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
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.
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 slightly odd notation in the abbreviated signature). The full function signature is the same as that of the Popen constructor - this functions passes all supplied arguments directly through to that interface.
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.
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 slightly odd notation in the abbreviated signature). The full function signature is the same as that of the Popen constructor - this functions passes all supplied arguments directly through to that interface.
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
New in version 2.5.
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.
Run command with arguments and return its output as a byte string.
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 slightly odd notation in the abbreviated signature). The full function signature is largely the same as that of the Popen constructor, except that stdout is not permitted as it is used internally. All other supplied arguments are passed directly through to the Popen constructor.
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 2.7.
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.
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.
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.
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, 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 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.
When stdout or stderr are pipes and universal_newlines is True then the output data is assumed to be encoded as UTF-8 and will automatically be decoded to text. All line endings will be converted to '\n' as described for the universal newlines 'U' mode argument to open().
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 access to other shell features such as filename wildcards, shell pipes and environment variable expansion.
Warning
Executing shell commands that incorporate unsanitized input from an untrusted source makes a program vulnerable to shell injection, a serious security flaw which can result in arbitrary command execution. For this reason, the use of shell=True is strongly discouraged in cases where the command string is constructed from external input:
>>> from subprocess import call >>> filename = input("What file would you like to display?\n") What file would you like to display? non_existent; rm -rf / # >>> call("cat " + filename, shell=True) # Uh-oh. This will end badly...shell=False disables all shell based features, but does not suffer from this vulnerability; see the Note in the Popen constructor documentation for helpful hints in getting shell=False to work.
These options, along with all of the other options, are described in more detail in the Popen constructor documentation.
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.
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
Enabling this option can be a security hazard if combined with untrusted input. See the warning under Frequently Used Arguments for details.
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 in a manner described in Converting an argument sequence to a string on Windows.
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 program’s 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 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 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.
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.
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.
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.
Instances of the Popen class have the following methods:
Check if child process has terminated. Set and return returncode attribute.
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.
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.
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.
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.
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.
If the stdin argument was PIPE, this attribute is a file object that provides input to the child process. Otherwise, it is None.
If the stdout argument was PIPE, this attribute is a file object that provides output from the child process. Otherwise, it is None.
If the stderr argument was PIPE, this attribute is a file object that provides error output from the child process. Otherwise, it is None.
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.
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).
The STARTUPINFO class and following constants are only available on Windows.
Partial support of the Windows STARTUPINFO structure is used for Popen creation.
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
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.
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.
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.
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.
The subprocess module exposes the following constants.
The standard input device. Initially, this is the console input buffer, CONIN$.
The standard output device. Initially, this is the active console screen buffer, CONOUT$.
The standard error device. Initially, this is the active console screen buffer, CONOUT$.
Hides the window. Another window will be activated.
Specifies that the STARTUPINFO.hStdInput, STARTUPINFO.hStdOutput, and STARTUPINFO.hStdError attributes contain additional information.
Specifies that the STARTUPINFO.wShowWindow attribute contains additional information.
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.
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.
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.
output=`mycmd myarg`
# becomes
output = check_output(["mycmd", "myarg"])
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)
sts = os.system("mycmd" + " myarg")
# becomes
sts = call("mycmd" + " myarg", shell=True)
Notes:
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)
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"})
(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")
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:
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.
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.
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.
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):