Doc/library/subprocess.rst

:mod:`subprocess` --- Subprocess management
===========================================

.. module:: subprocess
   :synopsis: Subprocess management.
.. moduleauthor:: Peter Åstrand <astrand@lysator.liu.se>
.. sectionauthor:: Peter Åstrand <astrand@lysator.liu.se>


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*

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

.. seealso::

   :pep:`324` -- PEP proposing the subprocess module


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 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
   :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 :data:`PIPE`, an existing file descriptor (a positive integer), an
   existing file object, and ``None``.  :data:`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 :data:`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 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 :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 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 :attr:`stdout`,
      :attr:`stdin` and :attr:`stderr` are not updated by the :meth:`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)


.. data:: PIPE

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


.. data:: STDOUT

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

Convenience Functions
^^^^^^^^^^^^^^^^^^^^^

This module also defines four 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"])


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

   Execute the string *cmd* in a shell with :func:`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 :cfunc:`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')


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

   Like :func:`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'


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.  Set and return :attr:`returncode`
   attribute.


.. method:: Popen.wait()

   Wait for child process to terminate.  Set and return :attr:`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 :meth:`communicate` to avoid that.


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

   :meth:`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.


.. method:: 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
      :meth:`terminate`.


.. method:: Popen.terminate()

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


.. method:: Popen.kill()

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


The following attributes are also available:

.. warning::

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


.. attribute:: Popen.stdin

   If the *stdin* argument was :data:`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 was :data:`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 was :data:`PIPE`, this attribute is a 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, set by :meth:`poll` and :meth:`wait` (and indirectly
   by :meth:`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).


.. _subprocess-replacements:

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 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 :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("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