2 :mod:`subprocess` --- Subprocess management
3 ===========================================
6 :synopsis: Subprocess management.
7 .. moduleauthor:: Peter Åstrand <astrand@lysator.liu.se>
8 .. sectionauthor:: Peter Åstrand <astrand@lysator.liu.se>
13 The :mod:`subprocess` module allows you to spawn new processes, connect to their
14 input/output/error pipes, and obtain their return codes. This module intends to
15 replace several other, older modules and functions, such as::
23 Information about how the :mod:`subprocess` module can be used to replace these
24 modules and functions can be found in the following sections.
27 Using the subprocess Module
28 ---------------------------
30 This module defines one class called :class:`Popen`:
33 .. 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)
37 *args* should be a string, or a sequence of program arguments. The program to
38 execute is normally the first item in the args sequence or string, but can be
39 explicitly set by using the executable argument.
41 On Unix, with *shell=False* (default): In this case, the Popen class uses
42 :meth:`os.execvp` to execute the child program. *args* should normally be a
43 sequence. A string will be treated as a sequence with the string as the only
44 item (the program to execute).
46 On Unix, with *shell=True*: If args is a string, it specifies the command string
47 to execute through the shell. If *args* is a sequence, the first item specifies
48 the command string, and any additional items will be treated as additional shell
51 On Windows: the :class:`Popen` class uses CreateProcess() to execute the child
52 program, which operates on strings. If *args* is a sequence, it will be
53 converted to a string using the :meth:`list2cmdline` method. Please note that
54 not all MS Windows applications interpret the command line the same way:
55 :meth:`list2cmdline` is designed for applications using the same rules as the MS
58 *bufsize*, if given, has the same meaning as the corresponding argument to the
59 built-in open() function: :const:`0` means unbuffered, :const:`1` means line
60 buffered, any other positive value means use a buffer of (approximately) that
61 size. A negative *bufsize* means to use the system default, which usually means
62 fully buffered. The default value for *bufsize* is :const:`0` (unbuffered).
64 The *executable* argument specifies the program to execute. It is very seldom
65 needed: Usually, the program to execute is defined by the *args* argument. If
66 ``shell=True``, the *executable* argument specifies which shell to use. On Unix,
67 the default shell is :file:`/bin/sh`. On Windows, the default shell is
68 specified by the :envvar:`COMSPEC` environment variable.
70 *stdin*, *stdout* and *stderr* specify the executed programs' standard input,
71 standard output and standard error file handles, respectively. Valid values are
72 ``PIPE``, an existing file descriptor (a positive integer), an existing file
73 object, and ``None``. ``PIPE`` indicates that a new pipe to the child should be
74 created. With ``None``, no redirection will occur; the child's file handles
75 will be inherited from the parent. Additionally, *stderr* can be ``STDOUT``,
76 which indicates that the stderr data from the applications should be captured
77 into the same file handle as for stdout.
79 If *preexec_fn* is set to a callable object, this object will be called in the
80 child process just before the child is executed. (Unix only)
82 If *close_fds* is true, all file descriptors except :const:`0`, :const:`1` and
83 :const:`2` will be closed before the child process is executed. (Unix only).
84 Or, on Windows, if *close_fds* is true then no handles will be inherited by the
85 child process. Note that on Windows, you cannot set *close_fds* to true and
86 also redirect the standard handles by setting *stdin*, *stdout* or *stderr*.
88 If *shell* is :const:`True`, the specified command will be executed through the
91 If *cwd* is not ``None``, the child's current directory will be changed to *cwd*
92 before it is executed. Note that this directory is not considered when
93 searching the executable, so you can't specify the program's path relative to
96 If *env* is not ``None``, it defines the environment variables for the new
99 If *universal_newlines* is :const:`True`, the file objects stdout and stderr are
100 opened as text files, but lines may be terminated by any of ``'\n'``, the Unix
101 end-of-line convention, ``'\r'``, the Macintosh convention or ``'\r\n'``, the
102 Windows convention. All of these external representations are seen as ``'\n'``
103 by the Python program.
107 This feature is only available if Python is built with universal newline support
108 (the default). Also, the newlines attribute of the file objects :attr:`stdout`,
109 :attr:`stdin` and :attr:`stderr` are not updated by the communicate() method.
111 The *startupinfo* and *creationflags*, if given, will be passed to the
112 underlying CreateProcess() function. They can specify things such as appearance
113 of the main window and priority for the new process. (Windows only)
116 Convenience Functions
117 ^^^^^^^^^^^^^^^^^^^^^
119 This module also defines two shortcut functions:
122 .. function:: call(*popenargs, **kwargs)
124 Run command with arguments. Wait for command to complete, then return the
125 :attr:`returncode` attribute.
127 The arguments are the same as for the Popen constructor. Example::
129 retcode = call(["ls", "-l"])
132 .. function:: check_call(*popenargs, **kwargs)
134 Run command with arguments. Wait for command to complete. If the exit code was
135 zero then return, otherwise raise :exc:`CalledProcessError.` The
136 :exc:`CalledProcessError` object will have the return code in the
137 :attr:`returncode` attribute.
139 The arguments are the same as for the Popen constructor. Example::
141 check_call(["ls", "-l"])
143 .. versionadded:: 2.5
149 Exceptions raised in the child process, before the new program has started to
150 execute, will be re-raised in the parent. Additionally, the exception object
151 will have one extra attribute called :attr:`child_traceback`, which is a string
152 containing traceback information from the childs point of view.
154 The most common exception raised is :exc:`OSError`. This occurs, for example,
155 when trying to execute a non-existent file. Applications should prepare for
156 :exc:`OSError` exceptions.
158 A :exc:`ValueError` will be raised if :class:`Popen` is called with invalid
161 check_call() will raise :exc:`CalledProcessError`, if the called process returns
162 a non-zero return code.
168 Unlike some other popen functions, this implementation will never call /bin/sh
169 implicitly. This means that all characters, including shell metacharacters, can
170 safely be passed to child processes.
176 Instances of the :class:`Popen` class have the following methods:
179 .. method:: Popen.poll()
181 Check if child process has terminated. Set and return :attr:`returncode`
185 .. method:: Popen.wait()
187 Wait for child process to terminate. Set and return :attr:`returncode`
191 .. method:: Popen.communicate(input=None)
193 Interact with process: Send data to stdin. Read data from stdout and stderr,
194 until end-of-file is reached. Wait for process to terminate. The optional
195 *input* argument should be a string to be sent to the child process, or
196 ``None``, if no data should be sent to the child.
198 :meth:`communicate` returns a tuple ``(stdout, stderr)``.
200 Note that if you want to send data to the process's stdin, you need to create
201 the Popen object with ``stdin=PIPE``. Similarly, to get anything other than
202 ``None`` in the result tuple, you need to give ``stdout=PIPE`` and/or
207 The data read is buffered in memory, so do not use this method if the data
208 size is large or unlimited.
211 The following attributes are also available:
213 .. attribute:: Popen.stdin
215 If the *stdin* argument is ``PIPE``, this attribute is a file object that
216 provides input to the child process. Otherwise, it is ``None``.
219 .. attribute:: Popen.stdout
221 If the *stdout* argument is ``PIPE``, this attribute is a file object that
222 provides output from the child process. Otherwise, it is ``None``.
225 .. attribute:: Popen.stderr
227 If the *stderr* argument is ``PIPE``, this attribute is file object that
228 provides error output from the child process. Otherwise, it is ``None``.
231 .. attribute:: Popen.pid
233 The process ID of the child process.
236 .. attribute:: Popen.returncode
238 The child return code, set by :meth:`poll` and :meth:`wait` (and indirectly
239 by :meth:`communicate`). A ``None`` value indicates that the process
240 hasn't terminated yet.
242 A negative value ``-N`` indicates that the child was terminated by signal
246 Replacing Older Functions with the subprocess Module
247 ----------------------------------------------------
249 In this section, "a ==> b" means that b can be used as a replacement for a.
253 All functions in this section fail (more or less) silently if the executed
254 program cannot be found; this module raises an :exc:`OSError` exception.
256 In the following examples, we assume that the subprocess module is imported with
257 "from subprocess import \*".
260 Replacing /bin/sh shell backquote
261 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
267 output = Popen(["mycmd", "myarg"], stdout=PIPE).communicate()[0]
270 Replacing shell pipe line
271 ^^^^^^^^^^^^^^^^^^^^^^^^^
275 output=`dmesg | grep hda`
277 p1 = Popen(["dmesg"], stdout=PIPE)
278 p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
279 output = p2.communicate()[0]
282 Replacing os.system()
283 ^^^^^^^^^^^^^^^^^^^^^
287 sts = os.system("mycmd" + " myarg")
289 p = Popen("mycmd" + " myarg", shell=True)
290 sts = os.waitpid(p.pid, 0)
294 * Calling the program through the shell is usually not required.
296 * It's easier to look at the :attr:`returncode` attribute than the exit status.
298 A more realistic example would look like this::
301 retcode = call("mycmd" + " myarg", shell=True)
303 print >>sys.stderr, "Child was terminated by signal", -retcode
305 print >>sys.stderr, "Child returned", retcode
307 print >>sys.stderr, "Execution failed:", e
315 pid = os.spawnlp(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg")
317 pid = Popen(["/bin/mycmd", "myarg"]).pid
321 retcode = os.spawnlp(os.P_WAIT, "/bin/mycmd", "mycmd", "myarg")
323 retcode = call(["/bin/mycmd", "myarg"])
327 os.spawnvp(os.P_NOWAIT, path, args)
329 Popen([path] + args[1:])
331 Environment example::
333 os.spawnlpe(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg", env)
335 Popen(["/bin/mycmd", "myarg"], env={"PATH": "/usr/bin"})
343 pipe = os.popen(cmd, mode='r', bufsize)
345 pipe = Popen(cmd, shell=True, bufsize=bufsize, stdout=PIPE).stdout
349 pipe = os.popen(cmd, mode='w', bufsize)
351 pipe = Popen(cmd, shell=True, bufsize=bufsize, stdin=PIPE).stdin
355 (child_stdin, child_stdout) = os.popen2(cmd, mode, bufsize)
357 p = Popen(cmd, shell=True, bufsize=bufsize,
358 stdin=PIPE, stdout=PIPE, close_fds=True)
359 (child_stdin, child_stdout) = (p.stdin, p.stdout)
365 child_stderr) = os.popen3(cmd, mode, bufsize)
367 p = Popen(cmd, shell=True, bufsize=bufsize,
368 stdin=PIPE, stdout=PIPE, stderr=PIPE, close_fds=True)
371 child_stderr) = (p.stdin, p.stdout, p.stderr)
375 (child_stdin, child_stdout_and_stderr) = os.popen4(cmd, mode, bufsize)
377 p = Popen(cmd, shell=True, bufsize=bufsize,
378 stdin=PIPE, stdout=PIPE, stderr=STDOUT, close_fds=True)
379 (child_stdin, child_stdout_and_stderr) = (p.stdin, p.stdout)
387 If the cmd argument to popen2 functions is a string, the command is executed
388 through /bin/sh. If it is a list, the command is directly executed.
392 (child_stdout, child_stdin) = popen2.popen2("somestring", bufsize, mode)
394 p = Popen(["somestring"], shell=True, bufsize=bufsize,
395 stdin=PIPE, stdout=PIPE, close_fds=True)
396 (child_stdout, child_stdin) = (p.stdout, p.stdin)
400 (child_stdout, child_stdin) = popen2.popen2(["mycmd", "myarg"], bufsize, mode)
402 p = Popen(["mycmd", "myarg"], bufsize=bufsize,
403 stdin=PIPE, stdout=PIPE, close_fds=True)
404 (child_stdout, child_stdin) = (p.stdout, p.stdin)
406 The popen2.Popen3 and popen2.Popen4 basically works as subprocess.Popen, except
409 * subprocess.Popen raises an exception if the execution fails
411 * the *capturestderr* argument is replaced with the *stderr* argument.
413 * stdin=PIPE and stdout=PIPE must be specified.
415 * popen2 closes all file descriptors by default, but you have to specify
416 close_fds=True with subprocess.Popen.