mirror of https://github.com/python/cpython
406 lines
13 KiB
ReStructuredText
406 lines
13 KiB
ReStructuredText
|
|
:mod:`subprocess` --- Subprocess management
|
|
===========================================
|
|
|
|
.. module:: subprocess
|
|
:synopsis: Subprocess management.
|
|
.. moduleauthor:: Peter Åstrand <astrand@lysator.liu.se>
|
|
.. sectionauthor:: Peter Åstrand <astrand@lysator.liu.se>
|
|
|
|
|
|
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*
|
|
|
|
Information about how the :mod:`subprocess` module can be used to replace these
|
|
modules and functions can be found in the following sections.
|
|
|
|
.. seealso::
|
|
|
|
:pep:`324` -- PEP proposing the subprocess module
|
|
|
|
|
|
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:
|
|
|
|
*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.
|
|
|
|
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
|
|
``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 :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 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 :meth:`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)
|
|
|
|
|
|
Convenience Functions
|
|
^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
This module also defines four 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
|
|
: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"])
|
|
|
|
|
|
.. function:: getstatusoutput(cmd)
|
|
Return ``(status, output)`` of executing *cmd* in a shell.
|
|
|
|
Execute the string *cmd* in a shell with :func:`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 :cfunc:`wait`. Example::
|
|
|
|
>>> import subprocess
|
|
>>> 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')
|
|
|
|
|
|
.. function:: getoutput(cmd)
|
|
Return output ``(stdout or stderr)`` of executing *cmd* in a shell.
|
|
|
|
Like :func:`getstatusoutput`, except the exit status is ignored and the return
|
|
value is a string containing the command's output. Example::
|
|
|
|
>>> import subprocess
|
|
>>> subprocess.getoutput('ls /bin/ls')
|
|
'/bin/ls'
|
|
|
|
|
|
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.
|
|
|
|
|
|
.. method:: Popen.wait()
|
|
|
|
Wait for child process to terminate. Set and return :attr:`returncode`
|
|
attribute.
|
|
|
|
|
|
.. 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 byte string to be sent to the child process, or
|
|
``None``, if no data should be sent to the child.
|
|
|
|
:meth:`communicate` returns a tuple ``(stdout, stderr)``.
|
|
|
|
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`.
|
|
|
|
|
|
.. 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.
|
|
|
|
|
|
.. 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`.
|
|
|
|
|
|
The following attributes are also available:
|
|
|
|
.. attribute:: Popen.stdin
|
|
|
|
If the *stdin* argument is ``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 is ``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 is ``PIPE``, this attribute is 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).
|
|
|
|
|
|
.. _subprocess-replacements:
|
|
|
|
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]
|
|
|
|
|
|
Replacing shell pipe line
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
::
|
|
|
|
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("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 os.spawn\*
|
|
^^^^^^^^^^^^^^^^^^^^
|
|
|
|
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\*
|
|
^^^^^^^^^^^^^^^^^^^^
|
|
|
|
::
|
|
|
|
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
|
|
|