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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 older modules and functions:

os.system
os.spawn*
os.popen*
popen2.*
commands.*

Information about how this module can be used to replace the older functions can be found in the subprocess-replacements section.

.. seealso::

   POSIX users (Linux, BSD, etc.) are strongly encouraged to install
   and use the much more recent subprocess32_ module instead of the
   version included with python 2.7.  It is a drop in replacement with
   better behavior in many situations.

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

Using the :mod:`subprocess` Module

The recommended way to launch subprocesses is to use the following convenience functions. For more advanced use cases when these do not meet your needs, use the underlying :class:`Popen` interface.

.. function:: call(args, *, stdin=None, stdout=None, stderr=None, shell=False)

   Run the command described by *args*.  Wait for command to complete, then
   return the :attr:`returncode` attribute.

   The arguments shown above are merely the most common ones, described below
   in :ref:`frequently-used-arguments` (hence the slightly odd notation in
   the abbreviated signature). The full function signature is the same as
   that of the :class:`Popen` constructor - this functions passes all
   supplied arguments directly through to that interface.

   Examples::

      >>> subprocess.call(["ls", "-l"])
      0

      >>> subprocess.call("exit 1", shell=True)
      1

   .. warning::

      Using ``shell=True`` can be a security hazard.  See the warning
      under :ref:`frequently-used-arguments` for details.

   .. note::

      Do not use ``stdout=PIPE`` or ``stderr=PIPE`` with this function
      as that can deadlock based on the child process output volume.
      Use :class:`Popen` with the :meth:`communicate` method when you
      need pipes.



.. function:: check_call(args, *, stdin=None, stdout=None, stderr=None, shell=False)

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

   The arguments shown above are merely the most common ones, described below
   in :ref:`frequently-used-arguments` (hence the slightly odd notation in
   the abbreviated signature). The full function signature is the same as
   that of the :class:`Popen` constructor - this functions passes all
   supplied arguments directly through to that interface.

   Examples::

      >>> subprocess.check_call(["ls", "-l"])
      0

      >>> subprocess.check_call("exit 1", shell=True)
      Traceback (most recent call last):
         ...
      subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1

   .. versionadded:: 2.5

   .. warning::

      Using ``shell=True`` can be a security hazard.  See the warning
      under :ref:`frequently-used-arguments` for details.

   .. note::

      Do not use ``stdout=PIPE`` or ``stderr=PIPE`` with this function
      as that can deadlock based on the child process output volume.
      Use :class:`Popen` with the :meth:`communicate` method when you
      need pipes.



.. function:: check_output(args, *, stdin=None, stderr=None, shell=False, universal_newlines=False)

   Run command with arguments and return its output as a byte string.

   If the return code was non-zero it raises a :exc:`CalledProcessError`. The
   :exc:`CalledProcessError` object will have the return code in the
   :attr:`~CalledProcessError.returncode` attribute and any output in the
   :attr:`~CalledProcessError.output` attribute.

   The arguments shown above are merely the most common ones, described below
   in :ref:`frequently-used-arguments` (hence the slightly odd notation in
   the abbreviated signature). The full function signature is largely the
   same as that of the :class:`Popen` constructor, except that *stdout* is
   not permitted as it is used internally. All other supplied arguments are
   passed directly through to the :class:`Popen` constructor.

   Examples::

      >>> subprocess.check_output(["echo", "Hello World!"])
      'Hello World!\n'

      >>> subprocess.check_output("exit 1", shell=True)
      Traceback (most recent call last):
         ...
      subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1

   To also capture standard error in the result, use
   ``stderr=subprocess.STDOUT``::

      >>> subprocess.check_output(
      ...     "ls non_existent_file; exit 0",
      ...     stderr=subprocess.STDOUT,
      ...     shell=True)
      'ls: non_existent_file: No such file or directory\n'

   .. versionadded:: 2.7

   .. warning::

      Using ``shell=True`` can be a security hazard.  See the warning
      under :ref:`frequently-used-arguments` for details.

   .. note::

      Do not use ``stderr=PIPE`` with this function as that can deadlock
      based on the child process error volume.  Use :class:`Popen` with
      the :meth:`communicate` method when you need a stderr pipe.



.. 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.



.. exception:: CalledProcessError

    Exception raised when a process run by :func:`check_call` or
    :func:`check_output` returns a non-zero exit status.

    .. attribute:: returncode

        Exit status of the child process.

    .. attribute:: cmd

        Command that was used to spawn the child process.

    .. attribute:: output

        Output of the child process if this exception is raised by
        :func:`check_output`.  Otherwise, ``None``.

Frequently Used Arguments

To support a wide variety of use cases, the :class:`Popen` constructor (and the convenience functions) accept a large number of optional arguments. For most typical use cases, many of these arguments can be safely left at their default values. The arguments that are most commonly needed are:

args is required for all calls and should be a string, or a sequence of program arguments. Providing a sequence of arguments is generally preferred, as it allows the module to take care of any required escaping and quoting of arguments (e.g. to permit spaces in file names). If passing a single string, either shell must be :const:`True` (see below) or else the string must simply name the program to be executed without specifying any arguments.

stdin, stdout and stderr specify the executed program's 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 the default settings of 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 child process should be captured into the same file handle as for stdout.

.. index::
   single: universal newlines; subprocess module

When stdout or stderr are pipes and universal_newlines is True then all line endings will be converted to '\n' as described for the :term:`universal newlines` 'U' mode argument to :func:`open`.

If shell is True, the specified command will be executed through the shell. This can be useful if you are using Python primarily for the enhanced control flow it offers over most system shells and still want convenient access to other shell features such as shell pipes, filename wildcards, environment variable expansion, and expansion of ~ to a user's home directory. However, note that Python itself offers implementations of many shell-like features (in particular, :mod:`glob`, :mod:`fnmatch`, :func:`os.walk`, :func:`os.path.expandvars`, :func:`os.path.expanduser`, and :mod:`shutil`).

Warning

Executing shell commands that incorporate unsanitized input from an untrusted source makes a program vulnerable to shell injection, a serious security flaw which can result in arbitrary command execution. For this reason, the use of shell=True is strongly discouraged in cases where the command string is constructed from external input:

>>> from subprocess import call
>>> filename = input("What file would you like to display?\n")
What file would you like to display?
non_existent; rm -rf / #
>>> call("cat " + filename, shell=True) # Uh-oh. This will end badly...

shell=False disables all shell based features, but does not suffer from this vulnerability; see the Note in the :class:`Popen` constructor documentation for helpful hints in getting shell=False to work.

When using shell=True, :func:`pipes.quote` can be used to properly escape whitespace and shell metacharacters in strings that are going to be used to construct shell commands.

These options, along with all of the other options, are described in more detail in the :class:`Popen` constructor documentation.

Popen Constructor

The underlying process creation and management in this module is handled by the :class:`Popen` class. It offers a lot of flexibility so that developers are able to handle the less common cases not covered by the convenience functions.

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 child's 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.

:func:`check_call` and :func:`check_output` 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 a system shell implicitly. This means that all characters, including shell metacharacters, can safely be passed to child processes. Obviously, if the shell is invoked explicitly, then it is the application's responsibility to ensure that all whitespace and metacharacters are quoted appropriately.

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:`~Popen.returncode` attribute.



.. method:: Popen.wait()

   Wait for child process to terminate.  Set and return
   :attr:`~Popen.returncode` attribute.

   .. warning::

      This will deadlock when using ``stdout=PIPE`` and/or
      ``stderr=PIPE`` and the child process generates enough output to
      a 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 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, SIGTERM is an alias for :meth:`terminate`. CTRL_C_EVENT and
      CTRL_BREAK_EVENT can be sent to processes started with a *creationflags*
      parameter which includes `CREATE_NEW_PROCESS_GROUP`.

   .. versionadded:: 2.6



.. method:: Popen.terminate()

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

   .. versionadded:: 2.6



.. 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`.

   .. versionadded:: 2.6

The following attributes are also available:

Warning

Use :meth:`~Popen.communicate` rather than :attr:`.stdin.write <Popen.stdin>`, :attr:`.stdout.read <Popen.stdout>` or :attr:`.stderr.read <Popen.stderr>` 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.

   Note that if you set the *shell* argument to ``True``, this is the process ID
   of the spawned shell.



.. 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).

Windows Popen Helpers

The :class:`STARTUPINFO` class and following constants are only available on Windows.

Partial support of the Windows STARTUPINFO structure is used for :class:`Popen` creation.

.. attribute:: dwFlags

   A bit field that determines whether certain :class:`STARTUPINFO`
   attributes are used when the process creates a window. ::

      si = subprocess.STARTUPINFO()
      si.dwFlags = subprocess.STARTF_USESTDHANDLES | subprocess.STARTF_USESHOWWINDOW


.. attribute:: hStdInput

   If :attr:`dwFlags` specifies :data:`STARTF_USESTDHANDLES`, this attribute
   is the standard input handle for the process. If
   :data:`STARTF_USESTDHANDLES` is not specified, the default for standard
   input is the keyboard buffer.


.. attribute:: hStdOutput

   If :attr:`dwFlags` specifies :data:`STARTF_USESTDHANDLES`, this attribute
   is the standard output handle for the process. Otherwise, this attribute
   is ignored and the default for standard output is the console window's
   buffer.


.. attribute:: hStdError

   If :attr:`dwFlags` specifies :data:`STARTF_USESTDHANDLES`, this attribute
   is the standard error handle for the process. Otherwise, this attribute is
   ignored and the default for standard error is the console window's buffer.


.. attribute:: wShowWindow

   If :attr:`dwFlags` specifies :data:`STARTF_USESHOWWINDOW`, this attribute
   can be any of the values that can be specified in the ``nCmdShow``
   parameter for the
   `ShowWindow <https://msdn.microsoft.com/en-us/library/ms633548(v=vs.85).aspx>`__
   function, except for ``SW_SHOWDEFAULT``. Otherwise, this attribute is
   ignored.

   :data:`SW_HIDE` is provided for this attribute. It is used when
   :class:`Popen` is called with ``shell=True``.

Constants

The :mod:`subprocess` module exposes the following constants.

.. data:: STD_INPUT_HANDLE

   The standard input device. Initially, this is the console input buffer,
   ``CONIN$``.


.. data:: STD_OUTPUT_HANDLE

   The standard output device. Initially, this is the active console screen
   buffer, ``CONOUT$``.


.. data:: STD_ERROR_HANDLE

   The standard error device. Initially, this is the active console screen
   buffer, ``CONOUT$``.


.. data:: SW_HIDE

   Hides the window. Another window will be activated.


.. data:: STARTF_USESTDHANDLES

   Specifies that the :attr:`STARTUPINFO.hStdInput`,
   :attr:`STARTUPINFO.hStdOutput`, and :attr:`STARTUPINFO.hStdError` attributes
   contain additional information.


.. data:: STARTF_USESHOWWINDOW

   Specifies that the :attr:`STARTUPINFO.wShowWindow` attribute contains
   additional information.


.. data:: CREATE_NEW_CONSOLE

   The new process has a new console, instead of inheriting its parent's
   console (the default).

   This flag is always set when :class:`Popen` is created with ``shell=True``.


.. data:: CREATE_NEW_PROCESS_GROUP

   A :class:`Popen` ``creationflags`` parameter to specify that a new process
   group will be created. This flag is necessary for using :func:`os.kill`
   on the subprocess.

   This flag is ignored if :data:`CREATE_NEW_CONSOLE` is specified.

Replacing Older Functions with the :mod:`subprocess` Module

In this section, "a becomes b" means that b can be used as a replacement for a.

Note

All "a" functions in this section fail (more or less) silently if the executed program cannot be found; the "b" replacements raise :exc:`OSError` instead.

In addition, the replacements using :func:`check_output` will fail with a :exc:`CalledProcessError` if the requested operation produces a non-zero return code. The output is still available as the :attr:`~CalledProcessError.output` attribute of the raised exception.

In the following examples, we assume that the relevant functions have already been imported from the :mod:`subprocess` module.

Replacing /bin/sh shell backquote

output=`mycmd myarg`

becomes:

output = check_output(["mycmd", "myarg"])

Replacing shell pipeline

output=`dmesg | grep hda`

becomes:

p1 = Popen(["dmesg"], stdout=PIPE)
p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
p1.stdout.close()  # Allow p1 to receive a SIGPIPE if p2 exits.
output = p2.communicate()[0]

The p1.stdout.close() call after starting the p2 is important in order for p1 to receive a SIGPIPE if p2 exits before p1.

Alternatively, for trusted input, the shell's own pipeline support may still be used directly:

output=`dmesg | grep hda`

becomes:

output=check_output("dmesg | grep hda", shell=True)

Replacing :func:`os.system`

status = os.system("mycmd" + " myarg")
# becomes
status = subprocess.call("mycmd" + " myarg", shell=True)

Notes:

  • Calling the program through the shell is usually not required.

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 as e:
    print >>sys.stderr, "Execution failed:", e

Replacing the :func:`os.spawn <os.spawnl>` 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 :func:`os.popen`, :func:`os.popen2`, :func:`os.popen3`

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

(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)

On Unix, os.popen2, os.popen3 and os.popen4 also accept a sequence as the command to execute, in which case arguments will be passed directly to the program without shell intervention. This usage can be replaced as follows:

(child_stdin, child_stdout) = os.popen2(["/bin/ls", "-l"], mode,
                                        bufsize)
==>
p = Popen(["/bin/ls", "-l"], bufsize=bufsize, stdin=PIPE, stdout=PIPE)
(child_stdin, child_stdout) = (p.stdin, p.stdout)

Return code handling translates as follows:

pipe = os.popen("cmd", 'w')
...
rc = pipe.close()
if rc is not None and rc >> 8:
    print "There were some errors"
==>
process = Popen("cmd", shell=True, stdin=PIPE)
...
process.stdin.close()
if process.wait() != 0:
    print "There were some errors"

Replacing functions from the :mod:`popen2` module

(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)

On Unix, popen2 also accepts a sequence as the command to execute, in which case arguments will be passed directly to the program without shell intervention. This usage can be replaced as follows:

(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)

:class:`popen2.Popen3` and :class:`popen2.Popen4` basically work as :class:`subprocess.Popen`, except that:

  • :class:`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 :class:`Popen`.

Notes

Converting an argument sequence to a string on Windows

On Windows, an args sequence is converted to a string that can be parsed using the following rules (which correspond to the rules used by the MS C runtime):

  1. Arguments are delimited by white space, which is either a space or a tab.
  2. A string surrounded by double quotation marks is interpreted as a single argument, regardless of white space contained within. A quoted string can be embedded in an argument.
  3. A double quotation mark preceded by a backslash is interpreted as a literal double quotation mark.
  4. Backslashes are interpreted literally, unless they immediately precede a double quotation mark.
  5. If backslashes immediately precede a double quotation mark, every pair of backslashes is interpreted as a literal backslash. If the number of backslashes is odd, the last backslash escapes the next double quotation mark as described in rule 3.