arguments are the same as for the Popen constructor.  Example:

      check_call(["ls", "-l"])

subprocess.getstatusoutput(cmd)
Return ``(status, output)`` of executing *cmd* in a shell.

   Execute the string *cmd* in a shell with ``os.popen()`` and return
   a 2-tuple ``(status, output)``.  *cmd* is actually run as ``{ cmd ;
   } 2>&1``, so that the returned output will contain output or error
   messages.  A trailing newline is stripped from the output.  The
   exit status for the command can be interpreted according to the
   rules for the C function ``wait``.  Example:

      >>> import subprocess
      >>> subprocess.getstatusoutput('ls /bin/ls')
      (0, '/bin/ls')
      >>> subprocess.getstatusoutput('cat /bin/junk')
      (256, 'cat: /bin/junk: No such file or directory')
      >>> subprocess.getstatusoutput('/bin/junk')
      (256, 'sh: /bin/junk: not found')

   Availability: UNIX.

subprocess.getoutput(cmd)
Return output ``(stdout or stderr)`` of executing *cmd* in a shell.

   Like ``getstatusoutput()``, except the exit status is ignored and
   the return value is a string containing the command's output.
   Example:

      >>> import subprocess
      >>> subprocess.getoutput('ls /bin/ls')
      '/bin/ls'

   Availability: UNIX.


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
``child_traceback``, which is a string containing traceback
information from the childs point of view.

The most common exception raised is ``OSError``.  This occurs, for
example, when trying to execute a non-existent file.  Applications
should prepare for ``OSError`` exceptions.

A ``ValueError`` will be raised if ``Popen`` is called with invalid
arguments.

check_call() will raise ``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 ``Popen`` class have the following methods:

Popen.poll()

   Check if child process has terminated.  Set and return
   ``returncode`` attribute.

Popen.wait()

   Wait for child process to terminate.  Set and return ``returncode``
   attribute.

   Warning: This will deadlock if the child process generates enough output
     to a stdout or stderr pipe such that it blocks waiting for the OS
     pipe buffer to accept more data.  Use ``communicate()`` to avoid
     that.

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 byte string to
   be sent to the child process, or ``None``, if no data should be
   sent to the child.

   ``communicate()`` returns a tuple ``(stdoutdata, stderrdata)``.

   Note that if you want to send data to the process's stdin, you need
   to create the Popen object with ``stdin=PIPE``.  Similarly, to get
   anything other than ``None`` in the result tuple, you need to give
   ``stdout=PIPE`` and/or ``stderr=PIPE`` too.

   Note: The data read is buffered in memory, so do not use this method if
     the data size is large or unlimited.

Popen.send_signal(signal)

   Sends the signal *signal* to the child.

   Note: On Windows only SIGTERM is supported so far. It's an alias for
     ``terminate()``.

Popen.terminate()

   Stop the child. On Posix OSs the method sends SIGTERM to the child.
   On Windows the Win32 API function ``TerminateProcess`` is called to
   stop the child.

Popen.kill()

   Kills the child. On Posix OSs the function sends SIGKILL to the
   child. On Windows ``kill()`` is an alias for ``terminate()``.

The following attributes are also available:

Warning: Use ``communicate()`` rather than ``stdin.write()``,
  ``stdout.read()`` or ``stderr.read()`` to avoid deadlocks due to any
  of the other OS pipe buffers filling up and blocking the child
  process.

Popen.stdin

   If the *stdin* argument was ``PIPE``, this attribute is a file
   object that provides input to the child process.  Otherwise, it is
   ``None``.

Popen.stdout

   If the *stdout* argument was ``PIPE``, this attribute is a file
   object that provides output from the child process.  Otherwise, it
   is ``None``.

Popen.stderr

   If the *stderr* argument was ``PIPE``, this attribute is a file
   object that provides error output from the child process.
   Otherwise, it is ``None``.

Popen.pid

   The process ID of the child process.

Popen.returncode

   The child return code, set by ``poll()`` and ``wait()`` (and
   indirectly by ``communicate()``).  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 ``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 pipeline
------------------------

   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 ``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("Child was terminated by signal", -retcode, file=sys.stderr)
       else:
           print("Child returned", retcode, file=sys.stderr)
   except OSError as e:
       print("Execution failed:", e, file=sys.stderr)


Replacing the os.spawn family
-----------------------------

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, 'r', bufsize)
   ==>
   pipe = Popen(cmd, shell=True, bufsize=bufsize, stdout=PIPE).stdout

   pipe = os.popen(cmd, 'w', bufsize)
   ==>
   pipe = Popen(cmd, shell=True, bufsize=bufsize, stdin=PIPE).stdin
