619 lines
21 KiB
ReStructuredText
619 lines
21 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 other, older modules and functions, such as::
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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 the :mod:`subprocess` module can be used to replace these
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modules and functions can be found in the following sections.
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.. seealso::
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:pep:`324` -- PEP proposing the subprocess module
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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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.. class:: Popen(args, bufsize=0, executable=None, stdin=None, stdout=None, stderr=None, preexec_fn=None, close_fds=False, shell=False, cwd=None, env=None, universal_newlines=False, startupinfo=None, creationflags=0)
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Arguments are:
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*args* should be a string, or a sequence of program arguments. The program
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to execute is normally the first item in the args sequence or the string if
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a string is given, but can be explicitly set by using the *executable*
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argument. When *executable* is given, the first item in the args sequence
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is still treated by most programs as the command name, which can then be
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different from the actual executable name. On Unix, it becomes the display
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name for the executing program in utilities such as :program:`ps`.
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On Unix, with *shell=False* (default): In this case, the Popen class uses
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:meth:`os.execvp` to execute the child program. *args* should normally be a
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sequence. If a string is specified for *args*, it will be used as the name
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or path of the program to execute; this will only work if the program is
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being given no arguments.
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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 Unix, with *shell=True*: If args is a string, it specifies the command
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string 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, *Popen* does the equivalent of::
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Popen(['/bin/sh', '-c', args[0], args[1], ...])
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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.
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On Windows: the :class:`Popen` class uses CreateProcess() to execute the child
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program, which operates on strings. If *args* is a sequence, it will be
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converted to a string using the :meth:`list2cmdline` method. Please note that
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not all MS Windows applications interpret the command line the same way:
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:meth:`list2cmdline` is designed for applications using the same rules as the MS
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C runtime.
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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 the program to execute. It is very seldom
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needed: Usually, the program to execute is defined by the *args* argument. If
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``shell=True``, the *executable* argument specifies which shell to use. On Unix,
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the default shell is :file:`/bin/sh`. On Windows, the default shell is
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specified by the :envvar:`COMSPEC` environment variable. The only reason you
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would need to specify ``shell=True`` on Windows is where the command you
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wish to execute is actually built in to the shell, eg ``dir``, ``copy``.
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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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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 ``None``, no redirection will occur;
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the child's file handles will be inherited from the parent. Additionally,
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*stderr* can be :data:`STDOUT`, which indicates that the stderr data from the
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applications 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. (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 *shell* is :const:`True`, the specified command will be executed through the
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shell.
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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: http://en.wikipedia.org/wiki/Side-by-Side_Assembly
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If *universal_newlines* is :const:`True`, the file objects stdout and stderr are
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opened as text files, but lines may be terminated by any of ``'\n'``, the Unix
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end-of-line convention, ``'\r'``, the old Macintosh convention or ``'\r\n'``, the
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Windows convention. All of these external representations are seen as ``'\n'``
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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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The *startupinfo* and *creationflags*, if given, will be passed to the
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underlying CreateProcess() function. They can specify things such as appearance
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of the main window and priority for the new process. (Windows only)
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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 two shortcut functions:
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.. function:: call(*popenargs, **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. Example::
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>>> retcode = subprocess.call(["ls", "-l"])
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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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.. function:: check_call(*popenargs, **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 :class:`Popen` constructor. Example::
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>>> subprocess.check_call(["ls", "-l"])
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0
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.. versionadded:: 2.5
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.. warning::
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See the warning for :func:`call`.
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.. function:: check_output(*popenargs, **kwargs)
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Run command with arguments and return its output as a byte string.
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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`
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attribute and output in the :attr:`output` attribute.
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The arguments are the same as for the :class:`Popen` constructor. Example::
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>>> subprocess.check_output(["ls", "-l", "/dev/null"])
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'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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'ls: non_existent_file: No such file or directory\n'
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.. versionadded:: 2.7
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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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check_call() will raise :exc:`CalledProcessError`, if the called process returns
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a non-zero return code.
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Security
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^^^^^^^^
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Unlike some other popen functions, this implementation will never call /bin/sh
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implicitly. This means that all characters, including shell metacharacters, can
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safely be passed to child processes.
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Popen Objects
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-------------
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Instances of the :class:`Popen` class have the following methods:
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.. method:: Popen.poll()
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Check if child process has terminated. Set and return :attr:`returncode`
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attribute.
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.. method:: Popen.wait()
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Wait for child process to terminate. Set and return :attr:`returncode`
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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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.. method:: Popen.communicate(input=None)
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Interact with process: Send data to stdin. Read data from stdout and stderr,
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until end-of-file is reached. Wait for process to terminate. The optional
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*input* argument should be a string to be sent to the child process, or
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``None``, if no data should be sent to the child.
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:meth:`communicate` returns a tuple ``(stdoutdata, stderrdata)``.
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Note that if you want to send data to the process's stdin, you need to create
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the Popen object with ``stdin=PIPE``. Similarly, to get anything other than
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``None`` in the result tuple, you need to give ``stdout=PIPE`` and/or
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``stderr=PIPE`` too.
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.. note::
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The data read is buffered in memory, so do not use this method if the data
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size is large or unlimited.
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.. method:: Popen.send_signal(signal)
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Sends the signal *signal* to the child.
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.. note::
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On Windows, SIGTERM is an alias for :meth:`terminate`. CTRL_C_EVENT and
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CTRL_BREAK_EVENT can be sent to processes started with a *creationflags*
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parameter which includes `CREATE_NEW_PROCESS_GROUP`.
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.. versionadded:: 2.6
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.. method:: Popen.terminate()
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Stop the child. On Posix OSs the method sends SIGTERM to the
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child. On Windows the Win32 API function :cfunc:`TerminateProcess` is called
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to stop the child.
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.. versionadded:: 2.6
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.. method:: Popen.kill()
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Kills the child. On Posix OSs the function sends SIGKILL to the child.
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On Windows :meth:`kill` is an alias for :meth:`terminate`.
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.. versionadded:: 2.6
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The following attributes are also available:
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.. warning::
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Use :meth:`communicate` rather than :attr:`.stdin.write <stdin>`,
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:attr:`.stdout.read <stdout>` or :attr:`.stderr.read <stderr>` to avoid
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deadlocks due to any of the other OS pipe buffers filling up and blocking the
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child process.
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.. attribute:: Popen.stdin
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If the *stdin* argument was :data:`PIPE`, this attribute is a file object
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that provides input to the child process. Otherwise, it is ``None``.
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.. attribute:: Popen.stdout
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If the *stdout* argument was :data:`PIPE`, this attribute is a file object
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that provides output from the child process. Otherwise, it is ``None``.
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.. attribute:: Popen.stderr
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If the *stderr* argument was :data:`PIPE`, this attribute is a file object
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that provides error output from the child process. Otherwise, it is
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``None``.
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.. attribute:: Popen.pid
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The process ID of the child process.
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Note that if you set the *shell* argument to ``True``, this is the process ID
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of the spawned shell.
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.. attribute:: Popen.returncode
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The child return code, set by :meth:`poll` and :meth:`wait` (and indirectly
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by :meth:`communicate`). A ``None`` value indicates that the process
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hasn't terminated yet.
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A negative value ``-N`` indicates that the child was terminated by signal
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``N`` (Unix only).
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.. _subprocess-replacements:
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Replacing Older Functions with the subprocess Module
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----------------------------------------------------
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In this section, "a ==> b" means that b can be used as a replacement for a.
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.. note::
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All functions in this section fail (more or less) silently if the executed
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program cannot be found; this module raises an :exc:`OSError` exception.
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In the following examples, we assume that the subprocess module is imported with
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"from subprocess import \*".
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Replacing /bin/sh shell backquote
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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::
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output=`mycmd myarg`
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==>
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output = Popen(["mycmd", "myarg"], stdout=PIPE).communicate()[0]
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Replacing shell pipeline
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^^^^^^^^^^^^^^^^^^^^^^^^
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::
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output=`dmesg | grep hda`
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==>
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p1 = Popen(["dmesg"], stdout=PIPE)
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p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
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p1.stdout.close() # Allow p1 to receive a SIGPIPE if p2 exits.
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output = p2.communicate()[0]
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The p1.stdout.close() call after starting the p2 is important in order for p1
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to receive a SIGPIPE if p2 exits before p1.
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Replacing :func:`os.system`
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^^^^^^^^^^^^^^^^^^^^^^^^^^^
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::
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sts = os.system("mycmd" + " myarg")
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==>
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p = Popen("mycmd" + " myarg", shell=True)
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sts = os.waitpid(p.pid, 0)[1]
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Notes:
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* Calling the program through the shell is usually not required.
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* It's easier to look at the :attr:`returncode` attribute than the exit status.
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A more realistic example would look like this::
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try:
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retcode = call("mycmd" + " myarg", shell=True)
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if retcode < 0:
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print >>sys.stderr, "Child was terminated by signal", -retcode
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else:
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print >>sys.stderr, "Child returned", retcode
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except OSError, e:
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print >>sys.stderr, "Execution failed:", e
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Replacing the :func:`os.spawn <os.spawnl>` family
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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P_NOWAIT example::
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pid = os.spawnlp(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg")
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==>
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pid = Popen(["/bin/mycmd", "myarg"]).pid
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P_WAIT example::
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retcode = os.spawnlp(os.P_WAIT, "/bin/mycmd", "mycmd", "myarg")
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==>
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retcode = call(["/bin/mycmd", "myarg"])
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Vector example::
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os.spawnvp(os.P_NOWAIT, path, args)
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==>
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Popen([path] + args[1:])
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Environment example::
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os.spawnlpe(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg", env)
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==>
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Popen(["/bin/mycmd", "myarg"], env={"PATH": "/usr/bin"})
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Replacing :func:`os.popen`, :func:`os.popen2`, :func:`os.popen3`
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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::
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pipe = os.popen("cmd", 'r', bufsize)
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==>
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pipe = Popen("cmd", shell=True, bufsize=bufsize, stdout=PIPE).stdout
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::
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pipe = os.popen("cmd", 'w', bufsize)
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==>
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pipe = Popen("cmd", shell=True, bufsize=bufsize, stdin=PIPE).stdin
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::
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(child_stdin, child_stdout) = os.popen2("cmd", mode, bufsize)
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==>
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p = Popen("cmd", shell=True, bufsize=bufsize,
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stdin=PIPE, stdout=PIPE, close_fds=True)
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(child_stdin, child_stdout) = (p.stdin, p.stdout)
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::
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(child_stdin,
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child_stdout,
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child_stderr) = os.popen3("cmd", mode, bufsize)
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==>
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p = Popen("cmd", shell=True, bufsize=bufsize,
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stdin=PIPE, stdout=PIPE, stderr=PIPE, close_fds=True)
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(child_stdin,
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child_stdout,
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child_stderr) = (p.stdin, p.stdout, p.stderr)
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::
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(child_stdin, child_stdout_and_stderr) = os.popen4("cmd", mode,
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bufsize)
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==>
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p = Popen("cmd", shell=True, bufsize=bufsize,
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stdin=PIPE, stdout=PIPE, stderr=STDOUT, close_fds=True)
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(child_stdin, child_stdout_and_stderr) = (p.stdin, p.stdout)
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On Unix, os.popen2, os.popen3 and os.popen4 also accept a sequence as
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the command to execute, in which case arguments will be passed
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directly to the program without shell intervention. This usage can be
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replaced as follows::
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(child_stdin, child_stdout) = os.popen2(["/bin/ls", "-l"], mode,
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bufsize)
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==>
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p = Popen(["/bin/ls", "-l"], bufsize=bufsize, stdin=PIPE, stdout=PIPE)
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(child_stdin, child_stdout) = (p.stdin, p.stdout)
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Return code handling translates as follows::
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pipe = os.popen("cmd", 'w')
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...
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rc = pipe.close()
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if rc is not None and rc >> 8:
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print "There were some errors"
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==>
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process = Popen("cmd", 'w', shell=True, stdin=PIPE)
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...
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process.stdin.close()
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if process.wait() != 0:
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print "There were some errors"
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Replacing functions from the :mod:`popen2` module
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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::
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(child_stdout, child_stdin) = popen2.popen2("somestring", bufsize, mode)
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==>
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p = Popen(["somestring"], shell=True, bufsize=bufsize,
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stdin=PIPE, stdout=PIPE, close_fds=True)
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(child_stdout, child_stdin) = (p.stdout, p.stdin)
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|
|
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::
|
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|
|
(child_stdout, child_stdin) = popen2.popen2(["mycmd", "myarg"], bufsize,
|
|
mode)
|
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==>
|
|
p = Popen(["mycmd", "myarg"], bufsize=bufsize,
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stdin=PIPE, stdout=PIPE, close_fds=True)
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(child_stdout, child_stdin) = (p.stdout, p.stdin)
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:class:`popen2.Popen3` and :class:`popen2.Popen4` basically work as
|
|
:class:`subprocess.Popen`, except that:
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|
* :class:`Popen` raises an exception if the execution fails.
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* the *capturestderr* argument is replaced with the *stderr* argument.
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|
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* ``stdin=PIPE`` and ``stdout=PIPE`` must be specified.
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* popen2 closes all file descriptors by default, but you have to specify
|
|
``close_fds=True`` with :class:`Popen`.
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|