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Doc/library/subprocess.rst
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[3.14] gh-141473: Document not calling Popen.wait after Popen.communicate times out. (GH-142101) (#142124)
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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>

**Source code:** :source:`Lib/subprocess.py`

--------------

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*

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

.. include:: ../includes/wasm-mobile-notavail.rst

Using the :mod:`subprocess` Module
----------------------------------

The recommended approach to invoking subprocesses is to use the :func:`run`
function for all use cases it can handle. For more advanced use cases, the
underlying :class:`Popen` interface can be used directly.


.. function:: run(args, *, stdin=None, input=None, stdout=None, stderr=None,\
                  capture_output=False, shell=False, cwd=None, timeout=None, \
                  check=False, encoding=None, errors=None, text=None, env=None, \
                  universal_newlines=None, **other_popen_kwargs)

   Run the command described by *args*.  Wait for command to complete, then
   return a :class:`CompletedProcess` instance.

   The arguments shown above are merely the most common ones, described below
   in :ref:`frequently-used-arguments` (hence the use of keyword-only notation
   in the abbreviated signature). The full function signature is largely the
   same as that of the :class:`Popen` constructor - most of the arguments to
   this function are passed through to that interface. (*timeout*,  *input*,
   *check*, and *capture_output* are not.)

   If *capture_output* is true, stdout and stderr will be captured.
   When used, the internal :class:`Popen` object is automatically created with
   *stdout* and *stderr* both set to :data:`~subprocess.PIPE`.
   The *stdout* and *stderr* arguments may not be supplied at the same time as *capture_output*.
   If you wish to capture and combine both streams into one,
   set *stdout* to :data:`~subprocess.PIPE`
   and *stderr* to :data:`~subprocess.STDOUT`,
   instead of using *capture_output*.

   A *timeout* may be specified in seconds, it is internally passed on to
   :meth:`Popen.communicate`. If the timeout expires, the child process will be
   killed and waited for. The :exc:`TimeoutExpired` exception will be
   re-raised after the child process has terminated. The initial process
   creation itself cannot be interrupted on many platform APIs so you are not
   guaranteed to see a timeout exception until at least after however long
   process creation takes.

   The *input* argument is passed to :meth:`Popen.communicate` and thus to the
   subprocess's stdin.  If used it must be a byte sequence, or a string if
   *encoding* or *errors* is specified or *text* is true.  When
   used, the internal :class:`Popen` object is automatically created with
   *stdin* set to :data:`~subprocess.PIPE`,
   and the *stdin* argument may not be used as well.

   If *check* is true, and the process exits with a non-zero exit code, a
   :exc:`CalledProcessError` exception will be raised. Attributes of that
   exception hold the arguments, the exit code, and stdout and stderr if they
   were captured.

   If *encoding* or *errors* are specified, or *text* is true,
   file objects for stdin, stdout and stderr are opened in text mode using the
   specified *encoding* and *errors* or the :class:`io.TextIOWrapper` default.
   The *universal_newlines* argument is equivalent  to *text* and is provided
   for backwards compatibility. By default, file objects are opened in binary mode.

   If *env* is not ``None``, it must be a mapping that defines the environment
   variables for the new process; these are used instead of the default
   behavior of inheriting the current process' environment. It is passed
   directly to :class:`Popen`. This mapping can be str to str on any platform
   or bytes to bytes on POSIX platforms much like :data:`os.environ` or
   :data:`os.environb`.

   Examples::

      >>> subprocess.run(["ls", "-l"])  # doesn't capture output
      CompletedProcess(args=['ls', '-l'], returncode=0)

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

      >>> subprocess.run(["ls", "-l", "/dev/null"], capture_output=True)
      CompletedProcess(args=['ls', '-l', '/dev/null'], returncode=0,
      stdout=b'crw-rw-rw- 1 root root 1, 3 Jan 23 16:23 /dev/null\n', stderr=b'')

   .. versionadded:: 3.5

   .. versionchanged:: 3.6

      Added *encoding* and *errors* parameters

   .. versionchanged:: 3.7

      Added the *text* parameter, as a more understandable alias of *universal_newlines*.
      Added the *capture_output* parameter.

   .. versionchanged:: 3.12

      Changed Windows shell search order for ``shell=True``. The current
      directory and ``%PATH%`` are replaced with ``%COMSPEC%`` and
      ``%SystemRoot%\System32\cmd.exe``. As a result, dropping a
      malicious program named ``cmd.exe`` into a current directory no
      longer works.

.. class:: CompletedProcess

   The return value from :func:`run`, representing a process that has finished.

   .. attribute:: args

      The arguments used to launch the process. This may be a list or a string.

   .. attribute:: returncode

      Exit status of the child process. Typically, an exit status of 0 indicates
      that it ran successfully.

      A negative value ``-N`` indicates that the child was terminated by signal
      ``N`` (POSIX only).

   .. attribute:: stdout

      Captured stdout from the child process. A bytes sequence, or a string if
      :func:`run` was called with an encoding, errors, or text=True.
      ``None`` if stdout was not captured.

      If you ran the process with ``stderr=subprocess.STDOUT``, stdout and
      stderr will be combined in this attribute, and :attr:`stderr` will be
      ``None``.

   .. attribute:: stderr

      Captured stderr from the child process. A bytes sequence, or a string if
      :func:`run` was called with an encoding, errors, or text=True.
      ``None`` if stderr was not captured.

   .. method:: check_returncode()

      If :attr:`returncode` is non-zero, raise a :exc:`CalledProcessError`.

   .. versionadded:: 3.5

.. data:: DEVNULL

   Special value that can be used as the *stdin*, *stdout* or *stderr* argument
   to :class:`Popen` and indicates that the special file :data:`os.devnull`
   will be used.

   .. versionadded:: 3.3


.. 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.  Most useful with :meth:`Popen.communicate`.


.. 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:: SubprocessError

    Base class for all other exceptions from this module.

    .. versionadded:: 3.3


.. exception:: TimeoutExpired

    Subclass of :exc:`SubprocessError`, raised when a timeout expires
    while waiting for a child process.

    .. attribute:: cmd

        Command that was used to spawn the child process.

    .. attribute:: timeout

        Timeout in seconds.

    .. attribute:: output

        Output of the child process if it was captured by :func:`run` or
        :func:`check_output`.  Otherwise, ``None``.  This is always
        :class:`bytes` when any output was captured regardless of the
        ``text=True`` setting.  It may remain ``None`` instead of ``b''``
        when no output was observed.

    .. attribute:: stdout

        Alias for output, for symmetry with :attr:`stderr`.

    .. attribute:: stderr

        Stderr output of the child process if it was captured by :func:`run`.
        Otherwise, ``None``.  This is always :class:`bytes` when stderr output
        was captured regardless of the ``text=True`` setting.  It may remain
        ``None`` instead of ``b''`` when no stderr output was observed.

    .. versionadded:: 3.3

    .. versionchanged:: 3.5
        *stdout* and *stderr* attributes added

.. exception:: CalledProcessError

    Subclass of :exc:`SubprocessError`, raised when a process run by
    :func:`check_call`, :func:`check_output`, or :func:`run` (with ``check=True``)
    returns a non-zero exit status.


    .. attribute:: returncode

        Exit status of the child process.  If the process exited due to a
        signal, this will be the negative signal number.

    .. attribute:: cmd

        Command that was used to spawn the child process.

    .. attribute:: output

        Output of the child process if it was captured by :func:`run` or
        :func:`check_output`.  Otherwise, ``None``.

    .. attribute:: stdout

        Alias for output, for symmetry with :attr:`stderr`.

    .. attribute:: stderr

        Stderr output of the child process if it was captured by :func:`run`.
        Otherwise, ``None``.

    .. versionchanged:: 3.5
        *stdout* and *stderr* attributes added


.. _frequently-used-arguments:

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 ``None``, :data:`PIPE`, :data:`DEVNULL`, an existing file descriptor (a
   positive integer), and an existing :term:`file object` with a valid file
   descriptor.  With the default settings of ``None``, no redirection will
   occur.  :data:`PIPE` indicates that a new pipe to the child should be
   created.  :data:`DEVNULL` indicates that the special file :data:`os.devnull`
   will be used.  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

   If *encoding* or *errors* are specified, or *text* (also known as
   *universal_newlines*) is true,
   the file objects *stdin*, *stdout* and *stderr* will be opened in text
   mode using the *encoding* and *errors* specified in the call or the
   defaults for :class:`io.TextIOWrapper`.

   For *stdin*, line ending characters ``'\n'`` in the input will be converted
   to the default line separator :data:`os.linesep`. For *stdout* and *stderr*,
   all line endings in the output will be converted to ``'\n'``.  For more
   information see the documentation of the :class:`io.TextIOWrapper` class
   when the *newline* argument to its constructor is ``None``.

   If text mode is not used, *stdin*, *stdout* and *stderr* will be opened as
   binary streams. No encoding or line ending conversion is performed.

   .. versionchanged:: 3.6
      Added the *encoding* and *errors* parameters.

   .. versionchanged:: 3.7
      Added the *text* parameter as an alias for *universal_newlines*.

   .. note::

      The newlines attribute of the file objects :attr:`Popen.stdin`,
      :attr:`Popen.stdout` and :attr:`Popen.stderr` are not updated by
      the :meth:`Popen.communicate` method.

   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`).

   .. versionchanged:: 3.3
      When *universal_newlines* is ``True``, the class uses the encoding
      :func:`locale.getpreferredencoding(False) <locale.getpreferredencoding>`
      instead of ``locale.getpreferredencoding()``.  See the
      :class:`io.TextIOWrapper` class for more information on this change.

   .. note::

      Read the `Security Considerations`_ section before using ``shell=True``.

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.


.. class:: Popen(args, bufsize=-1, executable=None, stdin=None, stdout=None, \
                 stderr=None, preexec_fn=None, close_fds=True, shell=False, \
                 cwd=None, env=None, universal_newlines=None, \
                 startupinfo=None, creationflags=0, restore_signals=True, \
                 start_new_session=False, pass_fds=(), *, group=None, \
                 extra_groups=None, user=None, umask=-1, \
                 encoding=None, errors=None, text=None, pipesize=-1, \
                 process_group=None)

   Execute a child program in a new process.  On POSIX, the class uses
   :meth:`os.execvpe`-like behavior to execute the child program.  On Windows,
   the class uses the Windows ``CreateProcess()`` function.  The arguments to
   :class:`Popen` are as follows.

   *args* should be a sequence of program arguments or else a single string
   or :term:`path-like object`.
   By default, the program to execute is the first item in *args* if *args* is
   a sequence.  If *args* is a string, the interpretation is
   platform-dependent and described below.  See the *shell* and *executable*
   arguments for additional differences from the default behavior.  Unless
   otherwise stated, it is recommended to pass *args* as a sequence.

   .. warning::

      For maximum reliability, use a fully qualified path for the executable.
      To search for an unqualified name on :envvar:`PATH`, use
      :meth:`shutil.which`. On all platforms, passing :data:`sys.executable`
      is the recommended way to launch the current Python interpreter again,
      and use the ``-m`` command-line format to launch an installed module.

      Resolving the path of *executable* (or the first item of *args*) is
      platform dependent. For POSIX, see :meth:`os.execvpe`, and note that
      when resolving or searching for the executable path, *cwd* overrides the
      current working directory and *env* can override the ``PATH``
      environment variable. For Windows, see the documentation of the
      ``lpApplicationName`` and ``lpCommandLine`` parameters of WinAPI
      ``CreateProcess``, and note that when resolving or searching for the
      executable path with ``shell=False``, *cwd* does not override the
      current working directory and *env* cannot override the ``PATH``
      environment variable. Using a full path avoids all of these variations.

   An example of passing some arguments to an external program
   as a sequence is::

     Popen(["/usr/bin/git", "commit", "-m", "Fixes a bug."])

   On POSIX, if *args* is a string, the string is interpreted as the name or
   path of the program to execute.  However, this can only be done if not
   passing arguments to the program.

   .. note::

      It may not be obvious how to break a shell command into a sequence of arguments,
      especially in complex cases. :meth:`shlex.split` can illustrate how to
      determine the correct tokenization for *args*::

         >>> import shlex, subprocess
         >>> command_line = input()
         /bin/vikings -input eggs.txt -output "spam spam.txt" -cmd "echo '$MONEY'"
         >>> args = shlex.split(command_line)
         >>> print(args)
         ['/bin/vikings', '-input', 'eggs.txt', '-output', 'spam spam.txt', '-cmd', "echo '$MONEY'"]
         >>> p = subprocess.Popen(args) # Success!

      Note in particular that options (such as *-input*) and arguments (such
      as *eggs.txt*) that are separated by whitespace in the shell go in separate
      list elements, while arguments that need quoting or backslash escaping when
      used in the shell (such as filenames containing spaces or the *echo* command
      shown above) are single list elements.

   On Windows, if *args* is a sequence, it will be converted to a string in a
   manner described in :ref:`converting-argument-sequence`.  This is because
   the underlying ``CreateProcess()`` operates on strings.

   .. versionchanged:: 3.6
      *args* parameter accepts a :term:`path-like object` if *shell* is
      ``False`` and a sequence containing path-like objects on POSIX.

   .. versionchanged:: 3.8
      *args* parameter accepts a :term:`path-like object` if *shell* is
      ``False`` and a sequence containing bytes and path-like objects
      on Windows.

   The *shell* argument (which defaults to ``False``) specifies whether to use
   the shell as the program to execute.  If *shell* is ``True``, it is
   recommended to pass *args* as a string rather than as a sequence.

   On POSIX with ``shell=True``, the shell defaults to :file:`/bin/sh`.  If
   *args* is a string, the string specifies the command
   to execute through the shell.  This means that the string must be
   formatted exactly as it would be when typed at the shell prompt.  This
   includes, for example, quoting or backslash escaping filenames with spaces in
   them.  If *args* is a sequence, the first item specifies the command string, and
   any additional items will be treated as additional arguments to the shell
   itself.  That is to say, :class:`Popen` does the equivalent of::

      Popen(['/bin/sh', '-c', args[0], args[1], ...])

   On Windows with ``shell=True``, the :envvar:`COMSPEC` environment variable
   specifies the default shell.  The only time you need to specify
   ``shell=True`` on Windows is when the command you wish to execute is built
   into the shell (e.g. :command:`dir` or :command:`copy`).  You do not need
   ``shell=True`` to run a batch file or console-based executable.

   .. note::

      Read the `Security Considerations`_ section before using ``shell=True``.

   *bufsize* will be supplied as the corresponding argument to the
   :func:`open` function when creating the stdin/stdout/stderr pipe
   file objects:

   - ``0`` means unbuffered (read and write are one
     system call and can return short)
   - ``1`` means line buffered
     (only usable if ``text=True`` or ``universal_newlines=True``)
   - any other positive value means use a buffer of approximately that
     size
   - negative bufsize (the default) means the system default of
     io.DEFAULT_BUFFER_SIZE will be used.

   .. versionchanged:: 3.3.1
      *bufsize* now defaults to -1 to enable buffering by default to match the
      behavior that most code expects.  In versions prior to Python 3.2.4 and
      3.3.1 it incorrectly defaulted to ``0`` which was unbuffered
      and allowed short reads.  This was unintentional and did not match the
      behavior of Python 2 as most code expected.

   The *executable* argument specifies a replacement program to execute.   It
   is very seldom needed.  When ``shell=False``, *executable* replaces the
   program to execute specified by *args*.  However, the original *args* is
   still passed to the program.  Most programs treat the program specified
   by *args* as the command name, which can then be different from the program
   actually executed.  On POSIX, the *args* name
   becomes the display name for the executable in utilities such as
   :program:`ps`.  If ``shell=True``, on POSIX the *executable* argument
   specifies a replacement shell for the default :file:`/bin/sh`.

   .. versionchanged:: 3.6
      *executable* parameter accepts a :term:`path-like object` on POSIX.

   .. versionchanged:: 3.8
      *executable* parameter accepts a bytes and :term:`path-like object`
      on Windows.

   .. versionchanged:: 3.12

      Changed Windows shell search order for ``shell=True``. The current
      directory and ``%PATH%`` are replaced with ``%COMSPEC%`` and
      ``%SystemRoot%\System32\cmd.exe``. As a result, dropping a
      malicious program named ``cmd.exe`` into a current directory no
      longer works.

   *stdin*, *stdout* and *stderr* specify the executed program's standard input,
   standard output and standard error file handles, respectively.  Valid values
   are ``None``, :data:`PIPE`, :data:`DEVNULL`, an existing file descriptor (a
   positive integer), and an existing :term:`file object` with a valid file
   descriptor.  With the default settings of ``None``, no redirection will
   occur.  :data:`PIPE` indicates that a new pipe to the child should be
   created.  :data:`DEVNULL` indicates that the special file :data:`os.devnull`
   will be used.  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.
   (POSIX only)

   .. warning::

      The *preexec_fn* parameter is NOT SAFE to use in the presence of threads
      in your application.  The child process could deadlock before exec is
      called.

   .. note::

      If you need to modify the environment for the child use the *env*
      parameter rather than doing it in a *preexec_fn*.
      The *start_new_session* and *process_group* parameters should take the place of
      code using *preexec_fn* to call :func:`os.setsid` or :func:`os.setpgid` in the child.

   .. versionchanged:: 3.8

      The *preexec_fn* parameter is no longer supported in subinterpreters.
      The use of the parameter in a subinterpreter raises
      :exc:`RuntimeError`. The new restriction may affect applications that
      are deployed in mod_wsgi, uWSGI, and other embedded environments.

   If *close_fds* is true, all file descriptors except ``0``, ``1`` and
   ``2`` will be closed before the child process is executed.  Otherwise
   when *close_fds* is false, file descriptors obey their inheritable flag
   as described in :ref:`fd_inheritance`.

   On Windows, if *close_fds* is true then no handles will be inherited by the
   child process unless explicitly passed in the ``handle_list`` element of
   :attr:`STARTUPINFO.lpAttributeList`, or by standard handle redirection.

   .. versionchanged:: 3.2
      The default for *close_fds* was changed from :const:`False` to
      what is described above.

   .. versionchanged:: 3.7
      On Windows the default for *close_fds* was changed from :const:`False` to
      :const:`True` when redirecting the standard handles. It's now possible to
      set *close_fds* to :const:`True` when redirecting the standard handles.

   *pass_fds* is an optional sequence of file descriptors to keep open
   between the parent and child.  Providing any *pass_fds* forces
   *close_fds* to be :const:`True`.  (POSIX only)

   .. versionchanged:: 3.2
      The *pass_fds* parameter was added.

   If *cwd* is not ``None``, the function changes the working directory to
   *cwd* before executing the child.  *cwd* can be a string, bytes or
   :term:`path-like <path-like object>` object.  On POSIX, the function
   looks for *executable* (or for the first item in *args*) relative to *cwd*
   if the executable path is a relative path.

   .. versionchanged:: 3.6
      *cwd* parameter accepts a :term:`path-like object` on POSIX.

   .. versionchanged:: 3.7
      *cwd* parameter accepts a :term:`path-like object` on Windows.

   .. versionchanged:: 3.8
      *cwd* parameter accepts a bytes object on Windows.

   If *restore_signals* is true (the default) all signals that Python has set to
   SIG_IGN are restored to SIG_DFL in the child process before the exec.
   Currently this includes the SIGPIPE, SIGXFZ and SIGXFSZ signals.
   (POSIX only)

   .. versionchanged:: 3.2
      *restore_signals* was added.

   If *start_new_session* is true the ``setsid()`` system call will be made in the
   child process prior to the execution of the subprocess.

   .. availability:: POSIX
   .. versionchanged:: 3.2
      *start_new_session* was added.

   If *process_group* is a non-negative integer, the ``setpgid(0, value)`` system call will
   be made in the child process prior to the execution of the subprocess.

   .. availability:: POSIX
   .. versionchanged:: 3.11
      *process_group* was added.

   If *group* is not ``None``, the setregid() system call will be made in the
   child process prior to the execution of the subprocess. If the provided
   value is a string, it will be looked up via :func:`grp.getgrnam` and
   the value in ``gr_gid`` will be used. If the value is an integer, it
   will be passed verbatim. (POSIX only)

   .. availability:: POSIX
   .. versionadded:: 3.9

   If *extra_groups* is not ``None``, the setgroups() system call will be
   made in the child process prior to the execution of the subprocess.
   Strings provided in *extra_groups* will be looked up via
   :func:`grp.getgrnam` and the values in ``gr_gid`` will be used.
   Integer values will be passed verbatim. (POSIX only)

   .. availability:: POSIX
   .. versionadded:: 3.9

   If *user* is not ``None``, the setreuid() system call will be made in the
   child process prior to the execution of the subprocess. If the provided
   value is a string, it will be looked up via :func:`pwd.getpwnam` and
   the value in ``pw_uid`` will be used. If the value is an integer, it will
   be passed verbatim. (POSIX only)

   .. availability:: POSIX
   .. versionadded:: 3.9

   If *umask* is not negative, the umask() system call will be made in the
   child process prior to the execution of the subprocess.

   .. availability:: POSIX
   .. versionadded:: 3.9

   If *env* is not ``None``, it must be a mapping that defines the environment
   variables for the new process; these are used instead of the default
   behavior of inheriting the current process' environment. This mapping can be
   str to str on any platform or bytes to bytes on POSIX platforms much like
   :data:`os.environ` or :data:`os.environb`.

   .. note::

      If specified, *env* must provide any variables required for the program to
      execute.  On Windows, in order to run a `side-by-side assembly`_ the
      specified *env* **must** include a valid ``%SystemRoot%``.

   .. _side-by-side assembly: https://en.wikipedia.org/wiki/Side-by-Side_Assembly

   If *encoding* or *errors* are specified, or *text* is true, the file objects
   *stdin*, *stdout* and *stderr* are opened in text mode with the specified
   *encoding* and *errors*, as described above in :ref:`frequently-used-arguments`.
   The *universal_newlines* argument is equivalent  to *text* and is provided
   for backwards compatibility. By default, file objects are opened in binary mode.

   .. versionadded:: 3.6
      *encoding* and *errors* were added.

   .. versionadded:: 3.7
      *text* was added as a more readable alias for *universal_newlines*.

   If given, *startupinfo* will be a :class:`STARTUPINFO` object, which is
   passed to the underlying ``CreateProcess`` function.

   If given, *creationflags*, can be one or more of the following flags:

   * :data:`CREATE_NEW_CONSOLE`
   * :data:`CREATE_NEW_PROCESS_GROUP`
   * :data:`ABOVE_NORMAL_PRIORITY_CLASS`
   * :data:`BELOW_NORMAL_PRIORITY_CLASS`
   * :data:`HIGH_PRIORITY_CLASS`
   * :data:`IDLE_PRIORITY_CLASS`
   * :data:`NORMAL_PRIORITY_CLASS`
   * :data:`REALTIME_PRIORITY_CLASS`
   * :data:`CREATE_NO_WINDOW`
   * :data:`DETACHED_PROCESS`
   * :data:`CREATE_DEFAULT_ERROR_MODE`
   * :data:`CREATE_BREAKAWAY_FROM_JOB`

   *pipesize* can be used to change the size of the pipe when
   :data:`PIPE` is used for *stdin*, *stdout* or *stderr*. The size of the pipe
   is only changed on platforms that support this (only Linux at this time of
   writing). Other platforms will ignore this parameter.

   .. versionchanged:: 3.10
      Added the *pipesize* parameter.

   Popen objects are supported as context managers via the :keyword:`with` statement:
   on exit, standard file descriptors are closed, and the process is waited for.
   ::

      with Popen(["ifconfig"], stdout=PIPE) as proc:
          log.write(proc.stdout.read())

   .. audit-event:: subprocess.Popen executable,args,cwd,env subprocess.Popen

      Popen and the other functions in this module that use it raise an
      :ref:`auditing event <auditing>` ``subprocess.Popen`` with arguments
      ``executable``, ``args``, ``cwd``, and ``env``. The value for ``args``
      may be a single string or a list of strings, depending on platform.

   .. versionchanged:: 3.2
      Added context manager support.

   .. versionchanged:: 3.6
      Popen destructor now emits a :exc:`ResourceWarning` warning if the child
      process is still running.

   .. versionchanged:: 3.8
      Popen can use :func:`os.posix_spawn` in some cases for better
      performance. On Windows Subsystem for Linux and QEMU User Emulation,
      Popen constructor using :func:`os.posix_spawn` no longer raise an
      exception on errors like missing program, but the child process fails
      with a non-zero :attr:`~Popen.returncode`.


Exceptions
^^^^^^^^^^

Exceptions raised in the child process, before the new program has started to
execute, will be re-raised in the parent.

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. Note that, when ``shell=True``, :exc:`OSError`
will be raised by the child only if the selected shell itself was not found.
To determine if the shell failed to find the requested application, it is
necessary to check the return code or output from the subprocess.

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.

All of the functions and methods that accept a *timeout* parameter, such as
:func:`run` and :meth:`Popen.communicate` will raise :exc:`TimeoutExpired` if
the timeout expires before the process exits.

Exceptions defined in this module all inherit from :exc:`SubprocessError`.

.. versionadded:: 3.3
   The :exc:`SubprocessError` base class was added.

.. _subprocess-security:

Security Considerations
-----------------------

Unlike some other popen functions, this library will not
implicitly choose to call a system shell.  This means that all characters,
including shell metacharacters, can safely be passed to child processes.
If the shell is invoked explicitly, via ``shell=True``, it is the application's
responsibility to ensure that all whitespace and metacharacters are
quoted appropriately to avoid
`shell injection <https://en.wikipedia.org/wiki/Shell_injection#Shell_injection>`_
vulnerabilities. On :ref:`some platforms <shlex-quote-warning>`, it is possible
to use :func:`shlex.quote` for this escaping.

On Windows, batch files (:file:`*.bat` or :file:`*.cmd`) may be launched by the
operating system in a system shell regardless of the arguments passed to this
library. This could result in arguments being parsed according to shell rules,
but without any escaping added by Python. If you are intentionally launching a
batch file with arguments from untrusted sources, consider passing
``shell=True`` to allow Python to escape special characters. See :gh:`114539`
for additional discussion.


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. Otherwise, returns ``None``.


.. method:: Popen.wait(timeout=None)

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

   If the process does not terminate after *timeout* seconds, raise a
   :exc:`TimeoutExpired` exception.  It is safe to catch this exception and
   retry the wait.

   .. note::

      This will deadlock when using ``stdout=PIPE`` 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:`Popen.communicate` when using pipes to avoid that.

   .. note::

      When the ``timeout`` parameter is not ``None``, then (on POSIX) the
      function is implemented using a busy loop (non-blocking call and short
      sleeps). Use the :mod:`asyncio` module for an asynchronous wait: see
      :class:`asyncio.create_subprocess_exec`.

   .. versionchanged:: 3.3
      *timeout* was added.

.. method:: Popen.communicate(input=None, timeout=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 and set the
   :attr:`~Popen.returncode` attribute.  The optional *input* argument should be
   data to be sent to the child process, or ``None``, if no data should be sent
   to the child.  If streams were opened in text mode, *input* must be a string.
   Otherwise, it must be bytes.

   :meth:`communicate` returns a tuple ``(stdout_data, stderr_data)``.
   The data will be strings if streams were opened in text mode; otherwise,
   bytes.

   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.

   If the process does not terminate after *timeout* seconds, a
   :exc:`TimeoutExpired` exception will be raised.  Catching this exception and
   retrying communication will not lose any output.  Supplying *input* to a
   subsequent post-timeout :meth:`communicate` call is in undefined behavior
   and may become an error in the future.

   The child process is not killed if the timeout expires, so in order to
   cleanup properly a well-behaved application should kill the child process and
   finish communication::

      proc = subprocess.Popen(...)
      try:
          outs, errs = proc.communicate(timeout=15)
      except TimeoutExpired:
          proc.kill()
          outs, errs = proc.communicate()

   After a call to :meth:`~Popen.communicate` raises :exc:`TimeoutExpired`, do
   not call :meth:`~Popen.wait`. Use an additional :meth:`~Popen.communicate`
   call to finish handling pipes and populate the :attr:`~Popen.returncode`
   attribute.

   .. note::

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

   .. versionchanged:: 3.3
      *timeout* was added.


.. method:: Popen.send_signal(signal)

   Sends the signal *signal* to the child.

   Do nothing if the process completed.

   .. 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``.


.. method:: Popen.terminate()

   Stop the child. On POSIX OSs the method sends :py:const:`~signal.SIGTERM` to the
   child. On Windows the Win32 API function :c:func:`!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 set by the class for you to access.
Reassigning them to new values is unsupported:

.. attribute:: Popen.args

   The *args* argument as it was passed to :class:`Popen` -- a
   sequence of program arguments or else a single string.

   .. versionadded:: 3.3

.. attribute:: Popen.stdin

   If the *stdin* argument was :data:`PIPE`, this attribute is a writeable
   stream object as returned by :func:`open`. If the *encoding* or *errors*
   arguments were specified or the *text* or *universal_newlines* argument
   was ``True``, the stream is a text stream, otherwise it is a byte stream.
   If the *stdin* argument was not :data:`PIPE`, this attribute is ``None``.


.. attribute:: Popen.stdout

   If the *stdout* argument was :data:`PIPE`, this attribute is a readable
   stream object as returned by :func:`open`. Reading from the stream provides
   output from the child process. If the *encoding* or *errors* arguments were
   specified or the *text* or *universal_newlines* argument was ``True``, the
   stream is a text stream, otherwise it is a byte stream. If the *stdout*
   argument was not :data:`PIPE`, this attribute is ``None``.


.. attribute:: Popen.stderr

   If the *stderr* argument was :data:`PIPE`, this attribute is a readable
   stream object as returned by :func:`open`. Reading from the stream provides
   error output from the child process. If the *encoding* or *errors* arguments
   were specified or the *text* or *universal_newlines* argument was ``True``, the
   stream is a text stream, otherwise it is a byte stream. If the *stderr* argument
   was not :data:`PIPE`, this attribute is ``None``.

.. 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.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. Initially ``None``, :attr:`returncode` is set by
   a call to the :meth:`poll`, :meth:`wait`, or :meth:`communicate` methods
   if they detect that the process has terminated.

   A ``None`` value indicates that the process hadn't yet terminated at the
   time of the last method call.

   A negative value ``-N`` indicates that the child was terminated by signal
   ``N`` (POSIX only).


Windows Popen Helpers
---------------------

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

.. class:: STARTUPINFO(*, dwFlags=0, hStdInput=None, hStdOutput=None, \
                       hStdError=None, wShowWindow=0, lpAttributeList=None)

   Partial support of the Windows
   `STARTUPINFO <https://msdn.microsoft.com/en-us/library/ms686331(v=vs.85).aspx>`__
   structure is used for :class:`Popen` creation.  The following attributes can
   be set by passing them as keyword-only arguments.

   .. versionchanged:: 3.7
      Keyword-only argument support was added.

   .. 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``.

   .. attribute:: lpAttributeList

      A dictionary of additional attributes for process creation as given in
      ``STARTUPINFOEX``, see
      `UpdateProcThreadAttribute <https://msdn.microsoft.com/en-us/library/windows/desktop/ms686880(v=vs.85).aspx>`__.

      Supported attributes:

      **handle_list**
         Sequence of handles that will be inherited. *close_fds* must be true if
         non-empty.

         The handles must be temporarily made inheritable by
         :func:`os.set_handle_inheritable` when passed to the :class:`Popen`
         constructor, else :class:`OSError` will be raised with Windows error
         ``ERROR_INVALID_PARAMETER`` (87).

         .. warning::

            In a multithreaded process, use caution to avoid leaking handles
            that are marked inheritable when combining this feature with
            concurrent calls to other process creation functions that inherit
            all handles such as :func:`os.system`.  This also applies to
            standard handle redirection, which temporarily creates inheritable
            handles.

      .. versionadded:: 3.7

Windows 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:: STARTF_FORCEONFEEDBACK

   A :attr:`STARTUPINFO.dwFlags` parameter to specify that the
   *Working in Background* mouse cursor will be displayed while a
   process is launching. This is the default behavior for GUI
   processes.

   .. versionadded:: 3.13

.. data:: STARTF_FORCEOFFFEEDBACK

   A :attr:`STARTUPINFO.dwFlags` parameter to specify that the mouse
   cursor will not be changed when launching a process.

   .. versionadded:: 3.13

.. data:: CREATE_NEW_CONSOLE

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

.. 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.

.. data:: ABOVE_NORMAL_PRIORITY_CLASS

   A :class:`Popen` ``creationflags`` parameter to specify that a new process
   will have an above average priority.

   .. versionadded:: 3.7

.. data:: BELOW_NORMAL_PRIORITY_CLASS

   A :class:`Popen` ``creationflags`` parameter to specify that a new process
   will have a below average priority.

   .. versionadded:: 3.7

.. data:: HIGH_PRIORITY_CLASS

   A :class:`Popen` ``creationflags`` parameter to specify that a new process
   will have a high priority.

   .. versionadded:: 3.7

.. data:: IDLE_PRIORITY_CLASS

   A :class:`Popen` ``creationflags`` parameter to specify that a new process
   will have an idle (lowest) priority.

   .. versionadded:: 3.7

.. data:: NORMAL_PRIORITY_CLASS

   A :class:`Popen` ``creationflags`` parameter to specify that a new process
   will have a normal priority. (default)

   .. versionadded:: 3.7

.. data:: REALTIME_PRIORITY_CLASS

   A :class:`Popen` ``creationflags`` parameter to specify that a new process
   will have realtime priority.
   You should almost never use REALTIME_PRIORITY_CLASS, because this interrupts
   system threads that manage mouse input, keyboard input, and background disk
   flushing. This class can be appropriate for applications that "talk" directly
   to hardware or that perform brief tasks that should have limited interruptions.

   .. versionadded:: 3.7

.. data:: CREATE_NO_WINDOW

   A :class:`Popen` ``creationflags`` parameter to specify that a new process
   will not create a window.

   .. versionadded:: 3.7

.. data:: DETACHED_PROCESS

   A :class:`Popen` ``creationflags`` parameter to specify that a new process
   will not inherit its parent's console.
   This value cannot be used with CREATE_NEW_CONSOLE.

   .. versionadded:: 3.7

.. data:: CREATE_DEFAULT_ERROR_MODE

   A :class:`Popen` ``creationflags`` parameter to specify that a new process
   does not inherit the error mode of the calling process. Instead, the new
   process gets the default error mode.
   This feature is particularly useful for multithreaded shell applications
   that run with hard errors disabled.

   .. versionadded:: 3.7

.. data:: CREATE_BREAKAWAY_FROM_JOB

   A :class:`Popen` ``creationflags`` parameter to specify that a new process
   is not associated with the job.

   .. versionadded:: 3.7

.. _call-function-trio:

Older high-level API
--------------------

Prior to Python 3.5, these three functions comprised the high level API to
subprocess. You can now use :func:`run` in many cases, but lots of existing code
calls these functions.

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

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

   Code needing to capture stdout or stderr should use :func:`run` instead::

       run(...).returncode

   To suppress stdout or stderr, supply a value of :data:`DEVNULL`.

   The arguments shown above are merely some common ones.
   The full function signature is the
   same as that of the :class:`Popen` constructor - this function passes all
   supplied arguments other than *timeout* directly through to that interface.

   .. note::

      Do not use ``stdout=PIPE`` or ``stderr=PIPE`` with this
      function.  The child process will block if it generates enough
      output to a pipe to fill up the OS pipe buffer as the pipes are
      not being read from.

   .. versionchanged:: 3.3
      *timeout* was added.

   .. versionchanged:: 3.12

      Changed Windows shell search order for ``shell=True``. The current
      directory and ``%PATH%`` are replaced with ``%COMSPEC%`` and
      ``%SystemRoot%\System32\cmd.exe``. As a result, dropping a
      malicious program named ``cmd.exe`` into a current directory no
      longer works.

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

   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.
   If :func:`check_call` was unable to start the process it will propagate the exception
   that was raised.

   Code needing to capture stdout or stderr should use :func:`run` instead::

       run(..., check=True)

   To suppress stdout or stderr, supply a value of :data:`DEVNULL`.

   The arguments shown above are merely some common ones.
   The full function signature is the
   same as that of the :class:`Popen` constructor - this function passes all
   supplied arguments other than *timeout* directly through to that interface.

   .. note::

      Do not use ``stdout=PIPE`` or ``stderr=PIPE`` with this
      function.  The child process will block if it generates enough
      output to a pipe to fill up the OS pipe buffer as the pipes are
      not being read from.

   .. versionchanged:: 3.3
      *timeout* was added.

   .. versionchanged:: 3.12

      Changed Windows shell search order for ``shell=True``. The current
      directory and ``%PATH%`` are replaced with ``%COMSPEC%`` and
      ``%SystemRoot%\System32\cmd.exe``. As a result, dropping a
      malicious program named ``cmd.exe`` into a current directory no
      longer works.


.. function:: check_output(args, *, stdin=None, stderr=None, shell=False, \
                           cwd=None, encoding=None, errors=None, \
                           universal_newlines=None, timeout=None, text=None, \
                           **other_popen_kwargs)

   Run command with arguments and return its output.

   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.

   This is equivalent to::

       run(..., check=True, stdout=PIPE).stdout

   The arguments shown above are merely some common ones.
   The full function signature is largely the same as that of :func:`run` -
   most arguments are passed directly through to that interface.
   One API deviation from :func:`run` behavior exists: passing ``input=None``
   will behave the same as ``input=b''`` (or ``input=''``, depending on other
   arguments) rather than using the parent's standard input file handle.

   By default, this function will return the data as encoded bytes. The actual
   encoding of the output data may depend on the command being invoked, so the
   decoding to text will often need to be handled at the application level.

   This behaviour may be overridden by setting *text*, *encoding*, *errors*,
   or *universal_newlines* to ``True`` as described in
   :ref:`frequently-used-arguments` and :func:`run`.

   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:: 3.1

   .. versionchanged:: 3.3
      *timeout* was added.

   .. versionchanged:: 3.4
      Support for the *input* keyword argument was added.

   .. versionchanged:: 3.6
      *encoding* and *errors* were added.  See :func:`run` for details.

   .. versionadded:: 3.7
      *text* was added as a more readable alias for *universal_newlines*.

   .. versionchanged:: 3.12

      Changed Windows shell search order for ``shell=True``. The current
      directory and ``%PATH%`` are replaced with ``%COMSPEC%`` and
      ``%SystemRoot%\System32\cmd.exe``. As a result, dropping a
      malicious program named ``cmd.exe`` into a current directory no
      longer works.


.. _subprocess-replacements:

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 :program:`/bin/sh` shell command substitution
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: bash

   output=$(mycmd myarg)

becomes::

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

Replacing shell pipeline
^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: bash

   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:

.. code-block:: bash

   output=$(dmesg | grep hda)

becomes::

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


Replacing :func:`os.system`
^^^^^^^^^^^^^^^^^^^^^^^^^^^

::

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

Notes:

* Calling the program through the shell is usually not required.
* The :func:`call` return value is encoded differently to that of
  :func:`os.system`.

* The :func:`os.system` function ignores SIGINT and SIGQUIT signals while
  the command is running, but the caller must do this separately when
  using the :mod:`subprocess` module.

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 :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`
^^^^^^^^^^^^^^^^^^^^^^^^^^

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, stdin=PIPE)
   ...
   process.stdin.close()
   if process.wait() != 0:
       print("There were some errors")


Legacy Shell Invocation Functions
---------------------------------

This module also provides the following legacy functions from the 2.x
``commands`` module. These operations implicitly invoke the system shell and
none of the guarantees described above regarding security and exception
handling consistency are valid for these functions.

.. function:: getstatusoutput(cmd, *, encoding=None, errors=None)

   Return ``(exitcode, output)`` of executing *cmd* in a shell.

   Execute the string *cmd* in a shell with :func:`check_output` and
   return a 2-tuple ``(exitcode, output)``.
   *encoding* and *errors* are used to decode output;
   see the notes on :ref:`frequently-used-arguments` for more details.

   A trailing newline is stripped from the output.
   The exit code for the command can be interpreted as the return code
   of subprocess.  Example::

      >>> subprocess.getstatusoutput('ls /bin/ls')
      (0, '/bin/ls')
      >>> subprocess.getstatusoutput('cat /bin/junk')
      (1, 'cat: /bin/junk: No such file or directory')
      >>> subprocess.getstatusoutput('/bin/junk')
      (127, 'sh: /bin/junk: not found')
      >>> subprocess.getstatusoutput('/bin/kill $$')
      (-15, '')

   .. availability:: Unix, Windows.

   .. versionchanged:: 3.3.4
      Windows support was added.

      The function now returns (exitcode, output) instead of (status, output)
      as it did in Python 3.3.3 and earlier.  exitcode has the same value as
      :attr:`~Popen.returncode`.

   .. versionchanged:: 3.11
      Added the *encoding* and *errors* parameters.

.. function:: getoutput(cmd, *, encoding=None, errors=None)

   Return output (stdout and stderr) of executing *cmd* in a shell.

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

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

   .. availability:: Unix, Windows.

   .. versionchanged:: 3.3.4
      Windows support added

   .. versionchanged:: 3.11
      Added the *encoding* and *errors* parameters.


Notes
-----

.. _subprocess-timeout-behavior:

Timeout Behavior
^^^^^^^^^^^^^^^^

When using the ``timeout`` parameter in functions like :func:`run`,
:meth:`Popen.wait`, or :meth:`Popen.communicate`,
users should be aware of the following behaviors:

1. **Process Creation Delay**: The initial process creation itself cannot be interrupted
   on many platform APIs. This means that even when specifying a timeout, you are not
   guaranteed to see a timeout exception until at least after however long process
   creation takes.

2. **Extremely Small Timeout Values**: Setting very small timeout values (such as a few
   milliseconds) may result in almost immediate :exc:`TimeoutExpired` exceptions because
   process creation and system scheduling inherently require time.

.. _converting-argument-sequence:

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.


.. seealso::

   :mod:`shlex`
      Module which provides function to parse and escape command lines.


.. _disable_posix_spawn:

Disable use of ``posix_spawn()``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

On Linux, :mod:`subprocess` defaults to using the ``vfork()`` system call
internally when it is safe to do so rather than ``fork()``. This greatly
improves performance.

::

   subprocess._USE_POSIX_SPAWN = False  # See CPython issue gh-NNNNNN.

It is safe to set this to false on any Python version. It will have no
effect on older or newer versions where unsupported. Do not assume the attribute
is available to read. Despite the name, a true value does not indicate the
corresponding function will be used, only that it may be.

Please file issues any time you have to use these private knobs with a way to
reproduce the issue you were seeing. Link to that issue from a comment in your
code.

.. versionadded:: 3.8 ``_USE_POSIX_SPAWN``