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# 17.5. `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

## 17.5.1. 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 `Popen` interface can be used directly.

`subprocess.``call`(args, *, stdin=None, stdout=None, stderr=None, shell=False, timeout=None)

  Run the command described by args. Wait for command to complete, then return the `returncode` attribute.

  The arguments shown above are merely the most common ones, described below in Frequently Used Arguments (hence the use of keyword-only notation in the abbreviated signature). The full function signature is largely the same as that of the `Popen` constructor - this function passes all supplied arguments other than timeout directly through to that interface.

  The timeout argument is passed to `Popen.wait()`. If the timeout expires, the child process will be killed and then waited for again. The `TimeoutExpired` exception will be re-raised after the child process has terminated.

  Examples:

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

      >>> subprocess.call("exit 1", shell=True)
      1

  Warning

  Invoking the system shell with `shell=True` can be a security hazard if combined with untrusted input. See the warning under Frequently Used Arguments for details.

  Note

  Do not use `stdout=PIPE` or `stderr=PIPE` with this function. As the pipes are not being read in the current process, the child process may block if it generates enough output to a pipe to fill up the OS pipe buffer.

  Changed in version 3.3: timeout was added.

`subprocess.``check_call`(args, *, stdin=None, stdout=None, stderr=None, shell=False, timeout=None)

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

  The arguments shown above are merely the most common ones, described below in Frequently Used Arguments (hence the use of keyword-only notation in the abbreviated signature). The full function signature is largely the same as that of the `Popen` constructor - this function passes all supplied arguments other than timeout directly through to that interface.

  The timeout argument is passed to `Popen.wait()`. If the timeout expires, the child process will be killed and then waited for again. The `TimeoutExpired` exception will be re-raised after the child process has terminated.

  Examples:

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

      >>> subprocess.check_call("exit 1", shell=True)
      Traceback (most recent call last):
         ...
      subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1

  Warning

  Invoking the system shell with `shell=True` can be a security hazard if combined with untrusted input. See the warning under Frequently Used Arguments for details.

  Note

  Do not use `stdout=PIPE` or `stderr=PIPE` with this function. As the pipes are not being read in the current process, the child process may block if it generates enough output to a pipe to fill up the OS pipe buffer.

  Changed in version 3.3: timeout was added.

`subprocess.``check_output`(args, *, stdin=None, stderr=None, shell=False, universal_newlines=False, timeout=None)

  Run command with arguments and return its output.

  If the return code was non-zero it raises a `CalledProcessError`. The `CalledProcessError` object will have the return code in the `returncode` attribute and any output in the `output` attribute.

  The arguments shown above are merely the most common ones, described below in Frequently Used Arguments (hence the use of keyword-only notation in the abbreviated signature). The full function signature is largely the same as that of the `Popen` constructor - this functions passes all supplied arguments other than timeout directly through to that interface. In addition, stdout is not permitted as an argument, as it is used internally to collect the output from the subprocess.

  The timeout argument is passed to `Popen.wait()`. If the timeout expires, the child process will be killed and then waited for again. The `TimeoutExpired` exception will be re-raised after the child process has terminated.

  Examples:

      >>> subprocess.check_output(["echo", "Hello World!"])
      b'Hello World!\n'

      >>> subprocess.check_output(["echo", "Hello World!"], universal_newlines=True)
      'Hello World!\n'

      >>> subprocess.check_output("exit 1", shell=True)
      Traceback (most recent call last):
         ...
      subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1

  By default, this function will return the data as encoded bytes. The actual encoding of the output data may depend on the command being invoked, so the decoding to text will often need to be handled at the application level.

  This behaviour may be overridden by setting universal_newlines to `True` as described below in Frequently Used Arguments.

  To also capture standard error in the result, use `stderr=subprocess.STDOUT`:

      >>> subprocess.check_output(
      ...     "ls non_existent_file; exit 0",
      ...     stderr=subprocess.STDOUT,
      ...     shell=True)
      'ls: non_existent_file: No such file or directory\n'

  New in version 3.1.

  Warning

  Invoking the system shell with `shell=True` can be a security hazard if combined with untrusted input. See the warning under Frequently Used Arguments for details.

  Note

  Do not use `stderr=PIPE` with this function. As the pipe is not being read in the current process, the child process may block if it generates enough output to the pipe to fill up the OS pipe buffer.

  Changed in version 3.3: timeout was added.

`subprocess.``DEVNULL`

  Special value that can be used as the stdin, stdout or stderr argument to `Popen` and indicates that the special file `os.devnull` will be used.

  New in version 3.3.

`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.

exception `subprocess.``SubprocessError`

  Base class for all other exceptions from this module.

  New in version 3.3.

exception `subprocess.``TimeoutExpired`

  Subclass of `SubprocessError`, raised when a timeout expires while waiting for a child process.

  `cmd`

    Command that was used to spawn the child process.

  `timeout`

    Timeout in seconds.

  `output`

    Output of the child process if this exception is raised by `check_output()`. Otherwise, `None`.

  New in version 3.3.

exception `subprocess.``CalledProcessError`

  Subclass of `SubprocessError`, raised when a process run by `check_call()` or `check_output()` returns a non-zero exit status.

  `returncode`

    Exit status of the child process.

  `cmd`

    Command that was used to spawn the child process.

  `output`

    Output of the child process if this exception is raised by `check_output()`. Otherwise, `None`.

### 17.5.1.1. Frequently Used Arguments

To support a wide variety of use cases, the `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 `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 `PIPE`, `DEVNULL`, 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. `DEVNULL` indicates that the special file `os.devnull` will be used. 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 `STDOUT`, which indicates that the stderr data from the child process should be captured into the same file handle as for stdout.

> If universal_newlines is `False` the file objects stdin, stdout and stderr will be opened as binary streams, and no line ending conversion is done.

> If universal_newlines is `True`, these file objects will be opened as text streams in universal newlines mode using the encoding returned by `locale.getpreferredencoding(False)`. For stdin, line ending characters `'\n'` in the input will be converted to the default line separator `os.linesep`. For stdout and stderr, all line endings in the output will be converted to `'\n'`. For more information see the documentation of the `io.TextIOWrapper` class when the newline argument to its constructor is `None`.

> Note

> The newlines attribute of the file objects `Popen.stdin`, `Popen.stdout` and `Popen.stderr` are not updated by the `Popen.communicate()` method.

> If shell is `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 convenient access to other shell features such as shell pipes, filename wildcards, environment variable expansion, and expansion of `~` to a user’s home directory. However, note that Python itself offers implementations of many shell-like features (in particular, `glob`, `fnmatch`, `os.walk()`, `os.path.expandvars()`, `os.path.expanduser()`, and `shutil`).

> Changed in version 3.3: When universal_newlines is `True`, the class uses the encoding `locale.getpreferredencoding(False)` instead of `locale.getpreferredencoding()`. See the `io.TextIOWrapper` class for more information on this change.

> 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 `Popen` constructor documentation for helpful hints in getting `shell=False` to work.

> When using `shell=True`, `shlex.quote()` can be used to properly escape whitespace and shell metacharacters in strings that are going to be used to construct shell commands.

These options, along with all of the other options, are described in more detail in the `Popen` constructor documentation.

### 17.5.1.2. Popen Constructor

The underlying process creation and management in this module is handled by the `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.

class `subprocess.``Popen`(args, bufsize=-1, 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=())

  Execute a child program in a new process. On Unix, the class uses `os.execvp()`-like behavior to execute the child program. On Windows, the class uses the Windows `CreateProcess()` function. The arguments to `Popen` are as follows.

  args should be a sequence of program arguments or else a single string. By default, the program to execute is the first item in args if args is a sequence. If args is a string, the interpretation is platform-dependent and described below. See the shell and executable arguments for additional differences from the default behavior. Unless otherwise stated, it is recommended to pass args as a sequence.

  On Unix, if args is a string, the string is interpreted as the name or path of the program to execute. However, this can only be done if not passing arguments to the program.

  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 Windows, if args is a sequence, it will be converted to a string in a manner described in Converting an argument sequence to a string on Windows. This is because the underlying `CreateProcess()` operates on strings.

  The shell argument (which defaults to False) specifies whether to use the shell as the program to execute. If shell is True, it is recommended to pass args as a string rather than as a sequence.

  On Unix with `shell=True`, the shell defaults to `/bin/sh`. If args is a string, the string specifies the command 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 with `shell=True`, the `COMSPEC` environment variable specifies the default shell. The only time you need to specify `shell=True` on Windows is when the command you wish to execute is built into the shell (e.g. dir or copy). You do not need `shell=True` to run a batch file or console-based executable.

  Warning

  Passing `shell=True` can be a security hazard if combined with untrusted input. See the warning under Frequently Used Arguments for details.

  bufsize will be supplied as the corresponding argument to the `io.open()` function when creating the stdin/stdout/stderr pipe file objects: `0` means unbuffered (read and write are one system call and can return short), `1` means line buffered, any other positive value means use a buffer of approximately that size. A negative bufsize (the default) means the system default of io.DEFAULT_BUFFER_SIZE will be used.

  Changed in version 3.2.4,: 3.3.1

  bufsize now defaults to -1 to enable buffering by default to match the behavior that most code expects. In 3.2.0 through 3.2.3 and 3.3.0 it incorrectly defaulted to `0` which was unbuffered and allowed short reads. This was unintentional and did not match the behavior of Python 2 as most code expected.

  The executable argument specifies a replacement program to execute. It is very seldom needed. When `shell=False`, executable replaces the program to execute specified by args. However, the original args is still passed to the program. Most programs treat the program specified by args as the command name, which can then be different from the program actually executed. On Unix, the args name becomes the display name for the executable in utilities such as ps. If `shell=True`, on Unix the executable argument specifies a replacement shell for the default `/bin/sh`.

  stdin, stdout and stderr specify the executed program’s standard input, standard output and standard error file handles, respectively. Valid values are `PIPE`, `DEVNULL`, 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. `DEVNULL` indicates that the special file `os.devnull` will be used. 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 `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 function changes the working directory to cwd before executing the child. In particular, the function looks for executable (or for the first item in args) relative to cwd if the executable path is a relative path.

  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 stdin, stdout and stderr are opened as text streams in universal newlines mode, as described above in Frequently Used Arguments, otherwise they are opened as binary streams.

  If given, startupinfo will be a `STARTUPINFO` object, which is passed to the underlying `CreateProcess` function. creationflags, if given, can be `CREATE_NEW_CONSOLE` or `CREATE_NEW_PROCESS_GROUP`. (Windows only)

  Popen objects are supported as context managers via the `with` statement: on exit, standard file descriptors are closed, and the process is waited for.

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

  Changed in version 3.2: Added context manager support.

### 17.5.1.3. 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()` and `check_output()` will raise `CalledProcessError` if the called process returns a non-zero return code.

All of the functions and methods that accept a timeout parameter, such as `call()` and `Popen.communicate()` will raise `TimeoutExpired` if the timeout expires before the process exits.

Exceptions defined in this module all inherit from `SubprocessError`.

> New in version 3.3: The `SubprocessError` base class was added.

### 17.5.1.4. 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.

## 17.5.2. 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`(timeout=None)

  Wait for child process to terminate. Set and return `returncode` attribute.

  If the process does not terminate after timeout seconds, raise a `TimeoutExpired` exception. It is safe to catch this exception and retry the wait.

  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.

  Changed in version 3.3: timeout was added.

`Popen.``communicate`(input=None, timeout=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 data to be sent to the child process, or `None`, if no data should be sent to the child. The type of input must be bytes or, if universal_newlines was `True`, a string.

  `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.

  If the process does not terminate after timeout seconds, a `TimeoutExpired` exception will be raised. Catching this exception and retrying communication will not lose any output.

  The child process is not killed if the timeout expires, so in order to cleanup properly a well-behaved application should kill the child process and finish communication:

      proc = subprocess.Popen(...)
      try:
          outs, errs = proc.communicate(timeout=15)
      except TimeoutExpired:
          proc.kill()
          outs, errs = proc.communicate()

  Note

  The data read is buffered in memory, so do not use this method if the data size is large or unlimited.

  Changed in version 3.3: timeout was added.

`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).

## 17.5.3. Windows Popen Helpers

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

class `subprocess.``STARTUPINFO`

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

  `dwFlags`

    A bit field that determines whether certain `STARTUPINFO` attributes are used when the process creates a window.

        si = subprocess.STARTUPINFO()
        si.dwFlags = subprocess.STARTF_USESTDHANDLES | subprocess.STARTF_USESHOWWINDOW

  `hStdInput`

    If `dwFlags` specifies `STARTF_USESTDHANDLES`, this attribute is the standard input handle for the process. If `STARTF_USESTDHANDLES` is not specified, the default for standard input is the keyboard buffer.

  `hStdOutput`

    If `dwFlags` specifies `STARTF_USESTDHANDLES`, this attribute is the standard output handle for the process. Otherwise, this attribute is ignored and the default for standard output is the console window’s buffer.

  `hStdError`

    If `dwFlags` specifies `STARTF_USESTDHANDLES`, this attribute is the standard error handle for the process. Otherwise, this attribute is ignored and the default for standard error is the console window’s buffer.

  `wShowWindow`

    If `dwFlags` specifies `STARTF_USESHOWWINDOW`, this attribute can be any of the values that can be specified in the `nCmdShow` parameter for the ShowWindow function, except for `SW_SHOWDEFAULT`. Otherwise, this attribute is ignored.

    `SW_HIDE` is provided for this attribute. It is used when `Popen` is called with `shell=True`.

### 17.5.3.1. Constants

The `subprocess` module exposes the following constants.

`subprocess.``STD_INPUT_HANDLE`

  The standard input device. Initially, this is the console input buffer, `CONIN$`.

`subprocess.``STD_OUTPUT_HANDLE`

  The standard output device. Initially, this is the active console screen buffer, `CONOUT$`.

`subprocess.``STD_ERROR_HANDLE`

  The standard error device. Initially, this is the active console screen buffer, `CONOUT$`.

`subprocess.``SW_HIDE`

  Hides the window. Another window will be activated.

`subprocess.``STARTF_USESTDHANDLES`

  Specifies that the `STARTUPINFO.hStdInput`, `STARTUPINFO.hStdOutput`, and `STARTUPINFO.hStdError` attributes contain additional information.

`subprocess.``STARTF_USESHOWWINDOW`

  Specifies that the `STARTUPINFO.wShowWindow` attribute contains additional information.

`subprocess.``CREATE_NEW_CONSOLE`

  The new process has a new console, instead of inheriting its parent’s console (the default).

  This flag is always set when `Popen` is created with `shell=True`.

`subprocess.``CREATE_NEW_PROCESS_GROUP`

  A `Popen` `creationflags` parameter to specify that a new process group will be created. This flag is necessary for using `os.kill()` on the subprocess.

  This flag is ignored if `CREATE_NEW_CONSOLE` is specified.

## 17.5.4. 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 `OSError` instead.

In addition, the replacements using `check_output()` will fail with a `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.

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

    output=`mycmd myarg`
    # becomes
    output = check_output(["mycmd", "myarg"])

### 17.5.4.2. 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)

### 17.5.4.3. Replacing `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("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)

### 17.5.4.4. 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"})

### 17.5.4.5. 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")

### 17.5.4.6. 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.

## 17.5.5. Legacy Shell Invocation Functions

This module also provides the following legacy functions from the 2.x `commands` module. These operations implicitly invoke the system shell and none of the guarantees described above regarding security and exception handling consistency are valid for these functions.

`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.

## 17.5.6. Notes

### 17.5.6.1. 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.

See also

`shlex`
  Module which provides function to parse and escape command lines.

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