:mod:`subprocess` --- Subprocess management =========================================== .. module:: subprocess :synopsis: Subprocess management. .. moduleauthor:: Peter Åstrand .. sectionauthor:: Peter Åstrand .. versionadded:: 2.4 The :mod:`subprocess` module allows you to spawn new processes, connect to their input/output/error pipes, and obtain their return codes. This module intends to replace several other, older modules and functions, such as:: os.system os.spawn* os.popen* popen2.* commands.* Information about how the :mod:`subprocess` module can be used to replace these modules and functions can be found in the following sections. .. seealso:: :pep:`324` -- PEP proposing the subprocess module Using the subprocess Module --------------------------- This module defines one class called :class:`Popen`: .. 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) Arguments are: *args* should be a string, or a sequence of program arguments. The program to execute is normally the first item in the args sequence or the string if a string is given, but can be explicitly set by using the *executable* argument. On Unix, with *shell=False* (default): In this case, the Popen class uses :meth:`os.execvp` to execute the child program. *args* should normally be a sequence. A string will be treated as a sequence with the string as the only item (the program to execute). On Unix, with *shell=True*: If args is a string, it specifies the command string to execute through the shell. If *args* is a sequence, the first item specifies the command string, and any additional items will be treated as additional shell arguments. On Windows: the :class:`Popen` class uses CreateProcess() to execute the child program, which operates on strings. If *args* is a sequence, it will be converted to a string using the :meth:`list2cmdline` method. Please note that not all MS Windows applications interpret the command line the same way: :meth:`list2cmdline` is designed for applications using the same rules as the MS C runtime. *bufsize*, if given, has the same meaning as the corresponding argument to the built-in open() function: :const:`0` means unbuffered, :const:`1` means line buffered, any other positive value means use a buffer of (approximately) that size. A negative *bufsize* means to use the system default, which usually means fully buffered. The default value for *bufsize* is :const:`0` (unbuffered). The *executable* argument specifies the program to execute. It is very seldom needed: Usually, the program to execute is defined by the *args* argument. If ``shell=True``, the *executable* argument specifies which shell to use. On Unix, the default shell is :file:`/bin/sh`. On Windows, the default shell is specified by the :envvar:`COMSPEC` environment variable. *stdin*, *stdout* and *stderr* specify the executed programs' standard input, standard output and standard error file handles, respectively. Valid values are :data:`PIPE`, an existing file descriptor (a positive integer), an existing file object, and ``None``. :data:`PIPE` indicates that a new pipe to the child should be created. With ``None``, no redirection will occur; the child's file handles will be inherited from the parent. Additionally, *stderr* can be :data:`STDOUT`, which indicates that the stderr data from the applications should be captured into the same file handle as for stdout. If *preexec_fn* is set to a callable object, this object will be called in the child process just before the child is executed. (Unix only) If *close_fds* is true, all file descriptors except :const:`0`, :const:`1` and :const:`2` will be closed before the child process is executed. (Unix only). Or, on Windows, if *close_fds* is true then no handles will be inherited by the child process. Note that on Windows, you cannot set *close_fds* to true and also redirect the standard handles by setting *stdin*, *stdout* or *stderr*. If *shell* is :const:`True`, the specified command will be executed through the shell. If *cwd* is not ``None``, the child's current directory will be changed to *cwd* before it is executed. Note that this directory is not considered when searching the executable, so you can't specify the program's path relative to *cwd*. If *env* is not ``None``, it must be a mapping that defines the environment variables for the new process; these are used instead of inheriting the current process' environment, which is the default behavior. .. note:: If specified, *env* must provide any variables required for the program to execute. On Windows, in order to run a `side-by-side assembly`_ the specified *env* **must** include a valid :envvar:`SystemRoot`. .. _side-by-side assembly: http://en.wikipedia.org/wiki/Side-by-Side_Assembly If *universal_newlines* is :const:`True`, the file objects stdout and stderr are opened as text files, but lines may be terminated by any of ``'\n'``, the Unix end-of-line convention, ``'\r'``, the old Macintosh convention or ``'\r\n'``, the Windows convention. All of these external representations are seen as ``'\n'`` by the Python program. .. note:: This feature is only available if Python is built with universal newline support (the default). Also, the newlines attribute of the file objects :attr:`stdout`, :attr:`stdin` and :attr:`stderr` are not updated by the communicate() method. The *startupinfo* and *creationflags*, if given, will be passed to the underlying CreateProcess() function. They can specify things such as appearance of the main window and priority for the new process. (Windows only) .. data:: PIPE Special value that can be used as the *stdin*, *stdout* or *stderr* argument to :class:`Popen` and indicates that a pipe to the standard stream should be opened. .. data:: STDOUT Special value that can be used as the *stderr* argument to :class:`Popen` and indicates that standard error should go into the same handle as standard output. Convenience Functions ^^^^^^^^^^^^^^^^^^^^^ This module also defines two shortcut functions: .. function:: call(*popenargs, **kwargs) Run command with arguments. Wait for command to complete, then return the :attr:`returncode` attribute. The arguments are the same as for the Popen constructor. Example:: retcode = call(["ls", "-l"]) .. warning:: Like :meth:`Popen.wait`, this will deadlock if the child process generates enough output to a stdout or stderr pipe such that it blocks waiting for the OS pipe buffer to accept more data. .. function:: check_call(*popenargs, **kwargs) Run command with arguments. Wait for command to complete. If the exit code was zero then return, otherwise raise :exc:`CalledProcessError`. The :exc:`CalledProcessError` object will have the return code in the :attr:`returncode` attribute. The arguments are the same as for the Popen constructor. Example:: check_call(["ls", "-l"]) .. versionadded:: 2.5 .. warning:: See the warning for :func:`call`. .. function:: check_output(*popenargs, **kwargs) Run command with arguments and return its output as a byte string. If the exit code was non-zero it raises a :exc:`CalledProcessError`. The :exc:`CalledProcessError` object will have the return code in the :attr:`returncode` attribute and output in the :attr:`output` attribute. The arguments are the same as for the :class:`Popen` constructor. Example:: >>> subprocess.check_output(["ls", "-l", "/dev/null"]) 'crw-rw-rw- 1 root root 1, 3 Oct 18 2007 /dev/null\n' The stdout argument is not allowed as it is used internally. To capture standard error in the result, use ``stderr=subprocess.STDOUT``:: >>> subprocess.check_output( ["/bin/sh", "-c", "ls non_existent_file ; exit 0"], stderr=subprocess.STDOUT) 'ls: non_existent_file: No such file or directory\n' .. versionadded:: 2.7 Exceptions ^^^^^^^^^^ Exceptions raised in the child process, before the new program has started to execute, will be re-raised in the parent. Additionally, the exception object will have one extra attribute called :attr:`child_traceback`, which is a string containing traceback information from the childs point of view. The most common exception raised is :exc:`OSError`. This occurs, for example, when trying to execute a non-existent file. Applications should prepare for :exc:`OSError` exceptions. A :exc:`ValueError` will be raised if :class:`Popen` is called with invalid arguments. check_call() will raise :exc:`CalledProcessError`, if the called process returns a non-zero return code. 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. Popen Objects ------------- Instances of the :class:`Popen` class have the following methods: .. method:: Popen.poll() Check if child process has terminated. Set and return :attr:`returncode` attribute. .. method:: Popen.wait() Wait for child process to terminate. Set and return :attr:`returncode` attribute. .. warning:: This will deadlock if the child process generates enough output to a stdout or stderr pipe such that it blocks waiting for the OS pipe buffer to accept more data. Use :meth:`communicate` to avoid that. .. method:: Popen.communicate(input=None) Interact with process: Send data to stdin. Read data from stdout and stderr, until end-of-file is reached. Wait for process to terminate. The optional *input* argument should be a string to be sent to the child process, or ``None``, if no data should be sent to the child. :meth:`communicate` returns a tuple ``(stdoutdata, stderrdata)``. Note that if you want to send data to the process's stdin, you need to create the Popen object with ``stdin=PIPE``. Similarly, to get anything other than ``None`` in the result tuple, you need to give ``stdout=PIPE`` and/or ``stderr=PIPE`` too. .. note:: The data read is buffered in memory, so do not use this method if the data size is large or unlimited. .. method:: Popen.send_signal(signal) Sends the signal *signal* to the child. .. note:: On Windows only SIGTERM is supported so far. It's an alias for :meth:`terminate`. .. versionadded:: 2.6 .. method:: Popen.terminate() Stop the child. On Posix OSs the method sends SIGTERM to the child. On Windows the Win32 API function :cfunc:`TerminateProcess` is called to stop the child. .. versionadded:: 2.6 .. method:: Popen.kill() Kills the child. On Posix OSs the function sends SIGKILL to the child. On Windows :meth:`kill` is an alias for :meth:`terminate`. .. versionadded:: 2.6 The following attributes are also available: .. warning:: Use :meth:`communicate` rather than :attr:`.stdin.write `, :attr:`.stdout.read ` or :attr:`.stderr.read ` to avoid deadlocks due to any of the other OS pipe buffers filling up and blocking the child process. .. attribute:: Popen.stdin If the *stdin* argument was :data:`PIPE`, this attribute is a file object that provides input to the child process. Otherwise, it is ``None``. .. attribute:: Popen.stdout If the *stdout* argument was :data:`PIPE`, this attribute is a file object that provides output from the child process. Otherwise, it is ``None``. .. attribute:: Popen.stderr If the *stderr* argument was :data:`PIPE`, this attribute is a file object that provides error output from the child process. Otherwise, it is ``None``. .. attribute:: Popen.pid The process ID of the child process. .. attribute:: Popen.returncode The child return code, set by :meth:`poll` and :meth:`wait` (and indirectly by :meth:`communicate`). A ``None`` value indicates that the process hasn't terminated yet. A negative value ``-N`` indicates that the child was terminated by signal ``N`` (Unix only). .. _subprocess-replacements: Replacing Older Functions with the subprocess Module ---------------------------------------------------- In this section, "a ==> 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. In the following examples, we assume that the subprocess module is imported with "from subprocess import \*". Replacing /bin/sh shell backquote ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: output=`mycmd myarg` ==> output = Popen(["mycmd", "myarg"], stdout=PIPE).communicate()[0] Replacing shell pipeline ^^^^^^^^^^^^^^^^^^^^^^^^ :: output=`dmesg | grep hda` ==> p1 = Popen(["dmesg"], stdout=PIPE) p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE) output = p2.communicate()[0] Replacing os.system() ^^^^^^^^^^^^^^^^^^^^^ :: sts = os.system("mycmd" + " myarg") ==> p = Popen("mycmd" + " myarg", shell=True) sts = os.waitpid(p.pid, 0) 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: retcode = call("mycmd" + " myarg", shell=True) if retcode < 0: print >>sys.stderr, "Child was terminated by signal", -retcode else: print >>sys.stderr, "Child returned", retcode except OSError, e: print >>sys.stderr, "Execution failed:", e Replacing the os.spawn family ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ P_NOWAIT example:: pid = os.spawnlp(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg") ==> pid = Popen(["/bin/mycmd", "myarg"]).pid P_WAIT example:: retcode = os.spawnlp(os.P_WAIT, "/bin/mycmd", "mycmd", "myarg") ==> retcode = call(["/bin/mycmd", "myarg"]) Vector example:: os.spawnvp(os.P_NOWAIT, path, args) ==> Popen([path] + args[1:]) Environment example:: os.spawnlpe(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg", env) ==> Popen(["/bin/mycmd", "myarg"], env={"PATH": "/usr/bin"}) Replacing os.popen, os.popen2, os.popen3 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: pipe = os.popen(cmd, 'r', bufsize) ==> pipe = Popen(cmd, shell=True, bufsize=bufsize, stdout=PIPE).stdout :: pipe = os.popen(cmd, 'w', bufsize) ==> pipe = Popen(cmd, shell=True, bufsize=bufsize, stdin=PIPE).stdin :: (child_stdin, child_stdout) = os.popen2(cmd, mode, bufsize) ==> p = Popen(cmd, shell=True, bufsize=bufsize, stdin=PIPE, stdout=PIPE, close_fds=True) (child_stdin, child_stdout) = (p.stdin, p.stdout) :: (child_stdin, child_stdout, child_stderr) = os.popen3(cmd, mode, bufsize) ==> p = Popen(cmd, shell=True, bufsize=bufsize, stdin=PIPE, stdout=PIPE, stderr=PIPE, close_fds=True) (child_stdin, child_stdout, child_stderr) = (p.stdin, p.stdout, p.stderr) :: (child_stdin, child_stdout_and_stderr) = os.popen4(cmd, mode, bufsize) ==> p = Popen(cmd, shell=True, bufsize=bufsize, stdin=PIPE, stdout=PIPE, stderr=STDOUT, close_fds=True) (child_stdin, child_stdout_and_stderr) = (p.stdin, p.stdout) Replacing functions from the popen2 module ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ .. note:: If the cmd argument to popen2 functions is a string, the command is executed through /bin/sh. If it is a list, the command is directly executed. :: (child_stdout, child_stdin) = popen2.popen2("somestring", bufsize, mode) ==> p = Popen(["somestring"], shell=True, bufsize=bufsize, stdin=PIPE, stdout=PIPE, close_fds=True) (child_stdout, child_stdin) = (p.stdout, p.stdin) :: (child_stdout, child_stdin) = popen2.popen2(["mycmd", "myarg"], bufsize, mode) ==> p = Popen(["mycmd", "myarg"], bufsize=bufsize, stdin=PIPE, stdout=PIPE, close_fds=True) (child_stdout, child_stdin) = (p.stdout, p.stdin) :class:`popen2.Popen3` and :class:`popen2.Popen4` basically work as :class:`subprocess.Popen`, except that: * :class:`Popen` raises an exception if the execution fails. * the *capturestderr* argument is replaced with the *stderr* argument. * ``stdin=PIPE`` and ``stdout=PIPE`` must be specified. * popen2 closes all file descriptors by default, but you have to specify ``close_fds=True`` with :class:`Popen`.