Frequently Used Arguments

To support a wide variety of use cases, the :class:`Popen` constructor (and the convenience functions) accept a large number of optional arguments. For most typical use cases, many of these arguments can be safely left at their default values. The arguments that are most commonly needed are:

args is required for all calls and should be a string, or a sequence of program arguments. Providing a sequence of arguments is generally preferred, as it allows the module to take care of any required escaping and quoting of arguments (e.g. to permit spaces in file names). If passing a single string, either shell must be :const:`True` (see below) or else the string must simply name the program to be executed without specifying any arguments.

?

stdin, stdout and stderr specify the executed program's standard input, standard output and standard error file handles, respectively. Valid values are :data:`PIPE`, an existing file descriptor (a positive integer), an existing file object, and None. :data:`PIPE` indicates that a new pipe to the child should be created. With the default settings of None, no redirection will occur; the child's file handles will be inherited from the parent. Additionally, stderr can be :data:`STDOUT`, which indicates that the stderr data from the child process should be captured into the same file handle as for stdout.

????

.. index::
   single: universal newlines; subprocess module

When stdout or stderr are pipes and universal_newlines is True then all line endings will be converted to '\n' as described for the :term:`universal newlines` 'U' mode argument to :func:`open`.

??

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`).

??????

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 disables all shell based features, but does not suffer from this vulnerability; see the Note in the :class:`Popen` constructor documentation for helpful hints in getting shell=False to work.

?

When using shell=True, :func:`pipes.quote` can be used to properly escape whitespace and shell metacharacters in strings that are going to be used to construct shell commands.

?

These options, along with all of the other options, are described in more detail in the :class:`Popen` constructor documentation.

Popen Constructor

The underlying process creation and management in this module is handled by the :class:`Popen` class. It offers a lot of flexibility so that developers are able to handle the less common cases not covered by the convenience functions.

.. class:: Popen(args, bufsize=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)

   Execute a child program in a new process.  On Unix, the class uses
   :meth:`os.execvp`-like behavior to execute the child program.  On Windows,
   the class uses the Windows ``CreateProcess()`` function.  The arguments to
   :class:`Popen` are as follows.

   *args* should be a sequence of program arguments or else a single string.
   By default, the program to execute is the first item in *args* if *args* is
   a sequence.  If *args* is a string, the interpretation is
   platform-dependent and described below.  See the *shell* and *executable*
   arguments for additional differences from the default behavior.  Unless
   otherwise stated, it is recommended to pass *args* as a sequence.

   On Unix, if *args* is a string, the string is interpreted as the name or
   path of the program to execute.  However, this can only be done if not
   passing arguments to the program.

   .. note::

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

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

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

   On Windows, if *args* is a sequence, it will be converted to a string in a
   manner described in :ref:`converting-argument-sequence`.  This is because
   the underlying ``CreateProcess()`` operates on strings.

   The *shell* argument (which defaults to *False*) specifies whether to use
   the shell as the program to execute.  If *shell* is *True*, it is
   recommended to pass *args* as a string rather than as a sequence.

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

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

   On Windows with ``shell=True``, the :envvar:`COMSPEC` environment variable
   specifies the default shell.  The only time you need to specify
   ``shell=True`` on Windows is when the command you wish to execute is built
   into the shell (e.g. :command:`dir` or :command:`copy`).  You do not need
   ``shell=True`` to run a batch file or console-based executable.

   .. warning::

      Passing ``shell=True`` can be a security hazard if combined with
      untrusted input.  See the warning under :ref:`frequently-used-arguments`
      for details.

   *bufsize*, if given, has the same meaning as the corresponding argument to the
   built-in open() function: :const:`0` means unbuffered, :const:`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 :const:`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 a replacement program to execute.   It
   is very seldom needed.  When ``shell=False``, *executable* replaces the
   program to execute specified by *args*.  However, the original *args* is
   still passed to the program.  Most programs treat the program specified
   by *args* as the command name, which can then be different from the program
   actually executed.  On Unix, the *args* name
   becomes the display name for the executable in utilities such as
   :program:`ps`.  If ``shell=True``, on Unix the *executable* argument
   specifies a replacement shell for the default :file:`/bin/sh`.

   *stdin*, *stdout* and *stderr* specify the executed program's standard input,
   standard output and standard error file handles, respectively.  Valid values
   are :data:`PIPE`, an existing file descriptor (a positive integer), an
   existing file object, and ``None``.  :data:`PIPE` indicates that a new pipe
   to the child should be created.  With the default settings of ``None``, no
   redirection will occur; the child's file handles will be inherited from the
   parent.  Additionally, *stderr* can be :data:`STDOUT`, which indicates that
   the stderr data from the child process should be captured into the same file
   handle as for stdout.

   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 :const:`0`, :const:`1` and
   :const:`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 *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
      :envvar:`SystemRoot`.

   .. _side-by-side assembly: http://en.wikipedia.org/wiki/Side-by-Side_Assembly

   If *universal_newlines* is ``True``, the file objects *stdout* and *stderr*
   are opened as text files in :term:`universal newlines` mode.  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
      :attr:`stdout`, :attr:`stdin` and :attr:`stderr` are not updated by the
      communicate() method.

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

Exceptions

Exceptions raised in the child process, before the new program has started to execute, will be re-raised in the parent. Additionally, the exception object will have one extra attribute called :attr:`child_traceback`, which is a string containing traceback information from the child's point of view.

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

A :exc:`ValueError` will be raised if :class:`Popen` is called with invalid arguments.

:func:`check_call` and :func:`check_output` will raise :exc:`CalledProcessError` if the called process returns a non-zero return code.

Security

Unlike some other popen functions, this implementation will never call a system shell implicitly. This means that all characters, including shell metacharacters, can safely be passed to child processes. Obviously, if the shell is invoked explicitly, then it is the application's responsibility to ensure that all whitespace and metacharacters are quoted appropriately.

Constants

The :mod:`subprocess` module exposes the following constants.

.. data:: STD_INPUT_HANDLE

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

.. data:: STD_OUTPUT_HANDLE

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

.. data:: STD_ERROR_HANDLE

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

.. data:: SW_HIDE

   Hides the window. Another window will be activated.

.. data:: STARTF_USESTDHANDLES

   Specifies that the :attr:`STARTUPINFO.hStdInput`,
   :attr:`STARTUPINFO.hStdOutput`, and :attr:`STARTUPINFO.hStdError` attributes
   contain additional information.

.. data:: STARTF_USESHOWWINDOW

   Specifies that the :attr:`STARTUPINFO.wShowWindow` attribute contains
   additional information.

.. data:: CREATE_NEW_CONSOLE

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

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

.. data:: CREATE_NEW_PROCESS_GROUP

   A :class:`Popen` ``creationflags`` parameter to specify that a new process
   group will be created. This flag is necessary for using :func:`os.kill`
   on the subprocess.

   This flag is ignored if :data:`CREATE_NEW_CONSOLE` is specified.

Replacing /bin/sh shell backquote

output=`mycmd myarg`
# becomes
output = check_output(["mycmd", "myarg"])

Replacing shell pipeline

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

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

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

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

Replacing :func:`os.system`

sts = os.system("mycmd" + " myarg")
# becomes
sts = call("mycmd" + " myarg", shell=True)

Notes:

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

A more realistic example would look like this:

try:
    retcode = call("mycmd" + " myarg", shell=True)
    if retcode < 0:
        print >>sys.stderr, "Child was terminated by signal", -retcode
    else:
        print >>sys.stderr, "Child returned", retcode
except OSError as e:
    print >>sys.stderr, "Execution failed:", e

Replacing the :func:`os.spawn <os.spawnl>` family

P_NOWAIT example:

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

P_WAIT example:

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

Vector example:

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

Environment example:

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

Replacing :func:`os.popen`, :func:`os.popen2`, :func:`os.popen3`

pipe = os.popen("cmd", 'r', bufsize)
==>
pipe = Popen("cmd", shell=True, bufsize=bufsize, stdout=PIPE).stdout

pipe = os.popen("cmd", 'w', bufsize)
==>
pipe = Popen("cmd", shell=True, bufsize=bufsize, stdin=PIPE).stdin

(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)

On Unix, os.popen2, os.popen3 and os.popen4 also accept a sequence as the command to execute, in which case arguments will be passed directly to the program without shell intervention. This usage can be replaced as follows:

(child_stdin, child_stdout) = os.popen2(["/bin/ls", "-l"], mode,
                                        bufsize)
==>
p = Popen(["/bin/ls", "-l"], bufsize=bufsize, stdin=PIPE, stdout=PIPE)
(child_stdin, child_stdout) = (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', shell=True, stdin=PIPE)
...
process.stdin.close()
if process.wait() != 0:
    print "There were some errors"

Replacing functions from the :mod:`popen2` module

(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)

On Unix, popen2 also accepts a sequence as the command to execute, in which case arguments will be passed directly to the program without shell intervention. This usage can be replaced as follows:

(child_stdout, child_stdin) = popen2.popen2(["mycmd", "myarg"], bufsize,
                                            mode)
==>
p = Popen(["mycmd", "myarg"], bufsize=bufsize,
          stdin=PIPE, stdout=PIPE, close_fds=True)
(child_stdout, child_stdin) = (p.stdout, p.stdin)

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

• :class:`Popen` raises an exception if the execution fails.

  ?

• the capturestderr argument is replaced with the stderr argument.

• stdin=PIPE and stdout=PIPE must be specified.

• popen2 closes all file descriptors by default, but you have to specify close_fds=True with :class:`Popen`.

  ?

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.