Skip to content
Permalink
Branch: master
Find file Copy path
Find file Copy path
Fetching contributors…
Cannot retrieve contributors at this time
1480 lines (1017 sloc) 53 KB

: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


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.

The :func:`run` function was added in Python 3.5; if you need to retain compatibility with older versions, see the :ref:`call-function-trio` section.

.. 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)

   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=PIPE`` and ``stderr=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, use ``stdout=PIPE`` and ``stderr=STDOUT``
   instead of *capture_output*.

   The *timeout* argument is passed 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 *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=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`.

   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.

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``.

    .. 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``.

    .. 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` or :func:`check_output` 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

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`, :data:`DEVNULL`, 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. :data:`DEVNULL` indicates that the special file :data:`os.devnull` will be used. 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

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.

.. versionadded:: 3.6
   Added *encoding* and *errors* parameters.

.. versionadded:: 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.

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.

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:`call` 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.


Security Considerations

Unlike some other popen functions, this implementation will never implicitly 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 vulnerabilities.

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

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::

      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.  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.

   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()

   .. 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.

   .. 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 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 available:

.. 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 *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 *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 *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, 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`` (POSIX only).


Windows Popen Helpers

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

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:: 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 an 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

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)

   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.

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

   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.

   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.


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

   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.
   However, explicitly passing ``input=None`` to inherit the parent's
   standard input file handle is not supported.

   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*.


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

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`

sts = os.system("mycmd" + " myarg")
# becomes
sts = 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("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`, :func:`os.popen2`, :func:`os.popen3`

(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)

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")

Replacing functions from the :mod:`popen2` module

Note

If the cmd argument to popen2 functions is a string, the command is executed through /bin/sh. If it is a list, the command is directly executed.

(child_stdout, child_stdin) = popen2.popen2("somestring", bufsize, mode)
==>
p = Popen("somestring", shell=True, bufsize=bufsize,
          stdin=PIPE, stdout=PIPE, close_fds=True)
(child_stdout, child_stdin) = (p.stdout, p.stdin)
(child_stdout, child_stdin) = popen2.popen2(["mycmd", "myarg"], bufsize, mode)
==>
p = Popen(["mycmd", "myarg"], bufsize=bufsize,
          stdin=PIPE, stdout=PIPE, close_fds=True)
(child_stdout, child_stdin) = (p.stdout, p.stdin)

: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` to guarantee this behavior on all platforms or past Python versions.

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)

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

   Execute the string *cmd* in a shell with :meth:`Popen.check_output` and
   return a 2-tuple ``(exitcode, output)``. The locale encoding is used;
   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:: POSIX & 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`.


.. function:: getoutput(cmd)

   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:: POSIX & Windows.

   .. versionchanged:: 3.3.4
      Windows support added


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.
.. seealso::

   :mod:`shlex`
      Module which provides function to parse and escape command lines.
You can’t perform that action at this time.