
``subprocess`` --- Subprocess management
****************************************

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 other, older modules and
functions, such as:

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

This module defines one class called ``Popen``:

class class subprocess.Popen(args, bufsize=0, executable=None, stdin=None, stdout=None, stderr=None, preexec_fn=None, close_fds=False, shell=False, cwd=None, env=None, universal_newlines=False, startupinfo=None, creationflags=0)

   Arguments are:

   *args* should be a string, or a sequence of program arguments.  The
   program to execute is normally the first item in the args sequence
   or the string if a string is given, but can be explicitly set by
   using the *executable* argument.  When *executable* is given, the
   first item in the args sequence is still treated by most programs
   as the command name, which can then be different from the actual
   executable name.  On Unix, it becomes the display name for the
   executing program in utilities such as **ps**.

   On Unix, with *shell=False* (default): In this case, the Popen
   class uses ``os.execvp()`` to execute the child program. *args*
   should normally be a sequence.  If a string is specified for
   *args*, it will be used as the name or path of the program to
   execute; this will only work if the program is being given no
   arguments.

   Note: ``shlex.split()`` can be useful when determining the correct
     tokenization for *args*, especially in complex cases:

        >>> import shlex, subprocess
        >>> command_line = input()
        /bin/vikings -input eggs.txt -output "spam spam.txt" -cmd "echo '$MONEY'"
        >>> args = shlex.split(command_line)
        >>> print(args)
        ['/bin/vikings', '-input', 'eggs.txt', '-output', 'spam spam.txt', '-cmd', "echo '$MONEY'"]
        >>> p = subprocess.Popen(args) # Success!

     Note in particular that options (such as *-input*) and arguments
     (such as *eggs.txt*) that are separated by whitespace in the
     shell go in separate list elements, while arguments that need
     quoting or backslash escaping when used in the shell (such as
     filenames containing spaces or the *echo* command shown above)
     are single list elements.

   On Unix, with *shell=True*: If args is a string, it specifies the
   command string to execute through the shell.  This means that the
   string must be formatted exactly as it would be when typed at the
   shell prompt.  This includes, for example, quoting or backslash
   escaping filenames with spaces in them.  If *args* is a sequence,
   the first item specifies the command string, and any additional
   items will be treated as additional arguments to the shell itself.
   That is to say, *Popen* does the equivalent of:

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

   Warning: Executing shell commands that incorporate unsanitized input from
     an untrusted source makes a program vulnerable to shell
     injection, a serious security flaw which can result in arbitrary
     command execution. For this reason, the use of *shell=True* is
     **strongly discouraged** in cases where the command string is
     constructed from external input:

        >>> from subprocess import call
        >>> filename = input("What file would you like to display?\n")
        What file would you like to display?
        non_existent; rm -rf / #
        >>> call("cat " + filename, shell=True) # Uh-oh. This will end badly...

     *shell=False* does not suffer from this vulnerability; the above
     Note may be helpful in getting code using *shell=False* to work.

   On Windows: the ``Popen`` class uses CreateProcess() to execute the
   child program, which operates on strings.  If *args* is a sequence,
   it will be converted to a string in a manner described in
   *Converting an argument sequence to a string on Windows*.

   *bufsize*, if given, has the same meaning as the corresponding
   argument to the built-in open() function: ``0`` means unbuffered,
   ``1`` means line buffered, any other positive value means use a
   buffer of (approximately) that size.  A negative *bufsize* means to
   use the system default, which usually means fully buffered.  The
   default value for *bufsize* is ``0`` (unbuffered).

   Note: If you experience performance issues, it is recommended that you
     try to enable buffering by setting *bufsize* to either -1 or a
     large enough positive value (such as 4096).

   The *executable* argument specifies the program to execute. It is
   very seldom needed: Usually, the program to execute is defined by
   the *args* argument. If ``shell=True``, the *executable* argument
   specifies which shell to use. On Unix, the default shell is
   ``/bin/sh``.  On Windows, the default shell is specified by the
   **COMSPEC** environment variable. The only reason you would need to
   specify ``shell=True`` on Windows is where the command you wish to
   execute is actually built in to the shell, eg ``dir``, ``copy``.
   You don't need ``shell=True`` to run a batch file, nor to run a
   console-based executable.

   *stdin*, *stdout* and *stderr* specify the executed programs'
   standard input, standard output and standard error file handles,
   respectively.  Valid values are ``PIPE``, 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.  With ``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.
   (Unix only)

   If *close_fds* is true, all file descriptors except ``0``, ``1``
   and ``2`` will be closed before the child process is executed.
   (Unix only). Or, 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*.

   If *shell* is ``True``, the specified command will be executed
   through the shell.

   If *cwd* is not ``None``, the child's current directory will be
   changed to *cwd* before it is executed.  Note that this directory
   is not considered when searching the executable, so you can't
   specify the program's path relative to *cwd*.

   If *env* is not ``None``, it must be a mapping that defines the
   environment variables for the new process; these are used instead
   of inheriting the current process' environment, which is the
   default behavior.

   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 *universal_newlines* is ``True``, the file objects stdout and
   stderr are opened as text files, but lines may be terminated by any
   of ``'\n'``, the Unix end-of-line convention, ``'\r'``, the old
   Macintosh convention or ``'\r\n'``, the Windows convention. All of
   these external representations are seen as ``'\n'`` by the Python
   program.

   Note: This feature is only available if Python is built with universal
     newline support (the default).  Also, the newlines attribute of
     the file objects ``stdout``, ``stdin`` and ``stderr`` are not
     updated by the ``communicate()`` method.

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

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.

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.


Convenience Functions
---------------------

This module also defines the following shortcut functions:

subprocess.call(*popenargs, **kwargs)

   Run command with arguments.  Wait for command to complete, then
   return the ``returncode`` attribute.

   The arguments are the same as for the Popen constructor.  Example:

      >>> retcode = subprocess.call(["ls", "-l"])

   Warning: Like ``Popen.wait()``, this will deadlock if the child process
     generates enough output to a stdout or stderr pipe such that it
     blocks waiting for the OS pipe buffer to accept more data.

subprocess.check_call(*popenargs, **kwargs)

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

   The arguments are the same as for the Popen constructor.  Example:

      >>> subprocess.check_call(["ls", "-l"])
      0

   Warning: See the warning for ``call()``.

subprocess.check_output(*popenargs, **kwargs)

   Run command with arguments and return its output as a byte string.

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

   The arguments are the same as for the ``Popen`` constructor.
   Example:

      >>> subprocess.check_output(["ls", "-l", "/dev/null"])
      b'crw-rw-rw- 1 root root 1, 3 Oct 18  2007 /dev/null\n'

   The stdout argument is not allowed as it is used internally. To
   capture standard error in the result, use
   ``stderr=subprocess.STDOUT``:

      >>> subprocess.check_output(
      ...     ["/bin/sh", "-c", "ls non_existent_file; exit 0"],
      ...     stderr=subprocess.STDOUT)
      b'ls: non_existent_file: No such file or directory\n'

   New in version 3.1.

subprocess.getstatusoutput(cmd)

   Return ``(status, output)`` of executing *cmd* in a shell.

   Execute the string *cmd* in a shell with ``os.popen()`` and return
   a 2-tuple ``(status, output)``.  *cmd* is actually run as ``{ cmd ;
   } 2>&1``, so that the returned output will contain output or error
   messages.  A trailing newline is stripped from the output.  The
   exit status for the command can be interpreted according to the
   rules for the C function ``wait()``.  Example:

      >>> subprocess.getstatusoutput('ls /bin/ls')
      (0, '/bin/ls')
      >>> subprocess.getstatusoutput('cat /bin/junk')
      (256, 'cat: /bin/junk: No such file or directory')
      >>> subprocess.getstatusoutput('/bin/junk')
      (256, 'sh: /bin/junk: not found')

   Availability: UNIX.

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: UNIX.


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() will raise ``CalledProcessError``, if the called process
returns a non-zero return code.


Security
--------

Unlike some other popen functions, this implementation will never call
/bin/sh implicitly.  This means that all characters, including shell
metacharacters, can safely be passed to child processes.


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

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

Popen.poll()

   Check if child process has terminated.  Set and return
   ``returncode`` attribute.

Popen.wait()

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

   Warning: This will deadlock if the child process generates enough output
     to a stdout or stderr pipe such that it blocks waiting for the OS
     pipe buffer to accept more data.  Use ``communicate()`` to avoid
     that.

Popen.communicate(input=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 a byte string to
   be sent to the child process, or ``None``, if no data should be
   sent to the child.

   ``communicate()`` returns a tuple ``(stdoutdata, stderrdata)``.

   Note that if you want to send data to the process's stdin, you need
   to create the Popen object with ``stdin=PIPE``.  Similarly, to get
   anything other than ``None`` in the result tuple, you need to give
   ``stdout=PIPE`` and/or ``stderr=PIPE`` too.

   Note: The data read is buffered in memory, so do not use this method if
     the data size is large or unlimited.

Popen.send_signal(signal)

   Sends the signal *signal* to the child.

   Note: On Windows only SIGTERM is supported so far. It's an alias for
     ``terminate()``.

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:

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.stdin

   If the *stdin* argument was ``PIPE``, this attribute is a *file
   object* that provides input to the child process.  Otherwise, it is
   ``None``.

Popen.stdout

   If the *stdout* argument was ``PIPE``, this attribute is a *file
   object* that provides output from the child process.  Otherwise, it
   is ``None``.

Popen.stderr

   If the *stderr* argument was ``PIPE``, this attribute is a *file
   object* that provides error output from the child process.
   Otherwise, it is ``None``.

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`` (Unix only).


Windows Popen Helpers
=====================

The ``STARTUPINFO`` class and following constants are only available
on Windows.

class class subprocess.STARTUPINFO

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

   dwFlags

      A bit field that determines whether certain ``STARTUPINFO``
      members 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 member
      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 member
      is the standard output handle for the process. Otherwise, this
      member is ignored and the default for standard output is the
      console window's buffer.

   hStdError

      If ``dwFlags`` specifies ``STARTF_USESTDHANDLES``, this member
      is the standard error handle for the process. Otherwise, this
      member is ignored and the default for standard error is the
      console window's buffer.

   wShowWindow

      If ``dwFlags`` specifies ``STARTF_USESHOWWINDOW``, this member
      can be any of the values that can be specified in the
      ``nCmdShow`` parameter for the ShowWindow function, except for
      ``SW_SHOWDEFAULT``. Otherwise, this member 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`` members
   contain additional information.

subprocess.STARTF_USESHOWWINDOW

   Specifies that the ``STARTUPINFO.wShowWindow`` member contains
   additional information.

subprocess.CREATE_NEW_CONSOLE

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

   This flag is always set when ``Popen`` is created with
   ``shell=True``.


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

In this section, "a ==> b" means that b can be used as a replacement
for a.

Note: All functions in this section fail (more or less) silently if the
  executed program cannot be found; this module raises an ``OSError``
  exception.

In the following examples, we assume that the subprocess module is
imported with "from subprocess import *".


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

   output=`mycmd myarg`
   ==>
   output = Popen(["mycmd", "myarg"], stdout=PIPE).communicate()[0]


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

   output=`dmesg | grep hda`
   ==>
   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.


Replacing ``os.system()``
-------------------------

   sts = os.system("mycmd" + " myarg")
   ==>
   p = Popen("mycmd" + " myarg", shell=True)
   sts = os.waitpid(p.pid, 0)[1]

Notes:

* Calling the program through the shell is usually not required.

* It's easier to look at the ``returncode`` attribute than the exit
  status.

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, 'w', 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``.


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.
