| :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. |
| |
| .. seealso:: |
| |
| :pep:`324` -- PEP proposing the subprocess module |
| |
| |
| Using the :mod:`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:`~CalledProcessError.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. |
| |
| If the return code was non-zero it raises a :exc:`CalledProcessError`. The |
| :exc:`CalledProcessError` object will have the return code in the |
| :attr:`~CalledProcessError.returncode` attribute and any output in the |
| :attr:`~CalledProcessError.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 ``False`` the file objects *stdin*, *stdout* and |
| *stderr* will be opened as binary streams, and no line ending conversion is |
| done. |
| |
| If *universal_newlines* is ``True``, these file objects |
| will be opened as text streams in :term:`universal newlines` mode |
| using the encoding returned by :func:`locale.getpreferredencoding(False) |
| <locale.getpreferredencoding>`. 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 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) <locale.getpreferredencoding>` |
| 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 |
| <http://en.wikipedia.org/wiki/Shell_injection#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. |
| |
| When using ``shell=True``, :func:`shlex.quote` can be used to properly |
| escape whitespace and shell metacharacters in strings that are going to |
| be used to construct shell commands. |
| |
| 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=-1, 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=()) |
| |
| Execute a child program in a new process. On Unix, the class uses |
| :meth:`os.execvp`-like behavior to execute the child program. On Windows, |
| the class uses the Windows ``CreateProcess()`` function. The arguments to |
| :class:`Popen` are as follows. |
| |
| *args* should be a sequence of program arguments or else a single string. |
| By default, the program to execute is the first item in *args* if *args* is |
| a sequence. If *args* is a string, the interpretation is |
| platform-dependent and described below. See the *shell* and *executable* |
| arguments for additional differences from the default behavior. Unless |
| otherwise stated, it is recommended to pass *args* as a sequence. |
| |
| On Unix, if *args* is a string, the string is interpreted as the name or |
| path of the program to execute. However, this can only be done if not |
| passing arguments to the program. |
| |
| .. 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 Windows, if *args* is a sequence, it will be converted to a string in a |
| manner described in :ref:`converting-argument-sequence`. This is because |
| the underlying ``CreateProcess()`` operates on strings. |
| |
| The *shell* argument (which defaults to *False*) specifies whether to use |
| the shell as the program to execute. If *shell* is *True*, it is |
| recommended to pass *args* as a string rather than as a sequence. |
| |
| On Unix with ``shell=True``, the shell defaults to :file:`/bin/sh`. If |
| *args* is a string, the string specifies the command |
| 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, :class:`Popen` does the equivalent of:: |
| |
| Popen(['/bin/sh', '-c', args[0], args[1], ...]) |
| |
| On Windows with ``shell=True``, the :envvar:`COMSPEC` environment variable |
| specifies the default shell. The only time you need to specify |
| ``shell=True`` on Windows is when the command you wish to execute is built |
| into the shell (e.g. :command:`dir` or :command:`copy`). You do not need |
| ``shell=True`` to run a batch file or console-based executable. |
| |
| .. warning:: |
| |
| Passing ``shell=True`` can be a security hazard if combined with |
| untrusted input. See the warning under :ref:`frequently-used-arguments` |
| for details. |
| |
| *bufsize* will be supplied as the corresponding argument to the :meth:`io.open` |
| function when creating the stdin/stdout/stderr pipe file objects: |
| :const:`0` means unbuffered (read and write are one system call and can return short), |
| :const:`1` means line buffered, any other positive value means use a buffer of |
| approximately that size. A negative bufsize (the default) means |
| the system default of io.DEFAULT_BUFFER_SIZE will be used. |
| |
| .. versionchanged:: 3.3.1 |
| *bufsize* now defaults to -1 to enable buffering by default to match the |
| behavior that most code expects. In versions prior to Python 3.2.4 and |
| 3.3.1 it incorrectly defaulted to :const:`0` which was unbuffered |
| and allowed short reads. This was unintentional and did not match the |
| behavior of Python 2 as most code expected. |
| |
| The *executable* argument specifies a replacement program to execute. It |
| is very seldom needed. When ``shell=False``, *executable* replaces the |
| program to execute specified by *args*. However, the original *args* is |
| still passed to the program. Most programs treat the program specified |
| by *args* as the command name, which can then be different from the program |
| actually executed. On Unix, the *args* name |
| becomes the display name for the executable in utilities such as |
| :program:`ps`. If ``shell=True``, on Unix the *executable* argument |
| specifies a replacement shell for the default :file:`/bin/sh`. |
| |
| *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 function changes the working directory to |
| *cwd* before executing the child. In particular, the function looks for |
| *executable* (or for the first item in *args*) relative to *cwd* if the |
| executable path is a relative path. |
| |
| 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`, otherwise they are |
| opened as binary streams. |
| |
| 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:`~Popen.returncode` attribute. |
| |
| |
| .. method:: Popen.wait(timeout=None) |
| |
| Wait for child process to terminate. Set and return |
| :attr:`~Popen.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 <Popen.stdin>`, |
| :attr:`.stdout.read <Popen.stdout>` or :attr:`.stderr.read <Popen.stderr>` 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 <http://msdn.microsoft.com/en-us/library/ms686331(v=vs.85).aspx>`__ |
| 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 <http://msdn.microsoft.com/en-us/library/ms633548(v=vs.85).aspx>`__ |
| 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 :mod:`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 |
| :attr:`~CalledProcessError.output` attribute of the raised exception. |
| |
| In the following examples, we assume that the relevant functions have already |
| been imported from the :mod:`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 <os.spawnl>` 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 :class:`Popen` and return a 2-tuple |
| ``(status, output)`` via :func:`Popen.communicate`. Universal newlines mode |
| is used; see the notes on :ref:`frequently-used-arguments` for more details. |
| |
| 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') |
| |
| .. versionchanged:: 3.3 |
| Availability: Unix & Windows |
| |
| |
| .. 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' |
| |
| .. versionchanged:: 3.3 |
| Availability: Unix & Windows |
| |
| |
| 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. |