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:mod:`subprocess` --- Subprocess management

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.

Using the subprocess Module

The recommended approach to invoking subprocesses is to use the following convenience functions for all use cases they can handle. For more advanced use cases, the underlying :class:`Popen` interface can be used directly.

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.

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

If shell is :const:`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 access to other shell features such as filename wildcards, shell pipes and environment variable expansion.

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.

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.

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.

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 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 child program, which operates on strings. If args is a sequence, it will be converted to a string in a manner described in :ref:`converting-argument-sequence`.

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 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 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 shell is :const:`True`, the specified command will be executed through the shell.

Warning

Enabling this option can be a security hazard if combined with untrusted input. See the warning under :ref:`frequently-used-arguments` for details.

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`.

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.

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.

Popen Objects

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

The following attributes are also available:

Warning

Use :meth:`communicate` rather than :attr:`.stdin.write <stdin>`, :attr:`.stdout.read <stdout>` or :attr:`.stderr.read <stderr>` to avoid deadlocks due to any of the other OS pipe buffers filling up and blocking the child process.

Windows Popen Helpers

The :class:`STARTUPINFO` class and following constants are only available on Windows.

Partial support of the Windows STARTUPINFO structure is used for :class:`Popen` creation.

Constants

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

Replacing Older Functions with the subprocess Module

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

Note

All "a" functions in this section fail (more or less) silently if the executed program cannot be found; the "b" replacements raise :exc:`OSError` instead.

In addition, the replacements using :func:`check_output` will fail with a :exc:`CalledProcessError` if the requested operation produces a non-zero return code. The output is still available as the output attribute of the raised exception.

In the following examples, we assume that the relevant functions have already been imported from the subprocess module.

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, 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`.

Notes

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.