:mod:`subprocess` --- Subprocess management =========================================== .. module:: subprocess :synopsis: Subprocess management. .. moduleauthor:: Peter Åstrand .. sectionauthor:: Peter Åstrand 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. .. seealso:: :pep:`324` -- PEP proposing the subprocess module Using the subprocess Module --------------------------- The recommended approach to invoking subprocesses is to use the following convenience functions for all use cases they can handle. For more advanced use cases, the underlying :class:`Popen` interface can be used directly. .. function:: call(args, *, stdin=None, stdout=None, stderr=None, shell=False, timeout=None) Run the command described by *args*. Wait for command to complete, then return the :attr:`returncode` attribute. The arguments shown above are merely the most common ones, described below in :ref:`frequently-used-arguments` (hence the use of keyword-only notation in the abbreviated signature). The full function signature is largely the same as that of the :class:`Popen` constructor - this function passes all supplied arguments other than *timeout* directly through to that interface. The *timeout* argument is passed to :meth:`Popen.wait`. If the timeout expires, the child process will be killed and then waited for again. The :exc:`TimeoutExpired` exception will be re-raised after the child process has terminated. Examples:: >>> subprocess.call(["ls", "-l"]) 0 >>> subprocess.call("exit 1", shell=True) 1 .. warning:: Invoking the system shell with ``shell=True`` can be a security hazard if combined with untrusted input. See the warning under :ref:`frequently-used-arguments` for details. .. note:: Do not use ``stdout=PIPE`` or ``stderr=PIPE`` with this function. As the pipes are not being read in the current process, the child process may block if it generates enough output to a pipe to fill up the OS pipe buffer. .. versionchanged:: 3.3 *timeout* was added. .. function:: check_call(args, *, stdin=None, stdout=None, stderr=None, shell=False, timeout=None) Run command with arguments. Wait for command to complete. If the return 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 shown above are merely the most common ones, described below in :ref:`frequently-used-arguments` (hence the use of keyword-only notation in the abbreviated signature). The full function signature is largely the same as that of the :class:`Popen` constructor - this function passes all supplied arguments other than *timeout* directly through to that interface. The *timeout* argument is passed to :meth:`Popen.wait`. If the timeout expires, the child process will be killed and then waited for again. The :exc:`TimeoutExpired` exception will be re-raised after the child process has terminated. Examples:: >>> subprocess.check_call(["ls", "-l"]) 0 >>> subprocess.check_call("exit 1", shell=True) Traceback (most recent call last): ... subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1 .. warning:: Invoking the system shell with ``shell=True`` can be a security hazard if combined with untrusted input. See the warning under :ref:`frequently-used-arguments` for details. .. note:: Do not use ``stdout=PIPE`` or ``stderr=PIPE`` with this function. As the pipes are not being read in the current process, the child process may block if it generates enough output to a pipe to fill up the OS pipe buffer. .. versionchanged:: 3.3 *timeout* was added. .. function:: check_output(args, *, stdin=None, stderr=None, shell=False, universal_newlines=False, timeout=None) Run command with arguments and return its output as a byte string. If the return code was non-zero it raises a :exc:`CalledProcessError`. The :exc:`CalledProcessError` object will have the return code in the :attr:`returncode` attribute and any output in the :attr:`output` attribute. The arguments shown above are merely the most common ones, described below in :ref:`frequently-used-arguments` (hence the use of keyword-only notation in the abbreviated signature). The full function signature is largely the same as that of the :class:`Popen` constructor - this functions passes all supplied arguments other than *timeout* directly through to that interface. In addition, *stdout* is not permitted as an argument, as it is used internally to collect the output from the subprocess. The *timeout* argument is passed to :meth:`Popen.wait`. If the timeout expires, the child process will be killed and then waited for again. The :exc:`TimeoutExpired` exception will be re-raised after the child process has terminated. Examples:: >>> subprocess.check_output(["echo", "Hello World!"]) b'Hello World!\n' >>> subprocess.check_output(["echo", "Hello World!"], universal_newlines=True) 'Hello World!\n' >>> subprocess.check_output("exit 1", shell=True) Traceback (most recent call last): ... subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1 By default, this function will return the data as encoded bytes. The actual encoding of the output data may depend on the command being invoked, so the decoding to text will often need to be handled at the application level. This behaviour may be overridden by setting *universal_newlines* to ``True`` as described below in :ref:`frequently-used-arguments`. To also capture standard error in the result, use ``stderr=subprocess.STDOUT``:: >>> subprocess.check_output( ... "ls non_existent_file; exit 0", ... stderr=subprocess.STDOUT, ... shell=True) 'ls: non_existent_file: No such file or directory\n' .. versionadded:: 3.1 .. warning:: Invoking the system shell with ``shell=True`` can be a security hazard if combined with untrusted input. See the warning under :ref:`frequently-used-arguments` for details. .. note:: Do not use ``stderr=PIPE`` with this function. As the pipe is not being read in the current process, the child process may block if it generates enough output to the pipe to fill up the OS pipe buffer. .. versionchanged:: 3.3 *timeout* was added. .. data:: DEVNULL Special value that can be used as the *stdin*, *stdout* or *stderr* argument to :class:`Popen` and indicates that the special file :data:`os.devnull` will be used. .. versionadded:: 3.3 .. 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. .. exception:: SubprocessError Base class for all other exceptions from this module. .. versionadded:: 3.3 .. exception:: TimeoutExpired Subclass of :exc:`SubprocessError`, raised when a timeout expires while waiting for a child process. .. attribute:: cmd Command that was used to spawn the child process. .. attribute:: timeout Timeout in seconds. .. attribute:: output Output of the child process if this exception is raised by :func:`check_output`. Otherwise, ``None``. .. versionadded:: 3.3 .. exception:: CalledProcessError Subclass of :exc:`SubprocessError`, raised when a process run by :func:`check_call` or :func:`check_output` returns a non-zero exit status. .. attribute:: returncode Exit status of the child process. .. attribute:: cmd Command that was used to spawn the child process. .. attribute:: output Output of the child process if this exception is raised by :func:`check_output`. Otherwise, ``None``. .. _frequently-used-arguments: Frequently Used Arguments ^^^^^^^^^^^^^^^^^^^^^^^^^ To support a wide variety of use cases, the :class:`Popen` constructor (and the convenience functions) accept a large number of optional arguments. For most typical use cases, many of these arguments can be safely left at their default values. The arguments that are most commonly needed are: *args* is required for all calls and should be a string, or a sequence of program arguments. Providing a sequence of arguments is generally preferred, as it allows the module to take care of any required escaping and quoting of arguments (e.g. to permit spaces in file names). If passing a single string, either *shell* must be :const:`True` (see below) or else the string must simply name the program to be executed without specifying any arguments. *stdin*, *stdout* and *stderr* specify the executed program's standard input, standard output and standard error file handles, respectively. Valid values are :data:`PIPE`, :data:`DEVNULL`, 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. :data:`DEVNULL` indicates that the special file :data:`os.devnull` will be used. With the default settings of ``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 child process should be captured into the same file handle as for *stdout*. .. index:: single: universal newlines; subprocess module If *universal_newlines* is ``True``, the file objects *stdin*, *stdout* and *stderr* will be opened as text streams in :term:`universal newlines` mode using the encoding returned by :func:`locale.getpreferredencoding(False) `. For *stdin*, line ending characters ``'\n'`` in the input will be converted to the default line separator :data:`os.linesep`. For *stdout* and *stderr*, all line endings in the output will be converted to ``'\n'``. For more information see the documentation of the :class:`io.TextIOWrapper` class when the *newline* argument to its constructor is ``None``. .. note:: The *universal_newlines* feature is supported only if Python is built with universal newline support (the default). Also, the newlines attribute of the file objects :attr:`Popen.stdin`, :attr:`Popen.stdout` and :attr:`Popen.stderr` are not updated by the :meth:`Popen.communicate` method. If *shell* is ``True``, the specified command will be executed through the shell. This can be useful if you are using Python primarily for the enhanced control flow it offers over most system shells and still want convenient access to other shell features such as shell pipes, filename wildcards, environment variable expansion, and expansion of ``~`` to a user's home directory. However, note that Python itself offers implementations of many shell-like features (in particular, :mod:`glob`, :mod:`fnmatch`, :func:`os.walk`, :func:`os.path.expandvars`, :func:`os.path.expanduser`, and :mod:`shutil`). .. versionchanged:: 3.3 When *universal_newlines* is ``True``, the class uses the encoding :func:`locale.getpreferredencoding(False) ` instead of ``locale.getpreferredencoding()``. See the :class:`io.TextIOWrapper` class for more information on this change. .. warning:: Executing shell commands that incorporate unsanitized input from an untrusted source makes a program vulnerable to `shell injection `_, a serious security flaw which can result in arbitrary command execution. For this reason, the use of *shell=True* is **strongly discouraged** in cases where the command string is constructed from external input:: >>> from subprocess import call >>> filename = input("What file would you like to display?\n") What file would you like to display? non_existent; rm -rf / # >>> call("cat " + filename, shell=True) # Uh-oh. This will end badly... ``shell=False`` disables all shell based features, but does not suffer from this vulnerability; see the Note in the :class:`Popen` constructor documentation for helpful hints in getting ``shell=False`` to work. These options, along with all of the other options, are described in more detail in the :class:`Popen` constructor documentation. Popen Constructor ^^^^^^^^^^^^^^^^^ The underlying process creation and management in this module is handled by the :class:`Popen` class. It offers a lot of flexibility so that developers are able to handle the less common cases not covered by the convenience functions. .. class:: Popen(args, bufsize=0, executable=None, stdin=None, stdout=None, stderr=None, preexec_fn=None, close_fds=True, shell=False, cwd=None, env=None, universal_newlines=False, startupinfo=None, creationflags=0, restore_signals=True, start_new_session=False, pass_fds=()) 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. When *executable* is given, the first item in the args sequence is still treated by most programs as the command name, which can then be different from the actual executable name. On Unix, it becomes the display name for the executing program in utilities such as :program:`ps`. On Unix, with *shell=False* (default): In this case, the Popen class uses :meth:`os.execvp` like behavior to execute the child program. *args* should normally be a sequence. If a string is specified for *args*, it will be used as the name or path of the program to execute; this will only work if the program is being given no arguments. .. note:: :meth:`shlex.split` can be useful when determining the correct tokenization for *args*, especially in complex cases:: >>> import shlex, subprocess >>> command_line = input() /bin/vikings -input eggs.txt -output "spam spam.txt" -cmd "echo '$MONEY'" >>> args = shlex.split(command_line) >>> print(args) ['/bin/vikings', '-input', 'eggs.txt', '-output', 'spam spam.txt', '-cmd', "echo '$MONEY'"] >>> p = subprocess.Popen(args) # Success! Note in particular that options (such as *-input*) and arguments (such as *eggs.txt*) that are separated by whitespace in the shell go in separate list elements, while arguments that need quoting or backslash escaping when used in the shell (such as filenames containing spaces or the *echo* command shown above) are single list elements. On Unix, with *shell=True*: If args is a string, it specifies the command string to execute through the shell. This means that the string must be formatted exactly as it would be when typed at the shell prompt. This includes, for example, quoting or backslash escaping filenames with spaces in them. If *args* is a sequence, the first item specifies the command string, and any additional items will be treated as additional arguments to the shell itself. That is to say, *Popen* does the equivalent of:: Popen(['/bin/sh', '-c', args[0], args[1], ...]) .. warning:: Enabling this option can be a security hazard if combined with untrusted input. See the warning under :ref:`frequently-used-arguments` for details. 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 in a manner described in :ref:`converting-argument-sequence`. *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). .. note:: If you experience performance issues, it is recommended that you try to enable buffering by setting *bufsize* to either -1 or a large enough positive value (such as 4096). 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. The only reason you would need to specify ``shell=True`` on Windows is where the command you wish to execute is actually built in to the shell, eg ``dir``, ``copy``. You don't need ``shell=True`` to run a batch file, nor to run a console-based executable. *stdin*, *stdout* and *stderr* specify the executed program's standard input, standard output and standard error file handles, respectively. Valid values are :data:`PIPE`, :data:`DEVNULL`, an existing file descriptor (a positive integer), an existing :term:`file object`, and ``None``. :data:`PIPE` indicates that a new pipe to the child should be created. :data:`DEVNULL` indicates that the special file :data:`os.devnull` will be used. With the default settings of ``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) .. warning:: The *preexec_fn* parameter is not safe to use in the presence of threads in your application. The child process could deadlock before exec is called. If you must use it, keep it trivial! Minimize the number of libraries you call into. .. note:: If you need to modify the environment for the child use the *env* parameter rather than doing it in a *preexec_fn*. The *start_new_session* parameter can take the place of a previously common use of *preexec_fn* to call os.setsid() in the child. 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). The default varies by platform: Always true on Unix. On Windows it is true when *stdin*/*stdout*/*stderr* are :const:`None`, false otherwise. 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*. .. versionchanged:: 3.2 The default for *close_fds* was changed from :const:`False` to what is described above. *pass_fds* is an optional sequence of file descriptors to keep open between the parent and child. Providing any *pass_fds* forces *close_fds* to be :const:`True`. (Unix only) .. versionadded:: 3.2 The *pass_fds* parameter was added. 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 *restore_signals* is True (the default) all signals that Python has set to SIG_IGN are restored to SIG_DFL in the child process before the exec. Currently this includes the SIGPIPE, SIGXFZ and SIGXFSZ signals. (Unix only) .. versionchanged:: 3.2 *restore_signals* was added. If *start_new_session* is True the setsid() system call will be made in the child process prior to the execution of the subprocess. (Unix only) .. versionchanged:: 3.2 *start_new_session* was added. If *env* is not ``None``, it must be a mapping that defines the environment variables for the new process; these are used instead of the default behavior of inheriting the current process' environment. .. 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 ``True``, the file objects *stdin*, *stdout* and *stderr* are opened as text streams in universal newlines mode, as described above in :ref:`frequently-used-arguments`. If given, *startupinfo* will be a :class:`STARTUPINFO` object, which is passed to the underlying ``CreateProcess`` function. *creationflags*, if given, can be :data:`CREATE_NEW_CONSOLE` or :data:`CREATE_NEW_PROCESS_GROUP`. (Windows only) Popen objects are supported as context managers via the :keyword:`with` statement: on exit, standard file descriptors are closed, and the process is waited for. :: with Popen(["ifconfig"], stdout=PIPE) as proc: log.write(proc.stdout.read()) .. versionchanged:: 3.2 Added context manager support. 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 child's 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. :func:`check_call` and :func:`check_output` will raise :exc:`CalledProcessError` if the called process returns a non-zero return code. All of the functions and methods that accept a *timeout* parameter, such as :func:`call` and :meth:`Popen.communicate` will raise :exc:`TimeoutExpired` if the timeout expires before the process exits. Exceptions defined in this module all inherit from :exc:`SubprocessError`. .. versionadded:: 3.3 The :exc:`SubprocessError` base class was added. Security ^^^^^^^^ Unlike some other popen functions, this implementation will never call a system shell implicitly. This means that all characters, including shell metacharacters, can safely be passed to child processes. Obviously, if the shell is invoked explicitly, then it is the application's responsibility to ensure that all whitespace and metacharacters are quoted appropriately. 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(timeout=None) Wait for child process to terminate. Set and return :attr:`returncode` attribute. If the process does not terminate after *timeout* seconds, raise a :exc:`TimeoutExpired` exception. It is safe to catch this exception and retry the wait. .. warning:: This will deadlock when using ``stdout=PIPE`` and/or ``stderr=PIPE`` and the child process generates enough output to a pipe such that it blocks waiting for the OS pipe buffer to accept more data. Use :meth:`communicate` to avoid that. .. versionchanged:: 3.3 *timeout* was added. .. method:: Popen.communicate(input=None, timeout=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 data to be sent to the child process, or ``None``, if no data should be sent to the child. The type of *input* must be bytes or, if *universal_newlines* was ``True``, a string. :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. If the process does not terminate after *timeout* seconds, a :exc:`TimeoutExpired` exception will be raised. Catching this exception and retrying communication will not lose any output. The child process is not killed if the timeout expires, so in order to cleanup properly a well-behaved application should kill the child process and finish communication:: proc = subprocess.Popen(...) try: outs, errs = proc.communicate(timeout=15) except TimeoutExpired: proc.kill() outs, errs = proc.communicate() .. note:: The data read is buffered in memory, so do not use this method if the data size is large or unlimited. .. versionchanged:: 3.3 *timeout* was added. .. method:: Popen.send_signal(signal) Sends the signal *signal* to the child. .. note:: On Windows, SIGTERM is an alias for :meth:`terminate`. CTRL_C_EVENT and CTRL_BREAK_EVENT can be sent to processes started with a *creationflags* parameter which includes `CREATE_NEW_PROCESS_GROUP`. .. method:: Popen.terminate() Stop the child. On Posix OSs the method sends SIGTERM to the child. On Windows the Win32 API function :c:func:`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: .. warning:: Use :meth:`~Popen.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 :term:`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 :term:`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 :term:`file object` that provides error output from the child process. Otherwise, it is ``None``. .. attribute:: Popen.pid The process ID of the child process. Note that if you set the *shell* argument to ``True``, this is the process ID of the spawned shell. .. 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). Windows Popen Helpers --------------------- The :class:`STARTUPINFO` class and following constants are only available on Windows. .. class:: STARTUPINFO() Partial support of the Windows `STARTUPINFO `__ structure is used for :class:`Popen` creation. .. attribute:: dwFlags A bit field that determines whether certain :class:`STARTUPINFO` attributes are used when the process creates a window. :: si = subprocess.STARTUPINFO() si.dwFlags = subprocess.STARTF_USESTDHANDLES | subprocess.STARTF_USESHOWWINDOW .. attribute:: hStdInput If :attr:`dwFlags` specifies :data:`STARTF_USESTDHANDLES`, this attribute is the standard input handle for the process. If :data:`STARTF_USESTDHANDLES` is not specified, the default for standard input is the keyboard buffer. .. attribute:: hStdOutput If :attr:`dwFlags` specifies :data:`STARTF_USESTDHANDLES`, this attribute is the standard output handle for the process. Otherwise, this attribute is ignored and the default for standard output is the console window's buffer. .. attribute:: hStdError If :attr:`dwFlags` specifies :data:`STARTF_USESTDHANDLES`, this attribute is the standard error handle for the process. Otherwise, this attribute is ignored and the default for standard error is the console window's buffer. .. attribute:: wShowWindow If :attr:`dwFlags` specifies :data:`STARTF_USESHOWWINDOW`, this attribute can be any of the values that can be specified in the ``nCmdShow`` parameter for the `ShowWindow `__ function, except for ``SW_SHOWDEFAULT``. Otherwise, this attribute is ignored. :data:`SW_HIDE` is provided for this attribute. It is used when :class:`Popen` is called with ``shell=True``. Constants ^^^^^^^^^ The :mod:`subprocess` module exposes the following constants. .. data:: STD_INPUT_HANDLE The standard input device. Initially, this is the console input buffer, ``CONIN$``. .. data:: STD_OUTPUT_HANDLE The standard output device. Initially, this is the active console screen buffer, ``CONOUT$``. .. data:: STD_ERROR_HANDLE The standard error device. Initially, this is the active console screen buffer, ``CONOUT$``. .. data:: SW_HIDE Hides the window. Another window will be activated. .. data:: STARTF_USESTDHANDLES Specifies that the :attr:`STARTUPINFO.hStdInput`, :attr:`STARTUPINFO.hStdOutput`, and :attr:`STARTUPINFO.hStdError` attributes contain additional information. .. data:: STARTF_USESHOWWINDOW Specifies that the :attr:`STARTUPINFO.wShowWindow` attribute contains additional information. .. data:: CREATE_NEW_CONSOLE The new process has a new console, instead of inheriting its parent's console (the default). This flag is always set when :class:`Popen` is created with ``shell=True``. .. data:: CREATE_NEW_PROCESS_GROUP A :class:`Popen` ``creationflags`` parameter to specify that a new process group will be created. This flag is necessary for using :func:`os.kill` on the subprocess. This flag is ignored if :data:`CREATE_NEW_CONSOLE` is specified. .. _subprocess-replacements: Replacing Older Functions with the subprocess Module ---------------------------------------------------- In this section, "a becomes b" means that b can be used as a replacement for a. .. note:: All "a" functions in this section fail (more or less) silently if the executed program cannot be found; the "b" replacements raise :exc:`OSError` instead. In addition, the replacements using :func:`check_output` will fail with a :exc:`CalledProcessError` if the requested operation produces a non-zero return code. The output is still available as the ``output`` attribute of the raised exception. In the following examples, we assume that the relevant functions have already been imported from the subprocess module. Replacing /bin/sh shell backquote ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: output=`mycmd myarg` # becomes output = check_output(["mycmd", "myarg"]) Replacing shell pipeline ^^^^^^^^^^^^^^^^^^^^^^^^ :: output=`dmesg | grep hda` # becomes p1 = Popen(["dmesg"], stdout=PIPE) p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE) p1.stdout.close() # Allow p1 to receive a SIGPIPE if p2 exits. output = p2.communicate()[0] The p1.stdout.close() call after starting the p2 is important in order for p1 to receive a SIGPIPE if p2 exits before p1. Alternatively, for trusted input, the shell's own pipeline support may still be used directly:: output=`dmesg | grep hda` # becomes output=check_output("dmesg | grep hda", shell=True) Replacing :func:`os.system` ^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: sts = os.system("mycmd" + " myarg") # becomes sts = call("mycmd" + " myarg", shell=True) Notes: * Calling the program through the shell is usually not required. 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 the :func:`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 :func:`os.popen`, :func:`os.popen2`, :func:`os.popen3` ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ :: (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) Return code handling translates as follows:: pipe = os.popen(cmd, 'w') ... rc = pipe.close() if rc is not None and rc >> 8: print("There were some errors") ==> process = Popen(cmd, 'w', stdin=PIPE) ... process.stdin.close() if process.wait() != 0: print("There were some errors") Replacing functions from the :mod:`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` to guarantee this behavior on all platforms or past Python versions. Legacy Shell Invocation Functions --------------------------------- This module also provides the following legacy functions from the 2.x ``commands`` module. These operations implicitly invoke the system shell and none of the guarantees described above regarding security and exception handling consistency are valid for these functions. .. 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 :c:func:`wait`. Example:: >>> 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') Availability: UNIX. .. function:: getoutput(cmd) Return output (stdout and 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:: >>> subprocess.getoutput('ls /bin/ls') '/bin/ls' Availability: UNIX. Notes ----- .. _converting-argument-sequence: Converting an argument sequence to a string on Windows ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ On Windows, an *args* sequence is converted to a string that can be parsed using the following rules (which correspond to the rules used by the MS C runtime): 1. Arguments are delimited by white space, which is either a space or a tab. 2. A string surrounded by double quotation marks is interpreted as a single argument, regardless of white space contained within. A quoted string can be embedded in an argument. 3. A double quotation mark preceded by a backslash is interpreted as a literal double quotation mark. 4. Backslashes are interpreted literally, unless they immediately precede a double quotation mark. 5. If backslashes immediately precede a double quotation mark, every pair of backslashes is interpreted as a literal backslash. If the number of backslashes is odd, the last backslash escapes the next double quotation mark as described in rule 3. .. seealso:: :mod:`shlex` Module which provides function to parse and escape command lines.