System.Cmd.Utils

Command invocation utilities.

Written by John Goerzen, [e-mail]

Please note: Most of this module is not compatible with Hugs.

Command lines executed will be logged using System.Log.Logger at the DEBUG level. Failure messages will be logged at the WARNING level in addition to being raised as an exception. Both are logged under "System.Cmd.Utils.funcname" -- for instance, "System.Cmd.Utils.safeSystem". If you wish to suppress these messages globally, you can simply run:

 updateGlobalLogger "System.Cmd.Utils.safeSystem"
                     (setLevel CRITICAL)

See also: updateGlobalLogger, System.Log.Logger.

It is possible to set up pipelines with these utilities. Example:

 (pid1, x1) <- pipeFrom "ls" ["/etc"]
 (pid2, x2) <- pipeBoth "grep" ["x"] x1
 putStr x2
 ... the grep output is displayed ...
 forceSuccess pid2
 forceSuccess pid1

Remember, when you use the functions that return a String, you must not call forceSuccess until after all data from the String has been consumed. Failure to wait will cause your program to appear to hang.

Here is an example of the wrong way to do it:

 (pid, x) <- pipeFrom "ls" ["/etc"]
 forceSuccess pid         -- Hangs; the called program hasn't terminated yet
 processTheData x

You must instead process the data before calling forceSuccess.

When using the hPipe family of functions, this is probably more obvious.

Most of this module will be incompatible with Windows.

High-Level Tools

data PipeHandle

Return value from pipeFrom, pipeLinesFrom, pipeTo, or pipeBoth. Contains both a ProcessID and the original command that was executed. If you prefer not to use forceSuccess on the result of one of these pipe calls, you can use (processID ph), assuming ph is your PipeHandle, as a parameter to getProcessStatus.

Constructors

PipeHandle

Fields

processID :: ProcessID

phCommand :: FilePath

phArgs :: [String]

phCreator :: String

Function that created it

Instances

Eq PipeHandle

Show PipeHandle

safeSystem :: FilePath -> [String] -> IO ()

Invokes the specified command in a subprocess, waiting for the result. If the command terminated successfully, return normally. Otherwise, raises a userError with the problem.

Implemented in terms of posixRawSystem where supported, and System.Posix.rawSystem otherwise.

forceSuccess :: PipeHandle -> IO ()

Uses getProcessStatus to obtain the exit status of the given process ID. If the process terminated normally, does nothing. Otherwise, raises an exception with an appropriate error message.

This call will block waiting for the given pid to terminate.

Not available on Windows.

posixRawSystem :: FilePath -> [String] -> IO ProcessStatus

Invokes the specified command in a subprocess, waiting for the result. Return the result status. Never raises an exception. Only available on POSIX platforms.

Like system(3), this command ignores SIGINT and SIGQUIT and blocks SIGCHLD during its execution.

Logs as System.Cmd.Utils.posixRawSystem

forkRawSystem :: FilePath -> [String] -> IO ProcessID

Invokes the specified command in a subprocess, without waiting for the result. Returns the PID of the subprocess -- it is YOUR responsibility to use getProcessStatus or getAnyProcessStatus on that at some point. Failure to do so will lead to resource leakage (zombie processes).

This function does nothing with signals. That too is up to you.

Logs as System.Cmd.Utils.forkRawSystem

Piping with lazy strings

pipeFrom :: FilePath -> [String] -> IO (PipeHandle, String)

Read data from a pipe. Returns a lazy string and a PipeHandle.

ONLY AFTER the string has been read completely, You must call either getProcessStatus or forceSuccess on the PipeHandle. Zombies will result otherwise.

Not available on Windows.

pipeLinesFrom :: FilePath -> [String] -> IO (PipeHandle, [String])

Like pipeFrom, but returns data in lines instead of just a String. Shortcut for calling lines on the result from pipeFrom.

Note: this function logs as pipeFrom.

Not available on Windows.

pipeTo :: FilePath -> [String] -> String -> IO PipeHandle

Write data to a pipe. Returns a ProcessID.

You must call either getProcessStatus or forceSuccess on the ProcessID. Zombies will result otherwise.

Not available on Windows.

pipeBoth :: FilePath -> [String] -> String -> IO (PipeHandle, String)

Like a combination of pipeTo and pipeFrom; forks an IO thread to send data to the piped program, and simultaneously returns its output stream.

The same note about checking the return status applies here as with pipeFrom.

Not available on Windows.

Piping with handles

hPipeFrom :: FilePath -> [String] -> IO (PipeHandle, Handle)

Read data from a pipe. Returns a Handle and a PipeHandle.

When done, you must hClose the handle, and then use either forceSuccess or getProcessStatus on the PipeHandle. Zombies will result otherwise.

This function logs as pipeFrom.

Not available on Windows or with Hugs.

hPipeTo :: FilePath -> [String] -> IO (PipeHandle, Handle)

Write data to a pipe. Returns a PipeHandle and a new Handle to write to.

When done, you must hClose the handle, and then use either forceSuccess or getProcessStatus on the PipeHandle. Zombies will result otherwise.

This function logs as pipeTo.

Not available on Windows.

hPipeBoth :: FilePath -> [String] -> IO (PipeHandle, Handle, Handle)

Like a combination of hPipeTo and hPipeFrom; returns a 3-tuple of (PipeHandle, Data From Pipe, Data To Pipe).

When done, you must hClose both handles, and then use either forceSuccess or getProcessStatus on the PipeHandle. Zombies will result otherwise.

Hint: you will usually need to ForkIO a thread to handle one of the Handles; otherwise, deadlock can result.

This function logs as pipeBoth.

Not available on Windows.

Low-Level Tools

data PipeMode

Constructors

ReadFromPipe

WriteToPipe

pOpen :: PipeMode -> FilePath -> [String] -> (Handle -> IO a) -> IO a

Open a pipe to the specified command.

Passes the handle on to the specified function.

The PipeMode specifies what you will be doing. That is, specifing ReadFromPipe sets up a pipe from stdin, and WriteToPipe sets up a pipe from stdout.

Not available on Windows.

pOpen3

Arguments

:: Maybe Fd

Send stdin to this fd

-> Maybe Fd

Get stdout from this fd

-> Maybe Fd

Get stderr from this fd

-> FilePath

Command to run

-> [String]

Command args

-> (ProcessID -> IO a)

Action to run in parent

-> IO ()

Action to run in child before execing (if you don't need something, set this to return ()) -- IGNORED IN HUGS

-> IO a

Runs a command, redirecting things to pipes.

Not available on Windows.

Note that you may not use the same fd on more than one item. If you want to redirect stdout and stderr, dup it first.

pOpen3Raw

Arguments

:: Maybe Fd

Send stdin to this fd

-> Maybe Fd

Get stdout from this fd

-> Maybe Fd

Get stderr from this fd

-> FilePath

Command to run

-> [String]

Command args

-> IO ()

Action to run in child before execing (if you don't need something, set this to return ()) -- IGNORED IN HUGS

-> IO ProcessID

Runs a command, redirecting things to pipes.

Not available on Windows.

Returns immediately with the PID of the child. Using waitProcess on it is YOUR responsibility!

Note that you may not use the same fd on more than one item. If you want to redirect stdout and stderr, dup it first.