936 lines
33 KiB
ReStructuredText
936 lines
33 KiB
ReStructuredText
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:mod:`subprocess` --- Subprocess management
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===========================================
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.. module:: subprocess
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:synopsis: Subprocess management.
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.. moduleauthor:: Peter Åstrand <astrand@lysator.liu.se>
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.. sectionauthor:: Peter Åstrand <astrand@lysator.liu.se>
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.. versionadded:: 2.4
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The :mod:`subprocess` module allows you to spawn new processes, connect to their
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input/output/error pipes, and obtain their return codes. This module intends to
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replace several older modules and functions::
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os.system
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os.spawn*
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os.popen*
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popen2.*
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commands.*
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Information about how this module can be used to replace the older
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functions can be found in the subprocess-replacements_ section.
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.. seealso::
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POSIX users (Linux, BSD, etc.) are strongly encouraged to install
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and use the much more recent subprocess32_ module instead of the
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version included with python 2.7. It is a drop in replacement with
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better behavior in many situations.
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:pep:`324` -- PEP proposing the subprocess module
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.. _subprocess32: https://pypi.python.org/pypi/subprocess32/
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Using the :mod:`subprocess` Module
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----------------------------------
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The recommended way to launch subprocesses is to use the following
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convenience functions. For more advanced use cases when these do not
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meet your needs, use the underlying :class:`Popen` interface.
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.. function:: call(args, *, stdin=None, stdout=None, stderr=None, shell=False)
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Run the command described by *args*. Wait for command to complete, then
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return the :attr:`returncode` attribute.
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The arguments shown above are merely the most common ones, described below
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in :ref:`frequently-used-arguments` (hence the slightly odd notation in
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the abbreviated signature). The full function signature is the same as
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that of the :class:`Popen` constructor - this functions passes all
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supplied arguments directly through to that interface.
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Examples::
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>>> subprocess.call(["ls", "-l"])
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0
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>>> subprocess.call("exit 1", shell=True)
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1
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.. warning::
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Using ``shell=True`` can be a security hazard. See the warning
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under :ref:`frequently-used-arguments` for details.
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.. note::
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Do not use ``stdout=PIPE`` or ``stderr=PIPE`` with this function
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as that can deadlock based on the child process output volume.
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Use :class:`Popen` with the :meth:`communicate` method when you
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need pipes.
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.. function:: check_call(args, *, stdin=None, stdout=None, stderr=None, shell=False)
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Run command with arguments. Wait for command to complete. If the return
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code was zero then return, otherwise raise :exc:`CalledProcessError`. The
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:exc:`CalledProcessError` object will have the return code in the
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:attr:`~CalledProcessError.returncode` attribute.
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The arguments shown above are merely the most common ones, described below
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in :ref:`frequently-used-arguments` (hence the slightly odd notation in
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the abbreviated signature). The full function signature is the same as
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that of the :class:`Popen` constructor - this functions passes all
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supplied arguments directly through to that interface.
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Examples::
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>>> subprocess.check_call(["ls", "-l"])
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0
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>>> subprocess.check_call("exit 1", shell=True)
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Traceback (most recent call last):
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...
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subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1
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.. versionadded:: 2.5
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.. warning::
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Using ``shell=True`` can be a security hazard. See the warning
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under :ref:`frequently-used-arguments` for details.
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.. note::
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Do not use ``stdout=PIPE`` or ``stderr=PIPE`` with this function
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as that can deadlock based on the child process output volume.
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Use :class:`Popen` with the :meth:`communicate` method when you
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need pipes.
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.. function:: check_output(args, *, stdin=None, stderr=None, shell=False, universal_newlines=False)
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Run command with arguments and return its output as a byte string.
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If the return code was non-zero it raises a :exc:`CalledProcessError`. The
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:exc:`CalledProcessError` object will have the return code in the
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:attr:`~CalledProcessError.returncode` attribute and any output in the
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:attr:`~CalledProcessError.output` attribute.
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The arguments shown above are merely the most common ones, described below
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in :ref:`frequently-used-arguments` (hence the slightly odd notation in
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the abbreviated signature). The full function signature is largely the
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same as that of the :class:`Popen` constructor, except that *stdout* is
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not permitted as it is used internally. All other supplied arguments are
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passed directly through to the :class:`Popen` constructor.
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Examples::
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>>> subprocess.check_output(["echo", "Hello World!"])
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'Hello World!\n'
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>>> subprocess.check_output("exit 1", shell=True)
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Traceback (most recent call last):
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...
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subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1
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To also capture standard error in the result, use
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``stderr=subprocess.STDOUT``::
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>>> subprocess.check_output(
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... "ls non_existent_file; exit 0",
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... stderr=subprocess.STDOUT,
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... shell=True)
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'ls: non_existent_file: No such file or directory\n'
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.. versionadded:: 2.7
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.. warning::
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Using ``shell=True`` can be a security hazard. See the warning
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under :ref:`frequently-used-arguments` for details.
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.. note::
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Do not use ``stderr=PIPE`` with this function as that can deadlock
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based on the child process error volume. Use :class:`Popen` with
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the :meth:`communicate` method when you need a stderr pipe.
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.. data:: PIPE
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Special value that can be used as the *stdin*, *stdout* or *stderr* argument
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to :class:`Popen` and indicates that a pipe to the standard stream should be
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opened.
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.. data:: STDOUT
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Special value that can be used as the *stderr* argument to :class:`Popen` and
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indicates that standard error should go into the same handle as standard
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output.
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.. exception:: CalledProcessError
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Exception raised when a process run by :func:`check_call` or
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:func:`check_output` returns a non-zero exit status.
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.. attribute:: returncode
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Exit status of the child process.
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.. attribute:: cmd
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Command that was used to spawn the child process.
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.. attribute:: output
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Output of the child process if this exception is raised by
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:func:`check_output`. Otherwise, ``None``.
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.. _frequently-used-arguments:
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Frequently Used Arguments
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^^^^^^^^^^^^^^^^^^^^^^^^^
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To support a wide variety of use cases, the :class:`Popen` constructor (and
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the convenience functions) accept a large number of optional arguments. For
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most typical use cases, many of these arguments can be safely left at their
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default values. The arguments that are most commonly needed are:
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*args* is required for all calls and should be a string, or a sequence of
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program arguments. Providing a sequence of arguments is generally
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preferred, as it allows the module to take care of any required escaping
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and quoting of arguments (e.g. to permit spaces in file names). If passing
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a single string, either *shell* must be :const:`True` (see below) or else
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the string must simply name the program to be executed without specifying
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any arguments.
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*stdin*, *stdout* and *stderr* specify the executed program's standard input,
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standard output and standard error file handles, respectively. Valid values
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are :data:`PIPE`, an existing file descriptor (a positive integer), an
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existing file object, and ``None``. :data:`PIPE` indicates that a new pipe
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to the child should be created. With the default settings of ``None``, no
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redirection will occur; the child's file handles will be inherited from the
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parent. Additionally, *stderr* can be :data:`STDOUT`, which indicates that
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the stderr data from the child process should be captured into the same file
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handle as for stdout.
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.. index::
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single: universal newlines; subprocess module
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When *stdout* or *stderr* are pipes and *universal_newlines* is
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``True`` then all line endings will be converted to ``'\n'`` as described
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for the :term:`universal newlines` ``'U'`` mode argument to :func:`open`.
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If *shell* is ``True``, the specified command will be executed through
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the shell. This can be useful if you are using Python primarily for the
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enhanced control flow it offers over most system shells and still want
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convenient access to other shell features such as shell pipes, filename
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wildcards, environment variable expansion, and expansion of ``~`` to a
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user's home directory. However, note that Python itself offers
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implementations of many shell-like features (in particular, :mod:`glob`,
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:mod:`fnmatch`, :func:`os.walk`, :func:`os.path.expandvars`,
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:func:`os.path.expanduser`, and :mod:`shutil`).
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.. warning::
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Executing shell commands that incorporate unsanitized input from an
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untrusted source makes a program vulnerable to `shell injection
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<http://en.wikipedia.org/wiki/Shell_injection#Shell_injection>`_,
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a serious security flaw which can result in arbitrary command execution.
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For this reason, the use of ``shell=True`` is **strongly discouraged**
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in cases where the command string is constructed from external input::
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>>> from subprocess import call
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>>> filename = input("What file would you like to display?\n")
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What file would you like to display?
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non_existent; rm -rf / #
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>>> call("cat " + filename, shell=True) # Uh-oh. This will end badly...
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``shell=False`` disables all shell based features, but does not suffer
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from this vulnerability; see the Note in the :class:`Popen` constructor
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documentation for helpful hints in getting ``shell=False`` to work.
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When using ``shell=True``, :func:`pipes.quote` can be used to properly
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escape whitespace and shell metacharacters in strings that are going to
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be used to construct shell commands.
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These options, along with all of the other options, are described in more
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detail in the :class:`Popen` constructor documentation.
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Popen Constructor
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^^^^^^^^^^^^^^^^^
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The underlying process creation and management in this module is handled by
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the :class:`Popen` class. It offers a lot of flexibility so that developers
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are able to handle the less common cases not covered by the convenience
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functions.
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.. class:: Popen(args, bufsize=0, executable=None, stdin=None, stdout=None, \
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stderr=None, preexec_fn=None, close_fds=False, shell=False, \
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cwd=None, env=None, universal_newlines=False, \
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startupinfo=None, creationflags=0)
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Execute a child program in a new process. On Unix, the class uses
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:meth:`os.execvp`-like behavior to execute the child program. On Windows,
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the class uses the Windows ``CreateProcess()`` function. The arguments to
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:class:`Popen` are as follows.
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*args* should be a sequence of program arguments or else a single string.
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By default, the program to execute is the first item in *args* if *args* is
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a sequence. If *args* is a string, the interpretation is
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platform-dependent and described below. See the *shell* and *executable*
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arguments for additional differences from the default behavior. Unless
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otherwise stated, it is recommended to pass *args* as a sequence.
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On Unix, if *args* is a string, the string is interpreted as the name or
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path of the program to execute. However, this can only be done if not
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passing arguments to the program.
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.. note::
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:meth:`shlex.split` can be useful when determining the correct
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tokenization for *args*, especially in complex cases::
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>>> import shlex, subprocess
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>>> command_line = raw_input()
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/bin/vikings -input eggs.txt -output "spam spam.txt" -cmd "echo '$MONEY'"
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>>> args = shlex.split(command_line)
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>>> print args
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['/bin/vikings', '-input', 'eggs.txt', '-output', 'spam spam.txt', '-cmd', "echo '$MONEY'"]
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>>> p = subprocess.Popen(args) # Success!
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Note in particular that options (such as *-input*) and arguments (such
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as *eggs.txt*) that are separated by whitespace in the shell go in separate
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list elements, while arguments that need quoting or backslash escaping when
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used in the shell (such as filenames containing spaces or the *echo* command
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shown above) are single list elements.
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On Windows, if *args* is a sequence, it will be converted to a string in a
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manner described in :ref:`converting-argument-sequence`. This is because
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the underlying ``CreateProcess()`` operates on strings.
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The *shell* argument (which defaults to *False*) specifies whether to use
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the shell as the program to execute. If *shell* is *True*, it is
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recommended to pass *args* as a string rather than as a sequence.
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On Unix with ``shell=True``, the shell defaults to :file:`/bin/sh`. If
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*args* is a string, the string specifies the command
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to execute through the shell. This means that the string must be
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formatted exactly as it would be when typed at the shell prompt. This
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includes, for example, quoting or backslash escaping filenames with spaces in
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them. If *args* is a sequence, the first item specifies the command string, and
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any additional items will be treated as additional arguments to the shell
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itself. That is to say, :class:`Popen` does the equivalent of::
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Popen(['/bin/sh', '-c', args[0], args[1], ...])
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On Windows with ``shell=True``, the :envvar:`COMSPEC` environment variable
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specifies the default shell. The only time you need to specify
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``shell=True`` on Windows is when the command you wish to execute is built
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into the shell (e.g. :command:`dir` or :command:`copy`). You do not need
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``shell=True`` to run a batch file or console-based executable.
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.. warning::
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Passing ``shell=True`` can be a security hazard if combined with
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untrusted input. See the warning under :ref:`frequently-used-arguments`
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for details.
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*bufsize*, if given, has the same meaning as the corresponding argument to the
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built-in open() function: :const:`0` means unbuffered, :const:`1` means line
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buffered, any other positive value means use a buffer of (approximately) that
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size. A negative *bufsize* means to use the system default, which usually means
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fully buffered. The default value for *bufsize* is :const:`0` (unbuffered).
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.. note::
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If you experience performance issues, it is recommended that you try to
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enable buffering by setting *bufsize* to either -1 or a large enough
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positive value (such as 4096).
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The *executable* argument specifies a replacement program to execute. It
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is very seldom needed. When ``shell=False``, *executable* replaces the
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program to execute specified by *args*. However, the original *args* is
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still passed to the program. Most programs treat the program specified
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by *args* as the command name, which can then be different from the program
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actually executed. On Unix, the *args* name
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becomes the display name for the executable in utilities such as
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:program:`ps`. If ``shell=True``, on Unix the *executable* argument
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specifies a replacement shell for the default :file:`/bin/sh`.
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*stdin*, *stdout* and *stderr* specify the executed program's standard input,
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standard output and standard error file handles, respectively. Valid values
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are :data:`PIPE`, an existing file descriptor (a positive integer), an
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existing file object, and ``None``. :data:`PIPE` indicates that a new pipe
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to the child should be created. With the default settings of ``None``, no
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redirection will occur; the child's file handles will be inherited from the
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parent. Additionally, *stderr* can be :data:`STDOUT`, which indicates that
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the stderr data from the child process should be captured into the same file
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handle as for stdout.
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If *preexec_fn* is set to a callable object, this object will be called in the
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child process just before the child is executed. (Unix only)
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If *close_fds* is true, all file descriptors except :const:`0`, :const:`1` and
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:const:`2` will be closed before the child process is executed. (Unix only).
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Or, on Windows, if *close_fds* is true then no handles will be inherited by the
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child process. Note that on Windows, you cannot set *close_fds* to true and
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also redirect the standard handles by setting *stdin*, *stdout* or *stderr*.
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If *cwd* is not ``None``, the child's current directory will be changed to *cwd*
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before it is executed. Note that this directory is not considered when
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searching the executable, so you can't specify the program's path relative to
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*cwd*.
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If *env* is not ``None``, it must be a mapping that defines the environment
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variables for the new process; these are used instead of inheriting the current
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process' environment, which is the default behavior.
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.. note::
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If specified, *env* must provide any variables required
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for the program to execute. On Windows, in order to run a
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`side-by-side assembly`_ the specified *env* **must** include a valid
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:envvar:`SystemRoot`.
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.. _side-by-side assembly: https://en.wikipedia.org/wiki/Side-by-Side_Assembly
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If *universal_newlines* is ``True``, the file objects *stdout* and *stderr*
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are opened as text files in :term:`universal newlines` mode. Lines may be
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terminated by any of ``'\n'``, the Unix end-of-line convention, ``'\r'``,
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the old Macintosh convention or ``'\r\n'``, the Windows convention. All of
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these external representations are seen as ``'\n'`` by the Python program.
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.. note::
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This feature is only available if Python is built with universal newline
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support (the default). Also, the newlines attribute of the file objects
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:attr:`stdout`, :attr:`stdin` and :attr:`stderr` are not updated by the
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communicate() method.
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If given, *startupinfo* will be a :class:`STARTUPINFO` object, which is
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passed to the underlying ``CreateProcess`` function.
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*creationflags*, if given, can be :data:`CREATE_NEW_CONSOLE` or
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:data:`CREATE_NEW_PROCESS_GROUP`. (Windows only)
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Exceptions
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^^^^^^^^^^
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Exceptions raised in the child process, before the new program has started to
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execute, will be re-raised in the parent. Additionally, the exception object
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will have one extra attribute called :attr:`child_traceback`, which is a string
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containing traceback information from the child's point of view.
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The most common exception raised is :exc:`OSError`. This occurs, for example,
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when trying to execute a non-existent file. Applications should prepare for
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:exc:`OSError` exceptions.
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A :exc:`ValueError` will be raised if :class:`Popen` is called with invalid
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arguments.
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:func:`check_call` and :func:`check_output` will raise
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:exc:`CalledProcessError` if the called process returns a non-zero return
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code.
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Security
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^^^^^^^^
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Unlike some other popen functions, this implementation will never call a
|
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system shell implicitly. This means that all characters, including shell
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metacharacters, can safely be passed to child processes. Obviously, if the
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shell is invoked explicitly, then it is the application's responsibility to
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ensure that all whitespace and metacharacters are quoted appropriately.
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Popen Objects
|
|
-------------
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Instances of the :class:`Popen` class have the following methods:
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|
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.. method:: Popen.poll()
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Check if child process has terminated. Set and return
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:attr:`~Popen.returncode` attribute.
|
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|
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.. method:: Popen.wait()
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Wait for child process to terminate. Set and return
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:attr:`~Popen.returncode` attribute.
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.. warning::
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This will deadlock when using ``stdout=PIPE`` and/or
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``stderr=PIPE`` and the child process generates enough output to
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a pipe such that it blocks waiting for the OS pipe buffer to
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accept more data. Use :meth:`communicate` to avoid that.
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|
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.. method:: Popen.communicate(input=None)
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Interact with process: Send data to stdin. Read data from stdout and stderr,
|
|
until end-of-file is reached. Wait for process to terminate. The optional
|
|
*input* argument should be a string to be sent to the child process, or
|
|
``None``, if no data should be sent to the child.
|
|
|
|
:meth:`communicate` returns a tuple ``(stdoutdata, stderrdata)``.
|
|
|
|
Note that if you want to send data to the process's stdin, you need to create
|
|
the Popen object with ``stdin=PIPE``. Similarly, to get anything other than
|
|
``None`` in the result tuple, you need to give ``stdout=PIPE`` and/or
|
|
``stderr=PIPE`` too.
|
|
|
|
.. note::
|
|
|
|
The data read is buffered in memory, so do not use this method if the data
|
|
size is large or unlimited.
|
|
|
|
|
|
.. method:: Popen.send_signal(signal)
|
|
|
|
Sends the signal *signal* to the child.
|
|
|
|
.. note::
|
|
|
|
On Windows, SIGTERM is an alias for :meth:`terminate`. CTRL_C_EVENT and
|
|
CTRL_BREAK_EVENT can be sent to processes started with a *creationflags*
|
|
parameter which includes `CREATE_NEW_PROCESS_GROUP`.
|
|
|
|
.. versionadded:: 2.6
|
|
|
|
|
|
.. method:: Popen.terminate()
|
|
|
|
Stop the child. On Posix OSs the method sends SIGTERM to the
|
|
child. On Windows the Win32 API function :c:func:`TerminateProcess` is called
|
|
to stop the child.
|
|
|
|
.. versionadded:: 2.6
|
|
|
|
|
|
.. method:: Popen.kill()
|
|
|
|
Kills the child. On Posix OSs the function sends SIGKILL to the child.
|
|
On Windows :meth:`kill` is an alias for :meth:`terminate`.
|
|
|
|
.. versionadded:: 2.6
|
|
|
|
|
|
The following attributes are also available:
|
|
|
|
.. warning::
|
|
|
|
Use :meth:`~Popen.communicate` rather than :attr:`.stdin.write <Popen.stdin>`,
|
|
:attr:`.stdout.read <Popen.stdout>` or :attr:`.stderr.read <Popen.stderr>` to avoid
|
|
deadlocks due to any of the other OS pipe buffers filling up and blocking the
|
|
child process.
|
|
|
|
|
|
.. attribute:: Popen.stdin
|
|
|
|
If the *stdin* argument was :data:`PIPE`, this attribute is a file object
|
|
that provides input to the child process. Otherwise, it is ``None``.
|
|
|
|
|
|
.. attribute:: Popen.stdout
|
|
|
|
If the *stdout* argument was :data:`PIPE`, this attribute is a file object
|
|
that provides output from the child process. Otherwise, it is ``None``.
|
|
|
|
|
|
.. attribute:: Popen.stderr
|
|
|
|
If the *stderr* argument was :data:`PIPE`, this attribute is a file object
|
|
that provides error output from the child process. Otherwise, it is
|
|
``None``.
|
|
|
|
|
|
.. attribute:: Popen.pid
|
|
|
|
The process ID of the child process.
|
|
|
|
Note that if you set the *shell* argument to ``True``, this is the process ID
|
|
of the spawned shell.
|
|
|
|
|
|
.. attribute:: Popen.returncode
|
|
|
|
The child return code, set by :meth:`poll` and :meth:`wait` (and indirectly
|
|
by :meth:`communicate`). A ``None`` value indicates that the process
|
|
hasn't terminated yet.
|
|
|
|
A negative value ``-N`` indicates that the child was terminated by signal
|
|
``N`` (Unix only).
|
|
|
|
|
|
Windows Popen Helpers
|
|
---------------------
|
|
|
|
The :class:`STARTUPINFO` class and following constants are only available
|
|
on Windows.
|
|
|
|
.. class:: STARTUPINFO()
|
|
|
|
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.
|
|
|
|
.. attribute:: dwFlags
|
|
|
|
A bit field that determines whether certain :class:`STARTUPINFO`
|
|
attributes are used when the process creates a window. ::
|
|
|
|
si = subprocess.STARTUPINFO()
|
|
si.dwFlags = subprocess.STARTF_USESTDHANDLES | subprocess.STARTF_USESHOWWINDOW
|
|
|
|
.. attribute:: hStdInput
|
|
|
|
If :attr:`dwFlags` specifies :data:`STARTF_USESTDHANDLES`, this attribute
|
|
is the standard input handle for the process. If
|
|
:data:`STARTF_USESTDHANDLES` is not specified, the default for standard
|
|
input is the keyboard buffer.
|
|
|
|
.. attribute:: hStdOutput
|
|
|
|
If :attr:`dwFlags` specifies :data:`STARTF_USESTDHANDLES`, this attribute
|
|
is the standard output handle for the process. Otherwise, this attribute
|
|
is ignored and the default for standard output is the console window's
|
|
buffer.
|
|
|
|
.. attribute:: hStdError
|
|
|
|
If :attr:`dwFlags` specifies :data:`STARTF_USESTDHANDLES`, this attribute
|
|
is the standard error handle for the process. Otherwise, this attribute is
|
|
ignored and the default for standard error is the console window's buffer.
|
|
|
|
.. attribute:: wShowWindow
|
|
|
|
If :attr:`dwFlags` specifies :data:`STARTF_USESHOWWINDOW`, this attribute
|
|
can be any of the values that can be specified in the ``nCmdShow``
|
|
parameter for the
|
|
`ShowWindow <https://msdn.microsoft.com/en-us/library/ms633548(v=vs.85).aspx>`__
|
|
function, except for ``SW_SHOWDEFAULT``. Otherwise, this attribute is
|
|
ignored.
|
|
|
|
:data:`SW_HIDE` is provided for this attribute. It is used when
|
|
:class:`Popen` is called with ``shell=True``.
|
|
|
|
|
|
Constants
|
|
^^^^^^^^^
|
|
|
|
The :mod:`subprocess` module exposes the following constants.
|
|
|
|
.. data:: STD_INPUT_HANDLE
|
|
|
|
The standard input device. Initially, this is the console input buffer,
|
|
``CONIN$``.
|
|
|
|
.. data:: STD_OUTPUT_HANDLE
|
|
|
|
The standard output device. Initially, this is the active console screen
|
|
buffer, ``CONOUT$``.
|
|
|
|
.. data:: STD_ERROR_HANDLE
|
|
|
|
The standard error device. Initially, this is the active console screen
|
|
buffer, ``CONOUT$``.
|
|
|
|
.. data:: SW_HIDE
|
|
|
|
Hides the window. Another window will be activated.
|
|
|
|
.. data:: STARTF_USESTDHANDLES
|
|
|
|
Specifies that the :attr:`STARTUPINFO.hStdInput`,
|
|
:attr:`STARTUPINFO.hStdOutput`, and :attr:`STARTUPINFO.hStdError` attributes
|
|
contain additional information.
|
|
|
|
.. data:: STARTF_USESHOWWINDOW
|
|
|
|
Specifies that the :attr:`STARTUPINFO.wShowWindow` attribute contains
|
|
additional information.
|
|
|
|
.. data:: CREATE_NEW_CONSOLE
|
|
|
|
The new process has a new console, instead of inheriting its parent's
|
|
console (the default).
|
|
|
|
This flag is always set when :class:`Popen` is created with ``shell=True``.
|
|
|
|
.. data:: CREATE_NEW_PROCESS_GROUP
|
|
|
|
A :class:`Popen` ``creationflags`` parameter to specify that a new process
|
|
group will be created. This flag is necessary for using :func:`os.kill`
|
|
on the subprocess.
|
|
|
|
This flag is ignored if :data:`CREATE_NEW_CONSOLE` is specified.
|
|
|
|
|
|
.. _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 /bin/sh shell backquote
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
.. 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`
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
::
|
|
|
|
status = os.system("mycmd" + " myarg")
|
|
# becomes
|
|
status = subprocess.call("mycmd" + " myarg", shell=True)
|
|
|
|
Notes:
|
|
|
|
* Calling the program through the shell is usually not required.
|
|
|
|
A more realistic example would look like this::
|
|
|
|
try:
|
|
retcode = call("mycmd" + " myarg", shell=True)
|
|
if retcode < 0:
|
|
print >>sys.stderr, "Child was terminated by signal", -retcode
|
|
else:
|
|
print >>sys.stderr, "Child returned", retcode
|
|
except OSError as e:
|
|
print >>sys.stderr, "Execution failed:", e
|
|
|
|
|
|
Replacing the :func:`os.spawn <os.spawnl>` family
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
P_NOWAIT example::
|
|
|
|
pid = os.spawnlp(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg")
|
|
==>
|
|
pid = Popen(["/bin/mycmd", "myarg"]).pid
|
|
|
|
P_WAIT example::
|
|
|
|
retcode = os.spawnlp(os.P_WAIT, "/bin/mycmd", "mycmd", "myarg")
|
|
==>
|
|
retcode = call(["/bin/mycmd", "myarg"])
|
|
|
|
Vector example::
|
|
|
|
os.spawnvp(os.P_NOWAIT, path, args)
|
|
==>
|
|
Popen([path] + args[1:])
|
|
|
|
Environment example::
|
|
|
|
os.spawnlpe(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg", env)
|
|
==>
|
|
Popen(["/bin/mycmd", "myarg"], env={"PATH": "/usr/bin"})
|
|
|
|
|
|
Replacing :func:`os.popen`, :func:`os.popen2`, :func:`os.popen3`
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
::
|
|
|
|
pipe = os.popen("cmd", 'r', bufsize)
|
|
==>
|
|
pipe = Popen("cmd", shell=True, bufsize=bufsize, stdout=PIPE).stdout
|
|
|
|
::
|
|
|
|
pipe = os.popen("cmd", 'w', bufsize)
|
|
==>
|
|
pipe = Popen("cmd", shell=True, bufsize=bufsize, stdin=PIPE).stdin
|
|
|
|
::
|
|
|
|
(child_stdin, child_stdout) = os.popen2("cmd", mode, bufsize)
|
|
==>
|
|
p = Popen("cmd", shell=True, bufsize=bufsize,
|
|
stdin=PIPE, stdout=PIPE, close_fds=True)
|
|
(child_stdin, child_stdout) = (p.stdin, p.stdout)
|
|
|
|
::
|
|
|
|
(child_stdin,
|
|
child_stdout,
|
|
child_stderr) = os.popen3("cmd", mode, bufsize)
|
|
==>
|
|
p = Popen("cmd", shell=True, bufsize=bufsize,
|
|
stdin=PIPE, stdout=PIPE, stderr=PIPE, close_fds=True)
|
|
(child_stdin,
|
|
child_stdout,
|
|
child_stderr) = (p.stdin, p.stdout, p.stderr)
|
|
|
|
::
|
|
|
|
(child_stdin, child_stdout_and_stderr) = os.popen4("cmd", mode,
|
|
bufsize)
|
|
==>
|
|
p = Popen("cmd", shell=True, bufsize=bufsize,
|
|
stdin=PIPE, stdout=PIPE, stderr=STDOUT, close_fds=True)
|
|
(child_stdin, child_stdout_and_stderr) = (p.stdin, p.stdout)
|
|
|
|
On Unix, os.popen2, os.popen3 and os.popen4 also accept a sequence as
|
|
the command to execute, in which case arguments will be passed
|
|
directly to the program without shell intervention. This usage can be
|
|
replaced as follows::
|
|
|
|
(child_stdin, child_stdout) = os.popen2(["/bin/ls", "-l"], mode,
|
|
bufsize)
|
|
==>
|
|
p = Popen(["/bin/ls", "-l"], bufsize=bufsize, stdin=PIPE, stdout=PIPE)
|
|
(child_stdin, child_stdout) = (p.stdin, p.stdout)
|
|
|
|
Return code handling translates as follows::
|
|
|
|
pipe = os.popen("cmd", 'w')
|
|
...
|
|
rc = pipe.close()
|
|
if rc is not None and rc >> 8:
|
|
print "There were some errors"
|
|
==>
|
|
process = Popen("cmd", shell=True, stdin=PIPE)
|
|
...
|
|
process.stdin.close()
|
|
if process.wait() != 0:
|
|
print "There were some errors"
|
|
|
|
|
|
Replacing functions from the :mod:`popen2` module
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
::
|
|
|
|
(child_stdout, child_stdin) = popen2.popen2("somestring", bufsize, mode)
|
|
==>
|
|
p = Popen("somestring", shell=True, bufsize=bufsize,
|
|
stdin=PIPE, stdout=PIPE, close_fds=True)
|
|
(child_stdout, child_stdin) = (p.stdout, p.stdin)
|
|
|
|
On Unix, popen2 also accepts a sequence as the command to execute, in
|
|
which case arguments will be passed directly to the program without
|
|
shell intervention. This usage can be replaced as follows::
|
|
|
|
(child_stdout, child_stdin) = popen2.popen2(["mycmd", "myarg"], bufsize,
|
|
mode)
|
|
==>
|
|
p = Popen(["mycmd", "myarg"], bufsize=bufsize,
|
|
stdin=PIPE, stdout=PIPE, close_fds=True)
|
|
(child_stdout, child_stdin) = (p.stdout, p.stdin)
|
|
|
|
:class:`popen2.Popen3` and :class:`popen2.Popen4` basically work as
|
|
:class:`subprocess.Popen`, except that:
|
|
|
|
* :class:`Popen` raises an exception if the execution fails.
|
|
|
|
* the *capturestderr* argument is replaced with the *stderr* argument.
|
|
|
|
* ``stdin=PIPE`` and ``stdout=PIPE`` must be specified.
|
|
|
|
* popen2 closes all file descriptors by default, but you have to specify
|
|
``close_fds=True`` with :class:`Popen`.
|
|
|
|
|
|
Notes
|
|
-----
|
|
|
|
.. _converting-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.
|
|
|