| |
| :mod:`subprocess` --- Subprocess management |
| =========================================== |
| |
| .. module:: subprocess |
| :synopsis: Subprocess management. |
| .. moduleauthor:: Peter Åstrand <astrand@lysator.liu.se> |
| .. sectionauthor:: Peter Åstrand <astrand@lysator.liu.se> |
| |
| |
| 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* |
| |
| 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 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. |
| |
| 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 :meth:`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 four 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"]) |
| |
| |
| .. 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 :cfunc:`wait`. Example:: |
| |
| >>> import subprocess |
| >>> 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') |
| |
| |
| .. function:: getoutput(cmd) |
| Return output ``(stdout or 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:: |
| |
| >>> import subprocess |
| >>> subprocess.getoutput('ls /bin/ls') |
| '/bin/ls' |
| |
| |
| 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. |
| |
| |
| .. 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 byte string to be sent to the child process, or |
| ``None``, if no data should be sent to the child. |
| |
| :meth:`communicate` returns a tuple ``(stdout, stderr)``. |
| |
| 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`. |
| |
| |
| .. 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. |
| |
| |
| .. 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`. |
| |
| |
| 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, 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 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("Child was terminated by signal", -retcode, file=sys.stderr) |
| else: |
| print("Child returned", retcode, file=sys.stderr) |
| except OSError as e: |
| print("Execution failed:", e, file=sys.stderr) |
| |
| |
| 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, '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 |
| |