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diff --git a/Doc/library/subprocess.rst b/Doc/library/subprocess.rst new file mode 100644 index 0000000..a3bc2cb --- /dev/null +++ b/Doc/library/subprocess.rst @@ -0,0 +1,340 @@ + +:mod:`subprocess` --- Subprocess management +=========================================== + +.. module:: subprocess + :synopsis: Subprocess management. +.. moduleauthor:: Peter Åstrand <astrand@lysator.liu.se> +.. sectionauthor:: Peter Åstrand <astrand@lysator.liu.se> + + +.. 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* + commands.* + +Information about how the :mod:`subprocess` module can be used to replace these +modules and functions can be found in the following sections. + + +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 string, 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 + ``PIPE``, an existing file descriptor (a positive integer), an existing file + object, and ``None``. ``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 ``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 defines the environment variables for the new + process. + + 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 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) + + +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"]) + + +.. 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 + + +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. Returns returncode attribute. + + +.. method:: Popen.wait() + + Wait for child process to terminate. Returns returncode attribute. + + +.. 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. + + communicate() returns a tuple (stdout, stderr). + + .. note:: + + The data read is buffered in memory, so do not use this method if the data size + is large or unlimited. + +The following attributes are also available: + + +.. attribute:: Popen.stdin + + If the *stdin* argument is ``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 is ``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 is ``PIPE``, this attribute is 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. 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). + + +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 pipe line +^^^^^^^^^^^^^^^^^^^^^^^^^ + +:: + + 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 as e: + print >>sys.stderr, "Execution failed:", e + + +Replacing os.spawn\* +^^^^^^^^^^^^^^^^^^^^ + +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\* +^^^^^^^^^^^^^^^^^^^^ + +:: + + pipe = os.popen(cmd, mode='r', bufsize) + ==> + pipe = Popen(cmd, shell=True, bufsize=bufsize, stdout=PIPE).stdout + +:: + + pipe = os.popen(cmd, mode='w', bufsize) + ==> + pipe = Popen(cmd, shell=True, bufsize=bufsize, stdin=PIPE).stdin + |