Source code:
———
The
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 older modules and functions:
os.systemos.spawn*Information about how the
module can be used to replace these modules and functions can be found in the following sections.
See also
– PEP proposing the subprocess module
: not Emscripten, not WASI.
This module does not work or is not available on WebAssembly platforms wasm32-emscripten and wasm32-wasi. See
for more information.
Using the
Module
The recommended approach to invoking subprocesses is to use the
function for all use cases it can handle. For more advanced use cases, the underlying
interface can be used directly.
subprocess.run(args, *, stdin=None, input=None, stdout=None, stderr=None, capture_output=False, shell=False, cwd=None, timeout=None, check=False, encoding=None, errors=None, text=None, env=None, universal_newlines=None, **other_popen_kwargs)
Run the command described by args. Wait for command to complete, then return a
instance.
The arguments shown above are merely the most common ones, described below in
(hence the use of keyword-only notation in the abbreviated signature). The full function signature is largely the same as that of the
constructor - most of the arguments to this function are passed through to that interface. (timeout, input, check, and capture_output are not.)
If capture_output is true, stdout and stderr will be captured. When used, the internal
object is automatically created with stdout and stdin both set to
. The stdout and stderr arguments may not be supplied at the same time as capture_output. If you wish to capture and combine both streams into one, set stdout to
and stderr to
, instead of using capture_output.
A timeout may be specified in seconds, it is internally passed on to
. If the timeout expires, the child process will be killed and waited for. The
exception will be re-raised after the child process has terminated. The initial process creation itself cannot be interrupted on many platform APIs so you are not guaranteed to see a timeout exception until at least after however long process creation takes.
The input argument is passed to
and thus to the subprocess’s stdin. If used it must be a byte sequence, or a string if encoding or errors is specified or text is true. When used, the internal
object is automatically created with stdin set to
, and the stdin argument may not be used as well.
If check is true, and the process exits with a non-zero exit code, a
exception will be raised. Attributes of that exception hold the arguments, the exit code, and stdout and stderr if they were captured.
If encoding or errors are specified, or text is true, file objects for stdin, stdout and stderr are opened in text mode using the specified encoding and errors or the
default. The universal_newlines argument is equivalent to text and is provided for backwards compatibility. By default, file objects are opened in binary mode.
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. It is passed directly to
. This mapping can be str to str on any platform or bytes to bytes on POSIX platforms much like
or
.
Examples:
>>> subprocess.run(["ls","-l"])# doesn't capture outputCompletedProcess(args=['ls', '-l'], returncode=0)>>> subprocess.run("exit 1",shell=True,check=True)Traceback (most recent call last):...subprocess.CalledProcessError: Command 'exit 1' returned non-zero exit status 1>>> subprocess.run(["ls","-l","/dev/null"],capture_output=True)CompletedProcess(args=['ls', '-l', '/dev/null'], returncode=0,stdout=b'crw-rw-rw- 1 root root 1, 3 Jan 23 16:23 /dev/null\n', stderr=b'')New in version 3.5.
Changed in version 3.6: Added encoding and errors parameters
Changed in version 3.7: Added the text parameter, as a more understandable alias of universal_newlines. Added the capture_output parameter.
Changed in version 3.11.3: Changed Windows shell search order for shell=True. The current directory and %PATH% are replaced with %COMSPEC% and %SystemRoot%\System32\cmd.exe. As a result, dropping a malicious program named cmd.exe into a current directory no longer works.
classsubprocess.CompletedProcess
The return value from
, representing a process that has finished.
args
The arguments used to launch the process. This may be a list or a string.
returncode
Exit status of the child process. Typically, an exit status of 0 indicates that it ran successfully.
A negative value -N indicates that the child was terminated by signal N (POSIX only).
stdout
Captured stdout from the child process. A bytes sequence, or a string if
was called with an encoding, errors, or text=True. None if stdout was not captured.
If you ran the process with stderr=subprocess.STDOUT, stdout and stderr will be combined in this attribute, and
will be None.
stderr
Captured stderr from the child process. A bytes sequence, or a string if
was called with an encoding, errors, or text=True. None if stderr was not captured.
check_returncode()
If
is non-zero, raise a
.
New in version 3.5.
subprocess.DEVNULL
Special value that can be used as the stdin, stdout or stderr argument to
and indicates that the special file
will be used.
New in version 3.3.
subprocess.PIPE
Special value that can be used as the stdin, stdout or stderr argument to
and indicates that a pipe to the standard stream should be opened. Most useful with
.
subprocess.STDOUT
Special value that can be used as the stderr argument to
and indicates that standard error should go into the same handle as standard output.
exceptionsubprocess.SubprocessError
Base class for all other exceptions from this module.
New in version 3.3.
exceptionsubprocess.TimeoutExpired
Subclass of
, 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 it was captured by
or
. Otherwise, None. This is always
when any output was captured regardless of the text=True setting. It may remain None instead of b'' when no output was observed.
stdout
Alias for output, for symmetry with
.
stderr
Stderr output of the child process if it was captured by
. Otherwise, None. This is always
when stderr output was captured regardless of the text=True setting. It may remain None instead of b'' when no stderr output was observed.
New in version 3.3.
Changed in version 3.5: stdout and stderr attributes added
exceptionsubprocess.CalledProcessError
Subclass of
, raised when a process run by
,
, or
(with check=True) returns a non-zero exit status.
returncode
Exit status of the child process. If the process exited due to a signal, this will be the negative signal number.
cmd
Command that was used to spawn the child process.
output
Output of the child process if it was captured by
or
. Otherwise, None.
stdout
Alias for output, for symmetry with
.
stderr
Stderr output of the child process if it was captured by
. Otherwise, None.
Changed in version 3.5: stdout and stderr attributes added
Frequently Used Arguments
To support a wide variety of use cases, the
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
(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
,
, an existing file descriptor (a positive integer), an existing file object with a valid file descriptor, and None.
indicates that a new pipe to the child should be created.
indicates that the special file
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
, which indicates that the stderr data from the child process should be captured into the same file handle as for stdout.
If encoding or errors are specified, or text (also known as universal_newlines) is true, the file objects stdin, stdout and stderr will be opened in text mode using the encoding and errors specified in the call or the defaults for
.
For stdin, line ending characters '\n' in the input will be converted to the default line separator
. For stdout and stderr, all line endings in the output will be converted to '\n'. For more information see the documentation of the
class when the newline argument to its constructor is None.
If text mode is not used, stdin, stdout and stderr will be opened as binary streams. No encoding or line ending conversion is performed.
Changed in version 3.6: Added the encoding and errors parameters.
Changed in version 3.7: Added the text parameter as an alias for universal_newlines.
Note
The newlines attribute of the file objects
,
and
are not updated by the
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,
,
,
,
,
, and
).
Changed in version 3.3: When universal_newlines is True, the class uses the encoding
locale.getpreferredencoding(False)
instead of locale.getpreferredencoding(). See the
class for more information on this change.
Note
Read the
section before using shell=True.
These options, along with all of the other options, are described in more detail in the
constructor documentation.
Popen Constructor
The underlying process creation and management in this module is handled by the
class. It offers a lot of flexibility so that developers are able to handle the less common cases not covered by the convenience functions.
classsubprocess.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=None, startupinfo=None, creationflags=0, restore_signals=True, start_new_session=False, pass_fds=(), *, group=None, extra_groups=None, user=None, umask=-1, encoding=None, errors=None, text=None, pipesize=-1, process_group=None)
Execute a child program in a new process. On POSIX, the class uses
-like behavior to execute the child program. On Windows, the class uses the Windows CreateProcess() function. The arguments to
are as follows.
args should be a sequence of program arguments or else a single string or
. 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.
Warning
For maximum reliability, use a fully qualified path for the executable. To search for an unqualified name on PATH, use
. On all platforms, passing
is the recommended way to launch the current Python interpreter again, and use the -m command-line format to launch an installed module.
Resolving the path of executable (or the first item of args) is platform dependent. For POSIX, see
, and note that when resolving or searching for the executable path, cwd overrides the current working directory and env can override the PATH environment variable. For Windows, see the documentation of the lpApplicationName and lpCommandLine parameters of WinAPI CreateProcess, and note that when resolving or searching for the executable path with shell=False, cwd does not override the current working directory and env cannot override the PATH environment variable. Using a full path avoids all of these variations.
An example of passing some arguments to an external program as a sequence is:
Popen(["/usr/bin/git","commit","-m","Fixes a bug."])On POSIX, 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
It may not be obvious how to break a shell command into a sequence of arguments, especially in complex cases.
can illustrate how to determine the correct tokenization for args:
>>> importshlex,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.
Changed in version 3.6: args parameter accepts a
if shell is False and a sequence containing path-like objects on POSIX.
Changed in version 3.8: args parameter accepts a
if shell is False and a sequence containing bytes and path-like objects on Windows.
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 POSIX 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,
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.
Note
Read the
section before using shell=True.
bufsize will be supplied as the corresponding argument to the
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 (only usable if text=True or universal_newlines=True)
any other positive value means use a buffer of approximately that size
negative bufsize (the default) means the system default of io.DEFAULT_BUFFER_SIZE will be used.
Changed in version 3.3.1: bufsize now defaults to -1 to enable buffering by default to match the behavior that most code expects. In versions prior to Python 3.2.4 and 3.3.1 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 POSIX, the args name becomes the display name for the executable in utilities such as ps. If shell=True, on POSIX the executable argument specifies a replacement shell for the default /bin/sh.
Changed in version 3.6: executable parameter accepts a
on POSIX.
Changed in version 3.8: executable parameter accepts a bytes and
on Windows.
Changed in version 3.11.3: Changed Windows shell search order for shell=True. The current directory and %PATH% are replaced with %COMSPEC% and %SystemRoot%\System32\cmd.exe. As a result, dropping a malicious program named cmd.exe into a current directory no longer works.
stdin, stdout and stderr specify the executed program’s standard input, standard output and standard error file handles, respectively. Valid values are
,
, an existing file descriptor (a positive integer), an existing
with a valid file descriptor, and None.
indicates that a new pipe to the child should be created.
indicates that the special file
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
, 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. (POSIX 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.
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 and process_group parameters should take the place of code using preexec_fn to call
or
in the child.
Changed in version 3.8: The preexec_fn parameter is no longer supported in subinterpreters. The use of the parameter in a subinterpreter raises
. The new restriction may affect applications that are deployed in mod_wsgi, uWSGI, and other embedded environments.
If close_fds is true, all file descriptors except 0, 1 and 2 will be closed before the child process is executed. Otherwise when close_fds is false, file descriptors obey their inheritable flag as described in
Inheritance of File Descriptors
.
On Windows, if close_fds is true then no handles will be inherited by the child process unless explicitly passed in the handle_list element of
, or by standard handle redirection.
Changed in version 3.2: The default for close_fds was changed from
to what is described above.
Changed in version 3.7: On Windows the default for close_fds was changed from
to
when redirecting the standard handles. It’s now possible to set close_fds to
when redirecting the standard handles.
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
. (POSIX only)
Changed 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. cwd can be a string, bytes or
object. On POSIX, the function looks for executable (or for the first item in args) relative to cwd if the executable path is a relative path.
Changed in version 3.6: cwd parameter accepts a
on POSIX.
Changed in version 3.7: cwd parameter accepts a
on Windows.
Changed in version 3.8: cwd parameter accepts a bytes object on Windows.
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. (POSIX 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.
Changed in version 3.2: start_new_session was added.
If process_group is a non-negative integer, the setpgid(0,value) system call will be made in the child process prior to the execution of the subprocess.
Changed in version 3.11: process_group was added.
If group is not None, the setregid() system call will be made in the child process prior to the execution of the subprocess. If the provided value is a string, it will be looked up via
and the value in gr_gid will be used. If the value is an integer, it will be passed verbatim. (POSIX only)
New in version 3.9.
If extra_groups is not None, the setgroups() system call will be made in the child process prior to the execution of the subprocess. Strings provided in extra_groups will be looked up via
and the values in gr_gid will be used. Integer values will be passed verbatim. (POSIX only)
New in version 3.9.
If user is not None, the setreuid() system call will be made in the child process prior to the execution of the subprocess. If the provided value is a string, it will be looked up via
and the value in pw_uid will be used. If the value is an integer, it will be passed verbatim. (POSIX only)
Note
Specifying user will not drop existing supplementary group memberships! The caller must also pass extra_groups=() to reduce the group membership of the child process for security purposes.
New in version 3.9.
If umask is not negative, the umask() system call will be made in the child process prior to the execution of the subprocess.
New in version 3.9.
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. This mapping can be str to str on any platform or bytes to bytes on POSIX platforms much like
or
.
Note
If specified, env must provide any variables required for the program to execute. On Windows, in order to run a
the specified envmust include a valid SystemRoot.
If encoding or errors are specified, or text is true, the file objects stdin, stdout and stderr are opened in text mode with the specified encoding and errors, as described above in
. The universal_newlines argument is equivalent to text and is provided for backwards compatibility. By default, file objects are opened in binary mode.
New in version 3.6: encoding and errors were added.
New in version 3.7: text was added as a more readable alias for universal_newlines.
If given, startupinfo will be a
object, which is passed to the underlying CreateProcess function.
If given, creationflags, can be one or more of the following flags:
pipesize can be used to change the size of the pipe when
is used for stdin, stdout or stderr. The size of the pipe is only changed on platforms that support this (only Linux at this time of writing). Other platforms will ignore this parameter.
Changed in version 3.10: Added the pipesize parameter.
Popen objects are supported as context managers via the
statement: on exit, standard file descriptors are closed, and the process is waited for.
withPopen(["ifconfig"],stdout=PIPE)asproc:log.write(proc.stdout.read())Popen and the other functions in this module that use it raise an
subprocess.Popen with arguments executable, args, cwd, and env. The value for args may be a single string or a list of strings, depending on platform.
Changed in version 3.2: Added context manager support.
Changed in version 3.6: Popen destructor now emits a
warning if the child process is still running.
Changed in version 3.8: Popen can use
in some cases for better performance. On Windows Subsystem for Linux and QEMU User Emulation, Popen constructor using
no longer raise an exception on errors like missing program, but the child process fails with a non-zero
.
Exceptions
Exceptions raised in the child process, before the new program has started to execute, will be re-raised in the parent.
The most common exception raised is
. This occurs, for example, when trying to execute a non-existent file. Applications should prepare for
exceptions. Note that, when shell=True,
will be raised by the child only if the selected shell itself was not found. To determine if the shell failed to find the requested application, it is necessary to check the return code or output from the subprocess.
A
will be raised if
is called with invalid arguments.
and
will raise
if the called process returns a non-zero return code.
All of the functions and methods that accept a timeout parameter, such as
and
will raise
if the timeout expires before the process exits.
Exceptions defined in this module all inherit from
.
New in version 3.3: The
base class was added.
Security Considerations
Unlike some other popen functions, this library will not implicitly choose to call a system shell. This means that all characters, including shell metacharacters, can safely be passed to child processes. If the shell is invoked explicitly, via shell=True, it is the application’s responsibility to ensure that all whitespace and metacharacters are quoted appropriately to avoid
vulnerabilities. On
, it is possible to use
for this escaping.
On Windows, batch files (*.bat or *.cmd) may be launched by the operating system in a system shell regardless of the arguments passed to this library. This could result in arguments being parsed according to shell rules, but without any escaping added by Python. If you are intentionally launching a batch file with arguments from untrusted sources, consider passing shell=True to allow Python to escape special characters. See
for additional discussion.
Popen Objects
Instances of the
class have the following methods:
Popen.poll()
Check if child process has terminated. Set and return
attribute. Otherwise, returns None.
Popen.wait(timeout=None)
Wait for child process to terminate. Set and return
attribute.
If the process does not terminate after timeout seconds, raise a
exception. It is safe to catch this exception and retry the wait.
Note
This will deadlock when using stdout=PIPE 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
when using pipes to avoid that.
Note
When the timeout parameter is not None, then (on POSIX) the function is implemented using a busy loop (non-blocking call and short sleeps). Use the
module for an asynchronous wait: see
asyncio.create_subprocess_exec
.
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 and set the
attribute. 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. If streams were opened in text mode, input must be a string. Otherwise, it must be bytes.
returns a tuple (stdout_data,stderr_data). The data will be strings if streams were opened in text mode; otherwise, bytes.
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
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)exceptTimeoutExpired: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.
Do nothing if the process completed.
Note
On Windows, SIGTERM is an alias for
. 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
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
is an alias for
.
The following attributes are also set by the class for you to access. Reassigning them to new values is unsupported:
Popen.args
The args argument as it was passed to
– a sequence of program arguments or else a single string.
New in version 3.3.
Popen.stdin
If the stdin argument was
, this attribute is a writeable stream object as returned by
. If the encoding or errors arguments were specified or the text or universal_newlines argument was True, the stream is a text stream, otherwise it is a byte stream. If the stdin argument was not
, this attribute is None.
Popen.stdout
If the stdout argument was
, this attribute is a readable stream object as returned by
. Reading from the stream provides output from the child process. If the encoding or errors arguments were specified or the text or universal_newlines argument was True, the stream is a text stream, otherwise it is a byte stream. If the stdout argument was not
, this attribute is None.
Popen.stderr
If the stderr argument was
, this attribute is a readable stream object as returned by
. Reading from the stream provides error output from the child process. If the encoding or errors arguments were specified or the text or universal_newlines argument was True, the stream is a text stream, otherwise it is a byte stream. If the stderr argument was not
, this attribute is None.
Warning
Use
rather than
,
or
to avoid deadlocks due to any of the other OS pipe buffers filling up and blocking the child process.
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. Initially None,
is set by a call to the
,
, or
methods if they detect that the process has terminated.
A None value indicates that the process hadn’t yet terminated at the time of the last method call.
A negative value -N indicates that the child was terminated by signal N (POSIX only).
Windows Popen Helpers
The
class and following constants are only available on Windows.
classsubprocess.STARTUPINFO(*, dwFlags=0, hStdInput=None, hStdOutput=None, hStdError=None, wShowWindow=0, lpAttributeList=None)
Partial support of the Windows
structure is used for
creation. The following attributes can be set by passing them as keyword-only arguments.
Changed in version 3.7: Keyword-only argument support was added.
dwFlags
A bit field that determines whether certain
attributes are used when the process creates a window.
si=subprocess.STARTUPINFO()si.dwFlags=subprocess.STARTF_USESTDHANDLES|subprocess.STARTF_USESHOWWINDOWhStdInput
If
specifies
, this attribute is the standard input handle for the process. If
is not specified, the default for standard input is the keyboard buffer.
hStdOutput
If
specifies
, 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
specifies
, 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
specifies
, this attribute can be any of the values that can be specified in the nCmdShow parameter for the
function, except for SW_SHOWDEFAULT. Otherwise, this attribute is ignored.
is provided for this attribute. It is used when
is called with shell=True.
lpAttributeList
A dictionary of additional attributes for process creation as given in STARTUPINFOEX, see
.
Supported attributes:
handle_listSequence of handles that will be inherited. close_fds must be true if non-empty.
The handles must be temporarily made inheritable by
when passed to the
constructor, else
will be raised with Windows error ERROR_INVALID_PARAMETER (87).
Warning
In a multithreaded process, use caution to avoid leaking handles that are marked inheritable when combining this feature with concurrent calls to other process creation functions that inherit all handles such as
. This also applies to standard handle redirection, which temporarily creates inheritable handles.
New in version 3.7.
Windows Constants
The
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
,
, and
attributes contain additional information.
subprocess.STARTF_USESHOWWINDOW
Specifies that the
attribute contains additional information.
subprocess.CREATE_NEW_CONSOLE
The new process has a new console, instead of inheriting its parent’s console (the default).
subprocess.CREATE_NEW_PROCESS_GROUP
A
creationflags parameter to specify that a new process group will be created. This flag is necessary for using
on the subprocess.
This flag is ignored if
is specified.
subprocess.ABOVE_NORMAL_PRIORITY_CLASS
A
creationflags parameter to specify that a new process will have an above average priority.
New in version 3.7.
subprocess.BELOW_NORMAL_PRIORITY_CLASS
A
creationflags parameter to specify that a new process will have a below average priority.
New in version 3.7.
subprocess.HIGH_PRIORITY_CLASS
A
creationflags parameter to specify that a new process will have a high priority.
New in version 3.7.
subprocess.IDLE_PRIORITY_CLASS
A
creationflags parameter to specify that a new process will have an idle (lowest) priority.
New in version 3.7.
subprocess.NORMAL_PRIORITY_CLASS
A
creationflags parameter to specify that a new process will have an normal priority. (default)
New in version 3.7.
subprocess.REALTIME_PRIORITY_CLASS
A
creationflags parameter to specify that a new process will have realtime priority. You should almost never use REALTIME_PRIORITY_CLASS, because this interrupts system threads that manage mouse input, keyboard input, and background disk flushing. This class can be appropriate for applications that “talk” directly to hardware or that perform brief tasks that should have limited interruptions.
New in version 3.7.
subprocess.CREATE_NO_WINDOW
A
creationflags parameter to specify that a new process will not create a window.
New in version 3.7.
subprocess.DETACHED_PROCESS
A
creationflags parameter to specify that a new process will not inherit its parent’s console. This value cannot be used with CREATE_NEW_CONSOLE.
New in version 3.7.
subprocess.CREATE_DEFAULT_ERROR_MODE
A
creationflags parameter to specify that a new process does not inherit the error mode of the calling process. Instead, the new process gets the default error mode. This feature is particularly useful for multithreaded shell applications that run with hard errors disabled.
New in version 3.7.
subprocess.CREATE_BREAKAWAY_FROM_JOB
A
creationflags parameter to specify that a new process is not associated with the job.
New in version 3.7.
Older high-level API
Prior to Python 3.5, these three functions comprised the high level API to subprocess. You can now use
in many cases, but lots of existing code calls these functions.
subprocess.call(args, *, stdin=None, stdout=None, stderr=None, shell=False, cwd=None, timeout=None, **other_popen_kwargs)
Run the command described by args. Wait for command to complete, then return the
attribute.
Code needing to capture stdout or stderr should use
instead:
run(...).returncodeTo suppress stdout or stderr, supply a value of
.
The arguments shown above are merely some common ones. The full function signature is the same as that of the
constructor - this function passes all supplied arguments other than timeout directly through to that interface.
Note
Do not use stdout=PIPE or stderr=PIPE with this function. The child process will block if it generates enough output to a pipe to fill up the OS pipe buffer as the pipes are not being read from.
Changed in version 3.3: timeout was added.
Changed in version 3.11.3: Changed Windows shell search order for shell=True. The current directory and %PATH% are replaced with %COMSPEC% and %SystemRoot%\System32\cmd.exe. As a result, dropping a malicious program named cmd.exe into a current directory no longer works.
subprocess.check_call(args, *, stdin=None, stdout=None, stderr=None, shell=False, cwd=None, timeout=None, **other_popen_kwargs)
Run command with arguments. Wait for command to complete. If the return code was zero then return, otherwise raise
. The
object will have the return code in the
attribute. If
was unable to start the process it will propagate the exception that was raised.
Code needing to capture stdout or stderr should use
instead:
run(...,check=True)To suppress stdout or stderr, supply a value of
.
The arguments shown above are merely some common ones. The full function signature is the same as that of the
constructor - this function passes all supplied arguments other than timeout directly through to that interface.
Note
Do not use stdout=PIPE or stderr=PIPE with this function. The child process will block if it generates enough output to a pipe to fill up the OS pipe buffer as the pipes are not being read from.
Changed in version 3.3: timeout was added.
Changed in version 3.11.3: Changed Windows shell search order for shell=True. The current directory and %PATH% are replaced with %COMSPEC% and %SystemRoot%\System32\cmd.exe. As a result, dropping a malicious program named cmd.exe into a current directory no longer works.
subprocess.check_output(args, *, stdin=None, stderr=None, shell=False, cwd=None, encoding=None, errors=None, universal_newlines=None, timeout=None, text=None, **other_popen_kwargs)
Run command with arguments and return its output.
If the return code was non-zero it raises a
. The
object will have the return code in the
attribute and any output in the
attribute.
This is equivalent to:
run(...,check=True,stdout=PIPE).stdoutThe arguments shown above are merely some common ones. The full function signature is largely the same as that of
- most arguments are passed directly through to that interface. One API deviation from
behavior exists: passing input=None will behave the same as input=b'' (or input='', depending on other arguments) rather than using the parent’s standard input file handle.
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 text, encoding, errors, or universal_newlines to True as described in
and
.
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.
Changed in version 3.3: timeout was added.
Changed in version 3.4: Support for the input keyword argument was added.
Changed in version 3.6: encoding and errors were added. See
for details.
New in version 3.7: text was added as a more readable alias for universal_newlines.
Changed in version 3.11.3: Changed Windows shell search order for shell=True. The current directory and %PATH% are replaced with %COMSPEC% and %SystemRoot%\System32\cmd.exe. As a result, dropping a malicious program named cmd.exe into a current directory no longer works.
Replacing Older Functions with the
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
instead.
In addition, the replacements using
will fail with a
if the requested operation produces a non-zero return code. The output is still available as the
attribute of the raised exception.
In the following examples, we assume that the relevant functions have already been imported from the
module.
Replacing /bin/sh shell command substitution
output=$(mycmdmyarg)becomes:
output=check_output(["mycmd","myarg"])Replacing shell pipeline
output=$(dmesg|grephda)becomes:
p1=Popen(["dmesg"],stdout=PIPE)p2=Popen(["grep","hda"],stdin=p1.stdout,stdout=PIPE)p1.stdout.close()# Allow p1 to receive a SIGPIPE if p2 exits.output=p2.communicate()[0]The p1.stdout.close() call after starting the p2 is important in order for p1 to receive a SIGPIPE if p2 exits before p1.
Alternatively, for trusted input, the shell’s own pipeline support may still be used directly:
output=$(dmesg|grephda)becomes:
output=check_output("dmesg | grep hda",shell=True)Replacing
sts=os.system("mycmd"+" myarg")# becomesretcode=call("mycmd"+" myarg",shell=True)Notes:
Calling the program through the shell is usually not required.
The
return value is encoded differently to that of
.
The
function ignores SIGINT and SIGQUIT signals while the command is running, but the caller must do this separately when using the
module.
A more realistic example would look like this:
try:retcode=call("mycmd"+" myarg",shell=True)ifretcode<0:print("Child was terminated by signal",-retcode,file=sys.stderr)else:print("Child returned",retcode,file=sys.stderr)exceptOSErrorase:print("Execution failed:",e,file=sys.stderr)Replacing the
family
P_NOWAIT example:
pid=os.spawnlp(os.P_NOWAIT,"/bin/mycmd","mycmd","myarg")==>pid=Popen(["/bin/mycmd","myarg"]).pidP_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
, 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()ifrcisnotNoneandrc>>8:print("There were some errors")==>process=Popen(cmd,stdin=PIPE)...process.stdin.close()ifprocess.wait()!=0:print("There were some errors")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
, except that:
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
to guarantee this behavior on all platforms or past Python versions.
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, *, encoding=None, errors=None)
Return (exitcode,output) of executing cmd in a shell.
Execute the string cmd in a shell with Popen.check_output() and return a 2-tuple (exitcode,output). encoding and errors are used to decode output; see the notes on
for more details.
A trailing newline is stripped from the output. The exit code for the command can be interpreted as the return code of subprocess. Example:
>>> subprocess.getstatusoutput('ls /bin/ls')(0, '/bin/ls')>>> subprocess.getstatusoutput('cat /bin/junk')(1, 'cat: /bin/junk: No such file or directory')>>> subprocess.getstatusoutput('/bin/junk')(127, 'sh: /bin/junk: not found')>>> subprocess.getstatusoutput('/bin/kill $$')(-15, '')Changed in version 3.3.4: Windows support was added.
The function now returns (exitcode, output) instead of (status, output) as it did in Python 3.3.3 and earlier. exitcode has the same value as
.
Changed in version 3.11: Added the encoding and errors parameters.
subprocess.getoutput(cmd, *, encoding=None, errors=None)
Return output (stdout and stderr) of executing cmd in a shell.
Like
, except the exit code is ignored and the return value is a string containing the command’s output. Example:
>>> subprocess.getoutput('ls /bin/ls')'/bin/ls'Changed in version 3.3.4: Windows support added
Changed in version 3.11: Added the encoding and errors parameters.
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):
Arguments are delimited by white space, which is either a space or a tab.
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.
A double quotation mark preceded by a backslash is interpreted as a literal double quotation mark.
Backslashes are interpreted literally, unless they immediately precede a double quotation mark.
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
Module which provides function to parse and escape command lines.
Disabling use of vfork() or posix_spawn()
On Linux,
defaults to using the vfork() system call internally when it is safe to do so rather than fork(). This greatly improves performance.
If you ever encounter a presumed highly unusual situation where you need to prevent vfork() from being used by Python, you can set the subprocess._USE_VFORK attribute to a false value.
subprocess._USE_VFORK=False# See CPython issue gh-NNNNNN.Setting this has no impact on use of posix_spawn() which could use vfork() internally within its libc implementation. There is a similar subprocess._USE_POSIX_SPAWN attribute if you need to prevent use of that.
subprocess._USE_POSIX_SPAWN=False# See CPython issue gh-NNNNNN.It is safe to set these to false on any Python version. They will have no effect on older versions when unsupported. Do not assume the attributes are available to read. Despite their names, a true value does not indicate that the corresponding function will be used, only that it may be.
Please file issues any time you have to use these private knobs with a way to reproduce the issue you were seeing. Link to that issue from a comment in your code.
New in version 3.8: _USE_POSIX_SPAWN
New in version 3.11: _USE_VFORK