Using the subprocess Module

This module defines one class called Popen:

class subprocess.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 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 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 list2cmdline() method. Please note that not all MS Windows applications interpret the command line the same way: 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: 0 means unbuffered, 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 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 /bin/sh. On Windows, the default shell is specified by the 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 0, 1 and 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 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 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 stdout, stdin and 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)

Popen Objects

Instances of the Popen class have the following methods:

Popen.poll()

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

Popen.wait()

Wait for child process to terminate. Set and return 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 communicate() to avoid that.

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.

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.

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 terminate().

New in version 2.6.

Popen.terminate()

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

New in version 2.6.

Popen.kill()

Kills the child. On Posix OSs the function sends SIGKILL to the child. On Windows kill() is an alias for terminate().

New in version 2.6.

The following attributes are also available:

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.

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.

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.

Popen.pid

The process ID of the child process.

Popen.returncode

The child return code, set by poll() and wait() (and indirectly by 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)

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_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)