Using the subprocess Module

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

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 :program:`ps`.

On Unix, with shell=False (default): In this case, the Popen class uses :meth:`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.

>>> 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!

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], ...])

On Windows: the :class:`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 using the :meth:`list2cmdline` method. Please note that not all MS Windows applications interpret the command line the same way: :meth:`list2cmdline` is designed for applications using the same rules as the MS C runtime.

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).

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 :file:`/bin/sh`. On Windows, the default shell is specified by the :envvar:`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 :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 None, no redirection will occur; the child's file handles will be inherited from the parent. Additionally, stderr can be :data:`STDOUT`, which indicates that the stderr data from the applications should be captured into the same file handle as for stdout.

If preexec_fn is set to a callable object, this object will be called in the child process just before the child is executed. (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 shell is :const:`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.

If universal_newlines is :const:`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.

The startupinfo and creationflags, if given, will be passed to the underlying CreateProcess() function. They can specify things such as appearance of the main window and priority for the new process. (Windows only)

.. 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.

.. 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.

.. function:: call(*popenargs, **kwargs)

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

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

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

.. function:: check_call(*popenargs, **kwargs)

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

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

      check_call(["ls", "-l"])

   .. versionadded:: 2.5

Popen Objects

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

.. method:: Popen.poll()

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

.. method:: Popen.wait()

   Wait for child process to terminate.  Set and return :attr:`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 :meth:`communicate` to avoid that.

.. method:: 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 string to be sent to the child process, or
   ``None``, if no data should be sent to the child.

   :meth:`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.

.. method:: 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
      :meth:`terminate`.

   .. versionadded:: 2.6

.. method:: Popen.terminate()

   Stop the child. On Posix OSs the method sends SIGTERM to the
   child. On Windows the Win32 API function :cfunc:`TerminateProcess` is called
   to stop the child.

   .. versionadded:: 2.6

.. 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`.

   .. versionadded:: 2.6

The following attributes are also available:

.. attribute:: Popen.stdin

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

.. attribute:: Popen.stdout

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

.. attribute:: Popen.stderr

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

.. attribute:: Popen.pid

   The process ID of the child process.

.. attribute:: Popen.returncode

   The child return code, set by :meth:`poll` and :meth:`wait` (and indirectly
   by :meth:`communicate`).  A ``None`` value indicates that the process
   hasn't terminated yet.

   A negative value ``-N`` indicates that the child was terminated by signal
   ``N`` (Unix only).

Replacing Older Functions with the subprocess Module

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

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

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

output=`dmesg | grep hda`
==>
p1 = Popen(["dmesg"], stdout=PIPE)
p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
output = p2.communicate()[0]

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

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, e:
    print >>sys.stderr, "Execution failed:", e

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

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

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

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

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)

(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)

pipe = os.popen("cmd", 'w')
...
rc = pipe.close()
if rc != None and rc % 256:
    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"

(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)