cpython/Doc/library/subprocess.rst

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:mod:`subprocess` --- Subprocess management
===========================================
.. module:: subprocess
:synopsis: Subprocess management.
.. moduleauthor:: Peter Åstrand <astrand@lysator.liu.se>
.. sectionauthor:: Peter Åstrand <astrand@lysator.liu.se>
.. versionadded:: 2.4
The :mod:`subprocess` module allows you to spawn new processes, connect to their
input/output/error pipes, and obtain their return codes. This module intends to
replace several other, older modules and functions, such as::
os.system
os.spawn*
os.popen*
popen2.*
commands.*
Information about how the :mod:`subprocess` module can be used to replace these
modules and functions can be found in the following sections.
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.. seealso::
:pep:`324` -- PEP proposing the subprocess module
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Using the subprocess Module
---------------------------
This module defines one class called :class:`Popen`:
.. 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)
Arguments are:
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*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.
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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. A string will be treated as a sequence with the string as the only
item (the program to execute).
On Unix, with *shell=True*: If args is a string, it specifies the command string
to execute through the shell. If *args* is a sequence, the first item specifies
the command string, and any additional items will be treated as additional shell
arguments.
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.
*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.
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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.
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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
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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.
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.
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Convenience Functions
^^^^^^^^^^^^^^^^^^^^^
This module also defines two shortcut functions:
.. 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
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: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
.. function:: 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"])
'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_existant_file ; exit 0"],
stderr=subprocess.STDOUT)
'ls: non_existant_file: No such file or directory\n'
.. versionadded:: 2.7
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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 childs 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.
check_call() 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 /bin/sh
implicitly. This means that all characters, including shell metacharacters, can
safely be passed to child processes.
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.
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.. method:: Popen.wait()
Wait for child process to terminate. Set and return :attr:`returncode`
attribute.
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.. 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.
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.. 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)``.
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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::
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The data read is buffered in memory, so do not use this method if the data
size is large or unlimited.
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.. 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
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:meth:`terminate`.
.. versionadded:: 2.6
.. method:: Popen.terminate()
Stop the child. On Posix OSs the method sends SIGTERM to the
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child. On Windows the Win32 API function :cfunc:`TerminateProcess` is called
to stop the child.
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.. versionadded:: 2.6
.. method:: Popen.kill()
Kills the child. On Posix OSs the function sends SIGKILL to the child.
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On Windows :meth:`kill` is an alias for :meth:`terminate`.
.. versionadded:: 2.6
The following attributes are also available:
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.. warning::
Use :meth:`communicate` rather than :meth:`.stdin.write`,
:meth:`.stdout.read` or :meth:`.stderr.read` to avoid deadlocks due
to any of the other OS pipe buffers filling up and blocking the child
process.
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.. 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``.
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.. 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``.
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.. 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``.
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.. 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.
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A negative value ``-N`` indicates that the child was terminated by signal
``N`` (Unix only).
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.. _subprocess-replacements:
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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 :exc:`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]
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Replacing shell pipeline
^^^^^^^^^^^^^^^^^^^^^^^^
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::
output=`dmesg | grep hda`
==>
p1 = Popen(["dmesg"], stdout=PIPE)
p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
output = p2.communicate()[0]
Replacing os.system()
^^^^^^^^^^^^^^^^^^^^^
::
sts = os.system("mycmd" + " myarg")
==>
p = Popen("mycmd" + " myarg", shell=True)
sts = os.waitpid(p.pid, 0)
Notes:
* Calling the program through the shell is usually not required.
* It's easier to look at the :attr:`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 >>sys.stderr, "Child was terminated by signal", -retcode
else:
print >>sys.stderr, "Child returned", retcode
except OSError, e:
print >>sys.stderr, "Execution failed:", e
Replacing the os.spawn family
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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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
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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::
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pipe = os.popen(cmd, 'r', bufsize)
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==>
pipe = Popen(cmd, shell=True, bufsize=bufsize, stdout=PIPE).stdout
::
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pipe = os.popen(cmd, 'w', bufsize)
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==>
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)
Replacing functions from the popen2 module
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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.. 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)
:class:`popen2.Popen3` and :class:`popen2.Popen4` basically work as
:class:`subprocess.Popen`, except that:
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* :class:`Popen` raises an exception if the execution fails.
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* the *capturestderr* argument is replaced with the *stderr* argument.
* ``stdin=PIPE`` and ``stdout=PIPE`` must be specified.
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* popen2 closes all file descriptors by default, but you have to specify
``close_fds=True`` with :class:`Popen`.
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