: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.
.. 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* and *stderr* both set to :data:`~subprocess.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,
set *stdout* to :data:`~subprocess.PIPE`
and *stderr* to :data:`~subprocess.STDOUT`,
instead of using *capture_output*.
A *timeout* may be specified in seconds, it is internally passed on 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 initial process
creation itself cannot be interrupted on many platform APIs so you are not
guaranteed to see a timeout exception until at least after however long
process creation takes.
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* set to :data:`~subprocess.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`. This mapping can be str to str on any platform
or bytes to bytes on POSIX platforms much like :data:`os.environ` or
:data:`os.environb`.
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.
.. versionchanged:: 3.12
Changed Windows shell search order for ``shell=True``. The current
directory and ``%PATH%`` are replaced with ``%COMSPEC%`` and
``%SystemRoot%\System32\cmd.exe``. As a result, dropping a
malicious program named ``cmd.exe`` into a current directory no
longer works.
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``. This is always
:class:`bytes` when any output was captured regardless of the
``text=True`` setting. It may remain ``None`` instead of ``b''``
when no output was observed.
.. 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``. This is always :class:`bytes` when stderr output
was captured regardless of the ``text=True`` setting. It may remain
``None`` instead of ``b''`` when no stderr output was observed.
.. 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`, :func:`check_output`, or :func:`run` (with ``check=True``)
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
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
None, :data:`PIPE`, :data:`DEVNULL`, an existing file descriptor (a positive integer), and an existing :term:`file object` with a valid file descriptor. With the default settings ofNone, no redirection will occur. :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. 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 moduleIf 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 isNone.If text mode is not used, stdin, stdout and stderr will be opened as binary streams. No encoding or line ending conversion is performed.
.. versionchanged:: 3.6 Added the *encoding* and *errors* parameters... versionchanged:: 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.
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.
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. Note that, when shell=True, :exc:`OSError`
will be raised by the child only if the selected shell itself was not found.
To determine if the shell failed to find the requested application, it is
necessary to check the return code or output from the subprocess.
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:`run` 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.
Unlike some other popen functions, this library will not
implicitly choose to 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
vulnerabilities. On :ref:`some platforms <shlex-quote-warning>`, it is possible
to use :func:`shlex.quote` for this escaping.
On Windows, batch files (:file:`*.bat` or :file:`*.cmd`) may be launched by the
operating system in a system shell regardless of the arguments passed to this
library. This could result in arguments being parsed according to shell rules,
but without any escaping added by Python. If you are intentionally launching a
batch file with arguments from untrusted sources, consider passing
shell=True to allow Python to escape special characters. See :gh:`114539`
for additional discussion.
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::
When the ``timeout`` parameter is not ``None``, then (on POSIX) 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 :py:const:`~signal.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 set by the class for you to access. Reassigning them to new values is unsupported:
.. 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 *text* or *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 *text* or *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 *text* or *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. Initially ``None``, :attr:`returncode` is set by a call to the :meth:`poll`, :meth:`wait`, or :meth:`communicate` methods if they detect that the process has terminated. A ``None`` value indicates that the process hadn't yet terminated at the time of the last method call. A negative value ``-N`` indicates that the child was terminated by signal ``N`` (POSIX only).
The :class:`STARTUPINFO` class and following constants are only available on Windows.
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:: STARTF_FORCEONFEEDBACK A :attr:`STARTUPINFO.dwFlags` parameter to specify that the *Working in Background* mouse cursor will be displayed while a process is launching. This is the default behavior for GUI processes. .. versionadded:: 3.13
.. data:: STARTF_FORCEOFFFEEDBACK A :attr:`STARTUPINFO.dwFlags` parameter to specify that the mouse cursor will not be changed when launching a process. .. versionadded:: 3.13
.. 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 a 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
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.
.. versionchanged:: 3.12
Changed Windows shell search order for ``shell=True``. The current
directory and ``%PATH%`` are replaced with ``%COMSPEC%`` and
``%SystemRoot%\System32\cmd.exe``. As a result, dropping a
malicious program named ``cmd.exe`` into a current directory no
longer works.
.. 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.
If :func:`check_call` was unable to start the process it will propagate the exception
that was raised.
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.
.. versionchanged:: 3.12
Changed Windows shell search order for ``shell=True``. The current
directory and ``%PATH%`` are replaced with ``%COMSPEC%`` and
``%SystemRoot%\System32\cmd.exe``. As a result, dropping a
malicious program named ``cmd.exe`` into a current directory no
longer works.
.. 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*.
.. versionchanged:: 3.12
Changed Windows shell search order for ``shell=True``. The current
directory and ``%PATH%`` are replaced with ``%COMSPEC%`` and
``%SystemRoot%\System32\cmd.exe``. As a result, dropping a
malicious program named ``cmd.exe`` into a current directory no
longer works.
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
output=$(mycmd myarg)becomes:
output = check_output(["mycmd", "myarg"])
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
retcode = call("mycmd" + " myarg", shell=True)
Notes:
- Calling the program through the shell is usually not required.
- The :func:`call` return value is encoded differently to that of :func:`os.system`.
- The :func:`os.system` function ignores SIGINT and SIGQUIT signals while the command is running, but the caller must do this separately when using the :mod:`subprocess` module.
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`
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")
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, *, encoding=None, errors=None)
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)``.
*encoding* and *errors* are used to decode output;
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:: Unix, 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`.
.. versionchanged:: 3.11
Added the *encoding* and *errors* parameters.
.. function:: getoutput(cmd, *, encoding=None, errors=None)
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:: Unix, Windows.
.. versionchanged:: 3.3.4
Windows support added
.. versionchanged:: 3.11
Added the *encoding* and *errors* parameters.
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):
- Arguments are delimited by white space, which is either a space or a tab.
- 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.
- A double quotation mark preceded by a backslash is interpreted as a literal double quotation mark.
- Backslashes are interpreted literally, unless they immediately precede a double quotation mark.
- 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.
On Linux, :mod:`subprocess` defaults to using the vfork() system call
internally when it is safe to do so rather than fork(). This greatly
improves performance.
subprocess._USE_POSIX_SPAWN = False # See CPython issue gh-NNNNNN.
It is safe to set this to false on any Python version. It will have no effect on older or newer versions where unsupported. Do not assume the attribute is available to read. Despite the name, a true value does not indicate the corresponding function will be used, only that it may be.
Please file issues any time you have to use these private knobs with a way to reproduce the issue you were seeing. Link to that issue from a comment in your code.
.. versionadded:: 3.8 ``_USE_POSIX_SPAWN``