process-1.1.0.0: Process libraries

Portabilitynon-portable (requires concurrency)
Stabilityexperimental
Maintainer[email protected]

System.Process

Contents

Description

Operations for creating and interacting with sub-processes.

Synopsis

Running sub-processes

createProcess :: CreateProcess -> IO (Maybe Handle, Maybe Handle, Maybe Handle, ProcessHandle)Source

This is the most general way to spawn an external process. The process can be a command line to be executed by a shell or a raw command with a list of arguments. The stdin, stdout, and stderr streams of the new process may individually be attached to new pipes, to existing Handles, or just inherited from the parent (the default.)

The details of how to create the process are passed in the CreateProcess record. To make it easier to construct a CreateProcess, the functions proc and shell are supplied that fill in the fields with default values which can be overriden as needed.

createProcess returns (mb_stdin_hdl, mb_stdout_hdl, mb_stderr_hdl, p), where

  • if std_in == CreatePipe, then mb_stdin_hdl will be Just h, where h is the write end of the pipe connected to the child process's stdin.
  • otherwise, mb_stdin_hdl == Nothing

Similarly for mb_stdout_hdl and mb_stderr_hdl.

For example, to execute a simple ls command:

   r <- createProcess (proc "ls" [])

To create a pipe from which to read the output of ls:

   (_, Just hout, _, _) <-
       createProcess (proc "ls" []){ std_out = CreatePipe }

To also set the directory in which to run ls:

   (_, Just hout, _, _) <-
       createProcess (proc "ls" []){ cwd = Just "\home\bob",
                                     std_out = CreatePipe }

shell :: String -> CreateProcessSource

Construct a CreateProcess record for passing to createProcess, representing a command to be passed to the shell.

proc :: FilePath -> [String] -> CreateProcessSource

Construct a CreateProcess record for passing to createProcess, representing a raw command with arguments.

data CreateProcess Source

Constructors

CreateProcess 

Fields

cmdspec :: CmdSpec

Executable & arguments, or shell command

cwd :: Maybe FilePath

Optional path to the working directory for the new process

env :: Maybe [(String, String)]

Optional environment (otherwise inherit from the current process)

std_in :: StdStream

How to determine stdin

std_out :: StdStream

How to determine stdout

std_err :: StdStream

How to determine stderr

close_fds :: Bool

Close all file descriptors except stdin, stdout and stderr in the new process (on Windows, only works if std_in, std_out, and std_err are all Inherit)

create_group :: Bool

Create a new process group

data CmdSpec Source

Constructors

ShellCommand String

a command line to execute using the shell

RawCommand FilePath [String]

the filename of an executable with a list of arguments

data StdStream Source

Constructors

Inherit

Inherit Handle from parent

UseHandle Handle

Use the supplied Handle

CreatePipe

Create a new pipe. The returned Handle will use the default encoding and newline translation mode (just like Handles created by openFile).

Specific variants of createProcess

runCommand :: String -> IO ProcessHandleSource

Runs a command using the shell.

runProcessSource

Arguments

:: FilePath

Filename of the executable

-> [String]

Arguments to pass to the executable

-> Maybe FilePath

Optional path to the working directory

-> Maybe [(String, String)]

Optional environment (otherwise inherit)

-> Maybe Handle

Handle to use for stdin (Nothing => use existing stdin)

-> Maybe Handle

Handle to use for stdout (Nothing => use existing stdout)

-> Maybe Handle

Handle to use for stderr (Nothing => use existing stderr)

-> IO ProcessHandle 

Runs a raw command, optionally specifying Handles from which to take the stdin, stdout and stderr channels for the new process (otherwise these handles are inherited from the current process).

Any Handles passed to runProcess are placed immediately in the closed state.

Note: consider using the more general createProcess instead of runProcess.

runInteractiveCommand :: String -> IO (Handle, Handle, Handle, ProcessHandle)Source

Runs a command using the shell, and returns Handles that may be used to communicate with the process via its stdin, stdout, and stderr respectively. The Handles are initially in binary mode; if you need them to be in text mode then use hSetBinaryMode.

runInteractiveProcessSource

Arguments

:: FilePath

Filename of the executable

-> [String]

Arguments to pass to the executable

-> Maybe FilePath

Optional path to the working directory

-> Maybe [(String, String)]

Optional environment (otherwise inherit)

-> IO (Handle, Handle, Handle, ProcessHandle) 

Runs a raw command, and returns Handles that may be used to communicate with the process via its stdin, stdout and stderr respectively.

For example, to start a process and feed a string to its stdin:

   (inp,out,err,pid) <- runInteractiveProcess "..."
   forkIO (hPutStr inp str)

The Handles are initially in binary mode; if you need them to be in text mode then use hSetBinaryMode.

readProcessSource

Arguments

:: FilePath

command to run

-> [String]

any arguments

-> String

standard input

-> IO String

stdout

readProcess forks an external process, reads its standard output strictly, blocking until the process terminates, and returns the output string.

Output is returned strictly, so this is not suitable for interactive applications.

Users of this function should compile with -threaded if they want other Haskell threads to keep running while waiting on the result of readProcess.

  > readProcess "date" [] []
  "Thu Feb  7 10:03:39 PST 2008\n"

The arguments are:

  • The command to run, which must be in the $PATH, or an absolute path
  • A list of separate command line arguments to the program
  • A string to pass on the standard input to the program.

readProcessWithExitCodeSource

Arguments

:: FilePath

command to run

-> [String]

any arguments

-> String

standard input

-> IO (ExitCode, String, String)

exitcode, stdout, stderr

readProcessWithExitCode creates an external process, reads its standard output and standard error strictly, waits until the process terminates, and then returns the ExitCode of the process, the standard output, and the standard error.

readProcess and readProcessWithExitCode are fairly simple wrappers around createProcess. Constructing variants of these functions is quite easy: follow the link to the source code to see how readProcess is implemented.

system :: String -> IO ExitCodeSource

Computation system cmd returns the exit code produced when the operating system runs the shell command cmd.

This computation may fail with

  • PermissionDenied: The process has insufficient privileges to perform the operation.
  • ResourceExhausted: Insufficient resources are available to perform the operation.
  • UnsupportedOperation: The implementation does not support system calls.

On Windows, system passes the command to the Windows command interpreter (CMD.EXE or COMMAND.COM), hence Unixy shell tricks will not work.

rawSystem :: String -> [String] -> IO ExitCodeSource

The computation rawSystem cmd args runs the operating system command cmd in such a way that it receives as arguments the args strings exactly as given, with no funny escaping or shell meta-syntax expansion. It will therefore behave more portably between operating systems than system.

The return codes and possible failures are the same as for system.

showCommandForUser :: FilePath -> [String] -> StringSource

Given a program p and arguments args, showCommandForUser p args returns a string suitable for pasting into sh (on POSIX OSs) or cmd.exe (on Windows).

Process completion

waitForProcess :: ProcessHandle -> IO ExitCodeSource

Waits for the specified process to terminate, and returns its exit code.

GHC Note: in order to call waitForProcess without blocking all the other threads in the system, you must compile the program with -threaded.

getProcessExitCode :: ProcessHandle -> IO (Maybe ExitCode)Source

This is a non-blocking version of waitForProcess. If the process is still running, Nothing is returned. If the process has exited, then Just e is returned where e is the exit code of the process.

terminateProcess :: ProcessHandle -> IO ()Source

Attempts to terminate the specified process. This function should not be used under normal circumstances - no guarantees are given regarding how cleanly the process is terminated. To check whether the process has indeed terminated, use getProcessExitCode.

On Unix systems, terminateProcess sends the process the SIGTERM signal. On Windows systems, the Win32 TerminateProcess function is called, passing an exit code of 1.

Note: on Windows, if the process was a shell command created by createProcess with shell, or created by runCommand or runInteractiveCommand, then terminateProcess will only terminate the shell, not the command itself. On Unix systems, both processes are in a process group and will be terminated together.

interruptProcessGroupOfSource

Arguments

:: ProcessHandle

Lead process in the process group

-> IO () 

Sends an interrupt signal to the process group of the given process.

On Unix systems, it sends the group the SIGINT signal.

On Windows systems, it generates a CTRL_BREAK_EVENT and will only work for processes created using createProcess and setting the create_group flag