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340 lines
11 KiB
ReStructuredText
340 lines
11 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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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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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 to
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execute is normally the first item in the args sequence or string, but can be
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explicitly set by using the executable argument.
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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. A string will be treated as a sequence with the string as the only
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item (the program to execute).
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On Unix, with *shell=True*: If args is a string, it specifies the command string
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to execute through the shell. If *args* is a sequence, the first item specifies
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the command string, and any additional items will be treated as additional shell
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arguments.
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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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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.
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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 are
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``PIPE``, an existing file descriptor (a positive integer), an existing file
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object, and ``None``. ``PIPE`` indicates that a new pipe to the child should be
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created. With ``None``, no redirection will occur; the child's file handles
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will be inherited from the parent. Additionally, *stderr* can be ``STDOUT``,
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which indicates that the stderr data from the applications should be captured
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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 defines the environment variables for the new
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process.
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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 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 support
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(the default). Also, the newlines attribute of the file objects :attr:`stdout`,
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:attr:`stdin` and :attr:`stderr` are not updated by the 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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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 Popen constructor. Example::
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retcode = call(["ls", "-l"])
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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 Popen constructor. Example::
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check_call(["ls", "-l"])
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.. versionadded:: 2.5
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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 childs 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. Returns returncode attribute.
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.. method:: Popen.wait()
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Wait for child process to terminate. Returns returncode attribute.
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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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communicate() returns a tuple (stdout, stderr).
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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 size
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is large or unlimited.
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The following attributes are also available:
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.. attribute:: Popen.stdin
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If the *stdin* argument is ``PIPE``, this attribute is a file object that
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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 is ``PIPE``, this attribute is a file object that
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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 is ``PIPE``, this attribute is file object that
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provides error output from the child process. Otherwise, it is ``None``.
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.. attribute:: Popen.pid
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The process ID of the child process.
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.. attribute:: Popen.returncode
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The child return code. A ``None`` value indicates that the process hasn't
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terminated yet. A negative value -N indicates that the child was terminated by
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signal N (Unix only).
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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 pipe line
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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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output = p2.communicate()[0]
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Replacing 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)
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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 as e:
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print >>sys.stderr, "Execution failed:", e
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Replacing os.spawn\*
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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 os.popen\*
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^^^^^^^^^^^^^^^^^^^^
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::
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pipe = os.popen(cmd, mode='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, mode='w', bufsize)
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==>
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pipe = Popen(cmd, shell=True, bufsize=bufsize, stdin=PIPE).stdin
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