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# 17.1. `subprocess` — Subprocess management

The `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 `subprocess` module can be used to replace these modules and functions can be found in the following sections.

See also

PEP 324 – PEP proposing the subprocess module

## 17.1.1. 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=True, shell=False, cwd=None, env=None, universal_newlines=False, startupinfo=None, creationflags=0, restore_signals=True, start_new_session=False, pass_fds=())

  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 ps.

  On Unix, with shell=False (default): In this case, the Popen class uses `os.execvp()` like behavior 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

  `shlex.split()` can be useful when determining the correct tokenization for args, especially in complex cases:

      >>> import shlex, subprocess
      >>> command_line = 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], ...])

  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 does not suffer from this vulnerability; the above Note may be helpful in getting code using shell=False to work.

  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).

  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 `/bin/sh`. On Windows, the default shell is specified by the `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 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)

  Warning

  The preexec_fn parameter is not safe to use in the presence of threads in your application. The child process could deadlock before exec is called. If you must use it, keep it trivial! Minimize the number of libraries you call into.

  Note

  If you need to modify the environment for the child use the env parameter rather than doing it in a preexec_fn. The start_new_session parameter can take the place of a previously common use of preexec_fn to call os.setsid() in the child.

  If close_fds is true, all file descriptors except `0`, `1` and `2` will be closed before the child process is executed. (Unix only). The default varies by platform: Always true on Unix. On Windows it is true when stdin/stdout/stderr are `None`, false otherwise. 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.

  Changed in version 3.2: The default for close_fds was changed from `False` to what is described above.

  pass_fds is an optional sequence of file descriptors to keep open between the parent and child. Providing any pass_fds forces close_fds to be `True`. (Unix only)

  New in version 3.2: The pass_fds parameter was added.

  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 restore_signals is True (the default) all signals that Python has set to SIG_IGN are restored to SIG_DFL in the child process before the exec. Currently this includes the SIGPIPE, SIGXFZ and SIGXFSZ signals. (Unix only)

  Changed in version 3.2: restore_signals was added.

  If start_new_session is True the setsid() system call will be made in the child process prior to the execution of the subprocess. (Unix only)

  Changed in version 3.2: start_new_session was added.

  If env is not `None`, it must be a mapping that defines the environment variables for the new process; these are used instead of the default behavior of inheriting the current process’ environment.

  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 `SystemRoot`.

  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 are supported as context managers via the `with` statement, closing any open file descriptors on exit.

      with Popen(["ifconfig"], stdout=PIPE) as proc:
          log.write(proc.stdout.read())

  Changed in version 3.2: Added context manager support.

`subprocess.``PIPE`

  Special value that can be used as the stdin, stdout or stderr argument to `Popen` and indicates that a pipe to the standard stream should be opened.

`subprocess.``STDOUT`

  Special value that can be used as the stderr argument to `Popen` and indicates that standard error should go into the same handle as standard output.

### 17.1.1.1. Convenience Functions

This module also defines four shortcut functions:

`subprocess.``call`(*popenargs, **kwargs)

  Run command with arguments. Wait for command to complete, then return the `returncode` attribute.

  The arguments are the same as for the `Popen` constructor. Example:

      >>> retcode = subprocess.call(["ls", "-l"])

  Warning

  Like `Popen.wait()`, this will deadlock when using `stdout=PIPE` and/or `stderr=PIPE` and the child process generates enough output to a pipe such that it blocks waiting for the OS pipe buffer to accept more data.

`subprocess.``check_call`(*popenargs, **kwargs)

  Run command with arguments. Wait for command to complete. If the exit code was zero then return, otherwise raise `CalledProcessError`. The `CalledProcessError` object will have the return code in the `returncode` attribute.

  The arguments are the same as for the `Popen` constructor. Example:

      >>> subprocess.check_call(["ls", "-l"])
      0

  Warning

  See the warning for `call()`.

`subprocess.``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"])
      b'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_existent_file; exit 0"],
      ...     stderr=subprocess.STDOUT)
      b'ls: non_existent_file: No such file or directory\n'

  New in version 3.1.

`subprocess.``getstatusoutput`(cmd)

  Return `(status, output)` of executing cmd in a shell.

  Execute the string cmd in a shell with `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 `wait()`. Example:

      >>> 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')

  Availability: UNIX.

`subprocess.``getoutput`(cmd)

  Return output (stdout and stderr) of executing cmd in a shell.

  Like `getstatusoutput()`, except the exit status is ignored and the return value is a string containing the command’s output. Example:

      >>> subprocess.getoutput('ls /bin/ls')
      '/bin/ls'

  Availability: UNIX.

### 17.1.1.2. 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 `child_traceback`, which is a string containing traceback information from the child’s point of view.

The most common exception raised is `OSError`. This occurs, for example, when trying to execute a non-existent file. Applications should prepare for `OSError` exceptions.

A `ValueError` will be raised if `Popen` is called with invalid arguments.

check_call() will raise `CalledProcessError`, if the called process returns a non-zero return code.

### 17.1.1.3. 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.

## 17.1.2. 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 when using `stdout=PIPE` and/or `stderr=PIPE` and the child process generates enough output to a 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 byte string to be sent to the child process, or `None`, if no data should be sent to the child.

  `communicate()` returns a tuple `(stdoutdata, stderrdata)`.

  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, SIGTERM is an alias for `terminate()`. CTRL_C_EVENT and CTRL_BREAK_EVENT can be sent to processes started with a creationflags parameter which includes CREATE_NEW_PROCESS_GROUP.

`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.

`Popen.``kill`()

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

The following attributes are also available:

Warning

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

`Popen.``stdin`

  If the stdin argument was `PIPE`, this attribute is a file object that provides input to the child process. Otherwise, it is `None`.

`Popen.``stdout`

  If the stdout argument was `PIPE`, this attribute is a file object that provides output from the child process. Otherwise, it is `None`.

`Popen.``stderr`

  If the stderr argument was `PIPE`, this attribute is a file object that provides error output from the child process. Otherwise, it is `None`.

`Popen.``pid`

  The process ID of the child process.

  Note that if you set the shell argument to `True`, this is the process ID of the spawned shell.

`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).

## 17.1.3. 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 `OSError` exception.

In the following examples, we assume that the subprocess module is imported with “from subprocess import *”.

### 17.1.3.1. Replacing /bin/sh shell backquote

    output=`mycmd myarg`
    ==>
    output = Popen(["mycmd", "myarg"], stdout=PIPE).communicate()[0]

### 17.1.3.2. Replacing shell pipeline

    output=`dmesg | grep hda`
    ==>
    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.

### 17.1.3.3. Replacing `os.system()`

    sts = os.system("mycmd" + " myarg")
    ==>
    p = Popen("mycmd" + " myarg", shell=True)
    sts = os.waitpid(p.pid, 0)[1]

Notes:

- Calling the program through the shell is usually not required.
- It’s easier to look at the `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)

### 17.1.3.4. Replacing the `os.spawn` 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"})

### 17.1.3.5. Replacing `os.popen()`, `os.popen2()`, `os.popen3()`

    (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)

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', stdin=PIPE)
    ...
    process.stdin.close()
    if process.wait() != 0:
        print("There were some errors")

### 17.1.3.6. Replacing functions from the `popen2` module

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)

`popen2.Popen3` and `popen2.Popen4` basically work as `subprocess.Popen`, except that:

- `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 `Popen` to guarantee this behavior on all platforms or past Python versions.

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