Issue #13237: Forward port from 3.2 of subprocess documentation updates. Needed quite a few adjustments to account for new features coming in 3.3
This commit is contained in:
Nick Coghlan
commit
217f05b3ed
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@ -25,7 +25,260 @@ modules and functions can be found in the following sections.
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Using the subprocess Module
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---------------------------
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This module defines one class called :class:`Popen`:
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The recommended approach to invoking subprocesses is to use the following
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convenience functions for all use cases they can handle. For more advanced
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use cases, the underlying :class:`Popen` interface can be used directly.
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.. function:: call(args, *, stdin=None, stdout=None, stderr=None, shell=False, timeout=None)
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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 use of keyword-only notation
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in the abbreviated signature). The full function signature is largely the
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same as that of the :class:`Popen` constructor - this function passes all
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supplied arguments other than *timeout* directly through to that interface.
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The *timeout* argument is passed to :meth:`Popen.wait`. If the timeout
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expires, the child process will be killed and then waited for again. The
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:exc:`TimeoutExpired` exception will be re-raised after the child process
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has terminated.
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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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Invoking the system shell with ``shell=True`` can be a security hazard
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if combined with untrusted input. See the warning under
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: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. As
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the pipes are not being read in the current process, the child
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process may block if it generates enough output to a pipe to fill up
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the OS pipe buffer.
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.. versionchanged:: 3.3
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*timeout* was added.
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.. function:: check_call(args, *, stdin=None, stdout=None, stderr=None, shell=False, timeout=None)
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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:`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 use of keyword-only notation
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in the abbreviated signature). The full function signature is largely the
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same as that of the :class:`Popen` constructor - this function passes all
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supplied arguments other than *timeout* directly through to that interface.
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The *timeout* argument is passed to :meth:`Popen.wait`. If the timeout
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expires, the child process will be killed and then waited for again. The
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:exc:`TimeoutExpired` exception will be re-raised after the child process
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has terminated.
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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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.. warning::
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Invoking the system shell with ``shell=True`` can be a security hazard
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if combined with untrusted input. See the warning under
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: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. As
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the pipes are not being read in the current process, the child
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process may block if it generates enough output to a pipe to fill up
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the OS pipe buffer.
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.. versionchanged:: 3.3
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*timeout* was added.
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.. function:: check_output(args, *, stdin=None, stderr=None, shell=False, universal_newlines=False, timeout=None)
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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:`returncode` attribute and any output in the :attr:`output`
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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 use of keyword-only notation
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in the abbreviated signature). The full function signature is largely the
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same as that of the :class:`Popen` constructor - this functions passes all
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supplied arguments other than *timeout* directly through to that interface.
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In addition, *stdout* is not permitted as an argument, as it is used
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internally to collect the output from the subprocess.
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The *timeout* argument is passed to :meth:`Popen.wait`. If the timeout
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expires, the child process will be killed and then waited for again. The
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:exc:`TimeoutExpired` exception will be re-raised after the child process
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has terminated.
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Examples::
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>>> subprocess.check_output(["echo", "Hello World!"])
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b'Hello World!\n'
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>>> subprocess.check_output(["echo", "Hello World!"], universal_newlines=True)
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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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By default, this function will return the data as encoded bytes. The actual
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encoding of the output data may depend on the command being invoked, so the
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decoding to text will often need to be handled at the application level.
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This behaviour may be overridden by setting *universal_newlines* to
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:const:`True` as described below in :ref:`frequently-used-arguments`.
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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:: 3.1
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.. warning::
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Invoking the system shell with ``shell=True`` can be a security hazard
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if combined with untrusted input. See the warning under
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:ref:`frequently-used-arguments` for details.
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.. note::
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Do not use ``stderr=PIPE`` with this function. As the pipe is not being
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read in the current process, the child process may block if it
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generates enough output to the pipe to fill up the OS pipe buffer.
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.. versionchanged:: 3.3
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*timeout* was added.
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.. data:: DEVNULL
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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 the special file :data:`os.devnull`
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will be used.
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.. versionadded:: 3.3
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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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.. _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`, :data:`DEVNULL`, an existing file descriptor (a positive
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integer), an existing file object, and ``None``. :data:`PIPE` indicates
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that a new pipe to the child should be created. :data:`DEVNULL` indicates
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that the special file :data:`os.devnull` will be used. With the default
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settings of ``None``, no redirection will occur; the child's file handles
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will be inherited from the parent. Additionally, *stderr* can be
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:data:`STDOUT`, which indicates that the stderr data from the child
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process should be captured into the same file handle as for *stdout*.
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When *stdout* or *stderr* are pipes and *universal_newlines* is
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:const:`True` then the output data is assumed to be encoded as UTF-8 and
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will automatically be decoded to text. All line endings will be converted
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to ``'\n'`` as described for the universal newlines `'U'`` mode argument
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to :func:`open`.
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If *shell* is :const:`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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access to other shell features such as filename wildcards, shell pipes and
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environment variable expansion.
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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** in cases
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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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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 Constuctor
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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, stderr=None, preexec_fn=None, close_fds=True, shell=False, cwd=None, env=None, universal_newlines=False, startupinfo=None, creationflags=0, restore_signals=True, start_new_session=False, pass_fds=())
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@ -78,22 +331,9 @@ This module defines one class called :class:`Popen`:
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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** in cases
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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* does not suffer from this vulnerability; the above Note may be
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helpful in getting code using *shell=False* to work. See also
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:func:`shlex.quote` for a function useful to quote filenames and commands.
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Enabling this option can be a security hazard if combined with untrusted
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input. See the warning under :ref:`frequently-used-arguments`
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for details.
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On Windows: the :class:`Popen` class uses CreateProcess() to execute the
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child program, which operates on strings. If *args* is a sequence, it will
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@ -122,16 +362,16 @@ This module defines one class called :class:`Popen`:
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You don't need ``shell=True`` to run a batch file, nor to run a console-based
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executable.
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*stdin*, *stdout* and *stderr* specify the executed programs' standard input,
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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`, :data:`DEVNULL`, an existing file descriptor (a positive
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integer), an existing :term:`file object`, and ``None``. :data:`PIPE`
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indicates that a new pipe to the child should be created. :data:`DEVNULL`
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indicates that the special file :data:`os.devnull` will be used. With ``None``,
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no redirection will occur; the child's file handles will be inherited from
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the parent. Additionally, *stderr* can be :data:`STDOUT`, which indicates
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that the stderr data from the applications should be captured into the same
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file handle as for stdout.
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indicates that the special file :data:`os.devnull` will be used. With the
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default settings of ``None``, no redirection will occur; the child's file
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handles will be inherited from the parent. Additionally, *stderr* can be
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:data:`STDOUT`, which indicates that the stderr data from the applications
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should be captured into the same file 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.
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@ -231,151 +471,6 @@ This module defines one class called :class:`Popen`:
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Added context manager support.
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.. data:: DEVNULL
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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 the special file :data:`os.devnull`
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will be used.
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.. versionadded:: 3.3
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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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Convenience Functions
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^^^^^^^^^^^^^^^^^^^^^
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This module also defines the following shortcut functions:
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.. function:: call(*popenargs, timeout=None, **kwargs)
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Run command with arguments. Wait for command to complete, then return the
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:attr:`returncode` attribute.
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The arguments are the same as for the :class:`Popen` constructor, with the
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exception of the *timeout* argument, which is given to :meth:`Popen.wait`.
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Example::
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>>> retcode = subprocess.call(["ls", "-l"])
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If the timeout expires, the child process will be killed and then waited for
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again. The :exc:`TimeoutExpired` exception will be re-raised after the child
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process has terminated.
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.. warning::
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Like :meth:`Popen.wait`, this will deadlock when using
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``stdout=PIPE`` and/or ``stderr=PIPE`` and the child process
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generates enough output to a pipe such that it blocks waiting
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for the OS pipe buffer to accept more data.
|
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.. versionchanged:: 3.3
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*timeout* was added.
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.. function:: check_call(*popenargs, timeout=None, **kwargs)
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Run command with arguments. Wait for command to complete. If the exit code was
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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:`returncode` attribute.
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The arguments are the same as for the :func:`call` function. Example::
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>>> subprocess.check_call(["ls", "-l"])
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0
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As in the :func:`call` function, if the timeout expires, the child process
|
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will be killed and the wait retried. The :exc:`TimeoutExpired` exception
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will be re-raised after the child process has terminated.
|
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.. warning::
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See the warning for :func:`call`.
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.. versionchanged:: 3.3
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*timeout* was added.
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.. function:: check_output(*popenargs, timeout=None, **kwargs)
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Run command with arguments and return its output as a bytes object.
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If the exit 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:`returncode` attribute and output in the :attr:`output` attribute.
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The arguments are the same as for the :func:`call` function. Example::
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>>> subprocess.check_output(["ls", "-l", "/dev/null"])
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b'crw-rw-rw- 1 root root 1, 3 Oct 18 2007 /dev/null\n'
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The stdout argument is not allowed as it is used internally.
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To capture standard error in the result, use ``stderr=subprocess.STDOUT``::
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>>> subprocess.check_output(
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... ["/bin/sh", "-c", "ls non_existent_file; exit 0"],
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... stderr=subprocess.STDOUT)
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b'ls: non_existent_file: No such file or directory\n'
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As in the :func:`call` function, if the timeout expires, the child process
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will be killed and the wait retried. The :exc:`TimeoutExpired` exception
|
||||
will be re-raised after the child process has terminated. The output from
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the child process so far will be in the :attr:`output` attribute of the
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exception.
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.. versionadded:: 3.1
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.. versionchanged:: 3.3
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*timeout* was added.
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.. function:: getstatusoutput(cmd)
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Return ``(status, output)`` of executing *cmd* in a shell.
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Execute the string *cmd* in a shell with :func:`os.popen` and return a 2-tuple
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``(status, output)``. *cmd* is actually run as ``{ cmd ; } 2>&1``, so that the
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returned output will contain output or error messages. A trailing newline is
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stripped from the output. The exit status for the command can be interpreted
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according to the rules for the C function :c:func:`wait`. Example::
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>>> subprocess.getstatusoutput('ls /bin/ls')
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(0, '/bin/ls')
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>>> subprocess.getstatusoutput('cat /bin/junk')
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(256, 'cat: /bin/junk: No such file or directory')
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>>> subprocess.getstatusoutput('/bin/junk')
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(256, 'sh: /bin/junk: not found')
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Availability: UNIX.
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.. function:: getoutput(cmd)
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Return output (stdout and stderr) of executing *cmd* in a shell.
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|
||||
Like :func:`getstatusoutput`, except the exit status is ignored and the return
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value is a string containing the command's output. Example::
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|
||||
>>> subprocess.getoutput('ls /bin/ls')
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'/bin/ls'
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|
||||
Availability: UNIX.
|
||||
|
||||
|
||||
Exceptions
|
||||
^^^^^^^^^^
|
||||
|
||||
|
|
@ -391,8 +486,9 @@ when trying to execute a non-existent file. Applications should prepare for
|
|||
A :exc:`ValueError` will be raised if :class:`Popen` is called with invalid
|
||||
arguments.
|
||||
|
||||
check_call() will raise :exc:`CalledProcessError`, if the called process returns
|
||||
a non-zero return code.
|
||||
: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
|
||||
|
|
@ -407,9 +503,11 @@ Exceptions defined in this module all inherit from :exc:`SubprocessError`.
|
|||
Security
|
||||
^^^^^^^^
|
||||
|
||||
Unlike some other popen functions, this implementation will never call /bin/sh
|
||||
implicitly. This means that all characters, including shell metacharacters, can
|
||||
safely be passed to child processes.
|
||||
Unlike some other popen functions, this implementation will never call a
|
||||
system shell implicitly. This means that all characters, including shell
|
||||
metacharacters, can safely be passed to child processes. Obviously, if the
|
||||
shell is invoked explicitly, then it is the application's responsibility to
|
||||
ensure that all whitespace and metacharacters are quoted appropriately.
|
||||
|
||||
|
||||
Popen Objects
|
||||
|
|
@ -663,15 +761,21 @@ The :mod:`subprocess` module exposes the following constants.
|
|||
Replacing Older Functions with the subprocess Module
|
||||
----------------------------------------------------
|
||||
|
||||
In this section, "a ==> b" means that b can be used as a replacement for a.
|
||||
In this section, "a becomes b" means that b can be used as a replacement for a.
|
||||
|
||||
.. note::
|
||||
|
||||
All functions in this section fail (more or less) silently if the executed
|
||||
program cannot be found; this module raises an :exc:`OSError` exception.
|
||||
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 the following examples, we assume that the subprocess module is imported with
|
||||
"from subprocess import \*".
|
||||
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 ``output`` attribute of
|
||||
the raised exception.
|
||||
|
||||
In the following examples, we assume that the relevant functions have already
|
||||
been imported from the subprocess module.
|
||||
|
||||
|
||||
Replacing /bin/sh shell backquote
|
||||
|
|
@ -680,8 +784,8 @@ Replacing /bin/sh shell backquote
|
|||
::
|
||||
|
||||
output=`mycmd myarg`
|
||||
==>
|
||||
output = Popen(["mycmd", "myarg"], stdout=PIPE).communicate()[0]
|
||||
# becomes
|
||||
output = check_output(["mycmd", "myarg"])
|
||||
|
||||
|
||||
Replacing shell pipeline
|
||||
|
|
@ -690,7 +794,7 @@ 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.
|
||||
|
|
@ -699,22 +803,27 @@ Replacing shell pipeline
|
|||
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")
|
||||
==>
|
||||
p = Popen("mycmd" + " myarg", shell=True)
|
||||
sts = os.waitpid(p.pid, 0)[1]
|
||||
# becomes
|
||||
sts = call("mycmd" + " myarg", shell=True)
|
||||
|
||||
Notes:
|
||||
|
||||
* Calling the program through the shell is usually not required.
|
||||
|
||||
* It's easier to look at the :attr:`returncode` attribute than the exit status.
|
||||
|
||||
A more realistic example would look like this::
|
||||
|
||||
try:
|
||||
|
|
@ -839,6 +948,48 @@ Replacing functions from the :mod:`popen2` module
|
|||
``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 ``(status, output)`` of executing *cmd* in a shell.
|
||||
|
||||
Execute the string *cmd* in a shell with :func:`os.popen` and return a 2-tuple
|
||||
``(status, output)``. *cmd* is actually run as ``{ cmd ; } 2>&1``, so that the
|
||||
returned output will contain output or error messages. A trailing newline is
|
||||
stripped from the output. The exit status for the command can be interpreted
|
||||
according to the rules for the C function :c:func:`wait`. Example::
|
||||
|
||||
>>> subprocess.getstatusoutput('ls /bin/ls')
|
||||
(0, '/bin/ls')
|
||||
>>> subprocess.getstatusoutput('cat /bin/junk')
|
||||
(256, 'cat: /bin/junk: No such file or directory')
|
||||
>>> subprocess.getstatusoutput('/bin/junk')
|
||||
(256, 'sh: /bin/junk: not found')
|
||||
|
||||
Availability: UNIX.
|
||||
|
||||
|
||||
.. function:: getoutput(cmd)
|
||||
|
||||
Return output (stdout and stderr) of executing *cmd* in a shell.
|
||||
|
||||
Like :func:`getstatusoutput`, except the exit status is ignored and the return
|
||||
value is a string containing the command's output. Example::
|
||||
|
||||
>>> subprocess.getoutput('ls /bin/ls')
|
||||
'/bin/ls'
|
||||
|
||||
Availability: UNIX.
|
||||
|
||||
|
||||
Notes
|
||||
-----
|
||||
|
||||
|
|
|
|||
|
|
@ -1780,6 +1780,9 @@ C-API
|
|||
Documentation
|
||||
-------------
|
||||
|
||||
- Issue #13237: Reorganise subprocess documentation to emphasise convenience
|
||||
functions and the most commonly needed arguments to Popen.
|
||||
|
||||
- Issue #13141: Demonstrate recommended style for socketserver examples.
|
||||
|
||||
- Issue #11818: Fix tempfile examples for Python 3.
|
||||
|
|
|
|||
Loading…
Reference in New Issue