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:mod:`subprocess` --- Subprocess management
===========================================
.. module:: subprocess
:synopsis: Subprocess management.
.. moduleauthor:: Peter Åstrand <astrand@lysator.liu.se>
.. sectionauthor:: Peter Åstrand <astrand@lysator.liu.se>
**Source code:** :source:`Lib/subprocess.py`
--------------
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 older modules and functions::
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 :func:`run`
function for all use cases it can handle. For more advanced use cases, the
underlying :class:`Popen` interface can be used directly.
The :func:`run` function was added in Python 3.5; if you need to retain
compatibility with older versions, see the :ref:`call-function-trio` section.
.. function:: run(args, *, stdin=None, input=None, stdout=None, stderr=None,\
capture_output=False, shell=False, cwd=None, timeout=None, \
check=False, encoding=None, errors=None, text=None, env=None, \
universal_newlines=None, **other_popen_kwargs)
Run the command described by *args*. Wait for command to complete, then
return a :class:`CompletedProcess` instance.
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 - most of the arguments to
this function are passed through to that interface. (*timeout*, *input*,
*check*, and *capture_output* are not.)
If *capture_output* is true, stdout and stderr will be captured.
When used, the internal :class:`Popen` object is automatically created with
``stdout=PIPE`` and ``stderr=PIPE``. The *stdout* and *stderr* arguments may
not be supplied at the same time as *capture_output*. If you wish to capture
and combine both streams into one, use ``stdout=PIPE`` and ``stderr=STDOUT``
instead of *capture_output*.
The *timeout* argument is passed to :meth:`Popen.communicate`. If the timeout
expires, the child process will be killed and waited for. The
:exc:`TimeoutExpired` exception will be re-raised after the child process
has terminated.
The *input* argument is passed to :meth:`Popen.communicate` and thus to the
subprocess's stdin. If used it must be a byte sequence, or a string if
*encoding* or *errors* is specified or *text* is true. When
used, the internal :class:`Popen` object is automatically created with
``stdin=PIPE``, and the *stdin* argument may not be used as well.
If *check* is true, and the process exits with a non-zero exit code, a
:exc:`CalledProcessError` exception will be raised. Attributes of that
exception hold the arguments, the exit code, and stdout and stderr if they
were captured.
If *encoding* or *errors* are specified, or *text* is true,
file objects for stdin, stdout and stderr are opened in text mode using the
specified *encoding* and *errors* or the :class:`io.TextIOWrapper` default.
The *universal_newlines* argument is equivalent to *text* and is provided
for backwards compatibility. By default, file objects are opened in binary mode.
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. It is passed directly
to :class:`Popen`.
Examples::
>>> subprocess.run(["ls", "-l"]) # doesn't capture output
CompletedProcess(args=['ls', '-l'], returncode=0)
>>> subprocess.run("exit 1", shell=True, check=True)
Traceback (most recent call last):
...
subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1
>>> subprocess.run(["ls", "-l", "/dev/null"], capture_output=True)
CompletedProcess(args=['ls', '-l', '/dev/null'], returncode=0,
stdout=b'crw-rw-rw- 1 root root 1, 3 Jan 23 16:23 /dev/null\n', stderr=b'')
.. versionadded:: 3.5
.. versionchanged:: 3.6
Added *encoding* and *errors* parameters
.. versionchanged:: 3.7
Added the *text* parameter, as a more understandable alias of *universal_newlines*.
Added the *capture_output* parameter.
.. class:: CompletedProcess
The return value from :func:`run`, representing a process that has finished.
.. attribute:: args
The arguments used to launch the process. This may be a list or a string.
.. attribute:: returncode
Exit status of the child process. Typically, an exit status of 0 indicates
that it ran successfully.
A negative value ``-N`` indicates that the child was terminated by signal
``N`` (POSIX only).
.. attribute:: stdout
Captured stdout from the child process. A bytes sequence, or a string if
:func:`run` was called with an encoding, errors, or text=True.
``None`` if stdout was not captured.
If you ran the process with ``stderr=subprocess.STDOUT``, stdout and
stderr will be combined in this attribute, and :attr:`stderr` will be
``None``.
.. attribute:: stderr
Captured stderr from the child process. A bytes sequence, or a string if
:func:`run` was called with an encoding, errors, or text=True.
``None`` if stderr was not captured.
.. method:: check_returncode()
If :attr:`returncode` is non-zero, raise a :exc:`CalledProcessError`.
.. versionadded:: 3.5
.. 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. Most useful with :meth:`Popen.communicate`.
.. 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 it was captured by :func:`run` or
:func:`check_output`. Otherwise, ``None``.
.. attribute:: stdout
Alias for output, for symmetry with :attr:`stderr`.
.. attribute:: stderr
Stderr output of the child process if it was captured by :func:`run`.
Otherwise, ``None``.
.. versionadded:: 3.3
.. versionchanged:: 3.5
*stdout* and *stderr* attributes added
.. 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. If the process exited due to a
signal, this will be the negative signal number.
.. attribute:: cmd
Command that was used to spawn the child process.
.. attribute:: output
Output of the child process if it was captured by :func:`run` or
:func:`check_output`. Otherwise, ``None``.
.. attribute:: stdout
Alias for output, for symmetry with :attr:`stderr`.
.. attribute:: stderr
Stderr output of the child process if it was captured by :func:`run`.
Otherwise, ``None``.
.. versionchanged:: 3.5
*stdout* and *stderr* attributes added
.. _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 *encoding* or *errors* are specified, or *text* (also known as
*universal_newlines*) is true,
the file objects *stdin*, *stdout* and *stderr* will be opened in text
mode using the *encoding* and *errors* specified in the call or the
defaults for :class:`io.TextIOWrapper`.
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``.
If text mode is not used, *stdin*, *stdout* and *stderr* will be opened as
binary streams. No encoding or line ending conversion is performed.
.. versionadded:: 3.6
Added *encoding* and *errors* parameters.
.. versionadded:: 3.7
Added the *text* parameter as an alias for *universal_newlines*.
.. 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.
.. note::
Read the `Security Considerations`_ section before using ``shell=True``.
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=None, \
startupinfo=None, creationflags=0, restore_signals=True, \
start_new_session=False, pass_fds=(), *, group=None, \
extra_groups=None, user=None, umask=-1, \
encoding=None, errors=None, text=None, pipesize=-1)
Execute a child program in a new process. On POSIX, the class uses
:meth:`os.execvpe`-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
or :term:`path-like object`.
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.
.. warning::
For maximum reliability, use a fully-qualified path for the executable.
To search for an unqualified name on :envvar:`PATH`, use
:meth:`shutil.which`. On all platforms, passing :data:`sys.executable`
is the recommended way to launch the current Python interpreter again,
and use the ``-m`` command-line format to launch an installed module.
Resolving the path of *executable* (or the first item of *args*) is
platform dependent. For POSIX, see :meth:`os.execvpe`, and note that
when resolving or searching for the executable path, *cwd* overrides the
current working directory and *env* can override the ``PATH``
environment variable. For Windows, see the documentation of the
``lpApplicationName`` and ``lpCommandLine`` parameters of WinAPI
``CreateProcess``, and note that when resolving or searching for the
executable path with ``shell=False``, *cwd* does not override the
current working directory and *env* cannot override the ``PATH``
environment variable. Using a full path avoids all of these variations.
An example of passing some arguments to an external program
as a sequence is::
Popen(["/usr/bin/git", "commit", "-m", "Fixes a bug."])
On POSIX, 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::
It may not be obvious how to break a shell command into a sequence of arguments,
especially in complex cases. :meth:`shlex.split` can illustrate how to
determine the correct tokenization for *args*::
>>> 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.
.. versionchanged:: 3.6
*args* parameter accepts a :term:`path-like object` if *shell* is
``False`` and a sequence containing path-like objects on POSIX.
.. versionchanged:: 3.8
*args* parameter accepts a :term:`path-like object` if *shell* is
``False`` and a sequence containing bytes and path-like objects
on Windows.
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 POSIX 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.
.. note::
Read the `Security Considerations`_ section before using ``shell=True``.
*bufsize* will be supplied as the corresponding argument to the
:func:`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
(only usable if ``universal_newlines=True`` i.e., in a text mode)
- any other positive value means use a buffer of approximately that
size
- 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 POSIX, the *args* name
becomes the display name for the executable in utilities such as
:program:`ps`. If ``shell=True``, on POSIX the *executable* argument
specifies a replacement shell for the default :file:`/bin/sh`.
.. versionchanged:: 3.6
*executable* parameter accepts a :term:`path-like object` on POSIX.
.. versionchanged:: 3.8
*executable* parameter accepts a bytes and :term:`path-like object`
on Windows.
*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.
(POSIX 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.
.. versionchanged:: 3.8
The *preexec_fn* parameter is no longer supported in subinterpreters.
The use of the parameter in a subinterpreter raises
:exc:`RuntimeError`. The new restriction may affect applications that
are deployed in mod_wsgi, uWSGI, and other embedded environments.
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. Otherwise
when *close_fds* is false, file descriptors obey their inheritable flag
as described in :ref:`fd_inheritance`.
On Windows, if *close_fds* is true then no handles will be inherited by the
child process unless explicitly passed in the ``handle_list`` element of
:attr:`STARTUPINFO.lpAttributeList`, or by standard handle redirection.
.. versionchanged:: 3.2
The default for *close_fds* was changed from :const:`False` to
what is described above.
.. versionchanged:: 3.7
On Windows the default for *close_fds* was changed from :const:`False` to
:const:`True` when redirecting the standard handles. It's now possible to
set *close_fds* to :const:`True` when redirecting the standard handles.
*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`. (POSIX only)
.. versionchanged:: 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. *cwd* can be a string, bytes or
:term:`path-like <path-like object>` object. In POSIX, the function
looks for *executable* (or for the first item in *args*) relative to *cwd*
if the executable path is a relative path.
.. versionchanged:: 3.6
*cwd* parameter accepts a :term:`path-like object` on POSIX.
.. versionchanged:: 3.7
*cwd* parameter accepts a :term:`path-like object` on Windows.
.. versionchanged:: 3.8
*cwd* parameter accepts a bytes object on Windows.
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.
(POSIX 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. (POSIX only)
.. versionchanged:: 3.2
*start_new_session* was added.
If *group* is not ``None``, the setregid() system call will be made in the
child process prior to the execution of the subprocess. If the provided
value is a string, it will be looked up via :func:`grp.getgrnam()` and
the value in ``gr_gid`` will be used. If the value is an integer, it
will be passed verbatim. (POSIX only)
.. availability:: POSIX
.. versionadded:: 3.9
If *extra_groups* is not ``None``, the setgroups() system call will be
made in the child process prior to the execution of the subprocess.
Strings provided in *extra_groups* will be looked up via
:func:`grp.getgrnam()` and the values in ``gr_gid`` will be used.
Integer values will be passed verbatim. (POSIX only)
.. availability:: POSIX
.. versionadded:: 3.9
If *user* is not ``None``, the setreuid() system call will be made in the
child process prior to the execution of the subprocess. If the provided
value is a string, it will be looked up via :func:`pwd.getpwnam()` and
the value in ``pw_uid`` will be used. If the value is an integer, it will
be passed verbatim. (POSIX only)
.. availability:: POSIX
.. versionadded:: 3.9
If *umask* is not negative, the umask() system call will be made in the
child process prior to the execution of the subprocess.
.. availability:: POSIX
.. versionadded:: 3.9
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: https://en.wikipedia.org/wiki/Side-by-Side_Assembly
If *encoding* or *errors* are specified, or *text* is true, the file objects
*stdin*, *stdout* and *stderr* are opened in text mode with the specified
encoding and *errors*, as described above in :ref:`frequently-used-arguments`.
The *universal_newlines* argument is equivalent to *text* and is provided
for backwards compatibility. By default, file objects are opened in binary mode.
.. versionadded:: 3.6
*encoding* and *errors* were added.
.. versionadded:: 3.7
*text* was added as a more readable alias for *universal_newlines*.
If given, *startupinfo* will be a :class:`STARTUPINFO` object, which is
passed to the underlying ``CreateProcess`` function.
*creationflags*, if given, can be one or more of the following flags:
* :data:`CREATE_NEW_CONSOLE`
* :data:`CREATE_NEW_PROCESS_GROUP`
* :data:`ABOVE_NORMAL_PRIORITY_CLASS`
* :data:`BELOW_NORMAL_PRIORITY_CLASS`
* :data:`HIGH_PRIORITY_CLASS`
* :data:`IDLE_PRIORITY_CLASS`
* :data:`NORMAL_PRIORITY_CLASS`
* :data:`REALTIME_PRIORITY_CLASS`
* :data:`CREATE_NO_WINDOW`
* :data:`DETACHED_PROCESS`
* :data:`CREATE_DEFAULT_ERROR_MODE`
* :data:`CREATE_BREAKAWAY_FROM_JOB`
*pipesize* can be used to change the size of the pipe when
:data:`PIPE` is used for *stdin*, *stdout* or *stderr*. The size of the pipe
is only changed on platforms that support this (only Linux at this time of
writing). Other platforms will ignore this parameter.
.. versionadded:: 3.10
The ``pipesize`` parameter was added.
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())
.. audit-event:: subprocess.Popen executable,args,cwd,env subprocess.Popen
Popen and the other functions in this module that use it raise an
:ref:`auditing event <auditing>` ``subprocess.Popen`` with arguments
``executable``, ``args``, ``cwd``, and ``env``. The value for ``args``
may be a single string or a list of strings, depending on platform.
.. versionchanged:: 3.2
Added context manager support.
.. versionchanged:: 3.6
Popen destructor now emits a :exc:`ResourceWarning` warning if the child
process is still running.
.. versionchanged:: 3.8
Popen can use :func:`os.posix_spawn` in some cases for better
performance. On Windows Subsystem for Linux and QEMU User Emulation,
Popen constructor using :func:`os.posix_spawn` no longer raise an
exception on errors like missing program, but the child process fails
with a non-zero :attr:`~Popen.returncode`.
Exceptions
^^^^^^^^^^
Exceptions raised in the child process, before the new program has started to
execute, will be re-raised in the parent.
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 Considerations
-----------------------
Unlike some other popen functions, this implementation will never
implicitly call a system shell. This means that all characters,
including shell metacharacters, can safely be passed to child processes.
If the shell is invoked explicitly, via ``shell=True``, it is the application's
responsibility to ensure that all whitespace and metacharacters are
quoted appropriately to avoid
`shell injection <https://en.wikipedia.org/wiki/Shell_injection#Shell_injection>`_
vulnerabilities. On :ref:`some platforms <shlex-quote-warning>`, it is possible
to use :func:`shlex.quote` for this escaping.
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. Otherwise, returns ``None``.
.. 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.
.. note::
This will deadlock when using ``stdout=PIPE`` 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:`Popen.communicate` when using pipes to avoid that.
.. note::
The function is implemented using a busy loop (non-blocking call and
short sleeps). Use the :mod:`asyncio` module for an asynchronous wait:
see :class:`asyncio.create_subprocess_exec`.
.. 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 and set the
:attr:`~Popen.returncode` attribute. 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. If streams were opened in text mode, *input* must be a string.
Otherwise, it must be bytes.
:meth:`communicate` returns a tuple ``(stdout_data, stderr_data)``.
The data will be strings if streams were opened in text mode; otherwise,
bytes.
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.
Do nothing if the process completed.
.. 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:
.. attribute:: Popen.args
The *args* argument as it was passed to :class:`Popen` -- a
sequence of program arguments or else a single string.
.. versionadded:: 3.3
.. attribute:: Popen.stdin
If the *stdin* argument was :data:`PIPE`, this attribute is a writeable
stream object as returned by :func:`open`. If the *encoding* or *errors*
arguments were specified or the *universal_newlines* argument was ``True``,
the stream is a text stream, otherwise it is a byte stream. If the *stdin*
argument was not :data:`PIPE`, this attribute is ``None``.
.. attribute:: Popen.stdout
If the *stdout* argument was :data:`PIPE`, this attribute is a readable
stream object as returned by :func:`open`. Reading from the stream provides
output from the child process. If the *encoding* or *errors* arguments were
specified or the *universal_newlines* argument was ``True``, the stream is a
text stream, otherwise it is a byte stream. If the *stdout* argument was not
:data:`PIPE`, this attribute is ``None``.
.. attribute:: Popen.stderr
If the *stderr* argument was :data:`PIPE`, this attribute is a readable
stream object as returned by :func:`open`. Reading from the stream provides
error output from the child process. If the *encoding* or *errors* arguments
were specified or the *universal_newlines* argument was ``True``, the stream
is a text stream, otherwise it is a byte stream. If the *stderr* argument was
not :data:`PIPE`, this attribute is ``None``.
.. 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.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`` (POSIX only).
Windows Popen Helpers
---------------------
The :class:`STARTUPINFO` class and following constants are only available
on Windows.
.. class:: STARTUPINFO(*, dwFlags=0, hStdInput=None, hStdOutput=None, \
hStdError=None, wShowWindow=0, lpAttributeList=None)
Partial support of the Windows
`STARTUPINFO <https://msdn.microsoft.com/en-us/library/ms686331(v=vs.85).aspx>`__
structure is used for :class:`Popen` creation. The following attributes can
be set by passing them as keyword-only arguments.
.. versionchanged:: 3.7
Keyword-only argument support was added.
.. 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 <https://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``.
.. attribute:: lpAttributeList
A dictionary of additional attributes for process creation as given in
``STARTUPINFOEX``, see
`UpdateProcThreadAttribute <https://msdn.microsoft.com/en-us/library/windows/desktop/ms686880(v=vs.85).aspx>`__.
Supported attributes:
**handle_list**
Sequence of handles that will be inherited. *close_fds* must be true if
non-empty.
The handles must be temporarily made inheritable by
:func:`os.set_handle_inheritable` when passed to the :class:`Popen`
constructor, else :class:`OSError` will be raised with Windows error
``ERROR_INVALID_PARAMETER`` (87).
.. warning::
In a multithreaded process, use caution to avoid leaking handles
that are marked inheritable when combining this feature with
concurrent calls to other process creation functions that inherit
all handles such as :func:`os.system`. This also applies to
standard handle redirection, which temporarily creates inheritable
handles.
.. versionadded:: 3.7
Windows 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).
.. 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.
.. data:: ABOVE_NORMAL_PRIORITY_CLASS
A :class:`Popen` ``creationflags`` parameter to specify that a new process
will have an above average priority.
.. versionadded:: 3.7
.. data:: BELOW_NORMAL_PRIORITY_CLASS
A :class:`Popen` ``creationflags`` parameter to specify that a new process
will have a below average priority.
.. versionadded:: 3.7
.. data:: HIGH_PRIORITY_CLASS
A :class:`Popen` ``creationflags`` parameter to specify that a new process
will have a high priority.
.. versionadded:: 3.7
.. data:: IDLE_PRIORITY_CLASS
A :class:`Popen` ``creationflags`` parameter to specify that a new process
will have an idle (lowest) priority.
.. versionadded:: 3.7
.. data:: NORMAL_PRIORITY_CLASS
A :class:`Popen` ``creationflags`` parameter to specify that a new process
will have an normal priority. (default)
.. versionadded:: 3.7
.. data:: REALTIME_PRIORITY_CLASS
A :class:`Popen` ``creationflags`` parameter to specify that a new process
will have realtime priority.
You should almost never use REALTIME_PRIORITY_CLASS, because this interrupts
system threads that manage mouse input, keyboard input, and background disk
flushing. This class can be appropriate for applications that "talk" directly
to hardware or that perform brief tasks that should have limited interruptions.
.. versionadded:: 3.7
.. data:: CREATE_NO_WINDOW
A :class:`Popen` ``creationflags`` parameter to specify that a new process
will not create a window.
.. versionadded:: 3.7
.. data:: DETACHED_PROCESS
A :class:`Popen` ``creationflags`` parameter to specify that a new process
will not inherit its parent's console.
This value cannot be used with CREATE_NEW_CONSOLE.
.. versionadded:: 3.7
.. data:: CREATE_DEFAULT_ERROR_MODE
A :class:`Popen` ``creationflags`` parameter to specify that a new process
does not inherit the error mode of the calling process. Instead, the new
process gets the default error mode.
This feature is particularly useful for multithreaded shell applications
that run with hard errors disabled.
.. versionadded:: 3.7
.. data:: CREATE_BREAKAWAY_FROM_JOB
A :class:`Popen` ``creationflags`` parameter to specify that a new process
is not associated with the job.
.. versionadded:: 3.7
.. _call-function-trio:
Older high-level API
--------------------
Prior to Python 3.5, these three functions comprised the high level API to
subprocess. You can now use :func:`run` in many cases, but lots of existing code
calls these functions.
.. function:: call(args, *, stdin=None, stdout=None, stderr=None, \
shell=False, cwd=None, timeout=None, **other_popen_kwargs)
Run the command described by *args*. Wait for command to complete, then
return the :attr:`~Popen.returncode` attribute.
Code needing to capture stdout or stderr should use :func:`run` instead::
run(...).returncode
To suppress stdout or stderr, supply a value of :data:`DEVNULL`.
The arguments shown above are merely some common ones.
The full function signature is the
same as that of the :class:`Popen` constructor - this function passes all
supplied arguments other than *timeout* directly through to that interface.
.. note::
Do not use ``stdout=PIPE`` or ``stderr=PIPE`` with this
function. The child process will block if it generates enough
output to a pipe to fill up the OS pipe buffer as the pipes are
not being read from.
.. versionchanged:: 3.3
*timeout* was added.
.. function:: check_call(args, *, stdin=None, stdout=None, stderr=None, \
shell=False, cwd=None, timeout=None, \
**other_popen_kwargs)
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.
Code needing to capture stdout or stderr should use :func:`run` instead::
run(..., check=True)
To suppress stdout or stderr, supply a value of :data:`DEVNULL`.
The arguments shown above are merely some common ones.
The full function signature is the
same as that of the :class:`Popen` constructor - this function passes all
supplied arguments other than *timeout* directly through to that interface.
.. note::
Do not use ``stdout=PIPE`` or ``stderr=PIPE`` with this
function. The child process will block if it generates enough
output to a pipe to fill up the OS pipe buffer as the pipes are
not being read from.
.. versionchanged:: 3.3
*timeout* was added.
.. function:: check_output(args, *, stdin=None, stderr=None, shell=False, \
cwd=None, encoding=None, errors=None, \
universal_newlines=None, timeout=None, text=None, \
**other_popen_kwargs)
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.
This is equivalent to::
run(..., check=True, stdout=PIPE).stdout
The arguments shown above are merely some common ones.
The full function signature is largely the same as that of :func:`run` -
most arguments are passed directly through to that interface.
One API deviation from :func:`run` behavior exists: passing ``input=None``
will behave the same as ``input=b''`` (or ``input=''``, depending on other
arguments) rather than using the parent's standard input file handle.
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 *text*, *encoding*, *errors*,
or *universal_newlines* to ``True`` as described in
:ref:`frequently-used-arguments` and :func:`run`.
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
.. versionchanged:: 3.3
*timeout* was added.
.. versionchanged:: 3.4
Support for the *input* keyword argument was added.
.. versionchanged:: 3.6
*encoding* and *errors* were added. See :func:`run` for details.
.. versionadded:: 3.7
*text* was added as a more readable alias for *universal_newlines*.
.. _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 :program:`/bin/sh` shell command substitution
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
.. code-block:: bash
output=$(mycmd myarg)
becomes::
output = check_output(["mycmd", "myarg"])
Replacing shell pipeline
^^^^^^^^^^^^^^^^^^^^^^^^
.. code-block:: bash
output=$(dmesg | grep hda)
becomes::
p1 = Popen(["dmesg"], stdout=PIPE)
p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
p1.stdout.close() # Allow p1 to receive a SIGPIPE if p2 exits.
output = p2.communicate()[0]
The ``p1.stdout.close()`` call after starting the p2 is important in order for
p1 to receive a SIGPIPE if p2 exits before p1.
Alternatively, for trusted input, the shell's own pipeline support may still
be used directly:
.. code-block:: bash
output=$(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, 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 ``(exitcode, output)`` of executing *cmd* in a shell.
Execute the string *cmd* in a shell with :meth:`Popen.check_output` and
return a 2-tuple ``(exitcode, output)``. The locale encoding is used;
see the notes on :ref:`frequently-used-arguments` for more details.
A trailing newline is stripped from the output.
The exit code for the command can be interpreted as the return code
of subprocess. Example::
>>> subprocess.getstatusoutput('ls /bin/ls')
(0, '/bin/ls')
>>> subprocess.getstatusoutput('cat /bin/junk')
(1, 'cat: /bin/junk: No such file or directory')
>>> subprocess.getstatusoutput('/bin/junk')
(127, 'sh: /bin/junk: not found')
>>> subprocess.getstatusoutput('/bin/kill $$')
(-15, '')
.. availability:: POSIX & Windows.
.. versionchanged:: 3.3.4
Windows support was added.
The function now returns (exitcode, output) instead of (status, output)
as it did in Python 3.3.3 and earlier. exitcode has the same value as
:attr:`~Popen.returncode`.
.. function:: getoutput(cmd)
Return output (stdout and stderr) of executing *cmd* in a shell.
Like :func:`getstatusoutput`, except the exit code is ignored and the return
value is a string containing the command's output. Example::
>>> subprocess.getoutput('ls /bin/ls')
'/bin/ls'
.. availability:: POSIX & Windows.
.. versionchanged:: 3.3.4
Windows support added
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.
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