Donald Bruce Stewart wrote:
christian.lean2:
Donald Bruce Stewart wrote:
christian.lean2:
I'm looking for a way to run an external program and get the results in
haskell. Something similar to HSH but that will work in windows. I don't
need anything too complex, just to provide the command to be run as a
string and get the result as a string. Is this possible?
One of the HSH examples is all I need:
runS $ "echo /etc/pass*"
-> "/etc/passwd /etc/passwd-"
System.Process is your friend. E.g.
import System.Process
import Control.Exception
import System.IO
import System.Exit
--
-- Run a normal shell command
--
run :: String -> IO String
run s = handle (fail . show) $ do
(ih,oh,eh,pid) <- runInteractiveCommand s
so <- hGetContents oh
se <- hGetContents eh
hClose ih
ex <- waitForProcess pid
case ex of
ExitFailure e -> fail $ "Failed with status: " ++ show e
_ | not (null se) -> fail se
| otherwise -> return so
*Process> run $ "echo *.hs"
"Main.hs Process.hs Shell.hs\n"
-- Don
Thank you for the speedy reply, I am trying to follow what the code is
trying to do, is this correct?
runInteractiveCommand:
This gives you handles for stdin, stdout, stderr, and the process
running the command.
You can then use functions like hPutStrLn with stdin or hGetContents
with stdout and stderr.
Right.
waitForProcess just stops that part of the code until the command has
finished.
Yep.
Is nClose ih necessary, none of the others are closed so I'm guessing
this is needed else waitForProcess will never finish.
They should be closed too I suspect.
Does this have to do with run_Interactive_Command, could I keep passing
commands and getting responses until I close the input.
Not with that code, but you could modify it to do so.
The two parts i'm having trouble following are:
run s = handle (fail . show) $ do
That's an exception handler that just shows the exception in the current
monad.
handle has type:
handle :: (Exception -> IO a) -> IO a -> IO a
Is (fail . show) composing a fuction of type
Exception -> IO a)
and what part is the middle IO a, is that the whole block that
follows(ie the do block)
Yes.
Also
case ex of
ExitFailure e -> fail $ "Failed with status: " ++ show e
_ | not (null se) -> fail se
| otherwise -> return so
I dont quite get this part. I understand that you are checking for
errors(not (null se)) otherwise returning everything gotten from stdout
but I don't get
the ExitFailure line.
That checks the exit status of the program, as reported by
waitForProcess, the other cases check for stderr.
I have one final question, how should you go about finding functions
like runInteractiveCommand?
hoogle!
http://www.haskell.org/hoogle/?q=Command
If I had that to begin with I might have had a chance to try some of the
above on my own but I find that it is difficult to know where to start.
Hoogle, and the library docs, are helpful here.
-- Don
Just a quick thankyou, the response has been very helpful.
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