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Python 3.6.5 Documentation > "subprocess" — Subprocess management

"subprocess" — Subprocess management
************************************

**Source code:** Lib/subprocess.py

======================================================================

The "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 "subprocess" module can be used to replace
these modules and functions can be found in the following sections.

See also: **PEP 324** – PEP proposing the subprocess module

Using the "subprocess" Module
=============================

The recommended approach to invoking subprocesses is to use the
"run()" function for all use cases it can handle. For more advanced
use cases, the underlying "Popen" interface can be used directly.

The "run()" function was added in Python 3.5; if you need to retain
compatibility with older versions, see the Older high-level API
section.

subprocess.run(args, *, stdin=None, input=None, stdout=None, stderr=None, shell=False, cwd=None, timeout=None, check=False, encoding=None, errors=None)

Run the command described by *args*. Wait for command to complete,
then return a "CompletedProcess" instance.

The arguments shown above are merely the most common ones,
described below in 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 "Popen"
constructor - apart from *timeout*, *input* and *check*, all the
arguments to this function are passed through to that interface.

This does not capture stdout or stderr by default. To do so, pass
"PIPE" for the *stdout* and/or *stderr* arguments.

The *timeout* argument is passed to "Popen.communicate()". If the
timeout expires, the child process will be killed and waited for.
The "TimeoutExpired" exception will be re-raised after the child
process has terminated.

The *input* argument is passed to "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
*universal_newlines* is true. When used, the internal "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 "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 *universal_newlines* is
true, file objects for stdin, stdout and stderr are opened in text
mode using the specified *encoding* and *errors* or the
"io.TextIOWrapper" default. Otherwise, file objects are opened in
binary mode.

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"], stdout=subprocess.PIPE)
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')

New in version 3.5.

Changed in version 3.6: Added *encoding* and *errors* parameters

class subprocess.CompletedProcess

The return value from "run()", representing a process that has
finished.

args

The arguments used to launch the process. This may be a list or
a string.

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).

stdout

Captured stdout from the child process. A bytes sequence, or a
string if "run()" was called with an encoding or errors. "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 "stderr" will
be "None".

stderr

Captured stderr from the child process. A bytes sequence, or a
string if "run()" was called with an encoding or errors. "None"
if stderr was not captured.

check_returncode()

If "returncode" is non-zero, raise a "CalledProcessError".

New in version 3.5.

subprocess.DEVNULL

Special value that can be used as the *stdin*, *stdout* or *stderr*
argument to "Popen" and indicates that the special file
"os.devnull" will be used.

New in version 3.3.

subprocess.PIPE

Special value that can be used as the *stdin*, *stdout* or *stderr*
argument to "Popen" and indicates that a pipe to the standard
stream should be opened. Most useful with "Popen.communicate()".

subprocess.STDOUT

Special value that can be used as the *stderr* argument to "Popen"
and indicates that standard error should go into the same handle as
standard output.

exception subprocess.SubprocessError

Base class for all other exceptions from this module.

New in version 3.3.

exception subprocess.TimeoutExpired

Subclass of "SubprocessError", raised when a timeout expires while
waiting for a child process.

cmd

Command that was used to spawn the child process.

timeout

Timeout in seconds.

output

Output of the child process if it was captured by "run()" or
"check_output()". Otherwise, "None".

stdout

Alias for output, for symmetry with "stderr".

stderr

Stderr output of the child process if it was captured by
"run()". Otherwise, "None".

New in version 3.3.

Changed in version 3.5: *stdout* and *stderr* attributes added

exception subprocess.CalledProcessError

Subclass of "SubprocessError", raised when a process run by
"check_call()" or "check_output()" returns a non-zero exit status.

returncode

Exit status of the child process. If the process exited due to
a signal, this will be the negative signal number.

cmd

Command that was used to spawn the child process.

output

Output of the child process if it was captured by "run()" or
"check_output()". Otherwise, "None".

stdout

Alias for output, for symmetry with "stderr".

stderr

Stderr output of the child process if it was captured by
"run()". Otherwise, "None".

Changed in version 3.5: *stdout* and *stderr* attributes added

Frequently Used Arguments
-------------------------

To support a wide variety of use cases, the "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
"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 "PIPE", "DEVNULL", an existing file
descriptor (a positive integer), an existing file object, and
"None". "PIPE" indicates that a new pipe to the child should be
created. "DEVNULL" indicates that the special file "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 "STDOUT", which indicates
that the stderr data from the child process should be captured into
the same file handle as for *stdout*.

If *encoding* or *errors* are specified, or *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 "io.TextIOWrapper".

For *stdin*, line ending characters "'\n'" in the input will be
converted to the default line separator "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
"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.

New in version 3.6: Added *encoding* and *errors* parameters.

Note: The newlines attribute of the file objects "Popen.stdin",
"Popen.stdout" and "Popen.stderr" are not updated by the
"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, "glob", "fnmatch", "os.walk()",
"os.path.expandvars()", "os.path.expanduser()", and "shutil").

Changed in version 3.3: When *universal_newlines* is "True", the
class uses the encoding "locale.getpreferredencoding(False)"
instead of "locale.getpreferredencoding()". See the
"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 "Popen" constructor documentation.

Popen Constructor
-----------------

The underlying process creation and management in this module is
handled by the "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 subprocess.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=(), *, encoding=None, errors=None)

Execute a child program in a new process. On POSIX, the class uses
"os.execvp()"-like behavior to execute the child program. On
Windows, the class uses the Windows "CreateProcess()" function.
The arguments to "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 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: "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 Converting an argument sequence to
a string on Windows. 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 POSIX with "shell=True", the shell defaults to "/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, "Popen"
does the equivalent of:

Popen(['/bin/sh', '-c', args[0], args[1], ...])

On Windows with "shell=True", the "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. **dir** or **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
"open()" function when creating the stdin/stdout/stderr pipe file
objects:

* "0" means unbuffered (read and write are one system call and
can return short)

* "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.

Changed in version 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 "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 **ps**. If "shell=True", on POSIX the
*executable* argument specifies a replacement shell for the default
"/bin/sh".

*stdin*, *stdout* and *stderr* specify the executed program’s
standard input, standard output and standard error file handles,
respectively. Valid values are "PIPE", "DEVNULL", an existing file
descriptor (a positive integer), an existing *file object*, and
"None". "PIPE" indicates that a new pipe to the child should be
created. "DEVNULL" indicates that the special file "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 "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.

If *close_fds* is true, all file descriptors except "0", "1" and
"2" will be closed before the child process is executed. (POSIX
only). The default varies by platform: Always true on POSIX. On
Windows it is true when *stdin*/*stdout*/*stderr* are "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*.

Changed in version 3.2: The default for *close_fds* was changed
from "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 "True". (POSIX only)

New in version 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 "str" and
*path-like* object. 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.

Changed in version 3.6: *cwd* parameter accepts a *path-like
object*.

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)

Changed in version 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)

Changed in version 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
"SystemRoot".

If *encoding* or *errors* are specified, the file objects *stdin*,
*stdout* and *stderr* are opened in text mode with the specified
encoding and *errors*, as described above in Frequently Used
Arguments. If *universal_newlines* is "True", they are opened in
text mode with default encoding. Otherwise, they are opened as
binary streams.

New in version 3.6: *encoding* and *errors* were added.

If given, *startupinfo* will be a "STARTUPINFO" object, which is
passed to the underlying "CreateProcess" function. *creationflags*,
if given, can be "CREATE_NEW_CONSOLE" or
"CREATE_NEW_PROCESS_GROUP". (Windows only)

Popen objects are supported as context managers via the "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())

Changed in version 3.2: Added context manager support.

Changed in version 3.6: Popen destructor now emits a
"ResourceWarning" warning if the child process is still running.

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
"child_traceback", which is a string containing traceback information
from the child’s point of view.

The most common exception raised is "OSError". This occurs, for
example, when trying to execute a non-existent file. Applications
should prepare for "OSError" exceptions.

A "ValueError" will be raised if "Popen" is called with invalid
arguments.

"check_call()" and "check_output()" will raise "CalledProcessError" if
the called process returns a non-zero return code.

All of the functions and methods that accept a *timeout* parameter,
such as "call()" and "Popen.communicate()" will raise "TimeoutExpired"
if the timeout expires before the process exits.

Exceptions defined in this module all inherit from "SubprocessError".

New in version 3.3: The "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
vulnerabilities.

When using "shell=True", the "shlex.quote()" function can be used to
properly escape whitespace and shell metacharacters in strings that
are going to be used to construct shell commands.

Popen Objects
=============

Instances of the "Popen" class have the following methods:

Popen.poll()

Check if child process has terminated. Set and return "returncode"
attribute. Otherwise, returns "None".

Popen.wait(timeout=None)

Wait for child process to terminate. Set and return "returncode"
attribute.

If the process does not terminate after *timeout* seconds, raise a
"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 "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 "asyncio" module for an
asynchronous wait: see "asyncio.create_subprocess_exec".

Changed in version 3.3: *timeout* was added.

Deprecated since version 3.4: Do not use the *endtime* parameter.
It is was unintentionally exposed in 3.3 but was left undocumented
as it was intended to be private for internal use. Use *timeout*
instead.

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. If streams were opened in text mode, *input* must be a
string. Otherwise, it must be bytes.

"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
"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.

Changed in version 3.3: *timeout* was added.

Popen.send_signal(signal)

Sends the signal *signal* to the child.

Note: On Windows, SIGTERM is an alias for "terminate()".
CTRL_C_EVENT and CTRL_BREAK_EVENT can be sent to processes
started with a *creationflags* parameter which includes
*CREATE_NEW_PROCESS_GROUP*.

Popen.terminate()

Stop the child. On Posix OSs the method sends SIGTERM to the child.
On Windows the Win32 API function "TerminateProcess()" is called to
stop the child.

Popen.kill()

Kills the child. On Posix OSs the function sends SIGKILL to the
child. On Windows "kill()" is an alias for "terminate()".

The following attributes are also available:

Popen.args

The *args* argument as it was passed to "Popen" – a sequence of
program arguments or else a single string.

New in version 3.3.

Popen.stdin

If the *stdin* argument was "PIPE", this attribute is a writeable
stream object as returned by "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 "PIPE", this attribute
is "None".

Popen.stdout

If the *stdout* argument was "PIPE", this attribute is a readable
stream object as returned by "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 "PIPE", this
attribute is "None".

Popen.stderr

If the *stderr* argument was "PIPE", this attribute is a readable
stream object as returned by "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 "PIPE", this
attribute is "None".

Warning: Use "communicate()" rather than ".stdin.write",
".stdout.read" or ".stderr.read" to avoid deadlocks due to any of
the other OS pipe buffers filling up and blocking the child process.

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.

Popen.returncode

The child return code, set by "poll()" and "wait()" (and indirectly
by "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 "STARTUPINFO" class and following constants are only available on
Windows.

class subprocess.STARTUPINFO

Partial support of the Windows STARTUPINFO structure is used for
"Popen" creation.

dwFlags

A bit field that determines whether certain "STARTUPINFO"
attributes are used when the process creates a window.

si = subprocess.STARTUPINFO()
si.dwFlags = subprocess.STARTF_USESTDHANDLES | subprocess.STARTF_USESHOWWINDOW

hStdInput

If "dwFlags" specifies "STARTF_USESTDHANDLES", this attribute is
the standard input handle for the process. If
"STARTF_USESTDHANDLES" is not specified, the default for
standard input is the keyboard buffer.

hStdOutput

If "dwFlags" specifies "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.

hStdError

If "dwFlags" specifies "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.

wShowWindow

If "dwFlags" specifies "STARTF_USESHOWWINDOW", this attribute
can be any of the values that can be specified in the "nCmdShow"
parameter for the ShowWindow function, except for
"SW_SHOWDEFAULT". Otherwise, this attribute is ignored.

"SW_HIDE" is provided for this attribute. It is used when
"Popen" is called with "shell=True".

Constants
---------

The "subprocess" module exposes the following constants.

subprocess.STD_INPUT_HANDLE

The standard input device. Initially, this is the console input
buffer, "CONIN$".

subprocess.STD_OUTPUT_HANDLE

The standard output device. Initially, this is the active console
screen buffer, "CONOUT$".

subprocess.STD_ERROR_HANDLE

The standard error device. Initially, this is the active console
screen buffer, "CONOUT$".

subprocess.SW_HIDE

Hides the window. Another window will be activated.

subprocess.STARTF_USESTDHANDLES

Specifies that the "STARTUPINFO.hStdInput",
"STARTUPINFO.hStdOutput", and "STARTUPINFO.hStdError" attributes
contain additional information.

subprocess.STARTF_USESHOWWINDOW

Specifies that the "STARTUPINFO.wShowWindow" attribute contains
additional information.

subprocess.CREATE_NEW_CONSOLE

The new process has a new console, instead of inheriting its
parent’s console (the default).

subprocess.CREATE_NEW_PROCESS_GROUP

A "Popen" "creationflags" parameter to specify that a new process
group will be created. This flag is necessary for using "os.kill()"
on the subprocess.

This flag is ignored if "CREATE_NEW_CONSOLE" is specified.

Older high-level API
====================

Prior to Python 3.5, these three functions comprised the high level
API to subprocess. You can now use "run()" in many cases, but lots of
existing code calls these functions.

subprocess.call(args, *, stdin=None, stdout=None, stderr=None, shell=False, cwd=None, timeout=None)

Run the command described by *args*. Wait for command to complete,
then return the "returncode" attribute.

This is equivalent to:

run(...).returncode

(except that the *input* and *check* parameters are not supported)

The arguments shown above are merely the most common ones. The full
function signature is largely the same as that of the "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.

Changed in version 3.3: *timeout* was added.

subprocess.check_call(args, *, stdin=None, stdout=None, stderr=None, shell=False, cwd=None, timeout=None)

Run command with arguments. Wait for command to complete. If the
return code was zero then return, otherwise raise
"CalledProcessError". The "CalledProcessError" object will have the
return code in the "returncode" attribute.

This is equivalent to:

run(..., check=True)

(except that the *input* parameter is not supported)

Changed in version 3.3: *timeout* was added.

subprocess.check_output(args, *, stdin=None, stderr=None, shell=False, cwd=None, encoding=None, errors=None, universal_newlines=False, timeout=None)

Run command with arguments and return its output.

If the return code was non-zero it raises a "CalledProcessError".
The "CalledProcessError" object will have the return code in the
"returncode" attribute and any output in the "output" attribute.

This is equivalent to:

run(..., check=True, stdout=PIPE).stdout

The arguments shown above are merely the most common ones. The full
function signature is largely the same as that of "run()" - most
arguments are passed directly through to that interface. However,
explicitly passing "input=None" to inherit the parent’s standard
input file handle is not supported.

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 above in 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'

New in version 3.1.

Changed in version 3.3: *timeout* was added.

Changed in version 3.4: Support for the *input* keyword argument
was added.

Changed in version 3.6: *encoding* and *errors* were added. See
"run()" for details.

Replacing Older Functions with the "subprocess" Module
======================================================

In this section, “a becomes b” means that b can be used as a
replacement for a.

Note: All “a” functions in this section fail (more or less) silently
if the executed program cannot be found; the “b” replacements raise
"OSError" instead.In addition, the replacements using
"check_output()" will fail with a "CalledProcessError" if the
requested operation produces a non-zero return code. The output is
still available as the "output" attribute of the raised exception.

In the following examples, we assume that the relevant functions have
already been imported from the "subprocess" module.

Replacing /bin/sh shell backquote
---------------------------------

output=`mycmd myarg`

becomes:

output = check_output(["mycmd", "myarg"])

Replacing shell pipeline
------------------------

output=`dmesg | grep hda`

becomes:

p1 = Popen(["dmesg"], stdout=PIPE)
p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
p1.stdout.close() # Allow p1 to receive a SIGPIPE if p2 exits.
output = p2.communicate()[0]

The p1.stdout.close() call after starting the p2 is important in order
for p1 to receive a SIGPIPE if p2 exits before p1.

Alternatively, for trusted input, the shell’s own pipeline support may
still be used directly:

output=`dmesg | grep hda`

becomes:

output=check_output("dmesg | grep hda", shell=True)

Replacing "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 "os.spawn" family
-------------------------------

P_NOWAIT example:

pid = os.spawnlp(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg")
==>
pid = Popen(["/bin/mycmd", "myarg"]).pid

P_WAIT example:

retcode = os.spawnlp(os.P_WAIT, "/bin/mycmd", "mycmd", "myarg")
==>
retcode = call(["/bin/mycmd", "myarg"])

Vector example:

os.spawnvp(os.P_NOWAIT, path, args)
==>
Popen([path] + args[1:])

Environment example:

os.spawnlpe(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg", env)
==>
Popen(["/bin/mycmd", "myarg"], env={"PATH": "/usr/bin"})

Replacing "os.popen()", "os.popen2()", "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 "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)

"popen2.Popen3" and "popen2.Popen4" basically work as
"subprocess.Popen", except that:

* "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 "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.

subprocess.getstatusoutput(cmd)

Return "(exitcode, output)" of executing *cmd* in a shell.

Execute the string *cmd* in a shell with "Popen.check_output()" and
return a 2-tuple "(exitcode, output)". The locale encoding is used;
see the notes on 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

Changed in version 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. See "WEXITSTATUS()".

subprocess.getoutput(cmd)

Return output (stdout and stderr) of executing *cmd* in a shell.

Like "getstatusoutput()", except the exit status is ignored and the
return value is a string containing the command’s output. Example:

>>> subprocess.getoutput('ls /bin/ls')
'/bin/ls'

Availability: POSIX & Windows

Changed in version 3.3.4: Windows support added

Notes
=====

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.