class QProcess#

The QProcess class is used to start external programs and to communicate with them. More…

Inheritance diagram of PySide6.QtCore.QProcess

Synopsis#

Methods#

Slots#

Signals#

Static functions#

Note

This documentation may contain snippets that were automatically translated from C++ to Python. We always welcome contributions to the snippet translation. If you see an issue with the translation, you can also let us know by creating a ticket on https:/bugreports.qt.io/projects/PYSIDE

Detailed Description#

Warning

This section contains snippets that were automatically translated from C++ to Python and may contain errors.

Running a Process#

To start a process, pass the name and command line arguments of the program you want to run as arguments to start() . Arguments are supplied as individual strings in a QStringList .

Alternatively, you can set the program to run with setProgram() and setArguments() , and then call start() or open() .

For example, the following code snippet runs the analog clock example in the Fusion style on X11 platforms by passing strings containing “-style” and “fusion” as two items in the list of arguments:

parent = QObject()            ...

program = "./path/to/Qt/examples/widgets/analogclock"
arguments = QStringList()
arguments << "-style" << "fusion"
myProcess = QProcess(parent)
myProcess.start(program, arguments)

QProcess then enters the Starting state, and when the program has started, QProcess enters the Running state and emits started() .

QProcess allows you to treat a process as a sequential I/O device. You can write to and read from the process just as you would access a network connection using QTcpSocket. You can then write to the process’s standard input by calling write() , and read the standard output by calling read() , readLine() , and getChar() . Because it inherits QIODevice , QProcess can also be used as an input source for QXmlReader, or for generating data to be uploaded using QNetworkAccessManager.

When the process exits, QProcess reenters the NotRunning state (the initial state), and emits finished() .

The finished() signal provides the exit code and exit status of the process as arguments, and you can also call exitCode() to obtain the exit code of the last process that finished, and exitStatus() to obtain its exit status. If an error occurs at any point in time, QProcess will emit the errorOccurred() signal. You can also call error() to find the type of error that occurred last, and state() to find the current process state.

Note

QProcess is not supported on VxWorks, iOS, tvOS, or watchOS.

Finding the Executable#

The program to be run can be set either by calling setProgram() or directly in the start() call. The effect of calling start() with the program name and arguments is equivalent to calling setProgram() and setArguments() before that function and then calling the overload without those parameters.

QProcess interprets the program name in one of three different ways, similar to how Unix shells and the Windows command interpreter operate in their own command-lines:

  • If the program name is an absolute path, then that is the exact executable that will be launched and QProcess performs no searching.

  • If the program name is a relative path with more than one path component (that is, it contains at least one slash), the starting directory where that relative path is searched is OS-dependent: on Windows, it’s the parent process’ current working dir, while on Unix it’s the one set with setWorkingDirectory() .

  • If the program name is a plain file name with no slashes, the behavior is operating-system dependent. On Unix systems, QProcess will search the PATH environment variable; on Windows, the search is performed by the OS and will first the parent process’ current directory before the PATH environment variable (see the documentation for CreateProcess for the full list).

To avoid platform-dependent behavior or any issues with how the current application was launched, it is advisable to always pass an absolute path to the executable to be launched. For auxiliary binaries shipped with the application, one can construct such a path starting with applicationDirPath() . Similarly, to explicitly run an executable that is to be found relative to the directory set with setWorkingDirectory() , use a program path starting with “./” or “../” as the case may be.

On Windows, the “.exe” suffix is not required for most uses, except those outlined in the CreateProcess documentation. Additionally, QProcess will convert the Unix-style forward slashes to Windows path backslashes for the program name. This allows code using QProcess to be written in a cross-platform manner, as shown in the examples above.

QProcess does not support directly executing Unix shell or Windows command interpreter built-in functions, such as cmd.exe's dir command or the Bourne shell’s export. On Unix, even though many shell built-ins are also provided as separate executables, their behavior may differ from those implemented as built-ins. To run those commands, one should explicitly execute the interpreter with suitable options. For Unix systems, launch “/bin/sh” with two arguments: “-c” and a string with the command-line to be run. For Windows, due to the non-standard way cmd.exe parses its command-line, use setNativeArguments() (for example, “/c dir d:”).

Environment variables#

The QProcess API offers methods to manipulate the environment variables that the child process will see. By default, the child process will have a copy of the current process environment variables that exist at the time the start() function is called. This means that any modifications performed using qputenv() prior to that call will be reflected in the child process’ environment. Note that QProcess makes no attempt to prevent race conditions with qputenv() happening in other threads, so it is recommended to avoid qputenv() after the application’s initial start up.

The environment for a specific child can be modified using the processEnvironment() and setProcessEnvironment() functions, which use the QProcessEnvironment class. By default, processEnvironment() will return an object for which inheritsFromParent() is true. Setting an environment that does not inherit from the parent will cause QProcess to use exactly that environment for the child when it is started.

The normal scenario starts from the current environment by calling systemEnvironment() and then proceeds to adding, changing, or removing specific variables. The resulting variable roster can then be applied to a QProcess with setProcessEnvironment() .

It is possible to remove all variables from the environment or to start from an empty environment, using the QProcessEnvironment() default constructor. This is not advisable outside of controlled and system-specific conditions, as there may be system variables that are set in the current process environment and are required for proper execution of the child process.

On Windows, QProcess will copy the current process’ "PATH" and "SystemRoot" environment variables if they were unset. It is not possible to unset them completely, but it is possible to set them to empty values. Setting "PATH" to empty on Windows will likely cause the child process to fail to start.

Communicating via Channels#

Processes have two predefined output channels: The standard output channel (stdout) supplies regular console output, and the standard error channel (stderr) usually supplies the errors that are printed by the process. These channels represent two separate streams of data. You can toggle between them by calling setReadChannel() . QProcess emits readyRead() when data is available on the current read channel. It also emits readyReadStandardOutput() when new standard output data is available, and when new standard error data is available, readyReadStandardError() is emitted. Instead of calling read() , readLine() , or getChar() , you can explicitly read all data from either of the two channels by calling readAllStandardOutput() or readAllStandardError() .

The terminology for the channels can be misleading. Be aware that the process’s output channels correspond to QProcess ‘s read channels, whereas the process’s input channels correspond to QProcess ‘s write channels. This is because what we read using QProcess is the process’s output, and what we write becomes the process’s input.

QProcess can merge the two output channels, so that standard output and standard error data from the running process both use the standard output channel. Call setProcessChannelMode() with MergedChannels before starting the process to activate this feature. You also have the option of forwarding the output of the running process to the calling, main process, by passing ForwardedChannels as the argument. It is also possible to forward only one of the output channels - typically one would use ForwardedErrorChannel , but ForwardedOutputChannel also exists. Note that using channel forwarding is typically a bad idea in GUI applications - you should present errors graphically instead.

Certain processes need special environment settings in order to operate. You can set environment variables for your process by calling setProcessEnvironment() . To set a working directory, call setWorkingDirectory() . By default, processes are run in the current working directory of the calling process.

The positioning and the screen Z-order of windows belonging to GUI applications started with QProcess are controlled by the underlying windowing system. For Qt 5 applications, the positioning can be specified using the -qwindowgeometry command line option; X11 applications generally accept a -geometry command line option.

Synchronous Process API#

QProcess provides a set of functions which allow it to be used without an event loop, by suspending the calling thread until certain signals are emitted:

  • waitForStarted() blocks until the process has started.

  • waitForReadyRead() blocks until new data is available for reading on the current read channel.

  • waitForBytesWritten() blocks until one payload of data has been written to the process.

  • waitForFinished() blocks until the process has finished.

Calling these functions from the main thread (the thread that calls QApplication::exec()) may cause your user interface to freeze.

The following example runs gzip to compress the string “Qt rocks!”, without an event loop:

gzip = QProcess()
gzip.start("gzip", QStringList() << "-c")
if not gzip.waitForStarted():
    return False
gzip.write("Qt rocks!")
gzip.closeWriteChannel()
if not gzip.waitForFinished():
    return False
result = gzip.readAll()

See also

QBuffer QFile QTcpSocket

class ProcessError#

This enum describes the different types of errors that are reported by QProcess .

Constant

Description

QProcess.FailedToStart

The process failed to start. Either the invoked program is missing, or you may have insufficient permissions or resources to invoke the program.

QProcess.Crashed

The process crashed some time after starting successfully.

QProcess.Timedout

The last waitFor…() function timed out. The state of QProcess is unchanged, and you can try calling waitFor…() again.

QProcess.WriteError

An error occurred when attempting to write to the process. For example, the process may not be running, or it may have closed its input channel.

QProcess.ReadError

An error occurred when attempting to read from the process. For example, the process may not be running.

QProcess.UnknownError

An unknown error occurred. This is the default return value of error() .

See also

error()

class ProcessState#

This enum describes the different states of QProcess .

Constant

Description

QProcess.NotRunning

The process is not running.

QProcess.Starting

The process is starting, but the program has not yet been invoked.

QProcess.Running

The process is running and is ready for reading and writing.

See also

state()

class ProcessChannel#

This enum describes the process channels used by the running process. Pass one of these values to setReadChannel() to set the current read channel of QProcess .

Constant

Description

QProcess.StandardOutput

The standard output (stdout) of the running process.

QProcess.StandardError

The standard error (stderr) of the running process.

See also

setReadChannel()

class ProcessChannelMode#

This enum describes the process output channel modes of QProcess . Pass one of these values to setProcessChannelMode() to set the current read channel mode.

Constant

Description

QProcess.SeparateChannels

QProcess manages the output of the running process, keeping standard output and standard error data in separate internal buffers. You can select the QProcess ‘s current read channel by calling setReadChannel() . This is the default channel mode of QProcess .

QProcess.MergedChannels

QProcess merges the output of the running process into the standard output channel (stdout). The standard error channel (stderr) will not receive any data. The standard output and standard error data of the running process are interleaved. For detached processes, the merged output of the running process is forwarded onto the main process.

QProcess.ForwardedChannels

QProcess forwards the output of the running process onto the main process. Anything the child process writes to its standard output and standard error will be written to the standard output and standard error of the main process.

QProcess.ForwardedErrorChannel

QProcess manages the standard output of the running process, but forwards its standard error onto the main process. This reflects the typical use of command line tools as filters, where the standard output is redirected to another process or a file, while standard error is printed to the console for diagnostic purposes. (This value was introduced in Qt 5.2.)

QProcess.ForwardedOutputChannel

Complementary to ForwardedErrorChannel. (This value was introduced in Qt 5.2.)

Note

Windows intentionally suppresses output from GUI-only applications to inherited consoles. This does not apply to output redirected to files or pipes. To forward the output of GUI-only applications on the console nonetheless, you must use SeparateChannels and do the forwarding yourself by reading the output and writing it to the appropriate output channels.

See also

setProcessChannelMode()

class InputChannelMode#

This enum describes the process input channel modes of QProcess . Pass one of these values to setInputChannelMode() to set the current write channel mode.

Constant

Description

QProcess.ManagedInputChannel

QProcess manages the input of the running process. This is the default input channel mode of QProcess .

QProcess.ForwardedInputChannel

QProcess forwards the input of the main process onto the running process. The child process reads its standard input from the same source as the main process. Note that the main process must not try to read its standard input while the child process is running.

See also

setInputChannelMode()

class ExitStatus#

This enum describes the different exit statuses of QProcess .

Constant

Description

QProcess.NormalExit

The process exited normally.

QProcess.CrashExit

The process crashed.

See also

exitStatus()

class UnixProcessFlag#

(inherits enum.Flag) These flags can be used in the flags field of UnixProcessParameters .

Constant

Description

QProcess.UnixProcessFlag.CloseFileDescriptors

Close all file descriptors above the threshold defined by lowestFileDescriptorToClose, preventing any currently open descriptor in the parent process from accidentally leaking to the child. The stdin, stdout, and stderr file descriptors are never closed.

QProcess.UnixProcessFlag.CreateNewSession

Starts a new process session, by calling setsid(2). This allows the child process to outlive the session the current process is in. This is one of the steps that startDetached() takes to allow the process to detach, and is also one of the steps to daemonize a process.

QProcess.UnixProcessFlag.DisconnectControllingTerminal

Requests that the process disconnect from its controlling terminal, if it has one. If it has none, nothing happens. Processes still connected to a controlling terminal may get a Hang Up (SIGHUP) signal if the terminal closes, or one of the other terminal-control signals (SIGTSTP, SIGTTIN, SIGTTOU). Note that on some operating systems, a process may only disconnect from the controlling terminal if it is the session leader, meaning the CreateNewSession flag may be required. Like it, this is one of the steps to daemonize a process.

QProcess.UnixProcessFlag.IgnoreSigPipe

Always sets the SIGPIPE signal to ignored (SIG_IGN), even if the ResetSignalHandlers flag was set. By default, if the child attempts to write to its standard output or standard error after the respective channel was closed with closeReadChannel() , it would get the SIGPIPE signal and terminate immediately; with this flag, the write operation fails without a signal and the child may continue executing.

QProcess.UnixProcessFlag.ResetIds

Drops any retained, effective user or group ID the current process may still have (see setuid(2) and setgid(2), plus QCoreApplication::setSetuidAllowed()). This is useful if the current process was setuid or setgid and does not wish the child process to retain the elevated privileges.

QProcess.UnixProcessFlag.ResetSignalHandlers

Resets all Unix signal handlers back to their default state (that is, pass SIG_DFL to signal(2)). This flag is useful to ensure any ignored (SIG_IGN) signal does not affect the child’s behavior.

QProcess.UnixProcessFlag.UseVFork

Requests that QProcess use vfork(2) to start the child process. Use this flag to indicate that the callback function set with setChildProcessModifier() is safe to execute in the child side of a vfork(2); that is, the callback does not modify any non-local variables (directly or through any function it calls), nor attempts to communicate with the parent process. It is implementation-defined if QProcess will actually use vfork(2) and if vfork(2) is different from standard fork(2).

See also

setUnixProcessParameters() unixProcessParameters()

New in version 6.6.

__init__([parent=None])#
Parameters:

parent – QObject

Constructs a QProcess object with the given parent.

arguments()#
Return type:

list of strings

Returns the command line arguments the process was last started with.

closeReadChannel(channel)#
Parameters:

channel – ProcessChannel

Closes the read channel channel. After calling this function, QProcess will no longer receive data on the channel. Any data that has already been received is still available for reading.

Call this function to save memory, if you are not interested in the output of the process.

closeWriteChannel()#

Warning

This section contains snippets that were automatically translated from C++ to Python and may contain errors.

Schedules the write channel of QProcess to be closed. The channel will close once all data has been written to the process. After calling this function, any attempts to write to the process will fail.

Closing the write channel is necessary for programs that read input data until the channel has been closed. For example, the program “more” is used to display text data in a console on both Unix and Windows. But it will not display the text data until QProcess ‘s write channel has been closed. Example:

more = QProcess()
more.start("more")
more.write("Text to display")
more.closeWriteChannel()
# QProcess will emit readyRead() once "more" starts printing

The write channel is implicitly opened when start() is called.

environment()#
Return type:

list of strings

Returns the environment that QProcess will pass to its child process, or an empty QStringList if no environment has been set using setEnvironment() . If no environment has been set, the environment of the calling process will be used.

error()#
Return type:

ProcessError

Returns the type of error that occurred last.

See also

state()

errorOccurred(error)#
Parameters:

error – ProcessError

This signal is emitted when an error occurs with the process. The specified error describes the type of error that occurred.

static execute(program[, arguments={}])#
Parameters:

  • program – str

  • arguments – list of strings

Return type:

int

Starts the program program with the arguments arguments in a new process, waits for it to finish, and then returns the exit code of the process. Any data the new process writes to the console is forwarded to the calling process.

The environment and working directory are inherited from the calling process.

Argument handling is identical to the respective start() overload.

If the process cannot be started, -2 is returned. If the process crashes, -1 is returned. Otherwise, the process’ exit code is returned.

See also

start()

exitCode()#
Return type:

int

Returns the exit code of the last process that finished.

This value is not valid unless exitStatus() returns NormalExit .

exitStatus()#
Return type:

ExitStatus

Returns the exit status of the last process that finished.

On Windows, if the process was terminated with TerminateProcess() from another application, this function will still return NormalExit unless the exit code is less than 0.

failChildProcessModifier(description[, error=0])#
Parameters:

  • description – str

  • error – int

This functions can be used inside the modifier set with setChildProcessModifier() to indicate an error condition was encountered. When the modifier calls these functions, QProcess will emit errorOccurred() with code FailedToStart in the parent process. The description can be used to include some information in errorString() to help diagnose the problem, usually the name of the call that failed, similar to the C Library function perror(). Additionally, the error parameter can be an <errno.h> error code whose text form will also be included.

For example, a child modifier could prepare an extra file descriptor for the child process this way:

process.setChildProcessModifier([fd, &process]() {
    if (dup2(fd, TargetFileDescriptor) < 0)
        process.failChildProcessModifier(errno, "aux comm channel");
});
process.start();

Where fd is a file descriptor currently open in the parent process. If the dup2() system call resulted in an EBADF condition, the process errorString() could be “Child process modifier reported error: aux comm channel: Bad file descriptor”.

This function does not return to the caller. Using it anywhere except in the child modifier and with the correct QProcess object is undefined behavior.

Note

The implementation imposes a length limit to the description parameter to about 500 characters. This does not include the text from the error code.

See also

setChildProcessModifier() setUnixProcessParameters()

finished(exitCode[, exitStatus=QProcess.ExitStatus.NormalExit])#
Parameters:

This signal is emitted when the process finishes. exitCode is the exit code of the process (only valid for normal exits), and exitStatus is the exit status. After the process has finished, the buffers in QProcess are still intact. You can still read any data that the process may have written before it finished.

See also

exitStatus()

inputChannelMode()#
Return type:

InputChannelMode

Returns the channel mode of the QProcess standard input channel.

kill()#

Kills the current process, causing it to exit immediately.

On Windows, kill() uses TerminateProcess, and on Unix and macOS, the SIGKILL signal is sent to the process.

See also

terminate()

static nullDevice()#
Return type:

str

The null device of the operating system.

The returned file path uses native directory separators.

processChannelMode()#
Return type:

ProcessChannelMode

Returns the channel mode of the QProcess standard output and standard error channels.

processEnvironment()#
Return type:

QProcessEnvironment

Returns the environment that QProcess will pass to its child process. If no environment has been set using setProcessEnvironment() , this method returns an object indicating the environment will be inherited from the parent.

See also

setProcessEnvironment() inheritsFromParent() Environment variables

processId()#
Return type:

int

Returns the native process identifier for the running process, if available. If no process is currently running, 0 is returned.

program()#
Return type:

str

Returns the program the process was last started with.

readAllStandardError()#
Return type:

QByteArray

Regardless of the current read channel, this function returns all data available from the standard error of the process as a QByteArray .

readAllStandardOutput()#
Return type:

QByteArray

Regardless of the current read channel, this function returns all data available from the standard output of the process as a QByteArray .

readChannel()#
Return type:

ProcessChannel

Returns the current read channel of the QProcess .

See also

setReadChannel()

readyReadStandardError()#

This signal is emitted when the process has made new data available through its standard error channel (stderr). It is emitted regardless of the current read channel .

readyReadStandardOutput()#

This signal is emitted when the process has made new data available through its standard output channel (stdout). It is emitted regardless of the current read channel .

setArguments(arguments)#
Parameters:

arguments – list of strings

Set the arguments to pass to the called program when starting the process. This function must be called before start() .

setEnvironment(environment)#
Parameters:

environment – list of strings

Warning

This section contains snippets that were automatically translated from C++ to Python and may contain errors.

Sets the environment that QProcess will pass to the child process. The parameter environment is a list of key=value pairs.

For example, the following code adds the environment variable TMPDIR:

process = QProcess()
env = QProcess.systemEnvironment()
env << "TMPDIR=C:\\MyApp\\temp" # Add an environment variable
process.setEnvironment(env)
process.start("myapp")

Note

This function is less efficient than the setProcessEnvironment() function.

See also

environment() setProcessEnvironment() systemEnvironment()

setInputChannelMode(mode)#
Parameters:

mode – InputChannelMode

Sets the channel mode of the QProcess standard input channel to the mode specified. This mode will be used the next time start() is called.

setProcessChannelMode(mode)#
Parameters:

mode – ProcessChannelMode

Warning

This section contains snippets that were automatically translated from C++ to Python and may contain errors.

Sets the channel mode of the QProcess standard output and standard error channels to the mode specified. This mode will be used the next time start() is called. For example:

builder = QProcess()
builder.setProcessChannelMode(QProcess.MergedChannels)
builder.start("make", QStringList() << "-j2")
if not builder.waitForFinished():
    print("Make failed:", builder.errorString())
else:
    print("Make output:", builder.readAll())

See also

processChannelMode() ProcessChannelMode setReadChannel()

setProcessEnvironment(environment)#
Parameters:

environment – QProcessEnvironment

Warning

This section contains snippets that were automatically translated from C++ to Python and may contain errors.

Sets the environment that QProcess will pass to the child process.

For example, the following code adds the environment variable TMPDIR:

process = QProcess()
env = QProcessEnvironment.systemEnvironment()
env.insert("TMPDIR", "C:\\MyApp\\temp") # Add an environment variable
process.setProcessEnvironment(env)
process.start("myapp")

Note how, on Windows, environment variable names are case-insensitive.

See also

processEnvironment() systemEnvironment() Environment variables

setProcessState(state)#
Parameters:

state – ProcessState

Sets the current state of the QProcess to the state specified.

See also

state()

setProgram(program)#
Parameters:

program – str

Set the program to use when starting the process. This function must be called before start() .

If program is an absolute path, it specifies the exact executable that will be launched. Relative paths will be resolved in a platform-specific manner, which includes searching the PATH environment variable (see Finding the Executable for details).

setReadChannel(channel)#
Parameters:

channel – ProcessChannel

Sets the current read channel of the QProcess to the given channel. The current input channel is used by the functions read() , readAll() , readLine() , and getChar() . It also determines which channel triggers QProcess to emit readyRead() .

See also

readChannel()

setStandardErrorFile(fileName[, mode=QIODeviceBase.OpenModeFlag.Truncate])#
Parameters:

Redirects the process’ standard error to the file fileName. When the redirection is in place, the standard error read channel is closed: reading from it using read() will always fail, as will readAllStandardError() . The file will be appended to if mode is Append, otherwise, it will be truncated.

See setStandardOutputFile() for more information on how the file is opened.

Note: if setProcessChannelMode() was called with an argument of MergedChannels , this function has no effect.

setStandardInputFile(fileName)#
Parameters:

fileName – str

Redirects the process’ standard input to the file indicated by fileName. When an input redirection is in place, the QProcess object will be in read-only mode (calling write() will result in error).

To make the process read EOF right away, pass nullDevice() here. This is cleaner than using closeWriteChannel() before writing any data, because it can be set up prior to starting the process.

If the file fileName does not exist at the moment start() is called or is not readable, starting the process will fail.

Calling setStandardInputFile() after the process has started has no effect.

setStandardOutputFile(fileName[, mode=QIODeviceBase.OpenModeFlag.Truncate])#
Parameters:

Redirects the process’ standard output to the file fileName. When the redirection is in place, the standard output read channel is closed: reading from it using read() will always fail, as will readAllStandardOutput() .

To discard all standard output from the process, pass nullDevice() here. This is more efficient than simply never reading the standard output, as no QProcess buffers are filled.

If the file fileName doesn’t exist at the moment start() is called, it will be created. If it cannot be created, the starting will fail.

If the file exists and mode is QIODevice::Truncate, the file will be truncated. Otherwise (if mode is QIODevice::Append), the file will be appended to.

Calling setStandardOutputFile() after the process has started has no effect.

If fileName is an empty string, it stops redirecting the standard output. This is useful for restoring the standard output after redirection.

setStandardOutputProcess(destination)#
Parameters:

destination – QProcess

Warning

This section contains snippets that were automatically translated from C++ to Python and may contain errors.

Pipes the standard output stream of this process to the destination process’ standard input.

The following shell command:

command1 | command2

Can be accomplished with QProcess with the following code:

process1 = QProcess()
process2 = QProcess()
process1.setStandardOutputProcess(process2)
process1.start("command1")
process2.start("command2")
setUnixProcessParameters(flagsOnly)#
Parameters:

flagsOnly – Combination of UnixProcessFlag

This is an overloaded function.

Sets the extra settings for the child process on Unix systems to flagsOnly. This is the same as the overload with just the flags field set.

Note

This function is only available on Unix platforms.

See also

unixProcessParameters() setChildProcessModifier()

setUnixProcessParameters(params)
Parameters:

params – UnixProcessParameters

Sets the extra settings and parameters for the child process on Unix systems to be params. This function can be used to ask QProcess to modify the child process before launching the target executable.

This function can be used to change certain properties of the child process, such as closing all extraneous file descriptors, changing the nice level of the child, or disconnecting from the controlling TTY. For more fine-grained control of the child process or to modify it in other ways, use the setChildProcessModifier() function. If both a child process modifier and Unix process parameters are set, the modifier is run before these parameters are applied.

Note

This function is only available on Unix platforms.

See also

unixProcessParameters() setChildProcessModifier()

setWorkingDirectory(dir)#
Parameters:

dir – str

Sets the working directory to dir. QProcess will start the process in this directory. The default behavior is to start the process in the working directory of the calling process.

static splitCommand(command)#
Parameters:

command – str

Return type:

list of strings

Splits the string command into a list of tokens, and returns the list.

Tokens with spaces can be surrounded by double quotes; three consecutive double quotes represent the quote character itself.

start(program[, arguments={}[, mode=QIODeviceBase.OpenModeFlag.ReadWrite]])#
Parameters:

  • program – str

  • arguments – list of strings

  • mode – Combination of OpenModeFlag

Starts the given program in a new process, passing the command line arguments in arguments. See setProgram() for information about how QProcess searches for the executable to be run. The OpenMode is set to mode. No further splitting of the arguments is performed.

The QProcess object will immediately enter the Starting state. If the process starts successfully, QProcess will emit started() ; otherwise, errorOccurred() will be emitted. Do note that on platforms that are able to start child processes synchronously (notably Windows), those signals will be emitted before this function returns and this QProcess object will transition to either Running or NotRunning state, respectively. On others paltforms, the started() and errorOccurred() signals will be delayed.

Call waitForStarted() to make sure the process has started (or has failed to start) and those signals have been emitted. It is safe to call that function even if the process starting state is already known, though the signal will not be emitted again.

Windows: The arguments are quoted and joined into a command line that is compatible with the CommandLineToArgvW() Windows function. For programs that have different command line quoting requirements, you need to use setNativeArguments() . One notable program that does not follow the CommandLineToArgvW() rules is cmd.exe and, by consequence, all batch scripts.

If the QProcess object is already running a process, a warning may be printed at the console, and the existing process will continue running unaffected.

Note

Success at starting the child process only implies the operating system has successfully created the process and assigned the resources every process has, such as its process ID. The child process may crash or otherwise fail very early and thus not produce its expected output. On most operating systems, this may include dynamic linking errors.

See also

processId() started() waitForStarted() setNativeArguments()

start([mode=QIODeviceBase.OpenModeFlag.ReadWrite])
Parameters:

mode – Combination of OpenModeFlag

This is an overloaded function.

Starts the program set by setProgram() with arguments set by setArguments() . The OpenMode is set to mode.

See also

open() setProgram() setArguments()

startCommand(command[, mode=QIODeviceBase.OpenModeFlag.ReadWrite])#
Parameters:

Warning

This section contains snippets that were automatically translated from C++ to Python and may contain errors.

Starts the command command in a new process. The OpenMode is set to mode.

command is a single string of text containing both the program name and its arguments. The arguments are separated by one or more spaces. For example:

process = QProcess()
process.startCommand("del /s *.txt")
# same as process.start("del", QStringList() << "/s" << "*.txt")
...

Arguments containing spaces must be quoted to be correctly supplied to the new process. For example:

process = QProcess()
process.startCommand("dir \"My Documents\"")

Literal quotes in the command string are represented by triple quotes. For example:

process = QProcess()
process.startCommand("dir \"Epic 12\"\"\" Singles\"")

After the command string has been split and unquoted, this function behaves like start() .

On operating systems where the system API for passing command line arguments to a subprocess natively uses a single string (Windows), one can conceive command lines which cannot be passed via QProcess ‘s portable list-based API. In these rare cases you need to use setProgram() and setNativeArguments() instead of this function.

static startDetached(program[, arguments={}[, workingDirectory=""]])#
Parameters:

  • program – str

  • arguments – list of strings

  • workingDirectory – str

Return type:

(retval, pid)

This function overloads startDetached() .

Starts the program program with the arguments arguments in a new process, and detaches from it. Returns true on success; otherwise returns false. If the calling process exits, the detached process will continue to run unaffected.

Argument handling is identical to the respective start() overload.

The process will be started in the directory workingDirectory. If workingDirectory is empty, the working directory is inherited from the calling process.

If the function is successful then *``pid`` is set to the process identifier of the started process.

See also

start()

startDetached([pid=None])
Parameters:

pid – qint64

Return type:

bool

Starts the program set by setProgram() with arguments set by setArguments() in a new process, and detaches from it. Returns true on success; otherwise returns false. If the calling process exits, the detached process will continue to run unaffected.

Unix: The started process will run in its own session and act like a daemon.

The process will be started in the directory set by setWorkingDirectory() . If workingDirectory() is empty, the working directory is inherited from the calling process.

If the function is successful then *``pid`` is set to the process identifier of the started process; otherwise, it’s set to -1. Note that the child process may exit and the PID may become invalid without notice. Furthermore, after the child process exits, the same PID may be recycled and used by a completely different process. User code should be careful when using this variable, especially if one intends to forcibly terminate the process by operating system means.

Only the following property setters are supported by startDetached():

All other properties of the QProcess object are ignored.

Note

The called process inherits the console window of the calling process. To suppress console output, redirect standard/error output to nullDevice() .

See also

start() startDetached(const QString &program, const QStringList &arguments, const QString &workingDirectory, qint64 *pid)

started()#

This signal is emitted by QProcess when the process has started, and state() returns Running .

state()#
Return type:

ProcessState

Returns the current state of the process.

stateChanged(state)#
Parameters:

state – ProcessState

This signal is emitted whenever the state of QProcess changes. The newState argument is the state QProcess changed to.

static systemEnvironment()#
Return type:

list of strings

Warning

This section contains snippets that were automatically translated from C++ to Python and may contain errors.

Returns the environment of the calling process as a list of key=value pairs. Example:

environment = QProcess.systemEnvironment()
# environment = {"PATH=/usr/bin:/usr/local/bin",
#                "USER=greg", "HOME=/home/greg"}

This function does not cache the system environment. Therefore, it’s possible to obtain an updated version of the environment if low-level C library functions like setenv or putenv have been called.

However, note that repeated calls to this function will recreate the list of environment variables, which is a non-trivial operation.

Note

For new code, it is recommended to use systemEnvironment()

See also

systemEnvironment() setProcessEnvironment()

terminate()#

Attempts to terminate the process.

The process may not exit as a result of calling this function (it is given the chance to prompt the user for any unsaved files, etc).

On Windows, terminate() posts a WM_CLOSE message to all top-level windows of the process and then to the main thread of the process itself. On Unix and macOS the SIGTERM signal is sent.

Console applications on Windows that do not run an event loop, or whose event loop does not handle the WM_CLOSE message, can only be terminated by calling kill() .

See also

kill()

unixProcessParameters()#
Return type:

UnixProcessParameters

Returns the UnixProcessParameters object describing extra flags and settings that will be applied to the child process on Unix systems. The default settings correspond to a default-constructed UnixProcessParameters .

Note

This function is only available on Unix platforms.

See also

setUnixProcessParameters() childProcessModifier()

waitForFinished([msecs=30000])#
Parameters:

msecs – int

Return type:

bool

Blocks until the process has finished and the finished() signal has been emitted, or until msecs milliseconds have passed.

Returns true if the process finished; otherwise returns false (if the operation timed out, if an error occurred, or if this QProcess is already finished).

This function can operate without an event loop. It is useful when writing non-GUI applications and when performing I/O operations in a non-GUI thread.

Warning

Calling this function from the main (GUI) thread might cause your user interface to freeze.

If msecs is -1, this function will not time out.

See also

finished() waitForStarted() waitForReadyRead() waitForBytesWritten()

waitForStarted([msecs=30000])#
Parameters:

msecs – int

Return type:

bool

Blocks until the process has started and the started() signal has been emitted, or until msecs milliseconds have passed.

Returns true if the process was started successfully; otherwise returns false (if the operation timed out or if an error occurred). If the process had already started successfully before this function, it returns immediately.

This function can operate without an event loop. It is useful when writing non-GUI applications and when performing I/O operations in a non-GUI thread.

Warning

Calling this function from the main (GUI) thread might cause your user interface to freeze.

If msecs is -1, this function will not time out.

Note

On some UNIX operating systems, this function may return true but the process may later report a FailedToStart error.

See also

started() waitForReadyRead() waitForBytesWritten() waitForFinished()

workingDirectory()#
Return type:

str

If QProcess has been assigned a working directory, this function returns the working directory that the QProcess will enter before the program has started. Otherwise, (i.e., no directory has been assigned,) an empty string is returned, and QProcess will use the application’s current working directory instead.